MnBi

MnBi is an intermetallic compound consisting of manganese and bismuth with the formula MnBi. It crystallizes in a hexagonal NiAs-type structure at room temperature, where manganese and bismuth occupy alternating lattice sites. The crystal arrangement leads to a strong uniaxial magnetic anisotropy along the c axis, which contributes to favorable magnetic stability and coercivity.

Magnetism in MnBi is ferromagnetic and the material exhibits a Curie temperature well above room temperature. The magnetocrystalline anisotropy is large and tends to increase as temperature decreases, helping to maintain magnetization against thermal demagnetization. As a rare-earth-free magnet candidate, MnBi can offer a moderate to strong energy product and coercivity in textured or fine-grained forms, making it of interest for certain permanent-magnet applications.

Processing and materials engineering play key roles in MnBi performance. It is typically synthesized by arc melting of Mn and Bi followed by homogenizing anneals. Powder metallurgy and rapid solidification techniques are used to achieve refined microstructures, control grain size, and improve coercivity. Alloying and microstructural tailoring, including small additions of other transition metals or metalloids, are explored to enhance thermal stability and magnetic properties. Thin films of MnBi are also investigated for fundamental studies and potential device integration.

Limitations and challenges include brittleness, oxidation sensitivity, and thermal stability concerns that complicate large-scale, long-term magnet applications. While MnBi remains a focus of research as a rare-earth-free permanent magnet option, practical deployment requires further optimization of phase stability, microstructure, and processing.