Role of electronic structure and oxygen defects in driving ferromagnetism in nondoped bulk $CeO2$

This article reports on reversible manipulation of room temperature ferromagnetism (RTFM) in nondoped bulk $CeO2$⁠. The magnetization measurements establish that paramagnetic $CeO2$ is driven to a ferromagnetic state, without change in structure, when vacuum annealed at $600°C$⁠. The Ce ions transform from $4+$ to $3+$ state, accompanied by evolution of oxygen vacancies $(VO)$ during the RTFM transition, as determined by x-ray photoemission. The $F+$ centers (the electrons in singly occupied oxygen vacancies) play key role in the exchange mechanism. The transition shows complete reversibility where the RTFM is removed by removing the vacancies through re-heating the vacuum-annealed $CeO2$ in air.