In this work, we study the local atomic and electronic structures as well as oxygen-vacancy-induced magnetic properties of electrodeposited V2O5 films. Unlike stoichiometric V2O5, which is a diamagnetic lamellar semiconductor, our oxygen-defective V2O5 films are ferromagnetic at room-temperature and their saturation magnetization decreases with air exposure time. X-ray absorption spectroscopy was used to monitor the aging effect on these films, revealing that freshly-made samples exhibit only local crystalline order, whereas the aged ones undoubtedly show an enhancement of crystallinity and coordination symmetry. The mean number of oxygen atoms around V tends to increase, indicating a decrease of oxygen vacancies with time. Concurrently with the decrease of oxygen vacancies, a loss of saturation magnetization is also observed. Hence, it can be concluded that the ferromagnetism of the V2O5 films originates from a vacancy-induced mechanism, confirming the universality of this class of ferromagnetism.
Local order, electronic structure, and magnetic properties of Co-doped VOx 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.
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