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Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Ecole Franco-RoumaineMagnétisme des systèmes nanoscopiques et structures hybrides
Brasov, septembre 2003
Spin tunnel and Spin Polarisation
Laurent Ranno
Laboratoire Louis Néel, Grenoble
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Summary
I- Introduction to Tunnel Effect
II-Magnetic Tunnel Effect
III-Bias Voltage and Temp Dependence
IV-Spin Polarisation
V- Half Metals
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
I- Introduction to Tunnel Effect
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
∆EwE
E0
ψψψ ExVdx
d
m=+− )(
2 2
22�
Tunnel Effect has a Quantum Mechanics Origin
A classical electron with energy E<E0 cannot enter the barrier zoneHowever a quantum electron obeys theSchrödinger equation !
(1D model)
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
2
2
�
Emq
∆±=tiqr
be ωψ −=
Off the barrier
In the barrier
)( tkrie ωψ −=
ψψ Edx
d
m=−
2
22
2
�
ψψ )(2 02
2
EEdx
d
m−=− �
and 2
2
�
mEk ±=
00 <− EEand
and
Planewaves
Evanescent waves
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
nmq
electronfreem
eVE
2.01
1
=�=
=∆
2
2
�
Emq
∆±=
Tunnel barriersmust be very thin insulating layers Width = w < 10 nm
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
ψψψ ExVdx
d
m=+− )(
2 2
22�
∆E
w
x
V(x)
�=
±x
duuk
ex 0
)(
)(ψW. K. B. Approximation
ψψψ )())((2 2
22
2
xkExVm
dx
d =−=�
For a more general barrier shape
E
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
What is neglected ? �=
±x
duuk
ex 0
)(
)(ψ
)()()(
xxkdx
xd ψψ ±=
)()()()(
)()()(
)()(
2
2
2
xxkxdx
xdkdx
xdxkx
dx
xdk
dx
xd
ψψ
ψψψ
+±=
±±=
))((2
)(2
ExVm
xk −=�
The barrier potential should vary smoothly
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Wearedealing with transport. What about the current ?
To passacurrent,wemust apply abiasvoltageacross the barrier.
the barrier has a transmission coefficient T
eV
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Simmons
The probability to find the electron (energy E) on the other sideofthe barrier is :
�===
−−w
duEuVm
eEP 02
))((22
2)(
�
ψψψ
dtdVEPVnVdN xxxx�∞=0
)()(
Electrons coming from the left to the right
�� ∞
=0
//
03
2
)()(4
dEEfdEEPh
m
dt
dN E
xx
π
dtVx
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
∆E
w
EE0No bias voltage
Current 1 2 = Current 2 1
J(V=0)=0
1 2
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
w
E
BiasvoltageeV<<E0
eV
�� ∞
+−=−=0
//
03
221 )]()([)(
4)( dEeVEfEfdEEP
h
me
dt
dN
dt
dNeJ
E
xx
π
E0
002.10
1010 16.3 ESeS
VEJ −=
Linear J(V) at low bias
Simmons (1963) has calculated approximate recipes
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
w
E
Biasvoltage 0<eV<E0
eVE0
Simmons ’ parabolic fit
)( 3VVJ βα +=
)31( 2VdV
dJ βα +=
β contains the barrier height and the barrier width
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
dV
dJWhy ?
