  Spin information carried by magnons is attractive for computing technology
and the development of magnon-based computing circuits is of great interest.
However, magnon transport in insulators has been challenging, different from
the clear physical picture for spin transport in conductors. Here we
investigate the lateral transport properties of thermally excited magnons in
yttrium iron garnet (YIG), a model magnetic insulator. Polarity reversals of
detected spins in non-local geometry devices have been experimentally observed
and are strongly dependent on temperature, YIG film thickness, and
injector-detector separation distance. A competing two-channel transport model
for thermally excited magnons is proposed, which is qualitatively consistent
with the spin signal behavior. In addition to the fundamental significance for
thermal magnon transport, our work furthers the development of magnonics by
creating an easily accessible magnon source with controllable transport
