Ultrathin transition-metal oxide films:: Thickness dependence of the electronic structure and local geometry in MnO

Ultrathin manganese monoxide films on silver substrates are studied as a function of the thickness using x- ray absorption spectroscopy and x- ray photoemission spectroscopy. By combining these techniques, an improved understanding of both the local geometry and the electronic structure is achieved. It was found that both electronic and geometric properties in ultrathin films differ significantly from the bulk. A tetragonal distortion of the oxide films is revealed by polarization- dependent x- ray absorption measurements; the extent is decreasing gradually with increasing film thickness. A relaxation of an epitaxial oxide layer induced by heating also leads to a decrease of the tetragonal interface strain. The electronic structure of ultrathin manganese oxide films changes in the valence- band region with dependence on thickness.