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Mesoscopic and strongly correlated systems Chernogolovka, 11-16.10. 2009. Double re-entrant superconductivity in SF-Hybrids A. S. Sidorenko Institute of Electronic Engineering ASM, Kishinev, Moldova In collaboration with: Kazan State University, Kazan, Russia - L. R. Tagirov

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Mesoscopic and strongly correlated systems

Chernogolovka, 11-16.10. 2009

Double re-entrant superconductivity in SF-Hybrids

A. S. Sidorenko

Institute of Electronic Engineering ASM, Kishinev, Moldova

In collaboration with:

Kazan State University, Kazan, Russia - L. R. Tagirov

Institute for Solid State Physics of RAS, Chernogolovka, Russia- V.V. Ryazanov, V.Oboznov

Universität Augsburg, Germany - M. Schreck, G.Obermeier, C. Müller, S. Horn, R. Tidecks

Karlsruhe Institute of Technology, Germany – H.Hahn, E.Nold

Moscow State University, Russia – M.Yu. Kupriyanov


ESF Exploratory Workshop

Paestum (Salerno), Italy, 20-21 June 2008

O U T L I N E

1. Coexistence of S-F, FFLO state

2. Proximity effect in S/F layers, quasi-1D FFLO

3. Novel technology-->Re-entrant superconductivity

4. Conclusions


1) FFLO state

P. Fulde, R. A. Ferrell Phys.Rev. 135 (1964) A550

A. I. Larkin, Yu. N. Ovchinnikov JETP 47 (1964) 1138

Non uniform SC state with:

- nonzero pairing momentum, q0=kF ≠ 0

- oscillating pairing function, F~cos(kFx).

Singlet pairs in a ferromagnet

- non uniformFFLO pairing

Exchange field splitsconduction band of ferromagnet


FFLO state:

Eex/0

Strict limitation: 0,71 0 < Eex < 0,76 0

Eex ~ 0.1-1 eV

0 ~ 0.001 eV


2) FFLO-like1D-state

Proximity-effect:

F

S

N

De Gennes, Rev.Mod.Phys.36 (1964)225

x

S

F

A. Buzdin, Z. Radović, PR B38 (1988) 2388

x

In F-layer: nonzero pairing momentum , q0 ~ Eex≠ 0,FFLO-like state

FF oscillates on magnetic coherence length, 2 =F = ħvF/Eex

and relaxes on decay length, 1=lF


Interference of Pairing Function

In F-layer:

Fabry-Perot interferometer

analogy

F

Vacuum

S

dF


Oscillations of superconducting Tc as a function of the ferromagnetic layer thickness in multilayers

Z. Radovich et al, PRB 44, 759 (1991)

π-phase

0-phase


non monotonous TC(dF) for S/F :

t=Tc/TcS

A. Buzdin, Z. Radović, PR B38 (1988) 2388

ln t = (½) – Re( ½ + r/t )

A lot of attempts – controversial results:

dF/F

Nb/Gd Ch.Strunk, PRB 49 (1994) 4053 (MBE) – nooscil.

J.Jiang, PRL 74 (1995) 314 (dc-magnetron) – oscil.

Nb/Fe G.Verbank,PRB57 (1998) 6029 (MBE) – nooscil. I.Garifullin, PRB 55 (1997) 8945 (dc-magnetron) - oscil.

Nb/CuMn C.Attanasio, PRB 57 (1998) 14411 (dc-magnetron) – oscil.

Nb/CuNi V.Ryazanov et al., JETP Lett. 77 (2003) 43(dc-magnetron) – oscil.


  • Experimentals

  • Our choice:

    • dc magnetron sputtering

    • atomic smooth substrate (flame polished glass )

    • Nb/Ni couple (Nb-Ni solubility less than 4 at.%)

      - single-run deposition process


magnetron sputtering

Nb/Ni samples

5-8nm

20-70 nm


XRD

RBS

Thickness measurement accuracy: dNi ± 0.03 nm

Roughness: rms < 0.3 nm


TC oscillation in Nb/Ni bilayer:quasi-1D FFLO state

L. R. Tagirov, Phys. C 307 (1998) 145

A. Sidorenko, V. Zdravkov, A.Prepelitsa et al., Ann. Phys. 12 (2003) 37.

dF/F

(Curves 1-5: variable interface transparency, Tm= 5, 2.5, 1.25, 0.5, 0.25)


3). Re-entrant superconductivity

Calculation for S/F sandwich – oscillations TC up to re-entrance:

dF/F

Tcs-The temperature of SC transition for single layer

S,M - SC coherence lengths in SC and FM

dS,M - thicknesses of SC and FM layers


Re-entrant superconductivity: pilot experiments with Nb/Cu43Ni57

V.Ryazanov et al., Pisma JETP Lett. 77, 43 (2003);

hint: CuNi layer thickness to observe the re-entrant Tc has to be 2 - 8 nm


Our pilot experiments with Nb/Cu0.41Ni0.59

1)dS, / S ~ 1

dS, ~ 10nm

2)alloy Cu0.41 Ni0.59ξF = ħvF/Eex~ 8nm

(allows larger thicknesses dF of about 5-10 nm )

