On the synthesis and microstructure analysis of high performance MnBi

Highly anisotropic MnBi powder with over 90 wt% low-temperature phase can be prepared using conventional arc-melting and 2 hour-low energy ball milling (BM) followed by magnetic separation. After proper alignment, the purified Mn55Bi45(Mn45Bi55) powder show remarkable magnetic properties: mass remanence of 71(65) Am2/kg and coercivity of 1.23(1.18) T at 300 K. The nominal maximum energy product of 120 kJ/m3 is achieved in the purified 2h-BM Mn55Bi45 powder, close to theoretical value of 140.8 kJ/m3. The Mn55Bi45(Mn45Bi55) bulk magnets show the highest volume remanence of 0.68(0.57) T at 300 K, while they were consolidated at 573(523) K by a pressure of 200 MPa for 5 minutes using hot-compaction method. In addition to the observed grain size, the coercivity of the hot-compacted samples at 300 K was found to be strongly related to the amount of metallic Mn and Bi residue at the grain-boundary. Our study proves that the magnetic properties of the Mn45Bi55 bulk magnets are stable up to 500 K, and the nominal (BH)max values are still above 40 kJ/m3 at 500 K showing the potential ability for high-temperature applications.