Using x-ray magnetic circular dichroism XMCD, x-ray absorption spectroscopy XAS, and energy filtered transmission electron microscopy, we provide evidence for a heterogeneous Co distribution in anatase Co:TiO2 magnetic semiconductor having a low Co concentration 1.4 at. %. Multiplet structure due to Co2+ is observed in XAS spectra, but suppressed due to the coexistence of metallic Co clusters and substitutional Co ions, whereas XMCD spectra resemble metallic Co. The presence of Co2+ can be correlated with earlier observations of impurity band conduction and Kondo behavior, whereas metallic Co clusters contribute to the ferromagnetism and anomalous Hall effect.
Lee, Y. J., de Jong, M. P., & Jansen, R. (2010). Magnetism and heterogeneity of Co in anatase Co:TiO2 magnetic semiconductor. Applied physics letters, 96(8), 0825061-0825063. https://doi.org/10.1063/1.3327805