Phase Transformations in Mn–Al and Mn–Bi Magnets by Repeated Heat Treatment

A stepped heating cycle in a tube furnace at 1000 °C for 1 h followed by holding at 400 °C for 1 h directly increases the amount of ferromagnetic τ-phase of manganese–aluminum (Mn–Al). Repeating this heating regime for 2 more times, further increase in the amount and uniform distribution of the τ-phase is obtained at the expense of the e-phase. The magnetization and coercivity are increased after each heating cycle, and the effect is more pronounced when 2% carbon (C) is added to Mn–Al. When the similar heat treatment cycle is applied to manganese–bismuth (Mn–Bi), the ferromagnetic α-phase is detected with larger magnetizations, but substantial Mn and Bi are segregated from the alloy. This solid-state diffusion in tube furnace offers a facile route to improve rare-earth-free magnets.