51 research outputs found
Non-linear spin Seebeck effect due to spin-charge interaction in graphene
The abilities to inject and detect spin carriers are fundamental for research
on transport and manipulation of spin information. Pure electronic spin
currents have been recently studied in nanoscale electronic devices using a
non-local lateral geometry, both in metallic systems and in semiconductors. To
unlock the full potential of spintronics we must understand the interactions of
spin with other degrees of freedom, going beyond the prototypical electrical
spin injection and detection using magnetic contacts. Such interactions have
been explored recently, for example, by using spin Hall or spin thermoelectric
effects. Here we present the detection of non-local spin signals using
non-magnetic detectors, via an as yet unexplored non-linear interaction between
spin and charge. In analogy to the Seebeck effect, where a heat current
generates a charge potential, we demonstrate that a spin current in a
paramagnet leads to a charge potential, if the conductivity is energy
dependent. We use graphene as a model system to study this effect, as recently
proposed. The physical concept demonstrated here is generally valid, opening
new possibilities for spintronics
Nonlinear interaction of spin and charge currents in graphene
We describe a nonlinear interaction between charge currents and spin currents
which arises from the energy dependence of the conductivity. This allows
nonmagnetic contacts to be used for measuring and controlling spin signals. We
choose graphene as a model system to study these effects and predict its
magnitudes in nonlocal spin valve devices. The ambipolar behavior of graphene
is used to demonstrate amplification of spin accumulation in p-n junctions by
applying a charge current through nonmagnetic contacts.Comment: minor changes, 4 pages, 3 figure
Spin transport in graphene nanostructures
Graphene is an interesting material for spintronics, showing long spin
relaxation lengths even at room temperature. For future spintronic devices it
is important to understand the behavior of the spins and the limitations for
spin transport in structures where the dimensions are smaller than the spin
relaxation length. However, the study of spin injection and transport in
graphene nanostructures is highly unexplored. Here we study the spin injection
and relaxation in nanostructured graphene with dimensions smaller than the spin
relaxation length. For graphene nanoislands, where the edge length to area
ratio is much higher than for standard devices, we show that enhanced spin-flip
processes at the edges do not seem to play a major role in the spin relaxation.
On the other hand, contact induced spin relaxation has a much more dramatic
effect for these low dimensional structures. By studying the nonlocal spin
transport through a graphene quantum dot we observe that the obtained values
for spin relaxation are dominated by the connecting graphene islands and not by
the quantum dot itself. Using a simple model we argue that future nonlocal
Hanle precession measurements can obtain a more significant value for the spin
relaxation time for the quantum dot by using high spin polarization contacts in
combination with low tunneling rates
Spin transport in high quality suspended graphene devices
We measure spin transport in high mobility suspended graphene (\mu ~ 10^5
cm^2/Vs), obtaining a (spin) diffusion coefficient of 0.1 m^2/s and giving a
lower bound on the spin relaxation time (\tau_s ~ 150 ps) and spin relaxation
length (\lambda_s=4.7 \mu m) for intrinsic graphene. We develop a theoretical
model considering the different graphene regions of our devices that explains
our experimental data.Comment: 22 pages, 6 figures; Nano Letters, Article ASAP (2012)
(http://pubs.acs.org/doi/abs/10.1021/nl301050a
Surface sensitivity of the spin Seebeck effect
We have investigated the influence of the interface quality on the spin
Seebeck effect (SSE) of the bilayer system yttrium iron garnet (YIG) - platinum
(Pt). The magnitude and shape of the SSE is strongly influenced by mechanical
treatment of the YIG single crystal surface. We observe that the saturation
magnetic field H_{sat} for the SSE signal increases from 55.3 mT to 72.8 mT
with mechanical treatment. The change in the magnitude of H_{sat} can be
attributed to the presence of a perpendicular magnetic anisotropy due to the
treatment induced surface strain or shape anisotropy in the Pt/YIG system. Our
results show that the SSE is a powerful tool to investigate magnetic anisotropy
at the interface.Comment: 5 pages, 4 figure
Relating Hysteresis and Electrochemistry in Graphene Field Effect Transistors
Hysteresis and commonly observed p-doping of graphene based field effect
transistors (FET) was already discussed in reports over last few years.
However, the interpretation of experimental works differs; and the mechanism
behind the appearance of the hysteresis and the role of charge transfer between
graphene and its environment are not clarified yet. We analyze the relation
between electrochemical and electronic properties of graphene FET in moist
environment extracted from the standard back gate dependence of the graphene
resistance. We argue that graphene based FET on a regular SiO2 substrate
exhibits behavior that corresponds to electrochemically induced hysteresis in
ambient conditions, and can be caused by charge trapping mechanism associated
with sensitivity of graphene to the local pH.Comment: 5 pages, 3 figure
Field induced quantum-Hall ferromagnetism in suspended bilayer graphene
We have measured the magneto-resistance of freely suspended high-mobility
bilayer graphene. For magnetic fields T we observe the opening of a field
induced gap at the charge neutrality point characterized by a diverging
resistance. For higher fields the eight-fold degenerated lowest Landau level
lifts completely. Both the sequence of this symmetry breaking and the strong
transition of the gap-size point to a ferromagnetic nature of the insulating
phase developing at the charge neutrality point.Comment: 7 pages, 5 figure
Temperature dependence of the effective spin-mixing conductance probed with lateral non-local spin valves
We report the temperature dependence of the effective spin-mixing conductance
between a normal metal (aluminium, Al) and a magnetic insulator
(, YIG). Non-local spin valve devices,
using Al as the spin transport channel, were fabricated on top of YIG and
SiO substrates. By comparing the spin relaxation lengths in the Al channel
on the two different substrates, we calculate the effective spin-mixing
conductance () to be ~
at 293~K for the Al/YIG interface. A decrease of up to 84\% in is
observed when the temperature () is decreased from 293~K to 4.2~K, with
scaling with . The real part of the
spin-mixing conductance (), calculated from the experimentally obtained
, is found to be approximately independent of the temperature. We
evidence a hitherto unrecognized underestimation of extracted from
the modulation of the spin signal by rotating the magnetization direction of
YIG with respect to the spin accumulation direction in the Al channel, which is
found to be 50 times smaller than the calculated value
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