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皮克宇 * Department of Physics and Astronomy UC Riverside 4 月 26 日, 2011 NTNU *Current location: Hitachi Global Storage Technologies Spintronic and electronic.

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Presentation on theme: "皮克宇 * Department of Physics and Astronomy UC Riverside 4 月 26 日, 2011 NTNU *Current location: Hitachi Global Storage Technologies Spintronic and electronic."— Presentation transcript:

1 皮克宇 * Department of Physics and Astronomy UC Riverside 4 月 26 日, 2011 NTNU *Current location: Hitachi Global Storage Technologies Spintronic and electronic transport properties in graphene – The cornerstone for spin logic devices.

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

3 Silicon electronics and the “end-of-the-roadmap”…. How to improve computers beyond the physics limits of existing technology? Motivation for Spintronics Spintronics: Utilize electron spin in addition to charge for information storage and processing. Spin up “1” Spin down “0” Spins for digital information OR

4 Logic: Silicon-based electronics are the dominant technology for microprocessors. Technological Approach Storage: Magnetic Hard Drives and Magnetic RAM use metal-based spintronics technologies. 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. Hanan Dery et al., arXiv (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. Carbon Nanotube 1D K. Tsukagoshi, B. W. Alphenaar, and H. Ago, Nature 401, 572 (1999). Graphite 3D M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, (2007). Graphene 2D Discover in 2004 !! K. S. Novoselov et al., Science 306, 666 (2004).

6 Properties of Graphene  High mobility -- up to 200,000 cm 2 /Vs (typically 1,000 – 10,000 cm 2 /Vs).  Zero gap semiconductor with linear dispersion: “massless Dirac fermions”.  Tunable hole/electron carrier density by gate voltage.  Possible for large scale device fabrication. Electronic Band Structure Physical Structure Atomic sheet of carbon 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 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, (2007). (Prof. Fuhrer’s group at Maryland) 4.M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, (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, (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 transport over long distances Long spin lifetimes Allows spin manipulation Gate-tunable spin transport High spin injection efficiency 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 Yes Spin Injector Spin Detector 0 + _ Lateral Spin Valve Ferromagnetic Electrodes Spin Transport Layer

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

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

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

12 Device characterization I (μA) dV/dI (kΩ) R 4pt R 3pt R 3pt – R 4pt V g = 0 V E1E2E3E4 I V R electrode + R contact < 300 ohms Transparent contact of Co/SLG Contact resistance Gate dependent resistance E1E2E3E4 I V MgO Co SLG E1E2E3E4 I V  ~ 2500 cm 2 /Vs

13 Spin Injection and Chemical Potential FM graphene Chemical Potential (Fermi level) e- Spin-dependent Chemical potential 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: Spin Injector Spin Detector charge current spin current I INJ V NL + - M. Johnson, and R. H. Silsbee, PRL, 55, 1790 (1985) Using lock-in detection

15 Nonlocal Magnetoresistance I INJ V NL L H InjectorDetector s V p >0 I INJ V NL L H InjectorDetector s V AP <0 ParallelAnti-Parallel Spin down Spin up Nonlocal MR = (V P - V AP )/I INJ Spin dependent chemical potential Spin dependent chemical potential

16 Spin Signal Nonlocal MR = ΔR NL = ΔV NL /I inj ΔR NL RT Nonlocal MR--- Temperature dependent Room temperature spin transport

17 L = 1 μm R Nl (mΩ) H (mT) Nonlocal MR—Spacing dependence R NL (mΩ) H (mT) L = 3 μm E1 SLG E2 E3 E4 E5 E6 E7 1 um 2μm 1 3μm L (  m) ΔR (m  ) λ S ~1.6 μm R NL (mΩ) H (mT) L = 2 μm Wei Han, K. Pi et al., APL. 94, (2009)

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

19 L = 3 μm H (mT) R NL (mΩ) Hanle spin precession – spin lifetime measurement I INJ V NL  L D = m 2 /s  s = 84 ps λ s = 1.5 μm Diffusion coefficient Spin Lifetime spin lifetime is “short”.

20 Challenges Create spin polarized current in graphene. Keep spin current polarized in graphene. How to increase the spin injection efficiency? What is the spin relaxation mechanism in graphene?

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

22 Theoretical analysis How to achieve efficient spin injection? Insert a thin tunnel barrier to make R 1, R 2 >> R G MgO Co SLG L=λ G =W=2 μm P F =0.5, P J =0.4 ρ G =2 kΩ Interface resistance (R 1, R 2 )(Ω) R NL (Ω) Tunneling contacts Transparent contacts How to fabricate pin-hole free tunnel barrier. Takahashi, et al, PRB 67, (2003)

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

24 Tunneling spin injection into SLG I V -+ Fabrication and Electrical characterization SLG SiO 2 Ti/MgO (0°) Ti/MgO (9°) Co (7°) MgO SLG SiO 2 TiO 2 I Co V DC (V) I DC (μA) 2-probe 300 K 3-probe 300 K I DC (  A) dV/dI (k  )

25 Tunneling spin injection into SLG  R NL =130 , P J =31 % Large Non-local MR with high spin injection efficiency Johnson & Silsbee, PRL, Jedema, et al, Nature, Wei Han, K. Pi et. al., PRL 105, (2010).

26 Comparison of Co/SLG and Co/MgO/SLG  R NL = 0.02  P ~ 1% Co 1nm SiO 2 MgO SLG 3nm Co SiO 2 MgO SLG 2nm Tunnel barrier increases spin signal by factor of ~1,000  R NL =130  P ~ 31% V g =0 V L=1  m L=2.1  m V g =0 V

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

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 contactPin-hole contact

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

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

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

33 Single-Layer Graphene (SLG) Si SiO 2 (backgate) Co electrode Graphene spin valve device I V + - 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. Experiment K. Pi, Wei Han et.al., Phys. Rev. Lett. 104, (2010).

