Current driven defect unbinding transition in an XY ferromagnet

TL;DR

This paper develops a theoretical framework to understand how electric current induces a defect-unbinding transition in an XY ferromagnet, highlighting non-equilibrium effects distinct from thermal transitions.

Contribution

It introduces a Keldysh-contour effective field theory for magnetic vortices under current flow, revealing how current-driven torques cause a nonequilibrium defect-unbinding transition.

Findings

Current flow causes a defect-unbinding phase transition.

Vortex density depends on current density.

Differences from superconducting vortex behavior are explained.

Abstract

A Keldysh-contour effective field theory is derived for magnetic vortices in the presence of current flow. The effect of adiabatic and non-adiabatic spin transfer torques on vortex motion is highlighted. Similarities to and differences from the superconducting case are presented and explained. Current flow across a magnetically ordered state is shown to lead to a defect-unbinding phase transition which is intrinsically nonequilibrium in the sense of not being driven by a variation in effective temperature. The dependence of the density of vortices on the current density is determined.