W. Tang, J. Wang, Y. Varma, Jun Cui
Ames National Lab of USDOE,
United States
Keywords: rare-earth free magnets, MnBi magnets, magnetic properties, thermal stability
Summary:
The growing demand for high-performance permanent magnets in electric drives, coupled with supply-chain vulnerabilities associated with rare-earth (RE) elements, underscores the urgent need for alternative magnet technologies. Among RE-free candidates, manganese–bismuth (MnBi) stands out for its unusual property of increasing coercivity with temperature—nearly doubling its room-temperature value at 120°C. This unique feature makes MnBi particularly attractive for high-temperature applications, such as industrial pumps and motors, where coercivity and thermal stability are more critical than peak magnetization. However, large-scale production of high-purity MnBi and its consolidation into dense, anisotropic bulk magnets remain challenging due to phase instability and compositional sensitivity. In this work, we address these challenges through a comprehensive investigation of the synthesis, microstructural evolution, magnetic behavior, and manufacturability of MnBi magnets. A new processing roadmap has been established to enable scalable fabrication of high-purity MnBi powders and magnets. The approach integrates optimized alloy design, controlled thermal processing, and targeted microstructure engineering to stabilize the low-temperature MnBi phase, suppress parasitic impurities, and retain high coercivity in both warm-sintered and bonded forms. The resulting MnBi materials exhibit strong thermal robustness, high intrinsic coercivity, and excellent phase purity, demonstrating clear potential for industrial motor applications. These findings provide a viable pathway for advancing MnBi from laboratory-scale innovation to system-level implementation, supporting the development of sustainable, rare-earth-free magnet technologies for next-generation electrified systems. This work was supported by the Critical Materials Innovation Hub, funded by the U.S. Department of Energy, and conducted at Ames National Laboratory, operated by Iowa State University for the U.S. Department of Energy.