Effective Exchange Energy in a Thin, Spatially Inhomogeneous CuNi Layer Proximized by Nb.

Thin films of diluted magnetic alloys are widely used in superconducting spintronics devices. Most studies rely on transport measurements and assume homogeneous magnetic layers. Here we examine on a local scale the electronic properties of the well-known two-layer superconductor/ferromagnet structure Nb/CuNi.

Second-Harmonic Current-Phase Relation in Josephson Junctions with Ferromagnetic Barriers.

We report the observation of a current-phase relation dominated by the second Josephson harmonic in superconductor-ferromagnet-superconductor junctions. The exotic current-phase relation is realized in the vicinity of a temperature-controlled 0-to-π junction transition, at which the first Josephson harmonic vanishes. Direct current-phase relation measurements, as well as Josephson interferometry, nonvanishing supercurrent and half-integer Shapiro steps at the 0-π transition self-consistently point to an intrinsic second harmonic term, making it possible to rule out common alternative origins of half-periodic behavior.

Thickness dependence of the Josephson ground States of superconductor-ferromagnet-superconductor junctions.

We report the first experimental observation of the two-node thickness dependence of the critical current in Josephson junctions with a ferromagnetic interlayer. Nodes of the critical current correspond to the transitions into the pi state and back into the conventional 0 state. From the experimental data the superconducting order parameter oscillation period and the pair decay length in the ferromagnet are extracted reliably.

Direct observation of vortex shells and magic numbers in mesoscopic superconducting disks.

We have studied vortex configurations in mesoscopic superconducting disks using the Bitter decoration technique. For a broad range of vorticities L the circular geometry is found to lead to the formation of concentric shells of vortices. From images obtained on disks of different sizes in a range of magnetic fields we traced the evolution of vortex states and identified stable and metastable configurations of interacting vortices.

Coupling of two superconductors through a ferromagnet: evidence for a pi junction.

We report measurements of the temperature dependence of the critical current, I(c), in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature I(c) generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which I(c) rises again.