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必不可少的 Nd-Fe-B 磁体中，Nd2Fe14B 晶粒周围的薄非晶晶界 (AGB) 相被证实对矫顽力有显着影响。但其根本机制尚未完全清楚。 Nd2Fe14B/AGB 界面的结构多样性和纳米级特征使得理解 AGB 对矫顽力的影响具有挑战性。 Nd-Fe-B磁体的矫顽力不仅依赖于Nd2Fe14B/AGB界面附近电子和原子水平的局部特性，而且还依赖于原子和微观水平的磁反转。在这个项目中，我们将通过集成第一原理、原子自旋模型和微磁学来进行多尺度模拟，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。然后根据第一原理计算添加/不添加元素的 Nd2Fe14B/AGB 界面附近的原子分辨磁特性（原子磁矩、Fe 原子 MAE、Nd 离子晶体场参数和原子对交换参数）。以它们作为输入，原子自旋模型将被参数化并以数值方式实现，以揭示原子尺度的磁反转。最后，结合原子自旋和微磁模拟将用于了解多晶粒情况下 AGB 对矫顽力的影响。原子自旋模型将用于通过原子计算界面区域同时用微磁离散化处理块体区域来与微磁模拟联系起来。该项目应揭示 Nd2Fe14B/AGB 系统中磁反转和矫顽力机制的多尺度图像，并激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.