Transcript Spin-polarized tunneling with magnetic oxide electrodes

Spin-polarized tunneling with
magnetic oxide electrodes and
barriers
Gervasi Herranz
Unité Mixte de Physique CNRS / Thales,
Route Départementale 128
91767 Palaiseau, France
[email protected]
Zaragoza, november 2005
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
M. Bibes 2, U. Lüders3,4, M. Gajek1,3, H. Béa1, R. Ranchal1,6,
M. Bowen1, K. Bouzehouane1, S. Fusil5, E. Jacquet1, J.-L.
Maurice1, A. Vaurès1, J.-F. Bobo4, M. Varela6, J. Fontcuberta3,
J.-P. Contour1, A. Barthélémy1, J. M. De Teresa, A. Fert1…
1 Unité Mixte de Physique CNRS / Thales, Orsay (FRANCE)
2 Institut d’Electronique Fondamentale, Université Paris-Sud, Orsay (FRANCE)
3 Institut de Ciència de Materials de Barcelona, CSIC, Campus de la UAB, Bellaterra
(SPAIN)
4 FRE CNRS-ONERA, Toulouse (FRANCE)
5 Université d’Evry (FRANCE)
6 Facultad de Física, Universidad Complutense de Madrid (SPAIN)
7 Facultat de Física Aplicada i Òptica, Universitat de Barcelona (SPAIN)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Outline
Spintronics: Spin polarization, Spin-dependent tunneling
Half-metallic ferromagnets
Spin Polarization
Interface (electrical bondings)
Barrier (electronic structure)
Voltage bias
Temperature
Artificial high-spin polarization: Spin-filter devices
Spin-injection: diluted magnetic systems (DMS)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Outline
Spintronics: Spin polarization, Spin-dependent tunneling
Half-metallic ferromagnets
Spin Polarization
Interface (electrical bondings)
Barrier (electronic structure)
Voltage bias
Temperature
Artificial high-spin polarization: Spin-filter devices
Spin-injection: diluted magnetic systems (DMS)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Spin-dependent conduction: spin polarization (SP)
Asymmetry in the DOS at the Fermi level
Metal with magnetic order
SP  0
N(EF)↑≠ N(EF)↓
Spin polarization (SP)
SP 
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
N(EF)  N(EF)
N(EF)  N(EF)
Determination of the spin polarization
Spin-resolved photoemission
Andreev reflection point contacts
Tunneling experiments:
FM/i/SC
FM/i/FM
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Physics of tunneling
Transmission coefficient through a barrier:
Exponential dependence on barrier thickness(d)
Exponential dependence on h1/2 (barrier height)
Conductance through a barrier:
Depends on the density of states (DOS) of the
electrodes
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
FM/I/SC tunnel junctions
Ferromagnet/Insulator/Superconductor
FM electrode:
Asymmetry: N(EF)↑≠ N(EF)↓
Determination of SP in FM/Al2O3/Al junctions
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
FM/I/FM: Magnetic tunnel junctions (MTJs)
Jullière’s model
Parallel magnetizations
M1
FM 1
V+
Antiparallel magnetizations
M2
NMI
M1
M2
Low resistance
High resistance
FM 2
Jullière: TMR depends only on
the electrode properties
M
R
Tunel magnetoresistance (TMR)
TMR =
H
H
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
R↑↓ – R↑↑
R↑↑
=
2SP1SP2
1– SP1SP2
TMR and applications
Moodera et al., PRL 74, 3273 (1995)
CoFe / Al2O3 / Co, TMR  10% @ RT
MRAMs
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
High-SP materials required:
Large TMR
Spin analyzers in spintronics devices
TMR =
2SP1
1– SP1
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
SP2 = 1
Materials for ferromagnetic electrodes: Half metals
De Groot et al.; Phys. Rev. Lett. 50, 2024 (1983)

EF
sf
sf
EF

SP=±100%  TMR
For classification of different HM see: J.M.D.Coey and M.Venkatesan; J. Appl. Phys. 91, 8345 (2002)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Materials for ferromagnetic electrodes: Half metals
Spin resolved
Photoemission
NiMnSb
SP
Andreev
reflection
SP by spin
tunneling
58%(1)
19%(8)
86%(2)
La2/3Ca1/3MnO3
CrO2
90%(5)
Yes: SPIPE(6)
95%(9)
Fe304
-80%(12)
La2/3Sr1/3MnO3
78%(1)
95%(4)
98%(7)
8%(11)
42%(10)
-90%(3)
Sr2FeMoO6
(1):
(2):
(3):
(4):
(5);
(6):
Soulen et al.; Science 282, 85 (1998)
Moon Ho Jo
Bibes et al. APL83
Bowen et al, APL.82, 233 (2003)
Park et al.; Nature (1998)
Bertacco et al.
