Simultaneous tuning of the magnetic anisotropy and thermal stability of $$\alpha ^{''}$$-phase Fe$$_{16}$$N$$_{2}$$

Simultaneously enhancing the uniaxial magnetic anisotropy ($$K_u$$ K u ) and thermal stability of $$\alpha ^{''}$$ α ′ ′ -phase Fe$$_{16}$$ 16 N$$_{2}$$ 2 without inclusion of heavy-metal or rare-earth (RE) elements has been a challenge over the years. Herein, through first-principles calculations and rigid-band analysis, significant enhancement of $$K_u$$ K u is proposed to be achievable through excess valence electrons in the Fe$$_{16}$$ 16 N$$_{2}$$ 2 unit cell. We demonstrate a persistent increase in $$K_u$$ K u up to 1.8 MJ m$$^{\text {-}3}$$ - 3 , a value three times that of 0.6 MJ m$$^{\text {-}3}$$ - 3 in $$\alpha ^{''}$$ α ′ ′ -Fe$$_{16}$$ 16 N$$_{2}$$ 2 , by simply replacing Fe with metal elements with more valence electrons (Co to Ga in the periodic table). A similar rigid-band argument is further adopted to reveal an extremely large $$K_u$$ K u up to 2.4 MJ m$$^{\text {-}3}$$ - 3 in (Fe$$_{0.5}$$ 0.5 Co$$_{0.5}$$ 0.5 )$$_{16}$$ 16 N$$_{2}$$ 2 obtained by replacing Co with Ni to Ga. Such a strong $$K_u$$ K u can also be achieved with the replacement by Al, which is isoelectronic to Ga, with simultaneous improvement of the phase stability. These results provide an instructive guideline for simultaneous manipulation of $$K_u$$ K u and the thermal stability in 3d-only metals for RE-free permanent magnet applications.