Field dependent exchange coupling in NiO/Co bilayers

Dynamic magnetization reversal measurements have revealed a strong dependence of the exchange-coupling strength on the maximum applied field for NiO/Co bilayers. Time-resolved Kerr measurements, performed at room temperature, show that the coercive field increases linearly with the maximum applied field between two reversals. If the maximum applied field is different in the positive and negative directions, an exchange bias is observed as well as an asymmetry in the magnetization reversal behavior between the two sides of the hysteresis loop. X-ray magnetic circular dichroism measurements at the Ni L_2,3 edges indicate that these effects are related to an increase of the net uncompensated Ni moment with increasing field. These effects should also be present in other low anisotropy antiferromagnetic/ferromagnetic systems.