Polycrystalline NiFe(10 nm)/NiO(1–8 nm) exchange-coupled bilayers show enhanced uniaxial anisotropy but no unidirectional anisotropy. Meanwhile, a two-step magnetization process was observed along the hard direction. These two phenomena cannot be simultaneously understood using the models for ferromagnetic/antiferromagnetic bilayers, in which a parallel alignment or a random distribution of the easy axes of antiferromagnetic grains is assumed. Therefore, a model, supposing a preferential distribution of the easy axes of antiferromagnetic grains, is proposed for understanding the mechanism of these phenomena.

• Received 4 January 2001

DOI:https://doi.org/10.1103/PhysRevB.65.014431