Effect of Mg content on the microstructure and magnetic properties of rare-earth-free MnBi alloys

The low-temperature phase MnBi (LTP-MnBi) has become one of the most promising rare-earth-free permanent magnetic alloys owing to its unique positive temperature coefficient of coercivity. However, so far, the maximum energy product of MnBi alloys is still lower than its theoretical value. In this study, Mn55Bi45-xMgx (x = 0, 1, 2, 3, and 4) alloys were prepared by induction melting followed by an annealing process. The effects of Mg content on the microstructure and magnetic properties of MnBi alloys were investigated. The results showed that the partial substitution of Mg for Bi inhibited the formation of LTP-MnBi. With the increase of Mg content, the coercivity increases from 8.1 kOe for x = 0 to 10.0 kOe for x = 4, but the saturation magnetization decreased from 71.7 emu/g for x = 0 to 61.3 emu/g for x = 4. Finally, the calculated maximum energy product of 11.1 MGOe was obtained in the annealed Mn55Bi43Mg2 powder.