Spintronic and electronic transport properties in
graphene – The cornerstone for spin logic devices.
皮克宇*
Department of Physics and Astronomy
UC Riverside
4月26日, 2011
NTNU
*Current location: Hitachi Global Storage Technologies
Outline
I.
Introduction.
II.
Gate tunable spin transport in signal layer graphene at
room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
IV. Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.
Motivation for Spintronics
Silicon electronics and the “end-of-the-roadmap”….
How to improve computers beyond the physics limits of
existing technology?
Spintronics: Utilize electron spin in addition to charge for
information storage and processing.
Spins for
digital
information
OR
Spin up
“1”
Spin down
“0”
Technological Approach
Storage:
Magnetic Hard Drives and
Magnetic RAM use metal-based
spintronics technologies.
Ferromagnetic Materials:
• Non-volatile
• Radiation hard
• Fast switching
Logic:
Silicon-based electronics are
the dominant technology for
microprocessors.
Semiconducting Materials:
• Tunable carrier concentration
• Bipolar (electrons & holes)
• Large on-off ratios for switches
Spintronics may enable the integration of storage and logic for new,
more powerful computing architectures.
Hanan Dery et al., arXiv 1101.1497 (2011).
Material
Good electrical properties and potential good spintronic properties.
Carbon Family (Z=6) ~ One of the candidates for the cornerstone of this bridge.
1D
Carbon Nanotube
K. Tsukagoshi, B. W. Alphenaar, and H.
Ago, Nature 401, 572 (1999).
2D
Graphene
Discover in 2004 !!
K. S. Novoselov et al., Science 306, 666 (2004).
3D
Graphite
M. Nishioka, and A. M. Goldman,
Appl. Phys. Lett. 90, 252505 (2007).
Properties of Graphene
Electronic Band Structure
Physical Structure
Atomic
sheet
of carbon
 High mobility -- up to 200,000 cm2/Vs (typically 1,000 – 10,000 cm2/Vs).
 Zero gap semiconductor with linear dispersion: “massless Dirac fermions”.
 Tunable hole/electron carrier density by gate voltage.
 Possible for large scale device fabrication.
Low intrinsic spin-orbit coupling
C. Berger et al., Science 312, 1191 (2006).
K. S. Kim et al., Nature 457, 706 (2009).
Possibility for long spin lifetime at RT
Graphene Spin transport
1.
E. W. Hill et al., IEEE Trans. Magn. 42, 2694 (2006). (Prof. Geim’s group at Manchester )
2.
M. Ohishi et al., Jpn. J. Appl. Phys 46, L605 (2007). (Prof. Suzuki’s group at Osaka)
3.
S. Cho et al., Appl. Phys. Lett. 91, 123105 (2007). (Prof. Fuhrer’s group at Maryland)
4.
M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, 252505 (2007). (Prof. Goldman’s group at Minnesota)
5.
N. Tombros et al., Nature, 571 (2007). (Prof. van Wees’ group at University of Groningen)
6.
W. H. Wang et al., Phys. Rev. B (Rapid Comm.) 77, 020402 (2008). (Prof. Kawakami’s group at Riverside)
Figure 2 in ref. 5.
Figure 3 in ref. 5.
Figure 4 in ref. 5.
•Demonstrated the first gate tunable spin transport in
graphene spin valve at room temperature.
Observed Local and nonlocal magnetoresistance.
Gate dependent non-local
magnetoresistance.
Hanle spin precession.
Hybrid Spintronic Devices
Spin Injector
Spin Detector
Lateral
Spin Valve
Ferromagnetic
Electrodes
_ 0 +
Spin Transport Layer
Desired Characteristics
Graphene (beginning in 2007)
Room temperature operation
Yes
High spin injection efficiency
Yes (With tunnel barrier)
Gate-tunable spin transport
Spin transport over long distances
Long spin lifetimes
Allows spin manipulation
Yes
OK, 5 microns. Small graphene flakes.
Theory: yes, Experiment: no
Good potential
Outline
I.
Introduction.
II.
Gate tunable spin transport in signal layer graphene at
room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
IV. Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.
Sample preparation
Raman
Identify single layer graphene with optical microscope
and confirm with Raman spectrum.
Sample preparation
Co (7°)
Co
MgO
(0°)
MgO
2nm
SLG
SiO2
SLG
Si
SiO2
Optical
Back Gate
SEM
SLG
SLG
Co
Standard ebeam lithography
500 nm
Device characterization
I
Contact resistance
V
1.5
dV/dI (kΩ)
Vg = 0 V
E1 E2 E3 E4
R3pt
1.0
0
I
R4pt
V
R3pt – R4pt
0.5
-200
0
I (μA)
200
E1 E2 E3 E4
Relectrode + Rcontact
< 300 ohms
Conductance (mG)
V
E1
E2
E3
E4
Co
MgO
SLG
1.5
Gate dependent resistance
I
Transparent
contact of Co/SLG
m ~ 2500 cm2/Vs
1.0
0.5
0.0
-60
-40
-20
0
20
Gate Voltage (V)
40
Spin Injection and Chemical Potential
graphene
FM
e-
Chemical
Potential
(Fermi level)

