Localized-delocalized crossover of spin-carriers and magnetization reversal in Co$_{2}$VO$_{4}$

TL;DR

This study investigates the temperature-driven magnetization reversal in Co₂VO₄, revealing a crossover from delocalized to localized spin-carriers that influences magnetic phases and is sensitive to external magnetic fields.

Contribution

It combines neutron diffraction, magnetization, muon spin relaxation, and DFT calculations to elucidate the microscopic mechanisms behind magnetization reversal in Co₂VO₄, highlighting the role of site-specific magnetic fluctuations.

Findings

Magnetization reversal occurs at around 65 K and is field-sensitive.

Delocalized to localized spin-carrier crossover drives magnetic phase changes.

Two antiparallel magnetic sub-lattices influence the net magnetization.

Abstract

Neutron diffraction, magnetization and muon spin relaxation measurements, supplemented by density functional theory (DFT) calculations are employed to unravel temperature-driven magnetization reversal (MR) in inverse spinel Co$_2$VO$_4$. All measurements show a second-order magnetic phase transition at $T_{\rm C} = 168$\,K to a collinear ferrimagnetic phase. The DFT results suggest the moments in the ferrimagnetic phase are delocalized and undergo gradual localization as the temperature is lowered below $T_{\rm C}$. The delocalized-localized crossover gives rise to a maximum magnetization at $T_{\rm NC} = 138$\,K and the continuous decrease in magnetization produces sign-change at $T_{\rm MR} \sim 65$\,K. Muon spectroscopy results support the DFT, as a strong $T_1$-relaxation is observed around $T_{\rm NC}$, indicating highly delocalized spin-carriers gradually tend to localization upon cooling. The magnetization reversal determined at zero field is found to be highly sensitive to the applied magnetic field, such that above $B\sim 0.25$\,T instead of a reversal a broad minimum in the magnetization is apparent at $T_{\rm MR}$. Analysis of the neutron diffraction measurements shows two antiparallel magnetic sub-lattice-structure, each belonging to magnetic ions on two distinct crystal lattice sites. The relative balance of these two structural components in essence determines the magnetization. Indeed, the order parameter of the magnetic phase on one site develops moderately more than that on the other site. Unusual tipping of the magnetic balance, caused by such site-specific magnetic fluctuation, gives rise to a spontaneous flipping of the magnetization as the temperature is lowered.