We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000 degrees C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000 degrees C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt% reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm(-3), cross-plane thermal conductivity of 5.65 W m(-1) K-1, and in-plane thermal conductivity of 2,025 +/- 25 W m(-1) K-1.

A macroscopic film (2.5 cm × 2.5 cm) made by layer-by-layer assembly of 100 single-layer polycrystalline graphene films is reported. The graphene layers are transferred and stacked one by one using a wet process that leads to layer defects and interstitial contamination. Heat-treatment of the sample up to 2800 °C results in the removal of interstitial contaminants and the healing of graphene layer defects. The resulting stacked graphene sample is a freestanding film with near-perfect in-plane crystallinity but a mixed stacking order through the thickness, which separates it from all existing carbon materials. Macroscale tensile tests yields maximum values of 62 GPa for the Young's modulus and 0.70 GPa for the fracture strength, significantly higher than has been reported for any other macroscale carbon films; microscale tensile tests yield maximum values of 290 GPa for the Young's modulus and 5.8 GPa for the fracture strength. The measured in-plane thermal conductivity is exceptionally high, 2292 ± 159 W m-1 K-1 while in-plane electrical conductivity is 2.2 × 105 S m-1 . The high performance of these films is attributed to the combination of the high in-plane crystalline order and unique stacking configuration through the thickness.

Oxide interfaces such as LaAlO 3 /SrTiO 3 (LAO/STO) are interesting platforms for the investigation of ‘spin–orbitronics’ because of their strongly coupled spin and orbital degrees of freedom due to the inversion asymmetry of the structure. In this investigation, we demonstrate a tunable Rashba spin–orbit field at the LAO/STO interface via the application of an external gate electric field. The strength of the Rashba field was indirectly estimated by measuring the planar angle dependence of the anisotropic magnetoresistance (AMR). The asymmetry of the planar AMR between θ = 0 and π indicates the existence of Rashba spin–orbit fields, which are tunable by adjusting the current density and gate electric field. From the AMR measurements, the effective Rashba field exhibits up to 4 T for the application of an external back-gate voltage of 30 V. This controllable and relatively high Rashba field suggests that the LAO/STO is an attractive 2-D interface for potential spin–orbitronic applications, such as spin-charge converters, spin-FETs, and spin–orbit torque devices.

Engineering the electron dispersion of graphene to be spin-dependent is crucial for the realization of spin-based logic devices. Enhancing spin-orbit coupling in graphene can induce spin Hall effect, which can be adapted to generate or detect a spin current without a ferromagnet. Recently, both chemically and physically decorated graphenes have shown to exhibit large nonlocal resistance via the spin Hall and its inverse effects. However, these nonlocal transport results have raised critical debates due to the absence of field dependent Hanle curve in subsequent studies. Here, we introduce Au clusters on graphene to enhance spin-orbit coupling and employ a nonlocal geometry to study the spin Hall induced nonlocal resistance. Our results show that the nonlocal resistance highly depends on the applied gate voltage due to various current channels. However, the spin Hall induced nonlocal resistance becomes dominant at a particular carrier concentration, which is further confirmed through Hanle curves. The obtained spin Hall angle is as high as similar to 0.09 at 2 K. Temperature dependence of spin relaxation time is governed by the symmetry of spin-orbit coupling, which also depends on the gate voltage: asymmetric near the charge neutral point and symmetric at high carrier concentration. These results inspire an effective method for generating spin currents in graphene and provide important insights for the spin Hall effect as well as the symmetry of spin scattering in physically decorated graphene.

A two-dimensional electron gas emerged at a LaAlO3/SrTiO3 interface is an ideal system for "spin-orbitronics" as the structure itself strongly couple the spin and orbital degree of freedom through the Rashba spin-orbit interaction. One of core experiments toward this direction is the nonlocal spin transport measurement, which has remained elusive due to the low spin injection efficiency to this system. Here we bypass the problem by generating a spin current not through the spin injection from outside but instead through the inherent spin Hall effect and demonstrate the nonlocal spin transport. The analysis on the nonlocal spin voltage, confirmed by the signature of a Larmor spin precession and its length dependence, displays that both D'yakonov-Perel' and Elliott-Yafet mechanisms involve in the spin relaxation at low temperature. Our results show that the oxide heterointerface is highly efficient in spin-charge conversion with exceptionally strong spin Hall coefficient γ ∼ 0.15 ± 0.05 and could be an outstanding platform for the study of coupled charge and spin transport phenomena and their electronic applications.