m
m
m


Density of states
Density of states
Spin-dependent
Chemical potential
Local and Nonlocal Magnetoresistance
Local spin transport measurement:
I
charge
current
V
Spin Injector
Spin Detector
spin current
Non-local spin transport measurement:
Spin Injector
charge
current
Spin Detector
+
IINJ
spin current
-
VNL
Using lock-in detection
M. Johnson, and R. H. Silsbee,
PRL, 55, 1790 (1985)
Nonlocal Magnetoresistance
IINJ
Parallel
IINJ
VNL
Anti-Parallel
VNL
H
H
L
L
m
Detector
s
Injector
Spin up
Vp>0

m
Spin down
Spin dependent
chemical potential

Spin dependent
chemical potential
Injector
m
Detector
s
Spin up
VAP<0
m
Nonlocal MR = (VP - VAP)/IINJ
Spin down
Nonlocal MR--- Temperature dependent
Spin Signal
Nonlocal MR = ΔRNL = ΔVNL/Iinj
80
200
0
ΔRNL
-40
RNL (m)
RNL (m)
40
100
RT
-80
-100
0
H (mT)
100
0
0
100
200
Temperature(K)
Room temperature spin transport
300
Nonlocal MR—Spacing dependence
E7
ΔR (m)
E6
E4
E2
SLG
E3
E1
E5
1 um
L (mm)
Wei Han, K. Pi et al., APL. 94, 222109 (2009)
3μm
RNl (mΩ)
H (mT)
L = 3 μm
RNL (mΩ)
L = 2 μm
RNL (mΩ)
2μm
L = 1 μm
1
λS ~1.6 μm
H (mT)
H (mT)
Graphene spin valve
Non-local signal (m)
40
spin injection
efficiency is low.
P~ 1%.
20
0
Non-local signal (m)
40
20
0
-20
-20
-40
-40
-600
-300
0
300
H (Oe)
600
-600
-300
0
300
H (Oe)
600
-600
-300
0
300
H (Oe)
Non-local signal (m)
40
20
0
-20
-40
Gate tunable non-local spin signal
600
Hanle spin precession – spin lifetime measurement
IINJ
RNL (mΩ)
H 
L = 3 μm
1.0
VNL
L
0.5

0
Diffusion
coefficient
-0.5
-1.0
Spin
Lifetime
-160
-80
0
H (mT)
RNL   

0
80
160
spin lifetime is “short”.
 L2 
exp 
cos(Lt ) exp(t /  s )dt

4 Dt
 4 Dt 
1
D = 0.025 m2/s
s = 84 ps
λs = 1.5 μm
Challenges
• Create spin polarized current in graphene.
How to increase the spin injection efficiency?
• Keep spin current polarized in graphene.
What is the spin relaxation mechanism in graphene?
Outline
I.
Introduction.
II.
Gate tunable spin transport in signal layer graphene at
room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
IV. Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.
Theoretical analysis
How to achieve efficient spin injection?
Ri
Ri
RF
RF
P
2
2
F
2
2
R
RG
RG
RG
 L / G
2 L / G 1
G
 4 RG e
(

