Dr I J Vera Marun's presentation Graphene Week 24th June

Graphene spintronics

Ivan J. Vera Marun

Outline

Motivation

Introduction to graphene spintronics

Limits on spin lifetime, substrate & functionalization

Spin transport in graphene nanostructures

From thermoelectrics to semiconductor spintronics

Potential spintronics applications

Motivation: electronic ABC

Explore different electronic degrees of freedom

Spin

HeatCharge

s

Qe-

Spintronics

20 yrs2007

Thermoelectrics200 yrs

transistor

Graphene an ideal system

2010

EU Flagship

Charge

Dirac spectrum Novoselov et al. Nature 438, 197 (2005)

Mobility > 100 000 cm2/Vs Bolotin et al. SSC 146, 351 (2008)

Spin

Long spin relaxation length Tombros et al. Nature 448, 571 (2007)

and spin lifetime Han et al. PRL 105, 167202 (2010)

Heat

Largest thermal conductivity Balandin et al. Nano Lett. 8, 902 (2008)

Large & tunable thermopower Zuev et al. PRL 102, 096807 (2009)

Long cooling length 2 μm Gabor et al. Science 334, 648 (2011)

Beyond charge: nonlocality

Novel functionality future electronics

Separate charge from other degrees of freedom

Nonlocal all-electrical measurements

Local charge current

Nonlocal pure spincurrents,valley,heat…

B: Spins

Magnetic contacts with polarization P for injection/detection

Nonlocal measurement of diffusing spins

Nonlocality very different from ballistic lmfp < λ < L

V+ -

I

Charge current

Nonlocal pure spin current

IPinj

μΔ

detPV

Only for magnetic materials P ≠ 0

L

w

RP=R

sq

NL exp2

2

FM1 FM2

FM1 FM2B

Rn

on

-local

Nonlocal spin valve

N. Tombros et al. Nature 448, 571 (2007 )

Bz

L

B

gD

t

B

S

S

2

Nonlocal Hanle spin precession

N. Tombros et al. Nature 448, 571 (2007 )

N↑ (E)N↓ (E)

E

FMNM

N↑ (E)N↓ (E)

E

EF

e_

λs

N↑ (E)N↓ (E)

E

Spin injection

Contact induced relaxation

T. Maassen, I. J. Vera-Marun, M. H. D. Guimaraes,and B. J. van Wees, PRB 86, 235408 (2012)

Low contact resistance ‘conductivity mismatch’

M.H.D. Guimarães et al., Nano Lett. 12 (7), 3512 (2012)

P. Zomer et al., APL 99, 232104 (2011)

Graphene on h-BN

Suspended graphene Few layer graphene

T. Maassen et al., PRB 83, 115410 (2011)

Epitaxial graphene

T. Maassen et al.,Nano Lett. 12 (3), 1498 (2012)

Influence of substrate

New fabrication process

Resist-based acid-free suspended graphene

Compatible with most (magnetic) materials

N. Tombros, A. Veligura, J. Junesch, J. J. van den Berg, P. J. Zomer, M. Wojtaszek, I. J. Vera-Marun, H. T. Jonkman, and B. J. van Wees, J. Appl. Phys. 109, 093702 (2011)

LOR = polydimethylglutaramidebased organic resist

High mobility (~105 cm2/Vs)

High quality graphene spintronics

M. H. D. Guimarães, A. Veligura, P. J. Zomer, T. Maassen, I. J. Vera-Marun, N. Tombros, and B. J. van Wees,Nano Lett. 12 (7), 3512–3517 (2012)

Enhanced diffussion coefficientLong λs up to 4.7 µm

SiO2 Susp.

Ds (m2/s) ~0.02 0.10

s (ps) ~150 200

s (m) ~1.7 4.5?

High quality graphene spintronics

Graphene on h-BN

5 m

~80 oC

P. J. Zomer, S. P. Dash, N. Tombros and B. J. van Wees, APL 99, 232104 (2011)

Alignment and transfer by optical mask aligner

Polymer (Tg 36 oC) melts on heated substrate

Anneal in Ar/H flow after processing

Graphene on h-BN

P. J. Zomer, M. H. D. Guimaraes, N. Tombrosand B. J. van Wees, PRB 86, 161416(R) (2012)

-60 -40 -20 0 20 40 60

-0.2

-0.1

0.0

c)

b)

retrace

CB

4 m 2 m

III

II

Rn

l()

B (mT)

I

IAC

V

L = 16 m

A

A

C

B

trace

graphene

-40 -20 0 20 40-0.16

-0.14

-0.12

-0.10 averaged R

nl

fit

Rn

l()

B (mT)

Ds = 0.052 m

2/s

s = 390 ps

= 4.5 m

a)

-40 -20 0 20 40-0.4

-0.3

-0.2 I

II

III

Rnl(

)

B (mT)

Spin valves and precession measured over ~ 20 μm

Longest spin relaxation length at room temperature!

