1. 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

2. Outline I. Introduction.
Gate tunable spin transport in signal layer graphene at room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.

3. 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.
Spin up
“1”
Spin down
“0”
Spins for
digital
information OR
Outline here

4. Technological Approach
Storage:
Magnetic Hard Drives and Magnetic RAM use metal-based spintronics technologies.
Logic:
Silicon-based electronics are the dominant technology for microprocessors.
Ferromagnetic Materials:
Non-volatile
Radiation hard
Fast switching
Semiconducting Materials:
Tunable carrier concentration
Bipolar (electrons & holes)
Large on-off ratios for switches
Outline here
Spintronics may enable the integration of storage and logic for new, more powerful computing architectures.
Hanan Dery et al., arXiv (2011).

5. Material 1D 2D 3D Discover in 2004 !!
Good electrical properties and potential good spintronic properties.
Carbon Family (Z=6) ~ One of the candidates for the cornerstone of this bridge.
Carbon Nanotube 1D
K. Tsukagoshi, B. W. Alphenaar, and H. Ago, Nature 401, 572 (1999).
Graphene 2D
Discover in 2004 !!
K. S. Novoselov et al., Science 306, 666 (2004).
Graphite 3D
M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, (2007).

6. Properties of Graphene
Physical Structure
Atomic
sheet
of carbon
Electronic Band Structure
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.
C. Berger et al., Science 312, 1191 (2006).
K. S. Kim et al., Nature 457, 706 (2009).
Possibility for long spin lifetime at RT
Low intrinsic spin-orbit coupling

7. Graphene Spin transport
E. W. Hill et al., IEEE Trans. Magn. 42, 2694 (2006). (Prof. Geim’s group at Manchester )
M. Ohishi et al., Jpn. J. Appl. Phys 46, L605 (2007). (Prof. Suzuki’s group at Osaka)
S. Cho et al., Appl. Phys. Lett. 91, (2007). (Prof. Fuhrer’s group at Maryland)
M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, (2007). (Prof. Goldman’s group at Minnesota)
N. Tombros et al., Nature, 571 (2007). (Prof. van Wees’ group at University of Groningen)
W. H. Wang et al., Phys. Rev. B (Rapid Comm.) 77, (2008). (Prof. Kawakami’s group at Riverside)
Figure 2 in ref. 5.
Observed Local and non-local magnetoresistance.
Figure 3 in ref. 5.
Gate dependent non-local magnetoresistance.
Figure 4 in ref. 5.
Hanle spin precession.
Demonstrated the first gate tunable spin transport in graphene spin valve at room temperature.

8. Hybrid Spintronic Devices
Spin Injector
Spin Detector
Lateral
Spin Valve
Ferromagnetic Electrodes M M _ +
Spin Transport Layer
Room temperature operation
Desired Characteristics
Graphene (beginning in 2007)
Yes
OK, 5 microns. Small graphene flakes.
Theory: yes, Experiment: no
Yes (With tunnel barrier)
Good potential
Outline here
High spin injection efficiency
Gate-tunable spin transport
Spin transport over long distances
Long spin lifetimes
Allows spin manipulation

9. Outline I. Introduction.
Gate tunable spin transport in signal layer graphene at room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.

10. Sample preparation
Raman
Identify single layer graphene with optical microscope and confirm with Raman spectrum.

11. Sample preparation Co SiO2 Si SLG Co Co (7°) MgO (0°) Back Gate SLG
2nm
SLG
SiO2
Back Gate
SLG Si
SiO2
Optical
Standard ebeam lithography Co
SLG
SEM
500 nm
SLG

12. Device characterization
Contact resistance E1 E2 E3 E4 I V
1.5
Transparent contact of Co/SLG
Vg = 0 V
R3pt
1.0 E1 E2 E3 E4 I V
R4pt
dV/dI (kΩ)
MgO Co
SLG
0.5
R3pt – R4pt
Relectrode + Rcontact
< 300 ohms
-200
200
I (μA)
Gate dependent resistance
m ~ 2500 cm2/Vs
Outline here I V E1 E2 E3 E4

13. Spin Injection and Chemical PotentialFM
graphene e-
Spin-dependent
Chemical potential
Chemical
Potential
(Fermi level)
Outline here
Density of states
Density of states

14. Local and Nonlocal Magnetoresistance
Local spin transport measurement:
Spin Injector
Spin Detector
charge current I V
spin current
Non-local spin transport measurement:
Outline here
Spin Injector
Spin Detector
charge current
spin current
IINJ
VNL + -
Using lock-in detection
M. Johnson, and R. H. Silsbee,
PRL, 55, 1790 (1985)