)

[
(1


)

e
]


2
2
2
2
1  PF
1  PJ 1  PF
i 1 1  PJ
i 1
PJ
RNL
Takahashi, et al, PRB 67, 052409 (2003)
Co
Tunneling
contacts
MgO
SLG
Insert a thin tunnel barrier
to make R1, R2 >> RG
How to fabricate pin-hole
free tunnel barrier.
RNL(Ω)
120
L=λG=W=2 μm
PF=0.5, PJ=0.4
ρG=2 kΩ
60
0
Transparent
contacts
0
20000
40000
Interface resistance
(R1, R2 )(Ω)
MgO Barrier with Ti adhesion layer
1 nm MgO on graphite (AFM)
MgO
Ti
No Ti
graphite
RMS roughness:
0.766nm
RMS roughness:
0.229nm
W. H. Wang, W. Han et. al. ,Appl. Phys. Lett. 93, 183107 (2008).
Tunneling spin injection into SLG
Fabrication and Electrical characterization
Co (7°)
Ti/MgO
Ti/MgO
(9°)
(0°)
I
Co
TiO2
MgO
I
SLG
SiO2
2-probe
3-probe
4
dV/dI (k)
IDC (μA)
SLG
SiO2
200
8
0
-4
-8
V
+ -
300 K
150
100
50
300 K
-0.6
-0.4
0
VDC(V)
0.3
0.6
0
-10
0
IDC (mA)
10
Tunneling spin injection into SLG
Large Non-local MR with high spin injection efficiency
PJ 2G  L / N
RNL 
e
W G
Johnson & Silsbee, PRL, 1985.
Jedema, et al, Nature, 2002 .
 (0V )  0.35mS ,
RNL (0V )  130.4,
W ~ 2.2 m m,
L  2.1m m,
G  2 m m,
RNL=130  , PJ=31 %
Wei Han, K. Pi et. al., PRL 105, 167202 (2010).
Comparison of Co/SLG and Co/MgO/SLG
Co
Co
MgO
2nm
MgO
1nm
SLG
SLG
SiO2
SiO2
L=1 mm
15
10
5
0
-5
-10
-15
-20
100
Non-local signal ()
Non-local signal (m)
20
Vg=0 V
-600
-300
300
0
H (Oe)
RNL= 0.02 
3nm
600
P ~ 1%
L=2.1 mm
50
0
-50
-100
Vg=0 V
-800
-400
400
0
H (Oe)
800
RNL=130  P ~ 31%
Tunnel barrier increases spin signal by factor of ~1,000
Theoretical analysis
For Ohmic spin injection with Co/SLG
Ri
Ri
RF
RF
P
2
2
F
2
2
RG
RG
RG
RG
 L / N
2 L / G 1
 4 RG e
(

)

[
(1


)

e
]


2
2
2
2
1  PF
1  PJ 1  PF
i 1 1  PJ
i 1
PJ
RNL
RNL
4 pF 2
RF 2 e L / G
4 pF 2 RF 2 e L / G
1

R
(
)

[
]
~ G
G
2 L / G
2 L / G
2 2
2 2
(1  pF )
RG 1  e
(1  pF ) 1  e
RG
For Tunneling spin injection with Co/MgO/SLG
RNL
Ri
Ri
RF
RF
P
2
2
F
2
2
R
RG
RG
RG
 L / G
2 L / G 1
G
 4 RG e
(

)

[
(1


)

e
]


