Study on the in-plane uniaxial anisotropy of high permeability granular films

We have investigated the uniaxial magnetic anisotropy of the high resistivity Fe-Al-O granular films prepared by sputtering. It is found that application of external de field during film deposition and post-annealing treatments give rise to no serious effects on the magnetic anisotropy. In contrast, the incident angle of sputtered particles to the substrate greatly affects the strength and the direction of the anisotropy. These results suggest that the magnetic anisotropy is induced by a shape anisotropy due to anisotropic morphology in the films. The temperature dependence of the anisotropy constant, however, does not obey the usual shape anisotropy relationship K-u(T) proportional to M-s(T)(2). In the present article, we propose an anisotropic coupling model in order to explain the uniaxial anisotropy found in these granular films, where the magnetic coupling between Fe grains is assumed to be anisotropic in the film plane due to the presence of low T-c intergranular phase which bridges the gap between Fe grains in one direction but not in the orthogonal direction. This model satisfactorily explains the temperature dependence of K-u(T)and M-s(T) for all Fe-Al-O granular films prepared in the present experiment. This agreement between the experiments and the theory implies that the magnetic anisotropy of the granular films is remarkably influenced by the magnetism and spatial distribution of the low T-c intergranular phase. (C) 1998 American Institute of Physics.