High frequency properties of Josephson π-junctions

Josephson junctions with ferromagnetic interlayer have attracted much interest due to their potential application as π-phase shift elements. We have developed a self-aligned multilayer process for the fabrication of superconductor/insulator/ferromagnet/superconductor Josephson junctions (S=Nb, I=AlOx, F=NiPd). Our junctions have RCSJ-like current-voltage characteristics and a Fraunhofer diffraction pattern for the magnetic field dependence of the critical current. The dependence of the IcRn-product on the ferromagnet thickness shows a clear crossover from the zero- to the π-state. We have determined the plasma frequency of π-coupled junctions from Fiske resonances. This data is used to explain the different excitations found in microwave spectroscopy experiments at milli-Kelvin temperature.