15. Nonlocal Magnetoresistance
Parallel
Anti-Parallel
IINJ
IINJ
VNL
VNL H H L L
Injector
Detectors
Injector
Detectors
Spin up
Spin up
Outline here
chemical potential
Spin dependent
Vp>0
chemical potential
Spin dependent
VAP<0
Spin down
Spin down
Nonlocal MR = (VP - VAP)/IINJ

16. Nonlocal MR--- Temperature dependent
Spin Signal
Nonlocal MR = ΔRNL = ΔVNL/Iinj
ΔRNL RT
Outline here
Room temperature spin transport

17. Nonlocal MR—Spacing dependence
L (mm)
ΔR (mW)
λS ~1.6 μm E1
SLG E2 E3 E4 E5 E6 E7
1 um
Wei Han, K. Pi et al., APL. 94, (2009)
2μm 1
3μm
L = 1 μm
RNl (mΩ)
H (mT)
RNL (mΩ)
H (mT)
L = 2 μm
RNL (mΩ)
H (mT)
L = 3 μm
Outline here

18. spin injection efficiency is low.
Graphene spin valve
spin injection efficiency is low.
P~ 1%.
Gate tunable non-local spin signal

19. Hanle spin precession – spin lifetime measurement
L = 3 μm
-160
-80 80
160
-1.0
-0.5
0.5
1.0
H (mT)
RNL (mΩ)
IINJ
VNL  L
Diffusion
coefficient
D = m2/s
ts = 84 ps
λs = 1.5 μm
Spin
Lifetime
Outline here
spin lifetime is “short”.

20. 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?

21. Outline I. Introduction.
Gate tunable spin transport in signal layer graphene at room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.

22. Interface resistance (R1, R2 )(Ω)
Theoretical analysis
How to achieve efficient spin injection?
Takahashi, et al, PRB 67, (2003) Co
L=λG=W=2 μm
PF=0.5, PJ=0.4
ρG=2 kΩ
20000
40000
Interface resistance (R1, R2 )(Ω)
RNL(Ω) 60
120
Tunneling
contacts
Transparent
MgO
SLG
Insert a thin tunnel barrier
to make R1, R2 >> RG
Outline here
How to fabricate pin-hole free tunnel barrier.

23. MgO Barrier with Ti adhesion layer
1 nm MgO on graphite (AFM) Ti
Outline here
MgO
No Ti
graphite
RMS roughness: 0.766nm
RMS roughness: 0.229nm
W. H. Wang, W. Han et. al. ,Appl. Phys. Lett. 93, (2008).

24. Tunneling spin injection into SLG
Fabrication and Electrical characterization
Co (7°) I V - +
Ti/MgO
(0°)
Ti/MgO
(9°) Co
MgO
TiO2 I
SLG
SLG
SiO2
SiO2
-0.6
-0.4
0.3
0.6 -8
VDC(V) -4 4 8
IDC (μA)
2-probe
300 K
3-probe
300 K
-10 10
IDC (mA) 50
100
150
200
dV/dI (kW)
Outline here

25. Tunneling spin injection into SLG
Large Non-local MR with high spin injection efficiency
Johnson & Silsbee, PRL, 1985.
Jedema, et al, Nature,
Outline here
DRNL=130 W , PJ=31 %
Wei Han, K. Pi et. al., PRL 105, (2010).

26. Comparison of Co/SLG and Co/MgO/SLG
2nm
MgO
1nm
MgO
3nm
SLG
SLG
SiO2
SiO2
L=1 mm
L=2.1 mm
Outline here
Vg=0 V
Vg=0 V
DRNL= 0.02 W P ~ 1%
DRNL=130 W P ~ 31%
Tunnel barrier increases spin signal by factor of ~1,000

27. Theoretical analysis For Ohmic spin injection with Co/SLG
For Tunneling spin injection with Co/MgO/SLG
Outline here

28. Gate Tuning of Spin Signal
Drift-Diffusion Theory for Different Types of Contacts
Proportional to
graphene conductivity
Outline here
Inversely proportional
to graphene conductivity

29. Gate Tuning of Spin Signal
Outline here
Transparent contact
Pin-hole contact

30. Gate Tuning of Spin Signal
Outline here
Tunneling contact
Characteristic gate dependence of tunneling spin injection is realized.

31. Outline I. Introduction.
Gate tunable spin transport in signal layer graphene at room temperature.
III. Enhanced spin injection efficiency: Tunnel barrier study.
Spin relaxation mechanism in graphene:
--- Charged impurities scattering.
--- Chemical doping on graphene spin valves.

