Organic Organic SpintronicsSpintronics

Himadri S MajumdarHimadri S Majumdar, , Sayani Majumdar Sayani Majumdar and Ronald Österbackaand Ronald Österbacka

Department of PhysicsDepartment of PhysicsCenter of Excellence for Functional Materials (FunMat)Center of Excellence for Functional Materials (FunMat)

Åbo Akademi University, Turku, FinlandÅbo Akademi University, Turku, Finland

OutlineOutline• Spin and Spintronics

• Why organics in spintronics

• Spins in Organics : Light-emitting diodes (OLED)

Sivu 231.8.2009

• Spins in Organics : Light-emitting diodes (OLED)

• Novel device concepts

• Summary

SpinSpin

Sivu 331.8.2009

Spin: In bulkSpin: In bulk

Paramagneticmaterials

Sivu 431.8.2009

Ferromagnetic materials

SpinSpin--based Electronics (Spintronics)based Electronics (Spintronics)

Sivu 531.8.2009

Hard-Disk technology

Binary information storage that has• Non-volatility• High integration density• Fast switching time• Low power consumption

Why Spins in Organics? Why Spins in Organics?

Two main perturbing factors of spin orientation inbulk are -

• Spin-Orbit interaction: Interaction betweenelectron spin and nuclear charge.

Sivu 631.8.2009

• Hyperfine interaction: Interaction betweenelectron spin and nuclear spin.

§Both effects are stronger for heavier atoms

§Organic molecules : Lighter atoms, better alternative

HSO α Z4

Spins in Organics Spins in Organics -- OLED OLED

ITOPEDOT:PSS

Organicsemiconductor

LiFAl

Sivu 731.8.2009

• Invented in 1987, this is the mostsuccessful organic electronic devicetill date.

http://www.sony.fi/article/id/1219306065993http://www.akihabaranews.com/en/news_9619.html

Tang CW, VanSlyke SA (Sept 1987)."Organic electroluminescent Diodes".Applied Physics Letters 51 (12): 913–915

Cathode

Anode

h

Spins in Organics Spins in Organics -- OLED OLED

ITOPEDOT:PSS

Organicsemiconductor

LiFAl

e hCoulombically

he

1,3

Sivu 831.8.2009

e hCoulombically bound e-h pair

e hExciton

e h e h

• One spin-singlet andthree spin-tripletsboth in e-h pairs andexcitons.

1 3

1,3

• Only excitonic spin-singlets radiatefluorescent light.

Limitations of OLED Limitations of OLED

Fluorescence Phosphorescence

Sivu 931.8.2009

• Theoretical limit of 25% on emittedfluorescent light.

• Experimental observation of > 25% invarious cases.

• Room for improvement

M. Wohlgenannt et al., Nature 409 (2001), p. 494

Why not singlets onlyWhy not singlets only

ITOPEDOT:PSS

Organicsemiconductor

LiFAl

Ferromagnetic electrodePEDOT:PSS

Organicsemiconductor

LiFFerro. electrode

Sivu 1031.8.2009

Spin Valve OpenCurrent High

Ferromagnet 2

Ferromagnet 1

Spacer layer

Spin Valve CloseCurrent Low

Spin-valves – as used in hard-disk technology!!

SpinSpin--valvevalve

Spin Valve OpenCurrent High

Ferromagnet 2

Ferromagnet 1

Spacer layer

Spin Valve CloseCurrent Low

Sivu 1131.8.2009

Magnetic field (B)

Magnetoresistance(MR)

Spin Valve OpenCurrent High

LSMO

CobaltP3HT

Spin Valve ClosedCurrent Low

Organic spinOrganic spin--valvesvalves

31.8.2009 Sivu 12

• La0.67Sr0.3MnO3 (LSMO) – a 100% spin polarized half-metal used as the bottom electrode

• Poly (3-hexylthiophene) (P3HT) as spacerüGood hole transporter

• Co as the top electrode.

Park et. al. Nature 392, 794 (1998).

Majumdar et al. Appl. Phys. Lett. 89, 122114 (2006)

R (M

Ω c

m-2)

40

80

120 T = 5K

0,3

0,4

0,5

0,6

MR

(%)

Spin polarized transportSpin polarized transport

• Spin-injection, spin-transport and spin-detection proved in organic electronic devices.

• Small but distinct signature

Sivu 1331.8.2009

00,3

MR

(%)

R (K

Ω c

m-2)

-300 -200 -100 0 100 200 300-2

0

2

4 T = 300K

B (mT)

23,5

24,0

24,5

Majumdar et al. Appl. Phys. Lett. 89, 122114 (2006)

• Small but distinct signature at room temperature.

Spin-polarized transportthrough organic materials ispossible!!

ITOPEDOT:PSS

Organicsemiconductor

LiFAl

Organic magnetoresistance (OMAR)Organic magnetoresistance (OMAR)

• Large room temperature magnetoresistance in OLEDs.

ITO

T. L. Francis, et al., New J. Phys. 6, 185 (2004)

© Univ. of Iowa

AlP3HT

Ag

Sivu 1531.8.2009

Charge pair(double carrier)

Bipolaronic (single carrier, double occupancy)

P. A. Bobbert et al., Phys. Rev. Lett. 99, 216801 (2007)

Understanding of OMARUnderstanding of OMAR

e h

he

1,3

16

(double carrier)Bin Hu et. al., Nature Materials 6, 985 (2007)

e h

e h e h1 3

Inter-systemcrossing

1,3

MIST model (spin – spin interaction)

Prigodin et al., Synth. Met. 156, 757 (2006)

10-1

100

101

| %

MR

|

P3HT

10-1

100

101

| %

MR

| No exciton, but e-h pair(MDMO-PPV:PCBM)

+

OMAR : our observationsOMAR : our observations

17

§ P3HT diodes compared with P3HT:PCBMdiodes.

§ Lesser probability of Electron (e)-hole (h) pairformation leads to reduced MR

S. Majumdar et al., Phys. Rev B (R), 79, 201202 (2009)

-300 -200 -100 0 100 200 30010-2

B (mT)No exciton, no e-h pair

(P3HT:PCBM)

-300 -200 -100 0 100 200 30010-2

B (mT)

+

e h

e h

e

OMAR : our observationsOMAR : our observations

1,3

1,3 10-1

100

101

| %

MR

|

e h

e h e h31

The formation process of the e-h pairsor the e-h pairs are the origin of OMAR

S. Majumdar et al., Phys. Rev B (R), 79, 201202 (2009)

-300 -200 -100 0 100 200 30010-2

10| % M

R |

B (mT)

SummarySummary

• Spin and Spintronics will pave way for future electronics.

• Organics in spintronics is a fundamentally logical and plausible goal.

Sivu 1931.8.2009

and plausible goal.

• Spin-polarized transport in organics is proved, even at room temperature.

• OMAR phenomenon opens possibilities of a new application of an existing technology.

Acknowledgements

– Academy of Finland Centre of Excellence Program

– Academy of Finland projects 116995 and 107684

– Wihuri Foundation

Thank you!

Sivu 2131.8.2009