Yoshihiro Matsumoto

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.

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.

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.

Contact-induced spin polarization in BNNT(CNT)/TM (TM=Co, Ni) nanocomposites

The interaction between carbon and BN nanotubes (NT) and transition metal Co and Ni supports was studied using electronic structure calculations. Several configurations of interfaces were considered, and the most stable ones were used for electronic structure analysis. All NT/Co interfaces were found to be more energetically favorable than NT/Ni, and conductive carbon nanotubes demonstrate slightly stronger bonding than semiconducting ones. The presence of contact-induced spin polarization was established for all nanocomposites. It was found that the contact-induced polarization of BNNT leads to the appearance of local conductivity in the vicinity of the interface while the rest of the nanotube lattice remains to be insulating.

Contact-induced spin polarization of monolayer hexagonal boron nitride on Ni(111)

Hexagonal boron nitride (h-BN) is a promising barrier material for graphene spintronics. In this Letter, spin-polarized metastable de-excitation spectroscopy (SPMDS) is employed to study the spin-dependent electronic structure of monolayer h-BN/Ni(111). The extreme surface sensitivity of SPMDS enables us to elucidate a partial filling of the in-gap states of h-BN without any superposition of Ni 3d signals. The in-gap states are shown to have a considerable spin polarization parallel to the majority spin of Ni. The positive spin polarization is attributed to the π-d hybridization and the effective spin transfer to the nitrogen atoms at the h-BN/Ni(111) interface.

Spin orientation transition across the single-layer graphene/nickel thin film interface

The spin-polarized electronic structures across the interface between single-layer graphene and a Ni(111) thin film are explored by employing depth-resolved X-ray absorption and magnetic circular dichroism spectroscopy with atomic layer resolution. The depth-resolved Ni L2,3-edge analysis clarifies that the Ni atomic layers adjacent to the interface show a transition of the spin orientation to the perpendicular one in contrast to the in-plane one in the bulk region. The C K-edge analysis reveals the intensification of the spin–orbit interactions induced by the π–d hybridization at the interface as well as out-of-plane spin polarization in the π band region of graphene. The present study indicates the importance of the interface design at the atomic layer level for graphene-based spintronics.

Uniformly dimerized C60 film prepared by deposition under in situ photoirradiation

C60 films with thicknesses of 100–480 nm were deposited on Si reed substrates under in situ photoirradiation. In anelasticity measurements, no internal friction peaks associated with rotational motions of the C60 molecules were observed, and Young’s modulus was 1.5 times larger than that of a pristine C60 material. X-ray diffraction patterns suggested that the face-centered cubic lattice was contracted by about 3% and locally distorted from the pristine C60 material. Raman spectra very similar to those reported for dimerized C60 were also obtained. These characteristics recovered to those of the pristine C60 materials after annealing the C60 films at 523 K. These results indicate uniform dimerization in C60 films deposited under in situ photoirradiation.

Quasi-monochromatic pencil beam of laser-driven protons generated using a conical cavity target holder

A 7 MeV proton beam collimated to 16 mrad containing more than 106 particles is experimentally demonstrated by focusing a 2 J, 60 fs pulse of a Ti:sapphire laser onto targets of different materials and thicknesses placed in a millimeter scale conical holder. The electric potential induced on the target holder by laser-driven electrons accelerates and dynamically controls a portion of a divergent quasi-thermal proton beam originated from the target, producing a quasi-monoenergetic “pencil” beam.

Composition dependence of magnetic and magnetotransport properties in C60–Co granular thin films

Composition dependence of magnetic and magnetotransport properties in C60Cox thin films exhibiting large magnetoresistance (MR) effect was investigated in the Co composition range of x=8–20, where x denotes the number of Co atoms per C60 molecule. From the superparamagnetic magnetization curves observed, the average diameter (dave) of Co nanoparticles dispersed in the matrix phase was evaluated to be approximately 1 nm for the sample of x=8, and increased with the Co composition, x. By analyzing the temperature (T) dependence of resistivity based on the model by Abeles et al. [Adv. Phys. 24, 407 (1975)], the average charging energies (⟨EC⟩) of Co nanoparticles were evaluated to be 2–9 meV for the samples of x=8–17 while the considerably weak temperature dependence suggested much smaller values of ⟨EC⟩ for the samples of x>17. The composition dependence of dave and ⟨EC⟩ revealed a structural transition from well-defined granular structures in the range of x=8–17 to magnetically and electronically couple...