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 High resistance due to spin-dependent scattering Strong H APPL  Fe Cr V Fe Cr Fe Cr V Low resistance due to spin-dependent scattering Fe/Cr stack: T = 4.2 K H APPL = H(saturation) Fe Spins parallel

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

From GMR effect is due to fact that electron scattering is less for spin aligned and spin antiparallel electrons. λ(↑↑)/λ(↑↓) ~ 20 in some systems

Spin valvesSpin 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 Technology in use in magnetic recording media/memories In plane spin valve

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)

Graphene Spin Valves—Early 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)

Spin diffusion—grain boundaries, substrate interactions lower the graphene mobilities to ~ 2000 cm 2 /V-s oxide Spin injection via tunneling, Not very efficient (< 10%) H applied P L P = [N↑ - N↓]/[N↑ + N↓] Length dependence—Device 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 band HAUGEN, HUERTAS-HERNANDO, AND BRATAAS PHYSICAL REVIEW B 77,

Spin relaxation rate in graphene much faster than predicted. Why: Interaction with “magnetic defects” in physically transferred graphene (Lundeberg, et al. PRL 110, (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 Co 3 O 4 (111)/Co(0001) MBE (graphite K: Layer-by-layer growth 11 1 st ML 2 nd ML 0.4 ML 3 ML M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) IEEE Nanodev. 2012

LEED: Oxide/Carbon Interface is incommensurate: Spinel is more stable than rocksalt (111) Graphene Domain Sized (from FWHM) ~1800 Å (comp. to HOPG) (a) (c) 65eV (d) (b) Oxide spots attenuated with increasing Carbon coverage 2.8 Å O-O surface repeat distance on Co 3 O 4 (111) W. Meyer, et al. JPCM 20 (2008) Å 2.5 Å 0.4 ML 3 ML graphene Co 3 O 4 (111) eV beam energy M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) eV beam energy IEEE Nanodev. 2012

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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! 14IEEE Nanodev. 2012

15 Graphene conduction electrons (unpolarized) Co +2 ions (unpolarized) Co(111) Co 3 O 4 (111) Sapphire(0001) Graphene conduction electrons (polarized) Co +2 ions (polarized) Co(111) Co 3 O 4 (111) Sapphire(0001) E exch > 300 K Magnetic polaron formation A 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!! T N Co 3 O 4 ~ 40 K Strong graphene/Co3O4/Co exchange!

Alternative: Cr 2 O 3 on Co(0001)—strong oxide perpendicular anisotropy T N ~ 300 K  Will Cr 2 O 3 (0001) on Co(0001) order at higher Temp?  Will it order with perpendicular anisotropy?  Can we grow Cr 2 O 3 (0001) on Co?  Can we grow graphene on Cr 2 O 3 (0001) on Co? Magnetoelectric Voltage control of magnetic behavior

Ox Gr Ox Gr (a) (b) (c) (d) Gr/Co 3 O 4 (111)/Co(111) Gr/Cr 2 O 3 (0001)/Co(111)

Can we grow Gr/Cr 2 O 3 by a method which does not involve leaving the Auger electron gun on overnight??????????????????????? Stay tuned!

Potential Spintronics Application Graphene on a Co 3 O 4 (111): Magnetic Polaron Formation for Spin Valves Coherent Spin Transport? Magnetic Polaron Formation Stabilized by Graphene/Co ion exchange interactions Coherent Spin-FET 20 IEEE Nanodev. 2012

Conventional Spin Valve Polarization is a function of source/drain distance (Tombros, et al., Nature 448(1007) 571 P = N↑ - N↓ N↑+N↓ Coherent Spin Valve Polarization is uniform Coherent spin transport: No spin injection No spin diffusion 21 Why is a coherent spin valve different? or Cr 2 O 3

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