Magnetism at Oxide Interface

Sanjay Kumar NayakPh.D. Student

Epitaxy Lab

CPMU Seminar

Outline

Introduction

Materials under Studies

Magnetic Measurements

Microscopic Origin

Approach towards Devices

Conclusion

Introduction

Conductance and Operation

Role of Electron in Modern Technology

Charge Spin Magnetism and

Information Storage

Nobel Prize (2007)

Peter Grünberg Albert FertNobel Prize (1956)

Role of Electron in Modern Technology

Goal Multifunctional devicePut 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-

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

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.

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

Materials under Studies

Emerging Oxide Materials

• LaAlO3 /SrTiO3

• La0.7Ca0.3MnO3/YBa2Cu3O7

• La0.7Ca0.3MnO3/PrBaCu3O7

• BiFeO3/La0.7Sr0.3MnO3

• CaMnO3/CaRuO3

• LaMnO3/SrMnO3

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

When both Oxides meet face to face

Reyren et al. Science 317,1196(2007) Ariando et al Nat commun. 2, 188 (2011)

H(kOe)

(kΩ

cm-2

)

Magnetic Studies

Magnetic Measurement Tools

• SQUID (Overall magnetization of sample)

• Torque magnetometer

• XMCD (Elemental sensitivity)

• Magneto resistance

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)

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

Torque τ = M HDeflection 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 HTorque = M×H H2

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

• 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?

Microscopic Origin

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 )

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

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

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

Theoretical Predictions on the Origin of Magnetism

In absence of extrinsic magnetic impurities, interface ferromagnetic

originate from Ti atom.

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)

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?

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)

• 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-

Polar Catastrophe

• ½ electron per unit cell• Carrier density:3.5×1014 cm-2

Muller et al ,Nat. Mat.,5,204(2006)

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

Symmetry breaking at interface

eg

dz2

47 meV

dxy

3d

t2g

dxz/yz

dx2

-y2

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)

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

Recent studies on Oxide Interface

Approach towards Applications

LaALO3

LaALO3

Co

Conventional 3-T measurement technique

Spin accumulation at interface

Hanle effect: Change the voltage due to spin dephasing

Spin Injection

N. Reyren et al. PRL 108 , 186802(2012)

Spin Injection

Spin relaxation time=50psSpin diffusion length=1micrometre

A.Ohtomo et al. NATURE 427,423(2004)

Suitable material for D-S channel

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

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

Some Other possible Applications

L Li et al. Science 2011;332:825-828

40%

enhancement

Sensors

Photo detectors

Some Other possible Applications

Multifunctional Oxide Heterostructures, Oxford University Press (2010)

Thermoelectric

Solar cells

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

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.

I = coupling strength

0.6 eV for early 3d element

1.0 eV for late 3d element

Stoner criterion

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

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