Real picture of magnetic domain dynamics along the magnetic hysteresis curve inside an advanced permanent magnet

In the long history of permanent magnet research for more than 100 years, three-dimensional magnetic microscopy has been eagerly awaited to elucidate the origin of the magnetic hysteresis of permanent magnets. In this study, we succeeded in observing the three-dimensional magnetic domain structure of an advanced high-coercivity Nd-Fe-B-based permanent magnet throughout the magnetic hysteresis curve using a recently developed hard X-ray magnetic tomography technique. Focused-ion-beam-based three-dimensional scanning electron microscopy was employed to study the relationship between the observed magnetic domains and the microstructure of the magnet for the same observing volume. Thermally demagnetized and coercivity states exhibit considerably different magnetic domain structures but show the same periodicity of 2.3 μm, indicating that the characteristic length of the magnetic domain is independent of the magnetization states. Further careful examination revealed some unexpected magnetic domain behaviors, such as running perpendicular to the magnetic easy axis and reversing back against the magnetic field. These findings demonstrate a wide variety of real magnetic domain behaviors along the magnetic hysteresis inside a permanent magnet.