Nitrogen as the best interstitial dopant among $X$=B, C, N, O and F for strong permanent magnet NdFe${}_{11}$Ti$X$: First-principles study

We study magnetic properties of NdFe${}_{11}$Ti$X$, where $X$=B, C, N, O, and F, by using the first-principles calculation based on the density functional theory. Its parent compound NdFe${}_{11}$Ti has the ThMn${}_{12}$ structure, which has the symmetry of space group $I4\mathrm{/}mmm$, No. 139. The magnetization increases by doping B, C, N, O, and F at the $2b$ site of the ThMn${}_{12}$ structure. The amount of the increase is larger for $X$=N, O, F than for $X$=B, C. On the other hand, the crystal field parameter $⟨r^2⟩A_0^2$, which controls the axial magnetic anisotropy of the Nd $4f$ magnetic moment, depends differently on the dopant. With increase of the atomic number from $X$=B, $⟨r^2⟩A_0^2$ increases, takes a maximum value for $X$=N, and then turns to decrease. This suggests that in NdFe${}_{11}$Ti$X$, nitrogen is the most appropriate dopant among B, C, N, O, and F for permanent magnets in terms of magnetization and anisotropy. The above calculated properties are explained based on the detailed analysis of the electronic structures of NdFe${}_{11}$Ti$X$.