2
2
2
2
1  PF
1  PJ 1  PF
i 1 1  PJ
i 1
RNL
RG G 2  L / G
1

PJ e
~
W
G
PJ
Gate Tuning of Spin Signal
Drift-Diffusion Theory for Different Types of Contacts
Proportional to
graphene conductivity
Inversely proportional
to graphene conductivity
Gate Tuning of Spin Signal
Transparent contact
Pin-hole contact
Gate Tuning of Spin Signal
Tunneling contact
Characteristic gate dependence of tunneling spin injection is realized.
Outline
I.
Introduction.
II.
Gate tunable spin transport in signal layer graphene at
room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
IV. Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.
Spin relaxation in graphene
Experiment:
Theory:
Spin lifetime ~ 500 ps
Spin lifetime ~ 100 ns – 1 ms
(for single layer graphene)
Two types of spin relaxation mechanisms:
Elliot-Yafet mechanism
D’yakonov-Perel mechanism
defects
Spin flip during momentum
scattering events.
Charged impurities (Coulomb)
are the most important type of
momentum scattering.
spins precess in internal
spin-orbit fields.
Are charged impurities important
for spin relaxation?
C. Jozsa, et al., Phys. Rev. B, 80, 241403(R) (2009).
N. Tombros, et al., Phys. Rev. Lett. 101, 046601 (2008).
Experiment
MBE cell
I
Co electrode
+
V
Single-Layer
Graphene (SLG)
SiO2
Si
(backgate)
Charged impurities
(we use Au in this study)
Graphene spin valve device
We add charged impurities onto a graphene spin valve
to study its effect on spin lifetime.
K. Pi, Wei Han et.al., Phys. Rev. Lett. 104, 187201 (2010).
Challenges
How to perform the experiment????
• With small amounts of adatom coverage, metal impurties
will oxidize.
• Clean environment and fine control of deposition rate.
In-situ Measurement.
Molecular beam epitaxy Growth.
The UHV System
Small MBE Chamber
•Measure Transport Properties
•Vary Temperature from 18K to
300K
•Ports for 4 different materials
•Apply a magnetic field
SLG
500 nm
Magnet
SEM image
In situ measurement
Au is selected for this study because Au behaves
as a point-like charged impurity on graphene.
T=18 K
m (cm2/Vs)
Conductivity (mS)
Gate dependent conductivity
vs.
Au deposition time
Au 2 s
Au 8 s Au 6 s Au 4 s
No Au
Au deposition (Sec)
Gate Voltage (V)
Deposition rate ~ 0.04 Å/min
(5x1011 atom/cm2s)
Coulomb scattering is the
dominant charge scattering
mechanism.
K. M. McCreary, K. Pi et al., Phys. Rev. B 81, 115453 (2010).
Without introducing
extra spin scattering.
Simulation
Conductivity (mS)
Effect of Au doping on non-local signal
Introducing extra spin
scattering.
Au 2 s
Au 8 s Au 6 s Au 4 s
No Au
Simulation
Rnl ()
Rnl ()
Gate Voltage (V)
Gate (V)
Au doping does not
introduce extra spin
scattering.
Gate (V)
Hanle precession
Directly compare spin
lifetime between different
amounts of Au doping.
data
fit
ΔRNL (Ω)
ΔRNL (Ω)
data
fit
Au = 8 s
Holes
-0.01
0
0.01
-0.01
H (T)
data
fit
Au = 0 s
Holes
-0.01
0
0
0.01
H (T)
ΔRNL (Ω)
ΔRNL (Ω)

Au = 8 s
Electrons
Au = 0 s
Electrons
-0.01
0.01
data
fit

0
H (T)
H (T)
data
fit
Au = 8 s
Dirac Pt.
-0.01
0
H (T)
data
fit
ΔRNL (Ω)
ΔRNL (Ω)

0.01
Au = 0 s
Dirac Pt.
-0.01
0
H (T)
0.01

0.01
Effect of charged impurities on spin lifetime
Spin lifetime and the diffusion coefficient are
determined from Hanle spin precession data
Spin relaxation
Momentum scattering
Spin lifetime (ps)
D (m2/s)
0.06
Dirac Pt.
Electrons
Holes
0.04
0.02
0.00
0
2
4
6
8
(2.9x1012 cm-2)
Au deposition (sec)
Au deposition (s)
Charged impurities are not the dominant spin relaxation mechanism.
Slight enhancement of spin lifetime
• Spin relaxation mechanisms are correlated.
1/  s  1/  C  1/  j
j
c : Spin relaxation by Coulomb scattering.
j : Spin relaxation by other defects (lattice

