On the origin of the reversed vortex ratchet motion

TL;DR

This paper experimentally investigates how the interplay between vortex interactions and the ratio of characteristic interaction distance to pinning potential period causes multiple reversals in vortex ratchet motion in an asymmetric magnetic dot array.

Contribution

It demonstrates experimentally that vortex ratchet reversals depend on the ratio of interaction distance to pinning period, confirming recent simulation predictions.

Findings

Reversal of vortex drift direction depends on size gradient d.

Number of reversals increases as d decreases.

Results align with recent computer simulations.

Abstract

We experimentally demonstrate that the origin of multiply reversed rectified vortex motion in an asymmetric pinning landscape is a consequence not only of the vortex-vortex interactions but also essentially depends on the ratio between the characteristic interaction distance and the period of the asymmetric pinning potential. Our system consists of an Al film deposited on top of a square array of size-graded magnetic dots with a constant lattice period a=2\mu m. Four samples with different periods of the size gradient d were investigated. For large d the dc voltage Vdc recorded under a sinusoidal ac excitation indicates that the average vortex drift is from bigger to smaller dots for all explored positive fields. As d is reduced a series of sign reversals in the dc response are observed as a function of field. We show that the number of sign reversals increases as d decreases. These findings are in agreement with recent computer simulations and illustrate the relevance of the different characteristic lengths for the vortex rectification effects.