2ND-ORDER AND 4TH-ORDER ANISOTROPY CONSTANTS NEAR THE SPIN REORIENTATION TRANSITION IN CO HO THIN-FILMS/

Epitaxial buried Co/Ho films have been studied ea: situ by means of po lar magneto-optical Kerr effect and X-ray magnetic circular dichroism (XMCD) at the rare-earth M-5 edge. The very weakly polarized Ho overla yer strongly enhances the perpendicular magnetic anisotropy of the Co layer and leads to a temperature-induced Spin Reorientation Transition (SRT). From a quantitative evaluation of the second-and fourth-order constants K-2eff, K-4 Of the magnetic uniaxial anisotropy as a functio n of temperature, it is shown that the continuous SRT results from the different thermal variations of these anisotropy constants, assuming a coherent rotation of the magnetization: K-2eff varies quasi-linearly with the temperature and changes sign near the SRT temperature, where as K-4 is positive and nearly temperature-independent.