UNIAXIAL MAGNETIC-ANISOTROPY OF CO FILMS ON SAPPHIRE

We have studied the magnetic properties of thin Co(0001) films on sapphire (1120BAR) 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.