The necessity of technology development for ultra-thin S and F layers preparation


Sample preparation- Novel Technology :Sidorenko A.S., Zdravkov V.I., “Instalaţie pentru obţinere peliculelor conductoare”, Patent of RM №3135 from 31.08 2006.

moving target

  • DC magnetron sputtering

    a) high deposition rate (4 nm/s)

    b) moving Nb target

    (precisely constant S-layer thickness)

    Nb-target with holder:


1. Superconducting properties of prepared Nb films

IEEIT

Critical temperatures for Nb

films with thickness 5.5-14 nm


Si

CuNi

Nb

Si

Nb

Substrate (Si)

Sample preparation - Novel Technology :Sidorenko A.S., Zdravkov V.I., “Instalaţie pentru obţinere peliculelor conductoare”, Patent of RM №4831 from 28 June 2006.

  • DC, RF- magnetron sputtering with high rate

  • Deposition in one run of the structure with constant «S» (Nb) and wedge-like «F» (CuNi on shifted substrate)layer

  • Deposition of long (80 mm) Nb films with constant thickness

  • Protection of the sample by covering Si-layer.


Si-Substrat

2. TEM of Nb/CuNi structures

Nb/CuNi

22#18

IEEIT

Si-Substrate

Si-Buffer

Nb

CuNi

Si-Cap

dCuNi= 14.1nm

dNb = 6nm


Si

N

Si

Sub

2. Investigation of the morphology of prepared Nb films and S/F nanostructures (AFM, XRD, SEM, RBS, Auger)

IEEIT

SEM measurements of Nb film


Workshop

Karlsruhe, 13-17 July 2008

2. SEM measurements of Nb/CuNi structures

IEEIT


IEEIT

2. Auger measurements of Nb/CuNi structures


Investigation of the morphology of S/F nanostructures (AFM, XRD,RBS)

AFM scan of Nb/CuNi( S15)

RBS-measurements


Si Cap

CuNi

Niob

Si Buffer

Substrate

3.Re-entrante behavior of superconductivity

in Nb/CuNi structures

IEEIT



Experimental observation of the re-entrant superconductivity

in Nb/Cu41Ni59bilayers (V.I. Zdravkov et al., PRL 97, 057004, 2006)

Non monotonous Tc (dF)

(dNb≈14.1, and 8.3 nm),

and re-entrant Tc (dF) behavior

for dNb≈7.3 nm < ξs  8 nm.

Measured down to 40 mK (dilution He3-He4)


First experimental observation of the double re-entrant superconductivity

in Nb/Cu41Ni59bilayers (A.S. Sidorenko et al.,. Quasi-One-Dimensional

Fulde-Ferrell-Larkin-Ovchinnikov-Like State

in Nb/Cu41Ni59 Bilayers. Pisma ZhETF, v.90, 149 (2009).)

Non monotonous Tc (dF)

(dNb≈14.1, and 7.8 nm),

and re-entrant Tc (dF) behavior

for dNb≈6.2nm < ξs  8 nm.

the next islandof superconductivity is possible to observe

in the range dCuNi ≈ 44-56 nm.


Solid curves are calculated based on the procedure: superconductivity

LR. Tagirov, Phys. C 307 (1998) 145

- with the common set of parameters for all curves :

ξS= 10.2 nm

NFvF/NSvS= 0.17, TF= 0.845,

lF/ξF0 =1.2 (closer to the “clean” case), ξF0= 8.6 nm,

lF  ≈ 10.3 nm – from <ρFlF> ≈ 2.5·10-5 μΩ·cm2, using measured ρF ≈ 25 μΩ·cm


AP superconductivity

IEEIT

SP

SAP

N

P


AP superconductivity

S/F spin-switch

IEEIT

P


4. CONCLUSIONS superconductivity

1. Novel technology of SF hybrids production (suitable for spintronics) is developed

2. The first pronounced observation of the re-entrant and double re-entrant superconductivity in S/F bilayers with thickness of the superconducting layer ds<ξs 10 nm is announced.

3. The experimentally-theoretical base for the

spintronic device design is developed.

Thank you for attention


Magnetic properties

Workshop superconductivity

Karlsruhe, 13-17 July 2008

Magnetic properties

Ferromagnetic ordering and spinglass-like behavior of magnetization for sample

34 from batch S15


Workshop superconductivity

Karlsruhe, 13-17 July 2008

IEEIT

ln t = (½) – Re( ½ +p /t )

FS ,


Workshop superconductivity

Karlsruhe, 13-17 July 2008

Si Cap

Nb

Si Buffer

Substr.

2. Auger measurements of Nb films

IEEIT


Si superconductivity

CuNi

Nb

Si

Nb

Substrate (Si)

Sample preparation- Novel Technology :Sidorenko A.S., Zdravkov V.I., “Instalaţie pentru obţinere peliculelor conductoare”, Patent of RM №4831 from 28 June 2006.

  • Equidistant cutting of long sample (~80 mm) along the wedge produces a batch of samples:


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