Phase structure evolution and coercivity mechanism of high-Co containing permanent magnets

Abstract

Abstract The phase structure and magnetic properties of high-Co containing permanent magnets with high thermal stability have been systematically studied in this work. It is abnormal that the coercivity of annealed samples is slightly lower than that of sintered samples, while the coercivity was usually enhanced after annealing in conventional Nd-Fe-B samples. Further analysis showed that in addition to RE2(Fe,Co)14B main phase and RE-rich grain boundary phase, there are also new Co-rich magnetic phase located in the grain boundary. During annealing, the phase structure of high-Co containing magnets were readjusted, especially the increasing Co-rich magnetic phase and emerging RE-rich particles precipitated from the main phase. Eventually, the isolated RE-rich particles would act as the pinning center of the domain wall movement in demagnetization process. It was confirmed that the coercivity of annealed high-Co containing magnets was controlled by both nucleation and pinning. Pinning mechanism can partially compensate for the weakening of magnetic isolation due to increased Co-rich magnetic phase, which explained the moderate decrease in coercivity of annealed high-Co containing magnets. The discovery of new coercivity mechanism contributed to in-depth understanding of high-Co containing magnets.