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Graphene-based spintronics has attracted much attention owing to the weak spin–orbit interaction and long spin relaxation length in graphene. For implementation of practical and high-density graphene-based spintronic devices, we need to define nanoscale areas with room-temperature ferromagnetism on graphene. Here we report the room-temperature ferromagnetism observed in nanoscale functionalized (oxidized and hydrogenated) graphene using atomic force microscope lithography without involving potential surface contamination and chemical agents. By performing magnetic force microscope (MFM) measurements, we can clearly distinguish the local ferromagnetic signal of selectively functionalized graphene from that of surrounding non-magnetic pristine graphene. The nanoscale functionalized graphene shows experimental evidence of room-temperature ferromagnetism: (1) larger MFM signal than that of graphene; (2) repulsive and attractive interaction with an MFM tip the magnetization of which points into and out of the graphene, respectively; and (3) MFM signal reversal after applying a high magnetic field at an elevated temperature of 400 K. Our first-principles calculations reveal that unpaired spins are present at non-passivated dangling bonds of vacancies on functionalized graphene and the stable ferromagnetic exchange interactions between them are favored. Therefore, nanoscale functionalized graphene is a good candidate for use as the spin injector or detector of high-density graphene-based spintronic devices.

We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp(2)-rich phase and high friction (low conductance) in the sp(3)-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp(2)/sp(3) carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices.

In free space, unshielded large volume of a low permanent magnetic field (MF) generating system is necessary for use in such areas of magnetic measurements as: calibration of magnetometers, testing of materials, the ecological and medical effects, etc.

We investigated the field-effect transistor (FET) characteristics of 15-μm graphene-covered copper wires (G-wires). Unlike the previously reported graphene FET, carries initially showed p -type like FET characteristics in two-terminal transport measurements. Our results indicate that the electrical transport processes in a G-wire FET occur in both the heavily p -doped contact and the p -doped radial graphene channel, as a p -channel. The interfacial potential barrier between the contact electrode and the radial graphene channel is small, but there is a radial potential barrier that limits electrical transport through the copper core in chemical vapor deposition (CVD) grown samples. The p -type FET characteristics appeared clearly after the oxidation of the G-wires.

Three-dimensional finite element analysis of arc-welding processes is presented with emphasis on practical applications for numerical simulation. We use an implicit numerical implementation for Leblond’s transformation plasticity constitutive equations, which are widely used in steel-structure welding. Several numerical examples, particularly including a large structure undergoing significant elastic–plastic deformations before welding, are presented to demonstrate the effectiveness of the three-dimensional analysis of welding processes.

Ultrathin Co films were deposited on Pt(100) at room temperature in ultrahigh vacuum, and investigated in situ by low-energy electron diffraction, scanning tunneling microscopy (STM), and surface magneto-optic Kerr effect. The Co film was grown into a wedge shape to provide a continuous change of the film thickness. We find that the Co film forms single-crystal ultrathin films at least up to 5 monolayers (ML). For as-grown films, we observe only in-plane magnetization. After annealing the film, the Co film develops a perpendicular magnetic anisotropy, leading to a spin reorientation transition at 2.7 ML Co thickness. STM measurements were performed at room temperature both before and after annealing the film. We found very different surface morphology and alloy formation after the film annealing, and attribute the perpendicular magnetic anisotropy to the formation of the Co-Pt alloy layer at the Co∕Pt(100) interface.Ultrathin Co films were deposited on Pt(100) at room temperature in ultrahigh vacuum, and investigated in situ by low-energy electron diffraction, scanning tunneling microscopy (STM), and surface magneto-optic Kerr effect. The Co film was grown into a wedge shape to provide a continuous change of the film thickness. We find that the Co film forms single-crystal ultrathin films at least up to 5 monolayers (ML). For as-grown films, we observe only in-plane magnetization. After annealing the film, the Co film develops a perpendicular magnetic anisotropy, leading to a spin reorientation transition at 2.7 ML Co thickness. STM measurements were performed at room temperature both before and after annealing the film. We found very different surface morphology and alloy formation after the film annealing, and attribute the perpendicular magnetic anisotropy to the formation of the Co-Pt alloy layer at the Co∕Pt(100) interface.

New evidence of the paired end-bridge configuration in the room-temperature adsorption geometries of C 2 H 2 molecules on Si(0 0 1) is presented by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. The distinct four-leaved feature occupying two adjacent Si dimer sites in the experimental empty-state STM images are well reproduced by simulations of the paired end-bridge adsorption configuration. Calculated energetics suggests that the Si(0 0 1) surface is covered by paired end-bridge structures at the saturation coverage of 1 ML, in agreement with the existing experiments.

We report the formation of atomically flat Si(0 0 1) surfaces with C incorporation using thermal dissociation of C 2 H 2 molecules. The resulting surface phases are investigated by scanning tunneling microscopy and found to contain C atoms as much as 0.2 monolayer (ML) without roughening. At low C concentrations, C-induced vacancy line defects are generated on the surface, forming a 2× n superstructure. As the C concentration increases, a new phase with a c(4×4) structure starts to appear and coexists with the 2× n phase, eventually covering the whole surface at about 0.2 ML. The C atoms in the 2× n superstructure are thought to occupy substitutional sites in a subsurface layer and constitute a δ-like distribution.

