Forschungszentrum Jülich
Combining Ferroelectricity, Magnetism, and Superconductivity in Tunnel JunctionsSupe co duct ty u e Ju ct o s
H. Kohlstedt1, N. A. Pertsev2, A. Petraru1,U. Poppe1, and R. Waser1
1CNI – Center of Nanoelectronic Systems for Information yTechnology (IFF-IEM)
2A.F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021, St. Petersburg, RussiaSciences, 194021, St. Petersburg, Russia
Arusha, Tansania August 2007
Layer Sequence of a Tunnel Junction
Top electrode(50 nm)Tunnel Barrier
(1 nm – 3 nm)Dielectric
Substrate
B ttBottom Electrode(50 nm)
Outline
• Electron Tunneling and Junctions (Overview)
F l t i T l J ti• Ferroelectric Tunnel Junctions
• Size Effects and Boundary Conditions
• Tunnel Junction: An Interfacial Device
• A novel “zoo” of Tunnel Junctions
Electron Tunneling and Junctions
Quantum Mechanical Electron Tunneling
real real
φ exk k xk
E
imaginary
realx xk xkCΨ
Ψ
x
AΨ
BΨ
Transmission coefficient
⎪⎫⎪⎧ t2⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧−= ∫ dxxmCT
t
0
)(22exp φh
Frenkel, Phys. Rev. 36 (1930).; A. Sommerfeld and H. Bethe, Handbuch der Physik, Springer 1933, XXIV, p.450R. Holm and W. Meissner, Z. Phys. 74, 715 (1932).
Tunnel Junctions: A short Survey
Me I Me Su I Su Mag I Mag Semic I Me
y
EgEg
I IΔR/R I
V V H V
Sommerfeld/Frenkel/Holm Giaever/Josephson Jullieré/Moodera/Parkin/(Sun/Fert for oxides)
Esaki
Superconducting -, Magnetic-, and Ferroelectric Tunnel Junctions
Dielectric barrier Density of states effects
Superconductor Superconductor Magnet Magnet
[ ]dEEfeVEfEneVEnETAeVI )()()()()(2)( 21 −−⋅−= ∫∞
h
π∫∞−h
Metal Metal
Ferroelectric tunnel junction:
Cooperative phenomenon
Ferroelectric Barrier
located in the barrier !
Ferroelectric Tunnel Junctions
Ferroelectric Tunnel Junction
Kohlstedt, Pertsev, Waser, Ferroelectric Thin Films X, Vol. 688 (M t i l R h S i t ) 2002 161
European Patent:0 657936 A1 1994
(Material Research Society) 2002, p. 161.
13, 2161 (1971).
0 657936 A1, 1994R. M. Wolf and P. W. M. Blom, Philips Electronics,
IBM Technical Disclosure Bulletin
p ,Eindhoven (NL).
Patent
Experiment
Ferroelectric Tunnel Junction
High-Resolution TEM, C. Jia, Jülich
ASrRuO3
+ + + + + + + + + + + + + + + + + + + + + + + +e-
V10 unit cells PZT
+ + + + + + + + + + + + + + + + + + + + + + + +
P
4 nm
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
SrRuO3
Tunneling matrix element:⎪⎬⎫⎪
⎨⎧
= ∫ dCTx1
)(22e p φ⎪⎭⎬
⎪⎩⎨−= ∫ dxxmCT
x0
)(2exp φh
Quantum Mechanical Electron Tunneling
Polarization State of PZTPolarization State of PZT
Current-Voltage Curve?
Possible Effects: Tunnel Current vs. Polarization
H. Kohlstedt et al., Phys. Rev. B 72, 125341 (2005).
Strain: Barrier Effects
t -t 0
VC-VC
a. Variation of barrier thickness
Vd Voltage
b. Shift of the conduction and valence band edgest ⎞⎛ *
Vdtt o 33+=
222 kkk z += ⊥
dzEEhmCT
t
zC ⎟⎟⎠
⎞⎜⎜⎝
⎛−π−= ∫
0
0*24exp
Deformation Potential:
)(0
02
*2 2 EEmk zCz
z
−⋅=−h
c Change of the electron effective mass
Brooks 1955, Herring 1956, Kane 1970
3330 SEE CC κ+= 3κ
c. Change of the electron effective massa: lattice parameter, Tight binding approx.
