Electron paramagnetic resonance measurements were presented to investigate paramagnetic oxygen vacancies (F+ centers) in Co-doped TiO2 nanocrystals. Surface and interior F+ centers were manipulated by washing or/and annealing. Anisotropic surface F+ centers were so instable as to be destroyed at high temperature, while isotropic interior F+ centers were rather stable even at 430C. The correlation of ferromagnetism with F+ centers was observed, which provided direct experimental evidence for F+-center-mediated coupling in diluted magnetic oxides.

Observation and Manipulation of Paramagnetic Oxygen Vacancies in Co-Doped TiO2 Nanocrystals. Pan, D., Xu, G., Lv, L., Yong, Y., Wang, X., Wan, J., Wang, G., Sui, Y.: Applied Physics Letters, 2006, 89[8], 082510