Spectroscopic investigation of the correlation between localization of electrons and ferromagnetism in CeO2 nanoparticles

Highlights: • Different V_O defect complex was obtained by doping with Pr³⁺ and performing annealing process. • Magnetic behavior was strongly correlated to differences in electron localization. • A normalization method was proposed and has been proven to be valid for several systems. Presence of oxygen vacancy (V_O) and the subsequent redistribution of electron are critical in the formation of ferromagnetism in oxides without magnetic impurities. However, the roles play by electron with diverse localization state in magnetism is still unclear. In this study, different V_O related defect complex was obtained in CeO₂ nanoparticles (NPs) by doping various content of Pr³⁺ and performing subsequent annealing process. Room temperature ferromagnetism was observed in all NPs. Combined systematic spectroscopy and microscopy analysis, it is demonstrated that magnetic behavior was strongly correlated to differences in electron localization. Two types of electron localization were revealed, which both result in the reduction of Ce ion. One is localized at the Ce ion adjacent to V_O, while the other distributes at farther shell. Ce³⁺ formed by the later one was found to play the major role in ferromagnetism. To estimate the amount of ferromagnetic active Ce³⁺ ion, a normalization method was proposed based on Raman and X-ray absorption spectroscopy analysis and has been proven to be valid for several systems, including un-doped and doped with Y/Pr/Sm CeO₂ NPs.