Josephson tunnel junctions with ferromagnetic interlayerJosephson tunnel junct ons w th ferromagnet c nterlayer

Seminar des Walther-Meissner-InstitutsSeminar des Walther-Meissner-InstitutsFriday, 01/19/2007

M WeidesM. WeidesInstitute for Electronic Materials, Institute for Solid State Research, Research Centre Juelich

d2 0 JJπ JJ

d

0-π JJ

1

d1

Cooperation withCooperation with

H. Kohlstedt and R. WaserI tit t f El t i M t i l I tit t f S lid St t R h R h C t J li hInstitute for Electronic Materials, Institute for Solid State Research, Research Centre Juelich

E. Goldobin, D. Doenitz, M. Kemmler, D. Kölle and R. KleinerUniversität Tübingen, Physikalisches Institut - Experimentalphysik II

A. Buzdin Institut Universitare de France and Condensed Matter Theory Group, CPMOH

University Bordeaux 1, UMR 5798, CNRS, F-33405 Talence Cedex, France

V. Oboznov and V.V. RyazanovInstitute for Solid State Physics, Chernogolovka, Russia

K. Madek, S. Beutner, G. Wild, Ch. Probst, A. Marx and R. GrossWalther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching, Germany

2

OutlineOutline

i. Theory: Coupling of Josephson junctions ith f ti i t lwith ferromagnetic interlayer

ii Experiment:ii. Experiment:a) Planar 0, π SIFS Josephson junctionsb) St d (0 0 0 ) SIFS J hb) Stepped (0-0, 0-π, π-π) SIFS Josephson junctions

iii. Outlook

3

Coupling of superconducting wavefunctionsCoupling of superconducting wavefunctions

SIS-type junction|Ψ( )|x

|Ψ |1 |Ψ |2

S-wave superconductorCooper pairs e↑+e↓ | |1 | |2

Θ2Θ1

Cooper pairs e +eBosonic system

xsuperconductor superconductorweak linkPhase difference jj

First Josephson equation

4

SIS Josephson Tunnel JunctionsSIS Josephson Tunnel Junctions•High quality Nb/Al2O3/Nb junctions •Josephson penetration length

•Critical current density jc=0.004-4 kA/cm²

•Oxidation for 30 min in various p(O2)

10

10 )) 0.015 mbar, 4 kA/cm2

2

1

10

50 mbar

0.015 mbar4 kA/cm2

0.45 mbar2y

j c (kA

/cm

²

j (kA

/cm

²

kneeregion

x1x1000

x5

0.45 mbar, 720 A/cm2 50 mbar UV, 4.5 A/cm2

0 01

0.15

216 A/cm2720 A/cm2

UV light+50 mbarrren

t den

sity

nt d

ensi

ty

jcregion x5

1E-3

0.01

0

UV light 50 mbar4.5 A/cm2

criti

cal c

ur

curr

en

5

0.01 0.1 1 10 1000 1 2 3voltage (mV) O2 pressure (mbar)

Vg

Proximity effect in SN and SFProximity effect in SN and SFBudzin et al., JETP 35Demler et at., PRB 55

order parameter Ψ

/( ) Nx ξ−Ψ ΨRe{ (x)}Ψ ξN

0/( ) Nxex ξΨ = Ψ

xsuperconductor normal metalsuperconductor normal metal

EexRe{ (x)}Ψ ξF1

Eex: exchange energy,magnetization in plane

1 20

/ /( ) F Fx ixex eξ ξ− −Ψ = Ψx

Re{ (x)}Ψ0π

ξF1 magnetization in plane

osc 22 Fλ πξ=superconductor magnetic metalξF2

63d magnets: ξF2<1 nm diluted magnets: CuNi, PdNi, PdFe

ξF2

Oscillating order parameter Ψ in F-layerOscillating order parameter Ψ in F layerExchange interaction in FMe↑ and e↓ different densit of states

