Koichi Hata

Mie University, Tu, Mie, Japan

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Publications (73)123.75 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Charge state distribution of the working gases of the Ar and Ne gas field ion source (GFIS) were investigated by the time-of-flight mass spectrometry using a newly designed experimental system. GIFS emitters having few atoms on the top of the (111) surface of tungsten were formed by a remolding and field evaporation sequence with electrochemically etched oriented tip. At a probe current of about 1 pA, the ratio of standard deviation of fluctuation and average ion current <2.5% was recorded from a single atom terminated emitter of the Ar GFIS. Mass analysis by the time-of-flight spectrometer showed that there were only singly charged ions both of the Ne and Ar GFIS. Furthermore, no impurity gas ion was detected in the mass spectrum.
    Japanese Journal of Applied Physics 04/2014; 53(5):058004. · 1.07 Impact Factor
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    ABSTRACT: Encapsulation of FePt nanoparticles in carbon nanotubes (CNTs) was attempted using a thermal chemical vapor deposition technique with a Fe/Pt bilayer catalyst. The metal nanoparticles were encapsulated at the tip of multi-walled CNTs. A selected area electron diffraction measurement of the nanoparticles at CNT tips indicated that diffraction spots attributed to an ordered L10 phase. Magnetic hysteresis loops indicated existence of magnetic nanoparticles having various coercivities. From numerical fittings assuming that high and low coercivity components contributed to the hysteresis loops, the high coercivity component was estimated to reach 11.3 kOe (902 kA/m).
    Vacuum. 01/2013; 87:187–190.
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    ABSTRACT: A high-resolution energy analyzer system for measuring energy distribution of field-emitted charged particles has been developed. For high resolution to be achieved, an electrostatic bipotential lens for deceleration of particles was newly designed by using an ion optics simulation software (SIMION 3D) and coupled with the cylindrical electrostatic spectrometer developed by C. Oshima et al. To evaluate the performance of the system, we measured energy distribution of thermal-field electrons emitted from a W(111) emitter and compared with those of theoretical calculation. As a result, it was found that our system has enough resolution to measure an energy distribution of field-emitted electrons. We also measured an energy spectrum of field-emitted electrons from a -oriented single crystalline magnetite whisker that has half-metallic property. As a result, the shoulder indicating the existence of peculiar density of state was observed below the Fermi level. Copyright © 2011 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 06/2012; 44(6). · 1.39 Impact Factor
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    ABSTRACT: The influence of geometry of a carbon nanotube (CNT) pillar array on its field-emission (FE) properties was investigated for the purpose of designing a high-efficiency CNT field-emitter array. The dependence of the FE properties (turn-on voltage and emission site distribution) on the pillar pitch and shape was examined. The examination of the FE properties using a square-shaped pillar array showed that the pillar pitch is not a dominant factor to lower the turn-on voltage and improve the uniformity of emission sites. On the other hand, the change of the pillar shape from a square to a ‘mountain shape’ improved the FE properties. This improvement is due to the dispersed growth morphology, which reduces the field-screening effect at the top of the mountain-shaped pillars. This result indicates that controlling the pillar shape is effective for improving the FE properties of the CNT pillar array. Copyright © 2011 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 06/2012; 44(6). · 1.39 Impact Factor
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    ABSTRACT: Low-pressure alcohol catalytic chemical vapor deposition (LP-ACCVD), which is an ACCVD method at a lower pressure (<1 Pa) than that in the conventional method, has been attracting much interest because it enables the low-temperature growth of single-wall carbon nanotubes (CNTs). However, the growth rate of CNTs by LP-ACCVD is markedly low owing to its low growth pressure. To alleviate this problem, we have examined the influence of the catalyst preparation and CVD conditions on the properties of CNTs grown by LP-ACCVD. It has been found that the oxidation of catalyst enhances the growth of CNTs by LP-ACCVD. Furthermore, the low flow rate of the ethanol enhances the growth yield of CNTs at lower growth temperature, which is understood to be the result of sufficient gas heating on the substrate.
    Japanese Journal of Applied Physics 01/2012; 51(1). · 1.07 Impact Factor
  • Journal of the Vacuum Society of Japan 01/2012; 55(9):430-436.
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    ABSTRACT: A 90° sector type spin rotator was developed for measurement of the z-direction component of a spin polarization, which is parallel to the emitter axis. The rotator enables us to measure all components of electron spins field-emitted from a single crystalline magnetite. In-plane component of spin polarization dominated of field-emitted electrons from single crystalline magnetite whisker, thus it is suggested that the magnetization of the magnetite whisker results from the anisotropy of crystalline structure rather than its shape.
