Effect of Co and O defects on ferromagnetism in Co-doped ZnO: An X-ray absorption spectroscopic investigation

Understanding of origin of ferromagnetism in dilute magnetic oxides (DMO's) has become one of the most challenging research problems in condensed matter physics. Here we are reporting a detailed study of magnetic properties and electronic structure of two 5% Co-doped ZnO samples (the as-prepared sample Zn_0.95Co_0.05O and the hydrogenated sample Zn_0.95Co_0.05O:H). The as-prepared sample is found to be paramagnetic while through hydrogenation, we observed inducement of remarkable ferromagnetism in it. The H-mediated magnetic transition is accompanied by electronic structure modifications with no structural deviations. To get in-depth information into electronic structure correlations of the observed ferromagnetism, we have investigated their electronic properties in detail. For this purpose, we have employed the site-selective and element-sensitive X-ray-absorption spectroscopy (XAS) in the vicinity of the Cobalt L_2,3 edge, the oxygen K edge, and the Zinc L₃ edge using synchrotron radiation. The Co L_2,3 edge spectra clearly show that Co dopants reside at the Zn sites for both these samples and that they are tetrahedrally coordinated with the ligand O atoms. Very minor changes are observed in the Zn L₃ edge spectra. However, the O 1s edge spectra display dominant additional components in the ferromagnetic hydrogenated sample Zn_0.95Co_0.05O:H, not observed in the as-prepared non-magnetic sample Zn_0.95Co_0.05O. We conclude that the observed spectral features can be attributed to the presence of O vacancies and the hybridization of Co 3d states with O 2p vacancy states. These two factors together are likely to play important role in inducement of ferromagnetic ordering in this Co-doped ZnO system. However, which of these two weighs more in this mechanism, cannot be pinpointed and more studies are required in this regard.