EδN

E

e↑ and e↓ : different density of states

(↑↓- ↓↑) Cooper pair aquires

Eex

EF

EF

(↑↓ ↓↑) Cooper pair aquiresfinite center of mass momentuminside magnet 2Eex k -qF k +qF

k(a) (b) kF2q=2Eex/vFℏ qF(a) (b) F

superconductor ferromagnet

Phase of the pair wave function

2q 2Eex/vFℏ

0

Δϕ= 2qxPhase of the pair wave function

k0

k0

-kF kF -k +qF k +qF

Demler et al., PRB 55 (1997)

7

Josephson junctions - Current-phase relationJosephson junctions Current phase relationsupercurrent IS energy EJ

I 2E0 JJ π JJ Ic 2EJ

EJϕ

0 JJ π JJ

-Ic

EJ-2π 2π-π πϕ

0-JJ (SFS) dF≪ ξF2

c

-π 2π-2π π

π-JJ (SFS) dF≈½ξF2

8

Determination of π coupling in SFS-JJsDetermination of π coupling in SFS JJs

Nb IJ=|Ic|sin(ϕ+π)=-|Ic|sin(ϕ)Ni60Cu40

Nb

NbdF

J | c| (ϕ ) | c| (ϕ)

IJ ∼ |Ψ|2

1.61.0

Nb d0-π= d0-π(ξF2), ξF2= ξF2(T)

1.0

1.2

1.4

0 6

0.8

I c [a.

U.]

I c/I c0

I c/I c0Ic(dF)0

Ic(H)Ic(T) 0-0 0-π

0.4

0.6

0.8

0 2

0.4

0.6

ππ

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0 1 2 3 4 50.0

0.2

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

0

dF T/Tc H [Φ

0]

9

Kontos et al., PRL 89 (2002) V. Ryazanov et al., PRL 86, (2001) Bauer et al., PRL 92 (2004)

Status in 2003: π coupled SFS/SIFS

Nb

Status in 2003: π coupled SFS/SIFS

Nb •High critical current• Thick Al2O3 barrier

Pd88Ni12

Al2O3NiCu

•High critical current•Low resistance

• jc(π)=10 mA/cm²• λJ=3.7 mm

NbV. Ryazanov et al., PRL 86 (2001)H. Sellier et al., PRB 68 (2003) T. Kontos et al., PRL 89 (2002)

Nb • A=1 mm², ( )

Wanted:

•Strongly underdamped (βc≫1) →Al2O3-layer•Small size →thin Al2O3•high IcR product in π state

10•Small junction to junction deviation

OutlineOutline

i. Theory: Coupling of Josephson junctions ith f ti i t lwith ferromagnetic interlayer

ii Experiment:ii. Experiment:a) Planar 0, π SIFS Josephson junctionsb) St d (0 0 0 ) SIFS J hb) Stepped (0-0, 0-π, π-π) SIFS Josephson junctions

iii. Outlook

11

Sputter machineSputter machineUnivex 450B LeyboldCl S

deposition byCluster System

p yAr-ion plasma in DC field

Momentum transfer fromMomentum transfer fromAr+ to target atom

Computer-controlled4-inch capable

12

Fabrication of junctionsFabrication of junctions(b) Ion beam etching(a) Multilayer deposition

NiCu 15 to 1 nm

Nb 40 nm

Al-Al O 5 nm2 3

Cu 2 nm

NiCu 15 to 1 nm

Nb 120 nm(d) Wiring deposition(c) Niobium anodization

--

++

13Weides et al., Physica C 437-438, 349 (2006)

Ni60Cu40 alloyNi60Cu40 alloy

Onset of ferromagnetism in NixCu1-x at x≈47%

Ryazanov: Ni53Cu47• kBT ≈ Eex

Ni l t i h FMSellier: Ni48Cu52• Ni cluster, inhomogeneous FM• strong Cooper pair breaking

Ni60Cu40

T 225 K age

(a.U

.)

