Investigation of the domain structure of sintered Nd-Fe-B permanent magnets by Bitter-pattern method

Abstract

The conventional Bitter-pattern technique and the colloid-scanning electron microscopy (colloid-SEM) method were used to study the domain structure of polycrystalline sintered Nd-Fe-B permanent magnets. In the thermally demagnetized state most of the grains are multidomain and the domain structures resemble those observed in bulk uniaxial crystals with strong magnetocrystalline anisotropy. Investigations of the magnetic microstructure during magnetizing cycle showed that the domain walls can easily be moved within the grains and that the magnetization reversal in sintered Nd-Fe-B magnets occurs predominantly by the nucleation and expansion of reverse domains at structural imperfections near the grain boundaries. It is also shown that the colloid-SEM method is more surface sensitive and reveals the domain structure with better resolution than the conventional Bitter technique. Thanks to the application of digital image processing systems, clear and high contrast domain images were obtained.