Topological Hall effect in diffusive ferromagnetic thin films with spin-flip scattering

TL;DR

This paper investigates how spin-flip scattering affects the topological Hall effect in ferromagnetic thin films, revealing that spin accumulation can suppress the Hall current unless skyrmions are very small, with implications for nanoscale devices.

Contribution

It derives a generalized spin diffusion equation including spin-flip scattering effects and provides a formula for topological Hall resistivity applicable to thin films with small skyrmions.

Findings

Spin accumulation can suppress the topological Hall current.

The Hall resistivity approaches the original value when skyrmions are much smaller than the spin diffusion length.

Corrections to Hall resistivity become significant for skyrmions at tens of nanometers.

Abstract

We study the topological Hall effect in a diffusive ferromagnetic metal thin film by solving a Boltzmann transport equation in the presence of spin-flip scattering. A generalized spin diffusion equation is derived, which contains an additional source term associated with the gradient of the emergent magnetic field that arises, and its solution shows that spin accumulation may build up in the vicinity of the magnetic skyrmions. We show that the spin accumulation gives rise to a spin polarized diffusion current that in general suppresses that bulk topological Hall current. Only when the spin diffusion length is much smaller than the skyrmion size, the topological Hall resistivity approaches to the one originally derived by Bruno \textit{et al.} [Phys. Rev. Lett. \textbf{93}, 096806 (2004)]. We derive a general expression of the TH resistivity that applies to thin-film geometries with spin-flip scattering, and show that the corrections to the TH resistivity become large when the size of room temperature skyrmions is further reduced to tens of nanometers.