defects, sp3 bound etc.).
Recent study shows that Co
contact plays an important role.
Y. Gan et al., Small 4, 587 (2008).
S. Molola et al., Appl. Phys. Lett. 94, 043106 (2009).
Wei Han et al., arXiv 1012.3435 (2011).
• Effect of D’yakonov-Perel mechanism.
E-Y mechanism: s ~ m
D-P mechanism: s ~ m-1
F. Guinea et al., Solid State comm. 149, 1140 (2009).
Further study is needed.
Enhancement of spin signal by chemical doping
• At fixed gate voltage, Au doping can enhance conductivity.
•No significant spin relaxation from charged impurities.
Conductivity (mS)
By Au doping we are able to enhance
spin life time from 50 ps to 150 ps.
2.0
1.5
1.0
0.5
0.0
Possible to tune spin properties by chemical doping
instead of applying high electric field (gate voltage).
Conclusion
Spin lifetime (ps)
Achieved tunneling contact on
graphene spin valves.
Demonstrated charged impurities are not
the dominant spin relaxation mechanism.
Au deposition (s)
Manipulation of spin transport in
graphene by surface chemical doping.
Acknowledgements
Roland Kawakami
Collaborators
Wei Han
Kathy McCreary
Postdoc: Wei-Hua Wang
(Academia Sinica in Taiwan)
Yan Li
Adrian Swartz
Jared Wong
Richard Chiang
Wenzhong Bao
Feng Miao
Jeanie Lau (PI)
Peng Wei
Jing Shi (PI)
Shan-Wen Tsai (PI)
Francisco Guinea (PI)
Mikhail Katsnelson (PI)
Thank you.
New physics in TM doped graphene system
• Adatoms on Graphene; Wave function hybridization between TM and
graphene may lead us to the new physics.
--- Fe on graphene is predicted to
result in 100% spin polarization.
Y. Mao et al., Journal of Physics: Condensed Matter 20, 2008 (2008).
--- Pt may induce localized magnetic states in Graphene.
B. Uchoa et al., Phys. Rev. Lett. 101, 026805 (2008).
• Hydrogen storage.
--- AI doped graphene as hydrogen
storage at room temperature.
Z. M. Ao et al., J. Appl. Phys. 105, 074307 (2009).
The UHV System
We use same system to
study the charge transfer
and charge scattering
mechanism of transition
metals doped graphene.
5 mm
Magnet
SEM image
Dirac point shift vs. Ti and Fe coverage
Øgraphene = 4.5 eV
No Ti (0 ML)
Conductivity (mS)
Conductivity (mS)
ØTi = 4.3 eV
0.0038 ML
0.0077 ML
0.015 ML
ØFe = 4.7 eV
No Fe (0 ML)
0.041 ML
0.123 ML
0.205 ML
Gate Voltage (V)
Dirac Point (V)
Dirac Point (V)
Gate Voltage (V)
0
-40
-80
0.00
0.01
0.02
0
-30
-60
0.0
Ti coverage (ML)
0.1
0.2
Fe coverage (ML)
Both Ti and Fe coverage show n-type doping
Keyu Pi et al., PRB 80, 075406 (2009).
Dirac point shift vs. Pt coverage
No Pt (0 ML)
0.025 ML
Dirac point shift (V)
Conductivity (mS)
ØPt = 5.9 eV
0.071 ML
0.127 ML
Dirac Point (V)
Gate Voltage (V)
0
-20
Pt-1
Pt-2
Fe-1
Fe-2
Fe-3
Ti-1
Ti-2
Ti-3
-40
TM coverage (ML)
0.00
0.05
0.10
0.15
Pt coverage (ML)
Regardless of the metal work
function, all TMs we have studied
result in n-type doping when making
contact with graphene.
• The trend of Dirac point shift follows
the work function.
