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Pt-4f and Fe-2p XPS spectra of 9.8 nm FePt particles before and after exposure to ambient air for 24 h (Fe-2p spectrum is vertically off-set for clarity). After exposure the majority of Fe atoms are found in the Fe3+ state, while Pt atoms remain metallic. All spectra were smoothed by 3-point moving window averaging and no further normalization was performed. Green solid curves through Fe-2p spectra in the lower panel are fits based on Fe0 and Fe3+ reference spectra.
