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.
@article{arxiv.cond-mat/0501459,
title = {Ferromagnetic 0-pi Junctions as Classical Spins},
author = {M. L. Della Rocca and M. Aprili and T. Kontos and A. Gomez and P. Spatkis},
journal= {arXiv preprint arXiv:cond-mat/0501459},
year = {2009}
}