Orbital contribution to perpendicular magnetic anisotropy in $Co80Pt20$ thin films

We have measured x-ray magnetic circular dichroism spectra at the Co and Pt $L2,3$-edges in $Co80Pt20$ perpendicular magnetization films, wherein the uniaxial magnetic anisotropy constant, $Ku$⁠, significantly increases with decreasing film thickness, $δ$⁠, and reaches $1.7×107erg/cm3$ for $δ=3nm$⁠. The anisotropy of the orbital magnetic moment (AOM) was estimated as a function of $δ$⁠, and the relationship between AOM and $Ku$ is discussed. When the thickness was reduced, the AOM for Pt $5d$ was nearly constant, whereas a weak increase was observed for Co $3d$⁠. Both AOMs are small in contrast to the large $Ku$⁠, and any proportionality between AOM and $Ku$ was hardly recognized. This result indicates that a large AOM is not always a necessary and sufficient condition for the appearance of perpendicular magnetization.