Magnetization reversal dynamics in exchange-coupled NiO-Co bilayers

We performed a detailed study of the magnetization reversal in polycrystall ine exchange-coupled NiO/Co bilayers over 10 decades of field sweep rate dH /dt for different NiO and Co thicknesses. For all sweep rates and thickness es, the symmetry of the hysteresis loops shows that an identical pinning st rength has to be overcome in both directions of the reversal. At low dH/dt the reversal is governed by domain wall displacement while domain nucleatio n is dominant at higher ones. The dH/dt at which the transition between the two regimes takes place depends on the relative thickness of the NiO and C o layers. It increases (decreases) when the Co (NiO) thickness is increased . Experimentally, it was found that the energy barrier varies linearly with the square root of the area corresponding to the activation (Barkhausen) v olume which is consistent with a random walk model of the coupling between antiferromagnetic and ferromagnetic layers. The results can be explained in terms of a thermally activated switching of the NiO magnetization dragged by the Co reversal. (C) 2001 American Institute of Physics.