Spin-diffusion, Tc suppression and proximity
effect in ferromagnet/superconductor
(LCMO/YBCO) bilayers and trilayers
Norbert M. Nemes
ICMM, POMT
contratado 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 Hoffmann
Materials Science Division, Argonne National Laboratory
high-res. TEM, STEM, EELS
Maria Varela
Oak Ridge National Laboratory
Seminario Alternativo, ICMM, 2006
What can happen when a ferromagnet and
a superconductor get too close?
Andreev reflection

NO with LCMO!
http://www.csr.umd.edu/csrpage
Proximity effect via exchange fields of ferromagnet
suppressing superconductivity
=1
F
Reflection (triplet correlations)
=0
Inverse proximity effect Bergeret et al PRB69 (2004)
(induced surface spin polarization in superconductor)
Magnetic exchange coupling (as in GMR devices)
spin imbalance in superconductor due to Takahashi et al PRL82 (1999)
injection or diffusion of spin-polarized carriers
stray dipole fields of ferromagnet
creating vortices in superconductor
M. Eschrig et al PRL 90, (2003); Volkov et al PRL 90, (2003);
S
Sa de Melo PRL79 (1997)
. . . . . .
Seminario Alternativo, ICMM, 2006
F
F
Spin transport between 2 ferromagnets through a superconductor
S. Takahashi, I. Imamura and S. Maekawa, Phys. Rev. Lett 82, 3911 (1999)
P Alignment
AP Alignment
Thus far the same is true with
normal metal spin-switch devices
Tc
What is extra with a superconductor?
Increased density of quasi-particles!!!
Current
in plane:
Tc
Exchange splitting vs bandwidth
EF
Eex=3 eV
Eex=0.6 eV
W=4.5 eV
Ni
Ferromagnet
EF
LCMO
Half-metallic Ferromagnet
W
Ferromagnets
400
300
Saturation Magnetisation
3
M (emu/cm )
200
100
0
-100
-200
Saturation Field
-300
-400
-2000
0
2000
H (Oe)
Coercive Field
Seminario Alternativo, ICMM, 2006
Superconductors
Perfect diamagnet,
expunges field
χ=-4π:
1200
4x10
-4
900
2x10
-4
m (emu/Nbil)
Resistance (  )
Perfect conductor
R=0:
600
300
0
0
100
200
300
T (K)
0
-2x10
-4
-4x10
-4
-4000 -2000
0
2000 4000
H (Oe)
Type I: coherence length > penetration depth  no vortices
Type II: coherence length < penetration depth  vortices
YBCO is extreme type II as coherence length is very short
Seminario Alternativo, ICMM, 2006
Hc1
[La0.7Ca0.3MnO3 / YBa2Cu3O7 ] Interface
LCMO
Ba
YBCO
Y
Ba
La
2 nm
LCMO terminates in MnO !!!
M. Varela and S.Pennycoock
La0.7Ca0.3MnO3 /YBa2Cu3O7./ La0.7Ca0.3MnO3
F/S/F trilayers
X-ray
STEM
Intensity (a.u.)
LYL 40 c.u. LC M O
35 c.u. YBCO
YBCO
17.5 c.u. YBCO
13.5 c.u. YBCO
10 c.u. YBCO
LCMO
8 c.u. YBCO
2
4
6
2q
8
10
Z-contrast
M. Varela and S.Pennycoock
Experiment
•Grow F/S/F trilayers
•dF La0.7Ca0.3MnO3 0.5453 nm/u.c. dS YBa2Cu3O7
•Measure transport:
current in plane (CIP)
1.1682 nm/u.c.
•Sweep magnetic field: in plane
Why manganite and HighTc-YBCO?
LCMO:
fully spin polarised, half-metal
small exchange fields
YBCO:
short coherence length
high Tc
Short range pair-breaking, high quality, smooth interfaces,
good lattice matching
Seminario Alternativo, ICMM, 2006
Polarised neutron reflectivity: AP
alignment in F/S/F trilayers
V. Peña et al. Phys Rev. Lett.
94 57002 (2005)
Peak in magnetoresistance occurs
for AP alignment of F layers
450
0.18
0.16
150
0
0.14
-150
0.12
-300
-450
-300
0
H (O e)
300
0.10
R ( )
3
M (emu/cm )
300
Neutron reflectometry: IPNS @ Argonne Ntl. Lab.
