Observation of an intersublattice exchange magnon in CoCr₂O₄ and analysis of magnetic ordering

We report on an investigation of optical properties of multiferroic CoCr₂O₄ at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr₂O₄ 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 Co²⁺ and Cr³⁺ ions), we find the inter-sublattice coupling constant, λ = −(18 ± 1) K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr³⁺ sublattice, θ₂ = (49 ± 2) K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr₂O₄ have been reported in literature.