Molecular SpintronicsMolecular Spintronics

S. K. NayakS. K. NayakDepartment of Physics, Applied Physics, and AstronomyDepartment of Physics, Applied Physics, and Astronomy

Rensselaer Polytechnic Institute, TroyRensselaer Polytechnic Institute, Troy

CollaboratorsCollaborators

Dr. R. PatiDr. R. PatiL. SenapatiL. Senapati

M. MailmannM. MailmannY. ZhangY. Zhang

PhysicsPhysics, , RPIRPI

Professors P. Ajayan and G. RamanathProfessors P. Ajayan and G. RamanathMat. Sci. and Eng.,Mat. Sci. and Eng., RPIRPI

Professor A. M. Rao,Professor A. M. Rao, Clemson University Clemson University

Y. Wu, Dr. P. Giannozzi, Professor R. CarY. Wu, Dr. P. Giannozzi, Professor R. CarPrinceton UniversityPrinceton University

Professor N. Marzari,Professor N. Marzari, MITMIT

Professors R. Reifenberger and Datta, PurdueProfessors R. Reifenberger and Datta, Purdue

Scope of the TalkScope of the Talk

• IntroductionIntroduction

• Fundamental QuestionsFundamental Questions

• Technological ApplicationsTechnological Applications

• Spintronics at the Molecular LevelSpintronics at the Molecular Level

• Experimental resultsExperimental results

• Theoretical ResultsTheoretical Results

NanoelectronicsNanoelectronics Moore’s LawMoore’s Law

Device sizes halve every 5 Device sizes halve every 5 yearsyears

This law, observed in This law, observed in the 60’s, still holds the 60’s, still holds todaytoday

By Moore’s law, devices By Moore’s law, devices should reach atomic scale should reach atomic scale by 2025by 2025

Moore’s law will Moore’s law will come to an end by come to an end by 2020.2020.

- - - - - - - - - - - - - - -

- - - - - - - - - - -

M

Si

SiO2

-

To achieve dramatic, innovative enhancements in the properties and performance of structures, materials, and devices that have controllable features on the nanometer scale (i.e., tens of Å).

The ability to affordably fabricate structures at the nanometer scale will enable new approaches and processes for manufacturing novel, more reliable, lower cost, higher performance and more flexible electronic, magnetic, optical, and mechanical devices.

NanoScience and NanoScience and NanoTechnologyNanoTechnology

DoD SRA

http://www.nanosra.nrl.navy.mil

Atoms in a small world

What I want to talk about is the problem of manipulating and controlling things on a small scale.

When we get to the very, very small world---say circuits of seven atoms---we have a lot of new things that would happen that represent completely new opportunities for design. Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics. So, as we go down and fiddle around with the atoms down there, we are working with different laws, and we can expect to do different things. We can manufacture in different ways. We can use, not just circuits, but some system involving the quantized energy levels, or the interactions of quantized spins, etc.

There's Plenty of Room at the Bottom

An Invitation to Enter a New Field of Physics

by Richard P. Feynman

December 29, 1959, APS Annual meeting

NanoScience and NanoScience and NanoTechnologyNanoTechnology

Materials and Phenomena at Nanometer-scale (10-100 Å) Offer the Opportunities to Realize Electronics Devices with Unprecedented Performance

--

--

-- ++

Self-assembled InAs QDs on GaAs substract

30 nm

30 nm

Dot feature size 5-6 nm

Nano-EnabledNano-EnabledRevolutionary Revolutionary

CapabilityCapability Nanoelectronics and Computer TechnologyNanoelectronics and Computer Technology

Monolithic electro optic devices that Monolithic electro optic devices that detect the entire infrared (SWIR-VLWIR) detect the entire infrared (SWIR-VLWIR) spectrumspectrum

Ultra-high performance massively parallel Ultra-high performance massively parallel data processors to allow downlinking data processors to allow downlinking target information directly to the target information directly to the warfighter (e.g., molecular computers)warfighter (e.g., molecular computers)