0,22
* =Δ
= kEa
mC
h
33*
*033
**0
* VdmmSmmm∂
+=∂
+=033
03333
0 tSmS
Smm
∂∂
Interfacial Effect
t
Symmetric barrier structure I
φ
φ´1φ´2
φ1 φ2
t
t`
Vφ1 φ2
Electrode Ferroelectric Electrode S t i I VElectrode Ferroelectric Electrode
Asymmetric barrier structure
I
Symmetric I-V
φ´2
fixedvariable I
2/)( φφφ +=
2/)( ´21 φφφ +=a
φ1 φ2
V2/)( 21 φφφ +=b
Asymmetric I-V
Origin of Giant Electroresistance(from E. Tsymbal, U Lincoln, Nebraska)
Metal
– +
Metal FE Metal–+Metal FE
P
– +
– +
– +
P
–+
–+
–+
–+Electrostatic
Potential E
–+EPotential E E
Tunneling EF EFPotential
F
Different potential (and barrier width) for transport electronsDifferent potential (and barrier width) for transport electronsM.Ye. Zhuravlev R. F. Sabirianov S. S. Jaswal and E.Y. Tsymbal, PRL 94, 246802 (2005)
BaTiO3: 5nm
BaTiO
SrRuO3
E (kV/ )
SrRuO3
SrTiO3
BaTiO3
1416
-1200 -800 -400 0 400 800 1200
E (kV/cm)
30
-1200 -800 -400 0 400 800 1200
E (KV/cm)
@100 H
101214
pF)
0
10
20
C/c
m2 )
@100 Hz
468 C
(p
-20
-10
0
P (μ
C
-0,6 -0,4 -0,2 0,0 0,2 0,4 0,624
f=1000 Hz@ 300K
U (V)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
-30
U (V)U (V)
Similar to: Y.S Kim et al., APL 86, 102907 (2005).
I-V curve of a SRO/BTO/SRO Junction
100
I (μA) 2.9 nm BTO
50
I (μA)at 1 kHz
0
-50
-1,0 -0,5 0,0 0,5 1,0-100
U (V)2.9 nm BTO
Could not detect displacement current
Current Transport Measurements
Electric Field [kV/cm]Pt
0.5
-2000 -1000 0 1000
3 2
m2 ]
Electric Field [kV/cm]SrRuO3
SrTiO3
PbZr0.52Ti0.48O3
0.0 0.0
0.24
1 ty [k
A/cm
t [m
A]
6 nm PZT
-0.5-0.2
8 5
ent D
ensi
Cur
rent
-1.0 -0.5 0.0 0.5 1.0
0.5
-0.47
6
Cur
re
@300 K
Voltage [V]
J. Rodriguez Contreras et al., APL 83 4959 (2003)
No direct (elastic) tunnlingAPL 83, 4959 (2003).
Switching not caused by ferroelectricity!K. Szot et al., Nature Mat. 2006
Size Effects and Boundary ConditionsSize Effects and Boundary Conditions
Ultra thin Ferroelectric Oxide Films
5 nm12
O)
(BTO
)an
(PZT
)
TO)
8
10
eige
r (P
TO
Kim
N
agar
aja
O)PTO
)
ev (P
TO)
(PZT
)
Rab
e (P
T
it ce
lls
on d
irect
ng re
gim
e
6
Lich
tens
te
O)
iffer
(PTO
nd
TO)
Stre
iffer
(P
Per
tse
Tybe
ll
sez
and
R
er o
f un
ra (B
TO)
Elec
tro
tunn
eli
2
4
L
appe
(PTO
Stre
i
nque
ra a
nho
sez
(BTS
Gho
s
Num
be
Ger
r
1 0.4 nm
1999 2000 2001 2002 2003 2004 2005 20060
2 Ra
Ju Gh
1999 2000 2001 2002 2003 2004 2005 2006
Year
Strain enhanced Ferroelectricity
N.A. Pertsev, et al., Phys. Rev. Lett. 80, 1988 (1998)K. J. Choi, et al., Science, 306 1005 (2004).