Acetylene molecules adsorbed on the Si(0 0 1) surface are studied by scanning tunneling microscope and ab initio pseudopotential calculations. In case of room-temperature adsorption, there are basically two different kinds of di-σ bonding geometries at low coverage: (i) on-top and (ii) end-bridge di-σ configurations. The tetra-σ configurations are not found. Theoretical calculations show that the paired end-bridge configuration is dominant at high coverage. Based on these findings, we discuss the results at saturation coverage in the existing literature consistently. Subsequent annealing of the C 2 H 2 /Si(0 0 1) surface induces different C-induced surface structures depending on the acetylene coverage. At low coverage, parallel lines of dimer vacancies are observed. As the amount of the adsorbed acetylene increases, the c(4×4) ordered phase replaces the dimer vacancy line phase. Near saturation, clusters are formed and the surface becomes rough.

Nonlinear dynamics of the magnetic vortex state in a circular nanodisk was studied under a perpendicular alternating magnetic field that excites the radial modes of the magnetic resonance. Here, we show that as the oscillating frequency is swept down from a frequency higher than the eigenfrequency, the amplitude of the radial mode is almost doubled to the amplitude at the fixed resonance frequency. This amplitude has a hysteresis vs. frequency sweeping direction. Our result showed that this phenomenon was due to a Duffing-type nonlinear resonance. Consequently, the amplitude enhancement reduced the vortex core-switching magnetic field to well below 10 mT. A theoretical model corresponding to the Duffing oscillator was developed from the Landau–Lifshitz–Gilbert equation to explore the physical origin of the simulation result. This work provides a new pathway for the switching of the magnetic vortex core polarity in future magnetic storage devices.

Heat is a familiar form of energy transported from a hot side to a colder side of an object, but not a notion associated with microscopic measurements of electronic properties. A temperature difference within a material causes charge carriers, electrons or holes, to diffuse along the temperature gradient inducing a thermoelectric voltage. Here we show that local thermoelectric measurements can yield high sensitivity imaging of structural disorder on the atomic and nanometre scales. The thermopower measurement acts to amplify the variations in the local density of states at the Fermi-level, giving high differential contrast in thermoelectric signals. Using this imaging technique, we uncovered point defects in the first layer of epitaxial graphene, which generate soliton-like domain wall line patterns separating regions of the different interlayer stacking of the second graphene layer., Comment: Main Text (19 pages including 5 figures) + Supplementary Information (6 pages including 10 figures, simplified version)

The C amount and the structure of the $\mathrm{S}\mathrm{i}(001)\mathrm{\text{\ensuremath{-}}}c(4\ifmmode\times\else\texttimes\fi{}4)$ surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The $c(4\ifmmode\times\else\texttimes\fi{}4)$ phase is found to contain $1/8$ monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The $c(4\ifmmode\times\else\texttimes\fi{}4)$ phase constitutes the subsurface ${\mathrm{S}\mathrm{i}}_{0.875}{\mathrm{C}}_{0.125}$ $\ensuremath{\delta}$ layer with two-dimensionally ordered C atoms.

We investigate the initial stage of the C incorporation into Si(001) using thermal dissociation of ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{2}}$. The scanning tunneling microscopy shows that C-induced dimer vacancies (DVs) with depressed adjacent dimers are generated on the surface and aligned in the dimer direction, forming the $2\ifmmode\times\else\texttimes\fi{}n$ structure. The ab initio pseudopotential calculations reveal that, with the presence of a DV in the surface, the $\ensuremath{\alpha}$ site in the fourth subsurface layer directly below the DV is the most favorable for the incorporated C atoms. The embedded C atoms align one dimensionally due to the interaction which is attractive in neighboring dimer rows but repulsive in the same dimer row.

Initial adsorption configurations of acetylene molecules on the Si(001) surface are investigated by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. Three distinctive features are observed in the STM images at low coverage. The comparison of the atomic resolution STM images with the simulation reveals that there are basically two kinds of di-$\ensuremath{\sigma}$ bonding configurations: (i) on-top over a single dimer and (ii) end-bridge between two successive dimers in the same dimer row. The third feature is identified to be a pair of end-bridge di-$\ensuremath{\sigma}$ configurations occupying the same two dimers. The tetra-$\ensuremath{\sigma}$ bonding configurations are not found. The existence of the two kinds of di-$\ensuremath{\sigma}$ bonding configurations can resolve the apparently conflicting results of the existing studies.

We report the experimental evidence of significant change of the valence band structure during crystallization of Ge2Sb2Te5 (GST). Amorphous GST, prepared by sputter deposition at room temperature (RT), transforms successively into face-centered-cubic (fcc) and a hexagonal-close-packed (hcp) structures at around 150 and 300°C, respectively, during a stepwise temperature increase from RT to 350°C. During temperature increase, ultraviolet photoemission spectra were in vacuo obtained using synchrotron radiation. The measurement of the amorphous and fcc GST shows that the difference between the maximum valence band edge and the Fermi level reduces by 0.35eV during crystallization. For the fcc to hcp phase transformation, no band gap reduction was observed.