    Ultramicroscopy 05/2011; 111(6):405-8. · 2.47 Impact Factor
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    ABSTRACT: We observed the Verwey transition, metal-insulator transition, in a spin-polarization of field-emitted electrons from a topmost surface of a ;-oriented magnetite whisker by means of field-emitted electrons polarimetry. The measurements of spin polarization of field-emitted electrons from a magnetite whisker were carried out ranging from 40 to 300 K of tip temperature under UHV condition of 3 × 10-8 Pa. Raising temperature of the whisker, the polarization increased from 4 to 11 % with a rotation of its direction. The rotation angle corresponds to a change of magnetic easy axis from ; to ; direction caused by the Verwey transition. Our experimental results clarified that the Verwey transition occurs even at a surface as well as in a bulk.
    01/2011;
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    ABSTRACT: Plasma enhanced chemical vapor deposition is a simple technique for preparing magnetic nanoparticles encapsulated in carbon nanotubes. We employed alloy catalysts when growing carbon nanotubes to control the composition and magnetic properties of encapsulated nanoparticles. Single-crystal nanoparticles were successfully encapsulated in carbon nanotubes, and their crystal structure varied depending on the composition of the alloy catalysts. The coercive force of nanoparticles also varied according to the composition of the catalysts.
    Journal of Magnetism and Magnetic Materials 01/2011; 323(24):3184-3188. · 2.00 Impact Factor
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    ABSTRACT: <110>-oriented single crystal magnetite (Fe3O4) whiskers 30 to 300 nm in diameter were synthesized on a stainless steel plate by means of the combustion flame thermal oxidation process. An electron source with this single magnetite whisker was fabricated by using a micro-sampling instrument in an FIB system. The temperature dependence of the spin polarization of field-emitted electrons from this electron source was measured by using a Mott electron polarimeter. The spin polarization of this sample was found to be 15% at room temperature. This emitter is an oxidative resistance, and can be handled in the atmosphere. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 07/2010; 7(10):2574 - 2577.
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    ABSTRACT: Quasicrystalline (QC) Al-Cu-Fe nanoclusters as catalysts for the synthesis of carbon nanotubes (CNTs) were characterized in this study. It was shown in previous work that armchair-type CNTs are preferentially synthesized by using Al-Cu-Fe ultrathin films as catalysts in catalytic-chemical vapour deposition. We postulated a plausible mechanism that facets of QC nanoclusters with fivefold symmetry play an important role in the nucleation of a CNT cap with specific six-pentagon-configuration, which gives rise to the selective synthesis of armchair-type CNTs. To verify this hypothesis, the morphology and structure of Al-Cu-Fe nanoclusters were investigated by using transmission electron microscopy and electron diffraction (ED) as well as X-ray diffraction. As a result, it was found that the morphology of their nanoclusters is generally faceted-polyhedra, and the average diameter is ~4 nm, ranging from 1 to 10 nm, which indicates to correlate with the diameter of CNTs synthesized by using these nanoclusters as catalysts. Their ED patterns could be attributed to an icosahedral (i) Al-Cu-Fe phase. It was also found that multi-walled CNTs are synthesized successfully by using i-Al-Cu-Fe, i-Al-Pd-Re, and decagonal-Al-Ni-Co bulk-alloy systems as catalysts, respectively.
    Journal of Physics Conference Series 05/2010; 226(1):012008.
  • e-Journal of Surface Science and Nanotechnology 01/2010; 8:321-324.
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    ABSTRACT: An adequate emitter shape for a gas field ion source was investigated to obtain a higher angular current density, dI/dΩ. Single-crystalline -oriented tungsten emitters having a similar tip radius of curvature and a different taper angle of emitter shank were prepared by dc- and ac-electrochemical polishing. The tip apex of the emitters was terminated with a tungsten trimer by means of remolding and a field evaporation processes. The helium ion currents emitted from these trimers were measured as a function of helium pressure. The ion current and the dI/dΩ obtained from the emitter with a taper angle of 1.5° were enhanced by factors of 2.7 and of 3.9, respectively, compared to those from an emitter with a taper angle of 14.5°. These experimental results suggest that to obtain a higher dI/dΩ, the emitter having a narrower taper angle is better.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2010; 28(2). · 1.36 Impact Factor
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    ABSTRACT: For focused ion beam technology in the next generation, an adequate emitter shape for field gas ion source was investigated to obtain a higher angular current density, dI/dΩ. A < 111 > -oriented tungsten emitter with a trimer-terminated nanostructure was fabricated by field-assisted oxygen etching method. Since this method does not utilize thermodynamic phenomena, e.g. faceting of crystal plane with a thermal treatment or pseudomorphic growth of surface layer, shape of the emitter tip has a rotational symmetry with no ridge lines which brings about a decrease of axial beam current. In the preliminary experiment for helium gas, an extraction of ion current was attempted from the trimer at the apex tip, and the increases in helium ion current and dI/dΩ in proportion to the gas pressure were observed. Copyright © 2010 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 01/2010; · 1.39 Impact Factor