(7) Anguelouch et al.; PRB64,180408 (2002)
(8) Tanaka et al.; JAP(1999)
(9) Kamper (1987)
(10) Seneor et al.;APL74, 4017 (1999)
(11) Gupta et al.; APL78, 1894 (2001)
(12) Dedkov et al.; PRB65, 064417 (2002)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
The sign of the spin polarization (SP)
All SP > 0, except for:
Co/STO
J. M. De Teresa et al., Science 286, 507 (1999).
FE3O4
Panchula, Ph.D. thesis, Stanford University, Stanford, 2003.
SrRuO3/STO
D. C. Worledge, PRL 85, 5182 (2000).
La0.7Ce0.3MnO3
Mitra, PRL 90, 017202 (2003)
Sr2FeMoO6
Bibes et al. APL83 (2003)
BUT contradictory with band
calculations of TM ferromagnets
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Outline
Spintronics: Spin polarization, Spin-dependent tunneling
Half-metallic ferromagnets
Spin Polarization
Interface (electrical bondings)
Barrier (electronic structure)
Voltage bias
Temperature
Artificial high-spin polarization: Spin-filter devices
Spin-injection: diluted magnetic systems (DMS)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Spin polarization depends on different factors
Jullière’s model is insufficient: Spin Polarization is NOT
ONLY a function of the electrode properties:
Interface bonding-types affect SP
The crystal symmetry of the barrier is important
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Effect of interface bondings on SP
Calculations in a tight-binding approximation
Tsymbal & Pettifor, J. Phys.: Condens. Matter 9 (1997) L411
The spin polarization of the tunnelling current
depends strongly on the type of covalent bonding
between the ferromagnet and the insulator.
sss bonding  tunnelling current carried by the
s electrons : SP > 0
sds bonding  tunnelling current carried by delectrons : SP < 0
sss
sss  sps, sds
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
4.2 10
6
4.1 10
6
4 10
LSMO/Al2O3/Co
Co
LSMO
6
3.9 10
6
3.8 10
6
3.7 10
6
3.6 10
6
3.5 10
6
-150 -100 -50 0
50
-6
-4
-2
0
2
4
6
8
10
Normal TMR
TMR (%)
Resistance (Ohms)
SP depends on the inteface
All MTJs with LSMO and Co electrodes
SPCo > 0
 de Teresa et al., Science 286, 509 (1999)
100 150
Magnetic Field (mT)
300
LSMO/STO/Co
280
-30
LSMO/TiO2/Co
1
Inverse
TMR
0
-20
240
-10
220
(a)
200
-100
0
10
-50
0
50
100
Magnetic field (mT)
-1
TMR (%)
260
TMR (%)
Resistance (Ohms)
 s electrons are responsible for tunneling
-2
-3
-4
-300
-150
0
150
300
SPCo < 0
Magnetic field (mT)
 d electrons with negative spin polarisation are responsible for tunneling
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Role of interfaces on SP
Co
Co
SrTiO3 (STO)
Al2O3(ALO)
Co
Al2O3 (ALO)
SrTiO3(STO)
LSMO
LSMO
LSMO/STO 2.5nm/Co
LSMO
LSMO/Al2O3 30nm/Co
LSMO/STO 1nm/Al2O3 1.5nm/Co
-10
220
(a)
200
0
10
-100
-50
0
50
Resistance (Ohm)
240
4.4 10
5
4.2 10
4 10
3.8 10
3.6 10
0
5
4
5
8
5
12
16
5
20
100
Magnetic field (mT)
Inverse TMR  SPCo <0
-100
-50
0
50
6
4.1 10
6
4 10
6
3.9 10
6
3.8 10
6
4
3.7 10
6
6
6
8
3.6 10
3.5 10
6
-6
Co
LSMO
2
10
-150 -100 -50 0
Magnetic Field (mT)
50
Hybridization at interface fixed the spin polarization
100 150
Magnetic Field (mT)
Normal TMR  SPCo >0
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
-2
0
100
★ de Teresa et al., Science 286, 509 (1999)
-4
TMR (%)
-20
-4
TMR (%)
260
5
Resistance (Ohms)
4.6 10
-30
280
TMR (%)
Resistance (Ohms)
300
4.2 10
Role of interfaces on SP
Co/Al2O3 interfaces: tunneling of s states through ALO
