Perovskite-type transition metal oxide interfaces
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Perovskite-type transition metal oxide interfaces. M. Matvejeff. 7.2.2011. Contents. Perovskites - Chemistry and properties Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs) Charge transfer at perovskite interfaces. A B O. Perovskites – Structure.

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Perovskite-type transition metal oxide interfaces

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Perovskite type transition metal oxide interfaces

Perovskite-type transition metal oxide interfaces

M. Matvejeff

7.2.2011


Perovskite type transition metal oxide interfaces

Contents

  • Perovskites - Chemistry and properties

  • Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs)

  • Charge transfer at perovskite interfaces


Perovskite type transition metal oxide interfaces

A

B

O

Perovskites – Structure

AO+BO2= ABO3

SrTiO3

(La,Sr)MnO3 (LSMO)

1 u.c.

AO

BO2

AO

BO2

AO


Perovskite type transition metal oxide interfaces

Perovskites – The Good

Highly flexible cation stoichiometry

Wide variety of functional properties through changes in cation stoichiometry

(La1-xSrx)MnO3 (LSMO)

Imada et al. Rev. Mod. Phys. 70


Perovskite type transition metal oxide interfaces

A

B

O

Perovskites – The Good

Highly flexible cation stoichiometry

Wide variety of functional properties through changes in cation stoichiometry

Highly flexible oxygen content

 Properties can be fine-tuned after synthesis

AO1- + BO2 = ABO3-

SrTiO3-

(La,Sr)MnO3- (LSMO)

1 u.c.

AO

BO2

AO

BO2

AO


Perovskite type transition metal oxide interfaces

The flexibility of perovskite structure and the easy tunability of the functional properties are definite bonuses as long as bulk material is suitable for applications

For example capacitors, catalytic converters and superconductors


Perovskite type transition metal oxide interfaces

However, significant number of industrial applications rely on device structures consisting of several different functional material layers, in some cases only few atomic layers in thickness

In these structures, such as field-effect transistors (FETs), the properties of the interface are often significantly more important to the correct function of the device than the properties of the bulk material


Perovskite type transition metal oxide interfaces

A

B

O

Perovskites – The Bad

Highly 3-dimensional structure

+

Strong hybridization of 3d orbital of the transition metal B to neighboring oxygen 2p orbitals

+

Highly sensitive to small changes in transition metal oxidation state

Properties at interfaces?

1 u.c.

AO

BO2

AO

BO2

AO


Perovskite type transition metal oxide interfaces

Contents

  • Perovskites - Chemistry and properties

  • Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs)

  • Charge transfer at perovskite interfaces


Perovskite type transition metal oxide interfaces

CMR in manganites

Manganites exhibit CMR i.e. strong change in resistivity under applied magnetic field

The CMR effect can be used for example for magnetic sensor applications

As the most properties of transition metal oxides, CMR is highly dependent on transition metal (Mn) oxidation state

Colossal MR (CMR) in La2/3Ba1/3MnO3

R. von Helmholt APL 1993


Perovskite type transition metal oxide interfaces

Electronic structure of manganites

General formula

AMnO3

A = divalent and/or trivalent cation

(Ca, Sr, La, Nd...)

To understand the origin of CMR phenomenon we need to first understand the electronic structure of manganites

Itinerant

electron

Local

electrons

(La,Sr)MnO3 (LSMO)

Mn3+

Mn4+

eg

eg

t2g

t2g


Perovskite type transition metal oxide interfaces

Chemical substitution means we’re directly playing with the average valence of Mn

+2 +4-2

+3 +3-2

SrMnO3

LaMnO3

General formula

AMnO3

A = divalent and/or trivalent cation

(Ca, Sr, La, Nd...)

+3 +2 3...4-2

La1-xSrxMnO3

x Mn4+

1-x Mn3+

Itinerant

electron

Local

electrons

(La,Sr)MnO3 (LSMO)

Mn3+

Mn4+

eg

eg

t2g

t2g


Perovskite type transition metal oxide interfaces

Mn3+/Mn4+-ratio (doping) has strong impact on magnetotransport properties

In double-exchange (DE) model

Itinerant eg electron is the charge carrier

whereas the t2g electrons are localized

+2 +4-2

+3 +3-2

SrMnO3

LaMnO3

+3 +2 3...4-2

La1-xSrxMnO3

x Mn4+

1-x Mn3+

Itinerant

electron

Local

electrons

(La,Sr)MnO3 (LSMO)

Mn3+

Mn4+

eg

eg

t2g

t2g


Perovskite type transition metal oxide interfaces

What are magnetic tunnel junctions (MTJs)?

Bulk CMR is not suitable for low field applications (magnetic field required is in order of several tesla)

How to increase sensitivity?