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    ABSTRACT: Magnetite (Fe3O4) is oxidative resistant and has a half metallic property and is expected to have spintronic applications. In this work, -oriented single crystal magnetite whiskers, 30–300 nm in diameter, were synthesized on a stainless steel plate by means of the combustion flame thermal oxidation process. An electron source with this single magnetite whisker was fabricated by using a microsampling instrument in a focused ion beam system. The spin polarization of the electron source was found to be 15% at room temperature by using a Mott electron polarimeter.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2010; 28(2). · 1.36 Impact Factor
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    ABSTRACT: The authors investigated the Verwey transition in field-emitted electrons from a -oriented magnetite whisker by means of temperature and emission-current-controlled spin-polarization measurements. Under a condition of total emission current of 1.5 nA, which is relatively high within the measured range, the spin polarization began to increase around 100 K due to the Verwey transition. At the low-temperature region, the spin polarization was sensitive to the emission current and the energy. Therefore, it is suggested that t2g band of Fe3O4 at the surface becomes narrower and possesses lower energy than that in an ideal bulky condition.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2010; 28(2). · 1.36 Impact Factor
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    ABSTRACT: We measured temperature dependence of a spin polarization of field-emitted electrons from a single-crystalline magnetite (Fe3O4) whisker with 〈110〉 orientation. The spin polarization of emitted electrons began to increase above 130K corresponding to the temperature of Verwey point (Tv). The increase is considered as reflection of the change of the spin state near the Fermi level due to the Verwey transition. Our experimental results support a localization of t2g orbital electrons below the Verwey point and a model of charge ordering for magnetite.
    Applied Surface Science 11/2009; 256(4):1058-1060. · 2.54 Impact Factor
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    ABSTRACT: We measured spin polarization of electrons field-emitted from half-metallic Co(2)MnSi thin film grown on a W(001) facet via chromium buffer layer using Mott scattering. For spontaneously magnetized samples, values of polarization at room temperature were observed in a range from 10% to 46% and the polarization direction was 110 orientation of substrate tungsten, which agreed with an easy axis of magnetization of bulky Co(2)MnSi. An enhancement of polarization was observed as a consequence of applying a magnetic field of 350G perpendicular to the emitter axis after the annealing at 800K. This result is considered to be caused by improvement in crystallinity of the evaporated film due to annealing.
    Ultramicroscopy 04/2009; 109(5):395-8. · 2.47 Impact Factor
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    ABSTRACT: In this study, a spin-polarized electron source which can operate at room temperature is reported. -oriented single crystal magnetite whiskers 30 to 200 nm in diameter were synthesized on a stainless steel plate by the combustion flame thermal oxidation process. Microscopic analyses revealed that the whiskers have some defects such as stacking faults and dislocations and also that manganese is the dominant impurity. An electron source with a single crystalline magnetite whisker with orientation was fabricated using a micro-sampling instrument in a FIB system.
    01/2009;
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    ABSTRACT: Now a day, focused ion beam (FIB) systems equipped with a gallium liquid metal ion source (Ga-LMIS) have been used in the wide areas, e.g., photo-mask repair for semiconductor devices, micro-fabrication for MEMS, sample preparation for TEM and so on. However, contaminations by the irradiated gallium ions are severe problems in these applications, thus for an FIB system in the next generation, a development of a noble gas field ion source (GFIS) with higher angular current density (dl/dOmega) are expected. Recently, the helium ion microscope by adopting a built-up W(111) emitter tip was developed as a powerful apparatus for surface analysis. To obtain an excellent emission properties, the shape of emitter tip having a microscopic protrusion which terminated by a single atom or trimer has been actively researched so far. On the other hand, the shank shape of emitter remains a matter of research for obtaining an ion beam with higher brightness. To improve a dl/dOmega of GFIS, we investigated the relationship between an ion current and a taper angle of emitter shank.
    01/2009;

Publication Stats

441 Citations
123.75 Total Impact Points

Institutions

  • 1995–2013
    • Mie University
      • • Graduate School of Engineering
      • • Department of Electrical and Electronic Engineering
      • • Faculty of Engineering
      • • Division of Electrical and Electronic Engineering
      Tu, Mie, Japan
  • 1994
    • Toyohashi Sozo College
      Toyohasi, Aichi, Japan