$\mathrm{L}{1}_0$ $\mathrm{Mn}\mathrm{Al}$ permanent-magnet alloys are promising candidates to plug the gap in performance and applications between rare-earth permanent magnets and ferrites; doping with a third atom is essential to achieve decent magnetic properties and good $\mathrm{L}{1}_0$-phase stability. In this work, the influence of substitutional atoms on intrinsic magnetic properties in $\mathrm{L}{1}_0$ $\mathrm{Mn}\mathrm{Al}$ alloys is investigated by combined experimental and theoretical studies. A highly pure $\mathrm{L}{1}_0$ phase is synthesized in ${\mathrm{Mn}}_{55}{\mathrm{Al}}_{45-x}{M}_x$ ($M$ = $\mathrm{Co}$, $\mathrm{Cu}$, $\mathrm{Ga}$) alloys by regulation of the heat treatment, on the basis of ternary phase diagrams. It is demonstrated by experimental measurements and calculations that $\mathrm{Cu}$ and $\mathrm{Co}$ tend to enter 1a (0, 0, 0) sites, where they degrade the intrinsic magnetic properties of the alloys, and $\mathrm{Ga}$ prefers the 1d (1/2, 1/2, 1/2) sites, where it improves them. The excess valence electrons of $\mathrm{Cu}$ or $\mathrm{Co}$ compared with $\mathrm{Mn}$ lead to stronger bonding with $\mathrm{Al}$, resulting in the preference of occupying 1a (0, 0, 0) sites. The similar valence-electron structure of $\mathrm{Ga}$ compared with $\mathrm{Al}$ makes $\mathrm{Ga}$ tend to occupy 1d (1/2, 1/2, 1/2) sites. On the basis of this understanding, we propose a strategy of replacing the antiferromagnetic $\mathrm{Mn}$ atoms by 3d atoms with fewer valence electrons than $\mathrm{Mn}$, which may effectively enhance the intrinsic magnetic properties. Moreover, the stability of the alloyed $\mathrm{L}{1}_0$ phase is investigated; $\mathrm{Co}$ is found to degrade the stability of the phase, but the addition of $\mathrm{Cu}$ or $\mathrm{Ga}$ can stabilize it. These results provide guidance for further performance optimization and composition design of $\mathrm{Mn}$-based rare-earth permanent magnets.

• Received 3 January 2019
• Revised 11 March 2019

DOI:https://doi.org/10.1103/PhysRevApplied.11.064008