Spin Transfer Torques Arising from Spin-Orbit Interactions

非技术摘要在过去两年中，已经发现了一种用于操纵磁性设备(例如磁性随机存取存储器)的新策略，其效率至少比任何已知技术高10倍。这种方法依赖于利用“自旋-轨道耦合”--电子的本征自旋与其在某些材料中的运动方向之间的强相互作用。然而，确切的机制尚未解决。该项目试图确定这种新效应背后的微观机制，调查如何最好地优化材料和设备以实现应用，并探索通过使用这一新策略来操纵以前没有研究过的各类磁性材料所能带来的科学机会。这项研究直接适用于改进磁记忆和逻辑技术，该项目还将产生额外的广泛影响，通过向研究生和本科生提供研究教育，帮助他们成为杰出的独立科学家。技术摘要本项目建立在主要研究人员和合作者的演示基础上，即由于重金属中的自旋霍尔效应，重金属/铁磁双层中的面内电流可以为铁磁的磁矩提供非常强的扭矩。其他研究小组也在类似的样本中提供了电流感应扭矩与另一种不同的物理机制--拉什巴效应有关的证据。该项目将寻求回答尚未解决的问题，即这些重金属/铁磁双层中电流感应扭矩的微观机制、对称性和强度，以及如何最好地优化材料和设备以实现这些扭矩的应用。所采用的方法包括测量纳米器件中电流诱导的磁矩重定向的技术，以及相关的自旋泵浦实验，在这些实验中，进动磁铁向重金属中注入自旋电流。这项研究涉及调查各种不同的材料和多层几何结构，并探索新的科学机会，通过使用新的电流感应扭矩来操纵绝缘亚铁磁体和反铁磁体中的自旋，这两类材料的自旋操纵无法使用先前已知的机制有效地实现。整个项目的目标既包括对电流和磁铁之间相互作用的新的基本理解，也包括可以使用比以前更低的功率或更高的频率运行的实用磁记忆和逻辑设备。

Non-technical abstractWithin the past two years, a new strategy has been discovered for manipulating magnetic devices (such as magnetic random access memories) that is at least a factor of 10 more efficient than any previously known technique. This approach relies on taking advantage of "spin-orbit coupling" -- a strong interaction between the intrinsic spin of an electron and its direction of motion in certain materials. However, the exact mechanism has not yet been resolved. This project seeks to determine the microscopic mechanism behind this new effect, to investigate how best to optimize materials and devices to enable applications, and to explore the scientific opportunities that can be enabled by using this new strategy to manipulate classes of magnetic materials that have not been investigated previously. This research is directly applicable to improving technologies for magnetic memory and logic, and the project will also yield additional broad impacts by providing research education to graduate students and undergraduates that helps teach them to be outstanding independent scientists.Technical abstractThis project builds upon demonstrations by the principal investigator and collaborators that an in-plane current in a heavy-metal/ferromagnet bilayer can provide a very strong torque to the magnetic moment of the ferromagnet on account of the spin Hall effect within the heavy metal. Other research groups have also provided evidence of current-induced torques associated with a different physical mechanism, the Rashba effect, in similar samples. The project will seek to answer unresolved questions about the microscopic mechanisms, symmetries, and strengths of current-induced torques in these heavy metal/ferromagnet bilayers, and how best to optimize materials and devices to enable applications of these torques. The methods employed include techniques to measure current-induced reorientations of magnetic moments in nanofabricated devices and related spin-pumping experiments in which a precessing magnet injects a spin current into the heavy metal. The research involves investigating a wide variety of different materials and multilayer geometries, and also the exploration of new scientific opportunities that can be enabled by using the new current-induced torques to manipulate spins within insulating ferrimagnets and antiferromagnets, two classes of materials in which spin manipulation could not be implemented efficiently using previously-known mechanisms. The overall project objectives include both new fundamental understanding about the interactions between electrical currents and magnets and also practical magnetic memory and logic devices that can operate using much lower power or at much higher frequencies than has been possible previously.