Composition-tunable magnon-polaron anomalies in spin Seebeck effects in epitaxial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Bi</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">Y</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Fe</mml:mi><mml:mn>5</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>12</mml:mn></mml:msub></mml:mrow></mml:…

Resonant enhancement of spin Seebeck effect (SSE) due to hybridized magnon-phonon excitation (magnon polarons) was recently observed in Y${}_{3}$Fe${}_{5}$O${}_{12}$ (YIG). The appears at high magnetic fields when the phonon dispersions are tangential magnon dispersion curve. Here, authors show that resonance field can be shifted by ~ 2 Tesla lower-field side Bi substitution YIG. result is attributed change doping. also observe Bi${}_{0.9}$Y${}_{2.1}$Fe${}_{5}$O${}_{12}$ an 500% greater than background magnonic SSE signal low temperature 3 K. Moreover, anisotropic magnon-polaron transport found through longitudinal and nonlocal measurements, which provides a clue further unraveling physics SSEs.