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
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