Spintronics and Graphene
Spin Valves and Giant Magnetoresistance
Graphene spin valves
Coherent spin valves with graphene
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
Cr
V
Fe
Cr
V
Cr
High resistance due to spin-dependent
scattering
Cr
Low resistance due to spin-dependent
scattering
Fe
Cr
V
Cr
Fe
F
Cr
e
V
Cr
Babitch, et al., PRL 61 (1988) 2472
Very large MR = [R(↑↓) – R(↑↑)]/R(↑↑) (different from TMR!)
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
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
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)
W. Han, et al. (Kawakami group) Proc. SPIE 7398(2009) 739819-1
General Results, uninspiring, MR ~ 10% at cryogenic temperatures!
WHY????????
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
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
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 dephasing 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 ML
3 ML
2nd ML
0.4 ML
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
IEEE Nanodev. 2012
11
Graphene Domain
Sized (from FWHM) (c)
65eV
~1800 Å (comp. to
HOPG)
Oxide spots
attenuated
with increasing
Carbon
coverage
65 eV beam energy
(b)
G1
40000
G2
graphene
35000
0.4 ML
Intensity
30000
Co3O4(111)
O1
25000
O2
20000
15000
10000
5000
400
300
200
100
0
Pixel Position
(d)
65 eV beam energy
40000
3 ML
G1
35000
Intensity
LEED:
(a) 65eV
Oxide/Carbon
Interface is
incommensurate:
Spinel is more
stable than rocksalt
(111)
G2
30000
2.5 Å
25000
O1
20000
2.8 Å
O2
15000
10000
5000
400
300
200
100
0
Pixel Position
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
12
http://iramis.cea.fr/sis2m/en/Phocea/Vie_des_lab
os/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
Néel Point!
IEEE Nanodev. 2012
14
A New Type of Spin Switch?
Graphene conduction electrons (unpolarized)
Unpolarized State (OFF)
Co+2
ions (unpolarized)
Co3O4(111)
Co(111)
Sapphire(0001)
Polarized State (ON)
Eexch > 300 K
Graphene conduction electrons (polarized)
Co3O4(111)
Magnetic polaron
formation
Co+2 ions (polarized)
Co(111)
Sapphire(0001)
15
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!
Alternative: Cr2O3 on Co(0001)—strong oxide perpendicular anisotropy
TN ~ 300 K
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?
Magnetoelectric
Voltage control of magnetic
behavior
60
C
O
Co
Co
dN/dE (a.u.)
30
0
(b)
-30
B
Gr
40000
35000
Ox
30000
-60
25000
(a)
-90
20000
Gr/Co3O4(111)/Co(111)
15000
10000
5000
0
100
200
300
400
500
600
700
50
100
150
200
250
300
800
Kinetic Energy (eV)
O
dN/dE (a.u.)
30
C
Cr
Co
(d)
0
40000
Gr
35000
30000
-30
(c)
Gr/Cr2O3(0001)/Co(111)
Ox
25000
20000
15000
100
200
300
400
500
600
700
800
10000
Kinetic Energy (eV)
5000
-50
0
50
100
150
200
250
300
350
400
Can we grow Gr/Cr2O3 by a method which does not involve
leaving the Auger electron gun on
overnight???????????????????????
Stay tuned!
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
20
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
P = N↑ - N↓
N↑+N↓
Coherent
Spin Valve
Polarization is uniform
or
Cr2O3
Coherent spin transport:
No spin injection
No spin diffusion
21
Coherent vs. Diffusive Spin FETS
200%
100%
?
Graphene/magnetic
oxide: coherent spin
transports
Graphene, but diffusive
spin transport
12%@4K
Kawakami group/7 K
(Wang, et al. PRB 77 (2008) 020402R
6%@4K
Cho, et al., APL 91
(2007) 123105
Band Gap
(NiO(111)/Ni(111)?
Other factors
Graphene/FM structure
>200%@300 K
(predicted)
22