Uniaxial magnetic anisotropy of Co films on sapphire

We have studied the magnetic properties of thin Co(0001) films on sapphire (11 2 0) substrates for thicknesses ranging from 10 to 40 nm. In this range the Co films, grown by molecular beam epitaxy techniques, exhibit a well defined orientational relationship to the sapphire substrate and a high structural coherence parallel to the film normal. Longitudinal magneto-optic Kerr effect measurements are presented which show that in addition to the usual in-plane anisotropy the magnetization strongly depends on the azimuthal angle between the magnetic field and an in-plane crystallographic direction. Instead of the expected six-fold crystal anisotropy we find a uniaxial anisotropy. This azimuthal anisotropy is solely due to an interface interaction and may be caused by internal oxidation of those Co atoms which are close to the sapphire substrate.