Revealing thermally driven distortion of magnon dispersion by spin Seebeck effect in Gd 3 Fe 5 O 12

We report a systematic study of the temperature and field dependences of the spin Seebeck effect (SSE) in a bilayer of $\mathrm{Pt}/{\mathrm{Gd}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. An anomalous structure is observed in the magnetic field dependent measurements at temperatures between \ensuremath{\sim}60 and \ensuremath{\sim}210 K. Unlike the ordinary SSE signal originating from the bare magnons, which changes sign at \ensuremath{\sim}95 and \ensuremath{\sim}266 K, the sign of the anomalies remains unchanged with increasing temperature. Moreover, the anomalies are found to show a temperature-sensitive double-peak structure between \ensuremath{\sim}116 and \ensuremath{\sim}143 K. We attribute these anomalies to the contribution from the quasiparticles hybridized between the Gd moment dominated spin wave (\ensuremath{\alpha} mode) and the transversal acoustic phonon, known as the magnon polarons. Given that the magnon polaron induced anomalies occur at the field where the linear phonon dispersion is tangential to the magnon dispersion curve, we explain these rich phenomena by an increase of the group velocity of the \ensuremath{\alpha}-mode magnon with increasing temperature and the nonparabolic magnon dispersion of ${\mathrm{Gd}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. Our results demonstrate that the magnon polaron induced SSE is helpful for the investigation of the magnon dispersion evolution with a simple transport approach.