Research Highlights

Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits

In this communication the authors report the investigation of the magnetization dynamics of a vanadyl complex with diethyldithiocarbamate (Et2dtc-) ligands in both solid-state and frozen solution. The investigation showed an anomalous and unprecedentedly observed field dependence of the relaxation time, which was modeled with three contributions to the relaxation mechanism. The temperature dependence of the weight of the two processes dominating at low fields was found to well correlate with the low energy vibrations as determined by THz spectroscopy. This detailed experimental comparative study represents a fundamental step to understand the spin dynamics of potential molecular quantum bits and enriches the guidelines to design molecule-based systems with enhanced quantum coherence.

Matteo Atzori, Lorenzo Tesi, Stefano Benci, Alessandro Lunghi, Roberto Righini, Andrea Taschin, Renato Torre, Lorenzo Sorace, Roberta Sessoli, Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits, J. Am. Chem. Soc., (2017) Article ASAP

The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets

The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here the authors propose spin-phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations the authors show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. The findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets.

Alessandro Lunghi, Federico Totti, Roberta Sessoli, Stefano Sanvito, The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets, Nature Communications 8, 14620 (2017)

Pressure induced enhancement of the magnetic ordering temperature in rhenium(IV) monomers

Materials that demonstrate long-range magnetic order are synonymous with information storage and the electronics industry, with the phenomenon commonly associated with metals, metal alloys or metal oxides and sulfides. A lesser known family of magnetically ordered complexes are the monometallic compounds of highly anisotropic d-block transition metals; the 'transformation' from isolated zero-dimensional molecule to ordered, spin-canted, three-dimensional lattice being the result of through-space interactions arising from the combination of large magnetic anisotropy and spin-delocalization from metal to ligand which induces important intermolecular contacts. Here the authors report the effect of pressure on two such mononuclear rhenium(IV) compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, with Tc controlled by the strength of the intermolecular interactions. As these are determined by intermolecular distance, 'squeezing' the molecules closer together generates remarkable enhancements in ordering temperatures, with a linear dependence of Tc with pressure.

Christopher H. Woodall, Gavin A. Craig, Alessandro Prescimone, Martin Misek, Joan Cano, Juan Faus, Michael R. Probert, Simon Parsons, Stephen Moggach, Jose Martinez-Lillo, Mark Murrie, Konstantin V. Kamenev, Euan K. Brechin, Pressure induced enhancement of the magnetic ordering temperature in rhenium(IV) monomers, Nature Communications 7, 13870 (2016)

Precision ESR measurements of transverse anisotropy in the single-molecule magnet Ni4

The authors present a method for precisely measuring the tunnel splitting in single-molecule magnets (SMMs) using electron-spin resonance, and use these measurements to precisely and independently determine the underlying transverse anisotropy parameter, given a certain class of transitions. By diluting samples of the SMM Ni4 via cocrystallization in a diamagnetic isostructural analog we obtain markedly narrower resonance peaks than are observed in undiluted samples. Using custom loop-gap resonators we measure the transitions at several frequencies, allowing a precise determination of the tunnel splitting. Because the transition under investigation occurs at zero field, and arises due to a first-order perturbation from the transverse anisotropy, we can determine the magnitude of this anisotropy independent of any other Hamiltonian parameters. This method can be applied to other SMMs with tunnel splittings arising from first-order transverse anisotropy perturbations.

Charles A. Collett, Rafael A. Allao Cassaro, and Jonathan R. Friedman, Precision ESR measurements of transverse anisotropy in the single-molecule magnet Ni4, Phys. Rev. B 94, 220402(R) (2016)

First cage-like pentanuclear Co(II)-silsesquioxane

A new pentanuclear cylinder-like cobalt(II) phenylsilsesquioxane exhibits a slow relaxation of the magnetization and a high catalytic activity and stereoselectivity in the oxidation of alkanes and alcohols.

A. N. Bilyachenko, A. I. Yalymov, M. M. Levitsky, A. A. Korlyukov, M. A. Es'kova, J. Long, J. Larionova, Y. Guari, L. S. Shul'pina, N. S. Ikonnikov, A. L. Trigub, Y. V. Zubavichus, I. E. Golub, E. S. Shubina and G. B. Shul'pin, First cage-like pentanuclear Co(II)-silsesquioxane, Dalton Trans., 2016,45, 13663-13666

Molecular Magnetic Materials: Concepts and Applications

A comprehensive overview of this rapidly expanding interdisciplinary field of research. After a short introduction to the basics of magnetism and molecular magnetism, the text goes on to cover specific properties of molecular magnetic materials as well as their current and future applications. Design strategies for acquiring molecular magnetic materials with desired physical properties are discussed, as are such multifunctional materials as high Tc magnets, chiral and luminescent magnets, magnetic sponges as well as photo- and piezo-switching magnets. The result is an excellent resource for materials scientists, chemists, physicists and crystal engineers either entering or already working in the field.

With contributions of: Maria Balanda, Robert Pelka, Michael Shatruk, Silvia Gomez-Coca, Kim R. Dunbar, Zhao-Ping Ni, Ming-Liang Tong, Masahiro Yamashita, Keiichi Katoh, Paolo Santini, Stefano Carretta, Giuseppe Amoretti, Kasper S. Pedersen, Alessandro Vindigni, Roberta Sessoli, Claude Coulon, Rodolphe Clerac, Joel S. Miller, Shin-ichi Ohkoshi, Andrea Cornia, Daniel R. Talham, Marco Affronte, Ana B. Gaspar, Birgit Weber, Wei-Xiong Zhang, Ming-Hua Zeng, Xiao-Ming Chen, Dawid Pinkowicz, Robet Podgajny, Barbara Sieklucka, Cyrille Train, Geert Rikken, Michel Verdaguer, Corine Mathoniere, Hiroko Tokoro, Shin-ichi Ohkoshi , Mauro Perfetti, Fabrice Pointillart, Olivier Cador, Lorenzo Sorace, Lahcene Ouahab, Yoshihiro Sekine, Wataru Kosaka, Kouji Taniguchi, Hitoshi Miyasaka, Thomas T. M. Palstra, Alexey O. Polyakov, Jordi Cirera, Eliseo Ruiz, Jürgen Schnack and Coen de Graaf.

Barbara Sieklucka (Editor), Dawid Pinkowicz (Editor), Molecular Magnetic Materials: Concepts and Applications, WILEY, 2016, ISBN: 978-3-527-33953-2

On Approaching the Limit of Molecular Magnetic Anisotropy: A Near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet

The authos report a monometallic dysprosium complex, that shows the largest effective energy barrier to magnetic relaxation of Ueff=1815 K. The massive magnetic anisotropy is due to bis-trans-disposed tert-butoxide ligands with weak equatorial pyridine donors, approaching proposed schemes for high-temperature single-molecule magnets (SMMs). The blocking temperature is 14 K, defined by zero-field-cooled magnetization experiments, and is the largest for any monometallic complex.

You-Song Ding, Nicholas F. Chilton, Richard E. P. Winpenny, Yan-Zhen Zheng, On Approaching the Limit of Molecular Magnetic Anisotropy: A Near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet, Angew. Chem. Int. Ed. (2016) early view

Research Update: The mechanocaloric potential of spin crossover compounds

The author presents a first evaluation of the potential for spin crossover (SCO) compounds to be considered as a new class of giant mechanocaloric effect materials. From literature data on the variation of the spin crossover temperature with pressure, we estimate the maximum available adiabatic temperature change for several compounds and the relatively low pressures that may be required to observe these effects.

Karl G. Sandeman, Research Update: The mechanocaloric potential of spin crossover compounds, APL Mater. 4, 111102 (2016)

Observation of Tunneling-Assisted Highly Forbidden Single-Photon Transitions in a Ni4 Single-Molecule Magnet

Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here the authors report observations of highly forbidden transitions between spin states in a Ni4 single-molecule magnet in which a single photon can induce the spin to change by several times hbar, nearly reversing the direction of the spin. These observations are understood as tunneling-assisted transitions that lift the standard Delta m=+/-1 selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon Delta m=+/-1 transitions.

Yiming Chen, Mohammad D. Ashkezari, Charles A. Collett, Rafael A. Allao Cassaro, Filippo Troiani, Paul M. Lahti, and Jonathan R. Friedman, Observation of Tunneling-Assisted Highly Forbidden Single-Photon Transitions in a Ni4 Single-Molecule Magnet, Phys. Rev. Lett. 117, 187202 (2016)

Experimental and theoretical investigation of the magnetization dynamics of an artificial square spin ice cluster

The authors study the magnetization dynamics of a spin ice cluster which is a building block of an artificial square spin ice fabricated by focused electron-beam-induced deposition both experimentally and theoretically. The spin ice cluster is composed of twelve interacting Co nanoislands grown directly on top of a high-resolution micro-Hall sensor. By employing micromagnetic simulations and a macrospin model, we calculate the magnetization and the experimentally investigated stray field emanating from a single nanoisland. The parameters determined from a comparison with the experimental hysteresis loop are used to derive an effective single-dipole macrospin model that allows us to investigate the dynamics of the spin ice cluster. Our model reproduces the experimentally observed non-deterministic sequences in the magnetization curves as well as the distinct temperature dependence of the hysteresis loop.

Merlin Pohlit, Irina Stockem, Fabrizio Porrati, Michael Huth, Christian Schröder and Jens Müller, Experimental and theoretical investigation of the magnetization dynamics of an artificial square spin ice cluster, J. Appl. Phys. 120, 142103 (2016)

Understanding Thermodynamic and Spectroscopic Properties of Tetragonal Mn12 Single-Molecule Magnets from Combined Density Functional Theory/Spin-Hamiltonian Calculations

The authors apply broken-symmetry density functional theory to determine isotropic exchange-coupling constants and local zero-field splitting (ZFS) tensors for the tetragonal Mn12tBuAc single-molecule magnet. The obtained parametrization of the many-spin Hamiltonian (MSH), taking into account all 12 spin centers, is assessed by comparing theoretical predictions for thermodynamic and spectroscopic properties with available experimental data. The magnetic susceptibility (calculated by the finite-temperature Lanczos method) is well approximated, and the intermultiplet excitation spectrum from inelastic neutron scattering (INS) experiments is correctly reproduced. In these respects, the present parametrization of the 12-spin model represents a significant improvement over previous theoretical estimates of exchange-coupling constants in Mn12, and additionally offers a refined interpretation of INS spectra. Treating anisotropic interactions at the third order of perturbation theory, the MSH is mapped onto the giant-spin Hamiltonian describing the S = 10 ground multiplet. Although the agreement with high-field EPR experiments is not perfect, the results clearly point in the right direction and for the first time rationalize the angular dependence of the transverse-field spectra from a fully microscopic viewpoint. Importantly, transverse anisotropy of the effective S = 10 manifold is explicitly shown to arise largely from the ZFS-induced mixing of exchange multiplets. This effect is given a thorough analysis in the approximate D2d spin-permutational symmetry group of the exchange Hamiltonian.

Shadan Ghassemi Tabrizi, Alexei V. Arbuznikov, and Martin Kaupp, Understanding Thermodynamic and Spectroscopic Properties of Tetragonal Mn12 Single-Molecule Magnets from Combined Density Functional Theory/Spin-Hamiltonian Calculations, J. Phys. Chem. A, 2016, 120 (34), pp 6864-6879

Electron paramagnetic resonance of individual atoms on a surface

Electron paramagnetic resonance (EPR) usually detects atoms with unpaired electrons as ensemble averages. The authors employed a spin-polarized scanning tunneling microscope tip to measure EPR spectra of single iron atoms adsorbed on a magnesium oxide surface at cryogenic temperatures. The high-energy resolution of conventional spin resonance was combined with scanning tunneling microscopy to measure electron paramagnetic resonance of individual iron (Fe) atoms placed on a magnesium oxide film. The spin resonance was driven with an oscillating electric field between tip and sample. The readout of the Fe atom's quantum state was performed by spin-polarized detection of the atomic-scale tunneling magnetoresistance. T1 and T2 times were determined. The spin resonance signals of different Fe atoms differ by much more than their resonance linewidth; in a traditional ensemble measurement, this difference would appear as inhomogeneous broadening.

Susanne Baumann, William Paul, Taeyoung Choi, Christopher P. Lutz, Arzhang Ardavan, Andreas J. Heinrich, Electron paramagnetic resonance of individual atoms on a surface, Science 23 Oct 2015, Vol. 350, Issue 6259, pp. 417-420

Quantum Einstein-de Haas effect

The classical Einstein-de Haas experiment demonstrates that a change of magnetization in a macroscopic magnetic object results in a mechanical rotation of this magnet. This experiment can therefore be considered as a macroscopic manifestation of the conservation of total angular momentum and energy of electronic spins. Since the conservation of angular momentum is a consequence of a system's rotational invariance, it is valid for an ensemble of spins in a macroscopic ferromaget as well as for single spins. Here we propose an experimental realization of an Einstein-de Haas experiment at the single-spin level based on a single-molecule magnet coupled to a nanomechanical resonator. We demonstrate that the spin associated with the single-molecule magnet is then subject to conservation of total angular momentum and energy, which results in a total suppression of the molecule's quantum tunnelling of magnetization.

Marc Ganzhorn, Svetlana Klyatskaya, Mario Ruben, Wolfgang Wernsdorfer, Quantum Einstein-de Haas effect, Nature Communications 7, 11443 (2016)

The classical and quantum dynamics of molecular spins on graphene

Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic and quantum computing devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics and electrical spin manipulation. However, the influence of the graphene environment on the spin systems has yet to be unravelled. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets on graphene. Whereas the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly developed model. Coupling to Dirac electrons introduces a dominant quantum relaxation channel that, by driving the spins over Villain's threshold, gives rise to fully coherent, resonant spin tunnelling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin manipulation in graphene nanodevices.

Christian Cervetti, Angelo Rettori, Maria Gloria Pini, Andrea Cornia, Ana Repolles, Fernando Luis, Martin Dressel, Stephan Rauschenbach, Klaus Kern, Marko Burghard, Lapo Bogani, The classical and quantum dynamics of molecular spins on graphene, Nature Materials 15, 164-168 (2016)

Making hybrid [n]-rotaxanes as supramolecular arrays of molecular electron spin qubits

Quantum information processing (QIP) would require that the individual units involved-qubits-communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic-inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear Cr7Ni coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron-electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates.

Antonio Fernandez, Jesus Ferrando-Soria, Eufemio Moreno Pineda, Floriana Tuna, Inigo J. Vitorica-Yrezabal, Christiane Knappke, Jakub Ujma, Christopher A. Muryn, Grigore A. Timco, Perdita E. Barran, Arzhang Ardavan, Richard E.P. Winpenny, Making hybrid [n]-rotaxanes as supramolecular arrays of molecular electron spin qubits, NATURE COMMUNICATIONS 7, 10240 (2016)

Studies on the Magnetic Ground State of a Spin Möbius Strip

The authors report the synthesis, structure and detailed characterisation of three n-membered oxovanadium rings. Their alternating heterometallic vanadium/sodium cyclic core structures were sandwiched between two CD moieties such that O-Na-O groups separated the neighbouring vanadyl ions. Antiferromagnetic interactions between the S=1/2 vanadyl ions led to S=0 ground states for the even-membered rings, but to two quasi-degenerate S=1/2 states for the spin-frustrated heptanuclear cluster. This is one of the rather rare examples of odd membered antiferromagnetic spin rings.

Graham N. Newton, Norihisa Hoshino, Takuto Matsumoto, Takuya Shiga, Motohiro Nakano, Hiroyuki Nojiri, Wolfgang Wernsdorfer, Yuji Furukawa, and Hiroki Oshio, Studies on the Magnetic Ground State of a Spin Möbius Strip, Chemistry - A European Journal 22 (2016) 14205

Dynamical effects on the magnetic properties of dithiazolyl bistable materials

The magnetic properties of molecule-based magnets are commonly rationalized by considering only a single nuclear configuration of the system under study (usually an X-ray crystal structure). Here, by means of a computational study, we compare the results obtained using such a static approach with those obtained by explicitly accounting for thermal fluctuations, and uncover the serious limitations of the static perspective when dealing with magnetic crystals whose radicals undergo wide-amplitude motions. As a proof of concept, these limitations are illustrated for the magnetically bistable 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) material. For its high-temperature phase at 300 K, we show that nuclear dynamics induce large fluctuations in the magnetic exchange interactions (J) between spins (up to 1000% of the average value). These deviations result in a about 20% difference between the 300 K magnetic susceptibility computed by explicitly considering the nuclear dynamics and that computed using the X-ray structure, the former being in better agreement with the experimental data. The unveiled strong coupling between J interactions and intermolecular vibrations reveals that considering J as a constant value at a given temperature (as always done in molecular magnetism) leads to a flawed description of the magnetism of TTTA. Instead, the physically relevant concept in this case is the statistical distribution of J values.

Sergi Vela, Merce Deumal, Motoyuki Shiga, Juan J. Novoa, Jordi Ribas-Arino, Dynamical effects on the magnetic properties of dithiazolyl bistable materials, Chem. Sci., 2015, 6, 2371-2381

Enhancing coherence in molecular spin qubits via atomic clock transitions

Quantum computing is an emerging area within the information sciences revolving around the concept of quantum bits (qubits). A major obstacle is the extreme fragility of these qubits due to interactions with their environment that destroy their quantumness. This phenomenon, known as decoherence, is of fundamental interest. When dealing with spin qubits, the strongest source of decoherence is the magnetic dipolar interaction. In This publication the authors present a way of enhancing coherence in solid-state molecular spin qubits without resorting to extreme dilution. It is based on the design of molecular structures with crystal field ground states possessing large tunnelling gaps that give rise to optimal operating points, or atomic clock transitions, at which the quantum spin dynamics become protected against dipolar decoherence. This approach is illustrated with a holmium molecular nanomagnet in which long coherence times (up to 8.4 microseconds at 5 kelvin) are obtained at unusually high concentrations.

Muhandis Shiddiq, Dorsa Komijani, Yan Duan, Alejandro Gaita-Arino, Eugenio Coronado, Stephen Hill, Enhancing coherence in molecular spin qubits via atomic clock transitions, Nature 531, 348-351 (2016)

A Mixed-Ligand Approach for a Gigantic and Hollow Heterometallic Cage {Ni64RE96} for Gas Separation and Magnetic Cooling Applications

Nanosized aggregations of metal ions shielded by organic ligands possessing both exquisite structural aesthetics and intriguing properties are fundamentally interesting. Three isostructural gigantic transition-metal-rare-earth heterometallic coordination cages are reported, abbreviated as {Ni64RE96} (RE=Gd, Dy, and Y) and obtained by a mixed-ligand approach, each possessing a cuboidal framework made of 160 metal ions and a nanosized spherical cavity in the center. Along with the structural novelty, these hollow cages show highly selective adsorptions for CO2 over CH4 or N2 at ambient temperatures. Moreover, the gadolinium analogue exhibits large magnetocaloric effect at ultralow temperatures.

