Exchange coupling in a half-metallic and hard-magnetic Fe3O4–antiferromagnetic CrMnPt system, and unidirectional anisotropy enhancement with the induction of stress-induced anisotropy in the CrMnPt film

Exchange coupling between half-metallic Fe3O4 and antiferromagnetic CrMnPt films was investigated, with the goal of inducing unidirectional anisotropy in the Fe3O4 film having hard magnetic properties. We succeeded in obtaining a large unidirectional anisotropy constant of ∼0.15 erg/cm2, a high blocking temperature of ∼320 °C, and an almost unidirectionally shifted M–H curve. The unidirectional anisotropy constant of the Fe3O4–CrMnPt system increased to ∼0.18 erg/cm2, while the high blocking temperature and the nearly unidirectionally shifted M–H curve were retained, when a NiO film was deposited by sputtering onto the CrMnPt film, and the resulting system was annealed at 230 °C for 3 h. This strong unidirectional anisotropy constant is thought to have been caused by large stress relief in the NiO film resulting from the thermal annealing that acted to increase the c/a ratio in the CrMnPt film (the a and c are lattice constants), inducing so-called stress-induced anisotropy in the CrMnPt film.