Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions

We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with barrier thickness implying repeated 0-pi phase-transitions in the superconducting order parameter. We show in particular that the Co barrier devices can be accurately modelled using existing clean limit theories and so that, despite the high exchange energy (309 meV), the large IcRN value in the pi-state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.Comment: 4 pages 3 figures 1 table. Revised version as accepted for publication. To appear in Physical Review Letter

Properties of tunnel Josephson junctions with a ferromagnetic interlayer

We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We formulate a quantitative model describing the oscillations of critical current as a function of thickness of the ferromagnetic layer and use this model to fit recent experimental data. We also calculate quantitatively the density of states (DOS) in this type of junctions and compare DOS oscillations with those of the critical current.Comment: 9 pages, 8 figures, to be published in Phys. Rev.

Spin-flip scattering and non-ideal interfaces in dirty ferromagnet/superconductor junctions

We study the proximity-induced superconducting correlations as well as the local density of states of a ferromagnet, in a ferromagnet/s-wave superconductor heterostructure. We include the effects of spin-flip scattering, non-ideal interfaces, and the presence of impurities in the sample. We employ the quasiclassical theory of superconductivity, solving the Usadel equation with emphasis on obtaining transparent analytical results. As our main result, we report that in a certain parameter regime the spatial oscillations of the anomalous (superconducting) part of the Green's function induced in the ferromagnet by the proximity effect from the s-wave superconductor, are damped out due to the presence of spin-flip processes. As a consequence, spin-flip scattering may under certain conditions actually enhance the local density of states due to the oscillatory behaviour of the latter in ferromagnet/superconductor structures. We also conjecture that the damping could be manifested in the behaviour of the critical temperature ($T_c$) of the s-wave superconductor in contact with the ferromagnet. More specifically, we argue that the non-monotonic decrease of $T_c$ in ferromagnet/s-wave superconductor junctions without magnetic impurities is altered to a monotonic, non-oscillatory decrease when the condition $1>16\tau_\text{sf}^2h^2$ is fulfilled, where $\tau_\text{sf}$ is the spin-flip relaxation time and $h$ is the exchange field.Comment: 11 pages, 5 figures. Accepted for publication in Phys. Rev. B. High-resolution figures to be available in published versio

Superconducting decay length in a ferromagnetic metal

The complex decay length xi characterizing penetration of superconducting correlations into a ferromagnet due to the proximity effect is studied theoretically in the frame of the linearized Eilenberger equations. The real part xi_1 and imaginary part xi_2 of the decay length are calculated as functions of exchange energy and the rates of ordinary, spin flip and spin orbit electronic scattering in a ferromagnet. The lengths xi_1,2 determine the spatial scales of, respectively, decay and oscillation of a critical current in SFS Josephson junctions in the limit of large distance between superconducting electrodes. The developed theory provides the criteria of applicability of the expressions for xi_1 and xi_2 in the dirty and the clean limits which are commonly used in the analysis of SF hybrid structures.Comment: 5 pages, 3 figure

Density of states in SF bilayers with arbitrary strength of magnetic scattering

We developed the self-consistent method for the calculation of the density of states $N(\epsilon)$ in the SF bilayers. It based on the quasi-classical Usadel equations and takes into account the suppression of superconductivity in the S layer due to the proximity effect with the F metal, as well as existing mechanisms of the spin dependent electron scattering. We demonstrate that the increase of the spin orbit or spin flip electron scattering rates results in completely different transformations of $N(\epsilon)$ at the free F layer interface. The developed formalism has been applied for the interpretation of the available experimental data.Comment: 5 pages, 8 figure

Scattering by magnetic and spin-orbit impurities and the Josephson current in superconductor-ferromagnet-superconductor junctions

We analyze the Josephson current in a junction consisting of two superconductors (S) and a ferromagnetic layer (F) for arbitrary impurity concentration. In addition to non-magnetic impurities, we consider also magnetic ones and spin-orbit scattering. In the limit of weak proximity effect we solve the linearized Eilenberger equation and derive an analytical expression for the Josephson critical current valid in a broad range of parameters. This expression enables us to obtain not only known results in the dirty and clean limits but also in a intermediate region of the impurity concentration, which may be very important for comparison with experimental data.Comment: revised versio

The role of interface transparency and spin-dependent scattering in diffusive ferromagnet/superconductor heterostructures

We present a numerical study of the density of states in a ferromagnet/superconductor junction and the Josephson current in a superconductor/ferromagnet/superconductor junction in the diffusive limit by solving the Usadel equation with Nazarov's boundary conditions. Our calculations are valid for an arbitrary interface transparency and arbitrary spin-dependent scattering rate, which allows us to explore the entire proximity-effect regime. We first investigate how the proximity-induced anomalous Green's function affects the density of states in the ferromagnet for several values of the exchange field. In each case, we consider the effect of the barrier transparency and allow for various concentrations of magnetic impurities. In particular, we address how the zero-energy peak and minigap observed in experiments may be understood in terms of the interplay between the singlet and triplet anomalous Green's function and their dependence on the concentration of magnetic impurities. We also investigate the role of the barrier transparency and spin-flip scattering in a superconductor/ferromagnet/superconductor junction. We suggest that such diffusive Josephson junctions with large residual values of the supercurrent at the 0-$\pi$ transition, where the first harmonic term in the current vanishes, may be used as efficient supercurrent-switching devices. It is also found that uniaxial spin-flip scattering has very different effect on the 0-$\pi$ transition points depending on whether one regards the width- or temperature-dependence of the current.Comment: 16 pages, 12 figures. Accepted for publication in Phys. Rev. B. High-resolution figures will be available in published version. Shortened abstract due to arXiv submissio

Multiple 0-pi transitions in superconductor/insulator/ferromagnet/superconductor Josephson tunnel junctions

We report on experimental studies about superconducting coupling through a thin Ni76Al24 film. A patterning process has been developed, which allows in combination with the wedge shaped deposition technique the in situ deposition of 20 single Nb/Al/Al2O3/Ni3Al/Nb multilayers, each with its own well-defined Ni3Al thickness. Every single multilayer consists of 10 different sized Josephson junctions, showing a high reproducibility and scaling with its junction area. Up to six damped oscillations of the critical current density against F-layer thickness were observed, revealing three single 0-pi transitions in the ground state of Josephson junctions. Contrary to former experimental studies, the exponential decay length is one magnitude larger than the oscillation period defining decay length. The theoretical predictions based on linearized Eilenberger equations results in excellent agreement of theory and experimental results