Dynamic magnetization observations and reversal mechanisms of sintered anddie-upset Nd-Fe-B magnets

In situ TEM magnetizing experiments combined with Lorentz microscopy in Fre snel and Foucault mode were used to study local features of magnetization r eversal in polycrystalline sintered and die-upset Nd-Fe-B magnets. The die- upset (DU) magnets featured well-aligned grain texture along the c-axis, wh ile the sintered (SI) magnets have a much wider distribution of grain orien tations. Despite the similarity of the hysteresis curves, we found their na noscale behavior of the magnetization reversal was quite different. The pre dominant mechanism controlling reversal in DU-magnets was nucleation of rev ersed domains, preferentially at misaligned grain interfaces, followed by d omain wan pinning at grain boundaries. In contrast, magnetization reversal in SI-magnet progressed first by nucleation and expansion of reversed domai n dipoles predominantly in misaligned grains at a high magnetic field, and then by the major irreversible process via nucleation and splitting of magn etic domains near the grain boundaries of well-aligned grains at more negat ive fields. This process is completed with lateral expansion of newly creat ed domain walls. A simple model for the strong pinning-trapping centers obs erved in DU-magnets is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.