Origin and Effects of Local Regions of Complex Biaxial Anisotropy in Thin Ferromagnetic Films with Uniaxial Anisotropy

Abstract

Besides the uniform field-induced anisotropy field in a thin ferromagnetic film, there are perturbation anisotropy fields due to the magnetocrystalline anisotropy of the randomly oriented crystallites, their shape anisotropy, the shape anisotropy of the film at the edge, and from the locally anisotropic strains on the crystallites. These perturbations are of great importance in determining the magnetic behavior of the film. Because of exchange and magnetostatic coupling, uniaxial perturbation anisotropy fields of great magnitude in crystallite-sized areas appear instead as complex biaxial perturbation anisotropy fields of small magnitude in areas the size of small domains. The size and shape of these regions are calculated. Most interactions between these areas are automatically included in the randomly oriented complex biaxial perturbation anisotropy fields. These fields can explain the various negative anisotropy effects, the internal biasing field, the relationship between the angular and magnitude inhomogeneity, the shape of the probability density function, the amount of ``negative'' anisotropy in a film, and the position of the negative anisotropy peak. The magnetization creep threshold is significantly larger in films with negative anisotropy than without. The susceptibility is explained quantitatively by including the magnetic stray field due to the net magnetic charge that builds up on the walls when the magnetization vector is rotated.