0
10
Bottom LCMO
300
3
M [emu/cm ]
600
Top LCMO
0
-300
-600
0
100
200
H [Oe]
Reflectivity
-2
T=58 K
T=50 K
300
10
H = 80 Oe
-4
1x10
There exists a region of
AP alignment
top and bottom
saturation magnetizations
are also different
+
I
I
H = 120 Oe
-6
10
0.01
0.02
0.03
-1
q [Å ]
0.04
0.05
Pena et al. Figure 3
Seminario Alternativo, ICMM, 2006
V. Peña et al. Phys Rev. Lett.
94 57002 (2005)
LYL: STO / 40LCMO / NYBCO / 40LCMO
400
4 YBCO
300
3
100
0
-100
-200
-100
-400
-400
-200
0
200
400
-400
-200
H (Oe)
11 YBCO
300
3
100
0
-100
-200
20 YBCO
-200
-400
-200
0
200
400
-400
200
3
M (emu/cm )
3
300
100
0
-100
-200
0
200
400
H (Oe)
15
20
25
30
35
40
45
d (YBCO unit cell)
top LCMO
bottom LCMO
5
10 15 20 25 30 35 40 45
d (YBCO unit cell)
40 YBCO
-400
400
10
0
-300
200
5
-100
-200
0
0
0
100
-300
-200
150
200
-200
-400
200
50
150
H (Oe)
30 YBCO
-400
200
0
0
H (Oe)
400
250
100
50
-100
-400
250
Hcoup-Hcodown
300
100
0
-300
-400
M (emu/cm )
400
100
-300
300
200
200
M (emu/cm )
3
M (emu/cm )
200
400
0
H (Oe)
400
350
300
-200
-400
400
350
0
-300
300
400
100
-300
400
Coercive fields of the top and bottom LCMO in trilayers
200
M (emu/cm )
3
M (emu/cm )
200
8 YBCO
Hcoerc ()
300
Hcoerc ()
400
T > Tc
-400
-200
0
H (Oe)
200
400
With increasing YBCO thickness
coercive fields change and
region of AP alignment diminishes
Seminario Alternativo, ICMM, 2006
LCMO
YBCO
LCMO
STO
310
10
R ()
0.18
Rmax
0.16
0.14
0.12
0.10
Rmin
-5000
MR 
0
H (Oe)
5000
Rmax  Rmin
Rmin
D
D
10
2
Temperature D
10 10-1
D
-2
D
110 D
10 10-3###
D
###
D
0 -4###
D
10 10 ###
-7500
0
7500
D
-1 ###
H (Oe)
D
10 ###
###
D
51### 54
57
60
D
###
D
###
T (K) increases
D
Magnetoresistance
###
D
###
0
R ()
MR (%)
0.20
1
with###
lower temperature
###
V. Peña et al. Phys Rev. Lett.
94 57002 (2005)
Seminario Alternativo, ICMM, 2006
3
10
20
How can we compare GMR
10
of various samples?
0
55
60
65
70
T(K)
YBCO 15uc
2
10
MR (%)
(Rmax-Rmin)/Rmin %
The
little issues
30
experimentalists
have:
YBCO 24uc
1
10
0
10
100
500
-1
100
10
0
5
10
15
5
10
15
20 25
T(K)
20 25
T(K)
30
30
35
YBCO 13.5uc
1
0.1
54
57
0.01
1E-3
60
1E-4
1E-5
T (K)
200
48
50
52
T(K)
100
40
0
35
R()
YBCO 6 uc
1
50
0
0
(Rmax-Rmin)/Rmin %
150
R ()
(Rmax-Rmin)/Rmin %
100
10
10 400
51
300
200
48
40
Seminario Alternativo, ICMM, 2006
50
52
T(K)
54
54
50A
100A
500A
1000A
5000A
56
LCMO
LCMO
YBCO
LCMO
YBCO
YBCO
LCMO
STO
STO
YBCO: 8 u.c.
