  In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down
magnons coexist and form an intrinsic degree of freedom resembling electrons.
When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can
be thermally generated in an AF. We explore thermal magnon transport in an
insulating F/AF/F trilayer where propagating magnons inside the AF spacer can
transfer angular momenta between the two Fs. We find that a sufficiently large
temperature gradient can switch the downstream F via magnonic spin-transfer
torque if it is initially antiparallel with the upstream F. A reciprocal
switching is achievable by reversing the temperature gradient. Using typical
material parameters, we estimate the threshold to be less than 1 K/nm at room
temperature, which can be reduced by raising temperature and enhancing the
interfacial exchange coupling.
