Jung-Woo Yoo

School of Materials Science and EngineeringUlsan National Institute of Science and Technology (UNIST)

2015년 동계학술대회

Outline

• Organic Semiconductor as a spin channel

• Organic-based magnet as a spin source

• Graphene – enhancing spin-orbit coupling

Applications:

Read Head, SensorsMRAMSpin-FET, Spin-LEDLogic Device

Phenomena:

GMR/TMRSpin transport and precessionSpin torque and dynamicsSpin-orbit coupling (Spin Hall,Topological Insulator,etc)Spin SeebeckMagnetic excitations (skyrmion,etc)

Why Organic for spintronic applications ?

• Low spin-orbit coupling ( ~ z4)- light constitute atoms, C and H

• Low hyperfine interaction ( ~ )- no nuclear spin in 12C- p electron carriers

• Greater flexibility of processing and new materials- chemical methodology

SI ×l

Spin transporting channel

Spin polarized carrier sources

• Conductivity mismatch• High spin polarization• Introduce greater functionality (magnetic bistability, photo- magnetism)• Greater flexibility of processing and new materials

- chemical methodology

Magnetoresistance of Organic spin valve (Nature 2004)

Organic SpintronicsMeasurement of spin diffusion via mSR (Nature Mater. 2009)

Organic spin-LED (Science 2012) Organic spin charge converter(Nature Mater. 2013)

Schematic Energy Diagram

LSMO Fe

f ~ 4.9 eV

Rubrene

f ~ 4.7 eV

f ~ 3.2 eV

E g ~ 2.3 eV

HOMO

LUMOAre the spins really transport through HOMO or LUMO ?

Need understanding of device characteristic associated with spin injection and transport in OSC !

Spin injection into HOMO and LUMO ?

LSMOLAO seed layer

OSC (Rubrene)Fe 30 nm

5-50 nm1.2 nm50 nm

Device size ~ 200 by 200 mm

PLDThermal depositionE-beam deposition

LSAT

Rubrene (C42H28) : only C & H

LSMO (La2/3Sr1/3MnO3) : well known half-metallic magnet

LAO (LaAlO3) :Improve metal/OSC interfacial quality (PRL 98 016601 (2007))

Organic spin valve

I vs V

T = 10 K

Vb (V)0.0001 0.001 0.01 0.1 1 10

log10

I (A

)

-11-10

-9-8-7-6-5-4-3

5 nm20 nm30 nm50 nm Instrumental limit

Low bias Rdc

T (K)0 50 100 150 200 250 300

log10

Rdc

5

6

7

8

9

20 nm30 nm50 nm

Tunneling vs Carrier Injection

R vs T

Vb = 0.1 V

T (K)0 50 100 150 200 250 300

log R

(Ohm

)

2

3

4

5

6

7

8

9

10

Instrumental limit

LSMO

Rubrene 5 nm

Rubrene 20 nm

Rubrene 50 nm

Rubrene 20 nm

1000/T (K-1)0 5 10 15 20

log(I) (A

)

-9

-8

-7

-6

-5

-4

DataFit

Vb = 0.1 V

Vb = 1 V

Vb = 0.5 V

Thermionic emission

Rubrene : 20 nm

Semiconductor

E

kBT

Metal

Thermionic field emission

Injection limit

Rubrene 50 nm

1000/T (K-1)0 5 10 15 20

log10

(I) (A)

-11

-10

-9

-8

-7

-6

-5

DataFit

Vb = 1 VVb = 0.5 V

Vb = 0.1 V

Instrumental limit

As T h a I ha thermionic emission

Bulk limit

Variable range hopping

Rubrene : 50 nm

Positive bias(LSMO +, Fe –)

Negative bias(LSMO +, Fe –)

Rubrene = 5 nm

Spin valve with thin OSC layer (TMR)

MR vs Vb

H (Oe)-1500 -1000 -500 0 500 1000 1500

MR

(%)

-202468

101214

V = 1 mV10 mV50 mV100 mV150 mV200 mV300 mV

MR vs Vb

H (Oe)-1500 -1000 -500 0 500 1000 1500

MR

(%)

-2

0

2

4

6

8

10

12 V = 1 mV10 mV50 mV100 mV150 mV200 mV300 mV

Yoo et al., PRB (2009)

H (Oe)-1000 -500 0 500 1000

MR

(%)

