Magnetic and microstructural properties of anisotropic MnBi magnets compacted by spark plasma sintering

In this study, we present that anisotropic MnBi bulk magnets can be obtained using spark-plasma-sintering technique (SPS). Grain misalignment was suppressed by either external field or addition of appropriate polymer binder. Various microstructures were effectively achieved during SPS processing, and lead to distinct magnetic performance. Microstructure analysis reveals that the coercive field of the SPS-compacted magnets declines strongly with increasing grain size. Bi-rich residue clearly plays a role in magnetic decoupling to enhance the coercive field, whereas the exchange coupling mediated by excess Mn reduces it dramatically. In addition, the amount and distribution of Mn and Bi residue has a profound influence on the thermal magnetic stability of sintered MnBi magnets. We suggest that fast consolidation methods like SPS technique enable to tailor the microstructure of bulk magnetic products to meet specific application requirements.