Novel communication devices providing Novel communication devices providing unheard-of frequencies and bandwidthunheard-of frequencies and bandwidth

Single-Molecule Electron Devices

•R. Metzger et al., Thin Solid Films, 327-329, 326 (1998)

•Rectificatoin of current demonstrated by LB films (mono and multilayers) of -(n-hexadecyl)quinolinium tricyanoquinodimethanide, C16H33Q-3CNQ

•Collier et al., Science 285, 391 (1999)

•Logic operations (AND and OR) demonstrated by Rotaxane monolayer sandwitched between Ti and Al2O3

•Moresco et al., Phys. Rev. Lett. 86, 672 (2000)

•Current switching by STM manipulation of Cu-tetra-3,5 di-ter-butyl-phenyl porphyrin (Cu-TBPP) on Cu (211) surface: Possibly change in intra-molecular conformation

•J. Chen, et al, Appl. Phys. Lett. 77, 1224 (2000)

•Room temperature negative differential resistance (NDR) exhibited by self-assembled monolayers of nitroamine and nitro substituted di(ethenylphenyl-benzene thiolate

Single-Molecule Devices

•C-Nanotube Based Electronic Devices•Explosion in the field: Andriotis et al, Phys. Rev. Lett. 87, (Aug 2001)

•Bio-molecules Based Electronic Devices•Fink and Schoenenberger, Nature 398, 407 (1999)Conduction through DNA molecules

•Photo-activated Molecular Devices•A. P. de Silva et al., J. Am. Chem. Soc. 122, 3965 (2000) Fluoresecent based moleular logic and arithmatic

•Nagatoshi et al., Nature 401, 152 (1999)Light-driven mono-directional molecular rotor

•Bermudez, et al., Nature 406, 608 (2000)AC-field induced molecular rotor

para-nitroaniline (PNA)

DA

-electron medium benzene ring

H2N- -NO2

+

qq

Organic molecules offer a natural medium for controlled electron transport

WHY ORGANICS?

Can be used as

Basic Device Elements

-wire (connectors)

-insulator

-diode (switch, memory)

-transistor

Molecular Electronics

CHALLENGES

•Understanding electron transport and device physics in molecular systems

•Conduction different from bulk: 1-electron, overlap of localized wavefunction, involvement of discrete energy levels, tunneling

•Interface with microscopic and real world

•Physics and chemistry of molecule-metal contact, assembly, fabrication, measurements and interpretation

OPPORTUNITIES

•True breakthroughs: Exploration of new science ==> Engineering

•Device Concepts

•Traditional electronics vs. new devices based on new physical mechanisms

Spintronics- (Spin Electronics): Telling the electron to Spintronics- (Spin Electronics): Telling the electron to remember its spinremember its spin

Electron has negative charge and spin (magnetic moment: 1/2).Electron has negative charge and spin (magnetic moment: 1/2).

Electron seen by an electronician:Electron seen by an electronician:

So far electronics industry are taking advantage of only its charge So far electronics industry are taking advantage of only its charge character to store and process information.character to store and process information.

Electronics ApplicationElectronics Application

Primary electronic device: MOSFET

Disadvantage

volatile of information

limited density information

reaching the fundamental limit

Spin Alone Phenomena- MagnetismSpin Alone Phenomena- Magnetism

Store information using spinStore information using spin

Alignment of spins are important Alignment of spins are important

Electrons seen by magnetician Electrons seen by magnetician

Magnetic ApplicationMagnetic Application

Primary magnetic Application: storage information

Disadvantage

mechanical access

Spin-electronics- Time for electron to take a spinSpin-electronics- Time for electron to take a spin

Exploit the quantum natureExploit the quantum nature

Combine charge and spin toCombine charge and spin to

store information in terms of spin orientation store information in terms of spin orientation (up/down)(up/down)

the spins will be attached to mobile electrons the spins will be attached to mobile electrons which will carry the information along with wirewhich will carry the information along with wire

the information will be read at the terminalthe information will be read at the terminal

