Reversible paramagnetism to ferromagnetism in transition metal-doped TiO2 nanocrystals prepared by microwave irradiation.

TiO2 nanoparticles doped with 1%, 5%, and 10% M (M=Co, Fe, and Ni) were prepd. by microwave irradn. and characterized using x-ray diffraction, transmission electron microscopy, and magnetometry. The as-prepd. samples are found to be paramagnetic at room temp., with the magnetic susceptibility following the Curie-Weiss law in the investigated range of 2-300 K. However, transformation from paramagnetism to room-temp. ferromagnetism (RTFM) was obsd. by hydrogenating the samples at 400 °C. Reheating in air converted the samples back to paramagnetic while rehydrogenating the samples again induced ferromagnetism. It is argued that the reversible RTFM obsd. is due to interaction between the dopant metal ions and oxygen vacancies produced during hydrogenation. X-ray diffraction of the hydrogenated Co- and Fe-doped samples shows only a single TiO2 phase suggesting that the obsd. RTFM may be intrinsic, but for the Ni-doped samples the magnetism may arise from metallic Ni on the surfaces of the TiO2 nanoparticles.