Combined spectroscopic and ab initio investigation of monolayer-range Cr oxides on Fe(001): The effect of ordered vacancy superstructure

We investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated $\mathrm{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ surface and characterized by the formation of an ordered array of Cr vacancies producing a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R{27}^{\ensuremath{\circ}}$ superstructure. We combined experimental techniques such as angle- and spin-resolved photoemission spectroscopy, low-energy electron diffraction, and scanning tunneling spectroscopy with ab initio calculations, focusing on (i) the peculiar energy dispersion of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ states and (ii) the orbital and spin character of $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ states. We show that the experimental $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ dispersion can be related to the presence of an ordered vacancy lattice. The comparison with the existing literature on the oxidation of bulk Cr(001), where a network of Cr vacancies with a short-range crystallographic order is present, reveals a similar effect on O states. The valence electronic structure of the Cr oxide layer is mostly composed by spin-minority Cr states, consistent with an antiferromagnetic coupling with the Fe substrate.