Grain boundary restructuring of multi-main-phase Nd-Ce-Fe-B sintered magnets with Nd hydrides

Multi-main-phase (MMP) Nd-Ce-Fe-B magnets prepared by sintering the mixture of Ce-free and Ce-containing RE₂Fe₁₄B (RE, rare earth) powders have been found to possess superior magnetic properties to the single-main-phase (SMP) ones. Purpose of this work is to further enhance the coercivity through restructuring the grain boundaries (GBs), which have strong influence on the short-range exchange coupling between adjacent grains. By incorporating 4 wt.% NdH_x powders as the intergranular additive, coercivity of the (Pr, Nd)_22.3Ce_8.24Fe_balB_0.98 (wt.%) MMP magnet has been substantially increased from 8.2 kOe to 13.1 kOe. Magnetic domain characterizations and the recoil loop measurements showed that the exchange coupling between adjacent grains has been weakened significantly due to the formation of continuous and smooth RE-rich GBs by the extra Nd after NdH_x dehydrogenation. Elemental distribution analysis revealed that thicker Nd-rich 2:14:1 shells with stronger magnetocrystalline anisotropy have been formed in the outer region of the Ce-rich main phase grains, which is beneficial to enhance their effective anisotropy. In addition, the Ce-rich 2:14:1 shells surrounding the Nd-rich 2:14:1 grain cores became thinner, which is beneficial to weaken the magnetic dilution effect of such grains. Micromagnetic simulation results also suggest that the coercivity can be further enhanced once the grain boundary is nonmagnetic. Technically, enriching RE elements or reducing Fe content can weaken the ferromagnetism of the grain boundary phase as well as the exchange coupling between adjacent grains. The above findings might shed new insights into enhancing the coercivity of low cost Nd-Ce-Fe-B magnets.