0-pi Josephson tunnel junctions with ferromagnetic barrier

TL;DR

This paper reports the fabrication and analysis of Nb/Al2O3/NiCu/Nb Josephson junctions with a ferromagnetic barrier, demonstrating a 0-pi phase shift and spontaneous vortex formation at the step-like ferromagnetic layer.

Contribution

It introduces a method to create 0-pi Josephson junctions with a ferromagnetic barrier and characterizes their ground state and magnetic flux properties.

Findings

Successful fabrication of high-quality Nb/Al2O3/NiCu/Nb junctions.

Observation of a spontaneous vortex carrying ~6.7% of Phi_0.

Critical current dependence shows a minimum near zero magnetic field.

Abstract

We fabricated high quality Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a step-like thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microns (1.3 lambda_J) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying ~6.7% of the magnetic flux quantum Phi_0. The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.