Hiroshi Naramoto

Tunnel magnetoresistance in Co nanoparticle/Co–C60 compound hybrid system

A nanometer-scale hybrid film of Co particle/Co–C60 compound was prepared by alternate deposition of Co and C60 under UHV condition. All of Raman spectra, magnetization curves, and tunnel conductivity concluded that the hybrid system has a granular structure consisting of Co nanoparticles embedded in a Co–C60 compound matrix. The magnetoresistance ratio of 26% was obtained at 2K and 10kOe for the electron tunneling across the Co–C60 compound barrier. In addition, anomalously large bias voltage dependence was found in the magnetotransport properties.

Preparation of MoO3 nanostructures and their optical properties

In this letter we report the synthesis of nanostructures of molybdenum trioxides by directly oxidizing a spiral coil of molybdenum, at ambient atmosphere, by passing a current through the coil. The advantage of this approach is that the temperature of the substrate is low (normally below 200?C), and that the nanostructures to be formed could be chosen, via controlling the current or, equivalently, the temperature of the coil. We show that, by adjusting the current through the coil, ?-MoO3 lamellas with a thickness of ~20?50?nm, and ?-MoO3 spheres of diameters down to nanometre scale can be synthesized at ambient atmosphere. These nanostructures exhibit a large optical band gap of ~3.05?eV, and room-temperature photoluminescence at ~395?nm. This study provides a simple, controllable way of fabricating metal oxide nanostructures of interest.

Luminescence of sapphire and ruby induced by He and Ar ion irradiation

The luminescence spectra of sapphire(α-Al2O3) and ruby (α-Al2O3:Cr, Cr2O3 content 0.02%) induced by 200 keV He+ and Ar+ ion irradiation were measured at room temperature in the wavelength range of 250 to 450 nm, as a function of irradiation dose. The analysis of the measured spectra gave the existence of three main luminescence features in this wavelength region, namely F-center, F+-center and a band observed around 315 nm. α-Al2O3 under He+ irradiation showed the growth and decay of F-center and F+-center with increase of irradiation dose. F-center appeared earlier than F+-center and reached its maximum intensity around 1 × 1014 He+/cm2 while F+-center reached its maximum around 1 × 1015 He+/cm2. It suggests that F-center is easier to be produced by ion irradiation than F+-center, and a concentration quenching on F-center luminescence occurred above 1 × 1014 He+/cm2. Furthermore, the F-center luminescence was strongly suppressed by the existence of impurity Cr. On the other hand, the luminescence band around 315 nm appeared in the early stage of irradiation and showed only its decay part. The dose dependent behavior was similar to that of Cr3+ emission (R-line) in ruby in both He+ and Ar+ irradiation.

Giant tunnel magnetoresistance in codeposited fullerene-cobalt films in the low bias-voltage regime

Magnetotransport properties in the low bias-voltage regime were investigated for codeposited C60–Co films. A giant tunnel magnetoresistance (MR) ratio (ΔR∕Rmax) of 80%, which is the highest in ferromagnetic metal/organic molecule systems, was found at low temperatures. The observed bias-voltage dependence of the MR ratio is expressed by an unusual exponential form, suggesting that the MR ratio of nearly 100% can be realized in the low bias-voltage limit.

Core level and valence band photoemission spectra of Au clusters embedded in carbon

X-ray photoelectron spectroscopy (XPS) has been applied for size estimation of Au clusters formed by ion implantation into glassy carbon. The 4f and 5d XPS spectra reveal the presence of the cluster 0.7–2.5nm in diameter, depending on the Au concentration. The relationship between XPS 4f-binding energy shift and 5d splitting is determined for the Au clusters embedded in the carbon and found to be significantly different from the previous data for the ones supported on a carbon substrate. We suppose that this difference results from the effect of the environment around a cluster on Coulomb charging during photoemission at the final state.

