Anisotropic magnetization of CO adsorbed on ferromagnetic metal thin filmsstudied by X-ray magnetic circular dichroism

We observed O K-edge X-ray magnetic circular dichroism (XMCD) of molecular CO adsorbed on Co and Ni epitaxial films grown on a Cu(001) substrate. The films examined are 6 ML Ni, thick (>100 ML) Ni, 4 ML fec Co which show in-p lane magnetic anisotropy, while the others, 10 ML Ni and similar to2 ML Co on 30 ML Ni, exhibit perpendicular magnetic anisotropy (PMA). In the PMA fi lms, negative XMCD signals were observed at the O 1s --> CO 2 pi* transitio n, while in the in-plane magnetization films, positive XMCD signals were de tected at the same transition. These findings imply the presence of orbital magnetic moments at the CO 2 pi* levels since the CO 2 pi* orbitals are pa rtially filled due to hybridization of metal 3d levels, in spite that CO is inherently diamagnetic in a gas phase. The observed XMCD signs indicate pa rallel and antiparallel alignments of the CO 2 pi* orbital magnetic moments for the PMA and in-plane magnetization films, respectively. The direction of the orbital moments is tried to interpret within a framework of chemical bonding between CO and metal atoms. (C) 2001 Elsevier Science B.V. All rig hts reserved.