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

The high content of ferromagnetic phase content delta 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, delta enhancement must be done using the annealing process. Since the ferromagnetic phase of MnBi is formed only at the temperature lower than 340 degrees C, the isothermal low-temperature anneal can enhance delta 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 delta and thus the spontaneous magnetization M-s of MnBi alloys. By this annealing process at 290 degrees C superposed by the temperature gradient gradT=1-3 degrees C/cm for 20 h, in comparison with the isothermal annealed sample, the content delta is enhanced from the value of 60.6 to 89.9 wt% resulting the M-s 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.