5 nm Ni60Cu40T=10 K

• TC = 225 K• dead magnetic layer ≈ 1-3 nm• ρ (10 K) = 53 97 μΩ cm

-6 -4 -2 0 2 4 6s

Hal

l vol

ta

• ρNiCu(10 K) = 53.97 μΩ cm• matom = 0.15 μB

• Ms= 123 emu Anom

alou

14

Ms 123 emu-100 -80 -60 -40 -20 0 20 40 60 80 100

Magnetic field H (mT)

Improved SINFS junctionsImproved SINFS junctionsCu layer improves uniformity of NiCu

01234

(a)

mA)

SIFS dF=2 nm

0 2

0.3

-100 -80 -60 -40 -20 0 20 40 60 80 100

ent I

c (m

SIFS dF=4 nm

Al Al O

Nb

Ni60Cu40

1.2

-140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140

0.2

al c

urre

(b)

Nb

Nb

Al-Al O2 3

0.6

0.8

1.0

criti

ca SINFS dF=4.7 nm

Al-Al O2 3

Cu

Ni60Cu40

-600 -400 -200 0 200 400 6000.0

0.2

0.4

(c)Nb

15Weides et al., Physica C 437-438, 349 (2006)

magnetic field (μT)

Low variation of parametersLow variation of parameters20

SIFS F-layer : 3 3 nm

10

15SIFS, F layer : 3.3 nm

Ic=14.30 mA δIc=2%R 26 97 mΩ δR 0 5%m

A)

5

16-600 -400 -200 0 200 400 600

Rn=26.97 mΩ δRn=0.5%

rren

t I (m

magnetic field ( T)

-5

0

10121416

bias

cur magnetic field (μT)

ent I

c (mA)

15

-10

468

10b

ritic

al c

urre

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-20

-1502 cr

16Weides et al., Physica C 437-438, 349 (2006)

voltage (mV)

Thickness induced 0 to π transition101

SIFS sample @4.2K(mA)

Thickness induced 0 to π transition

100thin Al2O3 barrier

thick A l2O

3 barrier

urre

nt I c (

Ni60Cu40

NbdF

10-1 ξF1

= 0.78 nmξF2= 1.35 nmritic

al c

uNi60Cu40

Al2O3

dFCu

10-2dd ea d= 3.09 nm

0 coupling π coupling

cr

Nb

3.0 3 .5 4.0 4.5 5.0 5. 5 6.0 6.5 7.0 7.5 8.0

thickness of the F-layer dF (nm)

Ic(dF)∼(γB2)-1 e-dF/ξF1 |cos(-dF/ξF2)|damping phase modulationtransparency

F

17Weides et al., Appl. Phys. Lett. 89, 122511 (2006)

Temperature dependences of IVC in π stateTemperature dependences of IVC in π state600

14 19 KSIS at 4 2 K)

200

4000

A)

4.19 K3.15 K

2.61 K2.40 K

SIS at 4.2 K

curre

nt (m

A)0 -3 -2 -1 0 1 2 3

-1

400

(mA

) SIFS

curre

nt (μ

A

SIFS JJvoltage (mV)

2.11 K

-400

-200

0

200

ritic

al c

urre

nt

bias

c SIFS JJπ coupled

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-600

-0.4 0.0 0.4cr

magnetic field (mT)

π JJ (dF =5.87 nm) @ 2.1 K:

voltage (mV)

18

jc=5 A/cm² βc=700 λJ=170 μm V=IcR≈400 μVWeides et al., Appl. Phys. Lett. 89, 122511 (2006)

Temperature induced 0 to π transition

400

500 π

μA)

