Nonlocal magnon-polaron transport in yttrium iron garnet

Magnon-polarons are coherently mixed quasiparticles -- half magnon, half phonon -- and are generated by magnetoelasticity. Though predicted by Kittel long ago, their effect on spin transport was discovered only recently in yttrium iron garnet (YIG) using optical and local spin Seebeck effect (SSE) experiments. In the latter, magnon-polarons are manifest as resonant peaks in the SSE signal as a function of magnetic field. Here, the authors show that they also show up in the n00exo00exn00exl00exo00exc00exa00exl SSE, with a surprising twist: measured nonlocally, the magnon-polaron peak turns into a dip. The authors show that this crossover is a consequence of the magnon physics underlying the SSE: thermal generation and diffusive backflow of magnons in YIG compete, which can generate any sign for the magnon-polaron anomaly in nonlocal experiments.