  The spin Seebeck effect (SSE) signal of magnon polarons in bulk-Y3Fe5O12
(YIG)/Pt heterostructures is found to drastically change as a function of
temperature. It appears as a dip in the total SSE signal at low temperatures,
but as the temperature increases, the dip gradually decreases before turning to
a peak. We attribute the observed dip-to-peak transition to the rapid rise of
the four-magnon scattering rate. Our analysis provides important insights into
the microscopic origin of the hybridized excitations and the overall
temperature dependence of the SSE anomalies.
