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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