Exchange anisotropy in polycrystalline and epitaxial (001)-oriented NiO/NiFe bilayers grown by ion beam sputtering (abstract)

The connection between the spin structure of antiferromagnetic NiO and the exchange anisotropy observed in NiO/NiFe bilayers is not well understood. For instance, the NiO bulk-terminated (001) surface is compensated, and therefore simple models predict no exchange bias in (001)-oriented NiFe/NiO bilayers.¹ Using a newly developed ion-beam sputtering (IBS) process to deposit NiO exchange-coupled films,² we have simultaneously grown polycrystalline and epitaxial NiO/NiFe bilayers. NiO grown on NiFe/MgO is polycrystalline, while NiO grown directly on MgO is epitaxial. The in-plane orientation of the epilayers was confirmed using $\text{(hk0)}$ $x$ -ray diffraction. The exchange anisotropy in epitaxial (001)-oriented bilayers is about half as large as that observed in polycrystalline bilayers. The size of the exchange anisotropy does not depend on the orientation of the bias field with respect to the in-plane NiFe/NiO crystallographic direction, indicating that the same interfacial spin structure is achieved regardless of the bias field direction. These results show that the surface NiO spin structure is different from that of the bulk, and is uncompensated at the interface independent of the crystalline orientation of the bilayer. Results on epitaxial Co/NiO and NiFe/NiCoO bilayers will also be discussed.