We investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated Fe(001)-p(1×1)O surface and characterized by the formation of an ordered array of Cr vacancies producing a ( √ 5× √ 5)R27 • 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

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... uliar energy dispersion of O 2p states and (ii) the orbital and spin character of Cr 3d states. We show that the experimental O 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.