Wei-Peng Chen, Pei-Qin Liao, Youzhu Yu, Zhiping Zheng, Xiao-Ming Chen, Yan-Zhen Zheng A Mixed-Ligand Approach for a Gigantic and Hollow Heterometallic Cage {Ni64RE96} for Gas Separation and Magnetic Cooling Applications, Angew. Chem. Int. Ed. 2016, 55, 9375-9379

Modelling spin Hamiltonian parameters of molecular nanomagnets

Molecular nanomagnets encompass a wide range of coordination complexes possessing several potential applications. A formidable challenge in realizing these potential applications lies in controlling the magnetic properties of these clusters. Microscopic spin Hamiltonian (SH) parameters describe the magnetic properties of these clusters, and viable ways to control these SH parameters are highly desirable. Computational tools play a proactive role in this area, where SH parameters such as isotropic exchange interaction (J), anisotropic exchange interaction (Jx, Jy, Jz), double exchange interaction (B), zero-field splitting parameters (D, E) and g-tensors can be computed reliably using X-ray structures. In this feature article the authors have attempted to provide a holistic view of the modelling of these SH parameters of molecular magnets. The determination of J includes various class of molecules, from di- and polynuclear Mn complexes to the {3d-Gd}, {Gd-Gd} and {Gd-2p} class of complexes. The estimation of anisotropic exchange coupling includes the exchange between an isotropic metal ion and an orbitally degenerate 3d/4d/5d metal ion. The double-exchange section contains some illustrative examples of mixed valance systems, and the section on the estimation of zfs parameters covers some mononuclear transition metal complexes possessing very large axial zfs parameters. The section on the computation of g-anisotropy exclusively covers studies on mononuclear DyIII and ErIII single-ion magnets. The examples depicted in this article clearly illustrate that computational tools not only aid in interpreting and rationalizing the observed magnetic properties but possess the potential to predict new generation MNMs.

Tulika Gupta, Gopalan Rajaraman, Modelling spin Hamiltonian parameters of molecular nanomagnets, Chem. Commun., 2016, 52, 8972-9008

A modular design of molecular qubits to implement universal quantum gates

The physical implementation of quantum information processing relies on individual modules - qubits - and operations that modify such modules either individually or in groups, i.e. quantum gates. Two examples of gates that entangle pairs of qubits are the controlled NOT-gate (CNOT) gate, which flips the state of one qubit depending on the state of another, and the SQRT(iSWAP) gate that brings a two-qubit product state into a superposition involving partially swapping the qubit states. Here we show that through supramolecular chemistry a single simple module, molecular {Cr7Ni} rings, which act as the qubits, can be assembled into structures suitable for either the CNOT or SQRT(iSWAP) gate by choice of linker, and we characterize these structures by electron spin resonance spectroscopy. We introduce two schemes for implementing such gates with these supramolecular assemblies and perform detailed simulations, based on the measured parameters including decoherence, to demonstrate how the gates would operate.

Jesus Ferrando-Soria, Eufemio Moreno Pineda, Alessandro Chiesa, Antonio Fernandez, Samantha A. Magee, Stefano Carretta, Paolo Santini, Inigo J. Vitorica-Yrezabal, Floriana Tuna, Grigore A. Timco, Eric J.L. McInnes & Richard E.P. Winpenny, A modular design of molecular qubits to implement universal quantum gates, NATURE COMMUNICATIONS | 7:11377 (2016)

see also:

Alessandro Chiesa, George F. S. Whitehead, Stefano Carretta, Laura Carthy, Grigore A. Timco, Simon J. Teat, Giuseppe Amoretti, Eva Pavarini, Richard E. P. Winpenny, Paolo Santini, Molecular nanomagnets with switchable coupling for quantum simulation, SCIENTIFIC REPORTS | 4:7423 (2014)

D5h makes the difference

Two more SIMs with record energy barrier and record magnetic hysteresis temperature have been reported recently.
Air-stable Dy(III) and Er(III) single-ion magnets (SIMs) with pseudo-D5h symmetry, where the Dy(III)-SIM exhibits a magnetization blocking (TB) up to 12 K, defined from the maxima of the zero-field cooled magnetization curve, with an anisotropy barrier (Ueff) as high as 735.4 K have been reported. The Dy(III)-SIM exhibits a magnetic hysteresis up to 12 K (30 K) with a large coercivity of ~0.9 T (~1.5 T) at a sweep rate of ~0.0018 T/s (0.02 T/s). These high values combined with persistent stability under ambient conditions, render this system as one of the best-characterized SIMs. Ab initio calculations have been used to establish the connection between the higher-order symmetry of the molecule and the quenching of quantum tunnelling of magnetization (QTM) effects. The relaxation of magnetization is observed via the second excited Kramers doublet owing to pseudo-high-order symmetry, which quenches the QTM. This study highlights fine-tuning of symmetry around the lanthanide ion to obtain new-generation SIMs and offers further scope for pushing the limits of Ueff and TB using this approach.

Sandeep K. Gupta, Thayalan Rajeshkumar, Gopalan Rajaraman, Ramaswamy Murugavel, An air-stable Dy(III) single-ion magnet with high anisotropy barrier and blocking temperature, Chem. Sci., 2016, Advance Article

Sandeep K. Gupta, Thayalan Rajeshkumar, Gopalan Rajaraman, Ramaswamy Murugavel, An unprecedented zero field neodymium(III) single-ion magnet based on a phosphonic diamide, Chem. Commun., 2016,52, 7168-7171

Symmetry-Supported Magnetic Blocking at 20 K in Pentagonal Bipyramidal Dy(III) Single-Ion Magnets

Two SIMs with record energy barrier and record magnetic hysteresis temperature up to 20 K, respectively, in two pentagonal bipyramidal Dy(III) SIMs have been reported recently.
Single-molecule magnets (SMMs) that can be trapped in one of the bistable magnetic states separated by an energy barrier are among the most promising candidates for high-density information storage, quantum processing, and spintronics. To date, a considerable series of achievements have been made. However, the presence of fast quantum tunnelling of magnetization (QTM) in most SMMs, especially in single-ion magnets (SIMs), provides a rapid relaxation route and often sets up a limit for the relaxation time. Here, we pursue the pentagonal bipyramidal symmetry to suppress the QTM. Magnetic characterizations reveal fascinating SMM properties with high energy barriers along with a record magnetic hysteresis temperature up to 20 K. These results, combined with the ab initio calculations, offer an illuminating insight into the vast possibility and potential of what the symmetry rules can achieve in molecular magnetism.

Yan-Cong Chen, Jun-Liang Liu, Liviu Ungur, Jiang Liu, Quan-Wen Li, Long-Fei Wang, Zhao-Ping Ni, Liviu F. Chibotaru, Xiao-Ming Chen, and Ming-Liang Tong, Symmetry-Supported Magnetic Blocking at 20 K in Pentagonal Bipyramidal Dy(III) Single-Ion Magnets, J. Am. Chem. Soc., 2016, 138 (8), pp 2829-2837

Jiang Liu, Yan-Cong Chen, Jun-Liang Liu, Veacheslav Vieru, Liviu Ungur, Jian-Hua Jia, Liviu F. Chibotaru, Yanhua Lan, Wolfgang Wernsdorfer, Song Gao, Xiao-Ming Chen, and Ming-Liang Tong, A Stable Pentagonal Bipyramidal Dy(III) Single-Ion Magnet with a Record Magnetization Reversal Barrier over 1000 K, J. Am. Chem. Soc., 2016, 138 (16), pp 5441-5450

Giant Hysteresis of Single-Molecule Magnets Adsorbed on a Nonmagnetic Insulator

It is demonstrated that TbPc2 single-molecule magnets adsorbed on a magnesium oxide tunnel barrier exhibit record magnetic remanence, record hysteresis opening, perfect out-of-plane alignment of the magnetic easy axes, and self-assembly into a well-ordered layer.

Christian Wäckerlin, Fabio Donati, Aparajita Singha, Romana Baltic, Stefano Rusponi, Katharina Diller, Francois Patthey, Marina Pivetta, Yanhua Lan, Svetlana Klyatskaya, Mario Ruben, Harald Brune, Jan Dreiser, Giant Hysteresis of Single-Molecule Magnets Adsorbed on a Nonmagnetic Insulator, Adv. Mater. 28 (2016) 5195-5199

Deciphering the origin of giant magnetic anisotropy and fast quantum tunnelling in Rhenium(IV) single-molecule magnets

Single-molecule magnets represent a promising route to achieve potential applications such as high-density information storage and spintronics devices. Among others, 4d/5d elements such as Re(IV) ion are found to exhibit very large magnetic anisotropy, and inclusion of this ion-aggregated clusters yields several attractive molecular magnets. Here, using ab intio calculations, the authors unravel the source of giant magnetic anisotropy associated with the Re(IV) ions by studying a series of mononuclear Re(IV) six coordinate complexes. The low-lying doublet states are found to be responsible for large magnetic anisotropy and the sign of the axial zero-field splitting parameter (D) can be categorically predicted based on the position of the ligand coordination. Large transverse anisotropy along with large hyperfine interactions opens up multiple relaxation channels leading to a fast quantum tunnelling of the magneti- zation (QTM) process. Enhancing the Re-ligand covalency is found to significantly quench the QTM process.

Saurabh Kumar Singh, Gopalan Rajaraman, Deciphering the origin of giant magnetic anisotropy and fast quantum tunnelling in Rhenium(IV) single-molecule magnets, NATURE COMMUNICATIONS 7, 10669 (2016)

Rotating Magnetocaloric Effect in an Anisotropic Molecular Dimer

In contrast to the mainstream research on molecular refrigerants that seeks magnetically isotropic molecules, we show that the magnetic anisotropy of dysprosium acetate tetrahydrate, can be efficiently used for cooling below liquid-helium temperature. This is attained by rotating aligned single-crystal samples in a constant applied magnetic field. The envisioned advantages are fast cooling cycles and potentially compact refrigerators.

Dr. Giulia Lorusso, Dr. Olivier Roubeau, Dr. Marco Evangelisti Rotating Magnetocaloric Effect in an Anisotropic Molecular Dimer, Angew. Chem. Int. Ed. 55 (2016) 3360-3363

Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear MnII2GdIII2 and CoII2EuIII2 Complexes

In view of a wide perspective of 3d-4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]arene tetranuclear heterometallic complexes MnII2GdIII2 and CoII2EuIII2. The energy pattern of the MnII2GdIII2 complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Lande intervals are affected by the biquadratic spin-spin interactions. The nonmonotonic temperature dependence of the Chi*T product observed for the MnII2GdIII2 complex is attributed to the competitive influence of the ferromagnetic Mn-Gd and antiferromagnetic Mn-Mn exchange interactions, the latter being stronger. The model for the CoII2EuIII2 complex includes uniaxial anisotropy of the seven-coordinate Co ions and an isotropic exchange interaction in the Co pair, while the Eu ions are diamagnetic in their ground states. Best-fit analysis of Chi*T versus T showed that the anisotropic contribution (arising from a large zero-field splitting in Co ions) dominates (weak-exchange limit) in the CoII2EuIII2. This complex is concluded to exhibit an easy plane of magnetization (arising from the Co pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-1/2 Hamiltonian that is deduced for the CoII2 pair in the weak-exchange limit.

Sergey M. Aldoshin, Nataliya A. Sanina, Andrew V. Palii, Boris S. Tsukerblat, Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear MnII2GdIII2 and CoII2EuIII2 Complexes, Inorg. Chem. (2016) Article ASAP

Molecular Nanomagnets and Related Phenomena

With contributions by M. Affronte, M.L. Baker, S.J. Blundell, L. Bogani, L.F. Chibotaru, R. Clerac, A. Cornia, C. Coulon, N. Domingo, M. Evangelisti, S. Gao, A. Ghirri, S. Hill, S.-D. Jiang, F. Luis, M. Mannini, C.J. Milios, V. Pianet, F. Troiani, M. Urdampilleta, R.E.P. Winpenny, B.-W. Wang,

Song Gao (Ed.), Molecular Nanomagnets and Related Phenomena, Structure and Bonding 164 (2015)

New single-ion magnets

Single-molecule magnets display magnetic bistability of molecular origin, which may one day be exploited in magnetic data storage devices. Recently it was realised that increasing the magnetic moment of polynuclear molecules does not automatically lead to a substantial increase in magnetic bistability. Attention has thus increasingly focussed on ions with large magnetic anisotropies. In spite of large effective energy barriers towards relaxation of the magnetic moment, this has so far not led to a big increase in magnetic bistability. Here we present a comprehensive study of a mononuclear, tetrahedrally coordinated cobalt(II) single-molecule magnet, which has a very high effective energy barrier and displays pronounced magnetic bistability. The combined experimental-theoretical approach enables an in-depth understanding of the origin of these favourable properties, which are shown to arise from a strong ligand field in combination with axial distortion. Our findings allow formulation of clear design principles for improved materials.

Yvonne Rechkemmer, Frauke D. Breitgoff, Margarethe van der Meer, Mihail Atanasov, Michael Hakl, Milan Orlita, Petr Neugebauer, Frank Neese, Biprajit Sarkar, Joris van Slageren A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier, Nature Communications 7, 10467 (2016)

More on single-ion magnets:
Yvonne Rechkemmer, Julia E. Fischer, Raphael Marx, María Dörfel, Petr Neugebauer, Sebastian Horvath, Maren Gysler, Theis Brock-Nannestad, Wolfgang Frey, Michael F. Reid, Joris van Slageren, Comprehensive Spectroscopic Determination of the Crystal Field Splitting in an Erbium Single-Ion Magnet, JACS 137, 13114 (2015)

Supramolecular aggregates of single-molecule magnets: exchange-biased quantum tunneling of magnetization in a rectangular [Mn3]4 tetramer

The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn3 units are reported. Magnetization vs. dc field sweeps on a single crystal of the tetrameric [Mn12O4(O2CR)12(pdpd)6](ClO4)4 (R = Me) gave hysteresis loops below 1 K that exhibit exchange-biased quantum tunneling of magnetization (QTM) steps with a bias field of 0.19 T. Simulation of the loops determined that each Mn3 unit is exchange-coupled to the two neighbors. The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.

Tu N. Nguyen, Wolfgang Wernsdorfer, Muhandis Shiddiq, Khalil A. Abboud, Stephen Hill, George Christou, Supramolecular aggregates of single-molecule magnets: exchange-biased quantum tunneling of magnetization in a rectangular [Mn3]4 tetramer, Chem. Sci. 7, 1156 (2016)

Thematic Issue: Phase Transition and Dynamical Properties of Spin Transition Materials

Great attention in the area of spin transition materials has been paid to the investigation of their various phase transitions and dynamical properties which can be modulated by external parameters, including temperature, pressure, photo-irradiation, or presence of guest molecules. The topics collected in the issue are focused on the four types of spin transition materials: (1) Fe(II)-based spin crossover compounds, (2) Dithiooxalato-bridged Fe-based charge transfer systems, (3) Charge transfer active Prussian Blue Analogues, and (4) Copper(II)-octacyanidomolybdate(IV) photomagnetic materials.

The download of this thematic issue is free of charge.

Norimichi Kojima, Shin-ichi Ohkoshi and Seiji Miyashita (Eds.), Thematic Issue: Phase Transition and Dynamical Properties of Spin Transition Materials, Current Inorganic Chemistry, vol. 6, issue 1 (2016)

Millisecond Coherence Time in a Tunable Molecular Electronic Spin Qubit

Millisecond coherence time for a coordination complex qubit was observed, surpassing the previous record for such a system by an order of magnitude. The results illuminate a path forward in synthetic design principles, which should unite CS2 solubility with nuclear spin free ligand fields to develop a new generation of molecular qubits.

Joseph M. Zadrozny, Jens Niklas, Oleg G. Poluektov, and Danna E. Freedman, Millisecond Coherence Time in a Tunable Molecular Electronic Spin Qubit, ACS Cent. Sci. 1, 488-492 (2015)

Ring, Ring

Long before molecular magnetism started, the advent of magnetic molecular rings was announce in a prophetic publication (Björn Benny & Agnetha Frida, Ring, Ring, Polar Music, 1973). In the early days only even-membered rings could be synthesized, odd rings were covered by strange theoretical prophecies (JMMM 220 (2000) 227; Phys. Rev. B 68 (2003) 054422).
Ring, ring, why don't you give me a call?
Ring, ring, the happiest sound of them all
Ring, ring, I stare at the phone on the wall
And I sit all alone impatiently
Won't you please understand the need in me
So, ring, ring, why don't you make an odd ring?

2016: here we go - an odd homometallic chromium ring is reported, which is the first regular Cr9 antiferromagnetic ring. Its electronic relaxation dynamics was probed by 1H-NMR, which allows to determine the spin-phonon coupling strength. At very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures.

E. Garlatti, S. Bordignon, S. Carretta, G. Allodi, G. Amoretti, R. De Renzi, A. Lascialfari, Y. Furukawa, G. A. Timco, R. Woolfson, R. E. P. Winpenny, and P. Santini, Relaxation dynamics in the frustrated Cr9 antiferromagnetic ring probed by NMR, Phys. Rev. B 93, 024424 (2016)

More stories on rings (Tell me, are we really through?):
McInnes, E. J. L., Timco, G. A., Whitehead, G. F. S. and Winpenny, R. E. P. Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks, Angew. Chem. Int. Ed., 54, 14244-14269 (2015)

Alternative interpretation (R.W.: The ring was made by Alberich?!):

Original publication:

Toward Tailored All-Spin Molecular Devices

Molecular based spintronic devices offer great potential for future energy-efficient information technology as they combine ultimately small size, high-speed operation, and low-power consumption. Recent developments in combining atom-by-atom assembly with spin-sensitive imaging and characterization at the atomic level have led to a first prototype of an all-spin atomic-scale logic device, but the very low working temperature limits its application. Here, we show that a more stable spintronic device could be achieved using tailored Co-Salophene based molecular building blocks, combined with in situ electrospray deposition under ultrahigh vacuum conditions as well as control of the surface-confined molecular assembly at the nanometer scale. In particular, we describe the tools to build a molecular, strongly bonded device structure from paramagnetic molecular building blocks including spin-wires, gates, and tails. Such molecular device concepts offer the advantage of inherent parallel fabrication based on molecular self-assembly as well as an order of magnitude higher operation temperatures due to enhanced energy scales of covalent through-bond linkage of basic molecular units compared to substrate- mediated coupling schemes employing indirect exchange coupling between individual adsorbed magnetic atoms on surfaces.

Maciej Bazarnik, Bernhard Bugenhagen, Micha Elsebach, Emil Sierda, Annika Frank, Marc H. Prosenc, Roland Wiesendanger, Toward Tailored All-Spin Molecular Devices, Nano Lett. 2016, 16, 577-582

The classical and quantum dynamics of molecular spins on graphene

The investigation of graphene has become a central field in contemporary science, because of its superior properties and the profusion of physical phenomena that it has revealed. Several central issues remains unexplored, though: how does graphene interact with spins? can one modulate the spin behaviour via interactions with electrons? Answering these questions is important for information storage and logic devices, because graphene is hailed as a potentially ideal material in spintronics, but is largely diamangetic.
The new research paper shows the effect of graphene phonons and electrons on the spin dynamics of molecules. The authors show that Dirac electrons introduce dominant quantum-relaxation channels and, allow reaching a quantum tunneling regime, called Villain's tunneling, that was predicted 20 years ago and never observed. A novel theoretical background is provided, fully explaining the data and setting the basis for the understanding of graphene-spin interactions. These observations have profound consequences for the design of graphene spin-processing devices and pave the way to coherent control of graphene-based spintronic devices that can be manipulated electrically.

