Evidence of Vacancy-Induced Room Temperature Ferromagnetism in Amorphous and Crystalline Al₂O₃ Nanoparticles

Amorphous and crystalline Al 2 O 3 nanoparticles were synthesized by a sol–gel method with postannealing at different temperatures. Magnetism measurements have indicated that all Al 2 O 3 nanoparticles exhibit intrinsic room temperature ferromagnetism, and the saturation magnetism of the samples increases after vacuum annealing, whereas bulk Al 2 O 3 presents paramagnetism. Electron spin resonance and fitting results of O 1s X-ray photoelectron spectroscopy reveal that the origin of the ferromagnetism in Al 2 O 3 nanoparticles could be attributed to the singly charged oxygen vacancies (F + centers). The variation of the relative area of oxygen vacancies and the number of free electrons is consistent with the change of saturation magnetization for the samples. Combined with these results, a direct correlation of ferromagnetism with F + centers exchange mechanism is established.