A study was made of the structural, magnetic, and optical properties, as well as the electronic structure, of Co-doped ZnO films prepared by magnetron sputtering. Magnetization measurements performed at different temperatures indicated ferromagnetic and paramagnetic behavior for samples prepared under O-poor conditions, whereas samples prepared under O-rich conditions exhibited only paramagnetic behaviour; confirming that the presence of O-related defects was essential for ferromagnetism in Zn1-xCoxO. X-ray absorption spectroscopy at the Co L2,3 edge together with optical transmittance measurements showed that Co ions were 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 X-ray absorption spectra of the samples prepared at different conditions exhibited substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as O vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. The findings indicated that the ferromagnetic properties of Co-doped ZnO samples were strongly correlated with the presence of O vacancies in the ZnO lattice; supporting the spin-split impurity band model.

Defect Induced Ferromagnetism in Co-Doped ZnO Thin Films. M.Fonin, G.Mayer, E.Biegger, N.Janssen, M.Beyer, T.Thomay, R.Bratschitsch, Y.S.Dedkov, U.Rüdiger: Journal of Physics - Conference Series, 2008, 100[4], 042034 (4pp)