Spintronics and Graphene

Spin Valves and Giant Magnetoresistance

Graphene spin valves

Coherent spin valves with graphene

Fe/Cr stack:T = 4.2 KHAPPL = 0Fe Spins anti-parallel for dCr< 30 High resistance due to spin-dependent scatteringStrong HAPPL

Low resistance due to spin-dependent scatteringFe/Cr stack:T = 4.2 KHAPPL = H(saturation)Fe Spins parallel

Babitch, et al., PRL 61 (1988) 2472Very large MR = [R() R()]/R() (different from TMR!)

From http://en.wikipedia.org/wiki/File:Spin-valve_GMR.svgGMR effect is due to fact that electron scattering is less for spin aligned and spin antiparallel electrons.

()/() ~ 20 in some systems

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 differenceFrom http://en.wikipedia.org/wiki/File:Spin-valve_GMR.svgTechnology in use in magnetic recording media/memoriesIn plane spin valve

Is graphene a good medium for spintronics?

High mobility should yield long spin diffusion length (~ 1-2 m, Tombros, et al., Nature 448, 571 (2007)

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

Spin diffusiongrain boundaries, substrate interactions lower the graphene mobilities to ~ 2000 cm2/V-s oxideSpin injection via tunneling, Not very efficient (< 10%)H appliedPLP = [N - N]/[N + N]Length dependenceDevice is dimension-dependent

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 bandHAUGEN, HUERTAS-HERNANDO, AND BRATAASPHYSICAL REVIEW B 77, 115406 2008

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

Graphene growth on Co3O4(111)/Co(0001)MBE (graphite source)@1000 K: Layer-by-layer growth*1st ML2nd ML0.4 ML3 MLM. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201IEEE Nanodev. 2012

IEEE Nanodev. 2012

LEED: Oxide/Carbon Interface is incommensurate:Spinel is more stable than rocksalt (111)

Graphene Domain Sized (from FWHM) ~1800 (comp. to HOPG)

Oxide spots attenuated with increasing Carbon coverage2.8 O-O surface repeat distance on Co3O4(111)W. Meyer, et al. JPCM 20 (2008) 2650112.8 2.5 0.4 ML3 MLgrapheneCo3O4(111)*65 eV beam energyM. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 07220165 eV beam energyIEEE Nanodev. 2012

IEEE Nanodev. 2012

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

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

IEEE Nanodev. 2012

*Eexch > 300 KMagnetic polaron formationA New Type of Spin Switch? Unpolarized State (OFF)Polarized State (ON)

Problem: Most samples appear to order in plane (oxide and graphene)Do not know why???????

NOTE: AF Ordering at > 420 K!!TN Co3O4 ~ 40 KStrong graphene/Co3O4/Co exchange!

Alternative: Cr2O3 on Co(0001)strong oxide perpendicular anisotropy

TN ~ 300 KWill 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?MagnetoelectricVoltage control of magnetic behavior

OxGrOxGr(a)(b)(c)(d)Gr/Co3O4(111)/Co(111)Gr/Cr2O3(0001)/Co(111)

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

Stay tuned!

Potential Spintronics ApplicationGraphene on a Co3O4(111):Magnetic Polaron Formation for Spin ValvesCoherent Spin Transport?Magnetic Polaron FormationStabilized by Graphene/Co ion exchange interactionsCoherent Spin-FET*IEEE Nanodev. 2012

IEEE Nanodev. 2012

Conventional Spin ValvePolarization is a function of source/drain distance(Tombros, et al., Nature 448(1007) 571Coherent Spin ValvePolarization is uniformCoherent spin transport:No spin injectionNo spin diffusion*Why is a coherent spin valve different?or Cr2O3

*Kawakami group/7 K(Wang, et al. PRB 77 (2008) 020402RCho, et al., APL 91 (2007) 12310512%@4K6%@4KGraphene, but diffusive spin transportGraphene/FM structure>200%@300 K(predicted)?Band Gap(NiO(111)/Ni(111)?Other factors100%200%Graphene/magnetic oxide: coherent spin transportsCoherent vs. Diffusive Spin FETS

**Fig.2 LEED image of graphene/Co3O4(111)/Co(111)/Al2O3(0001) (a). after the 1st PVD carbon growth; (b). after the 1st PVD carbon growth; (c). Line scan intensity in (a); (d). Line scan intensity in (b);

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