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We present a detailed study on the structural, magnetic and optical properties, as well as the electronic structure of epitaxial Co-doped ZnO films prepared by magnetron sputtering. Different preparation conditions were implemented in order to control the concentration of oxygen vacancies in the ZnO host lattice. Magnetization measurements indicate ferromagnetic behavior at low temper-
ature for samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions did not show ferromagnetic behavior corroborating that ferromagnetism in Co-doped ZnO is related to oxygen-related defects. X-ray absorption spectroscopy (XAS) at the Co L2,3 edge together with optical transmittance measurements show that Co ions are present in the high-spin Co2+ (d7) state under tetrahedral symmetry indicating a proper incorporation in the ZnO host
lattice. Comparison of the O K edge XAS spectra of the samples prepared at different conditions show substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as oxygen vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. Our findings indicate that the ferromagnetic properties of Co-doped ZnO samples are strongly correlated with the presence of oxygen vacancies in the ZnO lattice supporting the spin-split impurity band model. |