Enhanced uniaxial anisotropy and two-step magnetization process along the hard axis of polycrystalline NiFe/NiO bilayers

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 ease 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.