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Journal Articles

RBS study of disordering of Fe$$_{3-x}$$Mn$$_{x}$$Si/Ge(111) heteroepitaxial interfaces

Noguchi, Masaya*; Hirata, Tomoaki*; Kawakubo, Yuki*; Narumi, Kazumasa; Sakai, Seiji; Maeda, Yoshihito

Physica Status Solidi (C), 10(12), p.1732 - 1734, 2013/12

 Times Cited Count:0 Percentile:0.02

Journal Articles

Ion beam analysis of quaternary Heusler alloy Co$$_{2}$$(Mn$$_{1-x}$$Fe$$_{x}$$)Si(111) epitaxially grown on Ge(111)

Kawakubo, Yuki*; Noguchi, Masaya*; Hirata, Tomoaki*; Narumi, Kazumasa; Sakai, Seiji; Yamada, Shinya*; Hamaya, Kohei*; Miyao, Masanobu*; Maeda, Yoshihito

Physica Status Solidi (C), 10(12), p.1828 - 1831, 2013/12

 Times Cited Count:0 Percentile:0.02

Journal Articles

Cluster effect on damage accumulation in a Si crystal bombarded with 10-540-keV C$$_{60}$$ ions

Narumi, Kazumasa; Naramoto, Hiroshi*; Yamada, Keisuke; Chiba, Atsuya; Saito, Yuichi; Maeda, Yoshihito

JAEA-Review 2012-046, JAEA Takasaki Annual Report 2011, P. 166, 2013/01

no abstracts in English

Journal Articles

Photoluminescence properties of carbon-doped $$beta$$-FeSi$$_{2}$$ nanocrystals

Maeda, Yoshihito; Nishimura, Kentaro*; Nakajima, Takahito*; Matsukura, Bui*; Narumi, Kazumasa; Sakai, Seiji

Physica Status Solidi (C), 9(10-11), p.1884 - 1887, 2012/10

 Times Cited Count:4 Percentile:91.47

We report enhancement of intrinsic photoluminescence (PL) from $$beta$$-FeSi$$_{2}$$ nanocrystals by doping carbon. In the appropriate dose of C$$_{60}$$$$^{+}$$ ion implantation into the average nanocrystal size of 14 nm, the PL intensity was enhanced by 260% and increase of the exciton binding energy of 1.8 meV in comparison with that of the non-doped nanocrystal. Furthermore, we found that there was a clear correlation between the PL enhancement and increase of the exciton binding energy. This important result suggests that carbon atoms doped in the silicide lattice may play as an isoelectronic trap and probably form bound excitons with stable states as predicted theoretically. We found a new mechanism of the PL enhancement for $$beta$$-FeSi$$_{2}$$ nanocrystals.

Journal Articles

Enhancement of IR light emission from $$beta$$-FeSi$$_{2}$$ nanocrystals embedded in Si

Maeda, Yoshihito; Nishimura, Kentaro*; Nakajima, Takahito*; Matsukura, Bui*; Narumi, Kazumasa; Sakai, Seiji

Physica Status Solidi (C), 9(10-11), p.1888 - 1891, 2012/10

 Times Cited Count:3 Percentile:87.32

We have systematically investigated photoluminescence (PL) properties of $$beta$$-phase nanocrystals which are formed by a phase transition from metastable $$gamma$$-FeSi$$_{2}$$ with a Fluorite structure to $$beta$$-FeSi$$_{2}$$, and succeeded in enhancement of the PL intensity in the optimum conditions of double annealing process. For the PL enhancement, the time of postannealing at 800$$^{circ}$$C is dominated by the time of the preannealing at 400 or 500$$^{circ}$$C which is related to amount of the $$gamma$$-phase. After discussing some possible factors, we speculate that the PL enhancement observed in this study may be attributed mainly to improvement of the interface condition between the nanocrystal and Si, because the crystallographic epitaxial relationship among the phases, Si(111)//$$gamma$$(111)//$$beta$$(202)/(220) can be maintained during precipitation of the nanocrystal on Si(111).

