Himadri S Majumdar, Sayani Majumdar and Ronald Österbacka · • Spin and Spintronics will pave...
Transcript of Himadri S Majumdar, Sayani Majumdar and Ronald Österbacka · • Spin and Spintronics will pave...
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)
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• Spins in Organics : Light-emitting diodes (OLED)
• Novel device concepts
• Summary
SpinSpin
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Spin: In bulkSpin: In bulk
Paramagneticmaterials
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Ferromagnetic materials
SpinSpin--based Electronics (Spintronics)based Electronics (Spintronics)
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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.
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• 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
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• 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
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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
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• 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
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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
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Magnetic field (B)
Magnetoresistance(MR)
Spin Valve OpenCurrent High
LSMO
CobaltP3HT
Spin Valve ClosedCurrent Low
Organic spinOrganic spin--valvesvalves
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• 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
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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
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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
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§ 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.
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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!
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