Mössbauer study of permanent-magnet R₂Fe_17-xM_xC_y compounds

We report Fe⁵⁷ Mössbauer spectroscopy studies of the effect of carbon doping on the magnetic properties of the Fe sites of R₂Fe_17-xM_xC_y, and of the presence of multiple phases in these systems. In this study two systems, Sm₂Fe₁₄Al₃C_y, where y=0, 1.0, 2.0, and 3.0, and Nd₂Fe_14.54Si_2.46C_y, where y=0, 0.432, and 2.544, were measured at room temperature and analyzed. The ternary compounds R₂Fe_17-xM_x-C_y studied here have the rhombohedral Th₂Zn₁₇ structure with c-axis anisotropy. The Al and Si site occupation used in the fitting were taken from previous results. Analysis of the Mössbauer spectra showed that the hyperfine fields for the different sites decreased in the order 6c, 9d, 18f, and 18h. The average hyperfine field for Sm₂Fe₁₄Al₃C_y was found to increase with increasing carbon concentration, which is in qualitative agreement with magnetic measurements. For Nd₂Fe_14.54Si_2.46C_y. the average hyperfine field was found to increase for low C concentration and decrease for high C concentration; this behavior is in agreement with our magnetization measurements. The measured average isomer shift relative to α-iron was found to increase with y. In these compounds alloying with C and/or M can improve the magnetic properties such as Curie temperature and magnetization which make them potential candidates for permanent magnet development.