Journal Articles

Energy dependence of nonlinear effects of sputtering yields of Si bombarded with 10-540-keV C$$_{60}$$ ions

Narumi, Kazumasa; Naramoto, Hiroshi*; Yamada, Keisuke; Saito, Yuichi; Chiba, Atsuya; Takahashi, Yasuyuki*; Maeda, Yoshihito

JAEA-Review 2011-043, JAEA Takasaki Annual Report 2010, P. 155, 2012/01

no abstracts in English

Journal Articles

Ion channeling study of epitaxy of iron based Heusler alloy films on Ge(111)

Maeda, Yoshihito; Narumi, Kazumasa; Sakai, Seiji; Terai, Yoshikazu*; Hamaya, Kohei*; Sado, Taizo*; Miyao, Masanobu*

Thin Solid Films, 519(24), p.8461 - 8467, 2011/10

 Times Cited Count:7 Percentile:35.48(Materials Science, Multidisciplinary)

Journal Articles

Effect of cotunneling and spin polarization on the large tunneling magnetoresistance effect in granular C$$_{60}$$-Co films

Sakai, Seiji; Mitani, Seiji*; Sugai, Isamu; Takanashi, Koki; Matsumoto, Yoshihiro; Entani, Shiro; Naramoto, Hiroshi*; Avramov, P.; Maeda, Yoshihito

Physical Review B, 83(17), p.174422_1 - 174422_6, 2011/05

 Times Cited Count:8 Percentile:39.13(Materials Science, Multidisciplinary)

Journal Articles

Interface properties of Ag and Au/graphene heterostructures studied by micro-Raman spectroscopy

Entani, Shiro; Sakai, Seiji; Matsumoto, Yoshihiro; Naramoto, Hiroshi*; Hao, T.; Maeda, Yoshihito

Japanese Journal of Applied Physics, 50(4), p.04DN03_1 - 04DN03_5, 2011/04

 Times Cited Count:4 Percentile:20.9(Physics, Applied)

Journal Articles

Determination of silicon vacancy in ion-beam synthesized $$beta$$-FeSi$$_{2}$$

Maeda, Yoshihito; Ichikawa, Takayuki*; Jonishi, Takafumi*; Narumi, Kazumasa

Physics Procedia, 11, p.83 - 86, 2011/00

 Times Cited Count:0 Percentile:0.01

Journal Articles

Interface properties of metal/graphene heterostructures studied by micro-Raman spectroscopy

Entani, Shiro; Sakai, Seiji; Matsumoto, Yoshihiro; Naramoto, Hiroshi*; Hao, T.; Maeda, Yoshihito

Journal of Physical Chemistry C, 114(47), p.20042 - 20048, 2010/11

 Times Cited Count:36 Percentile:73.6(Chemistry, Physical)

Journal Articles

Composition dependence of magnetic and magnetotransport properties in C$$_{60}$$-Co granular thin films

Sugai, Isamu*; Sakai, Seiji; Matsumoto, Yoshihiro; Naramoto, Hiroshi*; Mitani, Seiji*; Takanashi, Koki; Maeda, Yoshihito

Journal of Applied Physics, 108(6), p.063920_1 - 063920_7, 2010/09

 Times Cited Count:8 Percentile:36.82(Physics, Applied)

Journal Articles

RBS study of diffusion under strong centrifugal force in bimetallic Au/Cu thin films

Hao, T.; Ono, Masao; Okayasu, Satoru; Sakai, Seiji; Narumi, Kazumasa; Hiraiwa, Yusuke*; Naramoto, Hiroshi*; Maeda, Yoshihito

Nuclear Instruments and Methods in Physics Research B, 268(11-12), p.1867 - 1870, 2010/06

 Times Cited Count:4 Percentile:36.62(Instruments & Instrumentation)

Journal Articles

Vicinage effect on secondary-electron emission in the forward direction from amorphous carbon foils induced by swift C$$_{2}$$$$^{+}$$ ions

Takahashi, Yasuyuki; Narumi, Kazumasa; Chiba, Atsuya; Saito, Yuichi; Yamada, Keisuke; Ishikawa, Norito; Sugai, Hiroyuki; Maeda, Yoshihito

EPL; A Letters Journal Exploring the Frontiers of Physics, 88(6), p.63001_1 - 63001_6, 2009/12

 Times Cited Count:6 Percentile:43.51(Physics, Multidisciplinary)

