First-principles study of intersite magnetic couplings in NdFe${}_{12}$ and NdFe${}_{12}$X (X = B, C, N, O, F)

We present a first-principles investigation of NdFe${}_{12}$ and NdFe${}_{12}$X (X = B, C, N, O, F) crystals with the ThMn${}_{12}$ structure. Intersite magnetic couplings in these compounds, so-called exchange couplings, are estimated by Liechtenstein’s method. It is found that the Nd–Fe couplings are sensitive to the interstitial dopant X, with the Nd–Fe(8j) coupling in particular reduced significantly for X = N. This suggests that the magnetocrystalline anisotropy decays quickly with rising temperature in the X = N system although nitrogenation has advantages over the other dopants in terms of enhancing low-temperature magnetic properties. The Curie temperature is also calculated from the magnetic couplings by using the mean field approximation. Introduction of X enhances the Curie temperature, with both structural changes and chemical effects found to play important roles in this enhancement.