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Spin Effects in OrganicSpin Effects in OrganicSemiconductorsSemiconductors
Sayani Majumdar
Wihuri Physical Laboratory, Department of Physics and AstronomyUniversity of Turku
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WihuriWihuri Physical LaboratoryPhysical Laboratory
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Magnetism and Spintronics study at WPLMagnetism and Spintronics study at WPL
• Preparation of bulk and thin film samples
• Structural characterization – XRD, AFM
• Magnetic Measurements (also under light and electric field)
• Transport and Magneto-transport Measurements
• Fabrication of Spintronic components (in collaboration with ÅA and MIT)
• Characterization of Spintronic components
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SpintronicsSpintronics
Spin Valve OpenCurrent High
Ferromagnet 2
Ferromagnet 1
Spacer layer
Spin Valve CloseCurrent Low
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Spin injection, transport and detection
• Spin polarized Injection
• Spin polarized transport
• Spin polarized detection
FM1 FM2NM
Injection Detectiontransport
spin flipspin polarized transport
FM2NM
Detection
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Spin injectionSpin injection –– Half metalsHalf metals
Park et. al. Nature 392, 794 (1998).
• Advantages:• Half metals (LSMO) have very high net spin
polarization ~ 100%• Stable oxides• Lesser conductivity mismatch
• Disavdantages:• Lower Tc and loss of spin polarization at the
surface• surface roughness
• Sollutions:• Higher Tc half metals (SFMO)• reducing surface roughness by optimization
of thin film growth parameters• protecting the surface
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Spin transporterSpin transporter• Spin-flips mainly due to –
• Spin-Orbit interaction: Interactionbetween electron spin magneticmoment and orbital angular momentum.
• Hyperfine interaction: Interactionbetween electron spin and nuclear spin.
Both effects are stronger for heavier atoms
Organic molecules : Lighter atoms, better alternative
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Organic SemiconductorsOrganic Semiconductors
• Advantages - chemical tuning of electronicfunctionality.
• Easy structural modification.• Ability of self assembly.• Mechanical flexibility.• Large Area and low cost electronic applications.• Thin film technology does not require high
temperatures and lattice matching.
• Challenges – unstable in air• Highly resistive• Reproducibility
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Hybrid spintronicsHybrid spintronics –– best of both worldbest of both world
• Half metallic spin injectors – La0.7Sr0.3MnO3, Sr2FeMoO6
• Better spin transporting materials – organic semiconductor small molecules,Polymer, Graphene, Carbon naotubes
• Different spintronic components – spin valves, magnetic tunnel junctions,magnetic sensors, spin LEDs, spin transistors and ....
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Organic SpintronicsOrganic Spintronics
• Organic spin valves
• Organic magnetic tunnel junctions
• Organic magneto resistance in OLED
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Organic SpinOrganic Spin--valvesvalves
Dediu et. al. Solid state comm. 122, 181 (2002).
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Organic SpinOrganic Spin--valvesvalves
Xiong et. al. Nature 427, 821 (2004).
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Polymeric SpinPolymeric Spin--valvesvalves
-300 -200 -100 0 100 200 3000.2
0.3
0.4
0.5
0.6
0.7
R (M
cm
-2)
B (mT)
0
40
80
120T = 5K
% M
R
RR-P3HT
• Room temperatureoperation of Organic spin valves.
-300 -200 -100 0 100 200 300108.0
108.4
108.8
109.2
109.6
110.0
110.4
-0.5
0.0
0.5
1.0
1.5
R (K
)
B (mT)
T = 300K
% M
R
Majumdar et al., Appl. Phys.Lett. 89, 122114 (2006).
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Spin injection in OrganicsSpin injection in Organics
• To achieve significant spin-current injection,the OS must be driven far out of localthermal equilibrium by an electric current.
• If the injecting contact has metallicconductivity, its electron distribution cannotbe driven far from thermal equilibrium bypractical current densities.
• Quasi-equilibration between the conjugatedOS and the metallic contact must besuppressed to achieve effective spininjection.
• This requires a spin-dependent barrier toelectrical injection that may be either due totunneling through the depletion region of alarge Schottky barrier or due to tunnelingthrough a thin, insulating, interface layer.
Ruden and Smith, J. Appl. Phys. 95, 4898 (2004).
