RELATING STRUCTURAL, MAGNETIC-MOMENT, AND HYPERFINE-FIELD BEHAVIOR TO A LOCAL-ENVIRONMENT MODEL IN FE3-XCOXSI

A detailed NMR, magnetization, x-ray, and neutron-diffraction study is reported for the Fe3-xCoxSi system over the entire range of Co concentration (0≤x≤3.00). As the Co concentration x is increased, the x-ray measurements indicate that a single phase having the fcc DO3-type structure is maintained (with some variation in lattice constant) up to x=2.15. It is found that Co selectively enters the (A,C) sites for the higher concentrations studied in this work (x≤2.00) with a high degree of order for x<1.50. The variation in lattice constant with Co concentration correlates well with the bulk magnetization and can be described by a simple empirical relation which is an extension of Vegard's law. Neutron-diffraction and magnetization measurements have enabled a determination of the Fe(B), Fe(A,C), and Co(A,C) magnetic-moment dependence on Co concentration for these alloys. In particular, we note that the moment on the substituted Co atoms appears well localized and remains essentially constant (+1.7μB) throughout the range 0≤x≤2.15. In addition, the variations of the internal hyperfine fields with Co concentration at all sites [Fe(B), Fe(A,C), Co(A,C), and Si(D)] have been studied by spin-echo NMR. In order to explain the magnetic-moment and internal field behavior, a model emphasizing the short range interaction approach is presented. Since Co enters the (A,C) sites, the short range interaction model involves only the 1nn configurations for the Fe(B) atoms. The model successfully describes the detailed behavior of the magnetic moments and internal fields at all sites and enables a subdivision of the observed internal field into contributions due to the 4s spin polarization transferred from neighboring moments and the polarization resulting from the on-site moment. Such an approach has already proved successful for transition metal substitutions into the B sites. © 1979 The American Physical Society.