Optimizing composition in MnBi permanent magnet alloys

Abstract

MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 × 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 °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 Mn 55 Bi 45 resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. In this paper, 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 Mn 52 Bi 48 ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5–5 µm 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 .