Feromagnetic "$\pi$"-junctions
  We investigate Josephson coupling through a ferromagnetic thin film using Superconductor-Insulator-Ferromagnet-Superconductor planar junctions. Damped oscillations of the critical current are observed as a function of the ferromagnetic layer thickness. We show that they result from the exchange energy gained or lost by a quasiparticle Andreev-reflected at the ferromagnet-superconductor interface. The critical current cancels out at the transition from positive ("0") to negative ("$\pi$") coupling, in agreement with theoretical calculations. 
T. Kontos, M. Aprili, J. Lesueur, F. Genet, B. Stephanidis and R.   Boursier