1. Magnetism at Oxide
Interface
Sanjay Kumar Nayak
Ph.D. Student
Epitaxy Lab
CPMU Seminar

2. Outline
Introduction
Materials under Studies
Magnetic Measurements
Microscopic Origin
Approach towards Devices
Conclusion

3. Introduction

4. Conductance and Operation
Role of Electron in Modern Technology
Charge Spin
 Magnetism and
Information Storage
Nobel Prize (2007)
Peter Grünberg Albert Fert
Nobel Prize (1956)

5. Role of Electron in Modern Technology
Goal Multifunctional device
Put both charge and spin together for faster and smaller devices
What is the obstacle ?
Lack of suitable material !
Does oxide have potential?
May be!
e-
±1/2 ħ ±1/2 ħ
e-

6. Conventional Semiconductor
Physics:
• Large overlap of s/p orbitals gives
extended wave functions
• No intrinsic magnetism or other
correlations
Technology:
• Quality: High - Can be fabricated into
complex structures
• Understanding: Semiconductor modeling
is straightforward
• Tunability: control charge with modest
doping/ E fields
Complex Oxide Materials
Physics:
• localization of 3d/2p orbitals gives strong
Coulomb interactions
• diverse magnetic and other strong
Correlations
Technology:
• Quality: Materials chemistry challenging;
fabrication less developed
• Understanding: Strong correlations
challenging to theoretical tools
• Tunability: High - due to competing
ordered states
Conventional Semiconductors
versus
Complex Oxides

7. Importance of Oxides
Co existence of charge, spin, orbital and lattice degree of freedom
Correlation between these degree of freedom and related coupling
generates rich varieties of phases which are highly tunable to
internal and external parameters
Virtually all phases of matter are found in Oxide family
 High temp. Superconductivity
 Metal insulator transition
 Colossal magneto resistance
 (anti-)Ferromagnetism
 (anti-)Ferro electricity
 Piezoelectricity
 Multiferroics
For understanding of fundamental nature of existing materials
well as application, oxides are important.

8. Why Surfaces or Interfaces ?
Herbert Kroemer:
“The interface is the device’’
Oxide based electronics:
Put the many-body properties of correlated
electrons: superconductivity, magnetism,
multiferroicity , metal-insulator transitions.....
to practical use.
Prof. Herbert Kroemer
Noble prize (2000) in
physics for developing
semiconductor
heterostructures used in
high-speed- and opto-
electronics
Image adopted from Wikipedia

9. Materials under Studies

10. Emerging Oxide Materials
• LaAlO3 /SrTiO3
• La0.7Ca0.3MnO3/YBa2Cu3O7
• La0.7Ca0.3MnO3/PrBaCu3O7
• BiFeO3/La0.7Sr0.3MnO3
• CaMnO3/CaRuO3
• LaMnO3/SrMnO3

11. LaAlO3 /SrTiO3
“Drosophila of Oxide Physics”
SrTiO3 (STO)
• Band insulator (band gap of 3.2 eV)
• Non magnetic
• Good for substrate
LaAlO3 (LAO)
• Band insulator (5.6 eV)
• Non magnetic
 Both have ABO3 (Perovskite) crystal structure

12. When both Oxides meet face to face
Reyren et al. Science 317,1196(2007) Ariando et al Nat commun. 2, 188 (2011)
H(kOe)
(kΩcm-2)

13. Magnetic Studies

14. Magnetic Measurement Tools
• SQUID (Overall magnetization of sample)
• Torque magnetometer
• XMCD (Elemental sensitivity)
• Magneto resistance

15. SQUID Results
Ariando et al Nat commun. 2, 188 (2011)
No sign of any magnetic impurity in SIMS measurement
H(kOe)
H(kOe) H(kOe)
H(kOe)

