Since the development of Nd-Fe-B based rare-earth permanent magnet in 1983, it has become indispensable materials in various industries, owing to their unparalleled magnetic properties among commercial magnets. Especially, the proliferation of electric/hybrid vehicles and demands for downsizing, lightweighting, and high efficiency of electric motors have further increased the reliance on high-performance Nd-Fe-B permanent magnets. However, it is predicted that the demand for neodymium (Nd) will surpass the supply by 2025, and considering the risks of rare earth supply/price due to the resource strategies of rare earth-rich countries, it is necessary to reduce the dependence on Nd-Fe-B permanent magnets. As a viable solution, active utilization of rare earth elements such as cerium (Ce), lanthanum (La), and samarium (Sm), which are abundant or oversupplied in the market, instead of Nd, is needed. Therefore, we will discuss the manufacturing technology and research trends of ‘Nd-reduced rare earth permanent magnets’ replacing Nd with Ce, La, and ‘Nd-free rare earth permanent magnets’ such as Sm-Co, Sm-Fe(-N), and explores their potential as commercial magnets.

This article explained the background and principles of electron holography that allows us to determine the phase shift of incident electrons and observe the magnetic domains from a nanometer-scaled area. We reviewed the studies on the application of the electron holography to various permanent magnets. In particular, we discussed a method for measurement the magnetic flux density of ultra-thin (~3 nm) grain boundaries within a Nd-Fe-B sintered magnet, which was determined from the phase information. Furthermore, we proposed a method to extract the information about the demagnetization field within a Nd₂Fe₁₄B single-crystalline specimen from the electron holography observation (i.e., the reconstructed phase image). The mapping of demagnetization field derived from this method showed a good agreement with the result of micromagnetic simulation. Therefore, these two methods proposed in this article can be promising tools for the development of high coercivity in permanent magnets.

Sm-Co magnets are attracting attention, especially in the military equipment and automobile market due to their high-temperature magnetic properties such as coercivity. Sm has an approximately three times lower price compared to Nd of Nd magnets which have been widely used. The ease of securing raw materials and its potential for high-temperature application make Sm-Co magnets highly valuable for research. In Sm₂Co₁₇ magnets, the supersaturated phase is formed by solution treatment after the sintering. Then it undergoes a two-step heat treatment process consisting of isothermal heat treatment at around 850 ℃ for about 20 hours, followed by annealing at around 400 ℃ for about 10 hours. By forming a cell structure through the formation of SmCo₅ boundary phase covering the Sm₂Co₁₇ main phase, high coercivity can be achieved. In this study, we find the optimal process conditions by investigating microstructures and magnetic properties with different heat treatment conditions including solution treatment, 1st isothermal aging, and 2nd isothermal aging step. We investigate the correlation between magnetic properties and microstructure such as the formation of the main phase, boundary phase, and z-phase, continuity, and distribution of solute atoms.

The demand for Nd-Fe-B permanent magnets is rapidly increasing as a most important material for traction motors of hybrid/electric vehicles along with the growth of the eco-friendly mobility field due to industrial changes. However, the issues on the price of heavy rare-earth (HRE) elements, added to improve the thermal stability of the magnet, are constantly arising. To manufacture high-performance magnets by reducing the amount of HRE or Nd, it is important to refine the grain size of the Nd₂Fe₁₄B phase (~300 nm). Hydrogen-based technologies such as hydrogenation-Decrepitation and the Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) process are essential for obtaining fine anisotropic powders in manufacturing of sintered and bonded magnets. Furthermore, it can also be applied to the recycling of Nd-Fe-B waste magnets to address future rare-earth resource issues. Herein, we introduce the trend in research and development of the hydrogen-based technology for Nd-Fe-B rare-earth permanent magnets.

Because both the motor system’s performance and price competitiveness are essential in small and medium-sized electric devices, single-phase or two-phase electric motor systems are suitable. Among them, TPHRM (Two Phase Hybrid Reluctance Motor) improves performance by adding permanent magnets to the SRM, but the cost increases due to the addition of permanent magnets, so this is complemented by reducing costs in the inverter to secure price competitiveness of the entire electric motor system. This paper proposes a modified bifilar winding of the bifilar inverter to TPHRM. The advantage of the bifilar inverter is that it has the fewest power switch elements. However, it is not used in large electric motor systems due to the limited slot for the winding. The proposed bifilar winding solved the disadvantage of low power density by securing space for the primary winding for excitation by winding the winding in a separate space.

The spin dynamics of antiferromagnets operates at THz frequencies, primarily driven by the robust exchange interaction between neighboring antiparallel spins, being typically 1000 times faster than that of ferromagnets. This dynamic characteristic lays the groundwork for potential applications to ultrafast switching devices. To unlock the THz spin dynamics for the sub-picosecond switching, it becomes imperative to both generate and actively manipulate the antiferromagnetic (AFM) spin currents because the spin currents play a pivotal role in determining the overall switching time. This review, particularly based on THz emission spectroscopy, delves into recent experimental findings of the generation of AFM spin currents in THz frequencies. These experiments serve as a cornerstone for the advancement of next-generation switching devices constructed entirely from AFM materials.