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Supercurrents through the
superconductor­ferromagnet­superconductor
(SFS) junctions
Alexander V. Veretennikov a,1 , Valery V. Ryazanov a , Vladimir A. Oboznov a ,
Alexander Yu. Rusanov a , Victor A. Larkin a , Jan Aarts b
a Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, 142432, Russia
b Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, the Netherlands
Abstract
We have observed and studied critical supercurrents, Ic , in cross­type Nb­Cu/Ni­Nb junctions with Josephson
layers prepared from diluted ferromagnetic alloys. For suitable values of the exchange energy in the ferromagnet
and the F­layer thickness we have observed Ic(T ) oscillation with vanishing Ic for the definite values of T and dF .
Keywords: Josephson e#ect; #­contact; superconductor­ferromagnet­superconductor junction
1. Introduction
M.V.Feigelman et al [1] have proposed a solid
state implementation of the quantum bit based
on the superconducting loop with ''0'' and ''#''
Josephson junctions (the latter is a weak link
with nonzero phase di#erence in the absence of
currents and magnetic fields). The idea of a #­
contact was first proposed for the superconductor­
ferromagnet­superconductor (SFS) Josephson
junctions [2], where exchange ferromagnetic field
was responsible for spatial oscillations of the in­
duced superconducting order parameter in the
F­layer. If the exchange energy E exc in the ferro­
magnet and the F­layer thickness dF are adjusted
by a suitable way the order parameter alternates
1 Corresponding author. E­mail: veretenn@issp.ac.ru
its sign as one goes from the one S­electrode of the
SFS junction to the other. Temperature changes
of E exc should result in oscillations of the critical
currents I c (T ) with vanishing it for definite values
of T and dF . This type of contacts has not been
realized and investigated so far.
An extra decay of the induced superconducting
order parameter in the F­layer of the SFS sand­
wich is increased with increasing of E exc in the fer­
romagnet. An estimation of the order parameter
decay length # F # (•hD/kTCurie ) 1/2 for ferromag­
netic metal layers (such as Co, Fe and Ni) yields
the amount of the order of 1 nm [3]. Since it is di#­
cult to prepare continuous uniform films with such
thickness, we used diluted Cu/Ni ferromagnetic al­
loys in order to decrease the exchange energy and
to increase the pair decay length in the F­layer.
Preprint submitted to Physica B 16 July 1999

2. Samples
Cross­type Nb--Cu/Ni--Nb thin film sandwiches
involved F­layers sputtered using Cu/Ni alloy tar­
gets with a Ni content near the critical concentra­
tion for ferromagnetism. The junctions had areas
of 50â50µm 2 . Using SQUID­magnetometer we ob­
served that the Curie temperature of Cu­52at.%Ni
films with thickness of 100 nmwas in accordance to
bulk alloy data (# 100 K). However TCurie sharply
dropped as the thickness decreased. Inset (a) in
Fig.1 shows the temperature dependencies of the
longitudinal magnetization and resistance of Cu­
52at.%Ni film with thickness of 19 nm. The tran­
sition to the ferromagnetic state at 18­20 K causes
sharp decrease of the resistance due to 'freezing­
out' of the spin­flip scattering.
3. Josephson characteristics
Because of low junction resistances (Rn #
10 -5
# Josephson characteristics of the SFS sand­
wiches were measured by SQUID picovoltmeter
with a sensitivity of 10 -11 V. The I­V character­
istics and magnetic field dependencies I c (H) were
classical (the inset (b) in Fig.1 illustrates I c (H)
dependencies for a sandwich with thickness of
Cu­52at.%Ni­layer of 22 nm at di#erent tempera­
# # # # # #
#
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###
#
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####
# #
#
E#
D#
,
F
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,
F
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7##.#
## ## ## # # # #
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Fig. 1.
tures). The freedom of the I c (H) dependence from
any appreciable distortions testifies on behalf of
high uniformity of the Josephson layer thickness
and properties along the junction. Though mag­
netic and resistive properties of the Cu/Ni­films
do not change practically with increase of the
thickness from 19 to 22 nm, Josephson behav­
ior of the SFS sandwiches undergoes a dramatic
change. The upper curve in Fig.1 shows that for
the SFS junction with the F­layer thickness of 19
nm the critical current increases monotonously as
temperature decreases. The SFS sandwich with a
22­nm­thickness ferromagnet of the same content
manifests a reentrant superconducting behavior
in the same temperature range (the lower curve in
Fig.1). We believe that the thickness and TCurie
of the junction F­layer are adjusted just so that
insignificant increase of the exchange field with
temperature decrease transfers the SFS junction
from ''0''­ to ''#''­state at the temperature close
to 3 K. Invariability of the 'Fraunhofer pictures'
position shown in the inset (b) of the Fig.1 proves
that the I c (T ) oscillation is not related to residual
magnetic induction changes.
Acknowledgement
This work was supported by NWO Grant No.
047­005­001, INTAS­RFBR Grant No. 97­11459
and partly by the Russian Scientific Program
''High Temperature Superconductivity''.
References
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