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

The induced uniaxial anisotropy field H/sub k/ in very thin Ni/sub 79/Fe/sub 21/ 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/sub 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/sub 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/sub u/ of as-deposited thin films decreases significantly with increasing temperature from 50/spl deg/C to 100/spl deg/C and the K/sub u/ reduction in a film of 5 nm reaches /spl sim/40%. It is likely that this K/sub 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/sub 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 /spl sim/0.8 nm and the isotropic surface-layer is likely to reduce the H/sub k/ value of very thin films.