Spin reorientation transitions of ultrathin Co/Pd(111) films induced by chemisorption: x-ray magnetic circular dichroism study

Spin reorientation transitions (SRTs) of ultrathin Co/Pd(111) films induced by adsorption of atoms and molecules have been investigated by means of Co L_III,II-edge x-ray magnetic circular dichroism (XMCD). We have examined CO, NO, O and H as chemisorbed species. We observed the adsorbate-induced SRT from surface parallel to perpendicular magnetization at 200 K in the cases of CO and NO on 4.5 ML Co/Pd(111), while O or H adsorption has been found not to lead to the transition on 4.5 ML Co. We have investigated the detailed Co-thickness dependence in the case of CO and have found that the critical thickness of the SRT in CO-adsorbed Co is ∼6.5 ML, which is greater by ∼3 ML than that of clean Co (∼3.5 ML). From the sum rule analysis, it has been found that the surface parallel orbital moment is reduced after CO adsorption, while the surface normal orbital moment is left unchanged. We can conclude that the observed stabilization of perpendicular magnetic anisotropy due to adsorption is ascribed to quenching of the surface parallel magnetic orbital moment.