Intrinsic ferromagnetism in CeO2: dispelling the myth of vacancy site localization mediated superexchange

Intrinsic ferromagnetism in CeO2 is a source of controversy in the literature and has been linked to the excess electrons left over upon oxygen vacancy formation on Ce sites neighbouring the vacancy. A recent theoretical study (Han et al 2009 Phys. Rev. B 79 100403) concluded that increased vacancy concentration changes the localization behaviour of CeO2, resulting in some degree of charge localization in the vacancy site itself, which leads to superexchange and polarization effects that enhance the stability of ferromagnetism. In this report, we show conclusively that oxygen vacancy concentrations of up to 12.5% do not cause localization in the vacancy site, and that this is not responsible for any enhanced ferromagnetism. Investigation of oxygen vacancies on the (111), (110) and (100) low index surfaces also show no evidence for ferromagnetic preference.