Spin Seebeck Effect in Helimagnetic Cu$_2$OSeO$_3$: Test of Bulk Magnon Spin Current Theory*

Cu$_2$OSeO$_3$ exhibits$^a$ the largest magnon thermal conductivity of any known ferro- or ferrimagnet. Here we report temperature ($T$) and magnetic field-dependent measurements of magnon thermal conductivity ($\kappa_m$) and longitudinal spin Seebeck coefficient ($S_{LSSE}$) in 10-nm Pt/Cu$_2$OSeO$_3$ heterostructures for 1~K $\leq T\leq$~15~K. Measurements for three different specimens, having spin-mixing conductances that vary by more than an order of magnitude, demonstrate a relationship between $\kappa_m$ and $S_{LSSE}$ in good agreement with bulk magnon spin current theory.
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$^a$ N. Prasai {\it et al.}, Phys. Rev. B {\bf 95}, 224407 (2017).