High magnetic performance of cobalt ferrite and anomalous magnetizing behavior of CoFe2/oxide derived from ferrite

Well-dispersed uniform CoFe2O4 nanoparticles were synthesized and then some nanoparticles were reduced in a H2/N2 gas mixture followed by oxidation in a pure O2 ambient to prepare CoFe2/oxide composite. The CoFe2O4 nanoparticles exhibited a high Hc value of 15,731 Oe and an approximately theoretical Mr/Ms ratio of 0.83 at T=10 K as a consequence of the strong surface anisotropy, which have seldom been obtained previously. The CoFe2/oxide composites exhibited the novel phenomena in their temperature derivative curves of the difference between the field cooling (FC) and zero field cooling (ZFC) magnetization, which presented two peaks at a low temperature T1 and at a high temperature T2. The low-temperature region below T1 corresponds to the completely frozen state. In such completely frozen temperature regime, the virgin magnetization curves exhibited an anomalous behavior that the magnetization decreases with increasing the applied field in the high field region due to the enhanced pinning action of disordered moments in oxides on the moments of CoFe2 alloy. These results revealed that the surface anisotropy and dipolar interaction as well as the pinning effect play a crucial rule in the magnetic properties of CoFe2O4 nanoparticles and CoFe2/oxide composite, and further investigations are in process.