*

We report measurements of the longitudinal spin-Seebeck coefficient (σ_LSSE) in single crystals^a of the helimagnetic insulator Cu₂OSeO₃ (with 10-nm thick Pt contacts) for the temperature range 0.5 K ≤ T ≤15 K. The magnitude of the maximum σ_LSSE ≈ 20 μV/K at T=5-6 K is the largest known for insulating magnets, consistent with the unprecedentedly large magnon thermal conductivity reported for this compound.^b The temperature dependence of σ_LSSE as well as the applicability of various spin-Seebeck theories will be discussed.

^a J. Panella, B. A. Trump, G. G. Marcus, T. M. McQueen, Cryst. Growth Des. 17, 4944-8 (2017).

^b N. Prasai, B. A. Trump, G. G. Marcus, A. Akopyan, S. X. Huang, T. M. McQueen, and J. L. Cohn, Phys. Rev. B 95, 224407 (2017).