Temperature Dependence of Induced Magnetic Anisotropy in Very Thin Permalloy Films

Abstract

The induced uniaxial anisotropy field H_k in very thin Ni₇₉Fe₂₁ films, sandwiched by seed and protective Ta layers, was investigated. Films prepared by using a UHV sputtering process show a very sharp fiber texture of fcc-<111> even with a film thickness of 3 nm. The H_k value was found to decrease as the film thickness decreases 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. The K_u reduction in a film of 5 nm reaches ∼40% of that of the original value. It is likely that the K_u reduction at temperatures below 100°C is mainly caused by alignment of atom pairs at film surfaces. The thickness of film surfaces with highly mobile atoms was estimated to be 5-6 A. The high mobility of surface atoms may disrupt the alignment of atom-pairs parallel to the field direction during the film deposition and annealing process, resulting in the smaller H_k of very thin films.