High mobility of surface atoms and induced anisotropy in very thin permalloy films

The induced uniaxial anisotropy field H_k in very thin Ni₇₉Fe₂₁ films, sandwiched by seed and protective Ta layers, was investigated. Films prepared by an ultra-high vacuum sputtering process show a very sharp fiber texture of fcc-<111> perpendicular to the film plane, even with a thin film of 3 nm. The H_k value was found to decrease as the film thickness decreased below 20 nm, even after an annealing procedure in a magnetic field. The change in the induced anisotropy energy K_u was measured as a function of the temperature while applying a dc magnetic field parallel to the hard axis of magnetization. The value of K_u of as-deposited thin films decreases significantly with increasing temperature from 50°C to 100°C and the K_u reduction in a film of 5 nm reaches ∼40%. It is likely that this K_u reduction is mainly caused by realignment of atom pairs at film surfaces along the field direction. This is supported by the low activation energy of the K_u reduction process. It is successfully shown that the high mobility of surface atoms should disrupt the alignment of atom-pairs parallel to the field direction during the film deposition and annealing process, resulting in the formation of magnetically isotropic layers at surfaces. The thickness of this isotropic layer was estimated to be ∼0.8 nm and the isotropic surface-layer is likely to reduce the H_k value of very thin films.