MICROSTRUCTURE AND MAGNETIC DOMAINS IN SINTERED ND-FE-B MAGNETS MADE BY HYDROGEN DECREPITATION AND CONVENTIONAL TECHNIQUES

The microstructures of magnets fabricated by the hydrogen decrepitation process and a conventional route have been compared using transmission electron microscopy. The results reveal that magnets produced by the conventional technique contain particles of neodymium-rich and boron-rich phases in the hard magnetic Nd2Fe14B grains, whilst this matrix phase is found to be almost free of neodymium-rich particles in the magnets produced by the hydrogen decrepitation route, but the boron-rich particles still exist within the matrix phase. In addition, in both routes the microstructure showed that the Nd2Fe14B grains have a clean and faultless structure. Magnetic domain walls have been observed within the Nd2Fe14B phase using Lorentz electron microscopy with no special stage. The result shows that all the domain walls end, and are impeded, at the neodymium-rich regions around the grain boundaries, confirming that Hci in the sintered Nd-Fe-B magnets is nucleation-controlled rather than pinning-controlled inside the matrix phase. © 1990.