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

摘要：铁磁材料具有长时间记忆、高存储密度、高性价比等优点，广泛应用于硬盘等数字信息存储设备中。与其他基于半导体的存储设备相比，硬盘驱动器的运行速度相对较慢，这促使了没有运动部件的磁存储器的发展。为了使这种存储技术具有竞争力，需要使用电流而不是磁场来有效地控制磁性。近年来，一种被称为自旋轨道转矩的新机制被证明可以在铁磁和反铁磁材料中控制磁性——后者在自然界中更为普遍，但目前尚未得到充分利用，尽管它比铁磁材料有几个优势，比如可能更高的速度和密度。然而，在多层磁系统中产生的自旋轨道转矩是由几种相互竞争的机制引起的，这些机制很难解开，这扩大了实验与理论之间的差距，阻碍了器件的优化。该合作项目旨在利用理论和实验方法确定单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的磁存储设备的开发。该项目培养研究生和本科生的各种研究技术，并为他们在科学和技术的劳动力做好准备。计划中的推广活动包括建立一个在线系列研讨会，邀请来自物理学和工程学中代表性不足的群体的研究人员将他们的前沿研究与他们追求科学事业的个人经验结合起来发表演讲。技术摘要：实现磁序的高效电气控制是存储技术发展的关键。自旋轨道转矩——在外加电场作用下角动量从晶体的原子晶格向磁序的转移——比以前的磁存储器写入机制更快速、更可靠、更节能。虽然利用自旋轨道转矩控制磁性已经在各种器件中得到证实，但铁磁层和反铁磁层在产生自旋轨道转矩中的作用尚不清楚，这导致实验与理论之间的不一致，并阻碍了器件的优化。这个合作项目从实验和理论上描述了单层磁性材料中产生的边界自旋轨道扭矩，消除了相邻层的竞争机制。单层磁性体系包括铁磁体、非共线反铁磁体和共线反铁磁体。利用磁光克尔效应测量了边界自旋轨道转矩，并用半经典模型和第一性原理输运计算对结果进行了理论解释。这项合作研究的目的是在为单层磁记忆铺平道路的同时，解开在多层中也必须发生的自旋扭矩贡献。它还首次在具有非共线和共线磁序的单反铁磁层中进行了自旋力矩的表征，拓宽了反铁磁体在自旋轨道耦合纳米结构中的作用。该项目培养研究生和本科生的各种研究技术，如薄膜生长、微加工、光学检测、第一性原理计算和半经典建模，为他们在科学和技术领域的劳动力做好准备。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。

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.