Anomalous domain wall velocity and Walker breakdown in hybrid systems with anisotropic exchange

TL;DR

This paper analytically investigates domain wall dynamics in magnetic systems with anisotropic exchange, revealing unique velocity limits and Walker breakdown behavior that differ from conventional models, with implications for spintronic applications.

Contribution

It provides analytical expressions for domain wall velocity and Walker breakdown in anisotropic exchange systems, highlighting their independence from the nonadiabaticity parameter beta.

Findings

Maximum domain wall velocity is independent of beta.

Walker breakdown threshold decreases with increasing beta.

Anisotropic exchange torque significantly enhances spin-transfer effects.

Abstract

It has recently been proposed that spin-transfer torques in magnetic systems with anisotropic exchange can be strongly enhanced, reducing the characteristic current density with up to four orders of magnitude compared to conventional setups. Motivated by this, we analytically solve the equations of motion in a collective-coordinate framework for this type of anisotropic exchange system, to investigate the domain wall dynamics in detail. In particular, we obtain analytical expressions for the maximum attainable domain wall velocity of such a setup and also for the occurrence of Walker breakdown. Surprisingly, we find that, in contrast to the standard case with domain wall motion driven by the nonadiabatic torque, the maximum velocity obtained via the anisotropic exchange torque is completely independent of the nonadiabaticity parameter beta, in spite of the torque itself being very large for small beta. Moreover, the Walker breakdown threshold has an opposite dependence on beta in these two cases; i.e., for the anisotropic exchange torque scenario, the threshold value decreases monotonically with beta. These findings are of importance to any practical application of the proposed giant spin-transfer torque in anisotropic exchange systems.