Spin-resolved photoemission from ultrathin fcc cobalt(100)-films and the influence of oxygen chemisorption

The spin dependent electronic structure of epitaxial Co overlayers on Cu (100) as well as the influence of O$_{2}$ chemisorption has been studied by spin and angle resolved photoemission spectroscopy with synchrotron radiation. Co layers with a thickness below 5 monolayers show an enhanced value of the exchange splitting at the center of the Brillouin zone due to quantum size effects. The experimental results are supported by FLAPW calculations for 1, 2 and 3 Co layers on Cu (100). These calculations show that the increase of the average exchange splitting is related to the enhancement of the magnetic moment in ultrathin Co layers. Oxygen adsorbs dissociatively on the Co (100) surface in an ordered c(2x2) structure for oxygen exposure between 2 and 7 L (at room temperature). The chemical interaction between the adsorbate and the substrate manifests itself by a reduction of the Co 3d surface state emission near E$_{F}$. The oxygen 2p-derived bands exhibit an induced exchange splitting at the center of the surface Brillouin zone. At higher oxygen exposure, corresponding to the formation of a CoO surface phase, the photoemission spectra show evidence for strong correlation effects in the electronic structure. The formation of the CoO surface oxide phase is accompanied by a decrease of the photoelectron spin polarization revealing the quenching of the surface ferromagnetic order. At low temperatures (150 K) and high exposures of O$_{2}$ the formation of a Co$_{3}$O$_{4}$ phase is observed