Spin-dependent tunneling through organic molecules: spin...

The University of Alabama Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Spin-dependent tunneling through organic molecules:spin scattering by organic radicals

P. LeClairPhysics & Astronomy, MINT, University of Alabama

G.J. Szulczewski, A. Gupta, S. StreetWeihao Xu

Chemistry, MINT, University of Alabama

Dina Genkina, Abel DemisseREU interns


Organic Spintronics• Tunneling processes more crucial as length scales decrease

• Control with multiple degrees of freedom

• Spin transport & more general applications

• enable organic-based spintronics

• Organic materials are unique:- properties tailored through synthesis

• Fundamental understanding of transport- one molecular building block at a time- multiple degrees of freedom- interplay with optoelectronics?

• Create toolkit for device design

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Organic semiconductors



Alq3



H2TPPAlq3



H2TPPAlq3

Pc



PTCDA

H2TPPAlq3

Pc


Sample Preparation

SrTiO3

La0.67Sr0.33MnO3film (50-100 nm)


Sample Preparation

SrTiO3


Pattern


Sample Preparation

SrTiO3


Pattern Clean with~0.1% Br2

in ethanolfor 30 sec


Sample Preparation

SrTiO3


Pattern

Vapor deposit organic

Clean with~0.1% Br2



Sample Preparation

SrTiO3


Pattern


Vapor deposit Co and Al through shadow mask

Clean with~0.1% Br2



Sample Preparation

SrTiO3


Pattern


Vapor deposit Co and Al through shadow mask

“Cross-bar” Area = 0.10 mm2

Clean with~0.1% Br2



Current-voltage and conductance curves

11 K LSMO

Co

15nm TPP

Xu et al. Appl. Phys. Lett. 90, 072506 (2007)

0.5 K

superconducting electrodesconfirms tunneling


Normalized MR versus LSMO surface polarization

0

0.25

0.50

0.75

1.00

1.25

0 60 120 180 240 300

Norm

aliz

ed M

R

Temperature (K)

LSMO surface polarization (Park)Normalized MR for LSMO/TPP/Co

Park et al. PRL 81, 1953 (1998)

no apparent spin loss through TPP

T dependence just LSMO

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

• thin film SC in H (~2-3T) Zeeman split quasiparticle DOSmeasure dI/dV ~ NscNfm

• nonmagnetic : symmetric -- equal # of spins

• magnetic: asymmetry = spin polarization

robust, microscopic theory

“directly” gives spin imbalance of tunneling current

Meservey-Tedrow tunnelingbarrier

FM

SC


T= 0.36 KH= 3 TH= 0 TT = 500mKP ~ 37%H = 0H = 3T

Al / AlOx / TPP / Copolarization is not decreased by TPP


increased conductance at low bias, low T ...

evidence of magnetic excitations!

dI/dV

Vmagnon scattering


Magnetoresistance of Co/20 nm TPP/LSMO

Xu et al. Appl. Phys. Lett. 90, 072506 (2007)

sizable MR observed

up to 40% at 10K


so what’s the problem?

• still to complicated• need a simpler starting point for structure-

property correlation

• so we go ‘back to the future’ … (again)


Molecular Tunnel Junctions• One layer of a simple, flexible molecule - benzoic acid derivatives• Model system for more complex molecules - ‘scaffolding’• Correlate transport and molecular structure

organic

pinned FM

free FM

Al2O3

TransportConductance (V,H,T)

Spin-polarized tunneling

StructurePhotoemission (UPS,XPS)

Raman/IRInelastic electron tunneling


Molecular Tunnel Junctions• One layer of a simple, flexible molecule - benzoic acid derivatives• Model system for more complex molecules - ‘scaffolding’• Correlate transport and molecular structure

organic

pinned FM

free FM

Al2O3

TransportConductance (V,H,T)

Spin-polarized tunneling

StructurePhotoemission (UPS,XPS)

Raman/IRInelastic electron tunneling

Self-assembly yields well-ordered monolayers

AlAl2O3


Sample Preparation

Al (~10 nm)shadow mask


Sample Preparation


deposit benzoic acid from solution


Sample Preparation



Vapor deposit Co or Pb through shadow mask


Sample Preparation



Vapor deposit Co or Pb through shadow mask “Cross-bar”

Area = 0.10 mm2


IETS: is there transport through the molecule?• tunneling electrons excite vibrational modes

– see Raman & IR modes– no strong selection rules

• when eV ≥ hv– stepped increase in dI/dV– new set of final states available– unambiguous - transport through molecule

• example: studying model catalysts– CO on Rh/Al2O3 -- CO vibrations

“tunneling energy loss spectroscopy”

detail: – tiny signals … challenging








d2I/dV2

dI/dV

Vhv

Vhv


How to measure a derivative?

