Spin-diffusion, Tc suppression and proximity effect in ferromagnet/superconductor (LCMO/YBCO) bilayers and trilayers

Norbert M. Nemes

Seminario Alternativo, ICMM, 2006

ICMM, POMTcontratado postdoctoral “Juan de la Cierva”

con Mar Garcia-Hernandez

Seminario Alternativo, ICMM, 2006

project ideas, sample-growth, characterization (x-ray reflectivity, TEM etc.), magnetization (SQUID):

Vanessa Pena, Zouhair Sefrioui, D. Arias, Carlos Leon, and Jacobo Santamaria

GFMC, Departamento Fisica Aplicada III, Universidad Complutense de Madrid

magnetoresistace:

Jose Luis Martinez, Mar Garcia-Hernandez Instituto de Ciencia de Materiales de Madrid

polarized neutron reflectivity

Susanne G. E. te Velthuis and Axel HoffmannMaterials Science Division, Argonne National Laboratory

high-res. TEM, STEM, EELS

Maria VarelaOak Ridge National Laboratory

What can happen when a ferromagnet and a superconductor get too close?

Seminario Alternativo, ICMM, 2006

http://www.csr.umd.edu/csrpageAndreev reflection NO with LCMO!

SF

=1

=0

F

FS

F

=1

=0

F

F

Proximity effect via exchange fields of ferromagnet suppressing superconductivity

stray dipole fields of ferromagnet creating vortices in superconductor

spin imbalance in superconductor due to Takahashi et al PRL82 (1999) injection or diffusion of spin-polarized carriers

Reflection (triplet correlations) M. Eschrig et al PRL 90, (2003); Volkov et al PRL 90, (2003); Inverse proximity effect Bergeret et al PRB69 (2004)

(induced surface spin polarization in superconductor)

Magnetic exchange coupling (as in GMR devices) Sa de Melo PRL79 (1997)

. . . . . .

Spin transport between 2 ferromagnets through a superconductor

P Alignment

Tc

S. Takahashi, I. Imamura and S. Maekawa, Phys. Rev. Lett 82, 3911 (1999)

AP Alignment

Tc

Current in plane:

Thus far the same is true with normal metal spin-switch devices

What is extra with a superconductor?

Increased density of quasi-particles!!!

Eex=0.6 eV

Exchange splitting vs bandwidth

EF

W=4.5 eV

NiFerromagnet

Eex=3 eV EFW

LCMOHalf-metallic Ferromagnet

-2000 0 2000-400

-300

-200

-100

0

100

200

300

400

M (

emu/

cm3 )

H (Oe)

Seminario Alternativo, ICMM, 2006

Ferromagnets

Coercive Field

Saturation Magnetisation

Saturation Field

Seminario Alternativo, ICMM, 2006

SuperconductorsPerfect conductor

R=0:Perfect diamagnet,

expunges fieldχ=-4π:

Type I: coherence length > penetration depth no vorticesType II: coherence length < penetration depth vorticesYBCO is extreme type II as coherence length is very short

-4000 -2000 0 2000 4000-4x10-4

-2x10-4

0

2x10-4

4x10-4

m (

em

u/N

bil)

H (Oe)

0 100 200 3000

300

600

900

1200

Re

sis

tan

ce

(

)

T (K) Hc1

LCMO terminates in MnO !!!

2 nm

LCMOYBCO

[La0.7Ca0.3MnO3 / YBa2Cu3O7 ] Interface

La

Y

Ba

Ba

M. Varela and S.Pennycoock

2 4 6 8 10

35 c.u. YBCO

17.5 c.u. YBCO

13.5 c.u. YBCO

10 c.u. YBCO

Inte

ns

ity

(a

.u.)

2

LYL 40 c.u. LCMO

8 c.u. YBCO

X-ray STEM

La0.7Ca0.3MnO3 /YBa2Cu3O7./ La0.7Ca0.3MnO3 F/S/F trilayers

Z-contrast

M. Varela and S.Pennycoock

YBCO

LCMO

•dF La0.7Ca0.3MnO3 0.5453 nm/u.c. dS YBa2Cu3O7 1.1682 nm/u.c.

•Grow F/S/F trilayers

Experiment

Why manganite and HighTc-YBCO?LCMO: fully spin polarised, half-metal

small exchange fieldsYBCO: short coherence length

high TcShort range pair-breaking, high quality, smooth interfaces, good lattice matching

•Measure transport: current in plane (CIP)

•Sweep magnetic field: in plane

Seminario Alternativo, ICMM, 2006

-300 0 300-450

-300

-150

0

150

300

450

0.10

0.12

0.14

0.16

0.18

M (

em

u/c

m3)

H (Oe)

R ()

Polarised neutron reflectivity: AP alignment in F/S/F trilayers

Peak in magnetoresistance occurs for AP alignment of F layers

V. Peña et al. Phys Rev. Lett. 94 57002 (2005)

Neutron reflectometry: IPNS @ Argonne Ntl. Lab.

