Microstructural insights into the coercivity enhancement of grain-boundary-diffusion-processed Tb-treated Nd-Fe-B sintered magnets beyond the core-shell formation mechanism

We propose a dominant core-shell formation mechanism for grain-boundary-diffusion-processed (GBDP), Tb-treated, Nd2Fe14B sintered magnets. A depth-sensitive analysis of Tb-treated samples, relative to a non-GBDP Nd2Fe14B magnet, showed a 30% increase of the coercivity in the central part of the magnet. A structure-chemistry-magnetic-property analysis revealed the dominant GBDP mechanism. On the surface of the Tb-treated magnet, the Tb is released from the starting precursor following a cascade of chemical reactions between the Tb oxide and the Nd and/or the Nd-Fe-B. The released Tb diffuses along the grain boundaries, forming a core-shell structure. The calculated optimum concentration for a 30% increase in the coercivity was 50 ppm of Tb. Off-axis electron-holography measurements were used to quantitatively map the characteristic magnetic states of the samples, confirming a different magnetic domain structure in the shell than in the core. The magnetic induction in the core was found to be 26% higher than that of the shell, which has a lower magnetic saturation due to the presence of Tb. The results show that the measured increase in the coercivity is due to a structural effect, and not the magnetic contribution of the Tb. Our results pave the way towards grain-boundary-engineering studies that can be used to increase the coercivity of Nd-Fe-B magnets for e-mobility and eco-power applications.