From the experimental point of view :
)cos()( 0 tvVtV ω+=
)()cos()())cos(()( 000 VdV
dJtvVJtvVJVJ ωω +=+=
Measuring the ω component of thesignal with a lock-in amplifiergives directly the differential conductance
Filter all theconstant voltageand non-ω noise
Using a voltage source : 0Vv <<
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Reduced effective w
E
Biasvoltage eV>E0
eVE0
J increases rapidly but in fact such a biasvoltage corresponds to theelectrical breakdown regime
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
-0.24 -0.16 -0.08 0 0.08 0.16
-0.24
-0.16
-0.08
0
0.08
0.16
0.24
0.32
courbe I(V) à 2 K et 52 K
V (2 K)V (52 K)
V (Volt)
I-V non linear curves (ferromagnetic/insulator/ferromagnetic)
Thèse E. Favre-Nicolin (Grenoble 2003)
No temperature dependenceof tunnel effect (1st order)
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
In 1972 Gittleman et al. measured the resistance and MR of Ni grains in a SiO2 matrix
Gittleman et al. Phys. Rev. 5 (1972) 3609
50% Ni
Longitudinal MR is <0 contrary to the sign of the bulk Ni AMR ρ// > ρ perp
R > RM J M J
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
221
10 )()(),(m
mmTTHT
�� ⋅+= σσσ
Gittleman et al. 1972 :
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Development of film deposition techniquesTrilayer : better characterisation of electrodes and control of magnetisation
R changes by 14% at low temperature depending on the magnetic configuration
Fe
CoGe(10-15nm) +dry oxygen
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
11.8% at 300 K24% at 24 K
φ=1.9 eV and t=1.6 nm
V50%=200mV
200µm x 300 µm
And also Miyazaki, Tezuka JMMM 139 (1995) L231
1995
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
IBM 1997
Ferro (free)
Ferro (pinned)
Insulator
Magnetic Tunnel Junction
Same technical solutions as GMR structures to get 2 different coercive fieldsi.e. well defined parallel and antiparallel states.
Hard - Soft materials (Co - NiFe)Different shape anisotropies for both electrodesPinning to AF layer (MnFe) or Artificial AF layer (Co/Ru/Co)
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Jullière ’s model (1975)
Not magnetisation BUT Polarisation of electrodes is the parameter )()(
)()(
FF
FF
ENEN
ENENP
↓↑
↓↑
+−
=
)()()()( 210210 FFFF ENENGENENGG ↓↓↑↑↑↑ +=
)()()()( 210210 FFFF ENENGENENGG ↑↓↓↑↑↓ +=
2
)1( iii
PNN
+=↑ 2
)1( iii
PNN
−=↓
Assume : No spin-flip transition across the barrier at low voltage2 parallel channels (spin up and spin down)
Conductance is the sum of spin up and down conductancesConductance is proportional to the density of state (d.o.s.) 1 and d.o.s. 2
)()( 2 electrode i 1 electrode i 0i FspinFspinspin ENENGG =
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Jullière ’s model (M. Jullière, Phys. Lett. 54 A, 225 (1975))
210 NNGGG =+ ↑↑↑↓
2211022102210 PNPNGPNNGPNNGGG =−=− ↓↑↑↓↑↑
21PPGG
GG=
+−
↑↓↑↑
↑↓↑↑
21
21
1
2
PP
PP
G
GG
+=
−
↑↑
↑↓↑↑or
Does depend on PiDoesnot depend on the barrier (height, width) because of assumption about G0
i.e. no spin dependenceof transmission
21
21
1
2
PP
PP
R
RR
−−=
−
↑↑
↑↓↑↑
TMR ratio :
(pick your definition )
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
69.1% at 4.2 K
From Jullière ’s formula
PCoFe=50.7%similar to expected
CoFeTMR junction (Tohoku 2000)
Fe/a-Ge/Co (Jullière)Exp : TMR=14%Theor : PCo34% +PFe44%
TMR 26%
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Slonczewski ’s model (1989)
Barrier in themodel
m ↑Fk
↓↑↓F
k
exchEm
kE ±=
2
22�
Ferromagnetic electrodes
↓↑
↓↑
↓↑
↓↑
+−
+−
=FF
FF
FF
FF
kk
kk
kkq
kkqP
2
2
SolveSchrödinger for both channels, calculateconductances
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
↓↑
↓↑
↓↑
↓↑
+−
+−
=FF
FF
FF
FF
kk
kk
kkq
kkqP
2
2
)()(
)()(
FF
FF
ENEN
ENENP
↓↑
↓↑
+−
=
2
2
�
Emq
∆±=
High barrier
↓↑
↓↑
