Hybridized magnon-phonon excitation in spin Seebeck effect in Bi_3-<i>x</i>Y_xFe₅O₁₂ films

The spin Seebeck effect (SSE) refers to the generation of a spin current in a magnetic material by a temperature gradient. The spin current is detected as an electric voltage via the inverse spin Hall effect in a metal attached to the magnet. Recently we reported that anomalous asymmetric structures appear in the magnetic field dependence of the SSE in Pt/Y₃Fe₅O₁₂ (YIG) systems when the magnon and phonon dispersion curves of YIG touch, which maximizes the phase space of hybridized magnon-phonon excitation (or magnon polarons) [1-3].
In this talk, we report the effect of the Bi substitution to YIG on the magnon-polaron features in the SSE. We found that the anomaly field as well as the amplitude of the anomaly change by changing the amount of the Bi substitution. We explain the result in terms of the modification of magnon/phonon dispersion relations and the enhanced magnetoelastic interaction by the Bi substitution. Our results demonstrate that the SSE provides a sensitive probe for coupled magnon and phonon dynamics.
[1] T. Kikkawa et al., Phys. Rev. Lett. 117, 207203 (2016).; [2] B. Flebus, K. Shen, T. Kikkawa et al., Phys. Rev. B 95, 144420 (2017).; [3] L. J. Cornelissen, K. Oyanagi, T. Kikkawa et al., Phys. Rev. B 96, 104441 (2017).