Propulsion of a domain wall in an antiferromagnet by magnons

TL;DR

This paper investigates how magnons can propel a domain wall in an antiferromagnet through two mechanisms, redshift and reflection, with the dominant mechanism depending on magnon intensity, supported by theoretical and simulation results.

Contribution

It introduces and analyzes the redshift mechanism as a new way magnons transfer momentum to domain walls, complementing the known reflection mechanism.

Findings

Redshift dominates at low magnon intensities.

Reflection becomes significant at higher magnon intensities.

Theoretical predictions align with micromagnetic simulations.

Abstract

We analyze the dynamics of a domain wall in an easy-axis antiferromagnet driven by circularly polarized magnons. Magnons pass through a stationary domain wall without reflection and thus exert no force on it. However, they reverse their spin upon transmission, thereby transferring two quanta of angular momentum to the domain wall and causing it to precess. A precessing domain wall partially reflects magnons back to the source. The reflection of spin waves creates a previously identified reactive force. We point out a second mechanism of propulsion, which we term redshift: magnons passing through a precessing domain wall lower their frequency by twice the angular velocity of the domain wall; the concomitant reduction of magnons' linear momentum indicates momentum transfer to the domain wall. We solve the equations of motion for spin waves in the background of a uniformly precessing domain wall with the aid of supersymmetric quantum mechanics and compute the net force and torque applied by magnons to the domain wall. Redshift is the dominant mechanism of propulsion at low spin-wave intensities; reflection dominates at higher intensities. We derive a set of coupled algebraic equations to determine the linear velocity and angular frequency of the domain wall in a steady state. The theory agrees well with numerical micromagnetic simulations.