↓↑
↓↑
+−
=+−
⋅=FF
FF
FF
FF
kk
kk
kk
kk
q
qP
2
2
Free electrons kmk
mEm
EDOS ∝==2223
2)(ππ ��
Back to Jullière ’s formula
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Improved models :
Bratkovsky :Correction to Slonczewski’smodel (different effective mass in the barrier)
mb/m=0.4 for Fe/Al2O3
Ab initio band structure calculations :to get the band structure close to the interface
↓↑
↓↑
↓↑
↓↑
+−
⋅+⋅−
=FF
FF
FFb
FFb
kk
kk
kkmq
kkmqP
22
22
2
2
�
Emq b∆±=
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
TMR temperature dependence
Resistance Temperature dependence
Miyazaki group (Tohoku) APL 2000
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Temperature dependence of resistance
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
2
2
�
Emq
∆=
qweTonTransmissi 2−∝=
w
∆E
Temperature dependence of the barrier transmissionGoing from 0 Kelvin to 300 Kelvin
Wavevector in the barrier (evanescent wave)
onTransmissieConductanc ∝
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
2
2
�
Emq
∆=
qweTonTransmissi 2−∝=
E
Edwqdqwqwd
T
dT
∆∆−=⋅−=−= 2)2(
meVkTEeVE
nmwq
25d ,2
, 1 ,11
==∆=∆=Α=
−°
%5.12=∆=∆=∆R
R
G
G
T
T
w
∆E
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Polarisation is related to magnetisation
)1)(()( 2/3TTMTM s α−=
)1)(()( 2/3TTPTP s α−=
Inelastic processes can appear
magnonee +� ↓↑
↓↑ �+ emagnone
Opens a spin-flip conductancechannelconductance increasesTMR decreases
Surfacemagnetisation is less robust to thermal fluctuations
No general results, dependson the studied system (Tc, surface state …)
Different contributionsmay rule this behaviour :
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Jullière, Phys. Lett. 1975
50% MR at 3mV
TMR50%=3mV
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
TMR - bias voltage dependence50% decreaseTMR for 400mV
R - bias voltage dependence
Miyazaki group (Tohoku) APL 2000
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Inelastic processes can be activated
magnonee +� ↓↑
↓↑ �+ emagnoneOpens a spin-flip conductancechannel
TMR decreases with V
Thevoltagedecrease dependson experimental systems and years
At largebiasvoltages, hot electronsare introduced inthesecond electrode : 0.1 V = 1200 Kelvin
1975 : TMR50%=2 mV1995 : TMR50%=200 mV2000 : TMR50%=450 mV2003 : TMR50%>1000 mV
May bedue to non perfect samples, which improve with time
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Epitaxial NiFe electrode : 50 % decreaseof TMR at 750 mV
Yu et al. APL 2003
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Ding et al. (M.P.I. Halle) PRL 2003
Cobalt (0001)
Magnetic amorphous tip
Tunnel junction with « perfect barrier » : STM with ferromagnetic electrodes in vacuum
Voltagedependenceof TMR not related to magnon excitations or surfacemagnetisation but more likely to defects in the barrier
to be confirmed ...
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
A few words about
The insulating barrier
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Making the barrier
Aluminium Film (0.7-2 nm) + thermal oxidation in oxygen atmosphereor air
Aluminium Film + oxygen plasma
Deposition of Alumina (Al2O3) producesa lessdensebarrierwith poor electrical properties
Good recipe#1
Good recipe#2
Bad recipe#1
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
SR
1∝
Constant thickness barrier
Gallagher et al. JAP 1997
5 ordersof magnitude
100 nm
100 µm
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Electrical Breakdown
1 Volt across a 1 nm barrier is E = 1 GigaV/m
This is the order of magnitude of the electrical field necessary to ionise an atom
The barrier should be uniform- flat- 0 roughness- fully oxidised, homogeneously - no oxidation of the ferromagnetic metals
A 0.1 nm fluctuation means 100 mV decrease of the breakdown voltage
!
