High coercivity permanent magnets without reliance on scarce elements

抄録

Permanent magnets are known as one of the enablers for achieving carbon neutrality due to their applications in green energy conversion. With the growing demand for permanent magnets, concerns arise regarding element criticality while maintaining the magnets’ functionality. Coercivity (resistance to magnetization reversal) is one of the most important extrinsic magnetic properties of permanent magnets, affecting their functionality. To date, coercivity enhancement has mostly been achieved by the addition of scarce elements, e.g. Dy is often used in Nd₂Fe₁₄B type permanent magnets, exacerbating the materials’ criticality. This review shows how fundamental research has provided alternative strategies for coercivity enhancement without reliance on scarce elements. Specifically, we showcase microstructural engineering, in particular fine-tuning the composition of grain boundary phase and its coverage of the matrix grains in the Nd₂Fe₁₄B type permanent magnets, has led to the development of high coercivity without reliance on Dy. Furthermore, based on micromagnetic simulations, we also discuss further microstructural modifications in the Nd₂Fe₁₄B type magnets required to push coercivity towards its physical limit. Lastly, we will demonstrate how the principles of microstructure engineering can be extended to improve the coercivity of other permanent magnets such as SmCo₅-type and recently-developed SmFe₁₂-based sintered magnets.