Electronic structures and Curie temperatures of iron-based rare-earth permanent-magnet compounds

The modification of the electronic structures of ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17\mathrm{-}\mathit{x}}$ ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{N}}_{\mathit{y}}$, ${\mathrm{NdFe}}_{11}$ ${\mathrm{TiN}}_{\mathit{y}}$, and ${\mathrm{YFe}}_{12\mathrm{-}\mathit{x}}$ ${\mathrm{Mo}}_{\mathit{x}}$ upon alloying and nitriding are examined with self-consistent spin-polarized calculations and soft-x-ray photoemission measurements between 18 and 135 eV. The changes in the Curie temperature ${\mathit{T}}_{\mathit{c}}$ with substitutional modifications and nitrogen addition are modeled with self-consistent spin-polarized electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth which relates the electronic structure to ${\mathit{T}}_{\mathit{c}}$. The calculations show that the spin-summed density of states at the Fermi energy is related to ${\mathit{T}}_{\mathit{c}}$. The photoemission spectra are dominated by the Fe 3d electrons within 3 eV of the Fermi energy in agreement with calculations. Changes in the density of states at the Fermi energy for interstitial and and substitutional modification compare well with calculations. Using photoemission results with experimental magnetic moments for the substitutional modification of the compound ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17\mathrm{-}\mathit{x}}$ ${\mathrm{Al}}_{\mathit{x}}$, the spin-fluctuation theory predicts a change in ${\mathit{T}}_{\mathit{c}}$ in agreement with the measured change in ${\mathit{T}}_{\mathit{c}}$. Spin-resolved photoemission spectra for c-axis oriented ${\mathrm{Sm}}_2$ ${\mathrm{Fe}}_{17}$ ${\mathrm{N}}_{2.6}$ with magnetization perpendicular to the surface are presented and compared to theoretical calculations.