Room temperature ferromagnetism of pure ZnO nanoparticles

We report the room temperature ferromagnetism (RTF) of pure ZnO nanoparticles, which were prepared by coprecipitation method. Magnetization measurement indicates that the ZnO nanoparticles annealed in air at 450, 550, 650, and $800°C$ exhibit the RTF and the decrease in the ferromagnetism is performed with the increase in annealed temperature. Selected area electron diffraction, x-ray diffraction, and x-ray photoelectron spectroscopy measurements show that all the samples possess a typical wurtzite structure and no other impurity phases are observed. The results of the Raman spectra indicate that there are lots of defects existing in the fabricated samples. It is also found that the ferromagnetism of ZnO nanoparticles increases after annealing in vacuum condition and decreases after annealing in a rich-oxygen atmosphere. These results confirm that the oxygen vacancies play an important role in introducing ferromagnetism for the ZnO nanoparticles in our case.