Christian Cervetti, Angelo Rettori, Maria Gloria Pini, Andrea Cornia, Ana Repolles, Fernando Luis, Martin Dressel, Stephan Rauschenbach, Klaus Kern, Marko Burghard, Lapo Bogani, The classical and quantum dynamics of molecular spins on graphene, Nature Materials 15 (2015) 164

Universal sequence of ground states validating the classification of frustration in antiferromagnetic rings with a single bond defect

The sequence of 2s+1 ground states in the frustrated antiferromagnetic rings with odd number n of local spins s resulting from a single bond defect strength is determined, and occurrence of the Lieb-Mattis energy level ordering, proven for the bipartite systems only, is confirmed for the rings in question. The sequence is universal, provides the theoretical basis for the recent classification of spin frustration in molecular magnets, and indicates that the Lieb-Mattis theorem (LMT) consequences exist for the nonbipartite rings. The states in the sequence are characterized by the total spin S fulfilling the constraint S smaller than s and are separated by 2s Kahn degenerate frustration points. The possible LMT consequences in other systems are discussed.

Grzegorz Kamieniarz, Wojciech Florek, and Micha Antkowiak, Universal sequence of ground states validating the classification of frustration in antiferromagnetic rings with a single bond defect, Phys. Rev. B 92, 140411 (2015)

Recent advances in the design of magnetic molecules for use as cryogenic magnetic coolants

This review outlines recent advances in the design of 3d-, 4f-, and 3d-4f type magnetic molecules for use as excellent cryogenic magnetic coolants based on the magnetocaloric effect (MCE), and the structure- magnetocaloric correlations of reported molecular coolants. Further improvements in the MCE values of molecular magnetic materials are also proposed based on assembly strategies from molecular chemistry and crystal engineering.

Jun-Liang Liu, Yan-Cong Chen, Fu-Sheng Guo, Ming-Liang Tong, Recent advances in the design of magnetic molecules for use as cryogenic magnetic coolants, Coordination Chemistry Reviews 281 (2014) 26-49

Pushing the limits of magnetic anisotropy in trigonal bipyramidal Ni(II)

Monometallic complexes based on 3d transition metal ions in certain axial coordination environments can exhibit appreciably enhanced magnetic anisotropy, important for memory applications, due to stabilisation of an unquenched orbital moment. For high-spin trigonal bipyramidal Ni(II), if competing structural distortions can be minimised, this may result in an axial anisotropy that is at least an order of magnitude stronger than found for orbitally non-degenerate octahedral complexes. Broadband, high-field EPR studies of [Ni(MDABCO)2Cl3]ClO4 confirm an unprecedented axial magnetic anisotropy, which pushes the limits of the familiar spin-only description. Crucially, compared to complexes with multidentate ligands that encapsulate the metal ion, we see only a very small degree of axial symmetry breaking. The compound displays field-induced slow magnetic relaxation, which is rare for monometallic Ni(II) complexes due to efficient spin–lattice and quantum tunnelling relaxation pathways.

Katie E. R. Marriott, Lakshmi Bhaskaran, Claire Wilson, Marisa Medarde, Stefan T. Ochsenbein, Stephen Hill, Mark Murrie, Pushing the limits of magnetic anisotropy in trigonal bipyramidal Ni(II), Chem. Sci., 6 (2015) 6823

A Single-Chain Magnet with a Very High Blocking Temperature and a Strong Coercive Field

Two isostructural 1-D complexes were synthesized. These stable chains exhibit very strong intramolecular antiferromagnetic metal-radical exchange coupling. One compound shows slow magnetic relaxation behavior with a high blocking temperature of about 13.2 K and a very high coercive field of 49 kOe at 4.0 K.

Rafael A. Allao Cassaro, Samira G. Reis, Thamyres S. Araujo, Paul M. Lahti, Miguel A. Novak, Maria G. F. Vaz, A Single-Chain Magnet with a Very High Blocking Temperature and a Strong Coercive Field, Inorg. Chem., 2015, 54 (19), 9381-9383

Thermodynamic observables of Mn12-acetate calculated for the full spin Hamiltonian

Thirty-five years after its synthesis, magnetic observables are calculated for the molecular nanomagnet Mn12-acetate using a spin Hamiltonian that contains all spins. Starting from a very advanced density functional theory parametrization [V. V. Mazurenko et al., Phys. Rev. B 89, 214422 (2014)], we evaluate magnetization and specific heat for this anisotropic system of 12 manganese ions with a staggering Hilbert space dimension of 100 000 000 using the finite-temperature Lanczos method. We compare the results with those obtained from other parametrizations. Our investigations demonstrate that it is now possible to assess the quality of parametrizations of effective spin Hamiltonians for rather large magnetic molecules.

Oliver Hanebaum and Jürgen Schnack, Thermodynamic observables of Mn12-acetate calculated for the full spin Hamiltonian, Phys. Rev. B 92, 064424 (2015)

Calorimetry of functional inorganic materials

Bentham Science published three volumes of the thematic issue of 'Current Inorganic Chemistry' on 'calorimetry of functional inorganic materials' edited by Prof. Michio Sorai and Prof. Yasuiro Nakazawa. The thematic issue includes several contributions on molecular magnets. You can download for free the three editorials to make you an opinion of the high level of the issue.

The three issues are available at:
The first volume is available on a free access basis.

Magnetic materials

Wiley-VCH opened a web page to present 'Magnetic Materials' as a Hot Topic. 'Magnetic phenomena are omnipresent in the modern world. The Nobel prize in Physics (2007) was awarded for studies of giant magnetoresistance to A. Fert and P. Grünberg; their discoveries were soon implemented in hard drives (see picture, courtesy of IBM Germany). Learn more about the current research into magnetic materials, basic or applied.'

Link to the page:

Tutorial Review: 3d single-ion magnets

One of the determining factors in whether single-molecule magnets (SMMs) may be used as the smallest component of data storage, is the size of the barrier to reversal of the magnetisation, Ueff. This physical quantity depends on the magnitude of the magnetic anisotropy of a complex and the size of its spin ground state. In recent years, there has been a growing focus on maximising the anisotropy generated for a single 3d transition metal (TM) ion, by an appropriate ligand field, as a means of achieving higher barriers. Because the magnetic properties of these compounds arise from a single ion in a ligand field, they are often referred to as single-ion magnets (SIMs). Here, the synthetic chemist has a significant role to play, both in the design of ligands to enforce propitious splitting of the 3d orbitals and in the judicious choice of TM ion. Since the publication of the first 3d-based SIM, which was based on Fe(II), many other contributions have been made to this field, using different first row TM ions, and exploring varied coordination environments for the paramagnetic ions.

Gavin A. Craig and Mark Murrie, Tutorial Review: 3d single-ion magnets, Chem. Soc. Rev. 44 (2015) 2135

Design Criteria for High-Temperature Single-Molecule Magnets

Design criteria to obtain slow magnetic relaxation are theoretically investigated for two-coordinate complexes of Dy(III). It is shown that large energy barriers to magnetic relaxation, Ueff, can be achieved in the absence of near-linearity and generally that any two-coordinate complex of Dy(III) is an attractive synthetic target that may possess Ueff > 1000 cm–1. These large Ueff values are immediately diminished if axial ligation is disrupted by solvent coordination.

Nicholas F. Chilton, Design Criteria for High-Temperature Single-Molecule Magnets, Inorg. Chem. 54 (2015) 2097

Self-trapping of charge polarized states in four-dot molecular quantum cellular automata: bi-electronic tetrameric mixed-valence species

Our interest in this article is prompted by the problem of the vibronic self-trapping of charge polarized states in the four-dot molecular quantum cellular automata (mQCA), a paradigm for nanoelectronics, in which binary information is encoded in charge configuration of the mQCA cell. We report the evaluation of the electronic states and the adiabatic potentials of mixed-valence (MV) systems in which two electrons (or holes) are shared among four sites. These systems are exemplified by the two kinds of tetra-ruthenium (2Ru(II)+ 2Ru(III)) clusters (assembled as two coupled Creutz-Taube dimers) for which molecular implementation of mQCA was proposed. The tetra-ruthenium clusters include two holes shared among four sites and correspondingly we employ the model which takes into account the electron transfer processes as well as the Coulomb repulsion in the different instant positions of localization. The vibronic self-trapping is considered within the conventional vibronic Piepho, Krausz and Schatz (PKS) model adapted to the bi-electronic MV species with the square topology. This leads to a complicated vibronic problem for spin-singlet and spin-triplet states correspondingly. The adiabatic potentials are evaluated with account for the low lying Coulomb levels in which the antipodal sites are occupied, the case just actual for utilization in mQCA. The conditions for the vibronic localization in spin-singlet and spin-triplet states are revealed in terms of the two actual transfer pathways parameters and strength of the vibronic coupling.

Boris Tsukerblat, Andrew Palii, Juan Modesto Clemente-Juan, Self-trapping of charge polarized states in four-dot molecular quantum cellular automata: bi-electronic tetrameric mixed-valence species, Pure and Applied Chemistry 87 (2014) 271

Supramolecular Recognition Influences Magnetism in [X@HVIV8VV14O54]6− Self-Assemblies with Symmetry-Breaking Guest Anions

Mixed-valence polyoxovanadates(IV/V) have emerged as one of the most intricate class of supramolecular all-inorganic host species, able to encapsulate a wide variety of smaller guest templates during their self-assembly formation process. As showcased herein, the incorporation of guests, though governed solely by ultra-weak electrostatic and van der Waals interactions, can cause drastic effects on the electronic and magnetic characteristics of the shell complex of the polyoxovanadate. The authors address the question of methodology for the magnetochemical analysis of virtually isostructural polyoxoanions of D2d symmetry enclosing diamagnetic template anions. These induce different polarization effects related to differences in their geometric structures, symmetry, ion radii, and valence shells, eventually resulting in a supramolecular modulation of magnetic exchange between the V(3d) electrons that are partly delocalized.

Kirill Yu. Monakhov, Oliver Linnenberg, Piotr Kozłowski, Jan van Leusen, Claire Besson, Tim Secker, Arkady Ellern, Xavier Lopez, Josep M. Poblet, Paul Kögerler, Supramolecular Recognition Influences Magnetism in [X@HVIV8VV14O54]6− Self-Assemblies with Symmetry-Breaking Guest Anions, Chem. Eur. J. 21, 2387 (2014)

Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements

Geometric spin frustration in low-dimensional materials, such as the two-dimensional kagome or triangular antiferromagnetic nets, can significantly enhance the change of the magnetic entropy and adiabatic temperature following a change in the applied magnetic field, that is, the magnetocaloric effect. In principle, an equivalent outcome should also be observable in certain high-symmetry zero-dimensional, that is, molecular, structures with frustrated topologies. Here we report experimental realization of this in a heptametallic gadolinium molecule. Adiabatic demagnetization experiments reach approximately 200 mK, the first sub-Kelvin cooling with any molecular nanomagnet, and reveal isentropes (the constant entropy paths followed in the temperature-field plane) with a rich structure. The latter is shown to be a direct manifestation of the trigonal antiferromagnetic net structure, allowing study of frustration-enhanced magnetocaloric effects in a finite system.

Joseph W. Sharples, David Collison, Eric J. L. McInnes, Jürgen Schnack, Elias Palacios, Marco Evangelisti, Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements, Nature Communications 5 (2014) 5321

Room temperature quantum coherence in a potential molecular qubit

The successful development of a quantum computer would change the world, and current internet encryption methods would cease to function. However, no working quantum computer that even begins to rival conventional computers has been developed yet, which is due to the lack of suitable quantum bits. A key characteristic of a quantum bit is the coherence time. Transition metal complexes are very promising quantum bits, owing to their facile surface deposition and their chemical tunability. However, reported quantum coherence times have been unimpressive. Here we report very long quantum coherence times for a transition metal complex of 68 microseconds at low temperature (qubit figure of merit QM=3,400) and 1 microseconds at room temperature, much higher than previously reported values for such systems. We show that this achievement is because of the rigidity of the lattice as well as removal of nuclear spins from the vicinity of the magnetic ion.

Katharina Bader, Dominik Dengler, Samuel Lenz, Burkhard Endeward, Shang-Da Jiang, Petr Neugebauer, Joris van Slageren, Room temperature quantum coherence in a potential molecular qubit, Nature Communications 5 (2014) 5304

Two is complicated enough!

Lanthanide compounds show much higher energy barriers to magnetic relaxation than 3d-block compounds, and this has led to speculation that they could be used in molecular spintronic devices. Prototype molecular spin valves and molecular transistors have been reported, with remarkable experiments showing the influence of nuclear hyperfine coupling on transport properties. Modelling magnetic data measured on lanthanides is always complicated due to the strong spin-orbit coupling and subtle crystal field effects observed for the 4f-ions; this problem becomes still more challenging when interactions between lanthanide ions are also important. Such interactions have been shown to hinder and enhance magnetic relaxation in different examples, hence understanding their nature is vital. Here the authors are able to measure directly the interaction between two dysprosium(III) ions through multi-frequency electron paramagnetic resonance spectroscopy and other techniques, and explain how this influences the dynamic magnetic behaviour of the system.
A series of water-bridged dinickel complexes has been synthesized and structurally characterized by X-ray crystallography. The magnetic properties have been probed by magnetometry and EPR spectroscopy, and detailed measurements show that the axial zero-field splitting, D, of the nickel(II) ions is on the same order as the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange interaction can be related to the angle between the nickel centers and the bridging water molecule, while the magnitude of D can be related to the coordination sphere at the nickel sites.

Eufemio Moreno Pineda, Nicholas F. Chilton, Raphael Marx, Maria Dörfel, Daniel O. Sells, Petr Neugebauer, Shang-Da Jiang, David Collison, Joris van Slageren, Eric J.L. McInnes & Richard E.P. Winpenny, Direct measurement of dysprosium(III)...dysprosium(III) interactions in a single-molecule magnet, Nature Communications 5 (2014) 5243

James P. S. Walsh, Stephen Sproules, Nicholas F. Chilton, Anne-Laure Barra, Grigore A. Timco, David Collison, Eric J. L. McInnes, and Richard E. P. Winpenny, On the Possibility of Magneto-Structural Correlations: Detailed Studies of Dinickel Carboxylate Complexes, Inorg. Chem. 53 (2014) 8464

First-principles modeling of magnetic excitations in Mn12

A fully microscopic theory of magnetic properties of the prototype molecular magnet Mn12 is developed. First, the intramolecular magnetic properties have been studied by means of first-principles density functional based methods, with local correlation effects being taken into account within the local density approximation plus U (LDA+U) approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results a distortion of ferrimagnetic ordering due to DM interactions is predicted. Further, the authors use an exact diagonalization approach allowing one to work with as large a Hilbert space dimension as 108 without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.

V. V. Mazurenko, Y. O. Kvashnin, Fengping Jin, H. A. De Raedt, A. I. Lichtenstein, and M. I. Katsnelson, First-principles modeling of magnetic excitations in Mn12, Phys. Rev. B 89 (2014) 214422

S=1/2 One-Dimensional Random-Exchange Ferromagnetic Zigzag Ladder, Which Exhibits Competing Interactions in a Critical Regime

The synthesis, crystal structure, and magnetic properties (from a combined experimental and First-Principles Bottom-Up theoretical study) of the new compound catena-dichloro(2-Cl-3Mpy)copper(II), 1, [2-Cl-3Mpy=2-chloro-3-methylpyridine] are described and rationalized. Crystals of 1 present well isolated magnetic 1D chains (no 3D order was experimentally observed down to 1.8 K) and magnetic frustration stemming from competing ferromagnetic nearest-neighbor (JNN) interactions and antiferromagnetic next-nearest neighbor (JNNN) interactions, in which JNNN/JNN <-0.25. These magnetic interactions give rise to a unique magnetic topology: a two-leg zigzag ladder composed of edge-sharing up-down triangles with antiferromagnetic interactions along the rails and ferromagnetic interactions along the zigzag chain that connects the rails. Crystals of 1 also present a random distribution of the 2-Cl-3Mpy groups, which are arranged in two different orientations, each with a 50 % occupancy. This translates into a random static structural disorder within each chain by virtue of which the value of the JNN magnetic interactions can randomly take one of three values. The structural disorder does not affect the JNNN value. A proper statistical treatment of this disorder provides a computed magnetic susceptibility curve that reproduces the main features of the experimental data.

Susan N. Herringer, Merce Deumal, Jordi Ribas-Arino, Juan J. Novoa, Christopher P. Landee, Jan L. Wikaira, Mark M. Turnbull, S=1/2 One-Dimensional Random-Exchange Ferromagnetic Zigzag Ladder, Which Exhibits Competing Interactions in a Critical Regime, Chem. Eur. J. 20 (2014) 8355

Review: A gentle introduction to magnetism: units, fields, theory, and experiment

In case you need a book on magnetism for your students, take this review:
We present an introduction to the workings, units of measure, and general properties of magnetic materials. This is intended as a üprimer to interpretation of magnetic dataý for those who are entering the field, or those who are encountering magnetic measurements in the literature. We expect this work will serve as an initial guide to the reader to familiarize them with the basics in the hope that those working in the field of magnetochemistry will wish to explore additional, more detailed literature as their specific investigations demand. Topics covered include: magnetic fields and units (SI and cgs), paramagnetism (magnetization and magnetic susceptibility), Curie and Curie-Weiss behavior, magnetic exchange interactions, magnetic anisotropy, dimeric systems and exchange-coupled networks (including chains, ladders, and layers), and long-range order.

Christopher P. Landee and Mark M. Turnbull, Review: A gentle introduction to magnetism: units, fields, theory, and experiment, Journal of Coordination Chemistry, 67 (2014) 375-439

Single-molecule magnet engineering: building-block approaches

Tailoring the specific magnetic properties of any material relies on the topological control of the constituent metal ion building blocks. Although this general approach does not seem to be easily applied to traditional inorganic bulk magnets, coordination chemistry offers a unique tool to delicately tune, for instance, the properties of molecules that behave as "magnets", the so-called single-molecule magnets (SMMs). Although many interesting SMMs have been prepared by a more or less serendipitous approach, the assembly of predesigned, isolatable molecular entities into higher nuclearity complexes constitutes an elegant and fascinating strategy. This Feature article focuses on the use of building blocks or modules (both terms being used indiscriminately) to direct the structure, and therefore also the magnetic properties, of metal ion complexes exhibiting SMM behaviour.

Kasper S. Pedersen, Jesper Bendix, Rodolphe Clerac, Single-molecule magnet engineering: building-block approaches, Chem. Comm. 33, 1084(2014)

Organometallic Single-Molecule Magnets

Single-molecule magnets (SMMs) display slow relaxation of the magnetization, purely of molecular origin, in the absence of an applied magnetic field. This review summarizes the important role played by organometallic chemistry in the recent development of SMMs. The broad applicability of organometallic synthesis has led to a series of organometallic SMMs containing transition metals, lanthanides, or actinides, with several examples accounting for some of the most fascinating low-temperature magnetism. The review has two main aims. The first aim is to provide organometallic chemists with an introduction to one of the most exciting areas of modern molecular magnetism and, in particular, to highlight how organometallic chemistry has allowed the field to evolve in new directions. The second aim is more of a clarion call: organometallic chemistry still has hugely underexploited potential in the development of single-molecule magnets, and it is reasonable to expect that different synthetic approaches will lead to new and unusual magnetic phenomena. By using this review as an entry point for studying the literature in more detail, hopefully more organometallic chemists will consider directing their synthetic repertoire toward the design and realization of new, and possibly improved, single-molecule magnets.

