Grain boundary and interface chemistry of an Nd-Fe-B-based sintered magnet

The compositions of grain boundaries (GBs) and other interfaces surrounding Nd ₂Fe ₁₄B grains in commercial Nd-Fe-B sintered magnets have been investigated by laser-assisted three-dimensional atom probe to understand the mechanism of the coercivity enhancement by post-sinter annealing. While only a slight segregation of Nd and Pr to the GBs was confirmed in the as-sintered sample, a thin Nd-rich amorphous phase layer was observed along the GBs with Cu segregation to the interfaces in the annealed sample. The segregation of Cu to NdO _x/Nd ₂Fe ₁₄B interfaces was also found, suggesting that the Nd ₂Fe ₁₄B grains are enveloped by a Cu-enriched layer after the annealing. The concentration of Fe + Co in the thin GB layer was found to be as high as 65 at.%, and a model amorphous film processed by sputtering with the same composition as the thin GB layer was found to be ferromagnetic. Ferromagnetic behavior of the thin GB layer suggested that Nd ₂Fe ₁₄B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings.