Koichi Hata

Mie University, Tu, Mie, Japan

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Publications (89)137.04 Total impact

  • [Show abstract] [Hide abstract] ABSTRACT: An enhancement of current of field-ionized He from a W⟨111⟩ gas field ion emitter in mixtures with Ne or Ar and its origin were investigated by a field ion microscope equipped with a time of flight (ToF) spectrometer. He ion current was increased up to three times by He-Ne mixture and two times by He-Ar mixture, respectively. Mass to charge ratio spectra show that only He atoms were field-ionized at a field of operating condition of a He gas field ion source. In both gas mixture conditions, a peak of singly charged He in ToF spectra shifted after the mixture of He and Ne or Ar, which corresponds to that He was field-ionized at lower potential energy. These observations indicate that He atoms field-adsorbed on a tungsten surface are exchanged for Ne or Ar atoms and the exchange of species of field-adsorbed atom is also related to the enhancement of He ion current. [DOI: 10.1380/ejssnt.2016.23]
    No preview · Article · Jan 2016 · e-Journal of Surface Science and Nanotechnology
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    [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.
    Preview · Article · Apr 2014 · Japanese Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: To analyze spin-polarized scanning tunneling microscopy (STM) studies quantitatively, we evaluated the atomic structure and spin polarization at the apex of Cr/W and Fe/W tips using field ion microscopy (FIM) and field-emitted electron polarimetry, respectively. The patchwork-patterned H-2-FIM images of the Cr/W tip indicated partially developed Cr planes, and the spin polarization at the surface was 10 +/- 3% at room temperature. H-2-FIM images of the Fe/W tip indicate the crystalline order of Fe layers on the W < 110 > tip, and its spin polarization was 41 +/- 2%. These first results allow us to quantify the spin polarization in spin-dependent STM measurements. (C) 2014 The Japan Society of Applied Physics
    No preview · Article · Feb 2014 · Applied Physics Express
  • [Show abstract] [Hide abstract] 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).
    No preview · Article · Jan 2013 · Vacuum
  • [Show abstract] [Hide abstract] ABSTRACT: A field-induced oxygen etching method (O2 etching) is a suitable method for preparing a single atom terminated field emitter tip. By modifying the etching conditions, we demonstrated that a nano-protrusion could be fabricated on a tungsten field emitter tip. The nano-protrusion brings about an enhancement of angular current intensity (dI/dΩ) for a gas field ion source (GFIS). The nano-protrusion is realized an ideal shape of GFIS emitter proposed by S. Kalbitzer, because the structure is well-defined at atomic level. In this paper, the model of progress process of O2 etching is verified experimentally. Next, we mention that a good agreement is obtained in the geometrical sizes of nano-protrusion measured by field ion microscopy (FIM) and by transmission electron microscopy (TEM). Finally, in the viewpoint of practical use, reduction of fabrication time of nano-protrusion is achieved by an O2 etching at the room temperature.
    No preview · Conference Paper · Jul 2012
  • [Show abstract] [Hide abstract] ABSTRACT: So far, focused ion beam (FIB) has been contributed to the development of nanotechnology. However, liquid metal ion source (LMIS) equipped on commercial FIB systems has a serious problem which is the pollution of sample surfaces due to irradiated ion species. To develop the pollution-free and high-brightness ion source, we have focused on noble-gas field ion source (GFIS) and have developed the practical use of Ne- or Ar-GFIS. In this study, we report the stability of Ne ion current emitted from a supertip prepared by a modified field-induced oxygen etching.
    No preview · Conference Paper · Jul 2012
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Jun 2012 · Surface and Interface Analysis
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Jun 2012 · Surface and Interface Analysis
  • [Show abstract] [Hide abstract] ABSTRACT: We observed the Verwey transition, metal-insulator transition, in a spin-polarization of field-emitted electrons from a topmost surface of a{110}-oriented magnetite whisker by means of field-emitted electron polarimetry. Raising temperature of the whisker to about 120 K, the polarization of field-emitted electrons increased from 4 to 11% with a rotation of its direction. The rotation angle corresponds to a change of magnetic easy axis from{001}to{111}direction caused by the Verwey transition. At magnetite temperature of 40 K, the spin polarization was sensitive to increase of the applied field. Eventually, the direction of spin polarization reversed, which can not be explained taking in to account conventional free electron model. These results reveal that the Verwey transition occurs even at a surface as well as in a bulk and that the energy width and level of t2g band become narrower and lower than that of bulk.
