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1 Electrical Characterization of Superconducting Microbridge Josephson Junctions with Ferromagnetic Strip Luis Gómez a, b , Shinichi Kitamura b , Takahiro Kubo b , Haruhisa Kitano b , and Atsutaka Maeda b a Japan Science and Technology Agency b The University of Tokyo

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  • 1Electrical Characterization of Superconducting Microbridge

    Josephson Junctions with Ferromagnetic Strip

    Luis Gmez a, b, Shinichi Kitamura b, Takahiro Kubo b, Haruhisa Kitano b, and Atsutaka Maeda b

    a Japan Science and Technology Agencyb The University of Tokyo

  • 2Sponsors and Collaborators

    Work supported by JST (Japan Science and Technology Agency)

    Through the CREST program (Core Research for Evolutional Science and Technology)

    Research area: Creation of Ultrafast, Ultralow-Power, Super-performance Nanodevices and Systems

    Project: Single-Flux-Quantum Terahertz Electronics

    Collaborators: Prof. Akira Fujimaki (Nagoya University) Prof. Masayoshi Tonouchi (Osaka University) and Prof. Yukio Tanaka (Nagoya University)

  • 3Outline Motivation Important Josephson Junction Parameters for

    SFQ Circuits Overview on Microbridges F/S Microbridges Results Summary and Future Work

  • 4These values are seldom achieved:

    Motivation

    ) to(close0T,KmV635.0)(

    0T,2/)0(

    ccNc

    Nc

    TTTRI

    eRI

    NcNc RIRI

    mVGHz500

    0

    For HTS with Tc 100 K

    Large Gaps 2=20-60meV

    IcRN 10s mV

    Ambegaokar and Baratoff, PRL10, 486 (1963)

    Fabricate HTS Josephson junctions with high IcRNproducts for SFQ circuits

    See for example: 10 mV reported by P. A. Rosenthal et. al., APL 63, 1984

    (1993) and 8 mV reported by Y. Divin et. al., Physica C 372-376, 115 (2002)

  • 5Important Josephson Junction Parameters for SFQ circuits

    Small spread of individual JJs Ic, RN values Minimal parasitic inductance and capacitance For Phase Mode SFQ logic

    used overdamped JJs (C

  • 6Overview on Microbridges

    Dolan and Lukens, IEEE Trans. on Magnetics. 1, 581 (1977)

    S. Shokhor, Appl. Phys.Lett. 67, 2869 (1995)

    R. Y. Chiao et. al., Rev. Phys. Appl. 9,

    183 (1974)

    A. L. Gudkov et. al., Sov. Tech. Phys.

    Lett. 5, 506 (1979)

    S. Tolpygo and M. Gurvitch, Appl. Phys. Lett. 69, 3914 (1996)

    LTS

    HTS

    Two metals overlay Magnetically form

    Direct E-beam written

    SFQ circuit

  • 7F/S MicrobridgesFabrication

    La2-xSrxCuO4 via PLD KrF excimer laser (=248nm) 250 mJ, 1 Hz 790C, O2 (10-1 torrs) On LaSrAlO4

    EBL, liftoff mask for Fe EBL, Mask for microbridge Wet etch in 0.05% HCl

    LSCO

    400nm FeLSAO

    W=2mts=65nml=400nmtf=60nm

    Fe

    LSCO

    LSAO

    Suppression of superconductivity

    directly underneath Fe strip

  • 8Results (RTs)

    5 10 15 20 25 30 35 40 45 500

    5k

    10k

    15k

    20k

    25k

    30k

    35k

    Without Fe With Fe, 300nm

    2006/09/13: LSCO15% 060811_7.5minC_45sec

    Temperature ( K )

    A400

    B300

    C200

    D100

    NO Fe A&B

    NO Fe C&D

    14 16 18 20 22 24 26 280

    50

    100

    150

    200

    250

    300

    350

    400

    Without Fe With Fe, 300nm

    2006/09/13: LSCO15% 060811_7.5minC_45sec

    Temperature ( K )

  • 9Results (IVs)

    0 3 6 9 12 150

    20

    40

    60

    80

    100

    120

    5.0 K 7.0 K 9.0 K 11. 0K 13.0 K 15.0 K 17.0 K 19.0 K

    Voltage ( mV )

    14 16 18 20 22 24 26 280

    50

    100

    150

    200

    250

    300

    350

    400

    Without Fe With Fe, 300nm

    2006/09/13: LSCO15% 060811_7.5minC_45sec

    Temperature ( K )

    4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 210

    10

    20

    30

    40

    50

    60

    IC = IC0 1 - T / Tc2

    Temperature

    K

    -100 -80 -60 -40 -20 0 20 40 60 80 100-1000

    -800

    -600

    -400

    -200

    0

    200

    400

    600

    800

    1000

    T @ 15.00K IcRN ~ 20mV

    Without Fe With Fe, 300nm

    2006/09/14: LSCO15% 060811_7.5minC_45sec

    Ic ~ 20 AJc ~ 15.4 kA/cm

    2

    RN ~ 1 kN ~ 65m -cm

    Voltage ( mV )

