The dependence of the critical current density j c on the ferromagnetic interlayer thickness d F was determined for Nb/ Al 2 O 3 / Cu/ Ni/ Nb Josephson tunnel junctions with ferromagnetic Ni interlayer thicknesses from very thin films ͑ϳ1 nm͒ upward and classified into F-layer thickness regimes showing a dead magnetic layer, exchange, exchange+ anisotropy and total suppression of j c . The Josephson coupling changes from 0 to as function of d F , and-very close to the crossover thickness-as function of temperature. The strong suppression of the supercurrent in comparison to nonmagnetic Nb/ Al 2 O 3 / Cu/ Nb junctions indicated that the insertion of a F layer leads to additional interface scattering. The transport inside the dead magnetic layer was in dirty limit. For the magnetically active regime fitting with both the clean and the dirty limit theories was carried out, indicating dirty limit condition, too. The results were discussed in the framework of literature.

We present experimental studies of static and dynamic properties of 0, π and 0-π superconductorinsulator-ferromagnet-superconductor (SIFS) Josephson junctions of small and intermediate length.In the underdamped limit these junctions exhibit a rich dynamical behavior such as resonant steps on the current-voltage characteristics. Varying the experimental conditions, zero field steps, Fiske steps and Shapiro steps are observed with a high resolution. A strong signature of the 0-π Josephson junction is demonstrated by measuring the critical current as a function of two components (Bx, By) of an in-plane magnetic field. The experimental observation of a half-integer zero field step in 0-π SIFS junctions is presented.

We present experiments and theoretical studies on the propagation of Josephson fluxons and electromagnetic waves in parallel arrays of Josephson junctions in the limit of small discreteness. Locking between the fluxon rotation frequency and the frequency of the radiated electromagnetic waves leads to a series of resonances, which we observe on the dc-current-voltage characteristics of the arrays. The arrays consist of small Josephson junctions embedded in an annular superconducting stripline. The experimental data are analyzed using the discrete sine-Gordon model and an extension by including a capacitive interaction between neighboring Josephson junctions. We compare experimental data with both analysis and numerical simulations and find an excellent quantitative agreement.

The behavior of topological solitons in a parallel array of a Josephson junction is studied experimentally. We observe the fusion of two relativistic solitons of the same polarity into a single soliton. The soliton carries two quanta of magnetic flux and, most strikingly, travels 18% faster than an ordinary soliton under the same driving force. We also find a variety of bunched states composed of solitons of the same polarity, moving with fixed separation.

We present a study on low-T c superconductorinsulator-ferromagnet-superconductor (SIFS) Josephson junctions. SIFS junctions have gained considerable interest in recent years because they show a number of interesting properties for future classical and quantum computing devices. We optimized the fabrication process of these junctions to achieve a homogeneous current transport, ending up with high-quality samples. Depending on the thickness of the ferromagnetic layer and on temperature, the SIFS junctions are in the ground state with a phase drop either 0 or π. By using a ferromagnetic layer with variable step-like thickness along the junction, we obtained a so-called 0-π Josephson junction, in which 0 and π ground states compete with each other. At a certain temperature the 0 and π parts of the junction are perfectly symmetric, i.e. the absolute critical current densities are equal. In this case the degenerate ground state corresponds to a vortex of supercurrent circulating clock-or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum Φ 0 .