Spin coherence length is large (~nm to Spin coherence length is large (~nm to m)m)

New ChallengesNew Challenges

Fundamental Questions- Injection of spins into semiconductors Spin coherence length (spin relaxation) Spin entanglement Interface effect

New Phenomena: Giant Magnetic Resistance: (GMR) Tunneling Magnetic Resistance (TMR)

New TechnologyNew Technology

magnetic disk heads (used in computer)

magnetic random access memories (M-RAM) (non volatile)

Forming Integrated Circuits- Ferromagnetic metal+semiconductor (still a challenge)

Spin-transistor

Spin Valve

Quantum Computer

Magnetic Tunneling Magnetic Tunneling Junction- MotorolaJunction- Motorola

Giant Magnetic Resistance (GMR)Giant Magnetic Resistance (GMR)

Baibich et al., PRL 61, 2472 (1988)Binasch et al., PRB 39, 4828 (1989)

GMR MechanismGMR Mechanism

RRFF R RAFAF

GMR Ratio = (RGMR Ratio = (RAFAF+R+RFF)/(R)/(RAFAF-R-RFF) )

could be larger than 50 %could be larger than 50 %

FerromagneticFerromagnetic

Anti-FerromagneticAnti-Ferromagnetic

Tunneling Magnetic Resistance (TMR)Tunneling Magnetic Resistance (TMR)

Moodera et al., PRL 74, 3273 (1995)

Applications of TMR: magnetic random access memories (M-RAM)

Injecting Spin Polarized Electrons in Injecting Spin Polarized Electrons in Semiconductor Semiconductor

Awschalom, Nature, 397 (1999)Awschalom, Nature, 397 (1999)

Spin-electronics at NanoscaleSpin-electronics at Nanoscale

Size of magnetic drive is Size of magnetic drive is also shrinking! also shrinking!

Reading the data through Reading the data through GMR needs to go to GMR needs to go to molecular scalemolecular scale

Spin-electronics at NanoscaleSpin-electronics at Nanoscale

Magnetic Reading HeadMagnetic Reading Head

Size of magnetic drive is also Size of magnetic drive is also shrinking! shrinking!

Reading the data through GMR needs Reading the data through GMR needs to go to molecular scaleto go to molecular scale

New Questions and New ChallengesNew Questions and New Challenges

Fundamental Interest:

Can we inject spins into molecules

Spin coherence length

Heating and time scale involved

Just a beginning ...

Coherent Spin polarized transport through carbon Coherent Spin polarized transport through carbon nanotubenanotube

Tsukagoshi Nature, 401, 572 (1999)Tsukagoshi Nature, 401, 572 (1999)

V I

Ni

SC

H

Gold

Molecules goes SpintronicsMolecules goes Spintronics

Schon, J. H., Science, Published online, I:10.1126/science.1070563 (2002).

Challenges- Challenges-

How to apply local magnetic field? How to apply local magnetic field?

First Principles Quantum Conductance Calculations of Spin Polarized Electron Transport in a Molecular Wire

THEORETICAL PROCEDURETHEORETICAL PROCEDURE

We solve Schrödinger equation:We solve Schrödinger equation: Hψ(xHψ(x11, x, x22, x, x33 …) = E ψ(x …) = E ψ(x11, x, x22, x, x33 …) …)

ψ is an N-electron wave function.ψ is an N-electron wave function.

A simple but accurate way of solving the above equation is to A simple but accurate way of solving the above equation is to use use density functional theory.density functional theory.

Here we work with ρ(r) : 3N to 3Here we work with ρ(r) : 3N to 3

Remarkable!Remarkable!