16. Scanning SQUID Results
Kalisky et al Nat.Comm.,3,922(2012)
Critical Thickness: 3.3 unit cell of LAO
Annealed STO 2 uc of LAO 5 uc of LAO 10 uc of LAO

17. Torque τ = M H
Deflection of cantilever
Torque
Lu Li et al. Nature Physics 7 ,762(2011)
Torque magnetometry
Sensitivity:10-13-10-12 A m-2 at 10T
M proportional to H
Torque = M×H H2

18.  For H → 0 , m → 5 10-10 A m-2
 0.3 to 0.4 μB per interface Unit Cell
Lu Li et al. Nature Physics 7 ,762(2011)
Torque magnetometry Results

19. • SQUID and Torque magnetometer can give idea about
whether material is magnetic or not
• Can not tell whether magnetic properties are intrinsic
or because of impurity
• Can not explain the origin of magnetism
Intrinsic Magnetism or Not?

20. Microscopic Origin

21. Microscopic Origin of
Interface Magnetism
X-ray Magnetic Circular Dichroism
XMCD = XAS with Polarized photons(circularly or linearly)
Element Specificity
Orbital Selectivity
 Sensitivity is very high (0.005 μB per atom )

22. Electron – Electron Interaction through Exchange Coupling
I N(Ef) > 1, I = coupling strength
ms= -2 <Sz> μB/ ħ = (N↑-N ↓) μB
Stoner’s model for Ferromagnetism

23. Principle Behind XMCD
 Core Electron excited in absorption process
in to empty state above the Fermi level
 Right Circular Photons (RCP) transfer
the opposite momentum to the electron as
Left Circular Photons (LCP)
www-ssrl.slac.stanford.edu/stohr/xmcd.htm
unoccupied, CB
occupied, VB
variable hn
core level


24.  Excitation of electron 2p core level to 3d unfilled state (L-edge x-ray
absorption spectra)
 Sum of IL3 and IL2 will give total vacant “d- hole”
Principle Behind XMCD

25. Theoretical Predictions on the Origin of
Magnetism
In absence of extrinsic magnetic impurities, interface ferromagnetic
originate from Tiatom.
Pentcheva et al., PRB ,74,035112(2006)
Popovic et al., PRL, 101,256801(2008)
Pavlenko et al., PRB, 85,020407(2012)
Micheali et al., PRL, 108,117003(2012)

26. Lee et al. Nature Materials 12, 703 (2013)
Observations
L-edge spectra of Ti atom
Experimental and theoretically
calculated spectra match for Ti3+
Bulk SrTiO3 : Ti4+ valence state
Ti4+ = d0 configuration
Are some extra electrons coming towards interface?

27.  High electron beam energy(200 keV)
 Spot size of 1-3 Å
• Free carrier at n type interface with density 3.5 1014 cm-2
• Confined within a few nm of the interface (quasi 2 DEG)
EELS
Muller et al. Nature, 5,206(2006)

28. • Oxygen vacancies at interface
• Cation intermixing (LaxSr1‐xTiO3)
• Electronic reconstruction at interface
Possibilities of Formation of 2DEG
?
?
Electronic Reconstruction
LaAlO3 on TiO2 terminated SrTiO3 (001) (n
type)
SrTiO3(001):Alternate layer of SrO and TiO2
LaAlO3(001):Alternate layer of charged LaO+
and AlO2-

29. Polar Catastrophe
• ½ electron per unit cell
• Carrier density:3.5×1014 cm-2
Muller et al ,Nat. Mat.,5,204(2006)

30. Is Electronics Reconstruction enough
for Interface Ferromagnetism?
SrTiO3 can be doped with p type or n type material
Metallic and Superconducting phases are observed
No Sign of ferromagnetism
Electronics Reconstruction is
Necessary but Not Sufficient

