Spin Transfer Torques Arising from Spin-Orbit Interactions

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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.