High-performance permanent magnets are essential in fields such as industrial machinery, medical equipment, and energy, and demand for them is expected to increase as we move toward carbon neutrality. It is essential to address the issues surrounding the supply of rare earth resources, and there is a need to improve the performance of Nd-Fe-B magnets and develop alternative materials that use less rare earth. In this paper, we review recent trends in the reduction of heavy rare earth elements, optimization of magnet microstructure, evaluation and analysis technology for magnet materials, development of innovative magnet materials, recycling technology for magnet motors, and application fields. Finally, we consider magnet material research’s future direction and role in achieving carbon neutrality.

Neodymium magnets (Nd₂Fe₁₄B), which are used in the drive motors of electric vehicles, pose high geopolitical risks, so there is active development of alternative magnets in Japan. Sm-Fe compounds have superior intrinsic magnetic properties to Nd₂Fe₁₄B compound and are therefore considered to be the leading candidates for the alternative magnets. Sm-Fe compounds are classified into Th₂Zn₁₇-type, TbCu₇-type, and ThMn₁₂-type based on differences in crystal structure, and have characteristics such as anisotropic magnetic field, saturation magnetization, and Curie temperature that exceed those of Nd₂Fe₁₄B compound. However, these Sm-Fe compounds have issues that neodymium magnets do not have, such as difficulty in sintering due to thermal decomposition and the difficulty in synthesizing single crystal powder. Recently, various new metallurgical technologies have been developed to address these issues. For example, Th₂Zn₁₇-type Sm₂Fe₁₇N₃ sintered magnets that do not degrade in magnetic properties have been reported using low-oxygen powder metallurgy and novel liquid phase sintering techniques. Moreover, it was reported that single crystal powder of metastable TbCu₇-type Sm-Fe, for which only polycrystalline powder was conventionally available, can be produced by reduction diffusion technique and the thermal plasma technique. In addition, Sm-Fe alloys could not achieve uniaxial anisotropy unless they were nitrided, but nitrogen-free Sm-Fe magnets have currently been invented.

We have investigated the mechanism of coercivity evolution in magnets through multi-scale microstructural characterization. Based on these findings, our goal is to enhance the coercivity of heavy rare-earth-lean magnets by optimizing their microstructure. We have developed several new processes and demonstrated improved magnet properties. This review paper describes (1) coercivity enhancement by the eutectic alloy diffusion process, (2) the development of light rare earth magnets, and (3) the development of high electrical resistance magnets. Recently, there has been an increasing demand for highly efficient magnet development that not only improves properties but also meets multiple requirements. Accordingly, (4) data-driven methods are being employed in ongoing magnet development.

Magnetic measurement of sintered Sr ferrite magnet in the hard-axis direction was executed, and it was found that the value of coercive force H_cJ in hard-axis direction is much smaller than H_cJD, which value is the deviation of dJ/dH on the hysteresis curve. The difference of H_cJ and H_cJD is compared and discussed in the demagnetization process phenomenon. The angular dependence of coercive force in all directions between θ=0 and 90 degree was also measured and observed.

In this study, we examined the sintering of Sm-Fe-N powder. The powder was consolidated into bulk form using the spark plasma sintering method, which resulted in the production of Sm-Fe-N bulk magnets. The magnetic properties of the bulk magnets prepared at 673 K under an applied pressure of 100 MPa using the spark plasma sintering method were much lower than those of the Sm-Fe-N powder. This was attributed to the partial decomposition of the Sm₂Fe₁₇N₃ phase. However, the magnetic properties of the bulk magnets prepared at 473 K under an applied pressure of 200 MPa using the spark plasma sintering method were almost comparable to those of the Sm-Fe-N powder. It was observed that the resulting Sm-Fe-N magnet retained the original Sm₂Fe₁₇N₃ phase.

W-type ferrites are easily decomposed into M-type ferrite and spinel ferrite during the sintering process, so it is difficult to obtain a W-type single-phase sintered bodies. Therefore, carbon as a reducing agent is added and heat treatment before sintering is performed. However, the effect of these treatments on the change in the crystal phase during sintering has not been clarified in detail. In this study, the effect of carbon addition and heat treatment before sintering on the change in the crystal phase during sintering was investigated in detail by removing the samples from the electrical furnace during sintering. Furthermore, similar experiments were carried out with the addition of CaCO₃ and SiO₂ as sintering aids.

Although we applied Nd-Fe-B thick-film magnets fabricated by pulsed laser deposition (PLD) with deposition rate higher than several tens microns per hour to DC brushless motors and small stepping motors, the deposition area was limited within 5 mm² due to increase in the distribution of thickness with enlarging deposition area. In this research, a laser scanning using a galvanometer mirror and inclined installation of substrates during the deposition were adopted. It was found that not only the thickness distribution but also composition distribution could be suppressed when enlarging the deposition area up to 10 mm in square.