Chemical potential of quasi-equilibrium magnon gas driven by pure spin current

V E Demidov,S Urazhdin,V V Ustinov,V V Ustinov,A B Rinkevich,B Divinskiy,S O Demokritov,V D Bessonov

doi:10.1038/s41467-017-01937-y

Abstract

Pure spin currents provide the possibility to control the magnetization state of conducting and insulating magnetic materials. They allow one to increase or reduce the density of magnons, and achieve coherent dynamic states of magnetization reminiscent of the Bose–Einstein condensation. However, until now there was no direct evidence that the state of the magnon gas subjected to spin current can be treated thermodynamically. Here, we show experimentally that the spin current generated by the spin-Hall effect drives the magnon gas into a quasi-equilibrium state that can be described by the Bose–Einstein statistics. The magnon population function is characterized either by an increased effective chemical potential or by a reduced effective temperature, depending on the spin current polarization. In the former case, the chemical potential can closely approach, at large driving currents, the lowest-energy magnon state, indicating the possibility of spin current-driven Bose–Einstein condensation.

Highlights

Pure spin currents provide the possibility to control the magnetization state of conducting and insulating magnetic materials
We utilize a Permalloy/Pt bilayer to study the effect of pure spin current on the magnon distribution over a significant spectral range, allowing us to demonstrate that this distribution can be described by the Bose–Einstein statistics expected for the quasi-equilibrium state, and determine the current-dependent chemical potential and effective temperature
We study the magnon population by the microfocus Brillouin light scattering (BLS) technique[25]

Summary

Introduction

Pure spin currents provide the possibility to control the magnetization state of conducting and insulating magnetic materials. Recent theoretical studies[17,18,19,20,21] suggest that spin current can drive the magnon gas into a quasi-equilibrium state described by the Bose–Einstein statistics with non-zero chemical potential, suggesting the possibility of BEC formation at sufficiently large currents. These theories have been supported by the successful application of the developed theoretical framework to incoherent magnon transport[20,22]. For the opposite polarization, the effective temperature remains nearly unaffected, whereas the chemical potential linearly increases with current until it closely approaches the lowest-energy magnon state

Methods

Results

Conclusion