Mixed-valence clusters: Prospects for single-molecule magnetoelectrics

Abstract In this review we summarize the results of recent studies of the mechanisms of magnetoelectric effect in mixed-valence molecules. Consideration of fairly different systems and emerging situations is united by a common physical concept of spin-dependent electric polarizability in a wide class of such systems. In many-electron dimers, which are on the borderline between the Robin and Day classes II and III, the electric field suppresses ferromagnetic double-exchange and induces electric dipole moment, while the antiferromagnetic Heisenberg-type exchange remains unaffected. This leads to stabilization of the low-spin states thus giving rise to the spin-switching effect. A short and to a large extent qualitative discussion of the role of the vibronic coupling that is inherent for mixed valence systems is given as well. In trigonal trimers with two delocalized excess electrons, the electric field suppresses the first-order electron transfer, and under some conditions also leads to the spin-switching effect. Magnetoelectric coupling of quite different nature occurs in linear two-electron trimers (such as linear triferrocenium complexes) and also in two-electron clusters of higher complexity such as polyoxoanion [GeV14O40]8−. In these systems electric field is shown to effectively approach the two remote electrons, thus forcing them to interact. This allows to enable/disable the exchange interaction by turning on/off the electric field. We also discuss a possibility to control spin-states in the two-electron square-planar systems acting as cells in molecular quantum cellular automata devices. Spin-singlets and spin-triplets exhibit different electric polarizabilities, which allows to control the spin-states by using polarized driver-cell. Finally, we briefly discuss the feasibility of the electric field control of mixed valence molecules and also give some additional examples of clusters exhibiting magnetoelectrical effect.