Proximity Effect and Interface Transparency in Nbbased SN and SF Layered Structures

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Proximity Effect and Interface Transparency in
Nb-based S/N and S/F Layered Structures

• Carmine Attanasio
• Dipartimento di Fisica E.R. Caianiello
• INFM-Laboratorio Regionale SUPERMAT
• Università degli Studi di Salerno
• Italy

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Carmine Attanasio Carla Cirillo Matteo
Salvato Serghej L. Prischepa Achille Angrisani
Armenio
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Outline
Proximity Effect and Transparency Coefficient
S/N and S/F layered systems
Experimental and transport measurements
Comments and Conclusions
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Motivation
Proximity effect has been known since the early
days of superconductivity, but its systematic
study becomes possible only through the
development of technology allowing to fabricate
heterostructure consisting of thin layers (with
thickness smaller than the coherence
lengths). Such structures behave as a single
superconductor with nontrivial properties, but
they have often been studied in the case of
ideally transparent interface, while the
properties of interfaces and the ability of
electrons to cross them are important in many
physical phenomena and applications. Recently,
great interest has been devoted to phenomena
involving spin-polarized quasiparticle transport
and to the study of superconducting spin-switch
devices. These theoretical and experimental
progresses require to take into account arbitrary
interface transparency. This crucial parameter
determines the strength of the proximity effect
and it is not directly measurable.
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Proximity effect (S/N)
When a superconductor (S) comes in contact with
another metal (X) proximity effect takes place.
The two metals influence each other on a spatial
scale of the order of the coherence lengths in
the vicinity of the interface. Superconductivity
is depressed in S and induced in X. In S/N
structure the Cooper pairs penetrating into the
normal metal are broken by thermal fluctuations.
S/N proximity structure can be used as
superconductors with tunable properties, varying
composition and thickness of layers.
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Proximity effect (S/F)
The physics of S/F systems is even more
intriguing. In this case the superconducting
order parameter does not simply decay in the
ferromagnetic metal, but also oscillates. This
behavior is due to the presence of the exchange
field in F, and reveals itself, for instance, in
a nonmonotonic dependence of the critical
temperature, TC, of S/F multilayers and bilayers
as a function of the F layers thickness.
Due to the large exchange energy, Eex, in
magnetic metals such as Fe, Cooper pairs are
broken over a coherence length of few Angstrom.
Making use of ferromagnetic alloys, such as CuNi
or PdNi, can avoid the technical problem of these
very thin thickness.
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What is interface transparency?
Interface transparency T takes into account all
effects which cause electrons to be reflected
rather than transmitted at the interfaces,
screening the proximity effect.
Fabrication methods
Fabrication methods
Interface
imperfections
Interface
imperfections
Lattice
Lattice
mismatches
mismatches
reduced
reduced T
Band
structure
Band
structure
mismatches
mismatches
Fermi
Fermi
Splitting
of the
spin
Splitting
of the
spin
velocities
velocities
Spin
-
dependent
Spin
-
dependent
subbands
subbands
mismatches
mismatches
impurity
scattering
impurity
scattering
Magnetic systems
Magnetic systems
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Samples
Two different samples typology
N/S/N and F/S samples to investigate the
behaviour of Tc(ds) and to extract the T value
S/N/S and S/F samples to study the variation of
Tc(dn,f) in order to estimate xn,f
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MBE deposition technique
4 samples in the same deposition run
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Sputtering
Deposition of Nb/Cu (Pd, PdNi) layered systems
2 Si (100)
Cu
Cu
...
Nb
increasing dCu
9 samples in the same deposition run
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Si\Cu (100 Å)\Nb (100 Å)
Reflectivity
Interface roughness
MBE
Sputtering
s 18 Å
s 25 Å
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Si\Cu\Nb
Rocking curves of the reflectivity
Lateral roughness correlation length
MBE samples 300 Å
Sputtered samples 200 Å
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Proximity effect in S/N multilayers for arbitrary
transparency (Golubovs model)
From Usadel equations, with Kupriyanov and
Lukichev boundary conditions, in Werthamer
approximation
Proximity effect is tuned by two parameters
g is a measure of the strength of the proximity
effect gb describes the effect of the boundary
transparency
A.A. Golubov, Proc. SPIE 2157, 353 (1994). M.Yu.
Kupriyanov, and V.F. Lukichev, Sov. Phys. JETP
67, 1163 (1988).
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Higher T
Stronger Proximity Effect
Lower Tc

