Valence state of Eu ions in C60 fullerides

We have prepared Eu-intercalated C60 films in ultra-high vacuum, with special attention to avoid oxygen contamination. X-ray photoelectron spectra were measured in situ. Eu 4f photoemission reveals that the valence state of Eu ions is divalent for all intercalation levels. C 1s photoemission exhibits evidence of both ionic and covalent contributions to the Eu–C60 bonding. The results eliminate the controversy about the valence state of Eu ions in the literature, and can help to understand the ferromagnetism (FM) and the giant magnetoresistance (GMR) of Eu6C60.