Oxygen-exposure mediated electron transfer and d0 ferromagnetism in metal oxide nanocrystals system

- The d0 ferromagnetism origin in CeO2 was studied by O-rich treatments.
- Both H2O2 treatment and O2 exposure significantly boost the dilute ferromagnetism.
- We illustrated that the interstitial oxygen facilitated the electron transfer and the dilute ferromagnetism of CeO2.

The room temperature d0 ferromagnetism origin is still highly controversial in oxide systems, due to the lack of subtle experimental set-up as well as sensitive defect detection. Herein, we developed O-rich treatments on Ceria nanocrystals, H2O2 and O2 ambient exposure, which creates intrinsic O defects without introducing trace level of impurities or changing crystallinity. Interestingly, results found that significant oxygen adsorption along with distinct augment of RT ferromagnetism were observed after both H2O2 treatment and O2 ambient exposure. The more the oxygen molecule adsorbed, the stronger the ferromagnetism was. Furthermore, Electron Spin Resonance (EPR) results illustrate that the Ce3+ concentration experienced reverse variation in H2O2 treated-and O2 exposed-samples, thus excluding the possibility of Ce cation as the cause of the ferromagnetism. Similar magnetism enhancements were also observed in La2O3, ZrO2 and SrTiO3 nanocrystals after H2O2 treatment. Based on these results, we surmise that the interstitial oxygen, rather than the other sources, facilitated the electron transfer and the dilute ferromagnetism of CeO2. This work established a general model in clarifying the origin of the disputed ferromagnetism in metal oxide system.