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

The compositions of grain boundaries (GBs) and other interfaces surrounding Nd2Fe14B 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 NdOx/Nd2Fe14B interfaces was also found, suggesting that the Nd2Fe14B 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 Nd2Fe14B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings.