Hard magneticSm(Fe,Si)9carbides: Structured and magnetic properties

Abstract

The structural and magnetic properties of the metastable hexagonal ${\mathrm{SmFe}}_{9\ensuremath{-}x}{\mathrm{Si}}_{x}\mathrm{C}$ compounds, where x is 0.25, 0.5, 0.75, and 1, have been investigated by means of powder x-ray diffraction, Curie temperature, magnetic moment and coercivity measurements, iron-57 M\"ossbauer spectroscopy, and high-resolution transmission electron microscopy. The Rietveld analysis points out a lattice expansion after carbon insertion. Upon carbonation the Curie temperatures are systematically 26 to 70 K higher than those of the homologous ${\mathrm{Sm}}_{2}(\mathrm{Fe},\mathrm{Si}{)}_{17}{\mathrm{C}}_{2}.$ The magnetic moment per iron atom increases with x but remains below that of the non-carbonated alloys. The statistical occupation of silicon in $3g$ site and the random distribution of the $2e$ dumbbell iron atoms have been taken into account to calculate the Wigner-Seitz cell volumes, which rule the hyperfine parameter assignment. The following sequence of isomer shift $\ensuremath{\delta}{2e}>\ensuremath{\delta}{3g}>\ensuremath{\delta}{6l}$ is used to deduce the following sequence of hyperfine field ${H}_{\mathrm{HF}}{2e}>{H}_{\mathrm{HF}}{6l}>{H}_{\mathrm{HF}}{3g}.$ From $\mathrm{Sm}(\mathrm{Fe},\mathrm{Si}{)}_{9}$ to their carbides, the increase of isomer shift is less pronounced for the $2e$ site with no carbon neighbor. The small volume effect on the weighted average isomer shift may indicate hybridization between $2p$ of carbon and $3d$ of iron stronger than that of nitrides. The best coercivity of 15 kOe is obtained for ${\mathrm{SmFe}}_{8.75}{\mathrm{Si}}_{0.25}\mathrm{C}$ with annealing temperature of the noncarbonated powder at $750\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ However, ${\mathrm{SmFe}}_{8.5}{\mathrm{Si}}_{0.5}\mathrm{C},$ with 13 kOe and an optimal grain size around 22 nm, presents a better thermal stability and might be suitable for permanent-magnet applications.