Rare earth free Mn-Al and Mn-Bi magnetic materials: A review

Permanent magnets are indispensable components of electromagnetic devices. They are widely used in many industries, e.g., electrical machines, computers, consumer electronics, power generators, wind turbines, electric vehicles, medical devices, etc. With the transition of internal combustion engine (ICE) vehicles to electric vehicles (EVs) in the transportation sector, the demand for permanent magnets is expected to increase rapidly. Currently, NdFeB magnets dominate the market, due to their superior magnetic properties (high coercivity, remanence, and maximum energy product). However, these magnets heavily depend on the supply of rare earth elements (REEs) like neodymium (Nd), praseodymium (Pr), dysprosium (Dy), and terbium (Tb), etc., which are subject to supply chain risks and price volatility. As an alternative to REE-based permanent magnets, rare-earth-free (REF) magnets have attracted much attention in both the scientific community and industry. Manganese-based compounds, particularly Mn-Al and Mn-Bi, are some of the REF magnets that have gained significant interest due to the abundance of the Mn and Al elements and their reasonably good magnetic properties. Mn-Al alloys exhibit strong ferromagnetism with high anisotropy, showing large potential in magnetic applications. Mn-Bi alloys show desirable magnetic characteristics, including a high magnetic anisotropy, a positive temperature coefficient of coercivity, and a reasonably high Curie temperature, making them suitable candidates for many applications. However, for Mn-Al alloys, forming the magnetic τ-MnAl phase is limited to a very narrow composition range, making its mass-production very challenging. The commercial production of high-quality Mn-Bi magnet is also very limited, as the magnetic performance of this magnet is still not satisfactory, due to the issues stemming from the impurity phases and its complex microstructure. This paper provides a comprehensive review on the development of Mn-Al and Mn-Bi magnets, especially on the crystal structure and phase formation in the Mn-Al and Mn-Bi compounds and explores the underlying factors that hinder their mass production. Furthermore, various techniques aimed at enhancing the microstructure and magnetic properties of the Mn-Al and Mn-Bi compounds are discussed, with particular emphasis on their impacts on the phase purity, grain size, and overall performance. Future directions for the development of Mn-Al and Mn-Bi magnets as an alternative to REE magnets in industrial applications are pointed out.