UNDERLAYER-INDUCED PERPENDICULAR MAGNETIC-ANISOTROPY IN ULTRATHIN CO AU/CU(111) FILMS - A SPIN-WAVE BRILLOUIN-SCATTERING STUDY/

We have used spin-wave Brillouin scattering to study the perpendicular magnetic anisotropy of ultrathin Co/Au/Cu(111) films with various thi cknesses of Au underlayer. From the field dependence of the spin-wave frequency we find that the second-order (fourth power) uniaxial perpen dicular anisotropy increases monotonically with increasing Au-underlay er thickness ranging from 0 to 5 monolayers (ML), while the first-orde r (second power) anisotropy shows a nonmonotonic increase with a minim um at 1 ML Au. The ratio of the second-order anisotropy constant to th e first one also increases with increasing Au thickness, from 0.01 to 0.07. We observe saturation for both the perpendicular anisotropy cons tants for a 5 ML Au underlayer, which coincides with saturation of exp ansion of the in-plane Co lattice due to the coherent growth of Co at the interface between the Co and Au underlayer. When the thickness of Au is further increased beyond 5 ML, we find an anisotropy-independent increase in coercivity on the polar-Kerr hysteresis curves. We also h ave observed a field-dependent broadening of the spin-wave Brillouin s pectrum around a critical field between out-of-plane and in-plane magn etizations, which we explain by assuming a distribution of the first-o rder perpendicular anisotropy. As the result, the normalized distribut ion of the first-order anisotropy is shown to have a minimum at 1 ML A u thickness. We believe the observed Au-thickness dependences of the p erpendicular anisotropy and of the distribution are characteristic beh aviors due to an atomic-scale transition of the underlayer materials f rom Cu to Au.