Significantly weaker field (~coercive field of the material) required in MTJs

Tunneling current

Tunneling current

Magnetic tunnel junction (MTJ)

FM

FM

Insulator (t = nm-Å)

Insulator (t = nm-Å)

FM

FM


Perovskite type transition metal oxide interfaces

Magnetization

AP

Magnetic field required is in order of tens to hundreds of Oe instead of several Tesla as for bulk CMR  low field sensors

For maximum sensitivity RA-RAP has to be maximized

 Degree of spin polarization is important!

Junction

resistance

R

P

Applied field

Applied field

TMR

RA(AP)Resistance in parallel (antiparallel) configuration

P1,P2Polarizations of electrodes 1 and 2

Tunneling current

MTJ

1

FM

Tunneling current

Applied field

Insulator

2

FM


Perovskite type transition metal oxide interfaces

Half-metals – Because polarization does matter…

R. von Helmholt APL 1993

P. M. Tedrow and R. Meservey PRB 1973


Perovskite type transition metal oxide interfaces

Half-metallicity in bulk La0.7Sr0.3MnO3

Y. Lu, APL 1996

P ~ 95-100% in low T

LSMO is a good candidate material for MTJs

J.-H. Park Nature 1998


Perovskite type transition metal oxide interfaces

4.2K

Good TMR only at low T

TMR dissappears well below Tc

Why?

Tc ~ 350K

LSMO

STO

LSMO

T. Obata, APL 1999


Perovskite type transition metal oxide interfaces

Dead layer

La0.67Sr0.33MnO3 films grown on (110) NGO (NdGaO3) and (001) LAO (LaAlO3) substrates

Clear thickness dependence in resistivity

Dead (insulating) layer forms at the interface?

How can we study this?

J. Z. Sun APL 1999


Perovskite type transition metal oxide interfaces

Dead layer

(2-10 u.c. LSMO – 2 u.c. STO)10-20 superstructure

LSMO = La1-xSrxMnO3, 0.2  x  0.4

By changing the thickness of conducting layers (LSMO) separated by the insulator (SrTiO3) we can probe the critical thickness for transition from ferromagnetic metal (FM) to antiferromagnetic insulator (AFI)

STO (2 u.c.)

LSMO (2-10 u.c.)

STO (2 u.c.)

LSMO (2-10 u.c.)


Perovskite type transition metal oxide interfaces

Dead layer

For all doping doping levels, decrease in Tc and magnetization with decreasing LSMO thickness

Decrease is faster with higher x

 Samples which are closer to metal to insulator-phase diagram line loose metallicity and magnetic order already in thicker films

M. Izumi J. Phys. Soc. Jpn. 2002

Y. Tokura Rep. Prog. Phys. 2006


Perovskite type transition metal oxide interfaces

Dead layer

Same effect also observed in M-H measurements

Also, for thinner films M-H does not saturate

 This indicates competing FM and AFM interactions

FM+AFM

+ ext. field!

FM

FM+AFM

M. Izumi J. Phys. Soc. Jpn. 2002


Perovskite type transition metal oxide interfaces

Dead layer

So how does the dead layer actually form?

H. Fujishiro J. Phys. Soc. Jpn 1998

From phase diagram we see transition from FM to AF state at x ~ 0.5

Is this related to the formation of dead layer at the interface?

Y. Tokura Rep. Prog. Phys. 2006


Perovskite type transition metal oxide interfaces

So what does actually happen at the interface layer?

STO (2 u.c.)

LSMO (2-10 u.c.)

STO (2 u.c.)

LSMO (2-10 u.c.)


Perovskite type transition metal oxide interfaces

Dead layer

M. Izumi J. Phys. Soc. Jpn. 2002

  • Hole-doping at La1-xSrxMnO3-STO interface

  • x increases

  • The properties of the interface change

Effect is stronger when x in the original phase is higher (already closer to critical limit of x ~ 0.5)

Why does the hole-doping occur?

La0.4Sr0.4MnO3 (x = 0.4)

Bulk

High Tc

High magnetization

FM

La0.8Sr0.2MnO3 (x = 0.2)

Bulk

High Tc

High magnetization

FM

STO (2 u.c.)

STO (2 u.c.)

