Nonlocal Drag of Magnons in a Ferromagnetic Bilayer

TL;DR

This paper predicts a nonlocal magnon drag effect in ferromagnetic bilayers, where magnon currents in one layer induce chemical potential and temperature gradients in the other, driven by magnetic dipolar interactions.

Contribution

It introduces the concept of nonlocal magnon drag in ferromagnetic bilayers, extending Coulomb drag analogies to magnonic systems using semiclassical Boltzmann equations.

Findings

Magnon current induces measurable temperature gradients in the adjacent layer.

Maximum drag occurs when magnon flow is parallel to magnetization.

Transverse magnon currents emerge for oblique magnon flows.

Abstract

Quantized spin waves, or magnons, in a magnetic insulator are assumed to interact weakly with the surroundings, and to flow with little dissipation or drag, producing exceptionally long diffusion lengths and relaxation times. In analogy to Coulomb drag in bilayer two dimensional electron gases, in which the contribution of the Coulomb interaction to the electric resistivity is studied by measuring the interlayer resistivity (transresistivity), we predict a nonlocal drag of magnons in a ferromagnetic bilayer structure based on semiclassical Boltzmann equations. Nonlocal magnon drag depends on magnetic dipolar interactions between the layers and manifests in the magnon current transresistivity and the magnon thermal transresistivity, whereby a magnon current in one layer induces a chemical potential gradient and/or a temperature gradient in the other layer. The largest drag effect occurs when the magnon current flows parallel to the magnetization, however for oblique magnon currents a large transverse current of magnons emerges. We examine the effect for practical parameters, and find that the predicted induced temperature gradient is readily observable.