K-edge magnetic circular dichroism of O in CO/Ni/Cu(001): Dependence on substrate magnetic anisotropy and its interpretation

O K-edge x-ray magnetic circular dichroism (XMCD) was investigated for mole cular CO adsorbed on Ni thin films grown epitaxially on Cu(001). In the cas e of 10 monolayer (ML) Ni that shows perpendicular magnetic anisotropy (PMA ), a relatively strong negative XMCD signal was observed at the O 1s-->CO 2 pi* transition. Since the CO 2 pi* orbital is partially filled due to hybr idization of Ni 3d levels, this finding implies parallel alignment of the C O 2 pi* orbital moment with substrate Ni magnetization. The orbital moment is ascribed to the molecular orbital origin that is derived from degeneracy of the two pi* orbitals. On the contrary, in the cases of in-plane magneti zation of 6 ML and thick (>100 ML) Ni films, the XMCD signal at the CO 2 pi * resonance was found to be, weak and positive. Although the origin of the orbital moment is not yet clear in the in-plane magnetization case, this in dicates that the orbital moment shows antiparallel alignment with Ni. The d irection of the CO 2 pi* orbital moment in PMA is interpreted with a framew ork of chemical bonding between CO and Ni.