Coercivity enhancement of Nd-La-Ce-Fe-B sintered magnets: Synergistic effects of grain boundary regulation and chemical heterogeneity

Abstract

Retaining the chemical heterogeneity of 2:14:1 main phase grains and forming continuous grain boundary (GB) layers are considered two significant contributions to high coercivity in multi-main-phase (MMP) Nd-La-Ce-Fe-B sintered magnets. In this work, the trade-off between chemical heterogeneity and the GB layers in MMP Nd-La-Ce-Fe-B sintered magnets was tackled by designing the composition of LaCe-free alloys. The coercivity of the MMP LaCe-40 (LaCe/TRE = 40 wt.%, TRE: total rare earth) magnets prepared by replacing conventional Nd30.5FebalM1.8B0.96 (Nd-normal) with Nd42.1FebalB0.79 (Nd-rich) as a LaCe-free alloy increased by 52.7%, reaching an ultra-high coercivity of 11.10 kOe. Analysis of the microstructure showed that chemical heterogeneity was retained in the MMP LaCe-40 sintered magnets prepared with Nd42.1FebalB0.79. In addition, thick Nd-rich hard magnetic shells and continuous GB layers were constructed. On the one hand, magnetization reversal of the LaCe-rich grains was hindered by the adjacent strongly magnetic LaCe-free grains. On the other hand, this enhanced the anisotropic field of the LaCe-rich grains and weakened the ferromagnetic coupling effect. These provided a guarantee for preventing the nucleation and propagation of reversed domains, thereby achieving high coercivity. This study offers a new solution for the contradiction between the retention of chemical heterogeneity and the regulation of the GB structure in the MMP Nd-La-Ce-Fe-B sintered magnets, creating the conditions for further improving the coercivity of magnets.