Signature of strong ferromagnetism and optical properties of Co doped TiO2 nanoparticles

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Abstract
We report on the high temperature ferromagnetism of Co doped TiO2 nanoparticles (NPs) grown by a ball milling method with two different Co concentrations (3% and 8%). We also investigated the structural and optical properties of as-grown NPs to identify the defects which are responsible for the observed ferromagnetism (FM) at and above room temperature (RT). The obtained NPs of sizes 35–50 nm were identified as the anatase TiO2 structure with no signature of Co-cluster or any other oxides of Co by x-ray diffraction and high resolution transmission electron microscopy analysis. Raman scattering was employed to observe the defect related shifting and broadening of most intense Eg(1) mode at 142 cm−1 and higher frequency Eg(3) mode at 638 cm−1. UV-visible absorption spectra show that the incorporation of Co into the TiO2 lattice leads to redshift in the optical response, as well as lowering of bandgap energy. Photoluminescence study further confirms the doping of Co atoms into the TiO2 lattice as evidenced by redshifting of the band edge emission peak. The ferromagnetic behavior was clearly observed at room temperature in doped samples with saturation magnetization (Ms) of the order of 2.79–7.39 emu/g depending on the dopant concentrations. Importantly, the saturation magnetization is found to be decreased when the samples were post-annealed at 300 °C in air atmosphere, which might be due to reduction of oxygen vacancies in TiO2. Temperature dependence magnetization (M-T) shows sharp ferromagnetic to paramagnetic transition with a high Curie temperature (Tc) of ∼793 K for 3% doped NPs, whereas 8% doped sample does not exhibit a sharp magnetic transition. We believe that the observed FM at and above RT is most probably associated with defects related to oxygen vacancies.

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