Enhancement of Ferromagnetism in CeO2 Nanoparticles by Nonmagnetic Cr3+ Doping

This study presents the effect of Cr3+ on the formation of ferromagnetism in Cr-doped CeO2 nanoparticles (NPs). Systematic synchrotron X-ray spectroscopy analysis was utilized to investigate the defects in CeO2 NPs. X-ray absorption spectrum (XAS) revealed that the magnetic properties are correlated to the type and structure of defects. Both concentration of Ce3+ and the oxygen vacancy increased with increasing the content of Cr3+. The magnetism raises as the Cr increases and reaches maximum at Cr3+ approximate to 11% and, in turn, diminishes beyond this value. Interestingly, X-ray magnetic circular dichroism (XMCD) results indicate that ferromagnetism was contributed mainly by Ce3+ ions but not Cr3+. With comparing to the concentration dependence of Ce3+ in reduced undoped CeO2 nanoparticles, the relationships between the defects and magnetism were unraveled. The magnetism of CeO2 NPs was closely related to the oxygen deficiency. The major effect of doping Cr3+ on the formation of ferromagnetism in CeO2 nanoparticles is introducing the high degree of oxygen deficiency caused by the concomitant formation of Cr3+-V-O-Ce3+ bondings. At last, it is suggested that the higher M-s value may be attributed to lattice distortion caused by the smaller size of Cr3+.