A combined XRD, Moessbauer, STXM and NMR study of surface oxidised, ball milled Fe nano-powders exhibiting magnetoresistive properties is presented. The XRD patterns show the peaks of bcc-Fe phase with linewidths increasing with increasing milling time. This is attributed to a decrease of the effective size of crystallites and increased strain due to accumulation of defects. Thermal treatment causes a reduction of the line-widths to their initial values from before milling. A similar effect is observed for the line-widths in the Moessbauer spectra, whereas other parameters, i.e. hyperfine field, quadrupole splitting and isomer shift remain unchanged. Scanning Transmission X-ray Microscopy (STXM) measurements provided the oxygen maps of the particles and, hence, the information on the distribution of oxides on particles surface. The O;K and Fe:L₂,L₃ near-edge absorption spectra (XANES) of selected areas of particle surface show differences between oxides in samples prepared under different conditions. The ⁵⁷Fe spin echo NMR spectra consist of a dominant resonant line corresponding to bcc-Fe core and a much weaker resonance corresponding to the iron oxides in the surface layer of nanometric thickness. The resonant frequency of the main line of the oxide layer is slightly lower than that of the bulk magnetite. The effect can be attributed to the exchange coupling between the oxide layer and the bcc-Fe core of the particle and/or to the presence of strain in the oxide layer. A lack of the signals from oxides in the Moessbauer spectra can be attributed to a small thickness of the oxide layers and the related low probability for the recoiless absorption. The studies constitute a detailed investigation of the nature of the iron oxide surface layer and its interaction with the iron core in ball milled iron nanoparticles.

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