Mössbauer spectrometry study of the hyperfine fields and electronic structure of Fe-Co alloys

We obtained 57Fe hyperfine-field parameters from Mössbauer spectra of Fe-Co alloys at 77 and 295 K. We also obtained the numbers of 3d electrons at Fe atoms by electron-energy-loss spectrometry (EELS). Together with bulk-magnetization data, we deduce how the occupancies of the 3d↑ and 3d↓ states at Fe atoms change with Co concentration. Changes in the total numbers of 3d electrons at Fe atoms were found to be small, with the Mössbauer data indicating a maximum of about 0.2 electrons at 30 at. % Co, and the EELS data indicating a change of similar magnitude but of opposite sign. From both our isomer-shift and hyperfine-magnetic-field (HMF) data, we deduce a slight reduction in the occupancy of 4s states with Co concentration of dn4s/dc=-0.02 electrons, which is principally due to a loss of 4s↓ electrons. The model employed for the calculation of HMF distributions in terms of conduction- and core-electron polarizations worked well for disordered Fe-Co alloys, and was used to analyze the changes in HMF after the disorder→order transformation. It showed that these HMF changes upon ordering are primarily due to changes in conduction-electron polarization.