Eur. Phys. J. B 68, 73-77 (2009)
https://doi.org/10.1140/epjb/e2009-00074-x

Sudden critical current drops induced in S/F structures

¹ Materials Science Department, IRC in Superconductivity and IRC in Nanotechnology, University of Cambridge, Cambridge, UK
² Departamento de Física, Grupo de Física de la Materia Condensada, Universidad de los Andes, 4976 Bogotá, Colombia

Corresponding author: ^a ejpatino@gmail.com

Received: 6 December 2008
Revised: 21 January 2009
Published online: 3 March 2009

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 ≈2ξ_S. Nb/Co and Nb/Py (Permalloy Ni₈₀Fe₂₀) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_F > 30 nm 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 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to T_C 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 ~ 50 nm 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.