Nd–Fe–B Hydrogenation-Disproportionation-Dehydrogenation-Recombination (HDDR) Magnet for Variable Magnetic Flux Motor

The variable magnetic flux permanent magnet synchronous motor (VMF-PMSM) is one of the next-generation technologies to improve the efficiency of PMSM. VMF-PMSMs require magnets with high magnetizability and reversal curve flatness. In this work, the high carbon content (Nd0.9Sm0.1)–Fe–B hydrogenation-disproportionation-dehydrogenation-recombination (HDDR) magnet with a fine grain size $D_{50}$ of 0.562 $\mu \text{m}$ and a sharp grain size distribution ( $D_{90} - D_{10})/D_{50}$ of 0.806 was developed. Electron probe microanalyzer (EPMA) analysis shows the formation of R–Fe–Ga–C and R–Fe–B–C grain boundary phase in the high carbon content HDDR magnet, which is related to densification while maintaining sub-micrometer size grains with sharp grain size distribution. First-order reversal curve (FORC) analysis and Lorentz scanning transmission electron microscopy (Lorentz-STEM) suggest that the single-domain (SD) magnetization reversal is dominant to result in high magnetizability and reversal curve flatness.