Microstructure of fine-grained Nd–Fe–B sintered magnets with high coercivity

The microstructure of high-coercivity Nd–Fe–B sintered magnets with a grain size of about 1 μm, which was processed by helium jet milling and the pressless sintering process, was studied by scanning electron microscopy, transmission electron microscopy and atom probe tomography. The high coercivity can be attributed to a similar microstructure that is five times smaller than that of conventional sintered magnets. Initial magnetization curves showed a two-step magnetization process, suggesting that the grain boundary phase causes a pinning force.