Exchange anisotropy in epitaxial and polycrystalline NiO/NiFe bilayers

(001)-oriented NiO/NiFe bilayers were grown on single crystal MgO (001) substrates by ion beam sputtering in order to determine the effect that the crystalline orientation of the NiO antiferromagnetic layer has on the magnetization curve of the NiFe ferromagnetic layer. The simplest model predicts no exchange anisotropy for the (001)-oriented NiO surface, which in its bulk termination is magnetically compensated. Nonetheless exchange anisotropy is present in the epitaxial films, although it is approximately half as large as in polycrystalline films that were grown simultaneously. The surface anisotropy in the epitaxial films is found to contain cubic and unidirectional components, while that in the polycrystalline film is best described by a uniaxial plus unidirectional anisotropy. Experiments indicate that differences in exchange field and coercivity between polycrystalline and epitaxial NiFe/NiO bilayers couples arise due to variations in induced surface anisotropy. Implications of these observations for models of induced exchange anisotropy in NiO/NiFe bilayer couples will be discussed.