16 research outputs found
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.
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
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 () of the
s-wave superconductor in contact with the ferromagnet. More specifically, we
argue that the non-monotonic decrease of in ferromagnet/s-wave
superconductor junctions without magnetic impurities is altered to a monotonic,
non-oscillatory decrease when the condition is
fulfilled, where is the spin-flip relaxation time and 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 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 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- 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-
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