Local order and the dependence of magnetization on Co content in V₂O₅ layered films

Local order, electronic structure, and magnetic properties of Co-doped VO_x films electrochemically grown onto Si are investigated. The films are studied by means of X-ray absorption spectroscopy (XAS) and magnetic measurements. Freshly made films have V₂O₅·n(H₂O) structure with vanadium valence close to +5. XAS data show that insertion of Co in the films does not affect the close environment around V, when compared to the undoped sample, even varying the concentration of Co by ten times. The site symmetry of Co dopant atoms in the films is consistent with an octahedral coordination where Co is surrounded by six oxygen atoms, as supported by X-ray absorption near-edge structure simulations. Furthermore, there is no evidence of the presence of metallic cobalt (Co⁰) in the films. The incorporation of small amounts of Co turns ferromagnetic undoped samples into paramagnetic ones. The net magnetic moment per unit volume initially decreases with the incorporation of Co and enhances for higher concentrations. Such behavior is consistent with an O vacancy reduction process driven by the insertion of Co in the films