Relating structural, magnetic-moment, and hyperfine-field behavior to a local-environment model inFe3−xCoxSi

A detailed NMR, magnetization, x-ray, and neutron-diffraction study is reported for the ${\mathrm{Fe}}_{3-x}{\mathrm{Co}}_x\mathrm{Si}$ system over the entire range of Co concentration ($0\le x\le 3.00$). As the Co concentration $x$ is increased, the x-ray measurements indicate that a single phase having the fcc $D{O}_3$-type structure is maintained (with some variation in lattice constant) up to $x=2.15\mathrm{}$. It is found that Co selectively enters the ($A$,$C$) sites for the higher concentrations studied in this work ($x\le 2.00$) with a high degree of order for $x<\mathrm{}1.50\mathrm{}$. 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 $\mathrm{Fe}\mathrm{}\left(B\right)\mathrm{}$, $\mathrm{Fe}\mathrm{}(A,C)\mathrm{}$, and $\mathrm{Co}\mathrm{}(A,C)\mathrm{}$ 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 (+1.7${\mathrm{\mu }}_{\mathit{B}}$) throughout the range $0\le x\le 2.15$. In addition, the variations of the internal hyperfine fields with Co concentration at all sites [$\mathrm{Fe}\mathrm{}\left(B\right)\mathrm{}$, $\mathrm{Fe}\mathrm{}(A,C)\mathrm{}$, $\mathrm{Co}\mathrm{}(A,C)\mathrm{}$, and $\mathrm{Si}\mathrm{}\left(D\right)\mathrm{}$] 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 $\mathrm{Fe}\mathrm{}\left(B\right)\mathrm{}$ 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.