10
10
1
1
0.1
0.01
1E-3
0.1
LCMO moments
0.01
aligned:
1E-3
Rmax: antiparalel
STO
YBCO: 10
10 u.c.
YBCO: 40 u.c.
1
R ()
100
R ()
R ()
100
LCMO
0.1
LCMO moments
LCMO moments
0.01
aligned:
aligned:
Rmax: antiparalel
R : antiparalel
1E-4
Rmin: paralel
1E-4
1E-3
1E-5
1E-5
1E-4
max
Rmin: paralel
Rmin: paralel
30 32 34 36 38 40 42 42
44 44 46 48 66
50 52 68
54
70
T (K)
Pair-breaking
effect is largerTwith
(K) thin YBCO!
T (K)
Seminario Alternativo, ICMM, 2006
72
1.2
10
R ()
1
0.1
0.01
1E-3
1E-4
1E-5
YBCO: 81.0
u.c.
Tc = T[Rmax] - T[Rmin]
100
ΔTc
YBCO
40uc
30uc
24uc
17.5uc
15uc
8uc
6uc
0.8
0.6
LCMO moments
0.4 aligned:
Rmax: antiparalel
0.2 Rmin:
paralel
30 32 340.036 38 40 42 44
1E-6
1E-5
1E-4
1E-3
T (K)
R/R
normal
Seminario Alternativo, ICMM, 2006
0.01
0.1
1
Tc(R/Rn=0.0001)
1
0.1
0.01
~24nm
~9nm
0
5
10
15
20
25
30
35
40
dYBCO (unit cells)
Spin diffusion length
in YBCO: ~9nm >> ξGL
Seminario Alternativo, ICMM, 2006
Quasi-particles with
E>Δ may diffuse,
reflect in AP alignment
and pile up in
superconductor
they suppress Δ and Tc
self-consistently
LYL: STO / 40LCMO / NYBCO / 40LCMO
4 YBCO
300
8 YBCO
3
100
0
-100
-200
-300
100
-100
0
200
400
-400
-200
400
300
3
M (emu/cm )
3
M (emu/cm )
0
-100
-200
-300
20 YBCO
100
-100
-200
0
200
400
-400
3
3
300
M (emu/cm )
0
-100
-200
-200
0
200
400
150
H (Oe)
50
200
0
0
5
10
15
20
25
30
35
40
45
d (YBCO unit cell)
150
0
0
top LCMO
bottom LCMO
5
10 15 20 25 30 35 40 45
d (YBCO unit cell)
100
0
-100
-400
0
100
40 YBCO
-300
-400
-200
250
200
-200
-300
-400
200
H (Oe)
400
30 YBCO
100
250
50
-200
H (Oe)
200
Hc2-Hc1
300
100
0
-400
-400
M (emu/cm )
400
-300
-400
300
200
200
100
400
0
H (Oe)
11 YBCO
200
350
Antiferromagnetic
exchange coupling
of LCMO layers is
mediated by spin polarized carrier?
-200
H (Oe)
300
400
300
-200
-400
-400
Coercive fields of the top and bottom LCMO in trilayers
350
0
-300
-400
400
400
200
M (emu/cm )
3
M (emu/cm )
200
T > Tc
Hcoerc ()
300
400
Hcoerc ()
400
200
400
-400
-200
0
H (Oe)
200
400
24nm:
- electron mean free path in YBCO
- Change of linear behavior of ΔTc
- Length-scale of decay for thicker films
With increasing YBCO thickness
coercive fields change and
region of AP alignment diminishes
Seminario Alternativo, ICMM, 2006
24uc YBCO
8uc YBCO
0.6
Tc = T[Rmax] - T[Rmin]
R ()
1
0.1
0.01
1E-3
-60000
-40000
-20000
0
20000
40000
60000
0.4
0.2
0.0
1E-4
H (Oe)
15uc YBCO
I perp. H
I paralel H
1E-3
0.01
0.1
1
10
100
R(Ohm)
1.0
10
YBCO 13.5uc
1
400
R()
Tc = T[Rmax] - T[Rmin]
(Rmax-Rmin)/Rmin %
0.8
Peaks can not originate
from vortices
8uc YBCO
100
500
0.1
0.01
1E-3
0.6300
1E-4
1E-5
0.4200
100
0.2
0
48
50
0.0
1E-5 1E-4 1E-3 0.01
Seminario Alternativo, ICMM, 2006
0.1
R(Ohm)
48
based on H-sweep
500uA,
based on T-sweeps
5000uA
4000uA
transition
3000uA
50
52
54
50A
T(K)
2000uA
1000uA
100A
100uA
500A
10uA
1000A
1uA
5000A
0.1uA
52
1
10
T(K)
54
100
56
Effect small in bilayers
So, dipole-field effect is ruled out
Bilayers
H (Oe)
BYL04D 40-10-40
-1000
0.015
-800
-600
-400
-200
0
400
600
800
-1000
1000
202.8
-800
0.10
202.6
-600
-400
-200
0
200
400
600
800
1000
90.75K
86K
0.09
0.014
90.10
202.4
0.012
96K
202.0
56K
0.011
-1000 -800 -600 -400 -200
201.8
0
200 400 600 800 1000
H (Oe)
MR@Rmin=104Ω
100%, broad
R ()
202.2
0.08
R ()
0.013
R ()
0.07
0.06
What is wrong with this
sample?
