Extrinsic scattering induced magnon spin swapping effect in Bi-doped yttrium iron garnet

Extrinsic scattering by impurities can alter the directions of flow and spin polarization of electrons, significantly advancing the generation and manipulation of electronic spin currents. It should also enable control of these properties of magnons, realizing the generation and manipulation of magnonic spin currents. However, no experimental observation of extrinsic scattering induced analogous magnon phenomenon has been made so far. Here, we report the observation of a thermal voltage converted from the extrinsic scattering induced magnon spin swapping effect in a ferrimagnetic insulator Bi-doped yttrium iron garnet $\mathrm{B}{\mathrm{i}}_{1}{\mathrm{Y}}_{2}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$, triggered by an in-plane temperature gradient. The thermal voltage disappears without Bi dopants and shows nearly linear scaling with the Bi amount, leading to an unambiguous ascription of this magnon transport phenomenon to the extrinsic scattering by impurities and ruling out other possible thermoelectric mechanisms. Our study not only provides compelling evidence for the extrinsic scattering induced magnon spin swapping effect by Bi impurities doping, but it also expands the range of materials and the means for controlling magnon transport.