200 ps Vortex Core Reversal by Azimuthal Spin Waves

TL;DR

This study demonstrates 200 ps vortex core reversal in Permalloy discs using azimuthal spin waves, combining experimental imaging and micromagnetic simulations to understand the dynamics and limitations of the process.

Contribution

It provides the first direct imaging of spin wave mediated vortex core reversal and identifies a fundamental 200 ps lower limit for reversal time based on energy transfer constraints.

Findings

Unidirectional vortex core switching achieved with rotating GHz fields.

Good agreement between experimental results and micromagnetic simulations.

Reversal time limited to about 200 ps due to energy transfer dynamics.

Abstract

Spin wave mediated vortex core reversal has been investigated by time-resolved scanning transmission X-ray microscopy (STXM). Movies showing the development of the spin wave and vortex core magnetization dynamics during unidirectional vortex core reversal could be taken in Permalloy discs, 1.6 \mu m in diameter and 50 nm thick, during excitation with rotating ac field bursts of one period duration at 4.5 GHz and with amplitudes up to 4 mT. Unidirectional switching is achieved by taking advantage of an asymmetry for CW or CCW excitation caused by the gyrofield. The differences in the magnetization dynamics due to this asymmetry could be imaged during continuous excitation with multi-GHz rotating fields. All our experimental results are in good agreement with micromagnetic simulations. In addition, for the sample geometry given above, simulations reveal a lower limit of about 200 ps for the time of unidirectional vortex core switching, which cannot be overcome by shortening the excitation length or by increasing the excitation amplitude. We explain this limitation by the finite time needed for an energy transfer of the global excitation towards the center of the sample. Thus smaller samples will allow for much shorter vortex core reversal times.