Influence de la structure locale de l´anisotropie magnétique induite sur la dispersion de ĺaimantation dans les couches minces ferromagnétiques polycristallines

A study is made of the magnetization dispersion of polycrystalline thin ferromagnetic films with induced uniaxial magnetic anisotropy. The general formula for the so-called magnetization-induced anisotropy in a crystallite of cubic symmetry is established in order to prove that in a polycrystalline thin film this anisotropy necessarily varies from one crystallite to the next. A calculation is then made to see how these variations and the variations of the magnetocrystalline anisotropy directions affect the magnetization dispersion. It is found that in general the resulting dispersion is larger than that obtained from a superposition of magnetocrystalline anisotropy and a uniform uniaxial anisotropy. It also seems that, contrary to what is generally assumed, the dispersion does not decrease monotonically with increasing uniaxial anisotropy (Ku), but that it initially decreases (for Kn< magnetocrystalline anisotropy energy) until it reaches a minimum, and then increases with Ku. On the other hand, the dispersion does not vanish even if magnetostriction and magnetocrystalline energy are zero. Thus it is shown how the experimental results of Wolf [13], on the dispersion of Ni Fe Co thin films, could be qualitatively explained by this theory.