Cu doped ZnO pellets: study of structure and Cu specific magnetic properties.

Cu doped ZnO polycrystalline pellets were synthesized with Cu concentrations varying from 2 to 10 wt% by a solid state reaction route (mixing of ZnO and CuO powders). Global magnetization measurements showed that all the samples were paramagnetic. Fitting the temperature-dependence of the magnetization to the Curie-Weiss law revealed the presence of an antiferromagnetic interaction between magnetic moments. Structural characterizations were carried out by x-ray diffraction and x-ray absorption spectroscopy (XAS) at the Cu K-edge. By analyzing the XAS data, we found that at low Cu content most of the Cu atoms substitute for Zn inside the ZnO wurtzite lattice, while for higher Cu concentrations some unreacted CuO remains segregated from the Zn(1-x)Cu(x)O solid solution. Element-specific magnetic measurements were carried out by x-ray magnetic circular dichroism (XMCD) and compared to the results of ab initio calculations. The XMCD signal at the Cu K-edge originates from magnetic moments localized at Cu sites and, by monitoring the magnetic field dependence, we concur that these moments are associated with a paramagnetic state.