90.05
0.05
-1000 -800 -600 -400 -200
0
200 400 600 800 1000
H (Oe)
-1500
3.7
-1000
-500
0
500
1000
1500
268.9
268.8
268.7
3.6
268.6
R()
MR@Rmin=10-4Ω
25%, very narrow
R ()
R ()
9/26/2005 15:59:57
BLY03D STO-10YBCO-40LCMO
9/26/2005 15:36:55
200
268.5
268.4
PYL01D 6P-5Y-35L
3.5
-1500
10/3/2005 23:36:13
268.3
-1000
-500
0
H (Oe)
500
1000
1500
MR@Rmin=10-4Ω
Very small
In a particular bilayer:
evidence of proximity effect
resistivity drops when magnetisation of
ferromagnet is non-uniform
STO-6PBCO-4YBCO-40LCMO
65 K
upsweep
downsweep
0.0002
100 K
M(H)
100 K
dM/dH
-0.0002
0.045
-10000
0.0000
M(emu)
R ()
0.050
-5000
0
H (Oe)
5000
10000
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.)] superlattices
resistance
a.c. susceptibility
R(
10
5
10
3
10
n=3
0,0
-6
a.c.
)
n=1
n=2
n=5
-2,0x10
n=4
-6
-4,0x10
1
n=5
-6
n=8
-6,0x10
10
-1
10
-3
0
0
50
100
T (K)
150
200
25
50
T(K)
75
100
[La0.7Ca0.3MnO3 (15 u.c.)/ YBa2Cu3O7 (n u.c.)] superlattices
F/S interaction !!!
1/ 2
 hDS 

 S  
 k BTc 
100
PBCO spacer
80
Tc
60
LCMO spacer
40
20
0
dScr
0
2
4
6
8
10 12
N (unit cells)
Z. Sefrioui et al APL 81, 4568 (2002)
dScr ~
 0.6nm
2.4 nm = 4 S
dScr
= 25 nm V/FeV
dScr
= 70 nm Fe/Pb/Fe
[La0.7Ca0.3MnO3 (m u.c.)/ YBa2Cu3O7 (5 u.c.)] superlattices
Fixed YBa2Cu3O7 (5 u.c.) thickness
Varying La0.7Ca0.3MnO3 (3<m<90 u.c.) thickness
3
m=60
500
100
3
1
m=3
10
MS (emu/cm )
10
R (
)
m=30
-1
400
50
M
10
300
0
-50
-100
200
-4000
0
H (Oe)
4000
100
10
0
-3
0
50 100 150 200 250 300
T(K)
1
10
NM (unit cells)
100
[La0.7Ca0.3MnO3 (m u.c.)/ YBa2Cu3O7 (5 u.c.)] superlattices
 4DF
 F  
 Eex
100
Tc ( K )
80
1/ 2



 0.2nm
60
40
dFcr
20
0
1
10
N M ( unit cells )
~ 20 nm !!!
100
Very long length scale into the ferromagnet!!!
Z. Sefrioui et al cond-mat/0301235 (2003)
Seminario Alternativo, ICMM, 2006