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Ta2O5 -1023 HfO2 -1144Nb2O5 -949 ZrO2 -1100
CeO2 -1088Y2O3 -952 TiO2 -944
Gd2O3 -909 SiO2 -910Nd2O3 -903 NbO2 -796La2O3 -896 CrO2 -598Al2O3 -837 MnO2 -520V2O3 -609
Cr2O3 -570 MgO -601Ga2O3 -544 NbO -405Mn2O3 -479 MnO -385Fe2O3 -412 ZnO -350
FeO -272Mn3O4 -462 NiO -238Fe3O4 -372 CoO -237Co3O4 -297 CuO -157
SiO -99
Ga2O -178Cu2O -84
Formation enthalpy ∆H (298K)kJ/mol metallic atom
Al-O bond is stronger than3d metal -O bonds
Better oxides exist : HfO2, , Rare earth-O
But kinetics : (Al passivated « automatically » 1 nm thick),
local state : (amorphous, nanocrystallised,
crystallised) when RT deposition
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Crystallised barrier Amorphous barrier
Smith et al. JAP,83 ,5154 (1998)
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Role of annealing a tunnel junction
Sousa et al. (Lisbon)
Barrier (Simmons’ fit)
after anneal : thickness :0.87 to 0.77 nm widebarrier height :1.8 eV to 2.5 eV
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
RBS measurement of Al and O distributions
Sousa et al. APL 98
O from the CoFe electrodes goes back to Al2O3
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Miyazaki group (Tohoku) APL 2000
Ta (5 nm)/Ni79Fe21 (3 nm)/
Cu (20 nm)/Ni79Fe21(3 nm)/
Ir22Mn78(10nm)/Co75Fe25(4 nm)/
Al (0.8 nm)-oxide/Co75Fe25(4 nm)/
Ni79Fe21(20 nm)/Ta (5nm)
Free layer
Pinned layer
Best present TMR junctions : 50% Room temperature(non exotic materials)
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
We have been using values for P.
Wheredo they come from ?
How to measure them ?
)()(
)()(
FF
FF
ENEN
ENENP
↓↑
↓↑
+−
=
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Density of States Band structure calculations (spin resolved)
Moroni et al. PRB56 (1997)15629
[100][000] [000] [000] [000][110] [111] [100] [111][210] [110]
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Nickel integrated density of states
Polarisation at Fermi level should beNEGATIVE
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Be careful (M. B. Stearns JMMM,5 ,167 (1977))
2
2
2*
k
Em
∂∂
= �
Bandsdo not have the same effective mass at EF
Electrons at Fermi level have different mobilities
d-like electronsare more localised (narrow bands)s-like electrons are less localised (wide bands)
and more mobile
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
[100][000] [000] [000] [000][110] [111] [100] [111][210] [110]
s-liked-like
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Be careful (M. B. Stearns JMMM,5 ,167 (1977))
2
2
2*
k
Em
∂∂
= �
Bandsdo not have the same effective mass at EF
Electrons at Fermi level have different mobilities
d-like electronsare more localised (narrow bands)s-like electrons are less localised (wide bands)
and more mobile
Ferromagnetism comes from the3d bands but transport comes from s-electrons. s electrons are not supposed to be polarised !
Difficult to predict thepolarisation of conduction electrons
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Ferromagnetic / insulating / superconducting junctionsM. I. T. speciality (Tedrow/Meservey)
�∞ +−∝0
21
2)]()()[()( dEeVEfEfENENMJ
qweM 22 −∝
N(E) non magnetic metal
constant)( ∝EN
When it is difficult to predict, let us measure !
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
�∞ +−∝0
21 )]()()[()( dEeVEfEfENENJ
∆<= E if 0)(EN
∆>∆−
= E if E
)(22E
EN
N(E) in a superconductor
∆−∆ E
N(E)
f(E)
f(E+eV)
E
E
EF
f(E)-f(E+eV)E
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
�∞ +−∝0
supermetal )]()()[()( dEeVEfEfENENJ
∆/eV
J
∆/eV
dV
dJ
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
V
dV
dJ
If a magnetic field is applied BBEBE Bµ±== )0()(
If 50% electrons spin up, 50% electrons spin down
V
dV
dJ
V
dV
dJ
µB/eIf not 50%-50% ….