31. Symmetry breaking at interface
eg
dz
2
47 meV
dxy
3d
t2g
dxz/yz
dx
2
-y
2
26 meV
Crystal
field
Experimentally confirmed from
XAS data (J. Park et al., PRL,
110, 017401 (2013))
dxy is lowest energy state
Removal of Degeneracy and Orbital
Reconstruction
Lee et al. Nat material 12,703(2013)

32. Double Exchange
Ti3+ (t2g) Ti4+ (t2g )
O2-
dxy
dxy
dxz/yz
 Double exchange interaction leads to ferromagnetism
 Competition between Double exchange and Spiral
magnetism
Interface magnetism for LAO/STO originates from dxy
orbital of Ti t2g band
dxz/yz

33. Recent studies on Oxide Interface

34. Approach towards
Applications

35. LaALO3
LaALO3
Co
Conventional 3-T measurement technique
Spin accumulation at
interface
Hanle effect: Change the
voltage due to spin dephasing
Spin Injection

36. N. Reyren et al. PRL 108 , 186802(2012)
Spin Injection
Spin relaxation time=50ps
Spin diffusion length=1micrometre

37. A.Ohtomo et al. NATURE 427,423(2004)
 Suitable material for D-S channel

38. FETs
Forg et al , APL ,100 ,053506 (2012)
 LAO as gate dielectric (εr=18)
Electrical Contacts : Ar ion milled hole
refilled with sputtered Titanium for
source and drain
 Gold contact for Gate
A change of VGS 700 mV causing a
change of 4 order of magnitude of IDS

39. I-V characteristics
Forg et al , APL ,100 ,053506 (2012)
Temp. Dependence of I-V Characteristics
At +ve VGS decrease with temp
 Enhancement of IDS
 reduction of Turn On voltage
 G > 1 obtained
 G = 40, For IGS= 5μA and
VDS = 450 mV

40. Some Other possible Applications
L Li et al. Science 2011;332:825-828
40%
enhancement

41. Sensors
Photo detectors
Some Other possible Applications
Multifunctional Oxide Heterostructures, Oxford University Press (2010)
Thermoelectric
 Solar cells

42. Conclusions
 Advantages of Oxide Materials are discussed
 Interfaces of Oxide Materials show interesting
properties
 Ferromagnetism at room temperature is observed
 Spin injection and detection is successfully realized
 Very high mobility 2DEG is observed
 Standard FET device is demonstrated and have
advantages over scaling limits of silicon based
transistor
 Could be a very prominent candidate for spin based
devices

43. Acknowledgement
1. Prof. S.M. Shivaprasad For Topic
2. My Labmates, Satish, Malli, Arpan, Nagaraja, Varun, Shivkumar,
Sandheep, Ankit for useful discussion and preparing slides
3. My friends, Dheeraj, Sunil,Vikas, Sukanya, Shantanu.

45. I = coupling strength
0.6 eV for early 3d element
1.0 eV for late 3d element
Stoner criterion

46.  Using PLD 4-5 unit cell thickness of
LAO on TiO2 terminated STO
 Ferromagnetic Cobalt of Thickness 15
nm deposited at room temp by
sputtering then capped with Gold
Spin injection
 Spin polarized current passed from
ferromagnetic material Co through
tunnel contact and one of the Ohmic
contact
 LAO film a band insulator play the role
of tunnel barrier
 Induced imbalance of spin population at
the channel side (Spin accumulation)
creates additional voltage at contacts
 Electrical Henley effect Causes
decrease of voltage

47.  Growth of LAO on TiO2 terminated
STO (approx. 9 unit cell) using PLD –
780 C , P (O2) = 9 10-5 mbar
 Gate Contacts of 40 nm YBCO
deposited at 760 C at 0.11 mbar of O2
 Annealed for 1hr at 600 C, 30 min at
460 C & 30 min at 430 C at 400 mbar
of O2
Two approaches
1) SrTiO3 as gate, Turn On voltage 60 V
2) Using Tip of SPM to write line on
LAO/STO interface , Turn On Voltage
less than 1 volt
FETs