Microstructure evolution and coercivity enhancement mechanisms of Ga-doped Nd–Ce–Fe–B sintered magnets upon post-sinter annealing

The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd–Ce–Fe–B sintered magnets. In this work, we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd–Ce–Fe–B sintered magnets with 30 wt% Ce replacing Nd, demonstrating enormous potential. The Ga-doped Nd–Ce–Fe–B magnets with higher boron (HB) and lower boron (LB) content are designed. The coercivity of the HB magnet increases slightly from 10.80 to 12.26 kOe after annealing, attributed to the optimized distribution of grain boundary (GB) phases. In contrast, the coercivity of the LB magnet remarkably increases from 8.13 to 15.04 kOe after annealing. Microstructural observations indicate that the narrow GB phase in the as-sintered magnet is rich in Fe, and the strong exchange coupling of adjacent grains resulted in low coercivity. The evolution of Ga-rich phases reveals a potential formation mechanism of the RE₆Fe₁₃Ga phase, that is the RE-Fe amorphous phase and REGa phase in the as-sintered magnet combine to form the RE₆Fe₁₃Ga phase and RE-Ga amorphous phase during post-sinter annealing (RE: rare earth). Moreover, the GB phase of the annealed magnet transforms into a Fe-lean phase with a thickness of 16.4 nm. Magnetization and demagnetization behavior characterizations reveal that the exchange decoupling of adjacent grains induced by the optimized GB phases is the main reason for the remarkable coercivity enhancement, which is also validated by micromagnetic simulations.

Graphical abstract