Temperature-gradient-driven annealing process for formation of MnBi ferromagnetic phase

The high content of ferromagnetic phase content δ is crucial in establishing high performance of MnBi magnets, which is the potential alternative for NdFeB magnetic material in high-temperature applications. Because of the peritectic solidification, the ferromagnetic phase of as-cast MnBi alloys is low in content and always assisted with Mn and Bi phases, and therefore, δ enhancement must be done using the annealing process. Since the ferromagnetic phase of MnBi is formed only at the temperature lower than 340 °C, the isothermal low-temperature anneal can enhance δ only moderately. In this paper, the temperature-gradient-driven annealing process has been applied to accelerate the formation of the MnBi ferromagnetic phase. This annealing process is based on the advection-assisted diffusion process which allows to accelerate the formation of ferromagnetic phase and to enhance its content δ and thus the spontaneous magnetization Ms of MnBi alloys. By this annealing process at 290 °C superposed by the temperature gradient gradT = 1–3 °C/cm for 20 h, in comparison with the isothermal annealed sample, the content δ is enhanced from the value of 60.6 to 89.9 wt% resulting the Ms enhancement from 43.0 to 64.1 emu/g. The mentioned annealing process was described by using the time-dependent one-dimensional diffusion–advection equation. The obtained analytical solution allows controlling the parameters of the process. So, the temperature-gradient-driven annealing process is promising for the large-scale production of high-LTP content MnBi alloys.