No hybridization with Co-d orbitals (Al has no empty d-states to hybridize with Co-d orbitals)
Co/STO & Co/TiO2 interfaces: tunneling of d states through STO
DOS of
Co
Bias dependence
of TMR
Empty Ti d-shells from
STO and TiO2 hybridize
with Co-d orbitals
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
SP depends on the complex electronic bands of the
barrier
D. Wortmann, J. Phys.:Condens.
Matter 16, S5822 (2004).
Butler et al., PRB 2001
Coherent spin tunneling in single crystalline MTJs
Bloch states symmetry at the Fermi energy and their
relationship to the symmetry of the slowly decaying evanescent
states in the barrier layer affect tunnelling conductance.
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
SP depends on the complex electronic bands of the
barrier
Conventional MTJs with amorphous AlOx  TMR  70% @ RT
Fe/MgO(001)/Fe  TMR  180% @ RT
S. Parkin et al., Nat. Mater. 2004
CoFe/MgO(001)/CoFe  TMR  220% @ RT
S. Yuasa et al., Nat. Mater. 2004
Strong enhancement of TMR due to coherent
spin-polarized tunneling in highly oriented
MgO (0 0 1) barriers
Butler et al., PRB 2001
TMR oscillations (with thickness)
 coherent tunneling
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Relevance of the electronic structure of the barrier
LSMO/STO/Co
B-site ions: Ti-d orbitals (octahedral
sites) hybridize with Co-d orbitals
Similar TMR(V)
dependence
LSMO/LAO/Co
A-site ions (La): conduction band comes
from La-4d in dodecahedral environment
hybridizing with Co-d orbitals
B-site (Al): no empty d-states can hybridize
with Co-d
V. Garcia et al., APL in press
Electronic structure of the barrier is relevant to SP
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Relevance of the electronic structure of the barrier
STO: Ti 3d states
1 (Tieg), 5(Ti2g)
2(Ti2g)
LSMO
1(), 2()
LAO: La 4d states
1
(dz2)
Calculations of the band
structure of BO2/hcp0001-Co
interfaces
Complex band structure of LAO
2(dx2-y2)
V. Garcia et al., APL in press
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Outline
Spintronics: Spin polarization, Spin-dependent tunneling
Half-metallic ferromagnets
Spin Polarization
Interface (electrical bondings)
Barrier (electronic structure)
Voltage bias
Temperature
Artificial high-spin polarization: Spin-filter devices
Spin-injection: diluted magnetic systems (DMS)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
LSMO / STO / LSMO tunnel junctions
Growth of LSMO / STO / LSMO heterostructures
by pulsed laser deposition
TEM :
excellent crystalline structure
M. Bowen et al., APL 82, 233 (2003)
Junction processing : UV lithography and
SIMS-monitored ion-beam etching
I+
I-
Tunnel magnetoresistance (TMR)
Junction size : 6 to >100 µm²
2000
600
1800 % TMR at 4K
PLSMO400= 95%
1500
TMR (%)
V+
TMR (%)
V-
1000
500
0
-400 -200
0
200 400
H (Oe)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Confirmation of the HM
200state by transport
measurements
Assymetry related to the presence of a Co/CoO
0
bilayer deposited
onto the top LSMO electrode
0
100
200
300
T (K)
LSMO / STO / LSMO tunnel junctions
Bias dependence of the TMR
Normalized TMR
1.0
0.8
R(H)
I(V)
4.2K

0.6
 Sharp decrease due to electron-magnon
scattering at LSMO/STO interfaces
 Influence of the density of states
(as in theoretical calculation of TMR(V)
in HM / I / HM junctions)
A. Bratkovsky, PRB (1997)
