Defect induced ferromagnetism in Co-doped ZnO thin films

We present a study on 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 indicate ferromagnetic and paramagnetic behavior for the samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions show only paramagnetic behavior corroborating that the presence of oxygen-related defects is essential for ferromagnetism in Zn _1-xCo_xO. X-ray absorption spectroscopy (XAS) at the Co L_2,3 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.