Composition-tunable magnon-polaron anomalies in spin Seebeck effects in epitaxial Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ films

TL;DR

This study demonstrates how the composition of Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ films can tune magnon-polaron features in spin Seebeck effects, revealing their dependence on phonon-magnon interactions and sound velocities.

Contribution

It introduces the first systematic investigation of composition-dependent magnon-polaron anomalies in spin Seebeck effects in epitaxial Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ films.

Findings

Sharp peaks in LSSE voltages correlate with phonon-magnon dispersion intersections.

Peak positions shift with Bi content due to changes in sound velocities.

Nonlocal SSE measurements show anomalies with different signs and intensities.

Abstract

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.