C$$_{2}$$$$^{+}$$ and C$$_{1}$$$$^{+}$$ ions with 62.5-250 keV/u were incident on self-supporting amorphous carbon foils of 1.4-150 $$mu$$g/cm$$^{2}$$ (70-7500 ${AA}$). The secondary electrons emitted in the forward direction from a carbon foil were detected by a microchannel-plate detector placed at the exit side of the target. The vicinage effect on the secondary-electron yield was evaluated with the ratio of the secondary-electron yield R$$_{2}$$ = $$gamma$$$$_{2}$$/2$$gamma$$$$_{1}$$, where $$gamma$$$$_{2}$$ and $$gamma$$$$_{1}$$ are the yields induced by the C$$_{2}$$$$^{+}$$ and C$$_{1}$$$$^{+}$$ ion with the same velocity, respectively. For the first time the disappearance of the vicinage effect on the secondary-electron yield from amorphous carbon foils bombarded with 62.5 keV/u C$$_{2}$$$$^{+}$$ ions was observed for thick foils of 61-150 $$mu$$g/cm$$^{2}$$. The internuclear distance between the fragment ions at the exit of the target was evaluated by calculating trajectories of the fragment ions considering the Coulomb explosion. For a 62.5 keV/u C$$_{2}$$$$^{+}$$ ion, we have determined the threshold internuclear distance where the vicinage effect disappears exits between 6 ${AA}$ and 23 ${AA}$. It is expected that the vicinage effect on the energy loss (production process) in this velocity region disappears at the internuclear distance of a few ${AA}$. This result means that the transport or transmission process is important for the appearance of the vicinage effect. Moreover, the threshold internuclear distance depends on the velocity of the ion and increases as the velocity increases. The average charge of the ion increases with increase of the velocity of the ion. These mean that there is a possibility that a charge state plays an important role in the origin of the vicinage effect. In order to account for the experimental results, we discussed two models taking account of two kinds of potentials induced in response to the charge of the fragment ion in the transport process.

Journal Articles

X-ray absorption spectroscopy and magnetic circular dichroism in codeposited C$$_{60}$$-Co films with giant tunnel magnetoresistance

Matsumoto, Yoshihiro; Sakai, Seiji; Takagi, Yasumasa*; Nakagawa, Takeshi*; Yokoyama, Toshihiko*; Shimada, Toshihiro*; Mitani, Seiji*; Naramoto, Hiroshi*; Maeda, Yoshihito

Chemical Physics Letters, 470(4-6), p.244 - 248, 2009/03

 Times Cited Count:17 Percentile:54.64(Chemistry, Physical)

Electronic and spin states of the codeposited C$$_{60}$$-Co films with large tunnel magnetoresistance (TMR) effect were investigated with the X-ray absorption and magnetic circular dichroism (MCD) spectroscopies. It is revealed that a C$$_{60}$$-Co compound generated in the C$$_{60}$$-Co films shows the clear MCD signal attributed to the spin-polarized Co 3d states hybridized with C$$_{60}$$ $$pi$$ orbitals. The magnetic response of these Co 3d-derived states agrees well with temperature dependence of the observed MR ratios of the granular C$$_{60}$$-Co films. This suggests the incorporation of the spin-polarized Co 3d-derived states of the C$$_{60}$$-Co compound into the observed TMR effect.

Journal Articles

Theoretical DFT study of atomic structure and spin states of the Co$$_{x}$$(C$$_{60}$$)$$_{n}$$ (x=3-8, n=1,2) somplex nanoclusters

Avramov, P.*; Sakai, Seiji; Naramoto, Hiroshi*; Narumi, Kazumasa; Matsumoto, Yoshihiro; Maeda, Yoshihito

Journal of Physical Chemistry C, 112(36), p.13932 - 13936, 2008/08

 Times Cited Count:4 Percentile:18(Chemistry, Physical)

Atomic structure and spin states of a set of Co$$_{x}$$ and Co$$_{x}$$(C$$_{60}$$)$$_{n}$$ (x=3-8, n=1,2) clusters are studied using a sophisticated ab initio GGA calculations. It is shown that high-spin low-symmetry structure of free-standing Co$$_{x}$$ clusters is determined by Jahn-Teller distortions. Formation of $$eta$$2, $$eta$$2' or $$eta$$1 coordination bonds between Co$$_{x}$$ fragment and C$$_{60}$$ molecules through carbon hexagons results in stable complex nanoclusters with nonmonotonic change of average spin momentum upon the number of cobalt atoms in the Co$$_{x}$$ cores. The theoretical results are compared with corresponding experimental data.

