Computational design and modeling of topological insulator-based heterostructures for spin-orbitronics and skyrmionics

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Spintronics, which explores phenomena intertwining spin and charge carried by an electron, has exhibited a remarkable ability to re-energize itself in directions that germinate fertile subfields for basic research aimed at future applications. The first generation spintronics, which has led to revolutionary increase in the amount of digital information stored on hard drives, has also crucially relied on the discovery of new materials and heterostructures. The next generation spintronics is expected to lead to ultralow power dissipation memory and logic devices that can be integrated with conventional electronics. The projects in this proposal will explore emerging resource for such advances brought about by the special relativistic effects, termed spin-orbit coupling (SOC), in recently discovered topological insulator (TI) materials. The TIs are insulating in their bulk, but also host metallic surfaces with strong SOC. The very recent experiments have demonstrated that when current is injected into heterostructures where TI is attached to a ferromagnetic (FM) layer, magnetization dynamics of the FM layer can be ignited with potentially much less dissipation than in presently available technologies underlying magnetic random access memory. The interface between TI and FM layers with strong SOC and broken inversion symmetry could also generate swirling spin texture characterized by nano-scale size, topological stability against defects and impurities, and gyro-dynamics analogous to that of a charged particle under magnetic field. Using high performance computing (HPC) simulations to search for optimal combination of TI and FM materials for these phenomena can significantly shorten time needed to produce functional devices. Broader impact of the proposed research will, include training for graduate students in nonequilibrium quantum statistical mechanics, advanced scientific computing techniques and fundamental understanding of TI-based heterostructures under nonequilibrium conditions. Students will interact with the international collaborators. New modeling software and computational design for high-density data-storage and nonvolatile memory with ultra-low energy cost manipulation developed under the program will be available for researchers in the field.Using combination of nonequilibrium Green function theory (NEGF), noncollinear density functional theory (DFT), and semiclassical Langevin equation techniques, this research program will develop fundamental understanding of: SO torque; spin pumping and spin-to-charge conversion; and Gilbert damping and noise effects on magnetization dynamics in the presence of strong interfacial SOC brought by TIs. The proposed research will commence with first principles screening of TI/FM heterostructures in order to identify those with the largest SOC proximity effect onto the magnetic atoms around the interface or interfacial Dzyaloshinsky-Moriya interaction that could give rise to skyrmions at room temperature (all presently known examples of magnetic skyrmions occur below room temperature). In the second stage, SO torque for most promising heterostructures will be computed. The SO torque will be used as an input for the stochastic Landau-Lifshitz-Gilbert (LLG) equation to study slow dynamics of (classically described) magnetization or skyrmion spin texture in the presence of fast moving (quantum-mechanically described) electrons, including damping and nonequilibrium noise effects generated by them. This research will form the basis for next generation of ultralow power dissipation memory and logic spintronic devices based on topological insulators.