Magnetoelectric effect in the mixed valence polyoxovanadate cage V$_{12}$

TL;DR

This study shows that magnetoelectric coupling in mixed-valence polyoxovanadate cages V$_{12}$ is chiefly mediated by the relocation of itinerant electrons within the external VO$_4$ squares, sensitively modulated by electric-field direction. By combining density functional theory with an effective full t-J Hamiltonian, the authors extract parameter sets that fit experimental magnetic data and reproduce DFT spin distributions, revealing strong anisotropy and valence-dependence in the magnetoelectric response. They demonstrate both gradual (electric field parallel to ES) and abrupt (perpendicular to ES) spin-state changes, including field-induced spin crossovers and, in some cases, spin-state switching at fields accessible without ionization. The findings suggest practical routes to electrically manipulate molecular spins at or near room temperature, with implications for spintronics and potentially quantum information processing, and emphasize the importance of itinerant electrons in enabling robust magnetoelectric effects in mixed-valence POV systems.

Abstract

Development of spintronic and quantum computing devices increases demand for efficient, energy saving method of spin manipulation at molecular scale. Polyoxovanadate molecular magnets being susceptible to both electric and magnetic fields may serve here as a good base material. In this paper two isostructural anions [V$_{12}$As$_8$O$_{40}$(HCO$_2$)]$^{n-}$ (with $n=3,5$) featuring two different mixed-valence states with itinerant and localized valence electrons are studied. The impact of the electric field on their magnetic properties is investigated by means of two complementary methods informed by magnetic measurements: effective Hamiltonian calculations and density functional theory. It is demonstrated that the magnetoelectric effect in theses molecules is induced mostly by relocation of itinerant electrons, is highly anisotropic, depends on the valence state and can be detected even at room temperature. These findings can pave the way to practical applications in which an electric field control over spin state is required.

Figures (21)

• Figure 1: upper panel: Structure of $V_{12}$ molecule (CCDC - 1260088 after DFT geometric optimization), color code: grey - V, red - O, violet - As, white - H; lower panel: A schema depicting superexchange interactions between magnetic vanadium ions located in the corners of three squares: one black internal square (IS) - sites 9, 10, 11, 12 and two blue external squares (ES), sites 1, 2, 3, 4 and 5, 6, 7, 8.
• Figure 2: Spin density for molecule I (upper panel) and II (lower panel).
• Figure 3: Temperature dependence of $\chi T$ ($B=0.1$) for I (upper panel) and II (lower panel) Gatteschi1993 with the optimal fits (red lines) and the theoretical prediction of magnetic field dependence of magnetization at $T=2$ K (in the inserts).
• Figure 4: Local magnetizations and corelations at $T=2$ K for molecule I (left) and II (right).
• Figure 5: Temperature dependence of $\chi T$ ($B=0.1$ T) and field dependence of magnetization $M$ for molecule I in different electric fields $E$ [V/nm] applied along sites 4 and 2. Empty circles stand for experimental results Gatteschi1993 at field $E=0$.