Semifluxons in ferromagnetic Josephson junctions

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In a O-π Josephson junction, vortices carrying only a half of the magnetic flux quantum (semifluxons) may appear spontaneously. The physics of such vortices has attracted a lot of interest both in view of their novel fundamental physics and in view of potential applications in superconducting electronics. Using superconductor-insulator-ferromagnet-superconductor (SIFS) technology O- π Josephson junctions with low damping (required when using Josephson junctions as active elements) and of almost arbitrary shape of both the junction and the O- π discontinuity can be fabricated. However, the existing technology has drawbacks: the maximum supercurrent density jc π in the π parts of the junctions is low (40 A/cm² or less), leading to a vortex size ( ) often exceeding 60 µm. Consequently, structures containing arrangements of several semifluxons are almost impossible to realize.Within this project we focus on (a) the further development of the ferromagnetic O- π Josephson junction technology and (b) the experimental investigation of semifluxons in such junctions.The technological aim is to substantially increase jc π in the π state (ideally by at least one order of magnitude), still keeping the junction normal state resistance Rn high and thus damping low. This will make fractional vortices (and devices) much smaller and suitable for circuits operating in either the classical or quantum regime. This aim will be persued by the following two routes. First, the existing (S|I|F|S) Josephson junction technology will be modified to include a clean ferromagnet with low damping. Second, the existing Superconductor-Ferromagnetic lnsulator-Superconductor (S|FIlS) Josephson junction technology (e.g., FI=FeSi) will be optimized in terms of stoichiometry and the thickness of the Fl-layer. The crossover from S|FI|S to S|F|S limit at high concentration of the ferromagnet will be investigated.Using these structures experiments will be performed in the temperature range between 300 mK and 6 K where the fractional vortices can be considered as classical nonlinear objects. The reduced vortex size will enable investigation of not yet explored multi-vortex systems such as two- or three-vortex molecules and even ID vortex crystals. The interaction of fractional vortices with each other, e.g., mutual flipping or splitting of eigenfrequencies, as well as interaction with integer fluxons will be studied.As a result, this project will push forward the ferromagnetic 0- π Josephson junction technology, which is also useful for many other applications such as self-biased classical and quantum circuits. The project will improve the understanding of the fractional vortex physics and may suggest new applications.