Richard A. Layfield, Organometallic Single-Molecule Magnets, Organometallics 33, 1084(2014)

A Cobalt Pyrenylnitronylnitroxide Single-Chain Magnet with High Coercivity and Record Blocking Temperature

Coordination of a [Co(hfac)2] moiety (hfac=hexafluoroacetylacetonate) with a nitronylnitroxide radical linked to bulky, rigid pyrene (PyrNN) gives a helical 1:1 chain complex, in which both oxygen atoms of the radical NO. groups are bonded to CoII ions with strong antiferromagnetic exchange. The complex shows single-chain magnet (SCM) behavior with frequency-dependent magnetic susceptibility, field-cooled and zero-field-cooled susceptibility divergence with a high blocking temperature of around 14 K (a record among SCMs), and hysteresis with a very large coercivity of 32 kOe at 8 K. The magnetic behavior is partly related to good chain isolation induced by the large pyrene units. Two magnetic relaxation processes have been observed, a slower one attributable to longer, and a faster one attributable to short chains. No evidence of magnetic ordering has been found.

Maria G. F. Vaz, Rafael A. Allao Cassaro, Handan Akpinar, John A. Schlueter, Paul M. Lahti, Miguel A. Novak, A Cobalt Pyrenylnitronylnitroxide Single-Chain Magnet with High Coercivity and Record Blocking Temperature, Chemistry - A European Journal 20, 5460 (2014)

Magnetochemical Complexity of Hexa- and Heptanuclear Wheel Complexes of Late-3d Ions Supported by N,O-Donor Pyridyl-Methanolate Ligands

The scaffold geometries, stability and magnetic features of the (pyridine-2-yl)methanolate (L) supported wheel-shaped transition-metal complexes with compositions [M6L12] (1), [Na⊂(ML2)6]+ (2), and [M′⊂(ML2)6]2+ (3), in which M=CoII, NiII, CuII, and ZnII were investigated with density functional theory (DFT). The goals of this study are manifold:
1) To advance understanding of the magnetism in the synthesized compounds;
2) To disclose how the structural, electronic, and magnetic characteristics of the molecules change upon varying MII from d7 (Co2+) to d10 (Zn2+);
3) To estimate the influence of the Na+ and M′2+ ions (XQ+) occupying the central voids on the external and internal magnetic coupling interactions in these spin structures;
4) To assess the relative structural and electrochemical stabilities.
In particular, the authors focus here on the net spin polarization, the determination of the strength and the sign of the exchange coupling energies, the rationalization of the nature of the magnetic coupling, and the ground-state structures. The study combines the broken symmetry DFT approach and the model Hamiltonian methodology implemented in the computational framework CONDON 2.0 for the modeling of molecular spin structures and to interpret magnetic susceptibility measurements. The authors illustrate that whereas the structures, stability and magnetism of 1, 2, and 3 are indeed influenced by the nature of 3d transition-metals in the {M6} rims, the XQ+ ions in the inner cavities of 2 and 3 impact these properties to an even larger degree.

Kirill Yu. Monakhov, Xavier Lopez, Manfred Speldrich, Jan van Leusen, Paul Kögerler, Pierre Braunstein, Josep M. Poblet, Magnetochemical Complexity of Hexa- and Heptanuclear Wheel Complexes of Late-3d Ions Supported by N,O-Donor Pyridyl-Methanolate Ligands, Chemistry - A European Journal 20 (2014) 3769-3781

New Synthetic Route toward Heterometallic 3d-3d' and 3d-4f Single-Molecule Magnets. The First CoII-MnIII Heterometallic Complex

Four tetranuclear heterometallic complexes, [CoII2Mn2III(dpm)4(MeO)6] (1) and [LnIII2MnIII2(dpm)6(MeO)6(MeOH)n], where Ln = Gd (2, n = 2), Tb (3, n = 2), and Dy (4, n = 0), have been obtained following the same general synthetic route, namely, the one-pot reaction between 2,2,6,6-tetrametil-3,5-heptanodione (Hdpm), MnCl2 and CoCl2 or Ln(NO3)3 in the presence of sodium methoxide. Within the four compounds, the metal ions bridged by methoxide ligands display a defect-diheterocubane core. Compounds 1, 3, and 4 show slow relaxation of the magnetization below 4 K.

Guilherme P. Guedes, Stephane Soriano, Luiza A. Mercante, Nivaldo L. Speziali, Miguel A. Novak, Marius Andruh, and Maria G. F. Vaz, New Synthetic Route toward Heterometallic 3d-3d' and 3d-4f Single-Molecule Magnets. The First CoII-MnIII Heterometallic Complex, Inorg. Chem., 2013, 52, 8309-8311

Trend: Employing anisotropic exchange



In a series of recent magnetic molecules 3d-5d exchange interactions are being employed. These interactions, as for instance in Mn(III)-Os(III)-Mn(III), are highly anisotropic. The hope is that properties of single-molecule magnets (SMM), such as the blocking temperature, can be improved using such compounds.

Kasper S. Pedersen, Magnus Schau-Magnussen, Jesper Bendix, Hogni Weihe, Andrei V. Palii, Sophia I. Klokishner, Serghei Ostrovsky, Oleg S. Reu, Hannu Mutka, Philip L. W. Tregenna-Piggott, Enhancing the Blocking Temperature in Single-Molecule Magnets by Incorporating 3d-5d Exchange Interactions, Chemistry - A European Journal, 16, 13458-13464, 2010

Eva M. V. Kessler, Sebastian Schmitt and Christoph van Wüllen, Broken symmetry approach to density functional calculation of zero field splittings including anisotropic exchange interactions, J. Chem. Phys. 139, 184110 (2013)

Jan Dreiser, Kasper S. Pedersen, Alexander Schnegg, Karsten Holldack, Joscha Nehrkorn, Marc Sigrist, Philip Tregenna-Piggott, Hannu Mutka, Hogni Weihe, Vladimir S. Mironov, Jesper Bendix, Oliver Waldmann, Three-Axis Anisotropic Exchange Coupling in the Single-Molecule Magnets NEt4[MnIII2(5-Brsalen)2(MeOH)2MIII(CN)6] (M=Ru, Os), Chemistry - A European Journal, 19, 3693-3701, 2013

Saurabh Kumar Singh, Gopalan Rajaraman, Can Anisotropic Exchange Be Reliably Calculated Using Density Functional Methods? A Case Study on Trinuclear Mn(III)-M(III)-Mn(III) (M=Fe, Ru, and Os) Cyanometalate Single-Molecule Magnets, Chemistry - A European Journal, 20, 113-123, 2014

Veronika Hoeke, Anja Stammler, Hartmut Bögge, Jürgen Schnack, and Thorsten Glaser, Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [MnIII6OsIII]3+: Promoting SMM Behavior through 3d-5d Transition Metal Substitution, Inorg. Chem. 53 (2014) 257

A symbol approach for classification of molecule-based magnetic materials exemplified by coordination polymers of metal carboxylates

The ever-growing family of molecule-based magnetic materials calls for a systematic classification of these structurally sophisticated and magnetically diverse compounds. Using magnetically active coordination polymers of metal carboxylates as representatives, the authors propose and illustrate in this review a symbol approach for their organization and categorization. By adopting this approach, the seemingly complex magneto-structural correlations in a wide variety of molecular magnetic materials, including single-molecule magnets, single-chain magnets, geometrically frustrated magnets, and high Curie-temperature magnets, can be readily discerned.

Yan-Zhen Zheng, Zhiping Zheng,Xiao-Ming Chen, A symbol approach for classification of molecule-based magnetic materials exemplified by coordination polymers of metal carboxylates, Coordination Chemistry Reviews, 258-259 (2014) 1-15

An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes

Understanding the anisotropic electronic structure of lanthanide complexes is important in areas as diverse as magnetic resonance imaging, luminescent cell labelling and quantum computing. We present an intuitive strategy based on a simple electrostatic method, capable of predicting the magnetic anisotropy of dysprosium(III) complexes, even in low symmetry. The strategy relies only on knowing the X-ray structure of the complex and the well-established observation that, in the absence of high symmetry, the ground state of dysprosium(III) is a doublet quantized along the anisotropy axis with an angular momentum quantum number mJ ±15/2. The magnetic anisotropy axis of 14 low-symmetry mono-metallic dysprosium(III) complexes computed via high-level ab initio calculations are very well reproduced by our electrostatic model. Furthermore, we show that the magnetic anisotropy is equally well predicted in a selection of low-symmetry polymetallic complexes.

Nicholas F. Chilton, David Collison, Eric J. L. McInnes, Richard E. P. Winpenny, and Alessandro Soncini, An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes, Nature Communications, 2013, 4, 2551

Electrons in Molecules

This book provides the reader with essential keys to a unified understanding of the rapidly expanding field of molecular materials and devices: electronic structures and bonding, magnetic, electrical and photo-physical properties, and the mastering of electrons in molecular electronics. Chemists will discover how basic quantum concepts allow usto understand the relations between structures, electronic structures, and properties of molecular entities and assemblies, and to design elaborate new molecules and useful materials. Physicists and engineers will realize how the molecular world fits in with their need for systems flexible enough to check theories or provide original solutions to exciting new scientific and technological challenges. The non-specialist will find out how molecules behave in electronics at the most minute, sub- nanosize level.

Jean-Pierre Launay, Michel Verdaguer, Electrons in Molecules, Oxford University Press, 978-0-19-929778-8,
Flyer and order form

Clusters of clusters

In order to design and construct devices that take advantage of the magnetic properties of molecular nanomagnets, we must learn how to control the self-assembly of individual components. It has been demonstrated that {Cr7Ni} is a potential qubit candidate for quantum information processing and simulation with a well-defined two level quantum system with long phase memory lifetimes. Recent advances of the functionalisation of the periphery of {Cr7Ni} molecular nanomagnets with pyridyl and carboxylic acids groups, coupled with the controlled self-assembly of these with other molecular clusters and metal salts, has led to engineering of larger clusters-of-clusters constructs that could ultimately lead to the synthesis of devices using {Cr7Ni}.

George F. S. Whitehead, Fabrizio Moro, Grigore A. Timco, Wolfgang Wernsdorfer, Simon J. Teat, Richard E. P. Winpenny, A Ring of Rings and Other Multicomponent Assemblies of Cages, Angewandte Chemie International Edition, Volume 52, Issue 38, pages 9932-9935, September 16, 2013

George F. S. Whitehead, Jesus Ferrando-Soria, Lorna G. Christie, Nicholas F. Chilton, Grigore A. Timco, Fabrizio Moroa, Richard E. P. Winpenny, The acid test: the chemistry of carboxylic acid functionalised {Cr7Ni} rings, Chem. Sci., 2014, Advance Article

Please click on the pictures to see animations.

Frustration, frustration, frustration

The term frustration is heavily used to explain many properties of magnetic materials, but it also seems to cause frustration among scientists. Read the following contributions:

Key Role of Frustration in Suppression of Magnetization Blocking in Single-Molecule Magnets

State-of-the-art ab initio calculations of a series of endohedral fullerenes are performed, and their anisotropic magnetic properties are described from the first principles. Contrary to general expectation that molecules of higher nuclearity exhibit better magnetic blocking properties, we predict Dy3N@C80 to be a relatively weak single-molecule magnet due to its frustrated ground state. This finding, relevant to polynuclear magnetic molecules in general, demonstrates the crucial role played by magnetic frustration in the suppression of magnetization blocking in single-molecule magnets.

Veacheslav Vieru , Liviu Ungur , and Liviu F. Chibotaru, Key Role of Frustration in Suppression of Magnetization Blocking in Single-Molecule Magnets, J. Phys. Chem. Lett. 4, 3565 (2013)

A classification of spin frustration in molecular magnets from a physical study of large odd-numbered-metal, odd electron rings

The term "frustration" in the context of magnetism was originally used by P. W. Anderson and quickly adopted for application to the description of spin glasses and later to very special lattice types, such as the kagomé. The original use of the term was to describe systems with competing antiferromagnetic interactions and is important in current condensed matter physics in areas such as the description of emergent magnetic monopoles in spin ice. Within molecular magnetism, at least two very different definitions of frustration are used. Here we report the synthesis and characterization of unusual nine-metal rings, using magnetic measurements and inelastic neutron scattering, supported by density functional theory calculations. These compounds show different electronic/magnetic structures caused by frustration, and the findings lead us to propose a classification for frustration within molecular magnets that encompasses and clarifies all previous definitions.

Michael L. Baker, Grigore A. Timco, Stergios Piligkos, Jennifer S. Mathieson, Hannu Mutka, Floriana Tuna, Piotr Kozlowski, Michal Antkowiak, Tatiana Guidi, Tulika Gupta, Harapriya Rath, Robert J. Woolfson, Grzegorz Kamieniarz, Robin G. Pritchard, Hogni Weihe, Leroy Cronin, Gopalan Rajaraman, David Collison, Eric J. L. McInnes, and Richard E. P. Winpenny, A classification of spin frustration in molecular magnets from a physical study of large odd-numbered-metal, odd electron rings, PNAS November 20, 2012 vol. 109 no. 47 19113-19118

Effects of frustration on magnetic molecules: a survey from Olivier Kahn until today

In magnetism, of which molecular magnetism is a part, the term frustration is used rather sloppily. Sometimes one gains the impression that if the reason for some phenomenon is not quite clear then it is attributed to frustration. In this paper a discussion of the effects of frustration that are relevant for the field of molecular magnetism is presented. As will become clear later these effects indeed lead to a variety of unusual magnetic properties.

J. Schnack, Effects of frustration on magnetic molecules: a survey from Olivier Kahn until today, Dalton Trans., 2010,39, 4677-4686

Dynamic control of magnetic nanowires by light-induced domain-wall kickoffs

Magnetic relaxation processes were first discussed for a crystal of paramagnetic transition ions. It was suggested that mechanical vibrations of the crystal lattice (phonons) modulate the crystal electric field of the magnetic ion, thus inducing a direct relaxation between two different spin states. Direct relaxation has also been predicted for single-molecule magnets with a large spin and a high magnetic anisotropy and was first demonstrated in a Mn12 acetate crystal. The spin-lattice relaxation time for such a direct transition is limited by the phonon density of states at the spin resonance. In a three-dimensional system, such as a single-molecule magnet crystal, the phonon energy spectrum is continuous, but in a one-dimensional system, like a suspended carbon nanotube, the spectrum is discrete and can be engineered to an extremely low density of states. An individual single-molecule magnet, coupled to a suspended carbon nanotube, should therefore exhibit extremely long relaxation times and the system's reduced size should result in a strong spin-phonon coupling. Here, we provide the first experimental evidence for a strong spin-phonon coupling between a single molecule spin and a carbon nanotube resonator, ultimately enabling coherent spin manipulation and quantum entanglement.

W. Wernsdorfer, Dynamic control of magnetic nanowires by light-induced domain-wall kickoffs, Nature Nanomaterials 8, 165 (2013)

Emerging young scientists in molecular magnetism

Rodolphe Clerac (editor) writes: "Over the past thirty years, a range of nanosized metal complexes have been engineered by molecular chemists to exhibit original physical and magnetic properties, which include single-molecule magnet (SMM), single-chain magnet (SCM), spincrossover (SCO), high TC magnets and photomagnetism. More recent activity has focused on strategies to combine many of these properties to systematically produce multifunctional materials. Among these are chirality, conductivity, guest molecule adsorption, soft-matter properties (liquid-crystals, gels) and nanoparticle assembly, etc... Indeed by controlling self-assembly reactions and chemically modifying the molecular precursors utilized, creative chemists can finely tune these properties at the atomic scale, and contribute to a better understanding of the factors necessary for the integration of these molecular systems into future spintronics or molecular electronics applications. In this research field devoted to molecule-based magnetic materials, a new generation of scientists has recently emerged and it is the purpose of this special issue of Current Inorganic Chemistry on Molecular Magnetism to highlight some of the most recent work of these young researchers. Of course, not everyone was able to contribute to this issue and we hope to produce a second special issue on this topic to give a timely accounting of this flourishing research field in the future."

All papers are available on line for free: LINK

Surface-Confined Electroactive Molecules for Multistate Charge Storage Information

Bi-stable molecular systems with potential for applications in binary memory devices are raising great interest for device miniaturization. Particular appealing are those systems that operate with electrical inputs since they are compatible with existing electronic technologies. The processing of higher memory densities in these devices could be accomplished by increasing the number of memory states in each cell, although this strategy has not been much explored yet. In the article the authos highlight the recent advances devoted to the fabrication of charge-storage molecular surface-confined devices exhibiting multiple states. Mainly, this goal has been realized immobilizing a variety (or a combination) of electroactive molecules on a surface, although alternative approaches employing non-electroactive systems have also been described. Undoubtedly, the use of molecules with chemically tunable properties and nanoscale dimensions are raising great hopes for the devices of the future in which molecules can bring new perspectives such as multistability.

M. Mas-Torrent, C. Rovira, J. Veciana, Surface-Confined Electroactive Molecules for Multistate Charge Storage Information, Advanced Materials 25 (2013) 462

Dynamic control of magnetic nanowires by light-induced domain-wall kickoffs

Controlling the speed at which systems evolve is a challenge shared by all disciplines, and otherwise unrelated areas use common theoretical frameworks towards this goal. A particularly widespread model is Glauber dynamics, which describes the time evolution of the Ising model and can be applied to any binary system. In the article it is shown, using molecular nanowires under irradiation, that Glauber dynamics can be controlled by a novel domain-wall kickoff mechanism. In contrast to known processes, the kickoff has unambiguous fingerprints, slowing down the spin-flip attempt rate by several orders of magnitude, and following a scaling law. The required irradiance is very low, a substantial improvement over present methods of magneto-optical switching. These results provide a new way to control and study stochastic dynamic processes. Being general for Glauber dynamics, they can be extended to different kinds of magnetic nanowires and to numerous fields, ranging from social evolution to neural networks and chemical reactivity.

Eric Heintze, Fadi El Hallak, Conrad Clauß, Angelo Rettori, Maria Gloria Pini, Federico Totti, Martin Dressel, Lapo Bogani, Dynamic control of magnetic nanowires by light-induced domain-wall kickoffs, Nature Materials 12 (2013) 202

Magnetic cluster excitations

Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds and magnetic molecules to artificial metal structures formed on surfaces and metalloproteins. Their magnetic excitation spectra are determined by the nature of the spin centers and of the magnetic interactions, and the particular arrangement of the mutual interaction paths between the spin centers. Small clusters of up to four magnetic ions are ideal model systems in which to examine the fundamental magnetic interactions, which are usually dominated by Heisenberg exchange, but often complemented by anisotropic and/or higher-order interactions. In large magnetic clusters, which may potentially deal with a dozen or more spin centers, there is the possibility of novel many-body quantum states and quantum phenomena. In this review the necessary theoretical concepts and experimental techniques to study the magnetic cluster excitations and the resulting characteristic magnetic properties are introduced, followed by examples of small clusters, demonstrating the enormous amount of detailed physical information that can be retrieved.
The current understanding of the excitations and their physical interpretation in the molecular nanomagnets which represent large magnetic clusters is then presented, with a section devoted to the subclass of single-molecule magnets, distinguished by displaying quantum tunneling of the magnetization. Finally, there is a summary of some quantum many-body states which evolve in magnetic insulators characterized by built-in or field-induced magnetic clusters. The review concludes by addressing future perspectives in the field of magnetic cluster excitations.