There exists a region of AP alignment

top and bottomsaturation magnetizations are also different

Seminario Alternativo, ICMM, 2006

V. Peña et al. Phys Rev. Lett. 94 57002 (2005)

Seminario Alternativo, ICMM, 2006

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0

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400

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0

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-200

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0

100

200

300

400

-400 -200 0 200 400-400

-300

-200

-100

0

100

200

300

400

M (

emu/

cm3 )

H (Oe)

4 YBCO

M (

emu/

cm3 )

H (Oe)

30 YBCO

M (

emu/

cm3 )

H (Oe)

20 YBCO

M (

emu/

cm3 )

H (Oe)

40 YBCO

LYL: STO / 40LCMO / NYBCO / 40LCMO T > Tc

M (

emu/

cm3 )

H (Oe)

8 YBCO

M (

emu/

cm3 )

H (Oe)

11 YBCO

With increasing YBCO thickness coercive fields change and

region of AP alignment diminishes

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

300

350

400

Hco

erc (

)

d (YBCO unit cell)

top LCMO bottom LCMO

Coercive fields of the top and bottom LCMO in trilayers

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

300

350

400

Hco

erc (

)

d (YBCO unit cell)

Hcoup

-Hcodown

min

minmax

RRR

MR

Rmax

Rmin

LCMO

LCMOYBCO

STO

Temperature

Magnetoresistance increases with lower temperature

V. Peña et al. Phys Rev. Lett. 94 57002 (2005)

Seminario Alternativo, ICMM, 2006

The little issues experimentalists have:

How can we compare GMR of various samples?

Seminario Alternativo, ICMM, 2006

0 5 10 15 20 25 30 35 400

50

100

150

200

(Rm

ax-R

min)/

Rm

in %

T(K)

YBCO 6 uc

0 5 10 15 20 25 30 35 40

1

10

100

R ()

T(K)

55 60 65 700

10

20

30

(Rm

ax-R

min)/

Rm

in %

T(K)

YBCO 24uc

YBCO 15uc

48 50 52 54 560

100

200

300

400

500

(Rm

ax-R

min)/

Rm

in %

T(K)

R(

)

50A 100A 500A 1000A 5000A

YBCO 13.5uc

48 50 52 541E-5

1E-4

1E-3

0.01

0.1

1

10

100

T(K)

LCMO

LCMO

YBCO

STO

LCMO

LCMOYBCO

STO

LCMOYBCO

STO

30 32 34 36 38 40 42 441E-5

1E-4

1E-3

0.01

0.1

1

10

100

LCMO moments aligned:

Rmax

: antiparalel

Rmin

: paralel

YBCO: 8 u.c.

R ()

T (K)42 44 46 48 50 52 54

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

LCMO moments aligned:

Rmax

: antiparalel

Rmin

: paralel

R ()

YBCO: 10 u.c.

T (K)66 68 70 72

1E-4

1E-3

0.01

0.1

1

10

LCMO moments aligned:

Rmax

: antiparalel

Rmin

: paralel

R ()

T (K)

YBCO: 40 u.c.

Pair-breaking effect is larger with thin YBCO!

Seminario Alternativo, ICMM, 2006

30 32 34 36 38 40 42 441E-5

1E-4

1E-3

0.01

0.1

1

10

100

LCMO moments aligned:

Rmax

: antiparalel

Rmin

: paralel

YBCO: 8 u.c.

R

()

T (K)

ΔTc

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0

0.2

0.4

0.6

0.8

1.0

1.2

Tc =

T[R

max

] -

T[R

min]

R/Rnormal

YBCO 40uc 30uc 24uc 17.5uc 15uc 8uc 6uc

Seminario Alternativo, ICMM, 2006

Seminario Alternativo, ICMM, 2006

0 5 10 15 20 25 30 35 400.01

0.1

1

Tc(R

/Rn=

0.00

01)

dYBCO

(unit cells)

Quasi-particles with E>Δ may diffuse, reflect in AP alignmentand pile up in superconductorthey suppress Δ and Tc self-consistently

Spin diffusion length in YBCO: ~9nm >> ξGL

~9nm ~24nm

Seminario Alternativo, ICMM, 2006

-400 -200 0 200 400-400

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0

100

200

300

400

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-300

-200

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0

100

200

300

400

M (

emu/

cm3 )

H (Oe)

4 YBCO

M (

emu/

cm3 )

H (Oe)

30 YBCO

M (

emu/

cm3 )

H (Oe)

20 YBCO

M (

emu/

cm3 )

H (Oe)

40 YBCO

LYL: STO / 40LCMO / NYBCO / 40LCMO T > Tc

M (

emu/

cm3 )

H (Oe)

8 YBCO

M (

emu/

cm3 )

H (Oe)

11 YBCO

With increasing YBCO thickness coercive fields change and

region of AP alignment diminishes

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

300

350

400

Hco

erc (

)

d (YBCO unit cell)

top LCMO bottom LCMO

Coercive fields of the top and bottom LCMO in trilayers

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

300

350

400

Hco

erc (

)

d (YBCO unit cell)

Hc2-Hc1

24nm:- electron mean free path in YBCO - Change of linear behavior of ΔTc

- Length-scale of decay for thicker films

Antiferromagnetic exchange coupling of LCMO layers is

mediated by spin polarized carrier?