• drive system with Vdc + δVacsin ωt• measure response at ω

frequency / phase sensitive technique

… lock-in amplifiers (talk to Dr. Enders)

€

I(Vdc + δVac cosωt) = I(Vdc ) +dIdV

δVac cosωt( ) +d2IdV 2

δIac2

4cos 2ωt( )

+K


(1) original nice idea ...



DC switch20k40k400k1M

AC switch1k10k100k1M10M

pin board connector

1 ac input from SW

2 dc input from SW

3 BNC ground

4 -V

5 +V

6-10 NC

11 com/BNC ground

12 to pin 4 LH0063K

13to pin 3 LH0063K

feedback

U1-U3-U4-U5 = OP07U2 = LF351U6 = LH0063K

(2) the “Danny filter” ...



DC switch20k40k400k1M

AC switch1k10k100k1M10M

pin board connector

1 ac input from SW

2 dc input from SW

3 BNC ground

4 -V

5 +V

6-10 NC

11 com/BNC ground

12 to pin 4 LH0063K

13to pin 3 LH0063K

feedback

U1-U3-U4-U5 = OP07U2 = LF351U6 = LH0063K

(2) the “Danny filter” ...

(3) implementation ...

dI/dV: lock in to fd2I/dV2 (IETS): lock in to 2f

‘constant resolution’ modesoftware feedback to maintain

modulation

Rthing

Rprecise

DC

LIA

I & dI

V & dV+D

C

LIA

Vdc

Vac

Σ

IETS setup


Quantum Design PPMS R(T,H) T: 350mK -- 400K 3He fridge H: 0 -- 7T scriptable …

+

Home-built electronics dI/dV(V,T) d2I/dV2(V,T)

Transport facilities

The University of Alabama Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center IETS: we really have transport through the molecule

T = 2K


JASCO FT/IR-4100_ATRScan #: 128Resolution: 2 cm-1

ATR crystal

IR beamTo detector

Sample

1 umEvanescent wave

ATR-FTIRwhat is the state of the molecule?is it still there?


ν: stretch; β: in-plane bend; γ: out-of-plane bend.

COO- symmetric modes - monodentate

T = 2K


AlAl2O3

AlAl2O3

Co Co

I III

I 3d

N 1s Al/Al2O3/BA/(Co or Pb)

Co reacts with heavier halogens

Pb does not

halogen substitution leads to organic radicals


Conductance Through Benzoic Acids• Vary para- substituent

• Controlled introduction of radicals

AlAl2O3

AlAl2O3

Co Co

I III


size of zero-bias anomaly scales with degree of reactiononly very tiny effects for fluorobenzoic acid (or cyano)slightly larger for chlorobenzoic acidhuge for iodobenzoic acid


when we have a reaction …

… there is an unpaired electron

… which means an unpaired spin

… which gives a Kondo resonance(TK~20K)

(wacky quantum interference)

(many body effect)


H=0

H=7T

in a field, Kondo peak suppressed small “well” carved out of it ...

Al/Al2O3/ 4-iodobenzoic acid / Co T = 2K


Spin-dependent Scattering by Organic Radicals• Organic radicals = unpaired electron• Should have ‘normal’ spin flip scattering• Revealed through application of H

fit gives g~2 … consistent


More details: • fit to high field data gives g ~ 2 … expected• signature logarithmic (V,T) dependence of Kondo peak in H=0• Kondo, H-dependent part are small terms

V-independent parts much largerspin scattering dominates conductance


Building a Molecular Spin Transport Toolkit

• Porphyrin derivatives (E. Galoppini, Rutgers)

- control orientation- Dr. Choe ...

• Phthalocyanine derivatives -CoPc, F16CuPc-tune spin-orbit scattering via TM ion

• Lanthanide complexes- RE3+ controls spin dynamics- strong exchange


• new families of (known) moleculesspin transport is unknown

• build knowledge block by block

• systematic variation

• new tools and synergy transport ↔ structure

• designer toolkit for spin transport

• Spin-dependent scattering dominates with radicals- General mechanism!

Building a Molecular Spin Transport Toolkit

thanks to: NSF MRSEC grant No. DMR 0213985.


Organic Spintronics• Understand spin injection & transport• Enable organic-based spintronics









organicpinned FM

free FM

Two terminal: OLED, MTJ, spin valve






organicpinned FM

free FM

Two terminal: OLED, MTJ, spin valveThree terminal: FET, logic, memory

organic Al2O3

FM FM

gate

Spin-dependent tunneling through organic molecules: spin...

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