34 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. Challenges

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

36 In situ measurement Au 2 s Au 4 s Au 6 s Au 8 s No Au Gate Voltage (V) Conductivity (mS) Au is selected for this study because Au behaves as a point-like charged impurity on graphene. Gate dependent conductivity vs. Au deposition time Au deposition (Sec)  (cm 2 /Vs) Coulomb scattering is the dominant charge scattering mechanism. T=18 K Deposition rate ~ 0.04 Å/min (5x10 11 atom/cm 2 s) K. M. McCreary, K. Pi et al., Phys. Rev. B 81, (2010).

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

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

39 Spin lifetime and the diffusion coefficient are determined from Hanle spin precession data Effect of charged impurities on spin lifetime Au deposition (s) Spin lifetime (ps) (2.9x10 12 cm -2 ) Spin relaxation Charged impurities are not the dominant spin relaxation mechanism. Momentum scattering Au deposition (sec) D (m 2 /s) Dirac Pt. Electrons Holes

40 Spin relaxation mechanisms are correlated. Effect of D’yakonov-Perel mechanism.  c : Spin relaxation by Coulomb scattering.  j : Spin relaxation by other defects (lattice defects, sp 3 bound etc.). Slight enhancement of spin lifetime Y. Gan et al., Small 4, 587 (2008). S. Molola et al., Appl. Phys. Lett. 94, (2009). Wei Han et al., arXiv (2011). 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. Recent study shows that Co contact plays an important role.

41 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) Conductivity (mS) By Au doping we are able to enhance spin life time from 50 ps to 150 ps.

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 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) Acknowledgements Thank you.

44 Hydrogen storage. --- AI doped graphene as hydrogen storage at room temperature. Z. M. Ao et al., J. Appl. Phys. 105, (2009). 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. --- Pt may induce localized magnetic states in Graphene. Y. Mao et al., Journal of Physics: Condensed Matter 20, 2008 (2008). B. Uchoa et al., Phys. Rev. Lett. 101, (2008). New physics in TM doped graphene system

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

46 Dirac point shift vs. Ti and Fe coverage Conductivity (mS) Gate Voltage (V) Ø Ti = 4.3 eV Ø Fe = 4.7 eVØ graphene = 4.5 eV Both Ti and Fe coverage show n-type doping No Ti (0 ML) ML ML ML No Fe (0 ML) ML ML ML Dirac Point (V) Ti coverage (ML) Dirac Point (V) Fe coverage (ML) Keyu Pi et al., PRB 80, (2009).

47 Dirac point shift vs. Pt coverage TM coverage (ML) Dirac point shift (V) Pt-1 Pt-2 Fe-1 Fe-2 Fe-3 Ti-1 Ti-2 Ti-3 Conductivity (mS) Gate Voltage (V) Ø Pt = 5.9 eV 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. No Pt (0 ML) ML ML ML Dirac Point (V) Pt coverage (ML)

48 Interfacial dipole WMWM Metal d WGWG EFEF VV W EFEF Graphene +q-q  V(d) =  tr (d) +  c (d) d WGWG EFEF VV W EFEF Graphene +q-q d WGWG VV W EFEF Graphene +q-q Become n-type doping G. Giovannetti et al., Physical Review Letters 101, (2008).  tr (d) : The charge transfer between graphene and the metal (difference in work functions).  c (d) : the overlap of the metal and graphene wave functions  c (d) = e −  d (a 0 + a 1 d + a 2 d 2 ) Highly depends on d.

49 G. Giovannetti et al., Physical Review Letters 101, (2008). Possible reason for anomalous n-type doping --- 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. M. A. Pushkin et al, Bulletin of the Russian Academy of Science: Physics 72, 878 (2008). Transition metal Graphene d p-type n-type

50 Experimental evidence of interfacial dipole Pt Coverage (ML) Pt Coverage (Å) Dirac Point (V) AFM 1 AFM 2 0 nm 10 nm 3.19 ML0.62 ML AFM 1 AFM 2 K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77, Graphene d d d By Theoretical calculation, d increase as material coverage went from adatoms to continuous film. Interfacial dipole

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 The electron and hole mobilities (μ e, μ h ) are determined by taking a linear fit of the σ vs. n curve just away from the Dirac point (μ e,h = |Δσ/Δne| ) Fe data show strong electron hole asymmetry. Mobility change vs. TM coverage Dirac point shift with TM coverage: Ti >Fe >Pt Mobility drop with TM coverage: Ti >Fe >Pt Dirac point shift vs. Mobility change ? Mobility,  (10 3 cm 2 /Vs) Fe-2 Conductivity (mS) Pt-2 Ti n (10 12 cm -2 )Coverage (ML)

53 Hole Electron Dirac Point Shift (V) Normalized mobility, μ/μ 0 Dirac Point Shift (V) Pt-1 Pt-2 Ti-1 Ti-2 Fe-2 μ/μ 0 = (Γ 0 + Γ TM ) -1 /Γ 0 -1 = (1 + Γ TM /Γ 0 ) -1 Fitting equation: Electron data follows the universal curve. Hole data is significantly different. This implies some wave function hybridization in the Fe system. Ti and Pt fall on the universal curve. Coulomb scattering is the dominant effect. Mobility change vs. Dirac point shift 0.1 ML ML Γ TM /Γ 0 = (A VD,shift ) β μ/μ 0 Keyu Pi, K. M. McCreary et al., PRB 80, (2009).


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