Anomalous Josephson effect controlled by an Abrikosov vortex

The very special characteristic of the proximity effect in superconductor-ferromagnet systems is the damped oscillatory behavior of the Cooper pair wave function in a ferromagnet. In some sense, this is analogous to the inhomogeneous superconductivity, predicted long time ago by Larkin and Ovchinnikov (1964), and Fulde and Ferrell (1964), and constantly searched since that. After the qualitative analysis of the peculiarities of the proximity effect in the presence of the exchange field, the author provides a unified description of the properties of the superconductor-ferromagnet heterostructures. Special attention is paid to the striking non-monotonous dependance of the critical temperature of the multilayers and bilayers on the ferromagnetic layer thickness and conditions of the realization of the "Pi"- Josephson junctions. The recent progress in the preparation of the high quality hybrid systems permitted to observe on experiments many interesting effects, which are also discussed in the article. Finally, the author analyzes the phenomenon of the domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers.

We report measurements of the temperature dependence of the critical current in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a Cu\(_x\)Ni\(_{1-x}\) 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. Such a sharp cusp can only be explained by assuming that the junction changes from a 0-phase state at high temperatures to a \(\pi\)-phase state at low temperatures.