Evidence of Oxygen Vacancies Enhancing the Room Temperature Ferromagnetism in CeO $_{2{-}{\rm x}}$ Nanopowders

To investigate whether oxygen vacancies play an important role in the origin of ferromagnetism in undoped nanosized metal oxides, CeO_2-x nanopowders were prepared using sol-gel method, and anneal studies under various atmosphere were carried out on samples. The room temperature ferromagnetism in the samples was enhanced after being hydrogenated at 320degC in a forming gas (Ar90%+H₂10%) and weakened after reheating the sample in air. These variations of the magnetization have been observed for additional cycles by alternately heating in air and Ar/H₂. Raman spectroscopy study reveals that hydrogenation leads an increase in the amount of oxygen vacancies in these H₂ -annealed samples. It is demonstrated that this observed ferromagnetism is intrinsic and affected by the oxygen vacancies which are produced by hydrogenation but reduced by oxidation. A correlation of ferromagnetism with concentration of oxygen vacancies is obtained in CeO_2-x nanopowders.