Controlling the efficiency of spin injection into graphene by carrier drift

Electrical spin injection from ferromagnetic metals into graphene is hindered by the impedance mismatch between the two materials. This problem can be reduced by the introduction of a thin tunnel barrier at the interface. We present room temperature non-local spin valve measurements in cobalt/aluminum-oxide/graphene structures with an injection efficiency as high as 25%, where electrical contact is achieved through relatively transparent pinholes in the oxide. This value is further enhanced to 43% by applying a DC current bias on the injector electrodes, that causes carrier drift away from the contact. A reverse bias reduces the AC spin valve signal to zero or negative values. We introduce a model that quantitatively predicts the behavior of the spin accumulation in the graphene under such circumstances, showing a good agreement with our measurements.Comment: 4 pages, 3 color figure

Zero-bias conductance peak and Josephson effect in graphene-NbTiN junctions

We report electronic transport measurements of graphene contacted by NbTiN electrodes, which at low temperature remain superconducting up to at least 11 Tesla. In devices with a single superconducting contact, we find a more than twofold enhancement of the conductance at zero bias, which we interpret in terms of reflectionless tunneling. In devices with two superconducting contacts, we observe the Josephson effect, bipolar supercurrents and Fraunhofer patterns.Comment: 6 pages, 5 figure

Electronic spin drift in graphene field effect transistors

We studied the drift of electron spins under an applied DC electric field in single layer graphene spin valves in a field effect transport geometry at room temperature. In the metallic conduction regime ($n \simeq 3.5 \times 10^{16}$ m$^{-2}$), for DC fields of about $\pm$70 kV/m applied between the spin injector and spin detector, the spin valve signals are increased/decreased, depending on the direction of the DC field and the carrier type, by as much as $\pm$50%. Sign reversal of the drift effect is observed when switching from hole to electron conduction. In the vicinity of the Dirac neutrality point the drift effect is strongly suppressed. The experiments are in quantitative agreement with a drift-diffusion model of spin transport.Comment: 4 figure

Energy level alignment at Co/AlOx/pentacene interfaces

X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) experiments were performed in order to study the energy level alignment and electronic structure at Co/AlOx/pentacene interfaces as a function of the aluminum oxide (AlOx) tunnel barrier thickness and the oxidation state of Co. XPS was used to determine the oxygen exposure for the optimum oxidation of 6, 8, and 10 A thin layers of Al deposited on Co. The Fermi level (FL) position in the band gap of AlOx depends on the oxidation state of the underlying Co and on the thickness of the tunnel barrier. The energy level alignment at Co/AlOx interfaces is consistent with an interfacial dipole, its magnitude being sensitive to the oxidation of Co, and band bending phenomena in the thin AlOx tunnel barrier. UPS experiments revealed no chemical interaction at Co/AlOx/pentacene interface in contrast with hybridization effects found at Co/pentacene interface. The vacuum level of pentacene aligns with that of AlOx, following the position of AlOx energy levels with respect to FL. The hole injection barrier was found to increase with the thickness of the tunnel barrier and to decrease with the oxidation of Co at a fixed thickness of the AlOx layer

Linear scaling between momentum and spin scattering in graphene

Spin transport in graphene carries the potential of a long spin diffusion length at room temperature. However, extrinsic relaxation processes limit the current experimental values to 1-2 um. We present Hanle spin precession measurements in gated lateral spin valve devices in the low to high (up to 10^13 cm^-2) carrier density range of graphene. A linear scaling between the spin diffusion length and the diffusion coefficient is observed. We measure nearly identical spin- and charge diffusion coefficients indicating that electron-electron interactions are relatively weak and transport is limited by impurity potential scattering. When extrapolated to the maximum carrier mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase in the spin diffusion length should be possible

