| Popis: |
A detailed NMR, magnetization, x-ray, and neutron-diffraction study is reported for the ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ system over the entire range of Co concentration ($0\ensuremath{\le}x\ensuremath{\le}3.00$). 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 $x=2.15$. It is found that Co selectively enters the ($A$,$C$) sites for the higher concentrations studied in this work ($x\ensuremath{\le}2.00$) with a high degree of order for $xl1.50$. 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 $\mathrm{Fe}(B)$, $\mathrm{Fe}(A,C)$, and $\mathrm{Co}(A,C)$ 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 (+1.7${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) throughout the range $0\ensuremath{\le}x\ensuremath{\le}2.15$. In addition, the variations of the internal hyperfine fields with Co concentration at all sites [$\mathrm{Fe}(B)$, $\mathrm{Fe}(A,C)$, $\mathrm{Co}(A,C)$, and $\mathrm{Si}(D)$] 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 $\mathrm{Fe}(B)$ 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. |
