Annealing temperature induced phase evolution and exchange bias properties of Fe/CoO nanocomposites

Nanocomposites of Fe/CoO (10 at. wt.% of α-Fe) have been prepared successfully by high energy ball-milling process. The effect of annealing temperature on the structural and magnetic properties of the sample has been studied by annealing the sample at different temperatures (200/400/600/800 °C) for one hour. X-ray diffraction measurements show the formation of composite phase in nano regime and evolution of cobalt ferrite phase at higher annealing temperatures. Mossbauer studies show the formation of one sextet and one doublet pattern for the sample at room temperature. But with annealing at higher temperatures (400 °C & above) evolution of another sextet pattern is observed corresponding to the Fe atoms occupying different sites. The most interesting phenomena have been observed while recording magnetization measurements as a vertical shift (M _Shift ) in the M-H loop with enhancement in saturation magnetization and coercivity is observed. The exchange bias (H _EX ) increases at lower annealing temperatures (200/400 °C) but vanishes at higher annealing temperatures (600/800 °C). Upon annealing, FM alpha Fe and AFM CoO forms a ferrimagnetic Co-Ferrite, which lacks the FM-AFM interface required for exchange bias.