Observation of an intersublattice exchange magnon in CoCr sub(2)O sub(4) and analysis of magnetic ordering

We report on an investigation of optical properties of multiferroic CoCr sub(2) O sub(4) at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr sub(2) O sub(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 super(2+) and Cr super(3+) ions), we find the inter-sublattice coupling constant, [lambda] = -(18 + or - 1) K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr super(3+) sublattice, [Theta] sub(2) = (49 + or - 2) K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr sub(2) O sub(4) have been reported in literature.