Room-Temperature Ferromagnetism in Semiconducting TiO_2-δ Nanoparticles

TiO_2-δ nanoparticles are synthesized by the sol-gel method and annealed under different reducing atmosphere. The x-ray diffraction patterns show that
anatase is the dominant phase with small amounts of the rutile phase of TiO_2-δ for all the samples. Magnetic measurements indicate that the samples annealed in reducing atmosphere exhibit inprecedented room-temperature ferromagnetism, in particular, the saturation magnetization Ms is up to about 8.6×10^-3emu/g for the sample annealed in H₂/Ar mixture. Analysis of the x-ray photoelectron spectroscopy spectra for the samples processed under
different conditions indicates that the amounts of Ti³⁺ or Ti²⁺ cations, namely, the concentration of oxygen vacancies, increase with intensifying reducing atmosphere during processing, which shows that ferromagnetism in this material strongly depends on the concentration of oxygen vacancies. The relationships between the ferromagnetism and the crystal structure as well as the grain size in this material are also discussed.