Fabrication of MnBi alloys with high ferromagnetic phase content: effects of heat treatment regimes and dopants

The low-cost and rare-earth-free MnBi alloys were focused for investigation during last 5 years as a promising alternative for high-cost rare-earth-containing Nd–Fe–B in high-temperature permanent magnet applications. Because of the peritectic nature of solidification, MnBi alloys containing high ferromagnetic phase content could not be easily prepared for massive scale. In this paper, the roles of heat treatment regimes including the zone-travelling under temperature gradient, multi-time annealing (MTA) and the doping soft ferromagnetic Fe₆₅Co₃₅ phase in preparing MnBi alloys with high ferromagnetic phase content were investigated. The results showed that, for the case of applying the temperature gradient assisted zone-travelling technique, the ferromagnetic phase content in prepared alloys reached 85 wt%. By using the MTA technique, the ferromagnetic phase content is raised up 96 wt% when MnBi alloys were annealed three times at 300 °C for 20 h. The doping of Fe₆₅Co₃₅ as a soft ferromagnetic phase stabilized the ferromagnetic phase in the obtained MnBi_1−x(Fe₆₅Co₃₅)_x alloys (x = 0.1, 0.2, 0.3) with the magnetization measured at 13.5 kOe of 63.1, 70.8 and 80.5 emu/g, respectively.