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

Karlsruhe Institute of Technology, Germany – H.Hahn, E.NoldMoscow State University, Russia – M.Yu. Kupriyanov

Mesoscopic and strongly correlated systemsChernogolovka, 11-16.10. 2009

  O U T L I N E

1. Coexistence of S-F, FFLO state2. Proximity effect in S/F layers, quasi-1D FFLO3. Novel technology --> Re-entrant superconductivity 4. Conclusions

ESF Exploratory Workshop Paestum (Salerno), Italy, 20-21 June 2008

kF

min kF

max

k

E

EF

Eexc

-kF

m ax

+kFm ax

+k F

min

-kFm i n

= -kF 1

= +kF 2

1) FFLO state

P. Fulde, R. A. FerrellPhys.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).

Exchange field splits

conduction band of ferromagnet

Singlet pairs in a ferromagnet

- non uniform FFLO pairing

Eex/0 FFLO state:Strict limitation: 0,71 0 < Eex < 0,76 0

Eex ~ 0.1-1 eV0 ~ 0.001 eV

S

S

F

N

x

x

F

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

In F-layer: nonzero pairing momentum , q0 ~

Eex ≠ 0, FFLO-like state

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

2) FFLO-like 1D-stateProximity-effect:

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

and relaxes on decay length, 1= lF

)cos( 21 xeF x

F

 

SF Vacuum

dF

Interference of Pairing FunctionIn F-layer:Fabry-Perot interferometeranalogy

12cos( )x

FF e x

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

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

multilayers

π-phase

0-phase

ln t = (½) – Re( ½ + /t )

dF/F

non monotonous TC(dF) for S/F : t=Tc/TcS

A lot of attempts – controversial results:

Nb/Gd Ch.Strunk, PRB 49 (1994) 4053 (MBE) – no oscil.J.Jiang, PRL 74 (1995) 314 (dc-magnetron) – oscil.

Nb/Fe G.Verbank, PRB 57 (1998) 6029 (MBE) – no oscil. 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.

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

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

20-70 nm5-8nm

Nb/Ni samples magnetron sputtering

XRD RBS

Thickness measurement accuracy: dNi ± 0.03 nmRoughness: rms < 0.3 nm

dF/F

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

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

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

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

dF/F

3). Re-entrant superconductivity

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

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

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

1) dS, / S ~ 1 dS, ~ 10 nm

0 10 20 30 40 500

2

4

6

8

Tc (dNb)

dCuNi56 nm

Experiment Calculation

Tc

(K)

dNb (nm)

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

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

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

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.

• DC magnetron sputtering

a) high deposition rate (4 nm/s)

b) moving Nb target(precisely constant S-layer thickness)

Nb-target with holder:

moving target

1. Superconducting properties of prepared Nb films

Critical temperatures for Nb

films with thickness 5.5-14 nm

6,0 6,4 6,8 7,2 7,6 8,0

0

2

4

6

8

10

12

14

16

18

, O

hm *

cm

T, K

Nb, thickness: d(Nb)=6,8 nm

Thickness, (nm)

Critical temperature ,Tc (К)

Nb 5,5 5,7

10 7,4

28 8,25

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.

CuNi

Nb

Substrate (Si)Substrate (Si)

Si

SiNb

Si-Substrat

Si-Cap

CuNi

Nb

Si-Buffer

Nb/CuNi

22#18

dCuNi= 14.1nm

dNb = 6nm

2. TEM of Nb/CuNi structures

Si-Substrate

CuNi wedge

Niobium

Silicon cap and buffer layers

Silicon substrate

50 µ

m

50 µ m

Surface

Si Substrate

Nb

Si-oxide

Si

SEM 2 Crater Edge

SEM measurements of Nb film

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

Si

N

Si

Sub

S23_12_1004.sem: SEM: SEM 2 crateredge FZK IMF1

07 Dec 3 10.0 keV 0 FATSEM/Full

20 µ

m

50 µ m

Si Substrate

Nb

CuNi

Surface Si-oxid

Si

SEM 2 Crater Edge

200

µm

200 µm

SEM 1 Crater

2. SEM measurements of Nb/CuNi structures

Workshop Karlsruhe, 13-17 July 2008

S23_12_1001.pro: SEM 1 Profil FZK IMF1

07 Dec 3 10.0 keV 0 FRR 9.7957e+001 maxNb1/Full (Binom3)

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100S23_12_1001.pro

Sputter Time (min) Rate 1nm/min Si-ox.