-60000 -40000 -20000 0 20000 40000 600001E-3

0.01

0.1

1

R ()

H (Oe)

24uc YBCO 8uc YBCO

1E-4 1E-3 0.01 0.1 1 10 1000.0

0.2

0.4

0.6

Tc =

T[R

max

] -

T[R

min]

R(Ohm)

15uc YBCO I perp. H I paralel H

Seminario Alternativo, ICMM, 2006

1E-5 1E-4 1E-3 0.01 0.1 1 10 1000.0

0.2

0.4

0.6

0.8

1.0

Tc =

T[R

max

] -

T[R

min]

R(Ohm)

8uc YBCO

based on H-sweep500uA,

based on T-sweeps 5000uA 4000uA 3000uA 2000uA 1000uA 100uA 10uA 1uA 0.1uA

transition

Peaks can not originate from vortices

48 50 52 54 560

100

200

300

400

500

(Rm

ax-R

min)/

Rm

in %

T(K)

R(

)

50A 100A 500A 1000A 5000A

YBCO 13.5uc

48 50 52 541E-5

1E-4

1E-3

0.01

0.1

1

10

100

T(K)

MR@Rmin=10-4Ω25%, very narrow

-1000 -800 -600 -400 -200 0 200 400 600 800 10000.011

0.012

0.013

0.014

0.015

9/26/2005 15:36:55BYL04D 40-10-40

R ()

H (Oe)

56K

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

201.8

202.0

202.2

202.4

202.6

202.8

R ()

H (Oe)

96K

Effect small in bilayersSo, dipole-field effect is ruled out

MR@Rmin=10-4Ω100%, broadWhat is wrong with this sample?

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

0.05

0.06

0.07

0.08

0.09

0.10

BLY03D STO-10YBCO-40LCMO 9/26/2005 15:59:57

R ()

H (Oe)

86K

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

90.05

90.10

R ()

90.75K

MR@Rmin=10-4ΩVery small-1500 -1000 -500 0 500 1000 1500

3.5

3.6

3.7

10/3/2005 23:36:13

R ()

H (Oe)

PYL01D 6P-5Y-35L

-1500 -1000 -500 0 500 1000 1500

268.3

268.4

268.5

268.6

268.7

268.8

268.9

R(

)

Bilayers

In a particular bilayer: evidence of proximity effect

resistivity drops when magnetisation offerromagnet is non-uniform

-10000 -5000 0 5000 10000

0.045

0.050

R ()

H (Oe)

65 K upsweep downsweep

100 K dM/dH

STO-6PBCO-4YBCO-40LCMO

-0.0002

0.0000

0.0002

100 K M(H)

M(e

mu

)

Conclusions

Large magnetoresistance in FM/SC/FM trilayers due to polarised spin diffusion when FM are aligned AntiParalel

Tc is suppressed by AP alignment,this pair-breaking increases for thinner YBCO

Spin diffusion length can be estimated as 9 nm >> coherence length

Seminario Alternativo, ICMM, 2006

[La0.7Ca0.3MnO3 (15 u.c.)/ YBa2Cu3O7 (n u.c.)]

superlatticesresistance

n=1

n=5

n=3

n=2

a.c. susceptibility

0 25 50 75 100

-6,0x10-6

-4,0x10-6

-2,0x10-6

0,0

a.c.

T(K)

n=4 n=5n=8

0 50 100 150 20010 -3

10 -1

101

103

105

R (

)

T ( K )

[La0.7Ca0.3MnO3 (15 u.c.)/ YBa2Cu3O7 (n u.c.)]

superlattices

dScr

dScr~ 2.4 nm = 4 S

nmTk

hD

cB

SS 6.0

2/1

dScr = 25 nm V/FeV

dScr = 70 nm Fe/Pb/Fe

F/S interaction !!!

Z. Sefrioui et al APL 81, 4568 (2002)

-4000 0 4000

-100

-50

0

50

100

1 10 1000

100

200

300

400

500

M

H (Oe)

MS (

emu/

cm3 )

NM (unit cells)

0 50 100 150 200 250 30010 -3

10 -1

101

103

R (

)

T ( K )

m=60

m=30

m=3

Fixed YBa2Cu3O7 (5 u.c.) thickness

Varying La0.7Ca0.3MnO3 (3<m<90 u.c.) thickness

[La0.7Ca0.3MnO3 (m u.c.)/ YBa2Cu3O7 (5 u.c.)]

superlattices

nmE

D

ex

FF 2.0

42/1

1 10 1000

20

40

60

80

100

Tc

( K

)

NM ( unit cells )

dFcr ~ 20 nm !!!

Very long length scale into the ferromagnet!!!

Z. Sefrioui et al cond-mat/0301235 (2003)

[La0.7Ca0.3MnO3 (m u.c.)/ YBa2Cu3O7 (5 u.c.)]

superlattices

Seminario Alternativo, ICMM, 2006