Ferromagnetism and Anomalous Hall Effect in Graphene

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1 Ferromagnetism and Anomalous Hall Effect in Graphene Jing Shi Department of Physics & Astronomy, University of California, Riverside Graphene/YIG

2 Introduction Outline Proximity induced ferromagnetism Quantized anomalous Hall effect (QAHE) Graphene on Magnetic Insulator (MI) MI: atomically flat yttrium iron garnet (YIG) films Ferromagnetism in graphene & classical AHE Towards QAHE Tuning exchange: improved contact with YIG Enhancing Rashba SOC:TMD/graphene Reducing disorder: dry transfer Summary 2

3 Graphene for Spintronics Graphene Weak intrinsic SOC (<10 mev) Long spin diffusion length High mobility Gate tunable Open for modifications What about ferromagnetism? 3

4 QAHE in Graphene Graphene + Exchange Quantized anomalous Hall effect (QAHE) Qiao et al. PRB (10) Fermi level in gap: C=2 + Rashba SOC + Exchange + Rashba Topological band gap at K and K chiral edge states in band gap s AHE = ±2e 2 /h 4

5 QAHE-Based Spintronic Devices Quantized anomalous Hall effect (QAHE) s AHE = ±2e 2 /h Non-volatile memory Sensors (electric or magnetic field) 5

6 Magnetism in Graphene Pristine graphene: Strong Landau orbital diamagnetism Graphene nanoribbons: Spin-polarized edges Graphene nanoribbon Defects in graphene: Local magnetic moments from transition metals impurities or vacancies Possibly long-range magnetic order Our approach to ferromagnetism: graphene on FMI 6

7 Epitaxial Growth of Magnetic Films Laser MBE system for oxides Growth of [LSMO(7ML)/STO(3ML)] superlattice LSMO/STO (110) epitaxial growth Junction area ~ 3mmx3mm HRTEM image of (110)-oriented LSMO/STO superlattice 7 ML LSMO 3ML STO 7ML LSMO 3ML STO 2.7 nm XRD of LSMO/STO superlattices 7

8 Epitaxial, Atomically Flat YIG Films Yttrium iron garnet (YIG Y 3 Fe 5 O 12 ): Ferri-magnetic insulator- FMI (T c ~ 550 K) AFM of ~ 30 nm thick YIG/GGG (110) Lattice match with Gd 3 Ga 5 O 12 (GGG) RHEED pattern and oscillations HRTEM of YIG/GGG (110)

9 Layer-by-Layer Growth of YIG on GGG Ferromagnetic resonance (FMR) Continuous layer-by-layer growth of YIG for ~ 17 hours Ferromagnetic resonance (FMR) linewidth at 9.6 GHz ~ 4 Oe narrowest FMR peak for YIG Excellent magnetic properties 9

10 In-Plane Uniaxial Anisotropy In-plane rotation 10

11 Device Transfer Technique Graphene exfoliation SiO 2 Si Device fabrication SiO 2 Si Resist spin-coating SiO 2 Si PMMA Device transferred onto target substrate Target substrate PMMA Resist/device layer peel-off PMMA Si DI Water SiO 2 etching SiO 2 Si NaOH solution PMMA Resist lift-off Sachs et al. APL (2014) Sachs et al. Sci. Rep. (2014) Target substrate Gate dielectric: PMMA or h-bn

12 Device Transfer Technique (h-bn Top Gate) 12

13 Device Transfer Technique PMMA Target substrate Transferred devices Si DI Water PMMA/Graphene with contact pads floating in DI water High transfer yield Work for large-area graphene devices Any flat target substrates Sachs et al. Sci. Rep. (2014) Sachs et al. APL (2014)

14 Devices with h-bn Top Gate Before transfer Au/BN/Graphene/SiO 2 After transfer Au/BN/Graphene/YIG YIG Wet transfer h-bn h-bn SiO 2 Top gate Top gate BN-layer is ~30 nm thick. It is much better than PMMA! 14

15 Intensity (a.u.) Device Characterization Raman Room-temperature Raman spectra of single layer graphene Graphene Graphene on YIG/GGG Before Graphene on SiO 2 /Si YIG/GGG 5μm After Raman Shift (cm -1 ) Both graphene and YIG peaks are identified in the room temperature Raman spectra

16 Before and After Transfer (PMMA Top Gate) Before-transfer: Back-gated through SiO 2 s xx /C s (m 2 /s) Conductivity comparison SiO 2 293K YIG 300K YIG 2K V g (V) After-transfer: Top-gated through PMMA Room temperature mobility slightly increased (6,000 cm 2 /Vs) Transfer does not compromise device quality! 16

17 Before and After Transfer (BN Top Gate) Thin BN top gate BN SiO 2 No BN SiO 2 Much smaller applied gate voltage (a factor of 15) needed to tune the carrier density over a wide range With h-bn, mobility is enhanced by 6 times, reaching over 30,000 cm 2 /Vs! 17

18 Hall Effecf of Same Graphene on SiO 2, YIG Before transfer SiO 2 Rxy ( ) Overall Hall resistance Linear background H (Oe) After transfer YIG 18

19 Anomalous Hall Effect in Graphene/YIG Ordinary Hall effect R B xy 0 R M s Anomalous Hall effect Hall after linear background removal Magnetization of YIG Х OHE from two carriers Х Stray field effect Ferromagnetic graphene! 19

20 T-Dependence of AHE Temperature dependence 1.0 Magnetization of YIG M/M S B B (G) AHE disappears at ~ 300 K Z.Y.Wang et al. PRL (2015) 20

21 How Strong is Rashba Interaction? Topological band gap ~ smaller of l R and Intrinsic contribution h/t ~ 12 mev: disorder energy scale in our graphene is set to ~ 25 mev Maximum l R is set to 12 mev Observed AHE is likely caused by extrinsic mechanism: skew or side jump 21

22 Towards QAHE in Graphene Need stronger SOC Introduce sp 3 defects Proximity couple graphene to transition metal dichalcogenides Need weaker Disorder Dry transfer 22

23 WS 2 /Graphene Device for direct comparison WS2 Graphene 23

24 Tuning SOC Pristine vs. with WS 2 Conductance Magneto-conductance Suppressed weak-localization in WS 2 covered graphene! Consistent with a SO length of 250 nm 24

25 WS 2 /Graphene WS 2 /Graphene Magneto-conductance Weak anti-localization 25

26 Summary Graphene becomes ferromagnetic when it is exchange coupled to atomically flat YIG ferrimagnetic insulator. AHE is observed SOC already exists at interface. Exchange strength is shown ~26 mev (308 K). 1/5 of quantized anomalous Hall value is obtained. Rashba SOC interaction may be enhanced by introducing sp 3 defects, or by proximity coupling to TMD. All-proximity graphene devices may be needed.

27 Acknowledgement Z.Y. Wang, C. Tang, J.L. Jiang, B.W. Yang, Z.S. Lin, M. Aldosary, R. Sachs, and Y. Barlas B. Cheng and Bockrath group at UCR