Local hyperfine fields in the sigma phase of Fe-Cr alloys

Full text: Around 50 at% Fe the α-phase (bcc) of Fe-Cr alloys exhibits strong ferromagnetism, the σ-phase (space group P42/mnm), formed as high temperature modification in this concentration range, shows, however, distinct delocalized behaviour. To gain information about the magnetic ground state we performed neutron diffraction, magnetic, and high field 57Fe Moessbauer investigations on a series of σ-FexCr1-x samples, covering the full concentration range in which this phase forms (x 0.5 to 0.56). The occupation numbers were obtained for the five crystallographically different lattice sites from Rietveld analysis of neutron diffraction measurements performed at ILL (D1A). Fe prefers lattice sites with high local symmetry, whereas Cr occupies preferentially sites, with lower symmetry. For the determination of the local hyperfine fields from the in field 57Fe Moessbauer investigations (up to 13.5 T, 4.2 K) a new scheme was developed, which takes into account the different lattice sites, respective occupation numbers, and nearest neighbour environments. Large differences in hyperfine field are found for the individual sites and also for different numbers of Cr atoms in the nearest neighbour shell. (author)