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