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

TL;DR

This study investigates the terahertz optical properties of multiferroic CoCr₂O₄, revealing an inter-sublattice exchange magnon mode and analyzing magnetic ordering through a two-sublattice model.

Contribution

It introduces a simple two-sublattice model to describe magnetic interactions in CoCr₂O₄ and identifies the inter-sublattice exchange resonance mode.

Findings

Identification of a field-dependent magnon mode in CoCr₂O₄

Determination of inter-sublattice coupling constant, λ = -18 K

Correlation of Curie temperature with dielectric and magnetic anomalies

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.