Electrical control of ferromagnetism in semiconductor nanostructures offers the promise of nonvolatile functionality in future semiconductor spintronics. Here, we demonstrate a dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). Ferromagnetism in our ZnO NWs arose from oxygen vacancies, which constitute deep levels hosting unpaired electron spins. The magnetic transition temperature of the studied ZnO NWs was estimated to be well above room temperature. The in situ UV confocal photoluminescence (PL) study confirmed oxygen vacancy mediated ferromagnetism in the studied ZnO NW FET devices. Both the estimated carrier concentration and temperature-dependent conductivity reveal the studied ZnO NWs are at the crossover of the metal-insulator transition. In particular, gate-induced modulation of the carrier concentration in the ZnO NW FET significantly alters carrier-mediated exchange interactions, which causes even inversion of magnetoresistance (MR) from negative to positive values. Upon sweeping the gate bias from -40 to +50 V, the MRs estimated at 2 K and 2 T were changed from -11.3% to +4.1%. Detailed analysis on the gate-dependent MR behavior clearly showed enhanced spin splitting energy with increasing carrier concentration. Gate-voltage-dependent PL spectra of an individual NW device confirmed the localization of oxygen vacancy-induced spins, indicating that gate-tunable indirect exchange coupling between localized magnetic moments played an important role in the remarkable change of the MR.

The crystallinity of epitaxial graphene (EG) grown on a Hexagonal-SiC substrate is found to be enhanced greatly by capping the substrate with a molybdenum plate (Mo-plate) during vacuum annealing. The crystallinity enhancement of EG layer grown with Mo-plate capping is confirmed by the significant change of measured Raman spectra, compared to the spectra for no capping. Mo-plate capping is considered to induce heat accumulation on SiC surface by thermal radiation mirroring and raise Si partial pressure near surface by confining the sublimated Si atoms between SiC substrate and Mo-plate, which would be the essential contributors of crystallinity enhancement.

Thin film of Fe(TCNQ:tetracyanoquinodimethane)(x similar to 2) that exhibits both ferromagnetic and semiconducting behavior was developed by physical vapor deposition. The IR and photoemission spectra display clear evidences of coordination between metal and TCNQ based on charge transfer rather than physical mixture. The shifts of C and N 1s core-level spectra due to metal-TCNQ coordination reveal the local spin density of the TCNQ(-) anion, predicted by DFT calculation. The valence band spectra of the Fe(TCNQ)(x similar to 2) thin film also confirm the formation of a singly occupied molecular orbital (SOMO) in TCNQ(-), which leads to long-range magnetic order as confirmed by a vibrating sample magnetometer. (C) 2014 Elsevier B.V. All rights reserved.

Submitted for the MAR14 Meeting of The American Physical Society Alternative generation of spin current in graphene1 JUNG-WOO YOO, MI-JIN JIN, JUNGMIN PARK, VIJAYAKUMAR MODEPALLI, JUNHYEON JO, Ulsan Nation Institute of Science and Technology, Republic of Korea — The manipulation of spin current which can be achieved in various device configurations has been under intense research in recent years. The spin current is typically obtained by injecting electrons from the ferromagnetic electrodes. In this study, we employed alternative methods for the generation of spin current in graphene. The first method we studied is using spin Hall effect. In the spin Hall effect, the charge current generates spin current due to a relativistic spin-orbit coupling. Generally the spin-orbit coupling in graphene is extremely weak to produce substantial spin current. We employed physical doping of heavy atoms on top of the graphene layer for the spin Hall induced spin current in graphene. The second alternative method we investigated is seebeck spin tunneling. The ferromagnetic electrode together with thin tunnel barrier (1-3nm of Al2O3 layer) was employed to introduce thermally induced spin imbalance in graphene. The gate dependence of generated spin current reflects unique electronic structure of graphene. 1This work was supported in part by future challenge project of UNIST and Basic Science Research Program of NRF Korea. Jung-Woo Yoo Ulsan Natl Inst of Sci & Tech Date submitted: 13 Nov 2013 Electronic form version 1.4