DENSITY FUNCTIONAL THEORYDENSITY FUNCTIONAL THEORY

HOHENBERG-KOHN (1964):HOHENBERG-KOHN (1964):

Total energy of an interacting electron gas in presence of an external Total energy of an interacting electron gas in presence of an external potential Vpotential Vextext (r): (r):

functional independent of Vfunctional independent of Vextext

KOHN-SHAM (1965):KOHN-SHAM (1965):

kinetic energykinetic energy exchangeexchange non interacting non interacting correlationcorrelation

Local Density Approximation (LDA): Local Density Approximation (LDA):

Gradient Corrected Approximation (GGA):Gradient Corrected Approximation (GGA):

][)()( FdrrrVEext

][||)()(

21)()(][

XCextsErdrd

rrrr

drrrVTE

drrrXC

)]([)(

drrFr )](,[)(

WORKING SCHEMEWORKING SCHEME

1-electron equations:1-electron equations:

where where

These equations are known as These equations are known as Kohn-Sham orbital equations.Kohn-Sham orbital equations.

KS equations KS equations have the similar form as Hartree equations- but have have the similar form as Hartree equations- but have correlation, in principle can work for all systems.correlation, in principle can work for all systems.

In practice, In practice, the present formalism works great for systems where the present formalism works great for systems where bonding is primarily chemical.bonding is primarily chemical.

Not successfulNot successful for weakly bound systems- attempts are underway. for weakly bound systems- attempts are underway.

)()(]},[||2

21{ r

iir

iFdrrrrZ

i i

r 2||)(

Calculation of CurrentCalculation of Current

1)( RightLeftMolHIEG

HMol: Molecular Hamiltonian

)( GGtrT nmmn

)( i

leftleftleftleft CGC rightrightrightright CGC

)]()([),(2

21

)1(

EfEfVETdEh

eI

eVE

eVE

f

f

aW. Tian, et al., J. Chem. Phys. 109, 2874 (1998)

Non-equilibrium Green’s Function Methoda

: Self-energy functionT: Transmission function

Ferromagnetic

Electron Spin Density Plot for anti-parallel Spin Alignment

Anti-Ferromagnetic

AF is lower in energy !AF is lower in energy !

I-V Characteristics For Different Spin Alignment

Down spins Down spins majority carriersmajority carriers

Conductance-Voltage Curve

0

5

10

15

20

25

30

35

40

45

50

-6 .5 -6 -5 .5 -5 -4 .5

E nergy (eV )

DO

S (1

/eV)

N i-(ll)-S p in U pN i-(ll)-S p in D ow nN i-(an ti)-S p in U pN i-(an ti)-S p in D ow n

Conductance-Voltage Curve

(A)

(B)

EF

EF

Conductance-Voltage Curve Spin Up are majority carriers- Spin Up are majority carriers- Spin valve effect is less.Spin valve effect is less.

H. Ohnishi et al, “Quantized Conductance through a chain of Gold atoms” Nature 395, 780 (1998).

Oscillatory MR is Atomic WiresOscillatory MR is Atomic Wires

A. I. Yanson et al , “Formation of atomic gold wires” Nature, 395 783 (1998).

CCo Au

V I

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1 2 3 4 5 6

Number of C-atoms in Atomic wire

E

(eV

)

Ferro- - 1.687 Å 1.294 Å 1.294 Å 1.687 Å

Anti ferro-1.692 Å 1.293 Å 1.293 Å 1.692 Å

Ferro—1.736 Å 1.269 Å 1.317 Å 1.269 Å 1.736 Å

Anti ferro-1.82 Å 1.245 Å 1.353 Å 1.245 Å 1.82 Å

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6

Number of C- Atoms in Atomic wire

G(E

f)

(ll)-Spin Up

(ll)-Spin Down

(anti)-Spin Up

(anti)-Spin Down

(ll)-Total

(anti)-Total

Transistor with different Oxidation State Transistor with different Oxidation State State- Experimental Results:State- Experimental Results:

[Co(tpy-(CH[Co(tpy-(CH22))55-SH)-SH)22]]2+ 2+ (longer molecule)(longer molecule)

I–V curves of the single-electron transistor as gate voltage is varied: from -0.4 V (red) to -1.0 V (black) in increments of -0.15 V.

