Fraktionale Flusswirbel in Josephson Tunnelkontakten mit einer ferromagnetischen Zwischenschicht

In this thesis, we study Josephson tunnel junctions with a ferromagnetic interlayer, so-called SIFS (superconductor-insulator-ferromagnet-superconductor) Josephson junctions. Conventional 0 Josephson junctions have a current phase relation I=Ic sin(mu) with Ic>0. In contrast, SIFS Josephson junctions provide the possibility to realize pi junctions. In such structures the superconducting wave function changes its sign across the barrier, ie, shifts its phase by pi. The current phase relation of pi junctions reads I=Ic sin(mu), formally with Ic<0. Using a step-like thickness of the ferromagnetic barrier, allows to fabricate so-called 0-pi Josephson junctions. The ground state phase in such junctions has a value of 0 deep inside the 0-region, and a value of pi deep inside the pi region. Supposed that the critical current densities in both halves of the 0-pi junction are equal, the ground state of the system consists of a spontaneously formed vortex of supercurrent circulating around the 0-pi boundary. This supercurrent corresponds to a local magnetic flux Phi<Phi_0/2, where Phi_0=h/2e is the magnetic flux quantum. Thus, the localized magnetic field is called semifluxon. In the framework of this thesis, we examine triplets, 0-pi junctions with their respective 0 and pi reference junctions. Samples of different geometries (linear and annular) and of different lengths (ranging from the short to the long junction limit) are available. The junctions are realized in overlap geometry. The aim of this thesis was twofold: First, we wanted to figure out whether the additional ferromagnetic interlayer of SIFS junctions modifies the Josephson physics or leads to additional noise contributions in contrast to conventional SIS Josephson junctions, both in the thermal and in the quantum regime. As a second aim, we studied the properties of 0-pi junctions and characterized the associated fractional flux in detail. We determined the static and dynamic properties of our samples by measuring current-voltage characteristics, IVCs, and by measuring the critical current Ic vs. applied magnetic field B, Ic(B). The experiments were performed using a standard 4He- and 3He-cryostat. As a result, Ic(B) of the reference junctions and of the 0-pi junctions showed a small offset from zero magnetic field. Additionally, Ic(B) of the 0-pi Josephson junctions revealed an asymmetric height of the maxima and bumped side minima. This behavior is most likely due to a finite magnetization of the ferromagnetic layer. Regarding the dynamic properties of the reference junctions, we observed no discrepancy from standard Josephson physics. Depending on the respective experimental conditions, Fiske steps, zero field steps and Shapiro steps were verified, exactly as expected from theory. In case of short 0-pi junctions, half-integer zero field steps were experimentally verified on the IVCs for the first time. Additionally, we presented the first experimental observation of various metastable fluxon/semifluxon configurations in long 0-pi junctions. Switching current measurements were performed in a dilution refrigerator to study escape mechanisms of the Josephson phase in SIFS junctions. The effective potential height as a function of magnetic field and as a function of temperature was examined using samples in the short limit. Numerical simulations showed, that the activation energy of SIFS Josephson junctions vs. magnetic field can be described in the framework of standard short Josephson junction theory. Performing switching current measurements at different temperatures revealed, that the escape temperatures coincided perfectly with the bath temperatures, for 0, pi as well as 0-pi coupling. Using microwave spectroscopy, we observed harmonic, subharmonic and superharmonic pumping. The experimental data of the eigenfrequencies of short and intermediate length samples showed a perfect agreement with the pointlike junction theory. Overall we conclude, that we do observe peculiarities of the ferromagnetic interlayer in SIFS Josephson junction. Nevertheless, we did not find any indication for additional noise contributions due to the presence of the ferromagnetic layer. Thus, regarding quantum applications, the usability of SIFS Josephson junctions is not restricted due to poor noise properties. In many aspects, short SIFS samples are well described by the short Josephson junction theory. In the case of 0-pi junctions, we developed a deep understanding of the associated fractional flux. Its occurrence was experimentally observed in several experiments, as it had been predicted in theory.Im Rahmen dieser Arbeit wurden Josephson Tunnelkontakte untersucht, die eine ferromagnetische Zwischenschicht aufweisen, sogenannte SIFS (Supraleiter-Isolator-Ferromagnet-Supraleiter) Kontakte. Reguläre 0 Kontakte haben eine Strom-Phasen-Beziehung von I=Ic sin(mu) mit Ic>0. Zusätzlich bieten SIFS Kontakte die Möglichkeit, pi Josephsonkontakte zu realisieren. In diesen Strukturen ändert sich das Vorzeichen der supraleitenden Wellenfunktion über den Kontakt um pi. Die Strom-Phasen-Beziehung ist I=Ic sin(mu) mit Ic<0. Hat der Ferromagnet eine Stufe, so ist es möglich, 0-pi Kontakte herzustellen. Im Grundzustand eines 0-pi Kontaktes ist die Phasendifferenz im 0 Bereich 0, im pi Bereich ist sie pi. Sind die kritischen Stromdichten in beiden Bereichen eines 0-pi Kontaktes identisch, so zeichnet sich der Grundzustand durch einen um die Phasengrenze zirkulierenden Ringstrom aus. Der Ringstrom ist mit einem magnetischen Fluss Phi