Hard magnetic Sm(Fe,Si)9 carbides:: Structured and magnetic properties -: art. no. 064402

The structural and magnetic properties of the metastable hexagonal SmFe9-xSixC 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 Mossbauer 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 Sm-2(Fe,Si)(17)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 delta{2e}>delta{3g}>delta{6l} is used to deduce the following sequence of hyperfine field H-HF{2e}>H-HF{6l}>H-HF{3g}. From Sm(Fe,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 SmFe8.75Si0.25C with annealing temperature of the noncarbonated powder at 750degreesC. However, SmFe8.5Si0.5C, with 13 kOe and an optimal grain size around 22 nm, presents a better thermal stability and might be suitable for permanent-magnet applications.