Study of the magnetic microstructure of high‐coercivity sintered SmCo5 permanent magnets with the conventional Bitter pattern technique and the colloid‐SEM method

Abstract

Abstract The magnetic microstructure of high‐coercivity sintered SmCo5 permanent magnets was studied with the conventional Bitter pattern technique, and also for the first time with the colloid‐scanning electron microscopy (colloid‐SEM) method. Both techniques were supported by digital image acquisition, enhancement and analysis. Thanks to this, it was possible to obtain high‐contrast and clear images of the magnetic microstructure and to analyze them in detail, and consequently also to achieve improvements over earlier results. In the thermally demagnetized state the grains were composed of magnetic domains. On the surface perpendicular to the alignment axis, the main domains forming a maze pattern and surface reverse spikes were observed. Investigations on the surface parallel to the alignment axis, especially by the colloid‐SEM technique, provided a detailed insight into the orientation of grains. The alignment of grains was good, but certainly not perfect; there were also strongly misaligned grains, although generally very rare. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) the domain walls were observed to continue through the grain boundaries, indicating significant magnetostatic interaction between neighboring grains. Studies of the behavior of the magnetic microstructure under the influence of an external magnetic field, performed for the first time on the surface parallel to the alignment axis (with the conventional Bitter pattern method), showed that the domain walls move easily within the grains and that the magnetization reversal mechanism is mainly related to the nucleation and growth of reverse domains, i.e. that sintered SmCo5 magnets are nucleation‐dominated systems. Groupwise magnetization reversal of adjacent magnetically coupled grains was observed, an unfavorable effect for high‐coercivity magnets. Images obtained by the colloid‐SEM technique and the conventional Bitter pattern method were compared and some aspects related to the observation of magnetic microstructures by these two techniques were also discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)