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 squareness ratio (M_R/M_S), 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 KN (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.