Magnetocrystalline volume and interface anisotropies in epitaxial films: Universal relation and Neel's model (invited)

The thickness dependence of the fourfold in-plane magnetic anisotropy was first observed in epitaxial Fe(001) films and described by the volume anisotropy of bcc Fe with a positive anisotropy constant (K-1(vol)>0) superimposed by a negative interface term (K-1(int)<0). This results in a spin reorientation of the easy axis from [100] in thick films to [110] for thicknesses t(Fe)<t(crit)=6 ML. To clarify the origin of the interface term Fe and bcc Fe1-xCox alloy films (x<0.8) epitaxially grown on different substrates-Ag(001), Au(001), GaAs(001)-were studied. As a result, magnetoelastic interactions can be ruled out because exactly the same anisotropies are found for tensile strain [Fe on Ag(001) and Au(001)] and for compressive strain [Fe on GaAs(001)]. Also, K-1(int) is not affected by the particular interface material or overlayer, e.g., vacuum or Au. A universal correlation was found between the volume and interface anisotropy constants: -K-1(int)/K-1(vol)=t(crit)=6 ML. It is shown that this is indeed expected within Neel's phenomenological theory of magnetic anisotropy if contributions from nearest and next-nearest neighbors are taken into account. Electronic hybridization effects in this case seem to play a minor role compared to the local symmetry. It is suggested that observed effects of overlayers on perpendicular interface anisotropies might be related to surface relaxations which have not been sufficiently studied so far and which, on the other hand, would not affect the fourth-order in-plane anisotropy according to Neel's model in agreement with present experimental results. (C) 2003 American Institute of Physics.