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Spintronics and Graphene Spin Valves and Giant Magnetoresistance Graphene spin valves

Spintronics and Graphene Spin Valves and Giant Magnetoresistance Graphene spin valves Coherent spin valves with graphene. Fe/Cr stack: T = 4.2 K H APPL = 0 Fe Spins anti-parallel for d Cr < 30 Å. Fe/Cr stack: T = 4.2 K H APPL = H(saturation) Fe Spins parallel. Strong H APPL . Fe.

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Spintronics and Graphene Spin Valves and Giant Magnetoresistance Graphene spin valves

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  1. Spintronics and Graphene • Spin Valves and Giant Magnetoresistance • Graphene spin valves • Coherent spin valves with graphene

  2. Fe/Cr stack: T = 4.2 K HAPPL = 0 Fe Spins anti-parallel for dCr< 30 Å Fe/Cr stack: T = 4.2 K HAPPL = H(saturation) Fe Spins parallel Strong HAPPL  Fe Fe Fe Cr Cr V V Cr Cr Cr Low resistance due to spin-dependent scattering High resistance due to spin-dependent scattering

  3. Fe Fe Fe Cr Cr V V Cr Cr Babitch, et al., PRL 61 (1988) 2472 Very large MR = [R(↑↓) – R(↑↑)]/R(↑↑) (different from TMR!)

  4. GMR effect is due to fact that electron scattering is less for spin aligned and spin antiparallel electrons. λ(↑↑)/λ(↑↓) ~ 20 in some systems From http://en.wikipedia.org/wiki/File:Spin-valve_GMR.svg

  5. Technology in use in magnetic recording media/memories In plane spin valve Spin valves in the reading head of a sensor in the CIP (left) and CPP (right) geometries. Red: leads providing current to the sensor, green and yellow: ferromagnetic and non-magnetic layers. V: potential difference From http://en.wikipedia.org/wiki/File:Spin-valve_GMR.svg

  6. Is graphene a good medium for spintronics? • High mobility should yield long spin “diffusion” length • (~ 1-2 μm, Tombros, et al., Nature 448, 571 (2007)

  7. Graphene Spin Valves—Early attempts (Tombros, et al, Nature 448 (2007), Kawakami group (UCR), Fuhrer group, Umaryland) W. Han, et al. (Kawakami group) Proc. SPIE 7398(2009) 739819-1 General Results, uninspiring, MR ~ 10% at cryogenic temperatures! WHY????????

  8. Spin injection via tunneling, Not very efficient (< 10%) oxide H applied Spin diffusion—grain boundaries, substrate interactions lower the graphene mobilities to ~ 2000 cm2/V-s P = [N↑ - N↓]/[N↑ + N↓] Length dependence—Device is dimension-dependent… P L

  9. Basic Problem: Previous designs deal with transport of discrete spins Can we polarize spins in graphene near the Fermi Level? Prediction: Yes, predicted graphene/ferromg. Exchange interactions lead to polariztion of graphene conduction band HAUGEN, HUERTAS-HERNANDO, AND BRATAAS PHYSICAL REVIEW B 77, 115406 2008

  10. Spin relaxation rate in graphene much faster than predicted. Why: Interaction with “magnetic defects” in physically transferred graphene (Lundeberg, et al. PRL 110, 156601 (2013)) Spin de-phasing rate decreases in external magnetic field is applied. Data indicate a relaxation time for individual spins of ~ 5 ns

  11. Graphene growth on Co3O4(111)/Co(0001) MBE (graphite source)@1000 K: Layer-by-layer growth 1st ML 3 ML 2nd ML 0.4 ML M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 IEEE Nanodev. 2012

  12. LEED: Oxide/Carbon Interface is incommensurate: Spinel is more stable than rocksalt (111) Graphene Domain Sized (from FWHM) ~1800 Å (comp. to HOPG) 65 eV beam energy 65eV (a) (b) graphene 0.4 ML Co3O4(111) 65 eV beam energy (d) (c) 3 ML 65eV Oxide spots attenuated with increasing Carbon coverage 2.5 Å 2.8 Å M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 2.8 Å O-O surface repeat distance on Co3O4(111) W. Meyer, et al. JPCM 20 (2008) 265011 IEEE Nanodev. 2012

  13. http://iramis.cea.fr/sis2m/en/Phocea/Vie_des_labos/Ast/ast_sstechnique.php?id_ast=499http://iramis.cea.fr/sis2m/en/Phocea/Vie_des_labos/Ast/ast_sstechnique.php?id_ast=499

  14. Room temperature MOKE (blue) and Reflectivity (Red) Data (from Dowben group): Graphene ferromagnetic ordering perpendicular to sample plane! AF ordering 260 K above Néel Point! IEEE Nanodev. 2012

  15. A New Type of Spin Switch? Unpolarized State (OFF) Graphene conduction electrons (unpolarized) Graphene conduction electrons (polarized) Co+2 ions (unpolarized) Co3O4(111) Co3O4(111) Co+2 ions (polarized) Co(111) Co(111) Sapphire(0001) Sapphire(0001) Polarized State (ON) Eexch > 300 K Magnetic polaron formation

  16. Problem: Most samples appear to order in plane (oxide and graphene) Do not know why??????? NOTE: AF Ordering at > 420 K!! TN Co3O4 ~ 40 K Strong graphene/Co3O4/Co exchange!

  17. Alternative: Cr2O3 on Co(0001)—strong oxide perpendicular anisotropy TN ~ 300 K Magnetoelectric Voltage control of magnetic behavior • Will Cr2O3(0001) on Co(0001) order at higher Temp? • Will it order with perpendicular anisotropy? • Can we grow Cr2O3(0001) on Co? • Can we grow graphene on Cr2O3(0001) on Co?

  18. (b) Gr Ox (a) Gr/Co3O4(111)/Co(111) (d) Gr (c) Ox Gr/Cr2O3(0001)/Co(111)

  19. Can we grow Gr/Cr2O3 by a method which does not involve leaving the Auger electron gun on overnight??????????????????????? Stay tuned!

  20. Potential Spintronics Application Graphene on a Co3O4(111): Magnetic Polaron Formation for Spin Valves Coherent Spin Transport? Magnetic Polaron Formation Stabilized by Graphene/Co ion exchange interactions Coherent Spin-FET IEEE Nanodev. 2012

  21. Why is a coherent spin valve different? Conventional Spin Valve Polarization is a function of source/drain distance (Tombros, et al., Nature 448(1007) 571 Polarization is uniform Coherent Spin Valve P = N↑ - N↓ or Cr2O3 N↑+N↓ Coherent spin transport: No spin injection No spin diffusion

  22. Coherent vs. Diffusive Spin FETS ? Graphene/magnetic oxide: coherent spin transports 200% 100% Graphene, but diffusive spin transport Band Gap (NiO(111)/Ni(111)? Other factors 12%@4K 6%@4K Kawakami group/7 K (Wang, et al. PRB 77 (2008) 020402R Graphene/FM structure >200%@300 K (predicted) Cho, et al., APL 91 (2007) 123105

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