Size dependent ferromagnetism in cerium oxide (CeO2) nanostructures independent of oxygen vacancies

Nanosized cerium oxide (CeO2) powders of different sizes were synthesized through homogeneous precipitation. Ferromagnetism was observed only in sub-20 nm powders. Ferromagnetic transition temperatures above room temperature and saturation magnetization values of 0.023 mu(B) per CeO2 formula unit were measured. Chemical analysis of the powders showed that trace ferromagnetic impurities could not be responsible for the magnetic signals. Carefully controlled coulometric cyclic annealing studies combined with photoluminescence measurements showed that oxygen vacancies do not mediate the ferromagnetism in the samples.