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MFM Study of a Sintered Nd–Fe–B Magnet: Analyzing Domain Structure and Measuring Defect Size in 3-D View

This paper describes the use of magnetic force microscopy (MFM) to investigate the domain structure of sintered Nd-Fe-B magnet in a thermally demagnetized state. Observations are made at both perpendicular and parallel surfaces to the alignment axis (easy axis). Clear magnetic contrast images in 3-D view with defined domain structures are presented. Mazelike magnetic domains and stripe magnetic domains are observed in the perpendicular and parallel surfaces, respectively. Superficial spike domains and reverse spikes are observed within the main domains and analyzed in 3-D view. MFM images illustrate the grains of Nd-Fe-B magnets, and indicate the multi-domain structures within individual grains. The effect of interfacial defects upon the domain structure is studied in 3-D, and evident reverse magnetic domains (nucleation sites) are observed on the imperfection region. The height and width of the defect region are determined using the XEI imaging software. In addition, the volume and surface area of nucleation region are evaluated and validated theoretically by developing an approach involving coercivity versus the nucleus volume. Defect regions have been identified by using Raman spectroscopy as an oxide in character rather than metallic. The average domain width and domain wall energy have been determined for both directions.