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160 165 170 175
LSMO/ODTS/P3HT
LSMO/HMDS/P3HT
LSMO/P3HT
s2s1 S 2p
Pho
toel
ectro
n in
tens
ity (a
rb. u
nits
)Binding energy (eV)
(a)
The FMThe FM-- Organic interfaceOrganic interface
Pure P3HTP3HT/LSMO
Majumdar et al., Appl. Phys.Lett. 89, 122114 (2006).
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The FMThe FM-- Organic interfaceOrganic interface
Zhan et al., Phys. Rev. B 76, 045406 (2007).
• Introduction of Alq3 on LSMO creates a strong interface dipole of 0.9 eV.
• Energy level shift of Alq3 with respect to the vacuum level makes electron injectioninto Alq3 more favorable than hole injection.
• Interface of Alq3/Co on the detector side of the SV show a shift of about 1 eV.
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Loss of injection with temperatureLoss of injection with temperature
• Irrespective of the spin injecting electrode, the SV response with increasedtemperature decreased substantially.
• Spin polarizaion is lost at the LSMO/OS interface.
Majumdar et al., J. Appl. Phys. 104, 033910-1(2008).
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Loss of injection with temperatureLoss of injection with temperature
• OS Small molecules also showed similar temperature dependence of SVresponse.
• Spin polarizaion is lost at the LSMO/OS interface.
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Spin transport and relaxationSpin transport and relaxation
• SP carriers injected into the OS, travel bydrift and diffusion under the influence of anelectric field.
• During transport, the SP carriers interactwith their environment (trapping, spinprecession around a local hyperfine field)and their initial spin direction is lost.
• Different spin relaxation length in OS isreported.
• The effect of disorder and impurity of theOS can play a major role.
Majumdar et al., Comprehensive Nanoscience and Technology, 2011, Vol. 1, 109-142.
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Effect of impurity on spin transportEffect of impurity on spin transport
Vinzelberg et al., J. Appl. Phys. 103, 093720-1(2008).
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Spin detectionSpin detection
Majumdar et al., New J. Phys. 11, 013022-1(2009).
• Depending on top electrode penetration inOS, MR response changed substantially.
• Two spin transport channels were detected.
• For improved performance it is essential tohave a well-defined interface.
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Organic MTJOrganic MTJ
• Sizable Room-temperature TMR response was observed.
• SP tunneling transport measurement showed more than 35% spin polarization ofthe Py/OS interface and spin injection in OS.
• 13 nm spin relaxation length in OS small molecule Rubrene was measured.Santos et al., Phys. Rev. Lett. 98, 016601 (2007).
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LSMO based organicLSMO based organic MTJsMTJs
• LSMO/Alq3/Co nano MTJ
- 300% TMR with high bias and temp. dependence- TMR vanishes beyond 50 mV and 150K- Reproducibility - 65% working samples• Among those, 20% showed measurable magnetoresistance from 10 to 300%.
C. Barraud et al., Nature Phys.6, 615 (2010)
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LSMO based organicLSMO based organic MTJsMTJs
• LSMO/Rubrene/V(TCNE)x(J-W.Yoo et al., Nature Mat. 9, 638 (2010)
- ~2 % TMR at 100 K with high bias and temp. dependence- Below 100K, too high junction resistance- ~1% TMR reported till 150K
• LSMO/LaO/Rubrene/FeJ-W.Yoo et al., Synth Mat. 160, 216 (2010)
- ~10 % TMR with high bias and temp. dependence at 10 K- Above 200 K, TMR vanished
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LSMO based organicLSMO based organic MTJsMTJs
• LSMO/Rubrene/Co MTJ(Majumdar et al., Unpublished)
- ~ 8 - 14 % TMR observed at low temperature- Bias dependence is not consistent from device to device- Measurable TMR was observed at least until 150 K- LSMO thickness plays a big role in the TMR response of the MTJs
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MR without FM electrodesMR without FM electrodes -- OMAROMAR
Francis et al., New Journal of Physics 6: 185-1 (2004).
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Conclusions and future researchConclusions and future research
• Due to small spin orbit coupling and hyperfine interaction OS are promisingas spin transporting materials.
• Spin injection and transport have been successfully demonstrated inorganic spintronic devices.
• Any defect in the OS layer can modify the spin transport propertiessignificantly.
• Search is ON for higher spin injecting electrode and better spin transporterlike carbon nanotubes and graphene.
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• Turku Collegium for Science and Medicine• Wihuri Foundation• Academy of Finland
AcknowledgementsAcknowledgements