Oblique-Incidence Anisotropy in Evaporated Permalloy Films

Abstract

Magnetic anisotropy has been observed in evaporated Permalloy films deposited at oblique incidence of the vapor. Electron micrographs of such films reveal chains of crystallites whose long axes are oriented perpendicular to the vapor beam. A ``self-shadowing'' model is proposed to explain chain growth, i.e., the area behind a crystallite is left vacant because it is in the crystallite's shadow. Oriented crystallite chains are thus caused by a purely geometric process; their existence has also been demonstrated for Au, Pt, W, and Mg. Oriented chains give rise to a number of macroscopic effects: magnetic anisotropy, anisotropic resistance, dichroism, and anisotropic resonance linewidth. Experiments involving the stripping of oblique-incidence Permalloy films from their substrates indicate the presence of an anisotropic strain which, in conjunction with magnetostriction, gives another contribution to the magnetic anisotropy; this contribution explains the observed compositional dependence of the magnetic anisotropy. Surface tension, which tends to contract the crystallite chains, is postulated as the generating force of anisotropic strain; surface tension could similarly generate the isotropic strain which is found in normal-incidence films.