Spin-resolved photoemission study of the reaction ofO2with fcc Co(100)

The effect of ${\mathrm{O}}_2$ 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 ${\mathrm{O}}_2$, oxygen adsorbs dissociatively in an ordered c(2×2) structure. [1 langmuir (L) ≡${10}^{\mathrm{-}6}$ Torr sec.] The chemical interaction between the adsorbate and the substrate manifests itself through a reduction of the 3d surface-state emission at Γ near ${\mathit{E}}_{\mathit{F}}$. The oxygen bands exhibit an induced exchange splitting of 0.2±0.1 eV at the center of the surface Brillouin zone. With higher ${\mathrm{O}}_2$ 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 ${\mathit{E}}_{\mathit{F}}$ 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 ${\mathrm{O}}_2$ the formation of a ${\mathrm{Co}}_3$ ${\mathrm{O}}_4$ phase was observed, which again has a different electronic structure.