Observation of an inter-sublattice exchange magnon in CoCr${}_2$O${}_4$ and analysis of magnetic ordering

Abstract

We report on an investigation of optical properties of multiferroic CoCr${}_2$O${}_4$ at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr${}_2$O${}_4$ is dominated by a magnon mode, which shows a steep magnetic-field dependence. We ascribe this mode to an exchange resonance between two magnetic sublattices with different $g$-factors. In the framework of a simple two-sublattice model (the sublattices are formed by Co${}^{2+}$ and Cr${}^{3+}$ ions), we find the inter-sublattice coupling constant, $\lambda =-(18\pm 1)$ K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr${}^{3+}$ sublattice, ${\Theta }_2$ = $(49\pm 2)$ K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr${}_2$O${}_4$ have been reported in literature.

accepted to PRB