The magnetization reversal in CoFe2O4/CoFe2 granular systems

The temperature-dependent field cooling (FC) and zero-field cooling (ZFC) magnetizations, i.e., M FC and M ZFC, measured under different magnetic fields from 500 Oe to 20 kOe have been investigated on two exchange–spring CoFe2O4/CoFe2 composites with different relative content of CoFe2. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature T irr. For the sample with less CoFe2, the curves of −d(M FC − M ZFC)/dT versus temperature T exhibit a broad peak at an intermediate temperature T 2 below T irr , and the moments are suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the −d(M FC − M ZFC)/dT curves of the sample with more CoFe2, besides a broad peat at an intermediate temperature T 2, a rapid rise around the low temperature T 1~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T 2 and T irr towards a lower temperature, and the shift of T 2 is attributable to the moment reversal of CoFe2O4.