Grain growth effect of Cr ₂O ₃ thin film layer on exchange coupling of Cr ₂O ₃/Co interface

We have observed high exchange bias for a Cr ₂O ₃ film, which was crystallized from an amorphous Cr-oxide by annealing in O ₂ flow. To clarify the origin of the high exchange bias, we characterized the morphology of the Cr ₂O ₃ film. From X-ray diffraction, atomic force microscopy, and transmission electron microscopy measurements, we discovered that our polycrystalline Cr ₂ O ₃ film was found to grow not to be entirely random orientation, but to form large R-planes on the surface. That is, R-planes were self-organized during crystallization. Since uncompensated Cr spins exist on R-planes of Cr ₂O ₃, the origin of the high exchange bias would be the self-organized R-planes. We successfully explained the peculiar temperature-dependent exchange bias and coercivity of the Cr ₂O ₃ film. For the Cr ₂O ₃ film, perpendicular exchange bias was obtained whereas out of plane direction is hard magnetization axis. This is because of the oblique Cr spin directions from out of plane direction of the film. The results demonstrated that exchange bias higher than coercivity (μ ₀H _ex >> μ ₀H _c) was realized near room temperature.