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

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

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

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 $B>1$ 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 ($\text{Y}_3\text{Fe}_5\text{O}_{12}$, YIG). Non-local spin valve devices, using Al as the spin transport channel, were fabricated on top of YIG and SiO$_2$ substrates. By comparing the spin relaxation lengths in the Al channel on the two different substrates, we calculate the effective spin-mixing conductance ($G_\text{s}$) to be $3.3\times10^{12}$~$\Omega^{-1}\text{m}^{-2}$ at 293~K for the Al/YIG interface. A decrease of up to 84\% in $G_\text{s}$ is observed when the temperature ($T$) is decreased from 293~K to 4.2~K, with $G_\text{s}$ scaling with $(T/T_\text{c})^{3/2}$. The real part of the spin-mixing conductance ($G_\text{r}\approx 5.7\times10^{13}~ \Omega^{-1}\text{m}^{-2}$), calculated from the experimentally obtained $G_\text{s}$, is found to be approximately independent of the temperature. We evidence a hitherto unrecognized underestimation of $G_\text{r}$ 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