Ferromagnetic 0-pi Junctions as Classical Spins

TL;DR

This study demonstrates that highly damped ferromagnetic Josephson junctions exhibit classical spin behavior, with spontaneous supercurrents causing measurable magnetic shifts, resembling magnetic nanoparticles with uniaxial anisotropy.

Contribution

It provides experimental evidence that 0-pi ferromagnetic Josephson junctions behave as classical spins, expanding understanding of their magnetic and superconducting properties.

Findings

Spontaneous half quantum vortex observed in junctions.

Random sign of supercurrent shift at transition.

Reproducible magnitude and flux compensation possible.

Abstract

The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson junctions shows a spontaneous half quantum vortex, sustained by a supercurrent of undetermined sign. This supercurrent flows in the electrode of a Josephson junction used as a detector and produces a phi(0)/4 shift in its magnetic diffraction pattern. We have measured the statistics of the positive or negative sign shift occurring at the superconducting transition of such a junction. The randomness of the shift sign, the reproducibility of its magnitude and the possibility of achieving exact flux compensation upon field cooling: all these features show that 0-pi junctions behave as classical spins, just as magnetic nanoparticles with uniaxial anisotropy.