Analysis of Magnetization Reversal Process of Nd–Fe–B Sintered Magnets by Magnetic Domain Observation Using Kerr Microscope
We used a Kerr microscope, image processing, and photo editing to clarify magnetization reversal and its propagation in a sintered Nd-Fe-B magnet. Magnetic domain change was observed when a dc field from +20 to 20 kOe was applied to a sintered Nd-Fe-B magnet. Simultaneous magnetization reversal in several grains along the easy axis direction and its propagation to neighboring grains occurred. This indicates that the nucleation field in a grain and magnetic interaction between grains are important controlling factors of the coercivity of sintered Nd-Fe-B magnets.
Magnetic domain observation of Nd-Fe-B magnets with submicron-sized grains by high-resolution Kerr microscopy
A Kerr microscope that uses UV light for high-resolution observation of magnetic domains was developed, and the domain structure and magnetization process of hot-pressed magnets prepared from hydrogenation-disproportionation-desorption-recombination (HDDR) processed powders were examined. This microscope is capable of distinguishing nanometer-sized domain patterns. The coercivity of the hot-pressed HDDR magnet depends on the area of magnetization reversal during a reversal process. Magnetization reversal occurs simultaneously in a few grains in a low-coercivity magnet because the Nd-rich phase along the grain boundaries is absent. In contrast, in a high-coercivity magnet, magnetization reversal in a grain independently occurs because of an adequate Nd-rich phase along the grain boundaries. It follows that the high coercivity of an HDDR powder compacted by hot pressing is due to domain wall pinning at the grain boundaries.
Magnetic domain structure of anisotropic Nd2Fe14B-based magnets produced via the hydrogenation, decomposition, desorption and recombination (HDDR) process
Abstract Lorentz transmission electron microscopy and optical Kerr microscopy studies of the hydrogenation, decomposition, desorption and recombination (HDDR)-processed anisotropic Nd 2 Fe 14 B-based magnets show that a magnetic domain structure develops in which a unit domain includes a number of submicron Nd 2 Fe 14 B grains coupled by the intergrain-exchange interaction. Grain boundaries and particles of an intergranular phase have been observed to pin the domain boundary walls, and their magnetic characteristics and distribution may determine the size of magnetic domains forming in a thermally demagnetized state. Measurements of differential susceptibilities indicate that the motions of the domain walls become sluggish after magnetization and dc-field demagnetization, compared to that in a thermally demagnetized state.