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

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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.