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Epitaxial growth and structure of cobalt ferrite thin films with large inversion parameter on Ag(001)

AuthorsSantis, Maurizio De; Bailly, Aude; Coates, Ian; Grenier, Stéphane; Heckmann, Olivier; Hricovini, Karol; Joly, Yves; Langlais, Veronique; Ramos, Aline Y.; Richter, Christine; Torrelles, Xavier CSIC ORCID; Garaudée, Stéphanie; Geaymond, Olivier; Ulrich, Olivier
Issue DateFeb-2019
AbstractCobalt ferrite ultrathin films with the inverse spinel structure are among the best candidates for spin filtering at room temperature. High-quality epitaxial CoFe2O4 films about 4nm thick have been fabricated on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate and then treated twice with O-2, first at room temperature and then during annealing. The epitaxial orientation and the surface, interface and film structure were resolved using a combination of low-energy electron diffraction, scanning tunnelling microscopy, Auger electron spectroscopy and in situ grazing-incidence X-ray diffraction. A slight tetragonal distortion was observed, which should drive the easy magnetization axis in-plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in situ resonant X-ray diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using FDMNES, an ab initio program already extensively used to simulate X-ray absorption spectroscopy, and shows that the Co ions are predominantly located on octahedral sites with an inversion parameter of 0.88(5). Ex situ X-ray photoelectron spectroscopy gives an estimation in accordance with the values obtained through diffraction analysis.
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