Temperature induced 0 to π transition

1015

300

400

π

0

0curr

ent I

c (μ

510 π 0

dF= 5.11 nm 0 JJ

criti

cal

2.5

T [K] Vc=50nV

3.58 3.44 3.393 29

010

100 dF= 5.20 nm 0 or π JJ

dF= 5.87 nm π JJ

Cum

ber

met

er β

c

1.5

2.0

rren

t Ic (μ

A)

3.29 3.25 3.22 3.19 3.16 3.14 3.12 3.113 1

2.0 2.5 3.0 3.5 4.01M

cCpa

ra

temperature (K)

0.5

1.0

crit

ical

cur3.1

3.09 3.075 3.06 3.04π

19-200 -100 0 100 200

0.0

magnetic field (μT)

Weides et al., Appl. Phys. Lett. 89, 122511 (2006)

Macroscopic quantum tunneling on π coupled junctionMacroscopic quantum tunneling on π coupled junction

SIFS junctions @ 120 mK:j @•thermal to quantum regime transover•photon induced transitions between quantized energy levels

20Madek et al. submitted to PRL

OutlineOutline

i. Theory: Coupling of Josephson junctions ith f ti i t lwith ferromagnetic interlayer

ii Experiment:ii. Experiment:a) Planar 0, π SIFS Josephson junctionsb) St d (0 0 0 ) SIFS J hb) Stepped (0-0, 0-π, π-π) SIFS Josephson junctions

iii. Outlook

21

Fabrication of step in F-layerFabrication of step in F layerresist

SF6

ArNb

CuNi Cu0.6 0.4

Nbd2 Ni Cu0.6 0.4 d1

Nb

π 0ΔdF

Nb

CuAl-Al O2 3

(a) (b) (c)

0.4

0.6

0.8

-50

0

50

100

-0 .5

0. 0

0. 5

1. 0 JJ w ith d1

(shifted along I-axis)

JJ with d2

JJ with d1

JJ ith dbi

as cu

rrent

(mA)

bias

curre

nt (m

A)

Etched and non etched

0 2

0.0

0.2 -1000 -500 0 500 1000-100

-40 -20 0 20 40

-1 .0

curre

nt I c (

mA)

JJ w ith d2

voltage (μV) voltage (μV)

Etched and non-etchedplanar junctions

-0.6

-0.4

-0.2

T=4.2 Kcritic

al

0 coupled JJs without etching

dF=d2=5.05 nm after etching

dF=d1=4.75 nm

22Weides et al., J. Appl. Phys. (2007) [cond-mat/0612164]

-0.4 -0.2 0.0 0.2 0.4-0.8

magnetic field (mT)

SEM and AFM of stepped SIFS stacksSEM and AFM of stepped SIFS stacks

23Weides et al., J. Appl. Phys. (2007) [cond-mat/0612164]

0 π and 0-π JJs fabricated in single process

1

0, π and 0 π JJs fabricated in single process

1 SIFS junction with dF

SIFS junction with dF-Δd

F

mA

)nt

|Ic(d

F)| (m

0.1

π coupling0 couplingcal c

urre

n c

ritic

4.8 5.0 5.2 5.4 5.6 5.8

thickness of F-layer dF (nm)

24Weides et al., Phys. Rev. Lett. 97, 247001 (2006)

Stepped SIFS: 0-0 or 0-π JJs

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

2 )Stepped SIFS: 0 0 or 0 π JJs

100ξF1=0.78 nmξ =1 35 nm SIFS JJ @ 4.2Kc (

A/cm

2

10

j (d )

jc(d

1) ξ

F2=1.35 nm

ensi

ty j c

1 stepped JJ:0-π phase and

stepped JJ:uniform phase (0-0)

jc(d2)

Δdurre

nt d

0.1

symmetric jc'sand tailored j

c's

ΔdF

ΔdF

0 coupling π couplingritic

al c

u

F

d2

d2 d1d1

p g π coupling

c

thickness of the F-layer d (nm)

25

thickness of the F layer dF

(nm)