Contracted Interlayer Distance in Graphene/Sapphire Heterostructure

Direct growth of graphene on insulators is expected to yield significant improvements in performance of graphene-based electronic and spintronic devices. In this study, we successfully reveal the atomic arrangement and electronic properties of a coherent heterostructure of single-layer graphene and α-Al2O3(0001). The analysis of the atomic arrangement of single-layer graphene on α-Al2O3(0001) revealed an apparentcontradiction. The in-plane analysis shows that single-layer graphene grows not in a single-crystalline epitaxial manner, but rather in polycrystalline form, with two strongly pronounced preferred orientations. This suggests relatively weak interfacial interactions are operative. However, we demonstrate that unusually strong physical interactions between graphene and α-Al2O3(0001) exist, as evidenced by the small separation between the graphene and the α-Al2O3(0001) surface. The interfacial interaction is shown to be dominated by the electrostatic forces involved in the graphene π-system and the unsaturated electrons of the topmost O layer of α-Al2O3(0001), rather than the van der Waals interactions. Such features causes graphene hole doping and enable the graphene to slide on the α-Al2O3(0001) surface with only a small energy barrier despite the strong interfacial interactions.

Precise control of single- and bi-layer graphene growths on epitaxial Ni(111) thin film

In situ analysis was performed on the graphene growth in ultrahigh vacuum chemical vapor deposition by exposing the epitaxial Ni(111) thin film to benzene vapor at 873 K. It is shown that the highly uniform single- and bi-layer graphenes can be synthesized by the control of benzene exposure in the range of 10–105 langmuirs, reflecting a change in the graphene growth-rate by three orders of magnitude in between the first and second layer. Electron energy loss spectroscopy measurements of single- and bi-layer graphenes indicates that the interface interaction between bi-layer graphene and Ni(111) is weakened in comparison with that between single-layer graphene and Ni(111). It is also clarified from the micro-Raman analysis that the structural and electrical uniformities of the graphene film transformed on a SiO2 substrate are improved remarkably under the specific exposure conditions at which the growths of single- and bi-layer graphenes are completed.

Magnetotransport properties of a few-layer graphene-ferromagnetic metal junctions in vertical spin valve devices

Magnetotransport properties were studied for the vertical spin valve devices with two junctions of permalloy electrodes and a few-layer graphene interlayer. The graphene layer was directly grown on the bottom electrode by chemical vapor deposition. X-ray photoelectron spectroscopy showed that the permalloy surface fully covered with a few-layer graphene is kept free from oxidation and contamination even after dispensing and removing photoresist. This enabled fabrication of the current perpendicular to plane spin valve devices with a well-defined interface between graphene and permalloy. Spin-dependent electron transport measurements revealed a distinct spin valve effect in the devices. The magnetotransport ratio was 0.8% at room temperature and increased to 1.75% at 50 K. Linear current-voltage characteristics and resistance increase with temperature indicated that ohmic contacts are realized at the relevant interfaces.

Contact-induced spin polarization in graphene/h-BN/Ni nanocomposites

Atomic and electronic structure of graphene/Ni(111), h-BN/Ni(111) and graphene/h-BN/Ni(111) nanocomposites with different numbers of graphene and h-BN layers and in different mutual arrangements of graphene/Ni and h-BN/Ni at the interfaces was studied using LDA/PBC/PW technique. Using the same technique corresponding graphene, h-BN and graphene/h-BN structures without the Ni plate were calculated for the sake of comparison. It was suggested that C-top:C-fcc and N-top:B-fcc configurations are energetically favorable for the graphene/Ni and h-BN/Ni interfaces, respectively. The Ni plate was found to induce a significant degree of spin polarization in graphene and h-BN through exchange interactions of the electronic states located on different fragments.

Proximity-Induced Spin Polarization of Graphene in Contact with Half-Metallic Manganite

The role of proximity contact with magnetic oxides is of particular interest from the expectations of the induced spin polarization and weak interactions at the graphene/magnetic oxide interfaces, which would allow us to achieve efficient spin-polarized injection in graphene-based spintronic devices. A combined approach of topmost-surface-sensitive spectroscopy utilizing spin-polarized metastable He atoms and ab initio calculations provides us direct evidence for the magnetic proximity effect in the junctions of single-layer graphene and half-metallic manganite La0.7Sr0.3MnO3 (LSMO). It is successfully demonstrated that in the graphene/LSMO junctions a sizable spin polarization is induced at the Fermi level of graphene in parallel to the spin polarization direction of LSMO without giving rise to a significant modification in the π band structure.