Current-induced spin torques in III-V ferromagnetic semiconductors

TL;DR

This paper develops a theoretical framework for understanding current-induced spin torques in III-V ferromagnetic semiconductors, highlighting differences from metallic systems due to spin-orbit interactions and non-conservation of spin.

Contribution

It introduces a novel theory accounting for spin-3/2 carriers and spin-orbit effects, providing insights into spin dynamics and domain wall motion in ferromagnetic semiconductors.

Findings

Spin polarization is smaller than in metals.

Spin non-conservation complicates torque expressions.

Scalar and spin-dependent scattering do not alter results.

Abstract

We formulate a theory of current-induced spin torques in inhomogeneous III-V ferromagnetic semiconductors. The carrier spin-3/2 and large spin-orbit interaction, leading to spin non-conservation, introduce significant conceptual differences from spin torques in ferromagnetic metals. We determine the spin density in an electric field in the weak momentum scattering regime, demonstrating that the torque on the magnetization is intimately related to spin precession under the action of both the spin-orbit interaction and the exchange field characteristic of ferromagnetism. The spin polarization excited by the electric field is smaller than in ferromagnetic metals and, due to lack of angular momentum conservation, cannot be expressed in a simple closed vectorial form. Remarkably, scalar and spin-dependent scattering do not affect the result. We use our results to estimate the velocity of current-driven domain walls.