Modelling the properties of magnetic clusters with complex structures: how symmetry can help us

Reversible Optical Switching of Polyoxovanadates and Their Communication via Photoexcited States

The 2-bit Lindqvist-type polyoxometalate (POM) [V6O13((OCH2)3CCH2N3)2]2- with a diamagnetic {V6O19} core and azide termini shows six fully oxidized VV centers in solution as well as the solid state, according to 51V NMR spectroscopy. Under UV irradiation, it exhibits reversible switching between its ground S0 state and the energetically higher lying states in acetonitrile and water solutions. TD-DFT calculations demonstrate that this process is mainly initialized by excitation from the S0 to S9 state. Pulse radiolysis transient absorption spectroscopy experiments with a solvated electron point out photochemically induced charge disproportionation of VV into VIV and electron communication between the POM molecules via their excited states. The existence of this unique POM-to-POM electron communication is also indicated by X-ray photoelectron spectroscopy (XPS) studies on gold-metalized silicon wafers (Au//SiO2//Si) under ambient conditions. The amount of reduced vanadium centers in the "confined" environment increases substantially after beam irradiation with soft X-rays compared to non-irradiated samples. The excited state of one POM anion seems to give rise to subsequent electron transfer from another POM anion. However, this reaction is prohibited as soon as the relaxed T1 state of the POM is reached.

Thermal processes in anisotropic metal complexes induced by non-adiabatic switching of magnetic field

In this article we analyze the thermal processes in magnetically anisotropic metal complexes under the action of non-adiabatic switching of magnetic field. Using the non-stationary perturbation theory for the case of sudden perturbation, we show that this field can cause not only heat release, but also heat absorption, interconnected with the axial zero field splitting (parameter D) in a paramagnetic metal complex. As an illustrative example we consider the simplest S = 1-complexes having "easy axis" and "easy plane" types of anisotropy influenced by the magnetic field that is suddenly turned off. We demonstrate that the character of the thermal processes (heat dissipation or absorption) depends on the sign of D and direction of applied field and so the analysis of these processes can be in principle used as a complementary tool (in addition to SQIUD magnetometry, EPR spectroscopy and INS) for studying magnetic anisotropy. The conditions under which the non-adiabatic switching of the magnetic field gives rise to the heat absorption are revealed. This unusual phenomenon, which can be called "nonadiabatic field switching cooling", may have practical applications.

Oxovanadium electronics for in-memory, neuromorphic, and quantum computing applications

Vanadium is a critical raw material. In the nearby future, it may, however, become one of the key elements of computer devices based on two-dimensional arrays of spin qubits for quantum information processing or charge- and resistance-based data memory cells for non-volatile in-memory and neuromorphic computing. The research and development (R&D) of vanadium-containing electronic materials and methods for their responsible fabrication underpins the transition to innovative hybrid semiconductors for energy- and resource-efficient memory and information processing technologies. The combination of standard and emerging solid-state semiconductors with stimuli-responsive oxo complexes of vanadium(IV,V) is envisioned to result in electronics with a new room-temperature device nanophysics, and the ability to modulate and control it at the sub-nanometer level. The development of exponential (Boolean) logics based on the oxovanadium-comprising circuitry and crossbar arrays of individual memristive cells for in-memory computing, the implementation of basic synaptic functions via dynamic electrical pulses for neuromorphic computing, and the readout and control of spin networks and interfaces for quantum computing are strategically important future areas of molecular chemistry and applied physics of vanadium.

Structural and Magnetical Studies of Mixed-Valence Hexavanadate Hybrids: How Organic Ligands Affect the Magnetism of Polyoxometalates?

In this Communication, we illustrate the influence of organic ligands on magnetic structure and behavior by employing a mixed-valence Lindqvist-type hexavanadate as a research platform. Through covalently attaching to different halogen-containing organic ligands, the derived hybrid materials have different magnetism compared to their parent structure. Single-crystal X-ray analyses show that the introduction of organic ligands can modify the crystal packing manners of the derivatives, leading to further changes of the interaction between magnetic units. This work demonstrates that organic functionalization can remarkably affect the magnetism of polyoxometalates by adjusting the distance and location of the magnetic fractions.

Can the Double Exchange Cause Antiferromagnetic Spin Alignment?

The effect of the double exchange in a square-planar mixed-valence dn+1−dn+1−dn−dn–type tetramers comprising two excess electrons delocalized over four spin cores is discussed. The detailed analysis of a relatively simple d2−d2−d1−d1–type tetramer shows that in system with the delocalized electronic pair the double exchange is able to produce antiferromagnetic spin alignment. This is drastically different from the customary ferromagnetic effect of the double exchange which is well established for mixed-valence dimers and tetramers with one excess electron or hole. That is why the question “Can double exchange cause antiferromagnetic spin alignment?” became the title of this article. As an answer to this question the qualitative and quantitative study revealed that due to antiparallel directions of spins of the two mobile electrons which give competitive contributions to the overall polarization of spin cores, the system entirely becomes antiferromagnetic. It has been also shown that depending on the relative strength of the second-order double exchange and Heisenberg–Dirac–Van Vleck exchange the system has either the ground localized spin-triplet or the ground delocalized spin-singlet.