Synthesis and study of magnetic properties of Co-doped anatase TiO₂ nanoparticles

Anatase phase Ti_0.97Co_0.03O₂ nanoparticles were synthesized using two different preparation methods. The prepared samples were investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF) analysis, and magnetization measurements. The particle diameters were controlled by adjusting the annealing temperature, and they ranged from 4.2–7.0 nm for Ti_0.97Co_0.03O₂ encapsulated in amorphous SiO₂, and from 14–61 nm for Ti_0.97Co_0.03O₂ that was not encapsulated in amorphous SiO₂. Magnetization measurements were performed on the 7.0-nm sample from the former sample set, and on the 14-nm sample from the latter sample set, and both these samples exhibited an identical crystal structure and an equal number of magnetic ions, as confirmed by XRF and XRD measurements. The magnetization curves for the two samples indicated their ferromagnetic behavior at room temperature. However, the magnetic parameters of saturation magnetization M_S and coercive force H_C were clearly different for these two samples. We attribute this difference in magnetic properties between these samples to magnetic impurities present due to the different degrees of solubility of Co ions in the TiO₂ lattice.