Potential high-performance magnet materials: Co- and Al-alloyedSm2Fe17

Abstract

${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ has long been known as a potential high-performance magnet whose deficiencies---planar anisotropy and lower-than-optimal ${T}_{c}$---can be remedied by nitrogen addition, but which presents synthesis difficulties. Herein we apply first-principles calculations to search for alternative low-cost, high-performance permanent magnets in this family, by exploring simultaneous Fe and Al substitution. Specifically, the goal is to improve properties of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ easy-plane magnet at the stoichiometric composition. Density functional theory calculations were executed for three series of compounds, i.e., ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{14}{\mathrm{Al}}_{3}$, ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{15}{\mathrm{Al}}_{2}$, and ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{16}\mathrm{Al}$. We find that substitution of Fe with 12--18 of Co in $%\phantom{\rule{4pt}{0ex}}{\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ modifies the magnetic anisotropy type from easy plane to easy axis with a substantial anisotropy of 7.1 MJ/${\mathrm{m}}^{3}$. We also demonstrate that the largest part of magnetic anisotropy is introduced by $4f$ Sm atom electrons. Thus the rotation of magnetic moment orientation from $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ to $\ensuremath{\langle}111\ensuremath{\rangle}$ is followed by an increase of the occupied $4f$ state number and, as a result, the orbital part of the magnetic moment of one of the Sm atoms. This increase of the occupied $4f$ state number at an energy $\ensuremath{\sim}\ensuremath{-}$4.3 eV results in a significant reduction of band structure energy. The substitution of Fe by Co does not significantly reduce the magnetization of the compound and keeps it slightly above 1 T. This combination of magnetic anisotropy and magnetization makes the compound a promising candidate for a permanent magnet.