Advanced Fresnel- & Foucault-Lorentz microscopy were applied to analyze magnetic behavior of the grain boundaries in Nd-Fe-B hard magnets. In-situ TEM magnetizing experiments combined with these imaging methods revealed the process of magnetization reversal in polycrystalline sintered and die-upset Nd-Fe-B under various magnetic fields. Fine details of magnetic flux distribution, derived from the magnetic interferograms created by phase-coherent Foucault imaging, provide a quantitative description of the local variation of magnetic flux. Our study suggests that the grain boundaries play an important multi-functional role in the reversal of magnetization, by acting as (a) pinning centers of domain walls, (b)centers of nucleation of reversal domains, and (c) sinks or sources for migrating magnetostatic charges and/or dipoles. They also ensure a smooth transition for irreversible remagnetization in polycrystalline samples.
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