1. Perovskite-type transition metal oxide interfaces
M. Matvejeff

2. Perovskites - Chemistry and properties
Contents
Perovskites - Chemistry and properties
Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs)
Charge transfer at perovskite interfaces 2

3. Perovskites – StructureA B O AO
BO2
AO + BO2 = ABO3
SrTiO3
(La,Sr)MnO3 (LSMO)

4. Highly flexible cation stoichiometry
Perovskites – The Good
(La1-xSrx)MnO3 (LSMO)
Highly flexible cation stoichiometry
Wide variety of functional properties through changes in cation stoichiometry
Imada et al. Rev. Mod. Phys. 70

5. 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 AO
1 u.c.
BO2 AO
BO2 AO
AO1- + BO2 = ABO3-
SrTiO3-
(La,Sr)MnO3- (LSMO)

6. For example capacitors, catalytic converters and superconductors
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

8. 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

9. Properties at interfaces?
Perovskites – The Bad A B O
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? AO
1 u.c.
BO2 AO
BO2 AO

10. Perovskites - Chemistry and properties
Contents
Perovskites - Chemistry and properties
Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs)
Charge transfer at perovskite interfaces 10

11. CMR in manganites Colossal MR (CMR) in La2/3Ba1/3MnO3
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

12. 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
(La,Sr)MnO3 (LSMO)
Local
electrons
Mn3+
t2g eg
Mn4+

13. A = divalent and/or trivalent cation
Chemical substitution means we’re directly playing with the average valence of Mn
General formula
AMnO3
A = divalent and/or trivalent cation
(Ca, Sr, La, Nd...)
LaMnO3
SrMnO3
La1-xSrxMnO3
x Mn4+
1-x Mn3+
Itinerant
electron
(La,Sr)MnO3 (LSMO)
Local
electrons
Mn3+
t2g eg
Mn4+
t2g eg

14. LaMnO3 SrMnO3 La1-xSrxMnO3 In double-exchange (DE) model x Mn4+
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
LaMnO3
SrMnO3
La1-xSrxMnO3
x Mn4+
1-x Mn3+
Itinerant
electron
(La,Sr)MnO3 (LSMO)
Local
electrons
Mn3+
t2g eg
Mn4+
t2g eg

15. 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
Magnetic tunnel junction (MTJ) FM FM
Tunneling current
Tunneling current
Insulator (t = nm-Å)
Insulator (t = nm-Å) FM FM

16. RA(AP) Resistance in parallel (antiparallel) configuration
P1,P2 Polarizations of electrodes 1 and 2
TMR R P AP
Magnetization
Applied field
resistance
Junction
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!
Applied field
MTJ 1 FM
Tunneling current
Tunneling current
Applied field
Insulator 2 FM

17. Half-metals – Because polarization does matter…
P. M. Tedrow and R. Meservey PRB 1973
R. von Helmholt APL 1993

18. LSMO is a good candidate material for MTJs
J.-H. Park Nature 1998
Half-metallicity in bulk La0.7Sr0.3MnO3
Y. Lu, APL 1996
P ~ % in low T
LSMO is a good candidate material for MTJs

19. TMR dissappears well below Tc
4.2K
Tc ~ 350K
Good TMR only at low T
TMR dissappears well below Tc
Why?
LSMO
STO
LSMO
T. Obata, APL 1999 19

20. 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

21. 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)
LSMO (2-10 u.c.)
STO (2 u.c.)

22. 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
Y. Tokura Rep. Prog. Phys. 2006
M. Izumi J. Phys. Soc. Jpn. 2002

23. Dead layer Same effect also observed in M-H measurements
M. Izumi J. Phys. Soc. Jpn. 2002
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
FM+AFM
+ ext. field!

24. Dead layer So how does the dead layer actually form?
H. Fujishiro J. Phys. Soc. Jpn 1998
So how does the dead layer actually form?
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

25. So what does actually happen at the interface layer?
LSMO (2-10 u.c.)
STO (2 u.c.)

26. Dead layer Hole-doping at La1-xSrxMnO3-STO interface x increases
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?
x increases
STO (2 u.c.)
La0.4Sr0.4MnO3 (x = 0.4)
Bulk
High Tc
High magnetization FM
Hole-doped LSMO
(x  0.4)
Faster decrease in properties
(charge transfer)
x increases
STO (2 u.c.)
La0.8Sr0.2MnO3 (x = 0.2)
Bulk
High Tc
High magnetization FM
Hole-doped LSMO
(x  0.2)
FM+AFM
Lower Tc/magnetization
Y. Tokura Rep. Prog. Phys. 2006

27. Perovskites - Chemistry and properties
Contents
Perovskites - Chemistry and properties
Properties of perovskite interfaces - CMR and Magnetic Tunnel Junctions (MTJs)
Charge transfer at perovskite interfaces 27

28. 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)
SrTiO3
LaTiO3
Sr2+ and O2-
Ti4+
(2 + x + 3*(-2) = 0)
La3+ and O2-
Ti3+
(3 + x + 3*(-2) = 0)

29. La Sr Ti O
SrTiO3
LaTiO3
Ti4+
Ti3+
Ti4+

30. La Sr Ti O
However in practice it has been found out that Ti3+ oxidation state is not limited to the LTO layers…
SrTiO3
Ti3+ fraction
LaTiO3
Ohtomo A. et al., Nature, 2002
Ti4+
Ti3+
Ti3/4+
… i.e. charge transfer (transfer of electrons) occurs from LTO into STO layers forming mixed valence interface layer

31. Now, our ideal TMR device the LSMO/STO/LSMO tunnel junction
LSMO TC ~ 350 K in the bulk phase FM
Insul. FM

32. In practice, charge transfer over the interface
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

33. 3D structure is the problem!
Perovskite - recap A B O
Alternating AO and BO2 layers
Formula: ABO3
3D structure is the problem!
So what about structures which aren’t (fully) 3D? AO
1 u.c.
BO2 AO
BO2 AO

34. 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)

35. Perovskite: 3D structure vs Ruddlesden-Popper (RP): 2D
High anisotropy (ab-plane vs c-axis)
n = 2 RP
(A3B2O7) AO
1 formula unit
BO2 AO
Perovskite
(ABO3)
1 u.c.
BO2
c-axis

36. A = La, Sr B = Mn La1.4Sr1.6Mn2O7 AO 1 formula unit BO2
Charge carriers AO
1 formula unit
BO2
T. Kimura & Y. Tokura, Annu. Rev. Mater. Sci., 2000
La1.4Sr1.6Mn2O7
Charge carriers
A = La, Sr
B = Mn

37. Modulation of interface properties
Perovskite 1
Perovskite 2
Strong interaction
Modulation of interface properties
Perovskite 1
Perovskite RP
Weak interaction
Clean interface, little or no modulation

38. We can study the interface electronic structure in XPS…
So does it actually work?
In perovskite-type interface between (La,Sr)MnO3/(La,Sr)FeO3 electrons are transferred from Mn eg states to Fe eg states
We can study the interface electronic structure in XPS…
Kumigashira et al. APL 2004 38

39. t2g eg Mn3+ Mn4+ … 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)
t2g eg
LSMO
LSFO (t = 1-7 layers)
Itinerant
electron
Local
electrons
Mn3+
t2g eg
Mn4+ 39

40. Large change in LSMO valence Small change in LSMO valence?
LSFO
Strong interaction
Large change in LSMO valence
LSMO
LSFO
Weak interaction
Clean interface
Small change in LSMO valence?
LSMO

41. Perovskite
t2g eg
RP-type interface
(LSMO layer thickness = 3 u.c.) 41

42. 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 42