32. Spin relaxation in graphene
Experiment:
Spin lifetime ~ 500 ps
(for single layer graphene)
Theory:
Spin lifetime ~ 100 ns – 1 ms
Two types of spin relaxation mechanisms:
Elliot-Yafet mechanism
D’yakonov-Perel mechanism
defects
Outline here
Spin flip during momentum scattering events.
spins precess in internal spin-orbit fields.
Charged impurities (Coulomb) are the most important type of momentum scattering.
Are charged impurities important for spin relaxation?
C. Jozsa, et al., Phys. Rev. B, 80, (R) (2009).
N. Tombros, et al., Phys. Rev. Lett. 101, (2008).

33. Experiment
MBE cell
Charged impurities
(we use Au in this study)
We add charged impurities onto a graphene spin valve to study its effect on spin lifetime. I V
Co electrode + -
Single-Layer Graphene (SLG)
SiO2 Si
(backgate)
Graphene spin valve device
K. Pi, Wei Han et.al., Phys. Rev. Lett. 104, (2010).

34. How to perform the experiment????
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.

35. The UHV System SEM image Small MBE Chamber
Measure Transport Properties
Vary Temperature from 18K to 300K
Ports for 4 different materials
Apply a magnetic field
500 nm
SLG
Magnet
SEM image

36. Gate dependent conductivity
In situ measurement
Au is selected for this study because Au behaves as a point-like charged impurity on graphene.
Gate dependent conductivity
vs.
Au deposition time
No Au
Gate Voltage (V)
Conductivity (mS)
Au 6 s
Au 2 s
Au 4 s
Au 8 s
Au deposition (Sec)
m (cm2/Vs)
T=18 K
Coulomb scattering is the dominant charge scattering mechanism.
Deposition rate ~ 0.04 Å/min (5x1011 atom/cm2s)
K. M. McCreary, K. Pi et al., Phys. Rev. B 81, (2010).

37. Introducing extra spin scattering.
Effect of Au doping on non-local signal
No Au
Gate Voltage (V)
Conductivity (mS)
Au 6 s
Au 8 s
Au 4 s
Au 2 s
Without introducing extra spin scattering.
Gate (V)
DRnl (W)
Simulation
Introducing extra spin scattering.
Gate (V)
DRnl (W)
Simulation
Au doping does not introduce extra spin scattering.

38. Hanle precession
Directly compare spin lifetime between different amounts of Au doping.
data
fit
Au = 0 s
Holes
H (T)
ΔRNL (Ω)
-0.01
0.01
Au = 8 s
data
fit
Au = 0 s
Electrons
ΔRNL (Ω)
-0.01
0.01
H (T)
Au = 8 s
data
fit
Au = 0 s
Dirac Pt.
ΔRNL (Ω)
Au = 8 s
-0.01
0.01
H (T)

39. Effect of charged impurities on spin lifetime
Spin lifetime and the diffusion coefficient are determined from Hanle spin precession data
Au deposition (s)
Spin lifetime (ps)
(2.9x1012 cm-2)
Spin relaxation
Momentum scattering 2 4 6 8
Au deposition (sec)
0.00
0.02
0.04
0.06
D (m2/s)
Dirac Pt.
Electrons
Holes
Charged impurities are not the dominant spin relaxation mechanism.

40. Slight enhancement of spin lifetime
Spin relaxation mechanisms are correlated.
tc : Spin relaxation by Coulomb scattering.
tj : Spin relaxation by other defects (lattice
defects, sp3 bound etc.).
Y. Gan et al., Small 4, 587 (2008).
S. Molola et al., Appl. Phys. Lett. 94, (2009).
Wei Han et al., arXiv (2011).
Recent study shows that Co contact plays an important role.
Effect of D’yakonov-Perel mechanism.
E-Y mechanism: ts ~ tm
D-P mechanism: ts ~ tm-1
F. Guinea et al., Solid State comm. 149, 1140 (2009).
Further study is needed.

41. Enhancement of spin signal by chemical doping
At fixed gate voltage, Au doping can enhance conductivity.
No significant spin relaxation from charged impurities.
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
Conductivity (mS)
Possible to tune spin properties by chemical doping instead of applying high electric field (gate voltage).

42. Conclusion Achieved tunneling contact on graphene spin valves.
Au deposition (s)
Spin lifetime (ps)
Demonstrated charged impurities are not the dominant spin relaxation mechanism.
Manipulation of spin transport in graphene by surface chemical doping.

