Topological spin transport by Brownian diffusion of domain walls

TL;DR

This paper introduces a novel topological spin transport mechanism using thermally-generated domain walls in ferromagnetic insulators, which enables algebraic decay of spin signals over distance, contrasting with traditional magnon decay.

Contribution

It proposes a new topological spin transport method leveraging domain wall chirality and topological charge conservation, leading to algebraic decay of spin currents.

Findings

Topological domain walls can carry spin over long distances.

Spin transport exhibits algebraic decay, not exponential.

Chirality-dependent nucleation influences spin current direction.

Abstract

We propose thermally-populated domain walls (DWs) in an easy-plane ferromagnetic insulator as robust spin carriers between two metals. The chirality of a DW, which serves as a topological charge, couples to the metal spin accumulation via spin-transfer torque and results in the chirality-dependent thermal nucleation rates of DWs at the interface. After overpopulated DWs of a particular (net) chirality diffuse and leave the ferromagnet at the other interface, they reemit the spin current by spin pumping. The conservation of the topological charge supports an algebraic decay of spin transport as the length of the ferromagnet increases; this is analogous to the decaying behavior of superfluid spin transport but contrasts with the exponential decay of magnon spin transport. We envision that similar spin transport with algebraic decay may be implemented in materials with exotic spin phases by exploiting topological characteristics and the associated conserved quantities of their excitations.