    No preview · Article · Jan 2012 · Journal of the Vacuum Society of Japan
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Jan 2012 · Japanese Journal of Applied Physics
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Dec 2011 · Journal of Magnetism and Magnetic Materials
  • [Show abstract] [Hide abstract] ABSTRACT: We have been developing a next generation nely focused ion beam (FIB) system with noble gas eld ion source (GFIS), which could reduce the contamination of irradiated ion species. Ion current stability of GFIS is one of the most fundamental factors for FIB applications. In this study, we report a preliminary measurement of current stability of eld-ionized Ar + ions from a tungsten emitter-tip which has a specic nanoprotrusion terminated by a trimer on the tip-apex. As a measure of long-term stability of Ar + ion beam, current uctuation of 5% for 1800 seconds was typically obtained at the beam current of 1.3 pA and the argon gas pressure of 5×10 -3 Pa. Furthermore it was found in argon-GFIS that the spike-like current uctuation was correlated with the trimer-dimer transformation in a tungsten trimer-tip.
    No preview · Article · Oct 2011 · e-Journal of Surface Science and Nanotechnology
  • [Show abstract] [Hide abstract] ABSTRACT: Focused ion beam system with a liquid metal ion source has a serious problem which is the pollution of samples due to irradiated ion species. To solve the problem, we have developed a noble gas field ion source which scarcely gives rise to the pollution. In this case, a key issue for the field-ion emitter is the shape of tip-apex to obtain a higher angular current density. Better performance is expected for a shape of tip-apex having a nanoscale-protrusion on the emitter tip with large radius of curvature. For realization of such a shape of tip-apex, field-induced oxygen etching method, which was reported by Onoda et al., was modified and applied to a sharpened tungsten wire which was electrolitically polished. Observation of formed emitter tips by transmission electron microscopy revealed that a nanoprotrusion having a shape of truncated cone with ∼5 nm in diameter and ∼4 nm in height was formed at the center of an emitter tip with ∼90 nm in radius of curvature.
    No preview · Article · Sep 2011 · e-Journal of Surface Science and Nanotechnology
  • S. Nagai · K. Hata · M. Okada · H. Mimura
    [Show abstract] [Hide abstract] ABSTRACT: We observed the Verwey transition, i. e., metal-insulator transition, in a spin-polarization of field-emitted electrons from a topmost surface of a (110)-oriented magnetite whisker by means of field-emitted electron polarimetry. With a rise in temperature of the whisker to about 120 K, the polarization of field-emitted electrons increased from 4 to 11% with a rotation of its direction. The rotation angle corresponds to a change of magnetic easy axis from (001) to (111) direction caused by the Verwey transition. This result reveals that the Verwey transition occurs even at a surface as well as in a bulk.
    No preview · Article · Sep 2011 · e-Journal of Surface Science and Nanotechnology
  • S Nagai · H Sakakibara · K Hata · M Okada · H Mimura
    [Show abstract] [Hide abstract] 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.
    No preview · Article · May 2011 · Ultramicroscopy
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Jan 2011
  • Koichi Hata · Akihiro Takakura · Yahachi Saito
    [Show abstract] [Hide abstract] ABSTRACT: Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10-10 Torr, residual gas molecules, attracted by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for stepwise increases and decreases in the emission current, respectively.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Oct 2010 · Surface and Interface Analysis
  • [Show abstract] [Hide abstract] 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)
    No preview · Article · Oct 2010 · physica status solidi (c)
  • Koichi Hata · Ryosuke Yabushita
    [Show abstract] [Hide abstract] ABSTRACT: IntroductionMultiwalled CNT Field Emission CathodeConstruction of High-Resolution Transmission X-ray Microscope Equipped with the Function of SEMCharacteristic Evaluation of High-Resolution X-ray Microscope Provided with SEM FunctionFactors Limiting Resolution of X-ray Transmission ImageConclusion References
    No preview · Article · Jul 2010