The angle-resolved photoemission study for ultrathin NiO and CoO thin films on Ag(1 0 0) surfaces

We have performed a high resolution angle-resolved photoemission study on ultrathin NiO and CoO films on Ag(1 0 0) surfaces. The NiO and CoO thin films were grown on Ag(1 0 0) at room temperature and 180 ^{{ring operator}} C, respectively, for an O₂ pressure of 7.5 × 10^{- 7} Torr . The thickness of thin film was determined by a quartz monitor and RHEED. A clearest (2 × 1) low-energy electron diffraction (LEED) pattern was observed at a NiO coverage of 0.9 monolayer(ML), consistent with literature. LEED shows a clear (1 × 1) pattern indicating well-ordered domains for the 2 ML NiO and CoO thin films. Angler-integrated photoemission spectra (AIPES) show a finite density of states at the Fermi level for the 2 ML NiO thin film, suggesting a metallic character, while the 2 ML CoO thin film was insulating. Angle-resolved photoemission spectroscopy (ARPES) was used to detect the band dispersion of 2 ML NiO and CoO thin films. Very different band dispersions of the valence bands were observed. The insulating nature of bulk NiO and CoO is related to their antiferromagnetic order, but the unusual metallic behavior of ultra NiO thin film may affect its magnetic order. The detail comparison of electronic structure for 2 ML NiO and CoO thin films will be discussed.