Dynamic dielectricity of multiferroic spinel CdCr2S4

Dynamic dielectric properties were studied on the newly discovered multiferroic spinel, CdCr2S4. Two single crystals, grown under the same condition but from different batches were used, which showed identical behavior. Frequency-dependent dielectric constant as a function of temperature (c'(omega)) exhibits relaxation behavior only below ferromagnetic transition temperature, T-C similar to 84 K, which indicated that the charge dipoles were associated with ordered magnetic spins. By fitting with extended Debye model (Cole-Cole equation), the relaxation characteristic time (tau) at different temperatures was obtained. When 55 < T < 84 K, the tau value decreases with decreasing temperature. It is some how unusual as it doesn't obey the thermal activation model (Arrhenius law). These results suggest that the relaxation behavior is strongly affected by spin not by thermal fluctuation. Different ac voltages were also selected to study the field effect on the relaxation behavior. For V-ac = 20 V, epsilon'(omega) dramatically changes and tau value becomes difficult to obtain as T < 55 K, suggesting that some unknown phases could exist below 55 K. Complex relaxation behavior below TC could be originated from the coupling among spin, lattice and dipole via exchange striction.