Albert Furrer, Oliver Waldmann, Magnetic cluster excitations, Rev. Mod. Phys. 85 (2013) 367

Switchable materials: A new spin on bistability

Many different kinds of switchable molecules and materials are based on transition metal ions, but similar properties are also possible in organic materials. Now, two separate studies reveal new insights into the ability of organic radicals to associate reversibly and cooperatively in the solid state, and in so doing create bistable, hysteretic materials.
The solid-state electronic structure and properties of a phenalenyl-based butyl-substituted neutral radical, that shows a hysteretic phase transition just above room temperature have been investigated. The electron density distribution of this radical throughout both branches of the hysteretic phase transition was analyzid using solid-state X-ray structures and two distinct electronic states were found in the hysteresis loop that accompanies the phase transition. The bistability of the two electronic states was observed through a number of measurements, including IR transmittance spectra of single crystals in the vicinity of the phase transition.

Sushanta K. Pal, Pradip Bag, Arindam Sarkar, Xiaoliu Chi, Mikhail E. Itkis, Fook S. Tham, Bruno Donnadieu, and Robert C. Haddon, Hysteretic Spin and Charge Delocalization in a Phenalenyl-Based Molecular Conductor, JACS 132 (2010) 17258

Robin G. Hicks, Switchable materials: A new spin on bistability, Nature Chemistry 3 (2011) 189

Inelastic Neutron Scattering at its height

INS is as powerful as it has never been before. Read the three following highlights to understand its success and importance for nowadays research in quantum magnetism.

Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering

Molecular nanomagnets are among the first examples of finite-size spin systems and have been test beds for addressing several phenomena in quantum dynamics. In fact, for short-enough timescales the spin wavefunctions evolve coherently according to an appropriate spin Hamiltonian, which can be engineered to meet specific requirements. Unfortunately, so far it has been impossible to determine these spin dynamics directly. Here we show that recently developed instrumentation yields the four-dimensional inelastic-neutron scattering function in vast portions of reciprocal space and enables the spin dynamics to be determined directly. We use the Cr8 antiferromagnetic ring as a benchmark to demonstrate the potential of this approach which allows us, for example, to examine how quantum fluctuations propagate along the ring or to test the degree of validity of the Neel-vector-tunnelling framework.

Michael L. Baker, Tatiana Guidi, Stefano Carretta, Jacques Ollivier, Hannu Mutka, Hans U. Güdel, Grigore A. Timco, Eric J. L. McInnes, Giuseppe Amoretti, Richard E. P. Winpenny, Paolo Santini, Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering, Nature Physics 8 (2012) 906

See also this nice work: Michael L. Baker, Grigore A. Timco, Stergios Piligkos, Jennifer S. Mathieson, Hannu Mutka, Floriana Tuna, Piotr Kozlowski, Michal Antkowiak, Tatiana Guidi, Tulika Gupta, Harapriya Rath, Robert J. Woolfson, Grzegorz Kamieniarz, Robin G. Pritchard, Hogni Weihe, Leroy Cronin, Gopalan Rajaraman, David Collison, Eric J. L. McInnes, and Richard E. P. Winpenny, A classification of spin frustration in molecular magnets from a physical study of large odd-numbered-metal, odd electron rings, , PNAS (2012)

Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18

The low-temperature elementary spin excitations in the AFM molecular wheel Fe18 were studied experimentally by inelastic neutron scattering and theoretically by modern numerical methods, such as dynamical density matrix renormalization group or quantum Monte Carlo techniques, and analytical spin-wave theory calculations. Fe18 involves eighteen spin-5/2 FeIII ions with a Hilbert space dimension of 1014, constituting a physical system that is situated in a region between microscopic and macroscopic. The combined experimental and theoretical approach allowed us to characterize and discuss the magnetic properties of Fe18 in great detail. It is demonstrated that physical concepts such as the rotational-band or L and E-band concepts developed for smaller rings are still applicable. In particular, the higher-lying low-temperature elementary spin excitations in Fe18 or AFM wheels, in general, are of discrete antiferromagnetic spin-wave character.

J. Ummethum, J. Nehrkorn, S. Mukherjee, N. B. Ivanov, S. Stuiber, Th. Strässle, P. L. W. Tregenna-Piggott, H. Mutka, G. Christou, O. Waldmann, J. Schnack, Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18, Phys. Rev. B 86 (2012) 104403

Ferromagnetic Cluster Spin Waves in Molecular Disks Studied by Inelastic Neutron Scattering

Structurally, the two mixed-valence manganese disks Mn7-11 and Mn7-16 differ only in the peripheral ligand but, as a result of a subtle interplay of intramolecular exchange interactions, differ strongly in their magnetic properties, e.g., Mn7-11 possesses a ground-state spin of S=11 and Mn7-16 of S=16. The exchange interactions in the disks were studied by inelastic neutron scattering. The analysis of the Q dependence of the observed magnetic transition intensities reveals that ferromagnetic cluster spin-wave excitations were observed. In this framework, it was possible to successfully model the experimental data and provide a physical understanding of the magnetism in the two disks.

J. Nehrkorn, S. Mukherjee, S. Stuiber, H. Mutka, Th. Strässle, G. Christou, O. Waldmann, Ferromagnetic Cluster Spin Waves in Molecular Disks Studied by Inelastic Neutron Scattering, Phys. Rev. B 86 (2012) 134417

Hysteresis in the ground and excited spin state up to 10 T of a [MnIII6MnIII]3+ triplesalen single-molecule magnet

We have synthesized the triplesalen-based single-molecule magnet (SMM) [MnIII6MnIII]3+ as a variation of our SMM [MnIII6CrIII](BPh4)3. The use of the rod-shaped anion lactate (lac) was intended to enforce a rod packing and resulted in the crystallization of [MnIII6MnIII](lac)3 in the highly symmetric space group R-3-bar. This entails a crystallographic S6 symmetry of the [MnIII6MnIII]3+ molecules, which in addition are all aligned with the crystallographic c axis. Moreover, the molecular environment of each [MnIII6MnIII]3+ molecule is highly symmetric. Single-crystals of [MnIII6MnIII](lac)3 exhibit a double hysteresis at 0.3 K with a hysteretic opening not only for the spin ground state up to 1.8 T, but also for an excited state becoming the ground state at about 3.4 T with a hysteretic opening up to 10 T.
Ab initio calculations including spin-orbit coupling establish a non-magnetic behavior of the central MnIII low-spin (l.s.) ion at low temperatures, demonstrating that predictions from ligand-field theory are corroborated in the case of MnIII l.s. by ab intio calculations. Simulations of the field- and temperature-dependent magnetization data indicate that [MnIII6MnIII]3+ is in the limit of weak exchange with antiferromagnetic interactions in the trinuclear MnIII3 triplesalen subunits resulting in intermediate S* = 2 spins. Slight ferromagnetic interactions between the two trinuclear MnIII3 subunits lead to a ground state in zero-field that is approximately described by a total spin quantum number S = 4. This ground state exhibits only a very small anisotropy barrier due to the misalignment of the local zero-field splitting tensors. At higher magnetic fields of about 3.4 T, the spin configuration changes to an all-up orientation of the local MnIII spins, with the main part of the Zeeman energy needed for the spin-flip being required to overcome the local MnIII anisotropy barriers, while only minor contributions of the Zeeman energy are needed to overcome the antiferromagnetic interactions. These combined theoretical analyses provide a clear picture of the double-hysteretic behavior of the [MnIII6MnIII]3+ single-molecule magnet with hysteretic openings up to 10 T.

Veronika Hoeke, Klaus Gieb, Paul Müller, Liviu Ungur, Liviu F. Chibotaru, Maik Heidemeier, Erich Krickemeyer, Anja Stammler, Hartmut Bögge, Christian Schröder, Jürgen Schnack, Thorsten Glaser, Hysteresis in the ground and excited spin state up to 10 T of a [MnIII6MnIII]3+ triplesalen single-molecule magnet, Chem. Sci. 3 (2012) 2868

Electronic read-out of a single nuclear spin using a molecular spin transistor

Quantum control of individual spins in condensed-matter devices is an emerging field with a wide range of applications, from nanospintronics to quantum computing. The electron, possessing spin and orbital degrees of freedom, is conventionally used as the carrier of quantum information in proposed devices. However, electrons couple strongly to the environment, and so have very short relaxation and coherence times. It is therefore extremely difficult to achieve quantum coherence and stable entanglement of electron spins.
Alternative concepts propose nuclear spins as the building blocks for quantum computing, because such spins are extremely well isolated from the environment and less prone to decoherence. However, weak coupling comes at a price: it remains challenging to address and manipulate individual nuclear spins. In this publication it is shown that the nuclear spin of an individual metal atom embedded in a single-molecule magnet can be read out electronically. The observed long lifetimes (tens of seconds) and relaxation characteristics of nuclear spin at the single-atom scale open the way to a completely new world of devices in which quantum logic may be implemented.

Romain Vincent, Svetlana Klyatskaya, Mario Ruben, Wolfgang Wernsdorfer, Franck Balestro, Electronic read-out of a single nuclear spin using a molecular spin transistor, Nature 488 (2012) 357

How good can spin densities be calculated?

An approach for the calculation of spin density distributions for molecules is presented, that require very large active spaces for a qualitatively correct description of their electronic structure. It is based on the density-matrix renormalization group (DMRG) algorithm to calculate the spin density matrix elements as a basic quantity for the spatially resolved spin density distribution. The spin density matrix elements are directly determined from the second-quantized elementary operators optimized by the DMRG algorithm. As an analytic convergence criterion for the spin density distribution, we employ our recently developed sampling-reconstruction scheme to build an accurate complete-active-space configuration-interaction (CASCI) wave function from the optimized matrix product states. The spin density matrix elements can then also be determined as an expectation value employing the reconstructed wave function expansion. Furthermore, the explicit reconstruction of a CASCI-type wave function provides insight into chemically interesting features of the molecule under study such as the distribution of alpha and beta electrons in terms of Slater determinants, CI coefficients, and natural orbitals. The methodology is applied to an iron nitrosyl complex which we have identified as a challenging system for standard approaches.
Katharina Boguslawski, Konrad H. Marti, Örs Legeza, and Markus Reiher, Accurate ab Initio Spin Densities, J. Chem. Theory Comput., 2012, 8 (6), pp 1970-1982

Fragmenting Gadolinium: Mononuclear Polyoxometalate-Based Magnetic Coolers for Ultra-Low Temperatures

The polyoxometalate clusters with formula [Gd(W5O18)2] and [Gd(P5W30O110)] each carry a single magnetic ion of gadolinium, which is the most widespread element among magnetic refrigerant materials. In an adiabatic demagnetization, the lowest attainable temperature is limited by the presence of magnetic interactions that bring about magnetic order below a critical temperature. The authors demonstrate that this limitation can be overcome by chemically engineering the molecules in such a way to effectively screen all magnetic interactions, suggesting their use as ultra-low-temperature coolers.
Martinez-Perez, Maria-Jose and Montero, Oscar and Evangelisti, Marco and Luis, Fernando and Sese, Javier and Cardona-Serra, Salvador and Coronado, Eugenio, Fragmenting Gadolinium: Mononuclear Polyoxometalate-Based Magnetic Coolers for Ultra-Low Temperatures, Adv. Mater. 24 (2012) 4301-4305

Inelastic neutron scattering studies on the odd-membered antiferromagnetic wheel Cr8Ni

Roughly 50 % of Europe's molecular magnetism community contributed to the following article:

A detailed characterization of the magnetic properties of the odd-membered antiferromagnetic "wheel" Cr8Ni has been conducted. Inelastic neutron scattering (INS) is used to investigate the energy and momentum transfer dependence of the low-lying spin excitations, including excited states inaccessible by other experimental techniques. The richness of the INS data, in conjunction with microscopic spin Hamiltonian simulations, enables an accurate characterization of the magnetic properties of Cr8Ni. Nearest-neighbor exchange constants of JCrCr = 1.31 meV and JCrNi = 3.22 meV are determined, and clear evidence of axial single-ion anisotropy is found. The parameters determined by INS are shown to fit magnetic susceptibility. The spectroscopic identification of several successive S=1 excited total spin states and lowest spin band excitations show that the rotational band picture, valid for bipartite AFM wheels, breaks down for this odd-numbered wheel. The exchange constants determined here differ from previous efforts based on bulk measurements, and possible reasons are discussed. The large JCrNi/JCrCr ratio in Cr8Ni puts this wheel into a regime with strong quantum fluctuations in which the ground state can be described with a valence bond solid state picture.

Michael L. Baker, Oliver Waldmann, Stergios Piligkos, Roland Bircher, Olivier Cador, Stefano Carretta, David Collison, Felix Fernandez-Alonso, Eric J. L. McInnes, Hannu Mutka, Andrew Podlesnyak, Floriana Tuna, Stefan Ochsenbein, Roberta Sessoli, Andreas Sieber, Grigore A. Timco, Hogni Weihe, Hans U. Güdel, and Richard E. P. Winpenny, Inelastic neutron scattering studies on the odd-membered antiferromagnetic wheel Cr8Ni, Phys. Rev. B 86, 064405 (2012)

Self-Assembled Monolayer of Cr7Ni Molecular Nanomagnets by Sublimation

We show, by complementary spectroscopic and STM analysis, that Cr7Ni derivatives are suitable to be sublimed in UHV conditions. Cr7Ni-bu weakly bonds to gold surface and can diffuse relatively freely on it, forming monolayers with hexagonal 2D packing. Conversely, by adding a functional thiol group to the central dibutylamine, a covalent bond between the molecule and surface gold adatoms is promoted, leading to a strong molecular grafting and the formation of a disordered monolayer. These two examples demonstrate the possibility to control the assembly of a large molecular complex, as rationalized by DFT calculations that establish different energy scales in the deposition processes. Moreover, low-temperature XMCD sprectra show that the magnetic features of Cr7Ni rings deposited in UHV on gold remain unchanged with respect to those of the corresponding bulk sample.
Alberto Ghirri, Valdis Corradini, Valerio Bellini, Roberto Biagi, Umberto del Pennino, Valentina De Renzi, Julio C. Cezar, Christopher A. Muryn , Grigore A. Timco , Richard E. P. Winpenny , and Marco Affronte, Self-Assembled Monolayer of Cr7Ni Molecular Nanomagnets by Sublimation, ACS Nano, 2011, 5 (9), pp 7090-7099

Ultrafast Spin Avalanches in Crystals of Nanomagnets in Terms of Magnetic Detonation

Recent experiments [W. Decelle et al., Phys. Rev. Lett. 102, 027203 (2009)] have discovered ultrafast propagation of spin avalanches in crystals of nanomagnets, which is 3 orders of magnitude faster than the traditionally studied magnetic deflagration. The new regime has been hypothetically identified as magnetic detonation. Here we demonstrate unequivocally the possibility of magnetic detonation in the crystals, as a front consisting of a leading shock and a zone of Zeeman energy release. We study the key features of the process and find that the magnetic detonation speed only slightly exceeds the sound speed in agreement with the experimental observations. For combustion science, our results provide a unique physical example of extremely weak detonation.
M. Modestov, V. Bychkov, and M. Marklund, Ultrafast Spin Avalanches in Crystals of Nanomagnets in Terms of Magnetic Detonation, Phys. Rev. Lett. 107, 207208 (2011)

Theoretical Study of Exchange Coupling in 3d-Gd Complexes: Large Magnetocaloric Effect Systems

Polynuclear 3d transition metal-Gd complexes are good candidates to present large magnetocaloric effect. This effect is favored by the presence of weak ferromagnetic exchange interactions that have been investigated using methods based on Density Functional Theory.The results show the crucial role of the Gd 5d orbitals in the exchange interaction while the 6s orbital seems to have a negligible participation. The analysis of the atomic and orbital spin populations reveals that the presence of spin density in the Gd 5d orbital is mainly due to a spin polarization effect, while a delocalization mechanism from the 3d orbitals of the transition metal can be ruled out. We propose a numerical DFT approach using pseudopotentials to calculate the exchange coupling constants in four polynuclear first-row transition metal-Gd complexes. Despite the complexity of the studied systems, the numerical approach gives coupling constants in excellent agreement with the available experimental data and, in conjunction with exact diagonalization methods (or Monte Carlo simulations), it makes it possible to obtain theoretical estimates of the entropy change due to the magnetization/demagnetization process of the molecule.
Eduard Cremades , Silvia Gomez-Coca , Daniel Aravena , Santiago Alvarez , and Eliseo Ruiz, Theoretical Study of Exchange Coupling in 3d-Gd Complexes: Large Magnetocaloric Effect Systems, J. Am. Chem. Soc., 2012, 134 (25), pp 10532-10542

Molecular Cluster Magnets

This work covers new developments in the field of molecular nanomagnetism, complementing previous books in this area. The book is written by experts in the field and is intended as a compilation of critical reviews of new areas rather than a comprehensive text.


