Magnetic properties and microstructures of HDDR-treated (Nd,Ce)–Fe–B magnet powders

In recent years, to reduce manufacturing costs and decrease the rare-earth supply/demand imbalance in the Nd–Fe–B magnet market, (Nd,Ce)–Fe–B magnets, in which Nd is replaced by Ce, have gained significant attention. In this study, we investigated the magnetic properties and microstructures of anisotropic (Nd,Ce)–Fe–B powders treated by hydrogenation–disproportionation–desorption–recombination (HDDR). The demagnetization curve of Nd12.5Febal.B6.5Nb0.2 after the grain boundary diffusion (GBD) process exhibited high squareness, whereas that of (Nd0.5Ce0.5)12.5Febal.B6.5Nb0.2 exhibited low squareness with a knick. Generally, the low squareness of the demagnetization curve is caused by the presence of different HcJ regions in the material. The XRD, SEM, and STEM-EDX analyses revealed the generation of different HcJ regions owing to two factors. The first was the presence of a ferromagnetic RFe2 (R = Ce and Nd) phase surrounding the R2Fe14B phase, which was observed in the starting material after HDDR and GBD. Consequently, this region showed a lower HcJ than other typical regions in which the R2Fe14B grains were surrounded by an R-rich phase. The second was the inhomogeneous distribution of Ce and Nd in each R2Fe14B grain. This inhomogeneity stemmed from differences in the equilibrium conditions of the HDDR reaction between Nd2Fe14B and Ce2Fe14B, leading to variations in the magnetocrystalline anisotropy and HcJ in the R2Fe14B grains.