  We have investigated hybridized magnon-phonon excitation (magnon polarons) in
spin Seebeck effects (SSEs) in Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$
(Bi$_x$Y$_{3-x}$IG; $x=0$, $0.5$, and $0.9$) films with Pt contact. We observed
sharp peak structures in the magnetic field $H$ dependence of the longitudinal
SSE (LSSE) voltages, which appear when the phonon dispersions are tangential to
the magnon dispersion curve in Bi$_x$Y$_{3-x}$IG. By increasing the Bi amount
$x$, the peak fields in the LSSE shift toward lower $H$ values due to the
reduction of the sound velocities in Bi$_x$Y$_{3-x}$IG. We also measured the
SSE in a nonlocal configuration and found that magnon-polaron anomalies appear
with different signs and intensities. Our result shows composition-tunability
of magnon-polaron anomalies and provides a clue to further unravel the physics
of magnon-polaron SSEs.