-202468

101214

T = 10 KT = 50 KT = 100 KT = 150 K

Vb = 1 V

H (Oe)-1000 -500 0 500 1000

MR

(%)

-2

0

2

4

6

8

10

10 K50 K100 K

MR vs T

T (K)0 50 100 150 200 250 300

MR

(%)

0

2

4

6

8

10

12

14Rubrene 5 nm,Vb = 10 mVRubrene 20 nm,Vb = 1 VRubrene 30 nm,Vb = 0.6 V

TMR

GMR

TMR vs GMRRubrene 5 nm

Rubrene 20 nm

Yoo et al., PRB (2009)

Surface spin polarization profile of LSMO

PRL 81 1953 (1998)

TMR vs GMR

MR vs Vb

Vb (V)0.0 0.5 1.0 1.5 2.0

MR

(%)

0

5

10

15

20

25

30

Rubrene 20 nmRubrene 30 nm

MR vs Vb

Vb (V)-0.4 -0.2 0.0 0.2 0.4

MR

(%)

2

4

6

8

10

12

14

T = 20 K

Rubrene 5 nm Rubrene 20 nm

• TMR for thin rubrene layer (t = 5 nm)- Positive MR for all T and Vb , spin selective tunneling for thin layer

• GMR for thick rubrene layer (t > 20 nm)- Strong T dependence- Positive MR at high Vb- At low T, only high Vb limit is the current through the HOMO and LUMO

• Thermionic Field Emission

• For d = 20 nm a Injection a MR

• For d = 50 nm a bulk a no MR

Field emission

Thermionic Emission

Thermionic Field Emission

Yoo et al., PRB (2009)

Electrical bistability

Yoo et al., Org. Electron. (2010)

Bistable spin valve

Yoo et al., Org. Electron. (2010)

Two Channel Model(a) (b)

Ri Rh

RhRhRh

RiRi

Ri

(c) (d)

Ri Rh

RhRhRh

RiRi

RiRh;N

Ri;N

Rh;N

Ri;N

Rh Rh

Ri Ri

Rh Ri

Ri Rh

For rubrene 20 nm device a injection limited and interface R dominate device current

By adding insulating barrier or schottky barrier

For t > 50 nm, a injection limited, but bulk R dominated device currenta conductivity mismatch, hopping transport, thermionic emission !

Schmidt et al. PRB (2000)

Spin polarized carrier sources

• Conductivity mismatch• High spin polarization• Introduce greater functionality (magnetic bistability, photo- magnetism)• Greater flexibility of processing and new materials

- chemical methodology

Molecular magnets as a spin source ?

Molecule-based Magnets

• Building block : Molecular units

• Spins in p, s orbitals as well as d orbitals

• Magnetic interaction : superexchange, direct exchange, dipolar

• Spin polarized valence and conduction bands

• Low-T synthesis

• Modulation/tuning of properties by means of organic chemistry- Tc, Hc

- Magnetic ordering (ferro-, ferri-, antiferro-, spin glass) - Dimensions (1-D, 2-D, 3-D)- Types of spin (Ising, XY, Heisenberg)

M(TCNE)x magnets (x ~ 2)V

V

C

C

C

N

N

C

C

C

N

N

C

N

C

N

C

N

C

N

C

N

C

N

C

N

C

N

N N

TCNQ

TCNB TCNP

JACS 116,7243 (1994)

C C

C

C

C

C

NN

NN

• Building block: Tetracyanoethylene (TCNE)M = V, Cr, Mn, Fe, Co, Ni

• Modulation of magnetic properties:MxM1-x(TCNE)yS2-y, S = CH3CN, THF

• Photo-induced magnetism: Mn(TCNE)2, V(TCNE)2Yoo et al., PRL 97, 247205 (2006)Yoo et al., PRL 99, 157205 (2007)

• Magnetic bubble in 2D layer : [Fe(TCNE)(NCMe)]Yoo et al., PRL 101, 197206 (2008)

• V(TCNE)x~2, room temperature magnetic semi-con.- Tc ~ 400 K, sRT ~ 10-2 S/cm- Ferrimagnetic order with net spin S = ½ (V2+ : S = 3/2, [TCNE]– : S = 1/2)

• low-T CVD film growth (Pokhodnya et al. Adv. Mater. (2000))