Electronic spin transport in graphene field effect transistors

Spin transport experiments in graphene, a single layer of carbon atoms, indicate spin relaxation times that are significantly shorter than the theoretical predictions. We investigate experimentally whether these short spin relaxation times are due to extrinsic factors, such as spin relaxation caused by low impedance contacts, enhanced spin flip processes at the device edges or the presence of an aluminium oxide layer on top of graphene in some samples. Lateral spin valve devices using a field effect transistor geometry allowed for the investigation of the spin relaxation as a function of the charge density, going continuously from metallic hole to electron conduction (charge densities of $n\sim 10^{12}$cm$^{-2}$) via the Dirac charge neutrality point ($n \sim 0$). The results are quantitatively described by a one dimensional spin diffusion model where the spin relaxation via the contacts is taken into account. Spin valve experiments for various injector/detector separations and spin precession experiments reveal that the longitudinal (T$_1$) and the transversal (T$_2$) relaxation times are similar. The anisotropy of the spin relaxation times $\tau_\parallel$ and $\tau_\perp$, when the spins are injected parallel or perpendicular to the graphene plane, indicates that the effective spin orbit fields do not lie exclusively in the two dimensional graphene plane. Furthermore, the proportionality between the spin relaxation time and the momentum relaxation time indicates that the spin relaxation mechanism is of the Elliott-Yafet type. For carrier mobilities of 2-5$\times 10^3$ cm2^/Vs and for graphene flakes of 0.1-2 $\mu$m in width, we found spin relaxation times of the order of 50-200 ps, times which appear not to be determined by the extrinsic factors mentioned above.Comment: 11 pages, 13 figure

Anisotropic spin relaxation in graphene

Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the non-local geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field $B$ applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B = 0 and B = 2 T shows a 20 % decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin orbit effective fields in the in-plane and out-of-plane directions.Comment: 5 pages, 4 figure

Spin dependent quantum interference in non-local graphene spin valves

Spin dependent electron transport measurements on graphene are of high importance to explore possible spintronic applications. Up to date all spin transport experiments on graphene were done in a semi-classical regime, disregarding quantum transport properties such as phase coherence and interference. Here we show that in a quantum coherent graphene nanostructure the non-local voltage is strongly modulated. Using non-local measurements, we separate the signal in spin dependent and spin independent contributions. We show that the spin dependent contribution is about two orders of magnitude larger than the spin independent one, when corrected for the finite polarization of the electrodes. The non-local spin signal is not only strongly modulated but also changes polarity as a function of the applied gate voltage. By locally tuning the carrier density in the constriction we show that the constriction plays a major role in this effect and indicates that it can act as a spin filter device. Our results show the potential of quantum coherent graphene nanostructures for the use in future spintronic devices

Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature

We present a new fabrication method of graphene spin-valve devices which yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes onto Si$^{++}$/SiO$_2$. Thereafter, we mechanically transfer a graphene-hBN heterostructure onto the prepatterned electrodes. We show that room temperature spin transport in single-, bi- and trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching 10$\mu$m combined with carrier mobilities exceeding 20,000 cm$^2$/Vs.Comment: 15 pages, 5 figure

Ballistic Josephson junctions in edge-contacted graphene

Hybrid graphene-superconductor devices have attracted much attention since the early days of graphene research. So far, these studies have been limited to the case of diffusive transport through graphene with poorly defined and modest quality graphene-superconductor interfaces, usually combined with small critical magnetic fields of the superconducting electrodes. Here we report graphene based Josephson junctions with one-dimensional edge contacts of Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the graphene has a high quality due to its encapsulation in hexagonal boron nitride. This allows us to study and exploit graphene Josephson junctions in a new regime, characterized by ballistic transport. We find that the critical current oscillates with the carrier density due to phase coherent interference of the electrons and holes that carry the supercurrent caused by the formation of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are observed over unprecedented long distances of up to 1.5 $\mu$m. Finally, in the quantum Hall regime we observe broken symmetry states while the contacts remain superconducting. These achievements open up new avenues to exploit the Dirac nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material and ancillary file with source code for tight binding simulation