Weides et al., J. Appl. Phys. (2007) [cond-mat/0612164]

0-0 JJ with tailored j and centered step in F-layer0 0 JJ with tailored jc and centered step in F layer

0.8

1.0

symmetric stepped JJj2/j

1

shifted by 10% along I-axis

I max

0.2

0.4

0.6

0 80.6

0.40.2

0.0

criti

cal c

urre

nt I c/I Simulated Ic(H) pattern

0 1 2 3 4 5 6 7 8 9 100.0

1.00.8c

magnetic f ield h

0.6

0.8calculated

d1=4.68 nm, d2=4.98 nm d1=4.68 nm, d2=4.92 nmd =4 72 nm d =4 92 nmA)

stepped JJ with different jc(dF)100x100 μm2

centered step in F-layermeasured

0.4

d1=4.72 nm, d2=4.92 nm

T=4.2 K

curre

nt I c (

mA measured

Measured Ic(H) pattern

0 0

0.2

criti

cal

26Weides et al., J. Appl. Phys. (2007) [cond-mat/0612164]

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.80.0

magnetic field (mT)

0-π coupled Josephson junctions0 π coupled Josephson junctionsNb/Al2O3/Nb with current injectorsYBCO-Nb ramp zigzags

π0

π0π 0

(a) (b)Goldobin et al., Phys. Rev. Lett. 92 (2004)

Hil k l N 422 (2003)Hilgenkamp et al., Nature 422 (2003)

SFS/SIFS-like structures

0 4

0.5

/I c0d2

0 π

0.2

0.3

0.4

I (d )criti

cal c

urre

nt I c/

273 4 5 6 7 80.0

0.1

d1

Ic(d2)

d2

Ic(d1)

F-layer thickness dF (nm)

d1Weides et al.,Phys. Rev. Lett. 97 (2006)

0-π JJs: fractional flux (Semifluxons)0 π JJs: fractional flux (Semifluxons)sin( ) ( ) ( ) ( )xx tt t x xxx h x xφ φ φ αφ γ θ− − = − + +Perturbed sine-Gordon equation with 0-π phase boundaryGoldobin et al., Phys. Rev. B 66 (2002)

2π

πθ=πθ=0

φ(x)

hase

μ(x

) fie

ld μ

x(x)

π

2π

θ=πθ=0Φ(x,t)=μ(x,t)+θ(x)

-4 -2 0 2 4

π

0

00

0

θ πθ 0

j ti di t (λ )

phas

e m

agne

tic p

hm

agne

tic f π

Φ=±½Φ0

FLAT PHASE STATE: Energetically unfavorable SEMIFLUXON: formed at boundary (pinned)

-4 -2 0 2 400 junction coordinate (λJ)m

π0 π0 0γ γ

28

Fractional flux in 0-π SIFS junctionFractional flux in 0 π SIFS junction200

A] T=4.0K 0 JJ

0 or π

0 π100

150

, I0-

πc

(H) [

μA π JJ 0-π JJ

0-π

0

50

0 c(H),

Iπ c(H),

jc(0)=jc(π)≈2 A/cm2150

200 T=2.65K

curre

nts

I0 c

λJ=260 μm

A=330x33 μm250

100

crit

ical

c

μ

-150 -100 -50 0 50 100 1500

magnetic field (μT)

29

g (μ )

Weides et al., Phys. Rev. Lett. 97, 247001 (2006)Effective length of 0-π JJ: ℓ≈1.3λJ

Magnetic field of spontaneous fractional fluxMagnetic field of spontaneous fractional flux1.5

"semifluxon" in JJsemif luxon in JJ

1.0

semifluxon in JJwith L=1.3λ

J

semif luxon in JJwith L→ ∞

j ( ) j ( )0

S

F

0.5 z

x

x

jc( )π jc( )0FS

0.0

d 1d 2

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-0.5

coordinate x/λ (norm units)coordinate x/λJ

(norm. units)

