The origin of the coercivity reduction of Nd-Fe-B sintered magnet annealed below an optimal temperature

In order to understand the origin of the coercivity reduction in a sintered Nd-Fe-B magnet that is annealed below an optimal annealing temperature, we performed focused ion beam/scanning electron microscopy tomography of post-sinter annealed magnets. A number of grain boundary cracks were observed between Nd2Fe14B grains and Nd-rich phases in the sample annealed below the optimal temperature. We deduced micromagnetic parameters α and Neff by fitting the temperature dependence of the coercivity. While α was constant regardless of the annealing conditions, Neff increased in the sample annealed below the optimal temperature with the reduced coercivity. This indicates that the reduction of the coercivity is due to the local stray field at the cracks.