Green's function formalism for nonlocal elliptical magnon transport

TL;DR

This paper introduces a Green's function approach to analyze nonlocal magnonic spin transport in anisotropic ferromagnetic insulators, revealing a parasitic conductance and spin squeezing effects influenced by anisotropy and local spin bias.

Contribution

It presents a novel formalism for non-equilibrium magnon transport in anisotropic systems, highlighting the impact of elliptic polarization and spin squeezing.

Findings

Parasitic spin conductance dominates at high anisotropy.

Magnons become elliptically polarized due to anisotropy.

Local spin bias can reduce spin squeezing.

Abstract

We develop a non-equilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreover, the spin state of the ferromagnet becomes squeezed in the high-anisotropy regime. We show that the squeezing may be globally reduced by the application of a local spin bias.