Journal Articles

The Electronic structures of fullerene/transition-metal hybrid material

Matsumoto, Yoshihiro; Sakai, Seiji; Naramoto, Hiroshi*; Hirao, Norie*; Baba, Yuji; Shimada, Toshihiro*; Sugai, Isamu; Takanashi, Koki; Maeda, Yoshihito

Materials Research Society Symposium Proceedings, Vol.1081 (Internet), 6 Pages, 2008/03

Recently, we have found the appearance of substantial MR ratio (80%) in a C$$_{60}$$/Co hybrid material. Such the MR ratio cannot be explained enough only by tunnel conduction through Co grains. Therefore, to obtain information about electronic structures of C$$_{60}$$/Co hybrid material is necessary. Absorption spectra of C$$_{60}$$Co are different from that of pristine C$$_{60}$$. In particular, peak intensity corresponding to $$pi$$ $$^*$$(LUMO) $$leftarrow$$ C 1s excitation of C$$_{60}$$Co is clearly attenuated. In addition, C 1s photoelectron peak of C$$_{60}$$Co slightly shifted to lower binding energy compared to that of pristine C$$_{60}$$. These results indicate that 3d electron of Co transfers to $$pi$$ $$^*$$ orbital of C$$_{60}$$ and new electronic states are formed in the C$$_{60}$$-Co compound. In fact, XPS spectra of valence excitation region also demonstrate the formation of hybrid orbital near the Fermi level due to the coupling of C$$_{60}$$ and Co.

Journal Articles

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

Sakai, Seiji; Sugai, Isamu; Mitani, Seiji*; Takanashi, Koki; Matsumoto, Yoshihiro; Naramoto, Hiroshi*; Avramov, P.; Okayasu, Satoru; Maeda, Yoshihito

Applied Physics Letters, 91(24), p.242104_1 - 242104_3, 2007/12

 Times Cited Count:30 Percentile:73.93(Physics, Applied)

Journal Articles

Axial orientation of molecular-beam-epitaxy-grown Fe$$_{3}$$Si/Ge hybrid structures and its degradation

Maeda, Yoshihito; Jonishi, Takafumi*; Narumi, Kazumasa; Ando, Yuichiro*; Ueda, Koji*; Kumano, Mamoru*; Sado, Taizo*; Miyao, Masanobu*

Applied Physics Letters, 91(17), p.171910_1 - 171910_3, 2007/10

 Times Cited Count:31 Percentile:74.76(Physics, Applied)

The axial orientation of molecular-beam-epitaxy (MBE)-grown Fe$$_{3}$$Si(111)/Ge(111) hybrid structures was investigated by Rutherford backscattering spectroscopy. We confirmed that during MBE above 300$$^{circ}$$C, the interdiffusion of Fe and Ge atoms results in a composition change and the epitaxial growth of FeGe in Fe$$_{3}$$Si. Low-temperature ($$<$$200 $$^{circ}$$C) MBE can realize fully ordered DO$$_{3}$$-Fe$$_{3}$$Si with highly axial orientation [minimum yield ($$chi$$$$_{min}$$)=2.2%]. Postannealing above 400 $$^{circ}$$C results in a composition change and the degradation of axial orientation in the off-stoichiometric Fe$$_{3}$$Si. The significance of stoichiometry with regard to thermal stability and the interfacial quality of Fe$$_{3}$$Si(111)/Ge(111) hybrid structures was also discussed.

Journal Articles

Comparative study of structures and electrical properties in cobalt-fullerene mixtures by systematic change of cobalt content

Sakai, Seiji; Naramoto, Hiroshi*; Avramov, P.; Yaita, Tsuyoshi; Lavrentiev, V.; Narumi, Kazumasa; Baba, Yuji; Maeda, Yoshihito

Thin Solid Films, 515(20-21), p.7758 - 7764, 2007/07

 Times Cited Count:22 Percentile:68.12(Materials Science, Multidisciplinary)

A systematic study is performed for the Co content-dependent structure evolution and electrical properties in the cobalt(Co)-fullerene(C$$_{60}$$) mixtures in the broad content region. Raman analysis confirms that the mixture has a saturation composition of Co$$_{5}$$C$$_{60}$$ for the compoundformation. An important role of the covalent Co-C bonds for the characteristic structure and the properties in the Co-C$$_{60}$$ compounds is concluded from the content-dependent peak shifts and the symmetry lowering of C$$_{60}$$. The Co-dilute mixtures exhibit a semiconductive nature represented bythe activated transport from the shallow mid-gap states. The Co-dense mixtures are characterized by hopping conduction (including variable rangehopping) attributed to the Co clusters/nanoparticles in the compound matrix and by the percolated metallic conduction, depending on the Co content.

119 (Records 1-20 displayed on this page)