Sudden Critical Current Drops Induced in S/F Structures

TL;DR

This paper investigates how the magnetic domain state of ferromagnets influences the critical current in superconductor/ferromagnet structures, revealing sudden drops linked to vortex dynamics caused by domain walls.

Contribution

It demonstrates that domain wall-induced vortex formation causes abrupt critical current drops in S/F bilayers, a novel insight into controlling superconductor properties via ferromagnetic domain states.

Findings

Critical current drops occur along hard axes in Nb/Co structures with ferromagnet thickness >30nm.

Vortex flux flow accompanies the critical current drops.

Magnetoresistance measurements show sharp increases near ferromagnet saturation fields.

Abstract

In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_{s} ~ 2 xi_{S}. Nb/Co and Nb/Py (Permalloy Ni_{80}Fe_{20}) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_{F} > 30nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to Tc show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_{F} ~ 50nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.