Absence of ferromagnetism in bulk polycrystalline Zn0.9Co0.1O

The atomic structure and the magnetic properties of 5 and 10 at. % Co-doped ZnO samples prepared by a high-pressure and high-temperature method have been investigated by various techniques, including x-ray diffraction with Rietveld refinements, scanning electron microscopy, high-resolution transmission electron microscopy, nanometer-sized element mapping, x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and extended x-ray absorption fine structure and magnetization measurements. It is found that Co ions with a valence of 2+ substitute Zn ions in the wurtzite ZnO structure. No metallic or oxidic Co-rich clusters were detected in the samples. If Co2+ ions substitute Zn2+ ions, no intrinsic ferromagnetism is observed in ZnO at doping levels up to 10 at. % Co even down to 5 K. A paramagnetic behavior with partially antiferromagnetic interactions is observed. Thus, in bulk Co-doped ZnO system any observation of ferromagnetism is probably due to small clusters of second phase materials. However, in thin films the situation might be more complex, since other extrinsic influences, such as strain or proximity effects, may modify the electronic and magnetic properties.