Hole-doped LSMO

(x 0.4)

Faster decrease in properties

Hole-doped LSMO

(x 0.2)

FM+AFM

Lower Tc/magnetization

xincreases

(charge transfer)

xincreases

Y. Tokura Rep. Prog. Phys. 2006


Perovskite type transition metal oxide interfaces

Contents

  • Perovskites - Chemistry and properties

  • Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs)

  • Charge transfer at perovskite interfaces


Perovskite type transition metal oxide interfaces

Let’s study the following a quantum well structure…

In theory the Ti valence changes sharply at the interface between SrTiO3 (STO) and LaTiO3 (LTO)

  • Sr2+ and O2-

  • Ti4+

  • (2 + x + 3*(-2) = 0)

SrTiO3

  • La3+ and O2-

  • Ti3+

  • (3 + x + 3*(-2) = 0)

LaTiO3

SrTiO3


Perovskite type transition metal oxide interfaces

La

Sr

Ti

O

Ti4+

SrTiO3

Ti3+

Ti4+

LaTiO3

SrTiO3


Perovskite type transition metal oxide interfaces

La

Sr

Ti

O

However in practice it has been found out that Ti3+ oxidation state is not limited to the LTO layers…

Ti4+

SrTiO3

Ti3+ fraction

Ti3/4+

Ti3+

LaTiO3

Ti3/4+

Ti4+

SrTiO3

Ohtomo A. et al., Nature, 2002

… i.e. charge transfer (transfer of electrons) occurs from LTO into STO layers forming mixed valence interface layer


Perovskite type transition metal oxide interfaces

Now, our ideal TMR device the

LSMO/STO/LSMO tunnel junction

LSMO TC~ 350 K in the bulk phase

FM

LSMO

Insul.

FM

STO

LSMO


Perovskite type transition metal oxide interfaces

Y. Tokura Rep. Prog. Phys. 2006

In practice, charge transfer over the interface

Strong impact on carrier density (valence of Mn) at the interface

Instead of FM, LSMO at interface either P or AF

Formation of dead layer and

TC 100 K instead of 350 K!

FM

LSMO

P/AF

Insul.

STO

P/AF

FM

LSMO


Perovskite type transition metal oxide interfaces

A

B

O

Perovskite - recap

Alternating AO and BO2 layers

Formula: ABO3

3D structure is the problem!

So what about structures which aren’t (fully) 3D?

1 u.c.

AO

BO2

AO

BO2

AO


Perovskite type transition metal oxide interfaces

Ruddlesden-Popper structure

Closely related to perovskite structure

Alternating AO and BO2 layers

Formula: An+1BnO3n+1

(i.e. one extra AO-layer compared to perovskites, ABO3)


Perovskite type transition metal oxide interfaces

Perovskite: 3D structure

vs

Ruddlesden-Popper (RP): 2D

High anisotropy (ab-plane vsc-axis)

n = 2 RP

(A3B2O7)

1 formula unit

1 u.c.

AO

AO

AO

AO

AO

BO2

BO2

BO2

BO2

BO2

Perovskite

(ABO3)

BO2

AO

AO

AO

AO

AO

BO2

AO

BO2

AO

BO2

AO

BO2

c-axis

AO


Perovskite type transition metal oxide interfaces

Charge carriers

T. Kimura & Y. Tokura, Annu. Rev. Mater. Sci., 2000

La1.4Sr1.6Mn2O7

1 formula unit

Charge carriers

AO

AO

AO

AO

AO

BO2

BO2

BO2

BO2

BO2

BO2

AO

AO

AO

AO

A= La, Sr

B = Mn


Perovskite type transition metal oxide interfaces

Strong interaction

Modulation of interface properties

Perovskite 1

Perovskite 1

Perovskite

Perovskite 2

RP

Weak interaction

Clean interface, little or no modulation

Perovskite


Perovskite type transition metal oxide interfaces

So does it actually work?

In perovskite-type interface between (La,Sr)MnO3/(La,Sr)FeO3electrons are transferred from Mn eg states to Fe eg states

We can study the interface electronic structure in XPS…

Kumigashira et al. APL 2004


Perovskite type transition metal oxide interfaces

… to determine the occupation of eg and t2g states

As LSFO layer thickness is increased, the charge transfer increases and eg electron occupation decreases (Mn valance increases)

LSFO (t = 1-7 layers)

LSMO

t2g

eg

Itinerant

electron

Local

electrons

Mn3+

Mn4+

eg

eg

t2g

t2g


Perovskite type transition metal oxide interfaces

LSFO

Strong interaction

Large change in LSMO valence

LSMO

LSFO

Weak interaction

Clean interface

Small change in LSMO valence?

LSMO


Perovskite type transition metal oxide interfaces

Perovskite

t2g

eg

RP-type interface

(LSMO layer thickness = 3 u.c.)


Perovskite type transition metal oxide interfaces

Conclusions

  • Perovskite phases exhibit interesting functional properties in bulk form

  • Applications, however, are often based on device structures built from functional layers at times only few atomic layers in thickness

  • Interface effects arising from the 3-dimensional nature of the perovskite structure dominate the behavior of the devices

    Interface effects can be, at times, partially compensated for, but this leads to expensive production processes where device properties are difficult to predict and/or control

  • Best solutions would be based on integrating, property-wise, 2-dimensional materials into device structures to create not only structurally but also electronically sharp interface structures


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