Suppression of the magnetocrystalline bulk anisotropy in thin epitaxial Co(110) films on Cu(110)

We report on an unexpected suppression of the magnetocrystalline anisotropy contribution in epitaxial fee Co(110) films on Cu(110) below a thickness of d(c)=(50+/-10) Angstrom. For film thicknesses larger than d(c) the measured anisotropy value agrees with published data. Measurements on films with reduced strain indicate a large strain dependence of d(c). A model calculation based on a crystal-field formalism and discussed within the context of band theory, which explicitly takes tetragonal misfit strains into account, reproduces the experimentally observed anomalies. Our results indicate that the usually applied phenomenological description of anisotropies, assuming additive free energy terms for each anisotropy contribution, fails in this case.