Transverse instabilities of multiple vortex chains in superconductor-ferromagnet bilayers

TL;DR

This study investigates how magnetic stripe domains in a superconductor-ferromagnet bilayer influence vortex arrangements, revealing complex chain formations and instabilities through microscopy and simulations.

Contribution

It demonstrates the formation of vortex chains and their instabilities in superconductor-ferromagnet bilayers due to magnetic pinning effects, combining experimental and simulation approaches.

Findings

Vortex chains form in pinning-antipinning channels.

Transitions between multichain states involve kink propagation.

High fields induce non-linear vortex behavior and levitation.

Abstract

Using scanning tunneling microscopy and Ginzburg-Landau simulations we explore vortex configurations in magnetically coupled NbSe$_2$-Permalloy superconductor-ferromagnet bilayer. The Permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes non-linear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices `levitating' in the anti-pinning channels.