Collaborative Research: Spin Currents and Spin-orbit Torques in Single Layer Magnetic Systems

非技术摘要：铁磁材料被广泛用于数字信息存储设备，如硬盘驱动器，拥有长期记忆，高存储密度和成本效益。与其他基于磁存储器的存储设备相比，硬盘驱动器的操作速度相对较慢，这促使了没有移动部件的磁存储器的发展。为了使这种存储器技术具有竞争力，需要使用电流而不是磁场来有效地控制磁性。近年来，一种称为自旋轨道扭矩的新机制已经被证明可以在铁磁和反铁磁材料中电控磁性，后者在自然界中更为普遍，但目前尚未得到充分利用，尽管与铁磁表兄弟相比具有一些优势，如潜在的更高速度和密度。然而，在多层磁系统中产生的自旋-轨道转矩产生于几种难以解开的竞争机制，从而扩大了实验与理论之间的差距，并阻止了器件优化。该合作项目旨在使用理论和实验方法确定单个铁磁和反铁磁层内产生的自旋轨道扭矩的微观起源和行为。这项研究的成功将有助于优化自旋轨道扭矩，从而加快用于传统信息存储和人工智能应用的更快磁存储设备的开发。该项目培训研究生和本科生各种研究技术，并为他们在科学和技术的劳动力做好准备。计划的推广活动包括建立一个在线研讨会系列邀请研究人员从代表性不足的团体在物理学和工程学给他们的前沿研究与他们的个人经验相结合，在追求自己的科学生涯的讲座。技术摘要：实现有效的电控制的磁秩序是至关重要的存储器技术的发展。自旋-轨道转矩是在外加电场作用下，从晶体原子晶格到磁序的角动量转移，与以前的磁存储器写入机制相比，它有望实现更快、更可靠、更节能的切换。虽然已经在各种器件中证明了使用自旋轨道扭矩控制磁性，但铁磁和反铁磁层在产生自旋轨道扭矩中的作用仍然不清楚，导致实验和理论之间的不一致，并阻止器件优化。这个合作项目从实验和理论上表征了单层磁性材料内产生的边界自旋轨道扭矩，消除了相邻层的竞争机制。单层磁系统包括铁磁体、非共线反铁磁体和共线反铁磁体。边界自旋轨道力矩的测量使用磁光克尔效应和结果进行了理论解释，使用半经典模型和第一性原理输运计算。这项合作研究的目的是解开自旋扭矩的贡献，也必须发生在多层，同时为单层磁存储器铺平道路。它还承担了第一个表征的自旋扭矩在单一的反铁磁层与非共线和共线磁序，扩大了反铁磁在自旋轨道耦合纳米结构的作用。该项目旨在培养研究生和本科生掌握薄膜生长、微细加工、光学检测、第一性原理计算、半经典建模等各种研究技术，为科学技术人才培养做好准备。该奖项体现了NSF的法定使命，通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical Abstract:Ferromagnetic materials are widely used in digital information storage devices such as hard disk drives, which boast long-term memory, high storage density, and cost-effectiveness. Compared to other semiconductor-based memory devices, the operation speed of hard disk drives is relatively slow, prompting the development of magnetic memories without moving parts. To make such memory technologies competitive requires efficient control of magnetism using electrical currents rather than magnetic fields. In recent years, a novel mechanism called spin-orbit torque has been shown to electrically control magnetism in both ferromagnetic and antiferromagnetic materials—the latter of which are far more prevalent in nature but currently underutilized, despite offering several advantages over their ferromagnetic cousins like potentially higher speed and density. However, the spin-orbit torques generated in multilayer magnetic systems arise from several competing mechanisms that are difficult to disentangle, widening the gap between experiment and theory and preventing device optimization. This collaborative project aims to determine the microscopic origins and behavior of spin-orbit torques generated within single ferromagnetic and antiferromagnetic layers using both theoretical and experimental methods. Success in this research will help optimize spin-orbit torques, thus expediting the development of faster magnetic memory devices used for traditional information storage and for artificial intelligence applications. This project trains graduate and undergraduate students in a variety of research techniques and prepares them for the workforce in science and technology. Planned outreach activities include establishing an online seminar series inviting researchers from underrepresented groups in physics and engineering to give talks combining their frontier research with their personal experience in pursuit of their scientific career.Technical Abstract:Achieving efficient electrical control of magnetic order is crucial for the development of memory technology. Spin-orbit torque—which is a transfer of angular momentum from the atomic lattice of crystals to magnetic order under an applied electric field—promises faster, more reliable, and more energy-efficient switching than previous write mechanisms in magnetic memories. While control of magnetism using spin-orbit torque has been demonstrated in various devices, the role of the ferromagnetic and antiferromagnetic layers in generating spin-orbit torques remains unclear, creating inconsistencies between experiment and theory and preventing device optimization. This collaborative project experimentally and theoretically characterizes boundary spin-orbit torques generated within single-layer magnetic materials, eliminating competing mechanisms from adjacent layers. The single-layer magnetic systems include ferromagnets, non-collinear and collinear antiferromagnets. The boundary spin-orbit torques are measured using the magneto-optic-Kerr-effect and the results are theoretically interpreted using both semiclassical models and the first-principles transport calculations. This collaborative research aims to disentangle the spin torque contributions that must also occur in multilayers while paving the way for single layer magnetic memories. It also undertakes the first characterization of spin torques in single antiferromagnetic layers with non-collinear and collinear magnetic order, broadening the role of antiferromagnets in spin-orbit coupled nanostructures. This project trains graduate and undergraduate students in a variety of research techniques such as thin film growth, micro-fabrication, optical detection, first-principles calculations, and semiclassical modeling, preparing them for the workforce in science and technology.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.