Room-temperature ferromagnetism in pure and Co doped CeO(2) powders.

We report the room-temperature (RT) ferromagnetism (FM) observed in pure and Co doped CeO(2) powder. An insulating nonmagnetic CeO(2) single crystal, after grinding into fine powder, shows an RT-FM with a small magnetization of 0.0045 emu g(-1). However, the CeO(2) powder became paramagnetic after oxygen annealing, which strongly suggests an oxygen vacancy meditated FM ordering. Furthermore, by doping Co into CeO(2) powder the FM can significantly enhance through a F-centre exchange (FCE) coupling mechanism, in which both oxygen vacancies and magnetic ions are involved. As the Co content increases, the FM of Co doped CeO(2) initially increases to a maximum 0.47 emu g(-1), and then degrades very quickly. The complex correlation between the Co content and saturation magnetization was well interpreted by supposing the coexistence of three subsets of Co ions in CeO(2). Our results reveal that the large RT-FM observed in Co doped CeO(2) powder originates from a combination effect of oxygen vacancies and transition metal doping.