Clustering of impurity atoms in Co-doped anatase TiO2 thin films probed with soft x-ray fluorescence

The electronic structure of Co-doped anatase TiO2 epitaxial thin films grown at different partial oxygen pressures is investigated using soft x-ray emission spectroscopy. The resonantly excited Co L2,3 x-ray emission spectra of ferromagnetic Ti0.96Co0.04O2 samples for the oxygen-deficient regime show that the ratio of integral intensities for Co L2 and L3 emission lines significantly decreases with respect to nonmagnetic samples in the oxygen-rich regime. This is due to L2L3M4,5 Coster-Kronig transitions and suggests that ferromagnetic Ti0.96Co0.04O2 samples have n-type charge carriers and Co-Co bonds between substitutional and interstitial Co atoms are present while Co-O bonds are dominant in nonmagnetic Ti0.96Co0.04O2 samples in the oxygen-rich regime. Electronic structure calculations show that the presence of free charge carriers and Co segregation play a crucial role in strong ferromagnetism at room temperature in Co-doped TiO2.