Numerical Study of Enhanced Coercivity of a Magnetically Hard Grain with Thin Surface Layers due to Antiferromagnetic Coupling

The effects of a thin ferromagnetic layer covering a magnetically hard grain on the coercivity were calculated based on micromagnetic theory. Antiferromagnetic exchange coupling between this thin ferromagnetic surface layer (SF layer) and the hard grain was found to prevent nucleation of reverse domains at the surface of the hard grain and to increase the coercivity. This was effective for hard grains with a layer of reduced magnetic anisotropy on their surfaces whose thickness is comparable with the exchange length. It was also found that a SF layer thickness of several nanometers is suitable. The effects were simulated for a hard grain with SF layers coupled both ferromagnetically and antiferromagnetically. It was found that the coercivity can be enhanced by antiferromagnetically coupled SF layers lying parallel to the easy magnetization axis of the hard grain