  • Supramolecular Polymetallic 2D [n x n] Transition Metal Grids – Approaches to Ordered Molecular Assemblies and Functional Molecular Devices (Laurence K Thompson, Louise N Dawe and Konstantin V Shuvaev)
  • Recent Synthetic Results Involving Single Molecule Magnets (Guillem Aromí, Eric J L McInnes and Richard E P Winpenny)
  • The Nanoscopic V15 Cluster: A Unique Magnetic Polyoxometalate (Boris Tsukerblat and Alex Tarantul)
  • Neutron Spectroscopy of Molecular Nanomagnets (Tatiana Guidi)
  • Recent Developments in EPR Spectroscopy of Molecular Nanomagnets (Eric J L McInnes)
  • Simulating Computationally Complex Magnetic Molecules (Larry Engelhardt and Christian Schröder)
World Scientific Series in Nanoscience and Nanotechnology- Vol. 3, MOLECULAR CLUSTER MAGNETS, edited by Richard Winpenny (The University of Manchester, UK), 312pp, 978-981-4322-94-2 & 978-981-4322-95-9(ebook)

Single-molecule magnets in carbon nanotubes

Next-generation electronic, photonic or spintronic devices will be based on nanoscale functional units, such as quantum dots, isolated spin centres or single-molecule magnets. The key challenge is the coupling of the nanoscale units to the macroscopic world, which is essential for read and write purposes. Carbon nanotubes with one macroscopic and two nanoscopic dimensions provide an excellent means to achieve this coupling. Although the dimensions of nanotube internal cavities are suitable for hosting a wide range of different molecules, to our knowledge, no examples of molecular magnets inserted in nanotubes have been reported up to date. Here, we report on the successful encapsulation of single-molecule magnets in carbon nanotubes yielding a new type of hybrid nanostructure that combines all the key single-molecule magnet properties of the guest-molecules with the functional properties of the host-nanotube. This breakthrough paves the way for the construction of spintronic or ultrahigh-density magnetic data storage devices.
Maria del Carmen Gimenez-Lopez, Fabrizio Moro, Alessandro La Torre, Carlos J. Gomez-Garcia, Paul D. Brown, Joris van Slageren, Andrei N. Khlobystov, Encapsulation of single-molecule magnets in carbon nanotubes, Nature Communications 2, 407 (2011)

A delocalized arene-bridged diuranium single-molecule magnet

Single-molecule magnets (SMMs) are multinuclear clusters whose behaviour typically relies on intramolecular spin-coupling interactions between neighbouring metal ions. A diuranium-arene complex has now been prepared that shows behaviour characteristic of an SMM without relying on this type of superexchange mechanism. This may enable the construction of SMMs that maintain their magnetism at higher temperatures.
David P. Mills, Fabrizio Moro, Jonathan McMaster, Joris van Slageren, William Lewis, Alexander J. Blake, Stephen T. Liddle, A delocalized arene-bridged diuranium single-molecule magnet, Nature Chemistry, 3, 454 - 460 (2011)

Magnetic Ground-State and Systematic X-ray Photoreduction Studies of an Iron-Based Star-Shaped Complex

XPS methods are valuable tools in order to characterize magnetic substances, but radiation damages might spoil the investigation. In a recent paper a the star-shaped iron-based single-molecule magnet is investigated in order to clarify some open questions concerning the exact electronic and magnetic properties. To this end the internal magnetic structure is studied by applying X-ray magnetic circular dichroism to the Fe L2,3 edges and the occuring radiation photochemistry is investigated. A Fe3+ to Fe2+ photoreduction process, taking place under soft X-ray radiation, is reported.
Karsten Kuepper, Christian Taubitz, Daniel Taubitz, Ulf Wiedwald, Andreas Scheurer, Stefan Sperner, Rolf W. Saalfrank, Jean-Paul Kappler, Loc Joly, Paul Ziemann, and Manfred Neumann, Magnetic Ground-State and Systematic X-ray Photoreduction Studies of an Iron-Based Star-Shaped Complex, J. Phys. Chem. Lett., 2011, 2 (13), pp 1491-1496

Supramolecular spin valves

Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. In a new publication the authors present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc2 single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K.
M. Urdampilleta, S. Klyatskaya, J-P. Cleuziou, M. Ruben, W. Wernsdorfer, Supramolecular spin valves, Nature Materials 10, 502-506 (2011)

World Record: The Highest Magnetic Fields Are Created in Dresden

On June 22, 2011, the Helmholtz-Zentrum Dresden-Rossendorf set a new world record for magnetic fields with 91.4 Tesla. To reach this record, Sergei Zherlitsyn and his colleagues at the High Magnetic Field Laboratory Dresden (HLD) developed a coil weighing about 200 kilograms in which electric current create the giant magnetic field for a period of a few milliseconds. The coil survived the experiment unscathed.
Press release of the Helmholtz-Zentrum Dresden-Rossendorf

Strong exchange and magnetic blocking in N23--radical-bridged lanthanide complexes

Single-molecule magnets approach the ultimate size limit for spin-based devices. These complexes can retain spin information over long periods of time at low temperature, suggesting possible applications in high-density information storage, quantum computing and spintronics. Notably, the success of most such applications hinges upon raising the inherent molecular spin-inversion barrier. Although recent advances have shown the viability of lanthanide-containing complexes in generating large barriers, weak or non-existent magnetic exchange coupling allows fast relaxation pathways that mitigate the full potential of these species.
In a recent article the authors show that the diffuse spin of an N23--radical bridge can lead to exceptionally strong magnetic exchange in dinuclear Ln(III) (Ln = Gd, Dy) complexes. The Gd(III) congener exhibits the strongest magnetic coupling yet observed for that ion, while incorporation of the high-anisotropy Dy(III) ion gives rise to a molecule with a record magnetic blocking temperature of 8.3 K at a sweep rate of 0.08 T/s. Jeffrey D. Rinehart, Ming Fang, William J. Evans, Jeffrey R. Long, Strong exchange and magnetic blocking in N23--radical-bridged lanthanide complexes, Nature Chemistry (2011) Published online 22 May 2011

Chemical Society Reviews, Themed issue: Molecule-based magnets

Guest editors Joel Miller and Dante Gatteschi introduce the molecule-based magnets themed issue of Chemical Society Reviews. With contributions of Gatteschi, Cornia, Sessoli, Winpenny, Meisel, Dunbar, Tsukerblat, Coronado, Verdaguer, Oshio, ...
Chemical Society Reviews, Themed issue Molecule-based magnets, Chem. Soc. Rev., 2011, 40, Issue 6

The first magnetization experiment after March 11 from IMR, Tohoku

On 26th of April the first low temperature magnetization experiment after the earthquake was performed at the Institute for Materials Research, Tohoku University in collaboration with Leroy Cronin at Glasgow University. The material is newly synthesized clusters with multi-magentic centers. A clear temperature dependence of the hysteresis as well as the clean magnetization curve at 0.5 K exhibit the full recovery of the research. Luckily, no serious damage of the instrument was caused by the earthquake, however, the laboratory has worked carefully to built up the solid safety in the last 45 days. Moreover, some time is needed to exchange the transfer tube of Helium liquefier. Now IMR group declares the restart from the spring break with sincere thanks for all supports given to us in the last several weeks.
Recovery reports from the Institute for Materials Research, Tohoku University

Room-Temperature Electrical Addressing of a Bistable Spin-Crossover Molecular System

A switchable molecular device is constructed by contacting an individual nanoparticle based on spin-crossover molecules between nanometer-spaced electrodes. The switching and memory effects near room temperature are a consequence of the intrinsic bistability of the nanoparticle. Interestingly, for molecular spintronics, the spin crossover can also be induced by applying a voltage, showing that its magnetic state is electrically controllable.
Ferry Prins, Maria Monrabal-Capilla, Edgar A. Osorio, Eugenio Coronado, Herre S. J. van der Zant, Room-Temperature Electrical Addressing of a Bistable Spin-Crossover Molecular System, Advanced Materials, Volume 23, Issue 13, pages 1545-1549 (2011)

A Density-Functional Study of Heterometallic Cr-Based Molecular Rings

Density-functional theoretical investigation of the electronic and magnetic properties of octametallic Cr-based molecular antiferromagnetic rings are presented. The presence of the divalent magnetic ion M unbalances the charge and the spin of the parent Cr8 ring, leading to a finite total spin in the molecules. Exchange interaction parameters have been extracted from broken-symmetry calculations and compared with the available experiments; in agreement with them, we find that exchange parameters are rather similar in the two derivatives. The analysis of the electronic properties shows some differences depending on M, in particular in the size of the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gaps.
V. Bellini and M. Affronte, A Density-Functional Study of Heterometallic Cr-Based Molecular Rings, J. Phys. Chem. B, 2010, 114 (46), 14797-14806

A Double-Switching Multistable Fe4 Grid Complex with Stepwise Spin-Crossover and Redox Transitions

A Fe4 grid complex featuring two orthogonal switching modes, namely stepwise spin-crossover and sequential redox processes, is presented as the cover article of issue 48/2010 of Angewandte Chemie. Of particular interest are the resulting twofold-degenerate configurations [HS-LS-HS-LS] and [FeII2FeIII2], where two identical metal centers occupy opposite corners of the Fe4 square. Such an arrangement qualifies this new grid complex as a potential building block for quantum cellular automata.
B. Schneider, S. Demeshko, S. Dechert, F. Meyer, A Double-Switching Multistable Fe4 Grid Complex with Stepwise Spin-Crossover and Redox Transitions, Angew. Chem. 2010, 122, 9461-9464; Angew. Chem. Int. Ed. 2010, 49, 9274-9277.

A New Family of 1D Exchange Biased Heterometal Single-Molecule Magnets: Observation of Pronounced Quantum Tunneling Steps in the Hysteresis Loops of Quasi-Linear {Mn2Ni3} Clusters

First members of a new family of heterometallic Mn/Ni complexes [Mn2Ni3X2L4(LH)2(H2O)2] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H2L) have been synthesized. The molecular structures feature a quasi-linear Mn-Ni-Ni-Ni-Mn core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. The compounds exhibit a quite strong ferromagnetic exchange. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal Ueff = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops.
Animesh Das, Klaus Gieb, Yulia Krupskaya, Serhiy Demeshko, Sebastian Dechert, Rüdiger Klingeler, Vladislav Kataev, Bernd Büchner, Paul Müller, Franc Meyer, A New Family of 1D Exchange Biased Heterometal Single-Molecule Magnets: Observation of Pronounced Quantum Tunneling Steps in the Hysteresis Loops of Quasi-Linear {Mn2Ni3} Clusters, J. Am. Chem. Soc. (2011)

Tim And The Pan-Galactic Civil Servants

"One night Timothy Becket woke to an adventure that would take him beyond his wildest dreams. With the Pan-Galactic Civil Servants - Clarence Smith, Marvelious Fermionus, Coola Baloola, Garvin McGuffin and Colonel Glumm - at his side, Tim must find the all-powerful Cerulean Circlet before the forces of darkness can claim it for their own. Before the Big Bang, the Architects used the Cerulean Circlet to create the Universe. ..."

Damian J.J. Farnell, usually known for his scientific contributions to the physics of spin systems (coupled cluster method), invites us to enjoy his first work of fiction.

Damian J.J. Farnell, Tim And The Pan-Galactic Civil Servants, CreateSpace, ISBN-10: 1453776303, ISBN-13: 978-1453776308

Rational design of single-molecule magnets: a supramolecular approach

Rational design of single-molecule magnets with C3-symmetry is reported in a new Feature Article in the Emerging Investigators issue of Chemical Communications. C3-symmetry minimizes the quantum tunneling of the magnetization through the anisotropy barrier and thus stabilizes the magnetization.
Based on a purposely designed triplesalen ligand heptanuclear complexes of the form [Mt6Mc]n+ could be synthesized, among them the isostructural series [MnIII6CrIII]3+, [MnIII6FeIII]3+, and [MnIII6CoIII]3+ with [MnIII6CrIII]3+ being a SMM. A detailed analysis and comparison of the magnetic properties provides significant insight for further optimization of the SMM properties.

Thorsten Glaser, Rational design of single-molecule magnets: a supramolecular approach, Chem. Commun., 2011, 47, 116-130

Spin- and Energy-Dependent Tunneling through a Single Molecule with Intramolecular Spatial Resolution

The spin- and energy-dependent tunneling through a single organic molecule (CoPc) adsorbed on a ferromagnetic Fe thin film is investigated: spatially resolved by low-temperature spin-polarized scanning tunneling microscopy. Interestingly, the metal ion as well as the organic ligand show a significant spin dependence of tunneling current flow.
State-of-the-art DFT calculations including also van der Waals interactions reveal a strong hybridization of molecular orbitals and substrate 3d states. The molecule is anionic due to a transfer of one electron, resulting in a nonmagnetic (S=0) state. Nevertheless, tunneling through the molecule exhibits a pronounced spin dependence due to spin-split molecule-surface hybrid states.

Jens Brede, Nicolae Atodiresei, Stefan Kuck, Predrag Lazi, Vasile Caciuc, Yoshitada Morikawa, Germar Hoffmann,Stefan Blügel, and Roland Wiesendanger, Spin- and Energy-Dependent Tunneling through a Single Molecule with Intramolecular Spatial Resolution, Phys. Rev. Lett. 105, 047204 (2010)

X-ray absorption and magnetic circular dichroism investigation of bis(phthalocyaninato)terbium single-molecule magnets deposited on graphite

Magnetization as a function of the external field at T=7 K. Bisphthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures, which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In the present work the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite are investigated by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism. The comparison of the magnetization values and their dependence on the external magnetic field and temperature suggest that the magnetic properties of molecules are preserved when adsorbed onto the graphite surface.

R. Biagi, J. Fernandez-Rodriguez, M. Gonidec, A. Mirone, V. Corradini, F. Moro, V. De Renzi, U. del Pennino, J. C. Cezar, D. B. Amabilino, and J. Veciana, X-ray absorption and magnetic circular dichroism investigation of bis(phthalocyaninato)terbium single-molecule magnets deposited on graphite, Phys. Rev. B 82, 224406 (2010)

Probing Superexchange Interaction in Molecular Magnets by Spin-Flip Spectroscopy and Microscopy

The superexchange mechanism in cobalt phthalocyanine (CoPc) thin films was studied by a low temperature scanning tunneling microscope. The CoPc molecules were found to form one-dimensional antiferromagnetic chains in the film. Collective spin excitations in individual molecular chains were measured with spin-flip associated inelastic electron tunneling spectroscopy. By spatially mapping the spin-flipping channels with submolecular precision, the authors are able to explicitly identify the specific molecular orbitals that mediate the superexchange interaction between molecules.

Xi Chen, Ying-Shuang Fu, Shuai-Hua Ji, Tong Zhang, Peng Cheng, Xu-Cun Ma, Xiao-Long Zou, Wen-Hui Duan, Jin-Feng Jia, and Qi-Kun Xue, Probing Superexchange Interaction in Molecular Magnets by Spin-Flip Spectroscopy and Microscopy, Phys. Rev. Lett. 101, 197208 (2008)

Co-Gd phosphonate complexes as magnetic refrigerants

Three 3d-4f phosphonate complexes, [CoII8GdIII8(Ì3-OH)4(NO3)4(O3PtBu)8(O2CtBu)16], [CoII8GdIII4(O3PtBu)6(O2CtBu)16] and [CoII4GdIII6(O3PCH2Ph)6(O2CtBu)14(MeCN)2], have been synthesized and have structures that can be related to molecular grids. Magnetic studies show they have promise as low temperature magnetic refrigerants.

See also RCS Publishing, Chemical Science Blog.

Yan-Zhen Zheng, Marco Evangelisti and Richard E. P. Winpenny, Co-Gd phosphonate complexes as magnetic refrigerants, Chem. Sci., 2011, 2, 99-102

FIT-MART fits smart

The FIT-MART package is a "Fully Integrated Tool for Magnetic Analysis in Research & Teaching" (hence the acronym) which provides a very simple interface for defining complex quantum spin models, carrying out complex calculations, and visualizing the results using several graphical representations. These representations include plots of the energy spectrum as well as plots of the magnetization and magnetic susceptibility as a function of temperature and magnetic field. The FIT-MART package is an Open Source Physics package written to help students as well as researchers who are studying magnetism. It is distributed as a ready-to-run (compiled) Java archive.

Larry Engelhardt and Cameron Rainey, FIT-MART package

High-spin ground states via electron delocalization in mixed-valence imidazolate-bridged divanadium complexes

Structure of [(PY5Me2)VII(µ-Lbr) VIII(PY5Me2)]4+ Many magnetic molecules rely on the superexchange mechanism. The possibility of instead using a double-exchange mechanism (based on electron delocalization rather than Heisenberg exchange through a non-magnetic bridge) presents a tantalizing prospect for synthesizing molecules with high-spin ground states that are well isolated in energy. In the article it is demonstrated that magnetic double exchange can be sustained by simple imidazolate bridging ligands, known to be well suited for the construction of coordination clusters and solids. A series of mixed-valence vanadium dimers were synthesized and their electron delocalization probed through cyclic voltammetry and spectroelectrochemistry. Magnetic susceptibility data reveal a well-isolated S = 5/2 ground state arising from double exchange.

Bettina Bechlars, Deanna M. D'Alessandro, David M. Jenkins, Anthony T. Iavarone, Starla D. Glover, Clifford P. Kubiak, Jeffrey R. Long, High-spin ground states via electron delocalization in mixed-valence imidazolate-bridged divanadium complexes,
Nature Chemistry 2, 362 - 368 (2010)

Molecular Magnets themed issue of Dalton Transactions

Dalton Trans. 39 (2010) "The study of magnetic cluster complexes, from the fundamental chemistry and physics that governs their properties to their potential applicability in a range of technologies, is a topic that transcends traditional scientific boundaries, with current research producing ground-breaking science. With the Molecular Magnets themed issue, Dalton Transactions continues its efforts in covering all areas represented by the subject including synthesis, structural characterisation, functional properties, physics and materials science, theory and application."
Euan K. Brechin

Molecular Magnets themed issue, Dalton Trans. 39 (2010)

Review on cobalt(II) single-molecule magnets

Structure of [Co4(cit)4]8− This short tutorial review covers recent progress in the field of polynuclear cobalt(II)-based complexes, which display slow magnetic relaxation at low temperature. Cobalt(II) single-molecule magnets (SMMs) can display much larger magnetic anisotropies and hence, potentially higher blocking temperatures than SMMs based on ions where the zero-field splitting originates from a second order spin-orbit coupling, such as manganese(III).

Mark Murrie, Cobalt(II) single-molecule magnets,
Chem. Soc. Rev., 2010, 39, 1986-1995

Experimental and theoretical investigations of magnetic molecules with non-collinear local d-tensors

Mn3Cr: scetch of exchange interactions and directions of local easy axes In spite of its topologically simple magnetic structure, the [CrIIIMnII3 (PyA)6Cl3] (CrMn3) molecule exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. The present work elucidates the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn3 molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, ligand-field multiplet calculations as well as quantum and classical spin model calculations.

Manuel Prinz, Karsten Kuepper, Christian Taubitz, Michael Raekers, Sumit Khanra, Biplab Biswas, Thomas Weyhermüller, Marc Uhlarz, Joachim Wosnitza, Jürgen Schnack, Andrei V. Postnikov, Christian Schröder, Simon J. George, Manfred Neumann, and Phalguni Chaudhuri, A Star-shaped Heteronuclear CrIIIMnII3 Species and Its Precise Electronic and Magnetic Structure: Spin Frustration Studied by X-ray Spectroscopic, Magnetic and Theoretical Methods,
Inorg. Chem. 49 (2010) 2093-2102

V4: anisotropy easy axis and easy planes Magnetic studies of clusters [V4Cl6(thme)2(bipy)3] and related compounds reveal very weak intramolecular antiferromagnetic exchange and very strong local zero-field splitting effects that dominate the magnetization behavior.

Ian S. Tidmarsh, Luke J. Batchelor, Emma Scales, Rebecca H. Laye, Lorenzo Sorace, Andrea Caneschi, Jürgen Schnack and Eric J. L. McInnes, Tri-, tetra- and octa-metallic vanadium(III) clusters from new, simple starting materials: interplay of exchange and anisotropy effects,
Dalton Trans. (2009) 9402-9409

Co Dimers on Hexagonal Carbon Rings Proposed as Subnanometer Magnetic Storage Bits

Magnetic anisotropy energy of Co atoms in different chemical and structural environments. Black squares denote experimental data, blue circles and red diamonds denote lower and upper estimate theoretical data, In the article it is demonstrated by means of density functional and ab initio quantum chemical calculations, that transition-metal-carbon systems have the potential to enhance the presently available area density of magnetic recording by 3 orders of magnitude. As a model system, Co2 benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy of the order of 0.1 eV per molecule, large enough to store permanently 1 bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite.

Xiao, Ruijuan and Fritsch, Daniel and Kuzmin, Michael D. and Koepernik, Klaus and Eschrig, Helmut and Richter, Manuel and Vietze, Knut and Seifert, Gotthard, Co Dimers on Hexagonal Carbon Rings Proposed as Subnanometer Magnetic Storage Bits,
Phys. Rev. Lett. 103 (2009) 187201

Electronic Control of Spin Coupling in Keplerate-Type Polyoxomolybdates

In the article it could be shown that the partial reduction of polyoxomolybdate building blocks in Keplerate cluster structures allows the magnetic superexchange mediated by these diamagnetic blocks to be altered. The isolation of a 16-fold reduced {Mo72Fe30} species illustrates how Raman-monitored synthesis can result in such clusters comprising a well-defined assembly of spin centers.

Bogdan Botar, Arkady Ellern, Raphael Hermann, Paul Kögerler Electronic Control of Spin Coupling in Keplerate-Type Polyoxomolybdates,
Angew. Chem. Int. Ed. 2009, 48, 9080-9083

Ab initio investigation of the non-collinear magnetic structure and the lowest magnetic excitations in dysprosium triangles

The unusual magnetism exhibited by dysprosium triangles is explained using the recently developed ab initio methodology for the simulation of magnetic properties of complexes. The local anisotropy axes on the dysprosium sites are found to lie in the plane of the Dy3 triangle and to make angles of ca. 120° with each other. The small antiferromagnetic exchange interaction between sites leads to a non-magnetic Kramers doublet in the ground state of the complex. The arrangement of the local magnetization vectors in this state is close to toroidal. By contrast, the lowest excited states are characterized by a huge magnetic moments of ca. 20 Bohr magnetons and show very different behavior of magnetization for fields applied along and perpendicular to the plane of the Dy3 triangle.

