# Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids

## TL;DR

This paper theoretically demonstrates that viscous magnon fluids in ferromagnets cause a sign change in nonlocal resistance, revealing hydrodynamic behavior and backflow effects, thus providing a new way to probe magnon-fluid dynamics.

## Contribution

It introduces a theoretical framework linking viscous effects in magnon fluids to measurable sign changes in nonlocal spin transport signals.

## Key findings

- Sign change in nonlocal resistance due to viscous effects
- Detection of magnon-fluid hydrodynamics via nonlocal measurements
- Observation of magnon backflow near injector lead

## Abstract

Magnons in ferromagnets behave as a viscous fluid over a length scale, the momentum-relaxation length, below which momentum-conserving scattering processes dominate. We show theoretically that in this hydrodynamic regime viscous effects lead to a sign change in the magnon chemical potential, which can be detected as a sign change in the nonlocal resistance measured in spin transport experiments. This sign change is observable when the injector-detector distance becomes comparable to the momentum-relaxation length. Taking into account momentum- and spin-relaxation processes, we consider the quasiconservation laws for momentum and spin in a magnon fluid. The resulting equations are solved for nonlocal spin transport devices in which spin is injected and detected via metallic leads. Because of the finite viscosity we also find a backflow of magnons close to the injector lead. Our work shows that nonlocal magnon spin transport devices are an attractive platform to develop and study magnon-fluid dynamics.