Endre Tóvári
International Workshop on Electrical
Properties of New Materials (IWEPNM)
Kirchberg-in-Tirol, 2012. March 3-10.
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Tóvári Endre: Kirchberg IWEPNM 2012
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Topics
• synthesis of CNT and graphene
• CNT sorting and functionalization, nanopore
arrays
• graphene magnetism, spintronics,
1 mm
optoelectronics, transport in suspended SLG,
BLG and TLG, FQHE, graphene on hexagonal BN
• optical conductivity, Raman spectroscopy, intrinsic properties,
effect of strain, substrate, ESR
• functionalization, doping
Tóvári Endre: Kirchberg IWEPNM 2012
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A few interesting presentations
Growth of high-density CNT forests
Liquid-induced
densification of
different
SWNT forests bygel
soaking
the samples with ethanol and drying in air.
Sorting
of SWCNTs
using
multi-column
chomatography
structure-dependent interaction
strength of SWCNTs with an allyl
dextran-based gel
Japanese Journal of Applied Physics 51 (2012) 01AH01
NATURE COMMUNICATIONS | 2:309 | DOI: 10.1038/ncomms1313
Tóvári Endre: Kirchberg IWEPNM 2012
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A few interesting presentations
Repeated growth and bubbling transfer of graphene with millimetre-size singlecrystal grains using platinum
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mm-size single-crystal SLG
CVD on polycrystalline Pt
µ > 7000 cm2V-1s-1
bubbling transfer: nondestructive to
Pt and graphene both
increasing T or low conc. of CH4:
nucleation density decreases, grain
size increases (ambient pressure,
T>1000°C)
CH4/H2 flow ratio 4/700 sccm:
dominantly hexagonal grains with
smooth edges (suppressed
nucleation, and low stability edges are
etched away by active atomic H)
most grains: no reflex angle at edges,
no visible boundaries under SEM
no new nuclei with increasing growth
time
Nature Communications, 3:699 | DOI: 10.1038/ncomms1702
Raman D-band intensity
map: showing grain
boundary
1 mm
0,5 mm
d,e,f
g,h,i 100 µm
d,e,f: grain boundary
10 µm
10 µm
10 µm
10 µm
Tóvári Endre: Kirchberg IWEPNM 2012
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A few interesting presentations
Repeated growth and bubbling transfer of graphene with millimetre-size singlecrystal grains using platinum
Pt+Gr+PMMA cathode (-):
Bubbling transfer:
aqueous NaOH electrolysis cell
nondestructive to Pt: reusable
nondestructive to graphene, transfer to SiO2
• free of metal residues
• preserves the original shape
• mostly monolayer
• small Raman D-band (ID/IG<5%)
• µ > 7000 cm2V-1s-1
100 µm
400 µm
Nature Communications, 3:699 | DOI: 10.1038/ncomms1702
Tóvári Endre: Kirchberg IWEPNM 2012
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A few interesting presentations
Spin-half paramagnetism in graphene induced by point defects
SQUID magnetometry
graphene laminates: large collections of
electronically non-interacting, parallel SLG
and BLG crystallites (10-50 nm) before and
after fluorination
fluorination: clustering, only the atoms at cluster
edge without pairs on the other C sublattice
contribute ~10-3 µB/F
high conc. (x≈1): lower M, but still PM (still a
large number of defects in the CFx lattice)
(F conc.: Raman, XPS)
350-400 keV proton irradiation or 20 MeV C4+ irrad.:
~0,1-0,4 µB/defect (defect conc. calc. with simulation!)
Nature Physics Vol 8, 199, March 2012
Tóvári Endre: Kirchberg IWEPNM 2012
Review:
Spin transport and relaxation in graphene
• low intrinsic spin-orbit coupling (SOC) and hyperfine coupling (HFC)
• key words: spin injection, diffusion (spin-polarized currents), precession in
magnetic field, gate dependence, relaxation mechanisms
• extrinsic spin relaxation mechanisms (τS~µs expected, ~100 ps – ns
measured): Elliot-Yafet,local
Dyakonov-Perel
nonlocal
setup
setup
spin valves
spin diffusion is usually described by a spindependent chemical potential (µ↑ and µ↓),
where a splitting of the chemical potential
corresponds to the spin density in the
graphene.
