SPIN-RESOLVED PHOTOEMISSION-STUDY OF THE REACTION OF O2 WITH FCC CO(100)

The effect of O2 chemisorption on the electronic structure and magnetism of fcc Co overlayers, grown epitaxially on Cu(100), has been studied by use of spin- and angle-resolved photoelectron spectroscopy. At room temperature, for exposures between 2 and 7 L Of O2, oxygen adsorbs dissociatively in an ordered c (2 X 2) structure. [ 1 langmuir (L) = 10(-6) Torr sec.] The chemical interaction between the adsorbate and the substrate manifests itself through a reduction of the 3d surface-state emission at GAMMA near E(F). The oxygen bands exhibit an induced exchange splitting of 0.2 +/- 0.1 eV at the center of the surface Brillouin zone. With higher O2 exposure, corresponding to the formation of a CoO surface phase, the photoemission spectra show evidence for strong correlation effects in the electronic structure. A satellite appears about 10 eV below E(F) in the spectra. The formation of the CoO surface-oxide phase is accompanied by a decrease in the photoelectron spin polarization revealing the quenching of the surface ferromagnetic order. At low temperatures (150 K) and high exposures Of O2 the formation of a Co3O4 phase was observed, which again has a different electronic structure.