Computational design of magnon spintronic devices with multiscale approach by combining time-dependent quantum transport with classical micromagnetics

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Conventional spintronics employs spin currents of electrons to carry, transport and process information. However, they decay over short distances while also generating Joule heat losses. Electronic spin current impinging onto a magnetic insulator is converted into spin current carried by collective motion of localized magnetic moments. Such spin wave or magnon spin currents can propagate over long distances without Joule heat losses since electrons do not move through an insulator. Even slower spatial decay of spin currents becomes possible when they are transported by spiraling textures of precessing magnetic moments within room temperature magnetic insulators with easy-plane anisotropy, sharing many features of coherent and superfluid transport without dissipation at cryogenic temperatures. In addition, the spin waves can interfere and exhibit nonlinear wave interaction which can be exploited for novel wave-based logic gates, as well as for mixing of logic and memory on the same chip to evade the so-called memory wall as a bottleneck in data exchange between distant slow memory and fast logic gates in conventional electronics. The superfluid spin transport can be exploited to create magnetic analogues of superconducting Josephson junctions. The proposed research will employ newly developed computational tools to simulate interconversion between electronic spin currents and spin wave or superfluid spin currents, thereby offering a precise guidance for device fabrication with optimal control of long-distance and low-dissipation spin-encoded information flow across magnetic insulators and its probing in the bulk or near interfaces with normal metals contacts. Broader impact of the proposed research will include: design of building blocks for novel computing technologies with ultralow power consumption; training for graduate students in advanced quantum device modeling and supercomputing simulations; and creation of new publicly available device modeling software.The scalability of magnon spintronic devices requires efficient schemes to excite exchange spin waves of short wavelength, which will be investigated by simulating spin waves of nanoscale wavelength generated by annihilation of two magnetic domain walls or by a single current-driven domain wall. These setups will also underlie simulations of logic-in-memory devices where domain walls store binary information while spin waves perform logic operations by traversing them. Magnon valves and transistors which modulate spin wave spin current across ferro- and antiferromagnetic insulators with easy-axis anisotropy will be simulated. Schemes to modulate superfluid spin currents across magnetic insulators with easy-plane anisotropy will be investigated, including magnetic analogues of Josephson junctions. In all of these devices, readout of information processed by magnetic insulator requires to eventually convert spin currents carried by the dynamics of localized magnetic moments into conventional electronic spin and charge currents. The whole process will be modeled microscopically by utilizing and further advancing theoretical and computational capabilities of recently developed multiscale framework which self-consistently combines time-dependent nonequilibrium Green function algorithms for electrons with the Landau-Lifshitz-Gilbert equation for classical dynamics of localized magnetic moments. This framework makes it possible for the first time to directly, and in time-resolved fashion, investigate how injection of electronic spin current (steady or pulsed) excites spin waves or superfluid spin transport by spin torque and, how they excite electronic spin currents via the spin pumping effect.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.