Magnetic behavior of mechanically milled FeNi-CoO nanocomposites


The effects of crystallite size and ball-mill-induced defects on the magnetic properties of ferromagnetic (FM) FeNi-antiferromagnetic (AFM) CoO (Neel Temperature = 290 K) nanocomposites are assessed by magnetic measurements. Asymmetric zero-field-cooled (ZFC) hysteresis loops were observed for samples milled for 30h and longer. The field-cooled (FC) hysteresis loops were asymmetric and shifted in the direction opposite to the cooling field. The enhancement of coercivity and squarness ratio (MR/MS), along with a presence of a loop shift after field cooling indicates the presence of an exchange bias effect between the FM and AFM phases of the composite. The exchange bias field extracted from the 5 K FC measurement shows continuous enhancement with milling times up to 30 hours and a reduction upon prolonged milling. The temperature dependent exchange bias field measurement shows that the exchange bias field reduces to zero at temperature T < 200 K < T N (290 K) of CoO. Thus, in agreement with thin film systems, exchange bias properties can be obtained in FM-AFM fine powder nanocomposites upon mechanical milling. Further, the present study has the potential implication of improving magnetic properties of hard magnetic materials upon milling with antiferromagnetic materials.

Publication Title

IEEE Transactions on Magnetics