V

Half Semiconductor

Both spatially and energetically separated spin subbandsJ.-W. Yoo et al., Nature Mater. 9, 638 (2010)

a desired properties for the spin polarizer in the spintronic applications

Organic-based Magnet as a Spin Injector

LSMOLAO seed layer

OSC (Rubrene)Fe 30 nm

5-50 nm1.2 nm50 nm

Device size ~ 200 by 200 mm

PLDThermal depositionE-beam deposition

LSAT

Organic-based Magnet as a Spin Injector

LSMOLAO seed layer

OSC (Rubrene)V(TCNE)x

5 nm1.2 nm50 nm

Device size ~ 200 by 200 mm

PLDThermal deposition

LSAT

CVD deposited V(TCNE)x film~ 500 nm

Organic-based Magnet as a Spin Injector

LSMOLAO seed layer

OSC (Rubrene)V(TCNE)x

5 nm1.2 nm50 nm

Device size ~ 200 by 200 mm

PLDThermal deposition

LSAT

CVD deposited V(TCNE)x film~ 500 nm

DR/R vs H

T = 100 KVb = 0.5 V

H (Oe)-300 -200 -100 0 100 200 300

Magnetoresistance (%

)

0.0

0.5

1.0

1.5

2.0

2.5

M vs HV(TCNE)x

M vs HLSMO

0

1

-1

M(a. u.)

J.-W. Yoo et al., Nature Mater. 9, 638 (2010)

Hybrid Magnetic Tunnel Junction

DR/R vs H

T = 100 KVb = 0.5 V

H (Oe)-400 -200 0 200 400

Magnetoresistance (%

)

0

1

2

3LAO(3u.c.)LAO/Rubrene(3u.c.)/(5 nm)

I vs V

Vb (V)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

I (mA)

-100

-50

0

50

100

T = 100 KT = 120 KT = 140 KT = 160 KT = 180 KT = 200 K

R vs 1000/T

1000/T (K-1)4 6 8 10

logR

(Ohm

)

4

5

6

7

8

9

10

Vb = 0.1 VVb = 0.2 VVb = 0.5 V

Resistance (Ohm)104 105 106 107 108

MR

(%)

0.0

0.5

1.0

1.5

2.0

2.5Rubrene (5 nm)LAO (3 u. c.)LAO(3 u.c.)/Rubrene (5 nm)

Temperature dependence of MR

Vb = 0.5 V

T (K)50 100 150 200 250

MR

(%)

0.0

0.5

1.0

1.5

2.0

2.5

LAO/Rubrene(3 u.c.)/(5 nm)LAO(3 u.c.)

Surface spin polarization of LSMO

Hc vs T

Hc (Oe)-200 -100 0 100 200

T (K)

100

150

200V(TCNE)x LSMO

Rubrene (5 nm)/LAO (3 u.c.) hybrid barrier

Vb = 0.5 V

H (Oe)-200 -100 0 100 200

Magnetoresistance (%

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

90 K100 110120130150

LAO (3 u.c.) barrier

Vb = 0.5 V

H (Oe)-200 -100 0 100 200

Magnetoresistance (%

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 90 K100110120130

Hybrid barrier Single LAO barrier

1000/T2 4 6 8 10 12 14 16 18

Norm

alized resistance (W

cm)

102103104105106107108109101010111012

Normalized Device Resistance

Thermionic field emission

Device resistance

V(TCNE)x bulk

1000/T2 4 6 8 10 12 14 16 18

Norm

alized resistance (W

cm)

102103104105106107108109101010111012

Injection Bulk

~ 100 K

Normalized Device Resistance

Organic/Molecular Spintronics

- High spin polarization- Introduce greater functionality

(magnetic bistability, photo-magnetism)- Conductivity mismatch- Greater flexibility of processing and new materials

- chemical methodology

SI ×l- Weak spin-orbit coupling ( ~ z4)- Weak hyperfine interaction ( ~ )- Greater flexibility of processing and new materials

- chemical methodology

- Spin dependent resonant tunneling through the discrete molecular level

• Spin transporting through the organic thin film

• Spin transporting/filtering through the molecular unit

• Spin filtering via organic-based magnet

Graphene

)( yyxxF kkvH ss +±» h

Spin transport in GrapheneNature 448, 571 (2007)

In experimental observation

a μm 21~ -sl ps 100~st

Extremely long spin diffusion have been expected ( )mm 21~ -sl

Strains and ripples, moments from edges and adatom…….