0.4

0.2
0.0
-1.2 -0.9 -0.6 -0.3 0.0
0.3
0.6 0.9
1.2
VDC (V)
M. Bowen et al., PRL 95, 137203 (2005)
Inflection point reflects the presence of a gap
in the spin-down sub-band
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
LSMO / STO / LSMO tunnel junctions
d I/dV (mA/V ) G(V) (mA/V)
Bias dependence of the conductance
2
2
2
1
P
AP
t2g
eg
e-
d
eVDC > d
T=4K
0
2
0
0.0
P
AP
t2g
d
0.1
0.2
0.3
eVDC < d
0.4
e-
eg
d
0.5
VDC (V)
Spin-resolved spectra
taken at 100 K for a
STO/LSMO interface.
M. Bowen et al., PRL 95, 137203 (2005)
From the TMR(V) and G(V) data, we deduce Eg = 380 meV
(in agreement with spin-polarized inverse photo- emission)
R. Bertacco et al, JMMM (2002)
Spectroscopic nature of spin-dependent tunneling from a half-metal
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
LSMO / STO / LSMO tunnel junctions
Temperature dependence of the spin-polarization
TMR = 2P2/(1 - P2)
STO, TiO2, LAO
PLSMO/STO
PLSMO/TiO
PLSMO/LAO
1.0
Interfaces
P/P0 , M/M0
2
0.8
continuity of the oxygen octahedra
Bulk M
0.6
0.4
Surface
LSMO Surface
0.2
Air
Park et al, PRL 1999
0.0
0
50
100
150
200
250
300
350
400
LSMO
discontinuity of the oxygen octahedra
T (K)
The interfacial SP decays more slowly than the surface SP.
The temperature dependence is similar for all interfaces.
V. Garcia et al., PRB 69, 052403 (2004)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Outline
Spintronics: Spin polarization, Spin-dependent tunneling
Half-metallic ferromagnets
Spin Polarization
Interface (electrical bondings)
Barrier (electronic structure)
Voltage bias
Temperature
Artificial high-spin polarization: Spin-filter devices
Spin-injection: diluted magnetic systems (DMS)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Materials for the barrier : Spin filters
Non magnetic Ferro
HM or
metal
insulator Ferromagn
metal
Ti  e
 i

2
J=0
large J
☆ Hao et al., Phys. Rev. B42, 8235 (1990)

2
2
J small
J large
Parallel: large current

small J
J=0
2
Antiparallel: small current
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Spin filter devices
Non-magnetic
electrode
Parallel configuration:
Magnetic insulating
barrier
Magnetic electrode
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Antiparallel configuration:
Materials for the barrier : Spin filters
Leclair; APL80, 625 (2002)
Al/EuS/Gd
Tc=16K
=0.36eV
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
La1-xBixMnO3 – based spin filters
c
Mn
+3
Distorted perovskite structure
(Monoclinic symmetry)
d4
z
Ferromagnetic insulator with
Eg = 0.5 eV and Eg = 2.6 eV
y x
O
-2
Ferromagnetic order
TC=105K, Ms=3.6 µB
Bi +3
b
a
180
BMO x=0 ; 30nm
LBMO x=0.1 ; 30nm
300
200
140
-3
M (emu.cm )
3
M (emu/cm )
BMO x=0 ; 30nm
LBMO x=0.1 ; 30nm
160
100
0
-100
-200
120
100
80
60
40
20
-300
-10
0
-8
-6
-4
-2
0
2
4
6
8
10
0
50
100
T (K)
H (kOe)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
150
200
La1-xBixMnO3 – based spin filters
BiMnO3, LaBiMnO3
BiMnO3
M. Gajek PRB 72 020406 (2005)
TcBulk=105K
I
Au
7
5.0x10
7
4.5x10
7
LSMO
La0.1Bi0.9MnO3
1mV
187%
4.0x10
7
R ( )
3.5x10
7
3.0x10
7
2.5x10
7
2.0x10
7
1.5x10
7
1.0x10
-6000
-4000
-2000
0
2000
4000
6000
H (Oe)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
NiFe2O4 – based spin filters
B sites
A
A sites
Fe3+
3d5
5µB
B
Fe3+ Ni2+
3d5 3d7
5µB 2µB
F-coupling
AF-coupling
TC=850K
Ferrimagnet with a
saturation moment of
2 µB/f.u.
