Effective spin Hall properties of a mixture of materials with and without spin-orbit coupling: Tailoring the effective spin-diffusion length

TL;DR

This paper theoretically investigates how a mixture of materials with and without spin-orbit coupling can enhance effective spin Hall properties, revealing significant effects on spin diffusion length and voltage response to magnetic fields.

Contribution

It introduces a model for composite materials with mixed spin-orbit properties and demonstrates how the effective spin diffusion length can be greatly enhanced when the bare length is small.

Findings

Effective spin diffusion length is strongly enhanced beyond geometric expectations.

Voltage generated by spin current is highly sensitive to weak magnetic fields.

Spin precession outside grains influences voltage sign reversal.

Abstract

We study theoretically the effective spin Hall properties of a composite consisting of two materials with and without spin-orbit (SO) coupling. In particular, we assume that SO material represents a system of grains in a matrix with no SO. We calculate the effective spin Hall angle and the effective spin diffusion length of the mixture. Our main qualitative finding is that, when the bare spin diffusion length is much smaller than the radius of the grain, the effective spin diffusion length is strongly enhanced, well beyond the "geometrical" factor. The physical origin of this additional enhancement is that, with small diffusion length, the spin current mostly flows around the grain without suffering much loss. We also demonstrate that the voltage, created by a spin current, is sensitive to a very weak magnetic field directed along the spin current, and even reverses sign in a certain domain of fields. The origin of this sensitivity is that the spin precession, caused by magnetic field, takes place outside the grains where SO is absent.