Recent theoretical predictions have shown that transition metal doped ZnO can be ferromagnetic with a Curie temperature above room temperature. However experimentally diverse magnetic properties from paramagnetic behavior to ferromagnetism over large temperature range are observed. A common question is whether the magnetic properties are intrinsic, or arise from segregation phases from transition metal clusters.

In this paper we investigate cobalt doped ZnO thin films prepared via sol-gel methods from different groups. Within the concerned doping concentration (0.5%-10%) XRD shows only the diffraction peaks from wurtzite ZnO without secondary phase, as widely reported in literature. Magnetic measurements of the corresponding samples show evidence for ferromagnetic behavior in highly doped samples (> 5%), while normal paramagnetism is found in the lowly doped films. Raman spectroscopy reveals the existence of cobalt oxides like CoO and Co₃O₄ already in intermediately doped samples. In addition, the Raman shifts of the doped samples exhibit a shift to higher energy with increasing Co doping content. Presently temperature dependent Raman measurements are under way so as to clarify the phase transition of cobalt oxides. The influence of the segregation phases on the magnetic properties of ZnO:Co thin films will be extensively studied.

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