Cooling field and temperature dependence on training effect in NiFe_2O_4-NiO nanogranular system

摘要

The training effect has been systematically studied in exchange coupled NiFe_2O_4/NiO nanogranular system. Both exchange bias field (H_(EB)) and vertical magnetization shifts (M_(Shift)) can be observed after the system field cooled from 350 K to low temperatures, which decrease monotonically through consecutive loop cycles. During this procedure, linear dependence between H_(EB) and M_(Shift) is found for this system, revealing the critical role of the pinned uncompensated spins. With the increase of cooling field, the relative change of H_(EB) becomes more pronounced, which shows that the rapid reduction of the pinned uncompensated spins for high cooling field. Moreover, the reduction of H_(EB) becomes weakened with decreasing measured temperatures, which indicated the spin configuration at low temperatures possesses higher dynamic stability. The cooling field and temperature dependence on training effect is discussed in terms of the evolution of the metastable spin configurations at the interfaces and fitted by a recent theoretical model.