Observing Differential Spin Currents by Resonant Inelastic X-ray Scattering

TL;DR

This paper demonstrates that resonant inelastic x-ray scattering (RIXS) can directly measure spin currents carried by magnons in magnetic insulators under temperature gradients, advancing experimental techniques in spintronics.

Contribution

It introduces RIXS as a novel method to observe and quantify pure spin currents via magnons, providing new insights into magnon transport properties.

Findings

RIXS intensity is sensitive to non-equilibrium magnon distributions.

Magnon lifetime at finite momentum can be extracted from RIXS data.

The method enables direct measurement of spin currents in magnetic insulators.

Abstract

Controlling spin currents, i.e., the flow of spin angular momentum, in small magnetic devices is the principal objective of spin electronics, a main contender for future energy efficient information technologies. Surprisingly, a pure spin current has never been measured directly since the associated electric stray fields and/or shifts in the non-equilibrium spin-dependent distribution functions are too small for conventional experimental detection methods optimized for charge transport. Here we report that resonant inelastic x-ray scattering (RIXS) can bridge this gap by measuring the spin current carried by magnons -- the quanta of the spin wave excitations of the magnetic order -- in the presence of temperature gradients across a magnetic insulator. This is possible due to the sensitivity of the momentum- and energy-resolved RIXS intensity to minute changes in the magnon distribution under non-equilibrium conditions. We use the Boltzmann equation in the relaxation time approximation to extract transport parameters, such as the magnon lifetime at finite momentum, essential for the realization of magnon spintronics.