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皮克宇 *

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.

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皮克宇 *

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

  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 Spintronics may enable the integration of storage and logic for new, more powerful computing architectures. HananDery et al., arXiv 1101.1497 (2011).

  5. Material Good electrical properties and potential good spintronic properties. Carbon Family (Z=6) ~ One of the candidates for the cornerstone of this bridge. 3D 1D 2D Graphite M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, 252505 (2007). Graphene Carbon Nanotube Discover in 2004 !! K. Tsukagoshi, B. W. Alphenaar, and H. Ago, Nature 401, 572 (1999). K. S. Novoselov et al., Science 306, 666 (2004).

  6. 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. 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, 123105 (2007). (Prof. Fuhrer’s groupat Maryland) • M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, 252505 (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, 020402 (2008). (Prof. Kawakami’s group at Riverside) Figure 4 in ref. 5. Figure 2 in ref. 5. Figure 3 in ref. 5. • Demonstrated the first gate tunable spin transport in graphene spin valve at room temperature. Observed Local and non-local magnetoresistance. Gate dependent non-local magnetoresistance. Hanle spin precession.

  8. Hybrid Spintronic Devices Spin Injector Spin Detector Lateral Spin Valve Ferromagnetic Electrodes M M _ 0 + Spin Transport Layer Desired Characteristics Graphene (beginning in 2007) Room temperature operation Yes Yes (With tunnel barrier) High spin injection efficiency Gate-tunable spin transport Yes OK, 5 microns. Small graphene flakes. Spin transport over long distances Long spin lifetimes Theory: yes, Experiment: no Good potential 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 (7°) Co MgO (0°) 2nm MgO SLG SiO2 SLG Back Gate SLG 500 nm Si SiO2 Optical SEM SLG Co Standard ebeam lithography

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

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

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

  15. Nonlocal Magnetoresistance Parallel Anti-Parallel IINJ IINJ VNL VNL H H L L Injector Detectors Injector Detectors Spin up Spin up Vp>0 Spin dependent chemical potential Spin dependent chemical potential 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 Room temperature spin transport

  17. Nonlocal MR—Spacing dependence ΔR (mW) E7 E6 E4 λS ~1.6 μm E2 SLG E3 1 um E1 E5 Wei Han, K. Pi et al., APL. 94, 222109 (2009) L (mm) 1 3μm 2μm L = 1 μm L = 2 μm L = 3 μm RNl (mΩ) RNL(mΩ) RNL(mΩ) H (mT) H (mT) H (mT)

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

  19. Hanle spin precession – spin lifetime measurement IINJ VNL  L = 3 μm 1.0 L 0.5 Diffusion coefficient RNL(mΩ) 0 D = 0.025 m2/s ts= 84 ps λs= 1.5 μm -0.5 Spin Lifetime -1.0 -160 -80 0 80 160 spin lifetime is “short”. H (mT)

  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. Theoretical analysis How to achieve efficient spin injection? Takahashi, et al, PRB 67, 052409 (2003) Tunneling contacts Co MgO SLG Insert a thin tunnel barrier to make R1, R2 >> RG 120 How to fabricate pin-hole free tunnel barrier. L=λG=W=2 μm PF=0.5, PJ=0.4 ρG=2 kΩ 60 RNL(Ω) Transparent contacts 0 0 20000 40000 Interface resistance (R1, R2 )(Ω)

  23. MgO Barrier with Ti adhesion layer Ti 1 nm MgO on graphite (AFM) MgO No Ti graphite RMS roughness: 0.229nm RMS roughness: 0.766nm W. H. Wang, W. Han et. al. ,Appl. Phys. Lett. 93, 183107 (2008).

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

  25. Tunneling spin injection into SLG Large Non-local MR with high spin injection efficiency Johnson & Silsbee, PRL, 1985. Jedema, et al, Nature, 2002 . DRNL=130 W , PJ=31 % Wei Han, K. Piet. al., PRL 105, 167202 (2010).

  26. Comparison of Co/SLG and Co/MgO/SLG Co Co 2nm 3nm 1nm MgO MgO SLG SLG SiO2 SiO2 L=2.1 mm L=1 mm 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

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

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

  30. Gate Tuning of Spin Signal 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 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, 241403(R) (2009). N. Tombros, et al., Phys. Rev. Lett. 101, 046601 (2008).

