Magnon spin-transfer torque from antiferromagnetic insulator to ferromagnetic metal: Time-dependent quantum transport combined with atomistic spin dynamics approach

The recent experiments [Y. Wang {\em et al.}, Science {\bf 366}, 1125 (2019)] on {\em magnon-mediated} spin-transfer torque (MSTT) have been interpreted in terms of a qualitative picture where magnons are excited within an antiferromagnetic insulator (AFI), by applying nonequilibrium spin density on its left edge, so that propagating magnons (without any motion of electrons) across AFI eventually lead to reversal of magnetization of a ferromagnetic metal (FM) attached to the right edge of AFI. We employ a recently developed time-dependent nonequilibrium Green functions combined with the Landau-Lifshitz-Gilbert equation (TDNEGF+LLG) formalism to evolve electrons quantum-mechanically while they interact via self-consistent back-action with localized magnetic moments described classically by atomistic spin dynamics based on solving a system of LLG equations. Upon injection of square current pulse as the initial condition, TDNEGF+LLG simulations of localized magnetic moments switching within FM-analyzer of FM-polarizer/AFI/FM-analyzer junction show that it is less efficient, in the sense of requiring larger pulse height and its longer duration, than conventional electron-mediated STT (ESTT)-driven switching in standard FM-polarizer/normal-metal/FM-analyzer spin valves. Since {\em both} electronic (via spin pumping from AFI) and magnonic (via direct transmission from AFI) spin currents are injected into the FM-analyzer, its localized magnetic moments will experience combined MSTT and ESTT. Nevertheless, we demonstrate, by artificially turning off ESTT, that MSTT {\em dominates} the switching, so that direct exchange coupling between AFI layer and FM-analyzer is required to obtain complete reversal of localized magnetic moments within the FM-analyzer.