A sites : tetrahedral
B sites : octahedral
Cubic spinel
a=b=c=8.33 Å
Insulator with
Eg=1.8 eV and
Eg= 0.4 eV
Mismatched by ~6% with STO and LSMO
Inverse spinel structure
D. Szotek et al, JPCM 2005
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
NiFe2O4 – based spin filters
Insulating NFO grown in Ar/O2: magnetic barrier in spin-filter devices
3.5
4K
10mV
R(M  )
Au/NFO/La2/3Sr1/3MnO3 on STO
3.0
 maximum TMR  50%
 filter efficiency  22%
2.5
-4000
-2000
0
2000
H(Oe)
4000
Temperature dependence
30
1.6
4K
100K
140K
10mV
 R/R (%)
R/R p
20
1.4
1.2
1.0
-2.0
10
0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0
50
H(kOe)
100
T(K)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
150
200
NiFe2O4 – based spin filters
Parallel state
Antiparallel state
Spin 
Spin 
Insulator with
Eg=1.8 eV and
Eg= 0.4 eV
Spin 
Spin 
FM
FM
Low barrier for majority
spin carriers in FM
High barrier for majority
spin carriers in FM
Au/NFO/LSMO
3.5
4K
R(M  )
10mV
TMR > 0 !
3.0
BUT Negative TMR should be
expected
2.5
-4000
-2000
0
2000
4000
H(Oe)
Possible explanations:
Positive spin-filtering efficicency
 Eg > Eg (opposite of what is found with calculations)
Modified electronic structure at interface ?
Role of symmetry filtering (like with MgO) ?
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Outline
Spintronics: Spin polarization, Spin-dependent tunneling
Half-metallic ferromagnets
Spin Polarization
Interface (electrical bondings)
Barrier (electronic structure)
Voltage bias
Temperature
Artificial high-spin polarization: Spin-filter devices
Spin-injection: diluted magnetic systems (DMS)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Transition metals and the problem of spin injection in
semiconductors
Spin FET
Spin LED
Datta & Das, Appl. Phys. Lett., 56, 665 (1990)
Fiederling et al., Nature, 402, 787 (1999)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Ferromagnet/semiconductor interface
Non magnetic
( ex: Fe, Co )
( ex: Cu, GaAs )
s
EF,
Spin
accumulatio
n
EF,
Number of spin flips in F:
 Unpolarized
current in SC
Co/Cu
current polarization
+
-
-
Co/SC
+
NSFF>> NSFSC
splitting of the electrochemical potentials m+-m-
(µ -µ )/|e|J
 mI NEFlFsf /Fsf  mI / FlFsf
d
d
lsfFM lsfNM
NSFF
s
P=(J -J )/J
ferromagnetic
N
F
-0.3 -0.2 -0.1
0.0
0.1
0.2
0.3
0.4
0.5
z(µm)
Solution: to insert a thin Insulating barrier Or ferromagnetic semiconductors
Schmidt; PRB62, R4790 (2000); Fert; PRB64, 184420 (2001); Jaffres; JAP91, 8111 (2001)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Materials for spin injection in semiconductors: DMS
DMS