• The parameter g can be fully experimentally
  determined
• directly measuring rs,n, the low temperature
  resistivities
• estimating (or measuring) xs,n, the coherence
  lengths

?B (or T) is the only free parameter
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Estimating the coherence length xn
S layers separated by a thin N layer
S layers separated by a large N layer
the decay of Cooper pairs from both sides overlap
Tc reaches a limiting value
S layers are coupled
S layers are decoupled
Nb/Ag/Nb
dNb 220 Å
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Estimating the coherence length xs
Cu/Nb/Cu trilayers dCu 1500 Å Nb changes
SdHc2?/dTTTc
MBE
xNb 2x(0)/p 64 Å
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Cu/Nb/Cu
dNormal Metal 1500 Å
MBE
Sputtering
rNb 3.6 mW cm rCu 1.3 mW cm xCu 260 Å xNb
64 Å
rNb 4.6 mW cm rCu 1.8 mW cm xCu 170 Å xNb
67 Å
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Study in external magnetic field
C. Ciuhu and A. Lodder, Influence of the boundary
resistivity on the proximity effect, Phys. Rev.
B, 46, 224526 (2001)
In Hpar pairs cross boundaries and T plays an
imprtant role At fixed H, lower T values imply a
smaller proximity effect Higher Tc values In Hc2
parallel dimensional crossover goes to higher
temperatures
dNb 200 Å
tCRSputtering gt tCRMBE
Effect more pronounced in external magnetic
field For larger Nb thickness the effect of the
boundaries is smaller
A. Tesauro, A. Aurigemma, C. Cirillo, S.L.
Prischepa, M. Salvato, and C. Attanasio, in
press on Superconductor Science and Technology
(2004).
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Study in external magnetic field
Some features are also present in Hc2perp for
small dNb
A.S. Sidorenko et al., Physica C 370, 197 (2002)
Recently positive curvature of Hc2 close to Tc
has been ascribed to the presence of potential
barrier at the interface Effect pronounced in
N/S/N Effect absent in I/S/I
Curvature more pronounced in MBE than in
sputtering
T lower in sputtered samples
Nb behaves more like isolated layer
For high dNb Hc2perp is linear
The effect of the boundaries is negligible
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Ag/Nb/Ag MBE
dAg 1500 Å
rNb 3.5 mW cm rAg 4.0 mW cm xAg 190
Å xNb 66 Å
A. Tesauro, A. Aurigemma, C. Cirillo, S.L.
Prischepa, M. Salvato, and C. Attanasio, in
press on Superconductor Science and Technology
(2004).
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Pd/Nb/Pd
dPd 1500 Å
rNb 3.6 mW cm rPd 5.0 mW cm xPd 60 Å from
Tc(dPd) xNb 64 Å
Tcontinous 0.46 T 0.42 T 0.54
xPd much lower than what found for xCu and xAg
C. Cirillo, S.L. Prischepa, M. Salvato, and C.
Attanasio, Eur. Phys. J. B 38, 59 (2004).
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Validity of the theory (in the case of Nb/Pd)
xPd in agreement with the value obtained from the
measured rPd
lPd 60 Å
xPd from 73 Å to 115 Å for T 10 K and T 4 K
respectively
The ratio xPd/lPd always less than one (dirty
limit) in the considered temperature range In our
case g/gb 0.4 always less than Tc/Tcs
C. Cirillo, S.L. Prischepa, M. Salvato, and C.
Attanasio, Eur. Phys. J. B 38, 59 (2004).
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Comments
T not strongly influenced by the fabrication
methods, but more by intrinsic factors
(mismatches of the Fermi velocities and
differences between the band structures of the
two metals). Also electrons in Nb and Pd
d-character in Cu and Ag s-type Nb bcc Pd, Cu,
Ag fcc and lattice mismatch
Pd-based magnetic alloys are good candidates for
studying S/F proximity problem
dSCR/xS (exp) is higher than Nb/Cu and
Nb/Ag dSCR/xS (exp) is comparable to Nb/CuMn
because of higher transparency
        1                    M. Geers, PhD
Thesis. 2                    G. R. Boogaard,
Master Thesis. 3                    A.A. Golubov
et al., Phys. Rev. B 51, 1073 (1995). 4           
         Cirillo et al., Eur. Phys. J. B 38, 59
(2004).  
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S/F systems
Nb/PdNi bilayers Ni 10 Determined by WDS
analyses
dNb 180 Å dPdNi 235 Å
s 12 Å
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S/F systems
Nb/PdNi bilayers Ni 10
For our Ni concentration TCurie strongly depends
on the layer thickness. Bulk TCurie 160 K while
it varies from 100 K to 200 K from 50 Å to 150 Å
dPdNi 100 Å
The lowest temperature at which dr/dT shows a
maximum can be taken as Tcurie. These features
are exhibited in all our PdNi bilayers. In the
temperature range investigated for the
superconducting measurements the magnetic
ordering is well established.
J. A. Mydosh at al., PRL 21, 1346 (1968) M.P.
Kawatra at al., PRL 23, 83 (1969) M.P. Kawatra
at al., PRB 2, 1587 (1970).
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S/F systems
Nb/PdNi bilayers Ni 10
Ya. V. Fominov, N. M. Chtchelktchev, and A. A.
Golubov, PRB 66, 014507 (2002)
dcrit 110 Å dcrit/xNb 1.83
Same fitting (xf and gb) parameters to reproduce
the two curves
No Oscillations probably Nb too thick
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Nb/Fe bilayers (MBE)
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    1                    Aarts et al., Phys. Rev.
B 56, 2779 (1997). 2                    M. Geers,
PhD Thesis. 3                    G. Verbanck et
al., Phys. Rev. B 57, 6029 (1998). 4              
      A. Rusanov et al., Physica C 369, 300
(2002). 5                    G. R. Boogaard,
Thesis. 6                    Fominov et al.,
Phys. Rev. B 66, 14507 (2002). 7                  
  Sellier, PhD Thesis. 8                    Lazar
et al., Phys. Rev. B 61, 3711 (2000).            
() In base alla relazione riportata sulla tesi
di Sellier T ? 1/gb.
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Comments

