Abstract
Amorphous and crystalline Al2O3 nanoparticles were synthesized by a sol–gel method with postannealing at different temperatures. Magnetism measurements have indicated that all Al2O3 nanoparticles exhibit intrinsic room temperature ferromagnetism, and the saturation magnetism of the samples increases after vacuum annealing, whereas bulk Al2O3 presents paramagnetism. Electron spin resonance and fitting results of O 1s X-ray photoelectron spectroscopy reveal that the origin of the ferromagnetism in Al2O3 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.
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