Spin-transfer torques in antiferromagnets: efficiency and quantification method

TL;DR

This paper develops a comprehensive theory of spin-transfer torques in antiferromagnets, defining efficiency in terms of material parameters and analyzing spin-wave dispersions to understand current-induced effects.

Contribution

It introduces a unified theoretical framework for spin-transfer torques in antiferromagnets, covering various exchange coupling regimes and providing quantification methods based on spin-wave analysis.

Findings

Charge current couples to different order parameters depending on exchange coupling strength.

Spin-wave Doppler shift occurs in the large exchange coupling limit.

Only modes with sublattice-canting moments react to current in the small exchange limit.

Abstract

We formulate a theory of spin-transfer torques in antiferromagnets, which covers the small to large limits of the exchange coupling energy relative to the kinetic energy of the inter-sublattice electron dynamics. Our theory suggests a natural definition of the efficiency of spin-transfer torques in antiferromagnets in terms of well-defined material parameters, revealing that the charge current couples predominantly to the antiferromagnetic order parameter and the sublattice-canting moment in, respectively, the limits of large and small exchange coupling. The effects can be quantified by analyzing the antiferromagnetic spin-wave dispersions in the presence of charge current: in the limit of large exchange coupling the spin-wave Doppler shift always occurs, whereas, in the opposite limit, the only spin-wave modes to react to the charge current are ones that carry a pronounced sublattice-canting moment. The findings offer a framework for understanding and designing spin-transfer torques in antiferromagnets belonging to different classes of sublattice structures such as, e.g., bipartite and layered antiferromagnets.