Abstract The grain boundary diffusion process (GBDP) is now widely used to increase coercivity in Nd-Fe-B sintered magnets with a more efficient use of heavy rare earth elements (Dy, Tb). This process leads to a typical core-shell structure for the grains consisting of (Nd,Dy)2Fe14B shells at the outer grain regions and Nd2Fe14B cores. The thickness of the (Nd,Dy)2Fe14B shells decreases from the diffusion surface to the magnet core. This inhomogeneous distribution in Dy content gives rise to a coercivity gradient within the magnet and leads therefore to a reduced squareness of the demagnetization curve. The purpose of this work is to provide a quantitative understanding of the influence of composition profiles after GBDP on the shape of the demagnetization curve of Nd-Fe-B sintered magnets diffused with the Dy63Co37 (at. %) intermetallic compound. SEM/X-EDS analyses along the Fisher diffusion model allow the estimation of the Dy concentration in grains and at different depths. Then, after ascribing to the grains some critical values for the switching field that are related to the local Dy content, a macroscopic finite element model is implemented to provide a better understanding of the grain reversal sequence in the graded magnets tested in closed-circuit. Grain reversal patterns show that demagnetization starts from the less coercive grains in the magnet core, remains constricted in this zone thanks to a shielding effect from the external surface, and then propagates towards outer layers via magnetostatic interactions. When the coercivity gradient is large, the coercivity of the whole magnet measured in closed-circuit could be 100–200 kA/m lower than the value expected without considering magnetostatic interactions, suggesting that the shielding effect from the diffusion affected layers could be limited and counterbalanced by magnetostatic interactions.

中文翻译：

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重新审视 Dy 扩散 Nd-Fe-B 烧结磁体的退磁曲线

摘要 晶界扩散工艺 (GBDP) 现在被广泛用于提高 Nd-Fe-B 烧结磁体的矫顽力，更有效地利用重稀土元素 (Dy, Tb)。该过程导致晶粒的典型核壳结构由外部晶粒区域的 (Nd,Dy)2Fe14B 壳和 Nd2Fe14B 核组成。(Nd,Dy)2Fe14B 壳的厚度从扩散表面到磁芯减小。Dy 含量的这种不均匀分布导致磁体内的矫顽力梯度，因此导致退磁曲线的垂直度降低。这项工作的目的是定量了解 GBDP 后成分分布对扩散有 Dy63Co37 (at. %) 金属间化合物的 Nd-Fe-B 烧结磁体退磁曲线形状的影响。沿着 Fisher 扩散模型的 SEM/X-EDS 分析允许估计晶粒中和不同深度的 Dy 浓度。然后，在将与局部 Dy 含量相关的开关场的一些临界值归因于晶粒后，实施宏观有限元模型，以更好地理解闭路测试中分级磁体中的晶粒反转序列。晶粒反转模式表明，退磁从磁芯中矫顽力较小的晶粒开始，由于外表面的屏蔽效应而在该区域保持收缩，然后通过静磁相互作用向外层传播。当矫顽力梯度较大时，