Modifications of the electronic and magnetic properties of ultrathin Ni/Cu(100) films induced by stepwise oxidation

We report near-edge x-ray-absorption fine-structure (NEXAFS) and magnetic circular x-ray dichroism (MCXD) measurements of 4-ML Ni films grown on Cu(100). The films were exposed to oxygen which then adsorbed in different adsorption states. The changes in the density of states as probed at the Ni L(2,3) edges, the oxygen K edge, as well as the magnetic response of the sample are investigated simultaneously for the various adsorption states that range from O-2/Ni at 38 K to an ultrathin film of NiO. The NEXAFS spectra provide a characterization of the changes in the different nickel 3d, 4sp, and oxygen 2p density of states above the Fermi level. These states dominate the interaction of the oxygen and nickel atoms and can be studied separately using the element specificity of x-ray absorption. We find that 0.5 ML of atomic oxygen suppresses the magnetization corresponding to one layer of Ni, and for 1.5 ML no ferromagnetic response from a 4-ML film at 38 K could be detected.