FeCr2O4 and CoCr2O4 spinels: Multiferroicity in the collinear magnetic state?

Dielectric permittivity (epsilon') and electrical polarization (P) have been measured as a function of temperature for two polycrystalline ACr(2)O(4) spinels (A = Fe and Co). Anomalies on the epsilon'(T) curves are detected at the characteristic magnetic transition temperatures (T-C, T-S, and Tlock-in) for FeCr2O4 and CoCr2O4 and also at the Jahn-Teller (JT) transition for FeCr2O4. The P(T) curves of both spinels exhibit transitions at TC showing that polar and ferrimagnetic states coexist in these oxides. The link between the distortion of the spinel structure due to the Jahn-Teller Fe2+ and the larger polarization value at 8K, P = 35 mu C/m(2), against P = 3.5 mu C/m(2) for CoCr2O4, is also discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3656711]