Film
Substrate:side view E h f P iblside view Enhancement of P possible
Sm = (b – a0)/bc
out-of-plane
Sm (b a0)/b
b = Substrate lattice parametera0 = Equiv. cubic cell constant of
ab
in planefree film, Prototypic cell
Sm: Misfit strain
in-plane
Electrical Boundary Conditions
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Metal
0⇒DE0=⋅∫ dsE+ + + + + + + + + + + + + + + + + + + + +
EDP (only for
perfect screening!!)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
ΦMetal
+ + + + + + + + + + + + + + + + + + + + + Φ
tt
P. Würfel and I. P. Batra , Ferroelectrics 12, 55 (1976).J. Juncquera and Ph. Ghosez, Nature 422, 506 (2003).
Tunnel Junction: An Interfacial DeviceTunnel Junction: An Interfacial Device
Tunnel Junction: An interfacial device!
F i´ G ld R lFermi´s Golden Rule
InitialState
Travel Final State
Tunneling electrons are extremely sensitive to g ybarrier and interface properties!!
examples?…examples?
Tunneling electrons - coupling to excitations
(Inelastic) Electron Tunneling Spectroscopy
Molecule and Phonon Electron-Phonon C li 2 ( k)
MagnonsSpectroscopy Coupling α2 (ω,k)
P. Balk, JAP 1991 J. S. Moodera, PRL 1998E. L. Wolf, PRB 1985
n-Si/SiO2/Al Co/Al2O3/Ni80Fe20Nb/MgO/Ag
Magnetic Oxide Tunnel Junctions
LSMO/SrTiO3/LSMO @4.2 K
3 nm SrTiO3 barrier
Y. Lu et al., PRB 54, R8357 (1996).
3
Interface Effect in Magnetic Tunnel Junctions
Spinpolarization influenced by Barrier Material
J. M. De Teresa et al., Science 286, 507 (1999).
Tunneling Magneto Resistance – An Interface Effect!
Ch Heiliger et alFe MgO Fe
Ch. Heiliger et al., Phys. Rev. B 73, 214441 2006
J. S. Moodera, G. Mathon,JMMM 200, 248 (1999).
First layer adjacent to tunnel barrier is essential!
Drastically change in TMR!!
Electrical Boundary Conditions: An endless story
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Metal
0⇒DE0=⋅∫ dsE+ + + + + + + + + + + + + + + + + + + + +
EDP (only for
perfect screening!!)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
ΦMetal
+ + + + + + + + + + + + + + + + + + + + + Φ
tP. Würfel and I. P. Batra , Ferroelectrics 12, 55 (1976).J. Juncquera and Ph. Ghosez, Nature 422, 506 (2003).
t
Ionic Screening
Alternative Screening Mechanism
Fong, et al., Phys. Rev. B 71, 144112 (2005).
Ionic Screening
Theoretically confirmed: G. Gerra et al., PRL (2006).
Alternative Screening Mechanismmetal ferroelectric
Th F i i dThomas-Fermi screening andKretschmer-Binder effect
CTF CKB
B d h ti b fBond charge compensation by freecarriers in the ferroelectric
E t i f th i i l i tiExtension of the ionic polarizationinto the metal; Ionic distortion also in the metal
Sketch taken from G. Gerra et al.,PRL 96. 107603 (2006). Fig.1
Magnetoelectric Interface Effect
Fe/BaTiOFe/BaTiO3
Interface between a ferromagnet and a ferroelectric
PPg
Top interface
DO
SMinority-spin charge density Paraelectric BTO
Bottom interface Ferroelectric BTO
C.-G. Duan, S.S. Jaswal and E. Y. Tsymbal,PRL 97, 047201 (2006).
