  The recent experiment [Y. Wang et al., Science 366, 1125 (2019)] on
magnon-mediated spin-transfer torque (MSTT) was interpreted in terms of a
picture where magnons are excited within an antiferromagnetic insulator (AFI),
by applying nonequilibrium electronic spin density at one of its surfaces, so
that their propagation across AFI deprived of conduction electrons eventually
leads to reversal of magnetization of a ferromagnetic metal (FM) attached to
the opposite surface of AFI. We employ a recently developed time-dependent
nonequilibrium Green functions combined with the Landau-Lifshitz-Gilbert
equation (TDNEGF+LLG) formalism to evolve conduction electrons
quantum-mechanically while they interact via self-consistent back-action with
localized magnetic moments described classically by atomistic spin dynamics
solving a system of LLG equations. Upon injection of square current pulse as
the initial condition, TDNEGF+LLG simulations of FM-polarizer/AFI/FM-analyzer
junctions show that reversal of localized magnetic moments within FM-analyzer
is less efficient, in the sense of requiring larger pulse height and its longer
duration, than conventional electron-mediated STT (ESTT) driving magnetization
switching in standard FM-polarizer/normal-metal/FM-analyzer spin valve. Since
both electronic, generated by spin pumping from AFI, and magnonic, generated by
direct transmission from AFI, spin currents are injected into the FM-analyzer,
its localized magnetic moments will experience combined MSTT and ESTT.
Nevertheless, by artificially turning off ESTT we demonstrate that MSTT plays a
dominant role whose understanding, therefore, paves the way for
all-magnon-driven magnetization switching devices with no electronic parts.
