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A factor to consider in applicability assessment of the sintered Nd-Fe-B magnets prepared by grain boundary diffusion: Macroscopic Demagnetization

Jinghui Di   2024/06/12

Sintered Nd-Fe-B magnets, recognized as a key enabling material, have revolutionized the modern world with their exceptional performance and reliability, attributed to the continuous advancements in processing technology by generations of material engineers and researchers. These magnets have become an indispensable component in various industries, ranging from renewable energy to automotive and electronics, shaping the very foundation of our modern technological advancements. 

The grain boundary diffusion (GBD) technology has proved an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets and optimizing the utilization of Tb/Dy. Improved industrial processing of GBD technology has facilitated the creation of magnets characterized by expanded demagnetization curves. This advancement has yielded a new class of magnets with greater remanent magnetization and sufficient coercivity to sustain performance across a myriad of conditions. The conditions to consider encompass the operating environment, product dimensions, operating temperature, demagnetizing field, mechanical stress and chemical exposure etc. The operating temperature and demagnetization field usually refer to the magnetic properties at various temperatures. This was based on the nucleation theory of coercivity. With a strong determination to unravel the mystery of GBD magnet demagnetization, our engineers have put in continuous effort. Their discovery that demagnetization initiates at the center of the magnet has added complexity to the theory, especially at the macroscopic scale.


The coercivity mechanism of sintered Nd-Fe-B magnets is dominated by the nucleation of reversed magnetic domains. Under this mechanism, the magnetization reversal, are typically observed at the outer part of the grains on a microscopic scale and the magnet's surface on a macroscopic scale, where grain defects are more likely to occur. However, to fully understand the coercivity mechanism, it is crucial to consider the characteristics by GBD process, which affects both the grain shell and the distribution of Tb in sintered Nd-Fe-B magnets at microscopic and macroscopic levels. In the case of GBD magnets, the macroscopic demagnetization mechanism differs from that of non-GBD magnets. In GBD magnets, the magnetization reversal initiates at both the surface and the center region. As the external magnetic field (μ0Hext) increases to a certain threshold, the demagnetization spreads across the layers with comparable μ0Hc. However, under certain μ0Hext, it cannot breach the layers with higher μ0Hc near the surface regions, in contrast to non-GBD magnets where the magnetic domain reversal quickly propagates from one grain to another, from the surface to the interior. The propagation of demagnetization in GBD magnets is impeded by the disparity in coercivity. With a further increase in μ0Hext, the demagnetization region begins to expand towards the original N-S poles, causing the original N-S poles to weaken until they are completely supplanted by the magnetization reversal growing from the center. The magnetization reversal in the center of the GBD magnet during a demagnetization process can be attributed to the fact that the μ0Hc at the outermost surface is higher than that in the center.

You can refer to our article Macroscopic demagnetization of the sintered Nd-Fe-B magnets prepared by Tb grain boundary diffusion and contact us for more information.

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