• All the Pt and Fe samples show the ntype doping behavior.
Interfacial dipole
Become n-type doping
V(d) = tr(d) + c(d)
V
V
V
WM
WG
WG
d d
W
W
W
WG
E
EFF
tr(d) : The charge transfer between
graphene and the metal (difference
in work functions).
EF
Graphene
Graphene
Graphene
EF
Metal
-q
+q
+q+q
c(d) : the overlap of the metal and
graphene wave functions
c(d) = e−gd (a0 + a1d + a2d2)
Highly depends on d.
G. Giovannetti et al., Physical Review Letters 101, 026803 (2008).
Possible reason for anomalous n-type doping
p-type
Graphene
d
n-type
Transition metal
--- An interfacial dipole having 0.9eV extra barrier for an
equilibrium distance ~ 3.3 Å makes the required work function
for p-type doping > 5.4eV. ( This explains why Fe with ØFe = 4.7
eV dopes n-type).
--- Nano-clusters (smaller than ~ 3nm) have different work
function values when compared with bulk material.
G. Giovannetti et al., Physical Review Letters 101, 026803 (2008).
M. A. Pushkin et al, Bulletin of the Russian Academy of Science: Physics 72, 878 (2008).
Interfacial dipole
Pt Coverage (Å)
Dirac Point (V)
0
2
4
6
8
By Theoretical calculation, d increase as
material coverage went from adatoms to
continuous film.
d
d
d
AFM 2
Graphene
AFM 1
0.62 ML
3.19 ML
10 nm
0 nm
0
0.87
1.75
2.62
3.50
AFM 1
AFM 2
Pt Coverage (ML)
Experimental evidence of interfacial dipole.
K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77, 235430 2008.
Scattering introduced by TM
• Long range scattering. (Charge impurity)
• Short-range scattering.
(Point defect, wave function hybridization etc.)
• Surface corrugations. (Ripple)
F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, and K. S. Novoselov, Nature Mater. 6, 652 (2007).
J.-H. Chen, C. Jang, S. Adam, M. S. Fuhrer, E. D. Williams, and M. Ishigami, Nature phys. 4, 377 (2008).
Mobility change vs. TM coverage
The electron and hole mobilities (μe,
μh) are determined by taking a linear
fit of the σ vs. n curve just away
from the Dirac point (μe,h= |Δσ/Δne| )
3
2.0
2
1.0
0.0
Ti-1
0
1.5
3
1.0
2
0.5
1
0.0
Fe-2
0
3
2.0
2
1.0
0.0
1
Pt-2
-4
-2
n
0
(1012
2
cm-2)
4 0.000
0.015
0
0.030
Coverage (ML)
Mobility, m (103 cm2/Vs)
Conductivity (mS)
1
Fe data show strong electron
hole asymmetry.
Dirac point shift with TM coverage:
Ti >Fe >Pt
Mobility drop with TM coverage:
Ti >Fe >Pt
?
Dirac point shift vs. Mobility change
Mobility change vs. Dirac point shift
Normalized mobility, μ/μ0
Fitting equation:
0.1 ML
μ/μ0 = (Γ0 + ΓTM)-1/Γ0-1
= (1 + ΓTM/Γ0)-1
ΓTM/Γ0 = (AVD,shift)β
Pt-1
Pt-2
Ti-1
Ti-2
0.008 ML
Ti and Pt fall on the universal curve.
Coulomb scattering is the dominant
effect.
Dirac Point Shift (V)
Fe-2
Electron
Hole
• Electron data follows the universal curve.
μ/μ0
• Hole data is significantly different.
• This implies some wave function
hybridization in the Fe system.
Dirac Point Shift (V)
Keyu Pi, K. M. McCreary et al., PRB 80, 075406 (2009).