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Materials for spin injection in semiconductors: DMS
(II,Mn)-VI
(Zn,Mn)-Se
(Zn,Mn)-S
(Cd,Mn)-Te
Paramagnetic DMS
EF
Ferromagnetic DMS
No carrier for
coupling
Paramagnetic,
AF or spinglass
EG
EF
EF
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
EGG
(III,Mn)-V
(Ga, Mn)-As
(In, Mn)-As
(Ga, Mn)-Sb
Materials for spin injection in semiconductors: DMS
Field effect transistor
Ohno, Nature 408, 944 (2000)
Electric control of magnetism: evidence of ferromagnetism
induced by carriers
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Materials for spin injection in semiconductors: DMS
Dietl, Science 287, 1019 (2000)
IV
III-V
II-VI
Mn2+
AF coupling between holes and ions
β
itinerant
carriers
β
Hole induced ferromagnetism
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
DMMS: Co-doped (La,Sr)TiO3
DMS: semiconductor hosts doped with low concentrations of magnetic ions
n  1018 – 1020 cm-3
Doping a metallic host: Co-(La,Sr)TiO3, n  1022 cm-3
Existence of ferromagnetism in La0.5Sr0.5TiO3− doped with Co, TC  500 K
Y.G. Zhao et al., Appl. Phys. Lett. 83, 2199 (2003).
High carrier density: effect on the magnetic interactions
between magnetic ions
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
DMMS: Co-doped (La,Sr)TiO3
Growth by PLD on STO (001)
(T= 700°C, PO2=10-7-10-4 mbar)
target comp : La0.63Sr0.37Ti0.98Co0.02O3
6
Co, x = 1%
102
STO (004)
LSTO (004)
STO (003)
STO (002)
LSTO (003)
3
10
LSTO (002)
4
10
STO (001)
5
10
LSTO (001)
Intensity (counts)
10
Ti mapping
1
10
20
40
60
80
Co mapping
100
2 (°)
500 nm
Auger electron spectroscopy
La
1.5% Co
500 nm
La
La
4
y (a.u.)
5x10
4
4x10
4
1.5% Co
ity (a.u.) Auger Intensity (a.u.)
500 nm
Co
mapping
1.5%
Co
4
5x10
Sr
4
4
3x10
4
2x10
O
Co
3% Co
4
5x10
0
0
4
4x10
4
5
10
15
20
O
Co
25
Sputter time (min)
30
5x10
3% Co
O
3% Co
4
4x10
4
3x10
4
2x10
4
1x10
5x10
4
4x10
4
3x10
4
2x10
4
1x10
0
0
5
10
15
20
25
Sputter time (min)
30
0
5
4
4
4
2x10
Homogeneous Ti and Co distribution
No Co-rich clusters larger than 10 nm
4
1x10
0
Unité mixte
de Physique (UMR137)
0
5
10 15 20 25 30
associée à l’Université Paris-Sud
Sputter time (min)
10
15
20
25
Sputter time (min)
4x10
3x10
Co
4
0
4
4x10
Ti
4
1x10
Ti
Auger Intensity (a.u.)
Ti mapping
Sr
Auger Intensity (a.u.)
Co mapping
Ti
Auger Intensity (a.u.)
4
5x10
Ti mapping
Sr
30
DMMS: Co-doped (La,Sr)TiO3
Co-doped LSTO film
Transmission electron microscopy
STO substrate
g [020]
 Good structural quality
 Absence of Co-clusters
EELS
Ti
Co
La
Homogeneous element distribution;
Co content ~0.01
No Co-rich phases larger than 10 nm
detected
Black is zero; white is 50% in Ti and La images, 1% in Co image.