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Meservey et al; J.A.P. 50 (1979)
FeCoNiGd
+44%+34%+11%+4%
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Andreev reflexion : metal / superconductor clean interface
Soulen et al. J.A.P. 85 (1999)
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Nb gap
I(V)dI/dV
Normal metal : 1 electron arrives, 2 electrons enter
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Nb gap
I(V)dI/dV
No conductance at small bias
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
0102030405060708090
100
NiFe Co Ni Fe
NiMnS
bLaS
rMnO
CrO2
Spi
n P
olar
isat
ion
(%) Tunneling
Andreev
Soulen et al. 1999But it only works at 1 Kelvin !Andreev Reflexion doesnot give the sign of P
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Material Surface (UHV conditions)
Photon hν Photoemitted electronEnergy+spin analysis
Photon penetration length = 0.5 nmVery clean surface, UHV prepared surface
UV
To analyse theconduction electrons : spin resolved photoemission
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Jonker et al. PRL57 (1986)
Example : Epitaxial Fe on Ag
Spin-resolved photoemission
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Park et al. PRL 81 (1998)
Phot
oem
issi
onsi
gnal
La0.7Sr0.3MnO3 film
Photon hν = 40eV40K
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Polarisation at 0K for Ni, Co and Fe is known
Its temperature dependence is still unclear
Surface polarisation still amystery
(enough to keep you busy after Brasov school)
What about the other magnetic metals?
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
D(E)
E
↓↑ >ss
λλ
d
d
s s
Ferromagnetic band structure
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Half metallic ferromagnets
D(E)
E
d
d
s s
Fully spin polarised conduction band
No s-band at Fermi levelLarge band splitting
Gap = not usual metals = less metallic than metals
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Predicted HMF
1984 : ab initio calculcations (De Groot et al.)
NiMnSb (Tc=720 K), PtMnSb (Tc=575 K)
CrO2
La0.7Sr0.3MnO3
Fe3O4
FexCo1–xS2 ………….
magnetic semiconductors ?
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Compound Curie temperature Magnetisation µoMs Crystallographic structureLa0.7Sr0.3MnO3 350 K 0.74 T 3.7 µB/Mn PerovskiteNiMnSb 728 K 0.89 T 4 µB/Mn Semi HeuslerCrO2 396 K 0.81T 2 µB/Cr RutileFe3O4 860 K 0.63 T 4 µB/F.U. Inverse SpinelPtMnSb 572 K 0.9 T 4 µB/Mn Semi HeuslerSr2FeMoO6 415 K 0.73 T 4 µB/F.U. Double perovskite
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Spin-polarized band structure of La0:7Sr0:3MnO3. The majority bands are shown assolid lines, and the minority as dashed lines
Livesay et al. J. Phys.: Condens. Matter 11 (1999) L279–L285
Predicted Half metallic character of La0.7Sr0.3MnO3
Zero Kpicture
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
EF
∆δ
E
D (E)
D (E)
E(q)
q
E=Dq2
∆
δ
Stonercontinuum
Spin waves
Electronic excitations in a HMF
Still exist at T>0
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
HMF are avery interesting class of materials
Useful to study HMF state
Are they really 100% polarised ?Up to Tc ?Is thesurfacepolarised ?
Useful to study other materials
Source of d-like electronsHighly spin polarised source
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Inverse TMR Cobalt : positive P (previous studies)LaSrMnO : positive P (no down bands)
and negativeTMR !
DeTeresa et al. PRL 82(1999)
d electrons tunnel
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Conclusions :
Tunnel is old but TMR is quitearecent field
Spin-dependent transport propertiesare not fully understood
HMF is still amystery
A lot of material science is necessary to control thestructures
Only a few materials and structures have been controled
Bringing together magnetism and electronics isa fruitful playground ...
Spin Tunnel Course Brasov sept. 2003 Laurent Ranno (Lab. Louis Néel)
Improved structures : resonant junctions