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Flowerlike CuO nanostructures were prepared by the coprecipitation method with postannealing in air at different temperatures. The results of X-ray diffraction and Raman and X-ray photoelectron spectroscopies show that the samples annealed at 400, 600, and 800 °C have a typical monoclinic structure and are absent of impurity phases. Magnetic measurements indicate that all of the CuO nanostructures show room-temperature ferromagnetism, whereas CuO bulk presents paramagnetism. The saturation magnetization of the samples was found to increase with increasing annealing temperature. The fitting results of the O 1s XPS spectra for the three samples indicate that oxygen vacancies exist in the samples and that the variation of the oxygen vacancy concentration is in complete agreement with the variation of the saturation magnetization. When the samples were annealed in oxygen atmosphere, the ferromagnetism of the samples decreased enormously. These results confirm that the observed room-temperature ferromagnetism in flowerlike CuO nanostructures might originate from oxygen vacancies.
Gao et al. (2010) studied this question.