Dynamics of Antiferromagnets Driven by Spin Current

TL;DR

This paper develops a microscopic theory for how spin currents influence antiferromagnets, revealing mechanisms for domain wall motion and spin wave instabilities without relying on spin angular momentum conservation.

Contribution

It introduces a novel microscopic framework for AFM dynamics driven by spin current, independent of spin angular momentum conservation and induced ferromagnetic moments.

Findings

Domain walls are accelerated to a terminal velocity without Walker's breakdown.

Spin injection alters AFM resonance frequency.

Spin current injection can trigger spin wave instability.

Abstract

When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the background magnetization via spin-transfer torques. In antiferromagnetic (AFM) materials, however, the corresponding problem is unsolved. We derive microscopically the dynamics of an AFM system driven by spin current generated by an attached FM polarizer, and find that the spin current exerts a driving force on the local staggered order parameter. The mechanism does not rely on the conservation of spin angular momentum, nor does it depend on the induced FM moments on top the AFM background. Two examples are studied: (i) A domain wall is accelerated to a terminal velocity by purely adiabatic effect where the Walker's break-down is avoided; and (ii) Spin injection modifies the AFM resonance frequency, and spin current injection triggers spin wave instability of local moments above a threshold.