Ballistic magnon heat conduction and possible Poiseuille flow in the helimagnetic insulator Cu2OSeO3

We report on the observation of magnon thermal conductivity kappa(m) similar to 70 W/mK near 5 K in the helimagnetic insulator Cu2OSeO3, 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 2K < T < 10 K. These observations establish Cu2OSeO3 as a model system for studying long-wavelength magnon dynamics.