Enhanced formation and improved thermal stability of ferromagnetic τ phase in nanocrystalline Mn55Al45 alloys by Co addition

MnAl alloy (τ phase) is one of the most promising rare-earth-free permanent magnets due to its remarkable magnetic properties. The metastable τ phase is normally produced by rapid quenching technique and subsequent heat treatments. In this work, the direct formation of pure τ phase in (Mn_0.55Al_0.45)₉₇Co₃ alloy was achieved by using the melt-spinning process without heat treatments, which showed the magnetization of 5.0 × 10⁵ A/m, coercivity of 125.6 kA/m and maximum energy product of 6.2 kJ/m³. It was found that the negative mixing enthalpy and the different atomic radius between Co and Mn, Al played a key role in improving the formation and stability of the τ phase in MnAlCo alloys. As a result, Co doping promoted the formation of and enhanced the stability of the τ phase in MnAl alloy. The present results will be helpful for the direct formation of ferromagnetic τ phase in MnAl rare-earth-free permanent magnet.