Anatase and rutile TiO2-δ powders were synthesized via the sol-gel method. The hysteresis loops and the zero field-cooled and the field-cooled magnetization curves indicate that reduced TiO2-δ powders exhibit the room-temperature ferromagnetism that becomes stronger with prolonging annealing time in H2/Ar mixture. Analysis of Ti 2p X-ray photoelectron spectroscopy spectra indicates that Ti ions were all in the Ti4+ state and that Ti3+ or Ti2+ ions do not exist in all samples. In addition, analysis of O 1s X-ray photo-electron spectroscopy spectra indicates that the concentration of oxygen vacancies increased with prolonging annealing time. Analysis of ultraviolet-visible absorption spectra also further confirmed that the concentration of oxygen vacancies increased with prolonged annealing time. These results indicated that ferromagnetism in pure TiO2-δ powders arose from oxygen vacancies.

Activation of Room-Temperature Ferromagnetism in Non-Stoichiometric TiO2-δ Powders by Oxygen Vacancies. Zhao, Q., Wu, P., Li, B.L., Lu, Z.M., Jiang, E.Y.: Journal of Applied Physics, 2008, 104[7], 073911