Enormous improvement of the coercivity of Ga and Cu co-doping Nd-Fe-B sintered magnet by post-sinter annealing

•The effects of annealing on the microstructures and magnetic properties of Ga and Cu co-added Nd-Fe-B magnet were investigated.•The intrinsic coercivity increases from 9.17 kOe to 18.67 kOe after annealing, which shows an increment of 103.5%.•The main reasons for the substantial increase of intrinsic coercivity are discussed. Post-sinter annealing is essential to improve the microstructures and magnetic properties in NdFeB-based sintered permanent magnetic materials. In this paper, investigated systemically are the effects of post-sinter annealing on the microstructures and magnetic performance of Ga and Cu co-doping Nd2Fe14B based sintered magnets. It is found out that the intrinsic coercive force of the magnets increased from 9.17 kOe to 18.67 kOe after two stage post-sinter annealing (PSA), which shows an extraordinary increment of 103.5%. The remanence, with a decreasing percentage of 1.95%, slightly decreases from 13.87 kGs to 13.60 kGs. The substantial increase of intrinsic coercivity is mainly attributed to the improvement of clean and thin continuous grain boundary layers, the smoothness of hard magnetic matrix phase and the appearance of the RE6(Fe, M)14 antiferromagnetic phase during annealing. The reduction of the remanence is caused by the deteriorated orientation degree and reduced volume fraction of hard magnetic main phases after post-sinter annealing.