T. Yokoyama et al., K-edge magnetic circular dichroism of O in CO/Ni/Cu(001): Dependence on substrate magnetic anisotropy and its interpretation, PHYS REV B, 62(21), 2000, pp. 14191-14196
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.