First-Principles Studies of Spin-Orbit Torque and Magnetoresistance in Magnetic Nanostructures

该奖项支持计算研究和教育，旨在促进对纳米级磁性器件运行的微观机制的理解。重点是在电流引起磁矩动态重定向的设备上。PI将调查由两层构成的设备。一种是最小的微观磁体排列的铁磁体，或者是最小的微观磁体沿层中特定方向交替的反铁磁体。另一层是含有重原子的普通金属。电流诱导动力学的最终来源是自旋轨道耦合——这是爱因斯坦狭义相对论中出现的一种效应，在这种效应中，电子本身就像微小的旋转陀螺，与自身的运动相互作用。自旋轨道效应在由重元素组成的材料中最强。这些双层纳米结构有望应用于纳米电子器件，如新型磁存储器、可调谐高频纳米振荡器、逻辑门和其他用于数字信息处理和存储技术的构建块。提高对纳米级磁性器件运行机制的理解，可能有助于设计新的纳米级器件，并有助于提高现有原型的功能和效率。这项研究将使用最先进的计算工具进行。研究生将对研究及其传播的各个方面作出贡献；在此过程中，他们将接受先进凝聚态物质和材料理论以及建模技术方面的广泛培训。该项目包括额外的教育活动，包括基于现代主动学习和同伴指导方法重新设计核心研究生水平的物理课程。该奖项支持在由铁磁或反铁磁层和正常金属层组成的异质结构中，在平面内电流存在的情况下，由自旋-轨道耦合产生的非平衡自旋扭矩的计算研究和教育。本研究将利用第一性原理计算和非平衡格林函数技术，对无序结构进行直接超级单体平均。总体目标是更好地理解磁性纳米结构中的耦合电荷和自旋输运，以及自旋轨道扭矩和相关磁阻效应的机制。这一目标将通过研究自旋轨道扭矩的类阻尼、类场和高阶角分量对各种材料和器件参数的依赖关系来实现，这些参数包括层厚度、无序类型和强度、界面的晶体取向、表面氧化以及封盖层或间隔层的存在。将结果与实验数据进行比较将有助于确定捕捉观测中关键特征和趋势的潜在机制和现象学理论。在垂直于界面流动的异质结构中，金属界面上的自旋弛豫也将采用多层无序配置的直接平均方法进行研究。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis award supports computational research and education aimed to advance understanding of the microscopic mechanisms responsible for the operation of nanoscale magnetic devices. The focus is on devices where electric currents cause dynamic reorientation of the magnetic moments. The PI will investigate devices made of two layers. One is a ferromagnet for which smallest microscopic magnets are aligned or an antiferromagnet for which the direction of the smallest microscopic magnets alternates along particular directions in the layer. The other layer is a normal metal that contains heavy atoms. The ultimate source of current-induced dynamics is spin-orbit coupling – an effect that arises in Einstein's theory of special relativity in which electrons which themselves act like tiny spinning tops interact with their own motion. The spin-orbit effect is strongest in materials comprised of heavy elements. These bilayer nanostructures are promising for applications in nanoelectronic devices, such as new types of magnetic memories, tunable high-frequency nano-oscillators, logic gates, and other building blocks for digital information processing and storage technologies. Improved understanding of the underlying mechanisms by which nanoscale magnetic devices can operate may enable the design of new nanoscale devices and help enhance the functionality and efficiency of existing prototypes. This research will be carried out using state-of-the-art computational tools. Graduate students will contribute to all aspects of the research and its dissemination; in so doing, they will receive extensive training in advanced condensed matter and materials theory, and modeling techniques. This project includes additional education activities involving the redesign of core graduate-level courses in physics based on modern active-learning and peer-instruction approaches.TECHNICAL SUMMARYThis award supports computational research and education on nonequilibrium spin torques produced by spin-orbit coupling in the presence of an in-plane electric current in heterostructures consisting of a ferromagnetic or antiferromagnetic layer and a normal-metal layer. This study will utilize first-principles calculations and a nonequilibrium Green’s function technique with direct supercell averaging over disorder configurations. The overall goal is to develop better understanding of coupled charge and spin transport in magnetic nanostructures and mechanisms contributing to spin-orbit torques and related magnetoresistive effects. This goal will be achieved by investigating the dependence of the damping-like, field-like, and higher-order angular components of spin-orbit torque on various materials and device parameters, including layer thicknesses, disorder type and strength, crystallographic orientation of the interface, surface oxidation, and the presence of capping or spacer layers. Comparison of the results with experimental data will help identify the underlying mechanisms and phenomenological theories that capture the key features and trends in the observations. Spin relaxation at metallic interfaces in heterostructures with current flowing perpendicular to the interfaces will also be studied using direct averaging over disorder configurations in multilayers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.