Effect of Annealing Temperature on Grain Boundary Structure and Magnetic Properties of Nd–Ce–Gd–Fe–B Sintered Magnet

Post-sinter annealing (PSA) plays an important role in optimizing grain boundary phases (GBPs) of Nd–Fe–B magnets. The sintered magnet is usually processed by two-step annealing, which consists of a high-temperature annealing (first-step annealing) followed by a low-temperature annealing (second-step annealing). In this work, the magnetic properties and microstructure of Nd–Ce–Gd–Fe–B magnets have been investigated by different second-step annealing temperatures. Compared with the magnet annealed at 420 °C, the knee coercivity ( $H_{k}$ ) of the 460 °C annealed magnet was increased by 156.7 kA/m, and the squareness factor was improved significantly from 0.85 to 0.98. The volume fraction of RE-rich (rare earth [RE]) phases in the magnet has increased after second-step annealing at 460 °C. Microstructural analysis showed that the grain boundary (GB) layers in the 460 °C annealed magnets were thicker and evenly distributed around RE2Fe14B grains, which was helpful to decrease the exchange-coupling between RE2Fe14B grains and, therefore, enhance the coercivity.