Spin relaxation time still similar to graphene on SiO2

20 m

Hydrogenated graphene

Graphene on SiO2 + chemisorption in Ar/H2 plasma

Decrease 3X in mobility, low hydrogen coverage

M. Wojtaszek, I. J. Vera-Marun, T. Maassen,and B. J. van Wees, PRB 87, 081402(R) (2013)

Hydrogenated graphene

2X reductionin Hanle width

B

gD

t

B

S

S

2

M. Wojtaszek, I. J. Vera-Marun, T. Maassen,and B. J. van Wees, PRB 87, 081402(R) (2013)

Localized magnetic moments

Functionalization via fluorine ad atoms & irradiation point defects

Nair, R. R. et al. Spin-half paramagnetism in graphene induced by point defects. Nat Phys 8, 199–202 (2012)

Graphene nanostructures 0D

First experiments of spin transport in graphene nanostructures with L<λ

Towards 0D limit reflections from theedges cause a uniform spin accumulationand a Lorentzian Hanle lineshape

For uniform accumulation the spin resistance RS = ρλ2 /A > ρ

The increased RS in 0D limits observedlifetime, requiring more resistive contacts

Wojtaszek, M., Vera-Marun, I. J. & van Wees, B. J .Phys. Rev. B 89, 245427 (2014)

M.H.D. Guimarães, J.J. van den Berg, I.J. Vera-Marun, P.J. Zomer, and B. J. van Wees, Phys. Rev. B 90, 235428 (2014)

Graphene nanostructures 1D

Universal conductance fluctuations (UCF) at 4K

Δµ

Guimarães, M.H.D., Zomer, P.J., Vera-Marun, I.J. & van Wees, B.J. Nano Lett. 14, 2952 (2014)

PV

ε

σ

σP

1

Tε

σ

σS

1

TSV

What does heat teach us?

22)( I

Theory

Experiment

Nonlinear detection w/o FM

I V

F F NN

-50 0 50

-2

0

2

4

B|| (mT)

b

R1 (

)

a

-40 -20 0 200

2

4

6

dc

R

1 (

)

Vg (V)

-400 -200 0 200 400

0

2

4

R

1 (

)

B (mT)

-40 -20 0 200.0

0.5

1.0

1.5

(m

)

Vg (V)

0

2

4

Rsq (

k

)

VI

NFF N

-50 0 50

-200

0

200

B|| (mT)

b

V

2 (

nV

)a

-50 0 50

-2

0

2

c

V1 (V

)

B|| (mT)

-100 -80 -60 -40 -20 0 20 40-50

0

50

100

-3

0

3

6

R

2 (

k

/A)

V

2 (

nV

)

Vg (V)

ElectronsS<0

HolesS>0

I.J. Vera-Marun, V. Ranjan, B.J. van Wees, Nature Phys. 8, 313 (2012)

Nonlinear spin detection in mesoscopic structures

(tunnel junctions, quantum point contacts)

Electromotive force generated by spin

accumulation in FM/n-GaAs

Impact beyond graphene

P. Stano, J. Fabian & P. JacquodPRB 85, 241301(R) (2012)

C. C. Geppert, L. R. Wienkes, K. D. Christie, S. J. Patel, C. J. Palmstrøm, and P. A. Crowell,

arXiv:1402.2638 [cond-mat] (2014)

Potential applications

Graphene RT spintronic properties surpass all other materials

Already excellent for storage spintronic devices and transistors

Seneor, P. et al. Spintronics with graphene. MRS Bulletin 37, 1245–1254 (2012)

Potential applications

Demonstrated field-assisted spin transfer torque

Potential technology for magnetic random Access memory

Lin, C.-C. et al. Spin Transfer Torque in a Graphene Lateral Spin Valve Assisted by an External Magnetic Field. Nano Lett. 13, 5177–5181 (2013)

Potential applications

Strong potential for spin logic circuits

Han, W., Kawakami, R. K., Gmitra, M. & Fabian, J. Graphene spintronics. Nat Nano 9, 794–807 (2014)

Behin-Aein, B., Datta, D., Salahuddin, S. & Datta, S. Proposal for an all-spin logic device with built-in memory. Nat Nano 5, 266–270 (2010)

Take-home message

Graphene spintronics functionality future electronics

High quality / chemical modification enhanced spin transport

Bridge subdisciplines (heat+spin) semiconductor spintronics

Excellent spin transport properties with potential applications

Outlook spin caloritronicsSpin heat accumulation,heat assisted spin injection…

I. J. Vera-Marun, B. J. van Wees & R. Jansen PRL 112, 056602 (2014)

Thank you for your attention