Spectral Characteristics of Quasi-Equilibrium Magnon Gas Driven by Pure Spin Current

The discovery of the magnon Bose-Einstein condensate (BEC) in magnetic insulators driven by parametric pumping has spurred intense studies of thermodynamics of driven magnon gases. We utilized micro-focus Brillouin Light Spectroscopy to experimentally show that the spin current generated by the spin Hall effect can efficiently drive the magnon gas in a magnetic film into a quasi-equilibrium state well described by the Bose-Einstein statistics.

These effects are studied in relatively thick Permalloy (Py) where the steep spin wave dispersion allows optical access to a large spectral range. For spin currents resulting in the magnon injection, we observe a spin current-independent effective temperature, while chemical potential increases with current until it almost reaches the energy of the lowest magnon state. For the opposite spin current, we observe a decrease of the effective temperature at a constant chemical potential. We interpret our results in terms of the mechanisms of interaction between spin current and magnetization.

We also use an alternative approach enabling access to a large spectral range in thin Py films where spin current-induced auto-oscillation is possible, allowing us to analyze the relation between the auto-oscillations and BEC.