Adaptation of High-Energy Ball Milling and Heat Treatment Conditions for Improving Phase Purity of LTP-MnBi

The low-temperature phase (LTP) of MnBi is considered a viable alternative to rare earth permanent magnets. However, the synthesis of a single-phase LTP-MnBi with a specific crystallographic form is difficult because slight changes in process parameters can produce considerable variations in the crystallographic forms of the compound. In this work, a high-energy ball milling and subsequent heat treatment technique were employed to prepare highly pure LTP-MnBi. A purity of ~96 wt% of LTP-MnBi was achieved by two-stage milling and tailoring the heat treatment schedule. The effects of the heat treatment and the ball milling conditions on the magnetic performances of the material were investigated by analyzing the phase purity and the grain sizes of the sample. This high-purity LTP-MnBi exhibits the highest saturation magnetization of 63.47 emu/g, and its coercivity was measured to be 5.53 kOe.