Park, et. al. Nature, 417, Park, et. al. Nature, 417, 722, (2002)722, (2002)

Why Nanotube ?

Unique molecular structure, Highly Stable (Thermally and Chemically)

Very small dimension (nm-mm)

Some are metallic ;J~ 1011 Electrons per Sec-nm2 (Copper Wire, J~ 106

Electrons per Sec-nm2); Some are semi-conducting (Eg~1/DNT)

Technological applications: Nanoelectronic devices.

aJ. Kong et al, Science 287 (2000) 622.

21 amanc

Metallic:n=m, and n-m=3iSemiconducting:n-m 3i

Molecular and Nano-electronics

* Progress Towards Miniaturization* Searching for New Device Architectures* Developing Compatible Technology

Carbon Nanotubes

Metallic and Semiconducting

Conductivity found to be higher compared to the best metal

Transconductance of the nanotube is found to be twice that of conventioanal MOSFET

Arrays of nanotube transistors are shown to exhibit logic circuits.

New Devices and Geometries: Challenges

• 3-D Architectures, Growth, Integration

• Tailoring Nanotube Structure, Properties

• Making and Characterizing Junctions, Networks

Motivation

Achievements of Last Two Years

Fabricated aligned carbon nanotube arrays at desired locations on planar substrates using substrate templating and CVD with control over:

* Nucleation & Termination Sites of Nanotubes

* Surface-selectivity

Effect of Molecular Adsorbate on Transport Property: Nano Sensor

0 200 400 600

0.98

1.00

1.02

Res

ista

nce

nor

mal

ized

(O

hm

s)

Temperature (oC)

0 200 400 6000

10

20

30

40

O2

H2O

CO2

CO

H2

Par

tial

Pre

ssu

re (

Pa*

10-6)

Temperature (oC)

Theory: Effect of Molecular Adsorbate on Transport Property: Nano Sensor

Oxygen doping increases conductance.

Water decreases conductance.

-160

0

160

-5.5 0 5.5

Voltage (V)

Cu

rren

t (

A)

Ideal (3x3)Ideal (3x3-H2O)Ideal (3x3-2H2O)Ideal (3x3-5H2O)

-200

-150

-100

-50

0

50

100

150

200

-6 -4 -2 0 2 4 6Voltage (V)

Cur

rent

( A

)

Ideal (3x3)Ideal (3x3-3O2)

0

50

100

150

200

250

300

0 1 2 3 4 5

Voltage (V)

Cu

rren

t (

A)

Ideal (5x5)

Ideal (5x5-H2O)

Ideal (5x5-5H2O)

This shows that C60 provides additional path for current. Applied Physics Letters -- February 25, 2002 -- Volume 80, Issue 8, pp. 1450-1452

K. Harihara et al. PRL,85(2000)5384 has shown that GdC82 can be encapsulated inside nanotube. Experiment done by HRTEM.

Endohedral Doping-Magnetic atom inside nano-tube

V

Transport through Peapod

17x0 nanotube with buckyball inside

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

0.00 0.20 0.40 0.60 0.80 1.00

Voltage (V)

Cur

rent

(

A)

17x0

17x0-C60

0

10

20

30

40

50

60

70

-6.53 -6.03 -5.53 -5.03 -4.53

Energy (eV)

DO

S (1

/eV

)

17x0

17x0-C60

SummarySummary

• Spintronics offer a new way of store and transmit Spintronics offer a new way of store and transmit information.information.

• Molecules, Wires, Carbon Nanotubes are good Molecules, Wires, Carbon Nanotubes are good examples of studying spin assisted transport at the examples of studying spin assisted transport at the molecular levelmolecular level

Funding:Funding:

NSFNSF

NASANASA

SRCSRC