Effect of an Au underlayer on perpendicular magnetic anisotropy in Au/Co/Au(111) sandwich films

We have used spin-wave Brillouin scattering to study perpendicular magnetic anisotropy in Au/Co/Au(111) sandwiched films with varying thicknesses of Au-underlayers. By using 5-monolayer (ML)-thick Co films, the transition of spin-wave frequency between out-of-plane and in-plane magnetizations can be observed by varying the in-plane field. We find that the field-dependence of the spin-wave energy is explained by a calculation including both the first-order (second power) and the second-order uniaxial perpendicular anisotropies depending on the Au-underlayer. We observe saturation of the anisotropy at 5 ML of Au-underlayer thickness, which corresponds to the saturation of in-plane lattice expansion of Co on that Au-underlayer. The contribution of interface anisotropy is evaluated from the Co-thickness dependence of the anisotropy observed. We also have observed a field-dependent broadening of the spin-wave Brillouin spectra at a critical field between out-of-plane and in-plane magnetizations, which we attribute to a distribution of the first-order anisotropy.