Ato

mic

Con

cent

ratio

n (%

)

ONbSi

CuNiNC

O

Si

O

Nb

Cu

Ni

NC

Nb

Si

2. Auger measurements of Nb/CuNi structures

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

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 340

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 340

5

10

15

20

25

30

35

0

566064

Thi

ckne

ss o

f N

b an

d C

uNi (

nm)

Sample #

Nb

CuNi

Ni content

CuNi wedge

Niobium

Silicon cap and buffer layers

Silicon substrate

Ni c

once

ntra

tion

in C

uNi (

%)

S15

RBS-measurementsAFM scan of Nb/CuNi ( S15)

3. Re-entrante behavior of superconductivity in Nb/CuNi structures

Si Cap

Niob

CuNi

Si Buffer

Substrate

Superconducting transitions of Nb/CuNi bilayers

0 1 2 3 4 5 6 7 8

0,0

0,2

0,4

0,6

0,8

1,0

0,0

0,2

0,4

0,6

0,8

1,0

dNb7.3 nm

#

sample dCuNi

[nm]

———————— #04 1.0 #06 1.6 #07 2.6 #09 5.0 #14 9.5 #16 11.5 #20 16.7 #22 19.3 #29 30.3

R/R

n

T (K)

#S15

sample dCuNi[nm]

———————— #03 0.7 #06 2.3 #10 6.5 #14 11.4 #16 12.8 #18 17.2 #25 28.2 #31 35.6

dNb8.3 nm

R/R

nS16

Experimental observation of the re-entrant superconductivity in Nb/Cu41Ni59 bilayers (V.I. Zdravkov et al., PRL 97, 057004, 2006)

Measured down to 40 mK (dilution He3-He4)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.

First experimental observation of the double re-entrant superconductivity

in Nb/Cu41Ni59 bilayers (A.S. Sidorenko et al.,. Quasi-One-Dimensional

Fulde-Ferrell-Larkin-Ovchinnikov-Like State

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

the next island of superconductivity is possible to observe in the range dCuNi ≈ 44-56 nm.

Non monotonous Tc (dF) (dNb≈14.1, and 7.8 nm),

and re-entrant Tc (dF) behavior

for dNb≈6.2nm < ξs 8 nm.

Solid curves are calculated based on the procedure: 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

SP SAP N

APP

P

AP

0,0

0,2

0,4

0,6

0,8

1,0

R/R

n

BEXTERNAL

B

SWITCH

Superconducting state

Normal state

S/F spin-switch

4. CONCLUSIONS

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.

Ferromagnetic ordering and spinglass-like behavior of magnetization for sample 34 from batch S15

Magnetic properties

0

5,0x10-7

1,0x10-6

1,5x10-6

2,0x10-6

2,5x10-6

3,0x10-6

3,5x10-6

Mmol

= 1.000 g/mol

m = 30.72 mg

CuNi S15-34

(e

mu

/mo

l)

0 100 2000

1000000

2000000

3000000

4000000

H = 100 Oe

1/

(mo

l/em

u)

T (K)

Workshop Karlsruhe, 13-17 July 2008

Workshop Karlsruhe, 13-17 July 2008

SS dk S

M

F S ,

FFFFM

M llI

D 6,13

22

4

2

0

tanh( )tan ,

2 1 / (2 / ) tanh( )S BCSF F F F

S S S F F F F F

dN v k d

N v i l T k d

21 22

/ / ,S Sd

0 0/ 1 / .F F F Fk i l

ln t = (½) – Re( ½ +p /t )

nb7_1003.pro: SEM 1 full area x20000 profile 2 FZK IMF1

07 Nov 30 10.0 keV 0 FRR 2.7738e+001 maxSi1/Full (Binom3)

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70

80

90

100nb7_1003.pro

Sputter Depth (nm)

Ato

mic

Con

cent

ratio

n (%

)

Si1.ls1

O1.ls1

C1.ls1

O

Si

O

SiNb

C

Nb1.ls1

Nb

C

2. Auger measurements of Nb films

Si Cap

Nb

Si Buffer

Substr.

Workshop Karlsruhe, 13-17 July 2008

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:

CuNi

Nb

Substrate (Si)Substrate (Si)

Si

SiNb