Quantitative correlation between intrinsic coercivity and grain boundaries in three dimensions is critical to
further improve the performance of sintered Nd-Fe-B permanent magnets. Here, we quantitatively reveal the
local composition variation across and especially along grain boundaries using the powerful atomic-scale analysis
technique known as atom probe tomography. We also estimate the saturation magnetization, magnetocrystalline
anisotropy constant, and exchange stiffness of the grain boundaries on the basis of the experimentally determined
structure and composition. Finally, using micromagnetic simulations, we quantify the intrinsic coercivity
degradation caused by inhomogeneous grain boundaries. This approach can be applied to other magnetic materials
for the analysis and optimization of magnetic properties.