Latest Major Developments in Permanent Magnets

Abstract

For a steady growth of the permanent magnet industry, resilience to the supply chain of raw materials has become one of the primary issues in the materials research. To be resilient, it would be desirable to have a variety in portfolio of materials performing similarly. However, development of permanent magnets has not pursued such a direction. In this presentation, the speaker tries to summarize recent major developments in rare earth permanent magnet materials, paying attention not only to magnetic performance but also to a balanced usage of rare earth elements, and present possible directions of future R&D of permanent magnets. Guided by decades of fundamental studies on coercivity mechanism in REPMs, the basic strategy to save an indispensable magnetic rare earth elements in an REPM is now consisted of; (1) decoupling hard magnetic grains with a thin, non-ferromagnetic, boundary phase, (2) reduction of the grain size to suppress local demagnetization fields, and (3) allocation of the indispensable elements selectively at the most effective position, i.e., the surface atomic layers of the main phase. Grain boundary diffusion (GBD) process has already become a standard process to accomplish this strategy in the Nd-Fe-B sintered magnets with the high-end specifications. The GBD process for hot-deformed Nd-Fe-B, which requires low processing temperatures, has also been realized by introducing low melting-point alloys such as eutectic Nd-Cu. For a balanced usage of other rare earth elements, the Sm-based magnet are attracting revived attentions: The 2-17-type Sm-Co with increased Fe/Co ratio shows an improved magnetic energy density. For bonded magnets, Sm2Fe17N3 single crystalline powders are suitably used. Inducement of a large-enough coercivity in anisotropic bulk magnets with the “1-12” type high-magnetization Sm-Fe-Co compounds remains under struggle. On the other hand, dilution of Nd with Ce and La in the Nd-Fe-B magnets has become a practical solution. It may follow that the highest energy product will be no longer the top priority but the variations in the portfolio will. To strengthen resiliency in permanent magnet industry, it would be necessary to establish methodology to quickly develop alternatives in the materials portfolio. An important ingredient of these research activities would be construction of a multi-component data base for computational thermodynamic assessment of rare earth permanent magnet alloys. Another, probably the most important, would be usage of artificial intelligence to minimize the experimental verification.