    Ic [1V criteria]

  • 10Results (Ic vs. B)

    22

    0

    0 46.0457.20 m

    GmG

    BlWArea eff

    nmmm

    WArealeff 2302

    46.0 2

    -80 -60 -40 -20 0 20 40 60 80-10

    -8

    -6

    -4

    -2

    0

    2

    4

    6

    8

    10

    @18K

    B ( G )

    +Ic -Ic For 300nm

    wide Fe strip

  • 11

    I-Vs + microwaves (Shapiro & Photon induced steps)

    -150 -100 -50 0 50 100 150-1m

    -800

    -600

    -400

    -200

    0

    200

    400

    600

    800

    1m

    Voltage (V)

    0.0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    T=17.5 K2 GHz

    0.0 dBm

    Fe, 400nm

    2006/10/31: LSCO15%060818_5minC_15sec

    -50 -40 -30 -20 -10 0 10 20 30 40 50-800

    -600

    -400

    -200

    0

    200

    400

    600

    800

    Voltage (V)

    0.00

    0.05

    0.10

    0.15

    0.20

    0.25

    0.30

    0.35

    T=17.5 K2 GHz

    0.0 dBm

    Fe, 400nm

    2006/10/31: LSCO15%060818_5minC_15sec

    V

    mVGHz

    GHzV AC 4500

    201

  • 12Summary and Future work We are fabricating Fe / LSCO microbridge junctions

    With promising Josephson characteristics and large IcRN products.

    The fabrication method is: Simple; compatible with modern lithographic techniques. Suitable for large integration of Josephson junctions into circuits

    Junctions may address all the SFQ requirements Reproducibility of junctions is still an open question

    Mechanism for junctions is still an open question Method should work for both HTS and LTS

    Future work: Fabrication of SQUIDS and Antennas:

  • 13

    End of Presentation

    Additional information below

  • 14

    Ic vs. B (as a function of temperature)

    -80 -60 -40 -20 0 20 40 60 800

    2

    4

    6

    8

    10

    12

    14

    16

    18

    20

    2006/09/15: LSCO15%060811_7.5minC_45sec

    B ( G )

    Fe, 300nm 15K 18K 19K

  • 15Ferromagnet/Superconducting microbridges

    Sample fabrication (Ferromagnet evaporation)

  • 16Ferromagnet/Superconducting microbridges

    Sample fabrication (Etch)

  • 17Ferromagnet/Superconducting microbridges

    Top view

  • 18Ferromagnet/Superconducting microbridges

    Top view

  • 19Ferromagnet/Superconducting microbridge JJs

    Finished sample

  • 20Ferromagnet/Superconducting microbridge JJs

    Sample dimensions

  • 21Junctions electrical characterization: Experimental setup

    New Cryo-ProbeDC-electrical characterizations as a function of :Temperature range (4.2 K-100 K)

    Magnetic field up to (350G)

    Microwaves up to (20GHz)

    20 individual leads

  • 22R vs. T (Josephson junction signature).A

    400B

    300

    C200

    D100

    NO Fe A&B

    NO Fe C&D

    LSCO

    400nmLSAO

    20 25 30 35 40 45 500

    1000

    2000

    3000

    4000

    5000

    400nm Fe Strip

    2006/10/27: LSCO15% 060818_5minC_15sec

    Temperature (K)25 26 27 28

    0

    10

    20

    400nm Fe Strip

    2006/10/27: LSCO15% 060818_5minC_15sec

    Temperature (K)

    E-beam voltage = 50KV

    W=2mtS=65nml=400nmtF=60nm

  • 23Comparison with E-beam damaged JJs

    S. Tolpygo et al, Appl. Phys. Lett. 63, 1696 (1993)

    E-beam voltage = 120KV

  • 24Photolithography made samples

    26 28 30 32 34 36 380

    200

    400

    6002006/08/24: LSCO15% 060613_10min_90sec

    Without Fe With Fe

    Temperature (K)

    26 27 28 29 30 31 320

    2

    4

    6

    8

    10

    122006/08/24: LSCO15% 060613_10min_90sec

    Without Fe With Fe

    Temperature (K)

    26 27 28 29 30 31 321m

    10m

    100m

    1

    10

    100

    1k2006/08/24: LSCO15% 060613_10min_90sec

    Without Fe With Fe

    Temperature (K)

  • 25

    RTs and IVs

    -1000 -500 0 500 1000

    -3

    -2

    -1

    0

    1

    2

    3

    @29.0K

    IV (B=0 G) IV (B=400G)

    Voltage (mV)

    0

    100

    200

    300

    400

    500

    dV/dI (B=0 G) dV/dI (B=400G)

