ABSTRACT

Physically, the coercivity of permanent magnets should scale with the anisotropy field of ferromagnetic compounds, H_A; however, the typical coercivity values of commercial polycrystalline sintered magnets are only ~0.2 H_A, which is known as Brown’s paradox. Recent advances in multi-scale microstructure characterizations using focused ion beam scanning electron microscope (FIB/SEM), aberration corrected scanning transmission electron microscopy (C_s-corrected STEM), and atom probe tomography (APT) revealed detailed microstructural features of commercial and experimental Nd-Fe-B magnets. These investigations suggest the magnetism of a thin layer formed along grain boundaries (intergranular phase) is a critical factor that influences the coercivity of polycrystalline magnets. To determine the magnetism of the thin intergranular phase, soft X-ray magnetic circular dichroism and electron holography play critical roles. Large-scale micromagnetic simulations using the models that are close to real microstructure incorporating the recent microstructure characterization results gave insights on how the coercivity and its thermal stability is influenced by the microstructures. Based on these new findings, coercivity of Nd-Fe-B magnets is being improved to its limit. This review replies to the most frequently asked questions about the coercivity of Nd-Fe-B permanent magnets based on our recent studies.

摘要

在物理上，永磁体的矫顽力应与铁磁化合物的各向异性场H _{A 成}比例；然而，商业多晶烧结磁体的典型矫顽力值仅为~0.2  H _A，这被称为布朗悖论。使用聚焦离子束扫描电子显微镜 (FIB/SEM)、像差校正扫描透射电子显微镜 (C _s-校正的 STEM）和原子探针断层扫描 (APT) 揭示了商业和实验 Nd-Fe-B 磁体的详细微观结构特征。这些研究表明，沿晶界（晶间相）形成的薄层的磁性是影响多晶磁体矫顽力的关键因素。为了确定薄晶间相的磁性，软 X 射线磁圆二色性和电子全息术起着关键作用。使用接近真实微观结构的模型进行的大规模微磁模拟结合了最近的微观结构表征结果，深入了解了微观结构如何影响矫顽力及其热稳定性。基于这些新发现，Nd-Fe-B 磁体的矫顽力正在提高到极限。