Liviu Ungur, Willem Van den Heuvel and Liviu F. Chibotaru Ab initio investigation of the non-collinear magnetic structure and the lowest magnetic excitations in dysprosium triangles,
New J. Chem., 2009, 33, 1224-1230

Towards odd-membered spin rings

Structure of the oxovanadium heptagons sandwiched by beta-cyclodextrins (l.h.s.) and magnetization curves exhibiting characteristic steps (r.h.s.).
For a long time it seemed to be practically impossible to synthesize odd-membered spin rings due to steric hindrance of the (bulky) ligands. A recent article reports the successful synthesis and magnetic characterization of a seven-membered vanadium spin ring. Antiferromagnetically coupled odd-membered rings are expected to show a qualitatively different behavior due to spin frustration. For example, the ground state is fourfold degenerate for half-integer spins, but non-degenerate for integer ones, see Ref. Bärwinkel et al..
In the recent investigation the characteristic behavior for the half-integer case was investigated by means of magnetization measurements.

Norihisa Hoshino, Motohiro Nakano, Hiroyuki Nojiri, Wolfgang Wernsdorfer, and Hiroki Oshio, Templating Odd Numbered Magnetic Rings: Oxovanadium Heptagons Sandwiched by beta-Cyclodextrins,
J. Am. Chem. Soc. 131 (2009) 15100

K. Bärwinkel, P. Hage, H.-J. Schmidt, J. Schnack, Quantum numbers for relative ground states of antiferromagnetic Heisenberg spin rings,
Phys. Rev. B 68, 054422 (2003)

see also: Hong-Chang Yao, Jun-Jie Wang, Yun-Sheng Ma, Oliver Waldmann, Wen-Xin Du, You Song, Yi-Zhi Li, Li-Min Zheng, Silvio Decurtins and Xin-Quan Xin, An iron(III) phosphonate cluster containing a nonanuclear ring,
Chem. Commun. (2006) 1745

Tuning Molecule-Mediated Spin Coupling in Bottom-Up-Fabricated Vanadium-Tetracyanoethylene Nanostructures

Highly resolved STM images and structural models of (a) TCNE, (b) V-TCNE, (c) V(TCNE)2, (d) V2TCNE@27°, and (e) V2TCNE@11° on Ag(100).
In this article the fabrication of a hybrid magnetic complexes from V atoms and tetracyanoethylene ligands via atomic manipulation with a cryogenic scanning tunneling microscope is reported. Using tunneling spectroscopy one observes spin-polarized molecular orbitals as well as Kondo behavior. For complexes having two V atoms, the Kondo behavior can be quenched for different molecular arrangements, even as the spin-polarized orbitals remain unchanged. This is explained by variable spin-spin (i.e., V-V) ferromagnetic coupling through a single tetracyanoethylene (TCNE) molecule, as supported by density functional calculations.

Daniel Wegner and Ryan Yamachika and Xiaowei Zhang and Yayu Wang and Tunna Baruah and Mark R. Pederson and Bart M. Bartlett and Jeffrey R. Long and Michael F. Crommie, Tuning Molecule-Mediated Spin Coupling in Bottom-Up-Fabricated Vanadium-Tetracyanoethylene Nanostructures,
Phys. Rev. Lett. 103, 087205 (2009)

Condensed Matter Physics special issue on spin systems

Condensed Matter Physics special issue on spin systems,
CONDENSED MATTER PHYSICS, 2009, vol. 12, No. 3

Nowadays spin model theory is a very broad field in condensed physics and statistical physics. The Condensed Matter Physics special issue on spin systems touches only upon a small part of the existing studies and by no means exhausts numerous investigations on spin systems.
Editor of the issue: O.V. Derzhko

Deutsche Forschungsgemeinschaft Molecular Magnetism Research Report

Deutsche Forschungsgemeinschaft Molecular Magnetism Research Report,
Coordination Chemistry Reviews, Volume 253, Issues 19-20, Pages 2241-2460 (October 2009)

This volume of the Coordination Chemistry Reviews highlights the research results of the German priority program SPP 1137 on Molecular Magnetism (2002-2008) which was a joined effort of chemists and physicists to explore the new and very vivid field of molecular magnetism. The priority program was financed by the Deutsche Forschungsgemeinschaft.

Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates

We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional theory+U calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.

M. Bernien, J. Miguel, C. Weis, Md. E. Ali, J. Kurde, B. Krumme, P. M. Panchmatia, B. Sanyal, M. Piantek, P. Srivastava, K. Baberschke, P. M. Oppeneer, O. Eriksson, W. Kuch, and H. Wende, Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates,
Phys. Rev. Lett. 102, 047202 (2009)

Microscopic Understanding of Negative Magnetization in Cu, Mn, and Fe Based Prussian Blue Analogues

Rietveld refined neutron diffraction pattern for Cu0.73Mn0.77[Fe(CN)6]·zH2O at room temperature.
A crossover of the field-cooled magnetization from positive to negative has been observed below the magnetic ordering temperature (17.9 K) in a multimetal Prussian Blue analogue (PBA), Cu0.73Mn0.77[Fe(CN)6]·zH2O. The reverse Monte Carlo (RMC) modeling (using the program RMCPOW) has been used to derive the various scattering contributions (e.g., nuclear diffuse, nuclear Bragg, magnetic diffuse, and magnetic Bragg) from the observed neutron diffraction patterns. The RMC analysis combined with the Rietveld refinement technique show an antiferromagnetic ordering of Mn moments with respect to the Cu as well as the Fe moments. Our study gives the first neutron magnetic structure evidence towards the microscopic understanding of the negative magnetization in the PBAs.

Amit Kumar, S. M. Yusuf, L. Keller, and J. V. Yakhmi, Microscopic Understanding of Negative Magnetization in Cu, Mn, and Fe Based Prussian Blue Analogues,
Phys. Rev. Lett. 101, 207206 (2008)

Maximum use of symmetry

For small enough quantum systems numerical exact and complete diagonalization of the Hamiltonian enables one to evaluate and discuss all static, dynamic, and thermodynamic properties. In a recent article the range of applicability of the exact diagonalization method for Heisenberg spin systems is extend by showing how the irreducible tensor operator technique can be combined with an unrestricted use of general point-group symmetries. Results for numerical exact diagonalizations of Heisenberg spin systems of unprecedented size are presented as e.g. for the antiferromagnetic cuboctahedron with total Hilbert space dimension is 16,777,216.

Low-lying energy spectrum of the antiferromagnetic cuboctahedron with s=3/2 in subspaces of S=0,1,2,3. Symbols denote the irreducible representations of the Oh group.
R. Schnalle, J. Schnack, Numerically exact and approximate determination of energy eigenvalues for antiferromagnetic molecules using irreducible tensor operators and general point-group symmetries,
Phys. Rev. B 79 (2009) 104419

Also in this work, the spin system consisting of four Ni ions (s=1) was analyzed by making massive use of symmetries. Especially it is shown that in the molecular Ni4 clusters with S4 symmetry, not only the local anisotropy but also the antisymmetric exchange interaction contributes to the observed fourth-order anisotropy terms in the single-spin Hamiltonian.

N. Kirchner, J. van Slageren, B. Tsukerblat, O. Waldmann, M. Dressel, Antisymmetric exchange interactions in Ni4 clusters,
Phys. Rev. B 78, 094426 (2008)

Engineering the coupling between molecular spin qubits by coordination chemistry

The ability to assemble weakly interacting subsystems is a prerequisite for implementing quantum information processing and generating controlled entanglement. In a new article it is shown that Cr7Ni rings can be chemically linked to each other and that the coupling between their spins can be tuned by choosing the linker. Calculations that demonstrate how realistic microwave pulse sequences could be used to generate maximally entangled states in such molecules are presented.

Grigore A. Timco, Stefano Carretta, Filippo Troiani, Floriana Tuna, Robin J. Pritchard, Christopher A. Muryn, Eric J. L. McInnes, Alberto Ghirri, Andrea Candini, Paolo Santini, Giuseppe Amoretti, Marco Affronte, Richard E. P. Winpenny, Engineering the coupling between molecular spin qubits by coordination chemistry,
Nature Nanotechnology, Published online: 01 February 2009;

Magnetic memory of a single-molecule quantum magnet wired to a gold surface

In the field of molecular spintronics the use of magnetic molecules for information technology is a main target and the observation of magnetic hysteresis on individual molecules organized on surfaces is a necessary step to develop molecular memory arrays. So far no magnetic hysteresis has been reported for monolayers of SMMs on various non-magnetic substrates, most probably owing to the chemical instability of clusters on surfaces. Using X-ray absorption spectroscopy and X-ray magnetic circular dichroism synchrotron-based techniques, pushed to the limits in sensitivity and operated at sub-kelvin temperatures, it has been found that robust, tailor-made Fe4 complexes retain magnetic hysteresis at gold surfaces.

Matteo Mannini, Francesco Pineider, Philippe Sainctavit, Chiara Danieli, Edwige Otero, Corrado Sciancalepore, Anna Maria Talarico, Marie-Anne Arrio, Andrea Cornia, Dante Gatteschi, Roberta Sessoli, Magnetic memory of a single-molecule quantum magnet wired to a gold surface, Nature Materials 8, 194 - 197 (2009)

See also News and Views: Heiko Wende, Molecular magnets: How a nightmare turns into a vision, Nature Materials 8, 165 - 166 (2009)

Landing a manganese cluster on a POM

The functionalization of a pre-formed, high oxidation state {CeIVMnIV6} cluster with a lacunary phosphotungstate, [Alpha-P2W15O56]12-, exemplifies a straightforward route for grafting redox-active building blocks to existing Mn-carboxylate clusters and modeling their deposition onto metal oxide surfaces.

Xikui Fang and Paul Kögerler, A polyoxometalate-based manganese carboxylate cluster, Chem. Commun., 2008, 3396 - 3398

New polyoxometalates

Several new polyoxomatelate cluster could be synthesized. Among them are octa- and nonanuclear nickel(II) polyoxometalate clusters and a 16-Metal-Centre Iron Oxide Nanocluster that is the result of anucleation process in the Cavity of a 48-Tungstophosphate Wheel. Another cluster that contains a Cu3 spin triangle was investigated in detail using pulsed-field magnetization, electron spin resonance, and nuclear spin-lattice relaxation. The latter compound shows an unusual hysteresis loop.

Pichon, Celine; Mialane, Pierre; Dolbecq, Anne; Marrot, Jerome; Riviere, Eric; Bassil, Bassem; Kortz, Ulrich; Keita, Bineta; Nadjo, Louis; Secheresse, Francis, Octa- and Nonanuclear Nickel(II) Polyoxometalate Clusters: Synthesis and Electrochemical and Magnetic Characterizations Inorg. Chem., 2008, 47 (23), pp 11120-11128

Mal, Sib Sankar; Dickman, Michael H.; Kortz, Ulrich; Todea, Ana Maria; Merca, Alice; Boegge, Hartmut; Glaser, Thorsten; Mueller, Achim; Nellutla, Saritha; Kaur, Narpinder; van Tol, Johan; Dalal, Naresh S.; Keita, Bineta; Nadjo, Louis, Nucleation Process in the Cavity of a 48-Tungstophosphate Wheel Resulting in a 16-Metal-Centre Iron Oxide Nanocluster Chem. Eur. J. 2008, 14, 1186-1195

Kwang-Yong Choi and Naresh S. Dalal and Arneil P. Reyes and Philip L. Kuhns and Yasuhiro H. Matsuda and Hiroyuki Nojiri and Sib Sankar Mal and Ulrich Kortz, Pulsed-field magnetization, electron spin resonance, and nuclear spin-lattice relaxation in the {Cu3} spin triangle Phys. Rev. B 77, 024406 (2008)

Visualizing the Spin of Individual Cobalt-Phthalocyanine Molecules

CoPc molecules adsorbed on cobalt nanoislands grown on Cu(111).
Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single cobalt-phthalocyanine molecules adsorbed on well-characterized magnetic nanoleads. A spin-polarized electronic resonance is identified over the center of the molecule and exploited to spatially resolve stationary spin states. These states reflect two molecular spin orientations and, as established by density functional calculations, originate from a ferromagnetic molecule-lead exchange interaction.

C. Iacovita, M. V. Rastei, B. W. Heinrich, T. Brumme, J. Kortus, L. Limot, and J. P. Bucher Visualizing the Spin of Individual Cobalt-Phthalocyanine Molecules, Phys. Rev. Lett. 101, 116602 (2008)

Direct Observation of Quantum Coherence in Single-Molecule Magnets

Rabi oscillations obtained by recording the echo intensity as a function of nutation pulse length. For two positions in the Rabi cycle the corresponding path on the Bloch sphere is shown on the right.
Direct evidence of quantum coherence in a single-molecule magnet in a frozen solution is reported with coherence times as long as T2=630±30 ns. We can strongly increase the coherence time by modifying the matrix in which the single-molecule magnets are embedded. The electron spins are coupled to the proton nuclear spins of both the molecule itself and, interestingly, also to those of the solvent. The clear observation of Rabi oscillations indicates that we can manipulate the spin coherently, an essential prerequisite for performing quantum computations.

C. Schlegel, J. van Slageren, M. Manoli, E. K. Brechin, and M. Dressel, Direct Observation of Quantum Coherence in Single-Molecule Magnets, Phys. Rev. Lett. 101, 147203 (2008)

Spin Chirality in a Molecular Dysprosium Triangle: the Archetype of the Non-Collinear Ising Model

Molecular nanomagnetism has provided benchmark systems to investigate new and fascinating phenomena in magnetism like magnetic memory at the molecular level, quantum tunneling of the magnetization or destructive interferences in the tunneling pathways. In this field rare earth ions like dysprosium(III) are currently investigated because of their large magnetic anisotropy. In the course of our synthetic efforts to obtain new molecular nanomagnets we recently obtained trinuclear DyIII clusters with almost ideal trigonal symmetry but with the unusual property of possessing a non magnetic ground state. Cancellation of the magnetic moments, resulting in a non-magnetic ground doublet, originates from the non-collinearity of the single ion easy axes of magnetization, as demonstrated by single crystal magnetic studies. The peculiar chiral nature of the ground non-magnetic doublet and the resonant quantum tunneling of the magnetization at the crossings of the discrete energy levels open new perspectives in quantum computation and data storage in molecular nanomagnets.

Structure of the Dy3 cluster and its low energy levels as a function of the applied magnetic field. Below 8 kOe two degenerate non magnetic states with opposite spin chirality have the lowest energy.
J. Luzon, K. Bernot, I. J. Hewitt, C. E. Anson, A. K. Powell, and R. Sessoli, Spin Chirality in a Molecular Dysprosium Triangle: the Archetype of the Non-Collinear Ising Model, Phys. Rev. Lett. 100, 247205 (2008)

Strong magneto-chiral dichroism in enantiopure chiral ferromagnets

Temperature dependence of the MChD effect (diamonds) measured at 615 nm and field-cooled (squares) magnetization.
As materials science is moving towards the synthesis, the study and the processing of new materials exhibiting well-defined and complex functions, the synthesis of new multifunctional materials is one of the important challenges. One of these complex physical properties is magneto-chiral dichroism which arises, at second order, from the coexistence of spatial asymmetry and magnetization in a material.
Recently the first measurement of strong magneto-chiral dichroism in an enantiopure chiral ferromagnet was reported. The ab initio synthesis of the enantiopure chiral ferromagnet is based on an enantioselective self-assembly, where a resolved chiral quaternary ammonium cation imposes the absolute configurations of the metal centres within chromium–manganese two-dimensional oxalate layers. The ferromagnetic interaction between Cr(III) and Mn(II) ions leads to a Curie temperature of 7 K. The magneto-chiral dichroic effect is enhanced by a factor of 17 when entering into the ferromagnetic phase.

Cyrille Train, Ruxandra Gheorghe, Vojislav Krstic, Lise-Marie Chamoreau, Nikolai S. Ovanesyan, Geert L. J. A. Rikken, Michel Gruselle, Michel Verdaguer, Strong magneto-chiral dichroism in enantiopure chiral ferromagnets, Nature Materials 7, 729 - 734 (2008)

Observation of a quantum phase transition in a molecular scale transistor

Atomic Force Microscope image of the molecular transistor. The inset represents a x100 zoom view of the inserted C60 molecule.
As physical objects become smaller, quantum effects become dominant and easier to measure. Thus, nanometer size quantum objects (in this work a C60 molecule) are propitious for observation of the new quantum phenomena associated with spin electronics. Such objects act as artificial atoms and can be controlled by external parameters such as magnetic field, electric potential or light.
We have shown that a molecular transistor based on fullerene (C60) can be switched electrostatically between two different spin states, corresponding to distinct resistance properties of the nanocircuit. In this case, the magnetized state is associated with an entanglement of the spin of the molecule with the conduction electron spins. An electrostatic coupling induces the transition from a spin zero state to a spin 1/2 state. The quantum critical point is then characterized by a spin1/2 that is not entangled with the conduction electrons. This kind of physics is of great current interest and, in addition, our experimental results offer new possibilities for controlling and manipulating the states in molecular spintronics.

N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer and F. Balestro, Quantum phase transition in a single-molecule quantum dot, Nature, 453, 633 (2008)

Calculation of the Zero-Field Splitting Tensor

Prof. Dr. Frank Neese receives this year's Klung-Wilhelmy-Weberbank prize for chemistry. He receives the prize for his pioneering work on electronic structure calculations for (magnetic) molecules. We would like to highlight some of his recent results especially on the evaluation of zero-field splitting tensors.
The zero-field splitting (ZFS) (expressed in terms of the D tensor) is the leading spin-Hamiltonian parameter for systems with a ground state spin S>1/2. To first order in perturbation theory, the ZFS arises from the direct spin-spin dipole-dipole interaction. To second order, contributions arise from spin-orbit coupling (SOC). The latter contributions are difficult to treat since the SOC mixes states of different multiplicities. This is an aspect of dominant importance for the correct prediction of the D tensor. In Frank Neese's work, the theory of the D tensor is discussed from the point of view of analytic derivative theory.

F. Neese, Calculation of the Zero-Field Splitting Tensor Using Hybrid Density Functional and Hartree-Fock Theory, J. Chem. Phys. 127 (2007) 164112
Dmitry Ganyushin and F. Neese, First-principles calculations of zero-field splitting parameters, J. Chem. Phys. 125 (2006) 024103
F. Neese, Importance of Direct Spin-Spin Coupling and Spin-Flip Excitations for the Zero-Field Splittings of Transition Metal Complexes: A Case Study, J. Am. Chem. Soc. 128 (2006) 10213-10222

Polyoxometalate-Mediated Self-Assembly of Single-Molecule Magnets

The first two examples of MnII/III-based Single Molecule Magnets (SMMs) encapsulated within a polyoxometalate ligand system are reported. These clusters both contain a central, mixed-valence {Mn6} cluster core that is anchored between two rigid lacunary polyoxometalate clusters.

