A Moessbauer spectroscopy study of isotopically enriched surface oxidised iron particles is presented. Powder samples enriched with 20% ⁵⁷Fe were prepared by ball milling and subsequently thermally treated in vacuum or air. The Moessbauer spectra have been measured in transmission mode at the temperatures between 4.2 K and room temperature. Conversion electron Moessbauer spectra at room temperature were also obtained. The CEMS spectrum of the sample annealed in air at 300 ^oC consists of the contributions of iron, magnetite, hematite and maghemite in the relative amounts of 55%, 34%, 8% and 3%, respectively. This shows that the dominant oxide forming at the surface on heating to 300 ^oC at ambient atmosphere is magnetite. The spectra of the sample measured in transmission show the bulk amount of 10% magnetite, 2% hematite and 88% of metallic bcc iron. A much smaller amount of oxides than that observed in the CEMS measurement is related to the difference of probing depths, which is of 300 nm in CEMS, whereas it is of a few micrometers in transmission an probes the bulk of the particles. This also proves that oxides are located mainly at the surfaces of the particles. The low temperature spectra of the thermally untreated and the vacuum annealed at 300 ^oC samples measured in transmission show the presence of oxides, mostly magnetite, in bulk amount below 1%, which corresponds to the oxide surface layer of nanometric thickness. Such a small amount of oxides could not be detected in the corresponding iron powders of natural isotopic abundance of ⁵⁷Fe, i.e. 2%. The Moessbauer signal from the oxide layer decreases with increasing temperature and disappears well below room temperature. The effect corresponds to a decrease of the probability for the recoiless absorption with increasing temperature and is much stronger than that in the bulk iron oxides. The observed features are analysed and a possible relation to the magnetotransport properties of the materials is discussed.

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