Negative exchange interactions and Curie temperatures for ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$sand ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$ ${\mathrm{N}}_{\mathrm{y}}$s

Mössbauer spectra of ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$ and ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$ ${\mathrm{N}}_{\mathrm{y}}$ in a temperature range between 77 K and the Curie temperature have been studied. The exchange integrals, as a function of the distances between the Fe-Fe pairs, have been found by fitting the hyperfine field data at various temperatures for each Fe site. The results show that there exist both positive and negative exchange interactions for ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$ and ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$ ${\mathrm{N}}_{\mathrm{y}}$ . The magnitude of the negative interaction leads to a significant difference in the Curie temperature for the two materials. Based on a model that considers the competition between the positive and negative exchange interactions, the Curie temperatures of some 2:17-type compounds have been calculated; the results agree well with the experimental values.