Microstructure and magnetic properties of MnBi powders prepared by various ball milling processes

The MnBi intermetallic compound with high purity of low-temperature phase MnBi (LTP-MnBi) and high maximum energy product (BH)max were successfully prepared using various ball milling processes. The effects of ball milling speed and duration on the microstructures, phase compositions, and magnetic properties of MnBi alloys were systematically investigated. The particle size of MnBi and the purity of LTP-MnBi gradually decreased with increasing ball milling speed. An optimized ball milling speed of 175 rpm resulted in MnBi powder with the (BH)max of 9.22 MGOe and a squareness (M r/M s) of 93.7%. Furthermore, the ball milling duration was optimized to observe its impact on the MnBi alloy. As the duration increased, the average particle size of MnBi decreased from 27 mu m at 0 h to 1.84 mu m at 10 h. Simultaneously, the coercivity (H c) increased from 0.7 T at 0 h to 1.7 T at 10 h. Ultimately, the powder ball milled at 175 rpm for 5 h exhibited the highest (BH)max of 11.3 MGOe.