Optimizing composition in MnBi permanent magnet alloys

MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 x 10(6) J m(-3)), and good magnetization (81 emu g(-1)) at room temperature. Although the theoretical maximum energy product (BH)(max) of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 degrees C, which makes it a potential candidate for high temperature applications such as permanent magnet motors. However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn55Bi45 resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. Here we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (M-S) of 79 emu g(-1) at 300 K in the annealed Mn52Bi48 ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5-5 mu m particles with M-S of 75.5 emu g(-1), coercivity H-ci of 10.8 kOe, and (BH)(max) of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased H-ci by up to 21%, and also resulted in an increase in M-S up to 78.8 emu g(-1). (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.