• gb is lower (T higher) than obtained for
  ferromagnet Nb/Fe, Pb/Fe
• and V/Fe
• gb is order of what obtained in another weak
  ferromagnet CuNi
• - in Nb/PdNi gb is actually a bit higher (T
  lower) probably because of the
• higher obtained exchange energy
• - the critical thickness is high even for this
  weak ferromagnet probably
• due to the higher transparency

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Conclusions.
S/N layered systems were studied in the framework
of the proximity effect. Different systems and
different deposition techniques. T has been
determined as the only free parameter in the
theory.

T seems not to be strongly dependent from
extrinsic causes Mismatch between VF of the two
metals seems to play a fundamental role. High T
for Nb/Pd systems. Effect of T more pronounced in
external magnetic field than in Tc(dNb)
S/F layered systems were studied in the framework
of the proximity effect. Weak and strong
ferromagnet Nb/PdNi and Nb/Fe. High T for
Nb/PdNi Nb/PdNi good candidate for application
where high T is needed
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and perspectives

• Investigation of the dependence of the proximity
  effect on the strength of the weak ferromagnet
  varying the Ni percentage
• Sistematic study in external magnetic field
• - Study of the depairing currents in Nb/PdNi
  system which could be a more sensitive tool to
  estimate T

1 C. Cirillo, S.L. Prischepa, A. Romano, M.
Salvato, and C. Attanasio, Physica C 44, 95
(2004) 2 C. Cirillo, S.L. Prischepa, M. Salvato,
and C. Attanasio, Eur. Phys. J. B 38, 59 (2004) 3
A. Tesauro, A. Aurigemma, C. Cirillo, S.L.
Prischepa, M. Salvato, and C. Attanasio, in
press on Superconductor Science and Technology
(2004). 4 C. Cirillo, S.L. Prischepa, M. Salvato,
and C. Attanasio, to be published on Journal of
Physics and Chemistry of Solids (2004). 4 C.
Cirillo, S.L. Prischepa, M. Salvato, and C.
Attanasio, in preparation (2004).
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