Magnon contributions to thermal conductivity and thermopower in a metallic thin film

Recent theoretical and experimental work has renewed interest in the role of magnons in the transport and thermoelectric properties of metallic ferromagnets. Magnon Drag is one consequence of the electron-magnon interaction whereby the spin excitations in a magnetic material transfer momentum to the electron system and increase the thermopower. Recent theoretical approaches clarify that magnon drag understandably depends on the Gilbert damping, $\alpha $, present in a given material. [1] The simplest theory predicts a magnon drag thermopower S$_{md}$ that is maximized by reduction of $\alpha $. Here we show that a low-damping metal, such as the Co$_{25}$Fe$_{75}$ alloy thin film [2] that has intrinsic Gilbert damping approaching the 10$^{-4}$ level typically seen only in ferromagnetic insulators, has thermal conductivity that deviates strongly from typical metal films, with a significant peak in thermal conductivity at 225K. This material also has a large deviation from the expected Seebeck coefficient estimated from the alloy's composition and density of states. These results suggest a large contribution from a magnon or spin effect due to the intrinsic low damping of magnetization dynamics in the metal. [1] S. J. Watzman, et al. ``Magnon-drag thermopower and Nernst coefficient in Fe, Co, and Ni.'' PRB 94, 144407 (2016). [2] M. A. W. Schoen, et al. ``Ultra-low magnetic damping of a metallic ferromagnet.'' Nature Physics 12, 839 (2016).