Composition-tunable magnon-polaron anomalies in spin Seebeck effects in epitaxial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Bi</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">Y</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Fe</mml:mi><mml:mn>5</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>12</mml:mn></mml:msub></mml:mrow></mml:math> films
Takashi Kikkawa, Koichi Oyanagi, Tomosato Hioki, Masahiko Ishida, Zhiyong Qiu, R. Ramos, Yusuke Hashimoto, Eiji Saitoh
Abstract
Resonant enhancement of spin Seebeck effect (SSE) due to hybridized magnon-phonon excitation (magnon polarons) was recently observed in Y${}_{3}$Fe${}_{5}$O${}_{12}$ (YIG). The effect appears at high magnetic fields when the phonon dispersions are tangential to the magnon dispersion curve. Here, the authors show that the resonance field can be shifted by ~ 2 Tesla to the lower-field side by the Bi substitution in YIG. The result is attributed to the change in the phonon dispersions by the Bi doping. The authors also observe in Bi${}_{0.9}$Y${}_{2.1}$Fe${}_{5}$O${}_{12}$ an enhancement 500% greater than the background magnonic SSE signal at the low temperature of 3 K. Moreover, anisotropic magnon-polaron transport was found through the longitudinal and nonlocal SSE measurements, which provides a clue to further unraveling the physics of magnon-polaron SSEs.