Directional excitation of a high-density magnon gas using coherently driven spin waves

TL;DR

This paper demonstrates how coherent spin waves can be used to generate a highly directional, out-of-equilibrium magnon gas in a magnetic insulator, revealing new control mechanisms for spin transport.

Contribution

It introduces a method to create and image large, directional magnon gases driven by coherent spin waves, surpassing equilibrium density limits.

Findings

Magnon gas extends unidirectionally over hundreds of micrometers.

Gas density exceeds Bose-Einstein distribution expectations.

Driving coherent spin waves causes strong out-of-equilibrium occupation.

Abstract

Controlling magnon densities in magnetic materials enables driving spin transport in magnonic devices. We demonstrate the creation of large, out-of-equilibrium magnon densities in a thin-film magnetic insulator via microwave excitation of coherent spin waves and subsequent multi-magnon scattering. We image both the coherent spin waves and the resulting incoherent magnon gas using scanning-probe magnetometry based on electron spins in diamond. We find that the gas extends unidirectionally over hundreds of micrometers from the excitation stripline. Surprisingly, the gas density far exceeds that expected for a boson system following a Bose-Einstein distribution with a maximum value of the chemical potential. We characterize the momentum distribution of the gas by measuring the nanoscale spatial decay of the magnetic stray fields. Our results show that driving coherent spin waves leads to a strong out-of-equilibrium occupation of the spin-wave band, opening new possibilities for controlling spin transport and magnetic dynamics in target directions.