Han, McCreary, Pi, Wang, Li, Wen, Chen, Kawakami ; Journal of Magnetism and Magnetic Materials 324 (2012) 369–381
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Tóvári Endre: Kirchberg IWEPNM 2012
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Spin valves: introduction
changing in-plane magnetic field:
changing magnetic polarization of Co
electrodes (different widths, different
coercivities: switching at different fields)
nonlocal
local
Nature Letters Vol 448, 571 (Aug 2007)
Tóvári Endre: Kirchberg IWEPNM 2012
Spin valves: introduction
Transparent and tunneling contacts: difference in spin injection efficiency
(1% or 26-30%)
Spin relaxation: contact spacing
RG: graphene spin resistance
RF: FM contact’s spin resistance
RJ: contact resistance
PF: spin polarization in FM contact
PJ: polarization if interfacial current
L: contact spacing
λG: spin diffusion length
transparent/tunneling: relation of RJ and RG
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Spin valves: introduction
Hanle spin precession in a ┴ magnetic field: spin precession
sign: P or AP
contact polariz.
spin diffusion in the L
(contact spacing) length
spin precession
spin relaxation
L  gBH
S  DS ~ 1  4 m
Tóvári Endre: Kirchberg IWEPNM 2012
Spin transport and relaxation in graphene
𝜏S does not decrease, although D does:
Au doping is effective at generating
momentum scattering, but in SLG for
transparent contacts charged impurity
scattering is not the dominant process
behind spin relaxation
Tunneling contacts on SLG: 400-1000 ps
Spin diffusion from Co into SLG: escape time
1
1
1
S1  spin




flip
esc
spin flip in good tunnel barriers
For pinhole and transparent contacts (50-200 ps range) the dominant spin
relaxation is generated by the contacts (escape time, inhomogeneous fields,
interfacial scattering)
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Spin relaxation in single-layer graphene
tunnel barriers: suppress the contact-induced spin
relaxation
𝜏S ~ 400-1000 ps
300 K: no correlation with D
4 K: strong correlation of 𝜏S and D in SLG with
tunneling contacts: both increasing with carrier
conc.; similar behaviour as a function of T
D (10-2 m2/s)
𝜏S ~ D ~ 𝜏p: Elliott-Yafet spin relaxation dominant at low T in SLG: finite
probability of spin-flip during a momentum scattering event (possible sources:
long-range and short-range impurity scattering; at RT multiple sources are
possible, such as phonons, which ruin the linear relationship; see Ref. 5)
Tóvári Endre: Kirchberg IWEPNM 2012
Spin relaxation in bilayer graphene
300 K: : 𝜏S ~ 200-400 ps, and no
correlation with D
4 K: 𝜏S ~ 2-6 ns, strong
correlation with D: opposite
behaviour with D as a function of
gate voltage
𝜏S-1 ~ D ~ 𝜏p dominant spin relaxation mechanism in BLG at low T (with
tunneling contacts): random magnetic Rashba fields of the Dyakonov-Pereltype (can be generated by ripples, adatoms in the graphene sheet): spin
relaxation via precession in internal spin-orbit fields.
(Elliott-Yafet mechanisms negligible due to enhanced screening of scatterers)
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Tóvári Endre: Kirchberg IWEPNM 2012
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All of the above mainly from:
Han, McCreary, Pi, Wang, Li, Wen, Chen, Kawakami: Review - Spin transport
and relaxation in graphene, Journal of Magnetism and Magnetic Materials 324
(2012) 369–381
Related articles (among many; mainly from van Wees’ and Kawakami’s group) :
1. Electronic spin transport and spin precession in SLG at room temperature, Nature
Letters Vol 448, 571 (Aug 2007)
2. Tunneling spin injection into SLG, PRL 105, 167202 (2010)
3. Comparison between charge and spin transport in FLG, PRB 83, 115410 (2011)
4. Observation of long spin-relaxation times in BLG at room temperature, PRL 107,
047206 (2011)
5. Spin relaxation in single layer and bilayer graphene, PRL 107, 047207 (2011)
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Thank you for your attention!
Tóvári Endre: Kirchberg IWEPNM 2012
pictures from the first slide:
• arXiv:1202.3212v1
• PRL 107, 217203 (2011)
• SCIENCE VOL 334, 648 (2011)
• Nature Communications, 3:699 | DOI: 10.1038/ncomms1702
http://www.iwepnm.org/2012/calendar.php
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