Chris Ritchie, Alan Ferguson, Hiroyuki Nojiri, Haralampos N. Miras, Yu-Fei Song, De-Liang Long, Eric Burkholder, Mark Murrie, Paul Kögerler, Euan K. Brechin, Leroy Cronin, Polyoxometalate-Mediated Self-Assembly of Single-Molecule Magnets: {[XW9O34]2[MnIII4MnII2O4(H2O)4]}12-, Angewandte Chemie International Edition 47 (2008) 5609

Quantum oscillations in a molecular magnet

Rabi oscillations The observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave) of the molecular magnet "V15" in a hybrid system, in which discrete and well-separated magnetic clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/2 spins, leading to an S = 1/2 collective ground state. When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 micro seconds (at liquid helium temperatures) are a realistic prospect.

S. Bertaina, S. Gambarelli, T. Mitra, B. Tsukerblat, A. Müller, B. Barbara, Quantum oscillations in a molecular magnet, Nature 453, 203-206 (8 May 2008)

A star-shaped manganese-based molecule an S = 10 high-spin state

manganese star A comprehensive study of the electronic and magnetic properties of a star-shaped molecule comprising a MnII4O6 core is reported. One feature of this compound is weak magnetic coupling constants compared to other similar polyoxo compounds. This leads to complicated low-lying magnetic states in which the ground state is not well separated from the upper-lying states, yielding a high-spin molecule with a giant magnetic moment of up to 20 μB/formula unit.

Sumit Khanra, Karsten Kuepper, Thomas Weyhermüller, Manuel Prinz, Michael Raekers, Sebastian Voget, Andrei V. Postnikov, Frank M. F. de Groot, Simon J. George, Marin Coldea, Manfred Neumann, and Phalguni Chaudhuri, Star-Shaped Molecule of MnII4O6 Core with an St = 10 High-Spin State. A Theoretical and Experimental Study with XPS, XMCD, and Other Magnetic Methods, Inorg. Chem. 7 (11), 4605-4617 (2008)

Ground state tuning by high magnetic fields in Ni-compounds

relaxation An important property of a molecular magnet is the possibility to tune its spin state by an external parameter such as the magnetic field. To address this issue we have measured electron spin resonance (ESR) and magnetization in fields up to 55 T of the novel multicenter complex [L2Ni4(N3)O2CAda)4(ClO4)] (a) and the S = 1 two leg spin ladder [Na2Ni2(C2O4)3(H2O)2] (b). Both complexes show a non-magnetic ground state. However, by applicaton of a strong external magnetic field, H > Hc we can turn the ground state to a strongly magnetic one. At Hc the S = 0 singlet is crossed by the excited |S = 1, Sz = -1> spin state. From ESR and magnetization we find that the critical field Hc at which the spin-level crossings occur are at Hc=25 T in (a), Hc=30 T in (b).
Furthermore, appreciable zero-field splittings have been observed which are due to the large single-ion anisotropy (11.5 K (a) and -4.8 K (b)) of nickel.

  • C. Golze , A. Alfonsov, R. Klingeler, B. Büchner, V. Kataev , C. Mennerich, H.-H. Klauss, M. Goiran, J.-M. Broto, H. Rakoto, S. Demeshko, G. Leibeling and F. Meyer, Tuning the Magnetic Ground State of a Tetranuclear Nickel(II) Molecular Complex by High Magnetic Fields, Phys. Rev. B 73 (2006) 224403
  • C. Mennerich, H.-H. Klauss, M. Broekelmann, F. J. Litterst, C. Golze, R. Klingeler, V. Kataev, B. Büchner, S.-N. Grossjohann W. Brenig, M. Goiran, H. Rakoto, J.-M. Broto, O. Kataeva and D.-J. Price, AFM Dimers of Ni(II) in the S = 1 Spin-Ladder Na2Ni2(C2O4)3(H2O)2, Phys. Rev. B 73 (2006) 104415
  • S. Demeshko, G. Leibeling, W. Maringgele, F. Meyer, C. Mennerich, H.-H. Klauss, and H. Pritzkow. Structural Variety and Magnetic Properties of Tetranuclear Nickel(II) Complexes with a Central μ4-azide. Inorg. Chem. 44, 519 (2005).
  • D. J. Price, A.K. Powell, and P. T. Wood, A new series of layered transition metal oxalates: Hydrothermal synthesis, structural and magnetic properties. J. Chem. Soc., Dalton Trans., 2478-2482 (2003).

Magnetocaloric properties of frustrated antiferromagnetic molecules and low-dimensional spin systems

relaxation Antiferromagnetically coupled low-dimensional spin systems showing geometric frustration like the sawtooth chain and the kagome lattice as well as frustrated antiferromagnetic molecules have been investigated with respect to their magnetocaloric properties recently. A key feature is the remarkably large adiabatic cooling rate close to the saturation field as shown in the figure for the example of an antiferromagnetic icosahedron.

Density functional studies of molecular magnets

relaxation After a general introduction into the field of molecular magnets the discussion focuses on a more specific description of their most important representative species, single-molecule magnets incorporating transition metal ions. We overview traditional model approaches for the phenomenological description of such systems and outline some ways used to parameterize the corresponding models from experiment and from first-principle calculations. The latter can be either multi-determinantal quantum chemical schemes or those based on the density functional theory. In particular we discuss Heisenberg exchange parameters and magnetic anisotropy constants. As a practical example, an introduction into problems and properties of some single-molecule magnets which gained much attention within last years, namely Mn12-acetate, Fe8 and V15 systems, is given.
This introduction into systems is followed by a critical comparison of calculation schemes based on the density functional theory that are particularly well suited for the study of molecular magnets. For the above systems we select some benchmark results, obtained by different methods. Finally, we outline our recent progress in the study of other single-molecule magnets, including six-membered "ferric wheels", "ferric stars" and "Ni4" molecules, which we studied with the use of firstprinciples methods SIESTA and NRLMOL.

Andrei V. Postnikov, Jens Kortus, Mark R. Pederson, Density functional studies of molecular magnets, physica status solidi (b) 243, 2533-2572 (2006)

A Ferromagnetically Coupled Mn19 Aggregate with a Record S=83/2 Ground Spin State

relaxation We report on the realization of the maximum-spin ground state of 83/2 for a Mn ixed-valence compound. The combination of using an organic ligand which can bridge to multiple metal centers and using azido ligands has resulted in a Mn19 aggregate that exhibits dominant ferromagnetic interactions with the maximum-spin ground state possible. Achieving such a high ground spin state is clearly one of the elusive goals in the search for obtaining superior SMMs. However, although the MnIII centers in the compound show a high degree of Jahn-Teller distortion, their geometrical arrangement and the strong ferromagnetic interactions between spin carriers lead to a system with a very low anisotropy. The challenge now is to find a means of introducing not only maximum spin but also greater anisotropy.

Ayuk M. Ako, Ian J. Hewitt, Valeriu Mereacre, Rodolphe Clerac, Wolfgang Wernsdorfer, Christopher E. Anson, Annie K. Powell, Magnetic relaxation studies on a single-molecule magnet by time-resolved inelastic neutron scattering, Angew. Chem. Int. Ed. 45 (2006) 4926-4929

Magnetic relaxation studies on a single-molecule magnet by time-resolved inelastic neutron scattering

relaxation Time-resolved inelastic neutron scattering measurements on an array of single-crystals of the single-molecule magnet Mn12ac are presented. The data facilitate a spectroscopic investigation of the slow relaxation of the magnetization in this compound in the time domain.

O. Waldmann, G. Carver, C. Dobe, D. Biner, A. Sieber, H. U. Güdel, H. Mutka, J. Ollivier, and N. E. Chakov, Magnetic relaxation studies on a single-molecule magnet by time-resolved inelastic neutron scattering, Appl. Phys. Lett. 88 (2006) 042507

Field-Induced Magnetoelastic Instabilities in Antiferromagnetic Molecular Wheels

Néel-vector tunneling The magnetic torque of the antiferromagnetic molecular wheel CsFe8 was studied down to 50 mK and up to 28 T. Below about 0.5 K phase transitions were observed at the field-induced level crossings (LCs). Intermolecular magnetic interactions are very weak excluding field-induced magnetic ordering. A magnetoelastic coupling was considered. A generic model shows that the wheel structure is unconditionally unstable at the LCs, and the predicted torque curves explain the essential features of the data well.

Figure: (a) dtau/dB at the first LC for a single crystal of compound 1 for several temperatures from 55 mK to 1 K.
(b) B-T phase diagram as derived from the data shown in panel (a) (lines are guides to the eyes). The solid symbols indicate the critical fields, the open symbols the half-maximum fields for T >T_c. The dashed line indicates the field of the S=0 to S=1 LC. The dash-dotted lines indicate the halfmaximum fields as expected for thermal broadening.

O. Waldmann, C. Dobe, T. Ochsenbein, H. U. Güdel,and I. Sheikin, Field-Induced Magnetoelastic Instabilities in Antiferromagnetic Molecular Wheels, Phys. Rev. Lett. 96 (2006) 027206

Please see also: Johannes Richter, Oleg Derzhko, and Jörg Schulenburg, Magnetic-field induced spin-Peierls instability in strongly frustrated quantum spin lattices, Phys. Rev. Lett. 93, 107206 (2004)

Addressing the Metal Centers of [2x2] CoII4 Grid-Type Complexes by STM/STS

Co 2x2 grid In their Communication the authors describe the spectroscopic dissecting of a single molecular square into regions of different electronic properties by STM/STS techniques. Figure: a) DFT calculation of occupied orbitals within an energy window between EF and -0.7 eV and the electron density map in superposition with the crystal structure data.
b) 3D representation of DFT-calculated electron density maps within an energy window between EF and -0.55 eV and between EF and -1 eV (upper row). Lower row: Central section of the measured CITS maps of Figure.
Mohammad Sahabul Alam, Stefan Strömsdörfer, Viacheslav Dremov, Paul Müller, Jens Kortus, Mario Ruben, Jean-Marie Lehn, Addressing the Metal Centers of [2x2] CoII4 Grid-Type Complexes by STM/STS, Angewandte Chemie International Edition, Volume 44, Issue 48 , Pages 7896 - 7900 (2005)

[{(Mo)Mo5O21(H2O)3(SO4)}12(VO)30(H2O)20]36?: A molecular quantum spin icosidodecahedron

V30 Self-assembly of aqueous solutions of molybdate and vanadate under reducing, mildly acidic conditions results in a polyoxomolybdate-based {Mo72V30} cluster compound Na8K16(VO)(H2O)5[K10{(Mo)Mo5O21(H2O)3(SO4)}12 (VO)30(H2O)20] *150H2O, a quantum spin-based Keplerate structure.
Bogdan Botar, Paul Kögerler, and Craig L. Hill, [{(Mo)Mo5O21(H2O)3(SO4)}12(VO)30(H2O)20]36-: A molecular quantum spin icosidodecahedron, Chem. Commun. 2005, 3138-3140 (2002)

The inverse Landau-Zener-Problem

Hysteretic Magnetic Bistability When applying a magnetic field sweep that is linear in time to a two-level system the probability to tunnel can be evaluated according to the works of Landau, Zener, Stueckelberg, and Majorana. But what if one woulkd like to reverse the problem, i.e. how should the functional form of a field sweep W(t) look like in order to obtain a desired tunneling probability function p(t)? This problem was solved theoretically by D.A. Garanin and R. Schilling and awaits its experimental verification.
D.A. Garanin and R. Schilling, Inverse problem for the Landau-Zener effect, Europhys. Lett. 59, 7 (2002)
D.A. Garanin and R. Schilling, Effects of nonlinear sweep in the Landau-Zener-Stueckelberg effect, Phys. Rev. B 66, 174438 (2002)

Hysteretic Magnetic Bistability Based on a Molecular Azide Switch

Hysteretic Magnetic Bistability Like a molecular toggle switch, the bridging azide ion in a highly preorganized dinickel(II) complex turns strong antiferromagnetic coupling on or off. Single crystals of the compound stay intact over the entire temperature range studied and feature hysteretic magnetic bistability not far from ambient temperature.
Guido Leibeling, Serhiy Demeshko, Sebastian Dechert, Franc Meyer, Hysteretic Magnetic Bistability Based on a Molecular Azide Switch, Angew. Chem. 2005, 117, 7273-7276; Angew. Chem. Int. Ed. 2005, 44, 7111-7114.

Anions get wheely big

Cu20 A Cu20-containing polyoxotungstate of large size and high symmetry was synthesized by making use of the template effect. The wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- ion (see picture; black W, turquoise Cu, yellow P, violet Cl, red O) is the first transition-metal-substituted derivative of [H7P8W48O184]33- and incorporates more paramagnetic 3d metal ions than any other polyoxotungstate to date.
Sib Sankar Mal and Ulrich Kortz, The Wheel-Shaped Cu20 Tungstophosphate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- Ion, Angew. Chem. Int. Ed., 44 (2005) 3777-3780

Terahertz Faraday Effect in Single Molecule Magnets

Faraday effect For the first time, Faraday rotation in the terahertz frequency range was observed in molecular magnetic systems. The effect is strongest near the magnetic resonance of the single molecule magnet Mn12Ac at ν = 300 GHz, where the Faraday rotation exceeds 150°/mm. Below the magnetization blocking temperature, the effect was observed in the magnetized state of the sample in zero field. Surprisingly, it could even be detected in nonmagnetized states, applying small magnetic fields (H ≤ 1 T), that do not create net magnetization of the sample. All observations were quantitatively explained without fit procedures using known Mn12Ac spin-Hamiltonian parameters.

Figure: (top) Transmission spectrum recorded on a single crystal mosaic of Mn12Ac at T = 1.77 K. (middle) radiation ellipticities calculated from angle dependent measurements (c) Faraday rotation angle calculated from angle dependent measurements

J. van Slageren, S. Vongtragool, A. Mukhin, B. Gorshunov, M. Dressel, Terahertz Faraday Effect in Single Molecule Magnets, Phys. Rev. B. 72 (2005) 020401R

Néel-Vector Tunneling in Antiferromagnetic Molecular Clusters

Néel-vector tunneling From inelastic neutron scattering experiments quantum tunneling of the Néel vector in the antiferromagnetic molecular ferric wheel CsFe8 is demonstrated. Analysis of the linewidth of the tunneling transition evidences coherent tunneling.

Figure: (a) Crystal structure of CsFe8, (b) Classical ground state spin configuration for the two orientations of the Néel vector. (c) Low-lying energy spectrum. At small anisotropies the levels can be classified by quantum numbers S and M (blue shading). For large anisotropy S looses its significance, but M remains a good quantum number (green shading). This is the region of Néel-vector tunneling.

O. Waldmann, C. Dobe, H. Mutka, A. Furrer, and H. U. Güdel, Néel-Vector Tunneling in Antiferromagnetic Molecular Clusters, Phys. Rev. Lett. 95 (2005) 057202

[V30Mo72]: Quantum Keplerate

[V<sub>30</sub>Mo<sub>72</sub>] Adding vanadyl sulfate to an acidified molybdate solution, in presence of K+ cations, results in formation of the compound abbreviated as [V30Mo72]. The vanadyl ions form a slightly distorted icosadodecahedron. This fascinating, highly symmetric structure is a spherical arrangement of pentagons and triangles. The S = 1/2 vanadyl ions are strongly exchange coupled. Quantum Monte Carlo simulations show that the average interaction is around J = 245 K. Strong exchange interactions between vanadyl ions despite long exchange pathways is a phenomenon that has been found in other vanadyl polyoxomolybdates. It is much stronger than the exchange interaction in [Fe30Mo72]. The higher spin of the iron ions (S = 5/2) compared to vanadium allowed the application of various classical calculation methods in that case. In that sense [V30Mo72] can be compared as the quantum analogue of [Fe30Mo72]

Figure: (top) Combined polyhedral and ball-and-stick representation of the structure of [V30Mo72], showing the triangles and pentagons of the icosidodecahedron, and additionally the basic {VO5} and {VO6} units as green polyhedra; (Mo atoms blue, O atoms red, K atoms purple, and SO42- groups yellow tetrahedra, disordered S atoms yellow).

A. Müller, A.M. Todea, M. Dressel, J. van Slageren, H. Bögge, M. Schmidtmann, M. Luban, L. Engelhardt, M. Rusu, On Triangular Geometrical and Magnetic Motifs Uniquely Linked on a Spherical Capsule-Surface, Angew. Chem. Int. Ed. 44 (2005) 3857 - 3861

Fast switching of bistable magnets

hysteresis The use of magnetic nanowires as memory units is made possible by the exponential divergence of the characteristic time for magnetization reversal at low temperature, but the slow relaxation makes the manipulation of the frozen magnetic states difficult. We suggest that finite-size segments can show a fast switching if collective reversal of the spins is taken into account. This mechanism gives rise at low temperatures to a scaling law for the dynamic susceptibility that has been experimentally observed for the dilute molecular chain Co(hfac)2NitPhOMe. These results suggest a possible way of engineering nanowires for fast switching of the magnetization.
Alessandro Vindigni, Angelo Rettori, Lapo Bogani, Andrea Caneschi, Dante Gatteschi, Roberta Sessoli, and Miguel A. Novak, Fast switching of bistable magnetic nanowires through collective spin reversal, Appl. Phys. Lett. 87 (2005) 073102

Hysteresis without anisotropy

hysteresis Normally hysteretic behaviour of magnetic systems is the outcome of anisotropic terms in the Hamiltonian. In a recent work we could show that the classical Heisenberg icosahedral antiferromagnet exhibits a hysteresis loop when the external field is swept (see figure). In this hypothetical magnetic molecule the spins are mounted at the vertices of an icosahedron and interact solely via antiferromagnetic nearest neighbor coupling. In the corresponding quantum system the (T=0) magnetization curve exhibits anusual jumps. The whole behavior can be charcterized as a first order metamagnetic phase transition.
C. Schröder, H.-J. Schmidt, J. Schnack, and M. Luban, Metamagnetic phase transition of the antiferromagnetic Heisenberg icosahedron, Phys. Rev. Lett. 94 (2005) 207203

Competing Spin Phases in Fe30

dip Frustrated spin systems such as the kagome lattice antiferromagnet show a pronounced (T=0) magnetization plateau at one third saturation magnetization. Since the magnetic molecule Fe30 is built of spins mounted at the vertices of a perfect icosidodecahedron it is structurally a "little brother" of the kagome antiferromagnet and shares several properties. In a recent experiment we could show that the differential suscptibility dM/dB features a pronounced minimum around one third of the saturation field which gives clear evidence that in this zero-dimensional system the related plateau is produced by competing spin phases just in the same way as on the two-dimensional kagome lattice.
C. Schröder, H. Nojiri, J. Schnack, P. Hage, M. Luban, P. Kögerler, Competing Spin Phases in Geometrically Frustrated Magnetic Molecules, Phys. Rev. Lett. 94 (2005) 017205

Giant magnetization jumps

jump For a class of frustrated spin lattices including the Kagome lattice we construct exact eigenstates consisting of several independent, localized one-magnon states and argue that they are ground states for high magnetic fields. If the maximal number of local magnons scales with the number of spins in the system, which is the case for the Kagome lattice, the effect persists in the thermodynamic limit and gives rise to a macroscopic jump in the zero-temperature magnetization curve just below the saturation field. The effect decreases with increasing spin quantum number and vanishes in the classical limit. Thus it is a true macroscopic quantum effect.
J. Schulenburg, A. Honecker, J. Schnack, J. Richter, H.-J. Schmidt, Macroscopic magnetization jumps due to independent magnons in frustrated quantum spin lattices, Phys. Rev. Lett. 88 (2002) 167207