Permanent magnets and soft magnets are the indispensable materials for efficient energy converters. Both are optimized in their required property profile. However, microstructural imperfections influence the local magnetic properties. In Fe-Nd-B sintered magnets impurities such as oxides, nonmagnetic phases or manufacturing related imperfections such as inhomogeneous microstructures with nonuniform grain size may appear. In electrical steel sheets, the main defects are nonmetallic inclusions and a strained microstructure at the cutting edges. With conventional characterization methods (e.g. hysteresis graph), only the materials global bulk properties can be measured. Application of an in-situ Kerr microscope equipped with an electromagnet, overcomes this restriction and allows the direct observation and quantification of local demagnetization processes.

Several permanent magnets and electrical sheets were analyzed. For Fe-Nd-B sintered magnets, one focus was on the detailed investigation of abnormally large grains and their impact on the demagnetization process. Demagnetization starts with the formation of reverse domains in the abnormal grain and spreads out into the surrounding matrix along preferred pathways that are magnetically destabilized by the abnormal grain. In soft magnets it was possible to identify inclusions which impede domain wall movement in the grain, or favor the formation of reverse domains.

With analytical tools it is possible to process the Kerr images. This allows to generate a heatmap of the demagnetization process, which shows the locally resolved field dependent demagnetization process in a single image. This approach enables a deeper understanding of the impact of defects on the magnetic properties. This can help in the future development of stronger permanent magnets and more efficient electrical sheets.

Dominic Hohs