Spin orbit coupling in graphene

Spin-orbit coupling in pristine graphene

~D

Chemical methods

Physical methods

μeV20 Phys. Rev. B 82. 245412 (2010)

Hydrogenation, Fluorination, etc

Phys. Rev. Lett. 110. 246602 (2013)

Phys. Rev. Lett. 103. 026804 (2009)

Carbon. 48. 1405 (2010)

Nature Physics 9. 284 (2013)

Decoration with heavy atom (Au, Pt, W, In, etc)

Proximity effect (WS2, WSe2, etc)

Phys. Rev. B. 77. 235430 (2008)

Phys. Rev. B. 90. 035444 (2014)

Nature Comm. 5, 4875 (2014)

Nature Comm. 5. 4748 (2014)Nature Comm. 3. 1232 (2012)

Enhancing Spin-orbit coupling in graphene ( )VkH soso Ñ´×= sh

Non-local measurement for spin diffusion

PRB, 79, 035304 (2009)

Generation(spin Hall effect)

sq ´CSHS JJ ~

Detection(inverse spin Hall effect)

sq ´SSHC JJ ~

The non-local resistance induced byspin Hall effect and spin diffusion

S

L

SSH

CNL ew

IVR l

lrq

-

== 212

21

ü Spin current without ferromagnet

Phys. Rev. Lett. 103, 166601 (2009)

1 µm

w = 110 nm t = 60 nm

a) In diffusive limit, wle <<

c) In ballistic limit, wle >>

b) Spin Hall regime, se lwl <<<<

a

a

a

S

L

SSH

CNL ew

IVR l

lrq

-

== 212

21

Negative signal

wLsqNL eROhmicR /)( p-= L/w > 3

Enhancing spin orbit coupling in graphene

Au pad

Au adatoms

sq ´CSHS JJ ~ sq ´SSHC JJ ~

The non-local resistance induced byspin Hall effect and spin diffusion

S

L

SSH

CNL ew

IVR l

lrq

-

== 212

21

-30 -20 -10 0 10 20 300

1

2

3

4

Vg(V)

r xx(k

W)

300 K

0.4

0.8

1.2

1.6

2.0

s (m

S)

CVD graphene ~ 2000 cm2/Vs

-40 -20 0 20 40 60

1

2

3300 K

Vg(V)

I d(mA)

-30 -20 -10 0 10 20 30

0.4

0.8

1.2

1.6300 K

Vg(V)

I d(mA)

Au adatoms Au pad

-15 -10 -5 0 5 10 15-20

-10

0

10

20

Inverse way

300 K

Vg = 0V

I(mA)V nl

(mV)

-15 -10 -5 0 5 10 15

-4

-2

0

2

4 300 K

I(mA)

V nl(m

V)

Vg = 0V

-10 -5 0 5 10

-4

-2

0

2

4

300 K

Vg = 0V

I(mA)

V nl(m

V)

Non-local signalLocal

CVD garphene

Non-local

-30 -20 -10 0 10 20

0

1

2

3

V nl (m

V)

Metal pad Pristine

Vg-Vd (V)

300KIs= 1 mA

Non-local signalGate dependence Non-local Resistance

-10 -5 0 5 10

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

Vg = 0V 300 K

I(mA)V nl

(mV)

-60 -40 -20 0 20 40 60

0

1

2

3

4

5

Vg(V)

V nl(m

V)

Dirac pointIs= 20 mA300K

Non-local Resistance induced by thermal effect

Au pad

Acknowledgment

Advisor : Prof. Arthur J. Epstein

Collaborators :

The Ohio State University

Dr. V. N. PrigodinDr. N. RajuDr. C.-Y. ChenDr. C. KaoDr. B. LiDr. D. P. Pejakovic

University of Wisconsin

Prof. C. B. EomProf. H. W. JangProf. C. W. Bark

University of Wisconsin

Prof. J. S. MillerDr. V. Pohkodnya

Group Members

me

Jungmin Park:Graphene Spintronics

Mi-Jin Jin:LAO/STO

Junhyun Jo:Organic Spintronics

Vijayakumar:Spin transport in nanowire

Dae-Sung ChoiOxide Interface

In-Sun OhThermoelectricity