Ballistic magnon heat conduction and possible Poiseuille flow in the helimagnetic insulator Cu${}_2$OSeO${}_3$

We report on the observation of magnon thermal conductivity ${\kappa }_m\sim$ 70 W/mK near 5 K in the helimagnetic insulator Cu${}_2$OSeO${}_3$, exceeding that measured in any other ferromagnet by almost two orders of magnitude. Ballistic, boundary-limited transport for both magnons and phonons is established below 1 K, and Poiseuille flow of magnons is proposed to explain a magnon mean-free path substantially exceeding the specimen width for the least defective specimens in the range 2 K $<T<$ 10 K. These observations establish Cu${}_2$OSeO${}_3$ as a model system for studying long-wavelength magnon dynamics.