Merits of Pr80Ga20 grain boundary diffusion process towards high coercivity‒remanence synergy of Nd‒La‒Ce‒Fe‒B sintered magnet

Pursing high coercivity‒remanence synergy is a common challenge for permanent magnets, particularly for the La/Ce-based RE‒Fe‒B (RE: rare earth) magnets that usually exhibit drastic magnetic dilution. Here we apply the Pr80Ga20 grain boundary diffusion process (GBDP) to overcome the coercivity‒remanence trade-off of the LaCe-40 (La‒Ce/total RE = 40%) magnet and yield the record-high magnetic performance with Hcj = 16.1 kOe, Br = 13.1 kG and (BH)max = 41.4 MGOe upon high La‒Ce substitution. One of the prominent merits of the PrGa GBDP is to assist the formation of anti-ferromagnetic RE6Fe13Ga phase at triple junctions and grain boundaries (GBs), which is observed for the first time within the RE‒Fe‒B diffusion system. The synergetic effects of Pr to enlarge the volume fraction of RE-rich phase and Ga to optimize its wettability also facilitate the formation of continuous non-ferromagnetic RE-rich phase at GBs, being another prominent merit of the PrGa GBDP. The anti-ferromagnetic RE6Fe13Ga and non-ferromagnetic RE-rich GBs to decouple neighboring ferromagnetic matrix grains, and the Pr-rich shell to strengthen local magnetocrystalline anisotropy jointly enhance the coercivity, as demonstrated by the microstructural characterization and micromagnetic simulation. The first order reversal curve and Kerr microscopy further reveal the impedance of the nucleation of reversal domains in the PrGa GBDP LaCe-40 magnet, transforming from multi-domain reversal to single-domain reversal. Moreover, Pr infiltration into the matrix grains increases the local magnetization and leads to raised remanence by 0.2 kG. The above work provides a clear picture of the synergetic effects between Pr and Ga towards fully exploiting paragenetic La‒Ce alloy and developing the low-cost commercial-grade permanent magnets.