Short 0-π JJ: |Φ|= Φ0 ℓ2/8π

30

| | 0

Spontaneous flux in ground state: ±Φ=13%·Φ0/2Weides et al., Phys. Rev. Lett. 97, 247001 (2006)

OutlineOutline

i. Theory: Coupling of Josephson junctions ith f ti i t lwith ferromagnetic interlayer

ii Experiment:ii. Experiment:a) Planar 0, π SIFS Josephson junctionsb) St d (0 0 0 ) SIFS J hb) Stepped (0-0, 0-π, π-π) SIFS Josephson junctions

iii. Outlook

31

OutlookOutlook• Material Science: π SIFS junctions with F=Co, Fe, Ni,... possible? • Study of dynamics (spin waves & ac supercurrent)• Study of dynamics (spin waves & ac-supercurrent)• 0-π JJs in various geometries

Short to Long JJ limit, annular JJ, multiple 0-π steps ...g• Readout of fractional vortex by SQUID, LTSEM ...• Macroscopic quantum tunneling in π and 0-π JJs• Qubit based on π or 0-π JJs (flux/phase qubits)

"thanks for your0.4

0.6

0 coupling π coupling

t Ic (

mA

)

0 JJ π JJ 0-π JJthanks for your

attendance"4 5 5 0 5 5 6 0 6 5

0.0

0.2

d2d1

criti

cal c

urre

n

32

4.5 5.0 5.5 6.0 6.5

thickness dF (nm)

Etching of junction-SIMSEtching of junction SIMS

1000010000 Al

1000Ni

coun

t

100Nb

artic

le c

100CuPa

00:00 01:00 02:00 03:00 04:00 05:0010

33Etching time (min:sec)

Fabrication of wedge-shaped F-layerFabrication of wedge shaped F layer

Nb 40 nm 0 3 Ni60Cu40 sputter rate at 50 W/s)

NiCu 15 to 1 nm

Nb 40 nm

0.2

0.3 60 40 p

r-rat

e(n

m/

Al 4 nmAl O 1 nm2 3

0.1sput

ter

Nb 120 nm

0 20 40 60 80 1000.0

distance alongwedge(mm)4 inch, 100 mm

distance alongwedge(mm)

(a) SIFS cross section (b) NiCu sputter rate

34

SIFS junctions with FM in clean limitSIFS junctions with FM in clean limitF. Born et al. (cond-mat/0604277)similar reports by Robinson et al.Dirty limit: ℓ ≪ ξF1on SFS JJs (condmat/ 0606067)Clean limit: ℓ ≫ ξF1

Promising magnets:Promising magnets:Co, Ni, Fe, NiAl, CoFe etc.

0 630

35

300

350

)SFS33 Ni3Al

Nb/Al-Al2O3/Ni3Al/Nb (SIFS)Nb/Al-Al2O3/Cu/Ni3+δAl/Nb (SINFS)

0.2

0.4

0.6

20

25

30

200

250

300

dens

ity j c (A

/cm

2

tIc (

mA

)

3 5.87 nm 7.45 nm 8.74 nm

A=330*33 μm2

-900 -880 -860 -840 -820 -800 -780 -7600.0

10

15

100

150

criti

cal c

uren

t d

criti

cal c

uren

t

jc=100 A/cm2

λJ=36 μm

35-1000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 900 10000

5

0

50

magnetic field (μT)

Possible applicationsPossible applications

π shifter in Rapid Single Flux Quantum (SFQ ) logics• π shifter in Rapid Single Flux Quantum (SFQ ) logics• Memory (readout and changing of the state using SFQ

logic)logic)• Qubit: semifluxon (or two) in the quantum limit

– advantages: like spin but big and pinned.– Coupling: no problem

• Study of fractional quantization, including “heavy” vortices, non-symmetric vortices

• Study of heavily degenerated frustrated system, Ising-like model

36