Multiscale simulations toward understanding effects of amorphous grain boundary on coercivity in Nd-Fe-B magnets

在现代能源相关技术中必不可少的Nd-Fe-B磁体中，Nd2Fe14B晶粒周围的薄非晶晶界（AGB）相对矫顽力有显著影响。但其潜在的机制尚未被完全理解。Nd2Fe14B/AGB界面的结构多样性和纳米尺度特征使得了解AGB对矫顽力的影响具有挑战性。Nd-Fe-B磁体的矫顽力不仅取决于Nd2Fe14B/AGB界面附近电子和原子层面的局部性质，还取决于原子和微观层面的磁反转。在本项目中，我们将通过整合第一性原理，原子自旋模型和微磁学进行多尺度模拟，以多层次地了解AGB对Nd-Fe-B磁体矫顽力的影响。Nd2Fe14B/AGB结构将通过从头算MD方案结合进化算法生成。然后根据第一性原理计算了Nd2Fe14B/AGB界面附近有/没有附加元素的原子分辨磁性能（原子磁矩、Fe的原子MAE、Nd离子的晶体场参数和原子对交换参数）。以它们为输入，原子自旋模型将被参数化和数值实现，以揭示原子尺度的磁反转。最后，结合原子自旋和微磁模拟来了解多晶情况下AGB对矫顽力的影响。原子自旋模型将用于连接微磁模拟，通过原子计算界面区域，同时用微磁离散处理体区。该项目将揭示Nd2Fe14B/AGB体系的磁反转和矫顽力机制的多尺度图景，并启发局部磁性能的原子尺度工程。

In Nd-Fe-B magnets which are essential for modern energy-related technologies, the thin amorphous grain boundary (AGB) phase surrounding Nd2Fe14B grains is confirmed to notably affect the coercivity. But the underlying mechanism is not yet fully understood. The structural diversity and nanoscale feature of Nd2Fe14B/AGB interface make understanding AGB effects on coercivity challenging. Coercivity in Nd-Fe-B magnets relies on not only the local properties at the electronic and atomic level in the Nd2Fe14B/AGB interface vicinity, but also the magnetic reversal at both the atomic and microscopic levels. In this project, we will perform multiscale simulations by integrating first principles, atomistic spin model, and micromagnetics, toward a multilevel understanding of the AGB effects on the coercivity in Nd-Fe-B magnets. The Nd2Fe14B/AGB structure will be generated by an ab initio MD scheme combined with an evolutionary algorithm. Then atomic-resolved magnetic properties (atomic magnetic moment, atomic MAE of Fe, crystal field parameters of Nd ions, and atomic-pair exchange parameters) in the vicinity of Nd2Fe14B/AGB interface with/without additional elements will be calculated from first principles. With them as input, the atomistic spin model will be parameterized and numerically implemented to reveal the atomic-scale magnetic reversal. Finally, the combined atomistic spin and micromagnetic simulations will be applied to understand the AGB effects on coercivity in the multigrain cases. Atomistic spin model will be used to link with micromagnetic simulations by calculating interfacial region atomistically while treating the bulk region with a micromagnetic discretization. This project should disclose the multiscale picture of the magnetic reversal and coercivity mechanism in Nd2Fe14B/AGB systems, and inspire an atomic-scale engineering of local magnetic properties.