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
DMMS: Co-doped (La,Sr)TiO3
Co-doped (La,Sr)TiO3 thin films: XAS & XMCD
Multiplet structure in Co-L2,3 XAS and XMCD spectra
Ionic state of Co (Co2+ in Oh symmetry)
No evidence of metallic Co
0.0
XMCD
-0.5
L2,3 Co
1.1
-1.0
10K_5T
780
790
Energy (eV)
1.0
0.01
0.00
0.00
Lz = -0.428411
Sz = -0.734611
-0.05
760
780
800
820
-0.01
XMCD
XAS & XMCD
-1.5
770
Metallic Co
-0.02
-0.03
Energy (eV)
Ionic Co
-0.04
M  2 mB/Co
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
770
780
Energy (eV)
790
DMMS: Co-doped (La,Sr)TiO3
M(emu/cm
3
)
5
0
M[100]
M[110]
4
0
-4
-1000
-5
-10000
0
0
1000
-6
10000
pO = 1.5*10 mbar
2
2
Films are ferromagnetic @ RT
M/M300 K (a.u.)
H(Oe)
-6
pO = 5*10 mbar
2
-6
pO = 5*10 mbar
2
reported TC
1
0
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
0
200
400
600
Temperature (K)
DMMS: Co-doped (La,Sr)TiO3
Spin polarization of Co-doped (La,Sr)TiO3: tunnel junctions
TMR at low temperature
6.5
0.1
I( m A)
R(M  )
STO//Co-LSTO/LAO(1.5nm)/Co/CoO/Au
junctions grown at 5.10-7 mbar
0.0
Optical photolithography: Fabrication
of tunnel junctions (6 – 128 mm2)
-0.1
6.0
-0.2
0.0
CoO (3 nm)
0.2
Vbias(V)
Co (12 nm)
LAO (1.5 nm)
5.5
-4000
0
4000
8000
Co-LSTO (150 nm)
H(Oe)
G. Herranz et al., Phys. Rev. Lett., accepted
 A sizeable TMR is obtained at 4K
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Barrier: LaAlO3 (LAO)
DMMS: Co-doped (La,Sr)TiO3
Spin polarization
TMR (%)
20
15
10
 TMR(V) indicate impurity-assisted tunneling (growth at low PO2;
defects in the LAO barrier)
P =-0.2
L
P =-0.8
R
Impurity-assisted
tunneling
 =0.0036
L
 =0.025
R
5
ec=0.02
0
-0.2
-0.1
0.0
0.1
0.2
VDC (V)
Tss    
0
jss 
F  1eV 
 T ()d
F eV
Ls  Rs   W 
4Ls Rs 
Ls  Rs  2  EC 2  Ls  Rs   W 2
TMR (%)
-5
-10
-15
-20
-25
P =-0.3
L
P =-0.8
R
 =0.00127
L
 =0.078
R
ec=0.0219
-0.2
-0.1
0.0
0.1
0.2
VDC (V)
20
P =-0.3
L
15
P =-0.78
10
 =0.00088
TMR (%)
R
L
 =0.0057
R
5
ec=0.00165
Ni/NiO/Co: Tsymbal et
al., Phys. Rev. Lett. (2003)
MnAs/GaAs(AlAs)/MnAs:
