Magnetization reversal driven by electron localization-delocalization crossover in the inverse spinel Co2VO4

Neutron diffraction, magnetization, and muon spin relaxation measurements, supplemented by density functional theory (DFT) calculations are employed to unravel temperature-driven magnetization reversal in inverse spinel ${\mathrm{Co}}_{2}{\mathrm{VO}}_{4}$. All measurements show a second-order magnetic phase transition at ${T}_{\mathrm{C}}=168\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ to a collinear ferrimagnetic phase. Neutron diffraction measurements reveal two antiparallel ferromagnetic (FM) sublattices, belonging to magnetic ions on two distinct crystal lattice sites, where the relative balance between the two sublattices determine the net FM moment in the unit cell. As the evolution of the ordered moment with temperature differs between the two sublattices, the net magnetic moment reaches a maximum at ${T}_{\mathrm{NC}}=138\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and reverses its sign at ${T}_{\mathrm{MR}}=65\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The DFT results suggest that the underlying microscopic mechanism for the reversal is a delocalization of the unfilled $3d$-shell electrons on one sublattice just below ${T}_{\mathrm{C}}$, followed by a gradual localization as the temperature is lowered. This delocalized-localized crossover is supported by muon spectroscopy results, as strong ${T}_{1}$ relaxation observed below ${T}_{\mathrm{C}}$ indicates fluctuating internal fields.