Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B: a first-principles study

We calculate intersite magnetic couplings for Sm$_2$Fe$_{17}$, Nd$_2$Fe$_{14}$ and Nd$_2$Fe$_{14}$X (X = B, C, N, O, F) using Liechtenstein's formula on the basis of first-principles calculation, and analyze them to investigate the Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B. We find that the magnetic coupling in the dumbbell bond is strongly ferromagnetic in our calculation, which is against a previous conjecture explaining the low Curie temperature for Sm$_2$Fe$_{17}$. The calculated values of the couplings explain the experimentally observed difference in the Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B. We also address boron's effects on the Curie temperature of Nd$_2$Fe$_{14}$B, especially in connection with Kanamori's theory of cobaltization.