Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators

TL;DR

This paper investigates how vortex-antivortex pairs influence the magnetization dynamics in nanocontact magnetic vortex oscillators, revealing temperature-dependent behaviors and a new regime caused by vortex-antivortex creation.

Contribution

It introduces a combined experimental, micromagnetic, and analytical approach to understand vortex-antivortex effects in magnetic vortex oscillators, highlighting a novel dynamical regime.

Findings

Low temperature experiments show different dynamics at low and high currents.

A sudden frequency jump occurs at a critical current, linked to vortex-antivortex pair creation.

The vortex-antivortex pair affects the free layer vortex dynamics through spin torque.

Abstract

We study the magnetization dynamics in a nanocontact magnetic vortex oscillators as function of temperature. Low temperature experiments reveal that the dynamics at low and high currents differ qualitatively. At low currents, we excite a temperature independent standard oscillation mode, consisting in the gyrotropic motion of a free layer vortex about the nanocontact. Above a critical current, a sudden jump of the frequency is observed, concomitant with a substantial increase of the frequency versus current slope factor. Using micromagnetic simulation and analytical modeling, we associate this new regime to the creation of a vortex-antivortex pair in the pinned layer of the spin valve. The vortex-antivortex distance depends on the Oersted field which favors a separation, and on the exchange bias field, which favors pair merging. The pair in the pinned layer provides an additional spin torque altering the dynamics of the free layer vortex, which can be quantitatively accounted for by an analytical model.