The role of Cu addition in the coercivity enhancement of sintered Nd-Fe-B permanent magnets

To understand the mechanism of the coercivity enhancement by a trace addition of Cu in Nd-Fe-B sintered magnets, we investigated the microstructure difference between Cu-doped and Cu-free alloys using high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and laser assisted three dimensional atom probe (LA-3DAP). From a serial sectioning back scattered electron (BSE) images of the Nd-rich phase obtained by an integration of the focused ion beam (FIB) and HRSEM technique, it was found that Cu addition leads to a continuous formation of Nd-rich thin layers along the grain boundaries. 3DAP analysis has shown that a thin Cu-rich layer with a thickness of approximately 2 nm is present at the interface between the Nd₂Fe₁₄B and Nd-rich phase grains.