Universal link of magnetic exchange and structural behavior under pressure in chromium spinels

Abstract

We have conducted high-pressure x-ray diffraction studies on the $\mathrm{MnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{NiC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ spinels at room temperature. Both compounds undergo pressure-induced structural transitions into diverse tetragonal modifications. Based on these experimental observations and our ab initio calculations, we show that the Cr-oxide spinels with magnetic ${A}^{2+}$ cations $({A}^{2+}=\mathrm{Mn}$, Fe, Co, Ni) follow a similar trend as their chalcogenide counterparts with nonmagnetic ${A}^{2+}$ ions, i.e., the transition pressure is proportionally related with the magnitude of the Cr-Cr magnetic exchange interactions. Therefore, we reach the conclusion that the Cr-Cr magnetic exchange interactions alone suffice to account for the high-pressure behavior of these systems. Our results clearly depict the close relationship between the structural and magnetic degrees of freedom in Cr-bearing spinels.