  33. Experiment Charged impurities (we use Au in this study) We add charged impurities onto a graphene spin valve to study its effect on spin lifetime. MBE cell V I - + Co electrode Single-Layer Graphene (SLG) SiO2 Si (backgate) Graphene spin valve device • K. Pi, Wei Han et.al., Phys. Rev. Lett. 104, 187201 (2010).

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

  36. In situ measurement Au 4 s Au is selected for this study because Au behaves as a point-like charged impurity on graphene. Gate dependent conductivity vs. Au deposition time Conductivity (mS) Au 8 s No Au T=18 K Gate Voltage (V) Au 2 s Au 6 s Coulomb scattering is the dominant charge scattering mechanism. m (cm2/Vs) Deposition rate ~ 0.04 Å/min (5x1011 atom/cm2s) Au deposition (Sec) K. M. McCreary, K. Pi et al., Phys. Rev. B 81, 115453 (2010).

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

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

  39. Effect of charged impurities on spin lifetime Spin lifetime (ps) Au deposition (s) Spin lifetime and the diffusion coefficient are determined from Hanle spin precession data Momentum scattering Spin relaxation 0.06 0.04 Dirac Pt. Electrons D (m2/s) Holes 0.02 0.00 0 2 4 6 8 (2.9x1012 cm-2) Au deposition (sec) 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, sp3bound etc.). 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). 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. By Au doping we are able to enhance spin life time from 50 ps to 150 ps. 2.0 1.5 Conductivity (mS) 1.0 0.5 0.0 Enhancement of spin signal by chemical doping • At fixed gate voltage, Au doping can enhance conductivity. • No significant spin relaxation from charged impurities. Possible to tune spin properties by chemical doping instead of applying high electric field (gate voltage).

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

  43. 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, 026805 (2008). • Hydrogen storage. --- AI doped graphene as hydrogen storage at room temperature. Z. M. Ao et al., J. Appl. Phys. 105, 074307 (2009).

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

  46. 0 0 Dirac Point (V) Dirac Point (V) -40 -30 -80 -60 0.00 0.01 0.02 0.0 0.1 0.2 Fe coverage (ML) Ti coverage (ML) 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) 0.0038 ML 0.041 ML 0.0077 ML Conductivity (mS) 0.123 ML Conductivity (mS) 0.015 ML 0.205 ML Gate Voltage (V) Gate Voltage (V) Both Ti and Fe coverage show n-type doping Keyu Pi et al., PRB 80, 075406 (2009).

  47. Dirac point shift (V) Pt-1 Pt-2 Fe-1 Fe-2 Fe-3 Ti-1 Ti-2 Ti-3 0 -20 Dirac Point (V) -40 TM coverage (ML) 0.10 0.15 0.00 0.05 Pt coverage (ML) Dirac point shift vs. Pt coverage ØPt = 5.9 eV No Pt (0 ML) 0.025 ML Conductivity (mS) 0.071 ML 0.127 ML Gate Voltage (V) • 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 dopingwhen making contact with graphene.

  48. Interfacial dipole Become n-type doping DV(d) = Dtr(d) + Dc(d) DV DV DV WG WG W Dtr(d) : The charge transfer between graphene and the metal (difference in work functions). W WM d d DEF DEF d W WG EF EF EF -DEF Dc(d) : the overlap of the metal and graphene wave functions Graphene Graphene Metal Graphene Dc(d) = e−gd (a0 + a1d + a2d2) -q +q -q +q Highly depends on d. -q +q G. Giovannetti et al., Physical Review Letters 101, 026803 (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, 026803 (2008). M. A. Pushkin et al, Bulletin of the Russian Academy of Science: Physics 72, 878 (2008).

  50. Interfacial dipole By Theoretical calculation, d increase as material coverage went from adatoms to continuous film. Pt Coverage (Å) d d 0 2 4 6 8 d Graphene AFM 2 Dirac Point (V) 0.62 ML 3.19 ML AFM 1 10 nm 0 nm AFM 1 AFM 2 0 0.87 1.75 2.62 3.50 Pt Coverage (ML) Experimental evidence of interfacial dipole. K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77, 235430 2008.

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