V. Garcia et al., Phys. Rev.
B (2005)
MMM 05: EB-09
L
PL
-0.4
-0.2
0.0
0.2
0.4
R
EF
eV
PR
0
-0.6

W Ec
0.6
VDC (V)

A spin-polarization of –80% is
inferred for the Co-LSTO film
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
1
0
Metal Tunnel barrier Metal
Conclusions
Spin polarization in tunneling experiments is not uniquely determined for each
electrode:
Bondings at interface between FM and electrode
Electronic structure of barrier: complex band structure determine decay rates of
wavefunctions with different symmetries
Potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved
spectroscopic studies
Artificial highly spin polarized sources through the spin filter effect: NiFe2O4, La1xBixMnO3
Observation of large spin polarization in a diluted magnetic system: Co-doped (La,
Sr)TiO3
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Perspectives
Multiferroics
Evidence of coupling between ferromagnetic
and ferroelectric orders
Fiebig, nature 2002, YMnO3
Open the way to
Control magnetization by an electric field
4 states MRAM
Molecular tunnel junctions
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Fiebig, nature 2002, YMnO3
Mixed-valence manganites
La1-xSrx(Mn3+1-xMn4+x)O3
Mn
La2/3Sr1/3MnO3 : TC=360K
Double-exchange ferromagnet
A
Half-metallic character predicted
by band-structure calculations
O
Crystallizes in a distorted
perovskite structure (a=3.88 Å)
Imada, Fujimori and Tokura, Rev. Mod. Phys (1998)
Spin-polarized photoemission
EF
Experimental confirmation of
the half-metallic state (P > 90%)
J.H. Park et al, Nature (1998)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Double perovskites
A2BB’O6, A=Ca, Sr, Ba; B=Fe, Cr; B’=Mo, Re
Sr2FeMoO6
Fe
M
oS
r
O
eg ()
eg ()
t2g ()
t2g ()
eg ()
t2g ()
Fe : 3d5 - 3d6
m=4mB/f.u
Sr2FeMoO6
Sr2FeReO6
Sr2CrReO6
Tc=415K
Tc=538K
Tc=635K
Mo : 4d1 - 4d0
F
 Kobayashi et al., Nature 395, 667
(1998)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Charge localization near the interface
NMR spectra on LaCaMnO3 films
Charge localization near the interface
 Bibes ; PRL87,067210 (2001)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Double perovskites: Sr2FeMoO6
5.5x10
7
5.0x10
7
T=4K
V = 10 mV
But
TMR ~ 50 %
50
40
30
4.5x10
7
4.0x10
7
20
TMR (%)
R (ohms)
Tunnel junctions Sr2FeMoO6 / SrTiO3 / Co
10
0
- growth very difficult (presence of
parasitic phases)
- antisite defects destroy HM
character
-reactivity to water
 Besse et al.; J. of Crystal
growth 241, 448 (2002)
7
3.5x10
-2000
-1000
0
1000
2000
H (Gauss)
Bibes et al, APL83, 2629 (2003)
SP = -90%@ 4K
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
Other DP:
Sr2CrRe06: Tc=635K
Materials for the barrier : Spin filters
Double barrier

Non
magnetic
metal
Ferro
Ferro
Insultor1 Insultor2
Non
magnetic
metal
2
Worledge et Geballe; J.A.P.88, 5277 (2000)
GP
1 2d(
 e
G AP 2
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
       )
SP depends on tunneling matrix elements
CoPt/Al2O3, CoPt/AlN interfaces
CoO H = 237 kJ/mol
PtO H = 71 kJ/mol
CoO bond strength is larger than PtO
Electronic tunneling rates from Pt sites are
much lower than from Co sites
Kaiser et al., PRL (2005)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
SP depends on tunneling matrix elements
Co-Gd alloys electrdes with Al2O3 and
MgO barriers
Independent spin-polarized tunneling currents
from Gd and Co subnetwork magnetizations.
Zero magnetization but
strongly spin-polarized current.
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
LSMO / STO / LSMO tunnel junctions
200
200
150
150
TMR (%)
TMR (%)
LSMO/TiO2/LSMO
100
50
PLSMO/TiO2=68%
Lower spin-polarization maybe
due to electronic structure
modifications induced by local
distorsions.
100
50
0
-800 -400
0
400
0
800
0
100
H (Oe)
200
APL 82, 3269 (2003)
300
T (K)
PLSMO/LAO=77%
LSMO/LAO/Co/CoO
LSMO/LAO/LSMO
300
250
250
0
200
-2
TMR (%)
TMR (%)
200
150
100
50
TMR (%)
300
150
100
50
-4
-6
PCo/LAO<0
 same as PCo/STO
 opposite to PCo/am-
-8
-10
0
-1000 -500
0
H (Oe)
500 1000
0
0
100
200
300
-2000 -1000
T (K)
Unité mixte de Physique (UMR137)
associée à l’Université Paris-Sud
0
H (Oe)
1000 2000 Al2O3
APL, in press