Spin Effects in OrganicSpin Effects in OrganicSemiconductorsSemiconductors

Sayani Majumdar

Wihuri Physical Laboratory, Department of Physics and AstronomyUniversity of Turku

WihuriWihuri Physical LaboratoryPhysical Laboratory

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

SpintronicsSpintronics

Spin Valve OpenCurrent High

Ferromagnet 2

Ferromagnet 1

Spacer layer

Spin Valve CloseCurrent Low

Spin injection, transport and detection

• Spin polarized Injection

• Spin polarized transport

• Spin polarized detection

FM1 FM2NM

Injection Detectiontransport

spin flipspin polarized transport

FM2NM

Detection

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

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

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

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 ....

Organic SpintronicsOrganic Spintronics

• Organic spin valves

• Organic magnetic tunnel junctions

• Organic magneto resistance in OLED

Organic SpinOrganic Spin--valvesvalves

Dediu et. al. Solid state comm. 122, 181 (2002).

Organic SpinOrganic Spin--valvesvalves

Xiong et. al. Nature 427, 821 (2004).

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).

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).

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).

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.

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).

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.

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.

Effect of impurity on spin transportEffect of impurity on spin transport

Vinzelberg et al., J. Appl. Phys. 103, 093720-1(2008).

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.

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).

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)

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

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

MR without FM electrodesMR without FM electrodes -- OMAROMAR

Francis et al., New Journal of Physics 6: 185-1 (2004).

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.

• Turku Collegium for Science and Medicine• Wihuri Foundation• Academy of Finland

AcknowledgementsAcknowledgements