Perpendicular and in-plane magnetic anisotropy in epitaxial Cu/Ni/Cu/Si(111) ultrathin films

Epitaxial ultrathin Cu/Ni/Cu films with Ni thickness in the range 1.5–6 nm have been grown by UHV evaporation on the $\mathrm{Si}\mathrm{}\left(111\right)-7\mathrm{}\times 7$ surface. In situ characterization made by low-energy electron diffraction and Kikuchi electron diffraction revealed that both Ni and Cu films grow epitaxially on Si, with a (111) orientation and with their (1¯10) axis parallel to the (12¯1) axis of the Si substrate. Magneto-optical Kerr effect measurements performed at room temperature have shown that the preferential direction of magnetization lies in the film plane for Ni thickness above 3 nm, while it is perpendicular to the film plane for lower thickness. Brillouin light scattering was then exploited to study the spin-wave dispersion as a function of both the applied magnetic field and the wave vector direction on the surface plane. In order to interpret the Brillouin data, we have used a macroscopic model which takes into account both dipolar and exchange interactions, as well as bulk and interface anisotropy for the (111) plane of a cubic crystal. This enabled us to determine, in addition to the other magnetic parameters, both the in-plane and the out-of-plane anisotropy constants. The observed dependence of these constants on the film thickness indicates that the magnetic anisotropy is mainly of magnetoelastic origin.