EF
Interface without Ionic Screening
SrRuO3BaTiO3 +
-
Interface with Ionic Screening
Local atomic rearrangement at the
Magnet or S d t
Ferroelectric+
ginterface
Variation of DOSSuperconductor Variation of DOSat the interfaces
Tunneling current…
Tunneling current modified by interface properties
TMR vs. P?
-Josephson-Effects vs. P?
Magnetic and superconducting junctions with ferroelectric barrierwith ferroelectric barrier
La0.33Sr0.67MnO3
BaTiO P
LaxSr1-xCuO3
BaTiO
La0.33Sr0.67MnO3
BaTiO3P
LaxSr1-xCuO3
BaTiO3
Th (f l t i ) l i ti i ht
x 1 x 3
The (ferroelectric) polarization might modify the spin polarization and
superconducting order parameter ( ξ ≅ 0.1 nm) (at the interfaces)
Tunneling magneto resistance as well as quasiparticleTunneling magneto resistance as well as quasiparticle current and Josephson current
should depend on P!
An optimistic Outlook: A novel “zoo” of tunnel junctions
Ferroelectric
PyroelectricPiezoelectricDielectric
Paramagnet(Anti)-Ferromagnet
S d+ + +
- - -
Multiferroic (I l t )
Anti-ferroelectric Dielectric
P, MSuperconductor
+ + +
- - -
(Insulator)(Tunnel Barrier)
Magnetic
,
gAnti-ferromagnetic
Josephson Junction with a ferroelectric barrierJosephson-Junction with a ferroelectric barrier• dc and ac Josepson Effect vs. P?
Magnetic Tunnel Junction• Tunnel Magneto Resistance vs. P?
E. Y. Tsymbal and H. Kohlstedt, Science 2006
Multiferroic Tunnel Junctions
M. Gajek et al., Tunnel junctions with multiferroic barriers Nature Mat. 2007 La: BiMnO3multiferroic barriers Nature Mat. 2007
4 bit Memory:
3
Sheng Ju et al, PRB 75, 064419 2007
2 from Ferroelectricity2 from Magnetism
More about Multiferroics:N A Spaldin and M FiebigN. A. Spaldin and M. Fiebig, Science (2005).
R Ramesh et al
W. Eerenstein, N. D. Mathur,
R. Ramesh et al. Phil Mag. Lett. (2007).
e e ste , at u ,J. F. Scott Nature (2006).
ConclusionQuantum Mechanical Electron Tunneling
andM ltif i M t i lMultiferroic Materials:
• Development of new tunnel junctionsp j• New Functionalities
• Will propel exciting theoretical approachesT li El ill b li d l i l l• Tunneling Electrons will be applied as an analytical tool
• Better understanding of multiferroic materials onthe nm levelthe nm level
• Multiple size effects•…
Challenge: Defect free/ideal ferroelectric tunnel barriers,I- V curves alone are not sufficient to extractI V curves alone are not sufficient to extractthe underlying switching mechanism
There is a gap between theory and experiment!
Center ofNanoelectronic Systems forInformation Technology
AcknowledgementSponsors:
Volkswagen-Foundation:“N i d f l t i h b id ” d t t b I/77 737“Nano-sized ferroelectric hybrids” under contract number I/77 737
Joint NSF-DFG Project:University of Berkeley (Material Science Department)University of Berkeley (Material Science Department)
University of Aachen (RWTH)Research Center Juelich
DFG:„Displacive and Conductive Phenomena in Ferroelectric Thin Films:
Scaling effects and switching properties“Scaling effects and switching properties
“Synchrotronstrahlungsexperimente zu Skalierungseffekten und ungewöhnlichen Phasen epitaktischer, perowskitscher Schichten”g p , p