Origin of the Magnetic "Surface Anisotropy" of Thin Ferromagnetic Films

Abstract

A very thin crystal (i. e., single-crystal film) of a cubic metal such as nickel is no longer truly cubic. This departure from cubic symmetry is reflected in the nature of the electronic states. We study the appearance of terms of lower than cubic symmetry (i. e., of axial symmetry with respect to the film normal) in the magnetic anisotropy energy associated with this change. The present paper considers the origin and nature of this additional axial anisotropy within the framework of the itinerant electron theory of magnetism. Within the localized-moment picture, as treated by Néel, this axial anisotropy is associated with effects occurring only at the surface planes, and therefore has come to be known as "surface anisotropy." We retain this nomenclature even though in our itinerant-electron picture the changes in anisotropy can be associated with electronic states extending into the interior of the film. The most striking qualitative result of our model is the marked variation of surface anisotropy with thickness possible for very thin films. This constrasts with the behavior in the Néel model, where for very thin films the surface anisotropy energy is almost independent of the film thickness. Physically this difference in behavior can be understood, since the itinerantelectron model allows the presence of coupling between the behavior at the two surfaces of the film, while such coupling cannot exist in the Néel model. We discuss the relevance of this difference to the existing experimental observations and to possible future experiments.