43. Thank you. Acknowledgements Roland Kawakami Wei Han Kathy McCreary
Postdoc: Wei-Hua Wang
(Academia Sinica in Taiwan)
Yan Li
Adrian Swartz
Jared Wong
Richard Chiang
Collaborators
Wenzhong Bao
Feng Miao
Jeanie Lau (PI)
Peng Wei
Jing Shi (PI)
Shan-Wen Tsai (PI)
Francisco Guinea (PI)
Mikhail Katsnelson (PI)
Thank you.

44. 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, (2008).
Hydrogen storage.
--- AI doped graphene as hydrogen storage at room temperature.
Z. M. Ao et al., J. Appl. Phys. 105, (2009).

45. The UHV System 5 mm SEM image
We use same system to study the charge transfer and charge scattering mechanism of transition metals doped graphene.
5 mm
SEM image
Magnet

46. Dirac point shift vs. Ti and Fe coverage
ØTi = 4.3 eV
Øgraphene = 4.5 eV
ØFe = 4.7 eV
No Ti (0 ML)
No Fe (0 ML) ML
0.041 ML ML
Conductivity (mS)
Conductivity (mS)
0.123 ML
0.015 ML
0.205 ML
Gate Voltage (V)
Gate Voltage (V)
Dirac Point (V)
Ti coverage (ML)
-40
-80
0.00
0.01
0.02
Dirac Point (V)
Fe coverage (ML)
-30
-60
0.0
0.1
0.2
Both Ti and Fe coverage show n-type doping
Keyu Pi et al., PRB 80, (2009).

47. Dirac point shift vs. Pt coverage
ØPt = 5.9 eV
TM coverage (ML)
Dirac point shift (V)
Pt-1
Pt-2
Fe-1
Fe-2
Fe-3
Ti-1
Ti-2
Ti-3
No Pt (0 ML)
0.025 ML
Conductivity (mS)
0.071 ML
0.127 ML
Gate Voltage (V)
Dirac Point (V)
-20
-40
Pt coverage (ML)
0.00
0.05
0.10
0.15
The trend of Dirac point shift follows the work function.
All the Pt and Fe samples show the n-type doping behavior.
Regardless of the metal work function, all TMs we have studied result in n-type doping when making contact with graphene.

48. Interfacial dipole DV(d) = Dtr(d) + Dc(d) Become n-type dopingWG
DEF DV W EF
Graphene +q -q d WG
-DEF DV W EF
Graphene +q -q d WG
DEF DV W EF
Graphene +q -q
Dtr(d) : The charge transfer between graphene and the metal (difference in work functions). WM
Dc(d) : the overlap of the metal and graphene wave functions
Metal
Dc(d) = e−gd (a0 + a1d + a2d2)
Highly depends on d.
G. Giovannetti et al., Physical Review Letters 101, (2008).

49. Possible reason for anomalous n-type doping
Graphene
p-type 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, (2008).
M. A. Pushkin et al, Bulletin of the Russian Academy of Science: Physics 72, 878 (2008).

50. Experimental evidence of interfacial dipole.2 4 6 8
0.87
1.75
2.62
3.50
Pt Coverage (ML)
Pt Coverage (Å)
Dirac Point (V)
AFM 1
AFM 2
0 nm
10 nm
3.19 ML
0.62 ML
By Theoretical calculation, d increase as material coverage went from adatoms to continuous film. d d d
Graphene
Experimental evidence of interfacial dipole.
K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77,

51. 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).

52. Mobility change vs. TM coverage
Mobility, m (103 cm2/Vs) 3 2 1
Fe-2
Conductivity (mS)
Pt-2
Ti-1
2.0
1.0
0.0
1.5
0.5 -4 -2 4
0.000
0.015
0.030
n (1012 cm-2)
Coverage (ML)
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| )
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

53. Mobility change vs. Dirac point shift
Fitting equation:
0.1 ML
μ/μ0 = (Γ0 + ΓTM)-1/Γ0-1
= (1 + ΓTM/Γ0)-1
Normalized mobility, μ/μ0
ΓTM/Γ0 = (AVD,shift)β
Pt-1
Pt-2
Ti-1
Ti-2
Ti and Pt fall on the universal curve.
Coulomb scattering is the dominant effect.
0.008 ML
Dirac Point Shift (V)
Fe-2
Electron data follows the universal curve.
Hole data is significantly different.
This implies some wave function hybridization in the Fe system.
Electron
Hole
μ/μ0
Dirac Point Shift (V)
Keyu Pi, K. M. McCreary et al., PRB 80, (2009).