    2006/11/23: LSCO20%-061013_10min(A)-400nmFe

    28.0 28.5 29.0 29.5 30.0 30.5 31.01E-3

    0.01

    0.1

    1

    10

    100

    2006/11/21:LSCO20%061013_10min(A)_12sec

    NoFe 400nmFe

    IBias = 1A

    Temperature (K)

    IcRN ~ 4 mV

  • 26Ic vs. T (Wide vs. narrow junctions)

    mlJ Lc

    J

    4.122

    2/1

    0

    0

    For T=15K S. Tolpygo et al, Appl. Phys. Lett. 69, 3914 (1996)

    4 6 8 10 12 14 16 18 20 220

    10

    20

    30

    40

    50

    602006/09/14: LSCO15% 060811_7.5minC_45sec

    Fe, 300nm

    Narrow Junction

    Wide Junction

    Ic

    IC = IC0 ( 1 - T / Tc ) 2

    Tc 23.7 K

    Temperature (K)

    0.000

    0.002

    0.004

    0.006

    0.008

    0.010

    Ic1/2

  • 27Possible explanations for the junction formation (theory)

    Ferromagnetic proximity effect in F/S multilayer

    V. Pea et al, PRB. 69, 224502 (2004)

    Buzdin, RMP. 77, 77935 (2005)LTSHTS

  • 28Possible explanations for the junction formation (theory)

    Inverse proximity effect in S near F material

    M. A. Sillanpaa et al, Europhys. Lett. 56, 590 (2001)LTS

  • 29PLD growth of LSCO films

    Schematic Figure of PLD

    KrF Excimer Laser=248nm LSCO

    Substrate Holder

    250mJ,1Hz

    Deposition condition:

    79010-1torr

    Substrate: LSAO

  • 30

    PLD growth parameters

  • 31

    Film thickness

  • 32LSCO Structure

    H.Sato et al. : Phys. Rev. B61 12447.

    Y.Maeno et al. : Physica C173 322.

    J. B. Torrance et al. : Phys. Rev. Lett.68 3777.

  • 33LSCO characterization- T X-ray

    c-axis length: 13.16 (x=0.15)

    12.83 (x=0.20)

  • 34

    LSCO thin film properties

    M. Suzuki and M. Hikita, PRB 44, 249 (1991)

  • 35R-Ts forLSCO15%060811_7.5minC_45sec

    16 18 20 22 24 26 281m

    10m

    100m

    1

    10

    100

    1k

    10k

    100k

    1M

    10M

    400nm100nm

    300nm

    200nm2006/09/14:LSCO15%060811_7.5minC_45sec

    Fe, 400nm Fe, 300nm No Fe, 300&400 Fe, 200nm Fe, 100nm No Fe, 100&200

    Temperature (K)

  • 36

    0 10 20 30 40 50 60 70 80 90 1000.01

    0.10

    1.00

    La2-xSrxCuO4 (LSCO)

    x = 0.08x = 0.15x = 0.20

    Temperature (K)0 50 100 150 200 250 300

    10-1

    100

    101

    La2-x

    SrxCuO

    4 / LaSrAlO

    4 Thin Film

    Temperature (K)

    x = 0.07 (t = 460 nm)x = 0.12 (t = 230 nm)x = 0.14 (t = 270 nm)x = 0.16 (t = 140 nm)x = 0.18 (t = 110 nm)x = 0.20 (t = 90 nm)

    T. Ohashi, Master Thesis, The university of Tokyo, pp. 41 (2005)

  • 37

    -100 -80 -60 -40 -20 0 20 40 60 80 100-1000

    -800

    -600

    -400

    -200

    0

    200

    400

    600

    800

    1000

    T @ 15.00K IcRN ~ 20mV

    Without Fe With Fe, 300nm

    2006/09/14: LSCO15% 060811_7.5minC_45sec

    Ic ~ 20 A

    Jc ~ 15.4 kA/cm2

    RN ~ 1 kN ~ 65m -cm

    Voltage ( mV )-100 -50 0 50 100

    -250

    -200

    -150

    -100

    -50

    0

    50

    100

    150

    200

    250

    T @ 15.00K

    IcRN ~ 20mV

    2006/09/14: LSCO15% 060811_7.5minC_45sec

    I

    Voltage ( mV )

    0

    500

    1000

    1500

    2000

    dV/dI

    1E-4 1E-3 0.01 0.1 1 10 100 100010

    100

    1k

    Without Fe With Fe, 300nm

    T @ 15.00K

    2006/09/14: LSCO15% 060811_7.5minC_45sec

    IC [1V] ~ 18 A

    IC [1V] ~ 120 A

    Voltage ( mV )0 200 400 600 800 1000

    -25

    0

    25

    50

    75

    100

    125

    150

    175 Without Fe With Fe, 300nm

    T @ 15.00K

    2006/09/14: LSCO15% 060811_7.5minC_45sec

    RN [NoFe](380 +/- 7)

    RN [Fe-LineB]

    (1670 +/- 20)

    Current ( A )