Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides

Room-temperature ferromagnetism has been observed in nanoparticles (7-30 nm diam) of nonmagnetic oxides such as CeO2, Al2O3, ZnO, In2O3, and SnO2. The saturated magnetic moments in CeO2 and Al2O3 nanoparticles are comparable to those observed in transition-metal-doped wideband semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanoparticles of metal oxides.