Dependence of exchange bias on the field-cooled remanent magnetic state in Ni/NiO nanogranular samples

The magnitude of the exchange bias (EB) effect in nanogranular Ni/NiO samples (with Ni content between about 4 and 69 wt. % and mean size of the Ni crystallites of the order of 10 nm) has been found to be strictly related to the increase in the remanent magnetization measured after field-cooling, with respect to the value after zero-field-cooling, normalized to the saturation magnetization. This allows describing the EB mechanism in terms of the fraction of the magnetic moments of the Ni nanocrystallites that irreversibly has aligned in the field direction during field-cooling, due to the exchange anisotropy, and that is effectively involved in the loop shift. Hence, the possibility of tuning EB by controlling the field-cooled remanent magnetic state is shown.