The exchange anisotropy of NiO/CoFe bilayers

The antiferromagnetic/ferromagnetic NiO/CoFe bilayers are prepared by radio frequency magnetron sputtering method. Magnetization hysteresis curves were measured by a vibrating sample magnetometer (VSM). The exchange bias fields Hex of NiO/CoFe bilayers are studied by using different substrates and sputtering Ar gas pressures. When the glass, Si (100), Si (110) and Si (111) substrates are used, the exchange bias fields of the bilayers are different. The Hex is also influenced because of different sputtering Ar gas pressures. The crystal texture and surface roughness of the samples were analyzed by using X-ray diffraction (XRD) and atom force microscope (AFM). It is found that the exchange bias field strongly depends on the NiO/CoFe interface roughness. With the increase of the interface roughness, the exchange bias field Hex of NiO/CoFe bilayers decreases. It is not dependent on the existence of NiO (111) texture which is the spin uncompensated plane, believed to strongly correlate with exchange bias field according to the traditional understanding for the anisotropic exchange biasing mechanism. These results cannot be explained by the ideal interface model and Mauri et al.ʹs interfacial antiferromagnetic domain wall model, but the random-field model can interpret the results.