Defect Structure Guided Room Temperature Ferromagnetism of doped CeO2 Nanoparticles
William Lee1*, Shih-Yun Chen1, Chung-Li Dong2, Hong-Ji Lin2, Chien-Te Chen2
1Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan
2National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan
* presenting author:李維烈,
In this study, the relationship between defect structure and magnetic behavior of CeO₂ nanoparticles (NPs) doped with Sm and Y was compared systematically by using spectroscopy and microscopy. The doping level ranges from 0% to 20%. At first, it is demonstrated that both type and doping level governs the formation of oxygen vacancy. At low doping level, both Y3+ and Sm3+ randomly distributed throughout the particle. However, as doping level above 9%, Y3+ and Sm3+ aggregate at surface to form Y-rich clusters and a (CexSm1-x)O2-y layer, respectively. In addition, a charge delocalization occurs upon Y doping on the Ce(Y)-O(VO)-Ce(Y) orbitals was demonstrated by Raman and X-ray absorption near edge (XANES) spectroscopy. Then, all these doped CeO₂ NPs were found to be ferromagnetic at room temperature. It is also demonstrated that the value of saturation magnetization (Ms) was closely related to defect structure. In Y-doped ones, Ms increases until Y reaches 9%, then it decreases. In Sm-doped ones, the maximum Ms was obtained as doping level reaches 15%. Notably, the magnetism is evidenced by X-ray magnetic circular dichroism (XMCD) spectroscopy only on Ce orbitals. Combined with the results of XANES, the magnetism intensity is thus attributed to the amount of Ce3+ at the surface. These features plead for the presence of a defect band at the surface, which was believed to be the origin of the ferromagnetism.

Keywords: CeO2, defect, magnetism, nanoparticles