Ken Harada

Osaka Prefecture University, Sakai, Osaka-fu, Japan

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Publications (33)86.99 Total impact

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    ABSTRACT: Advanced split-illumination electron holography was developed by employing two biprisms in the illuminating system to split an electron wave into two coherent waves and two biprisms in the imaging system to overlap them. A focused image of an upper condenser-biprism filament was formed on the sample plane, and all other filaments were placed in its shadow. This developed system makes it possible to obtain precise reconstructed object waves without modulations due to Fresnel fringes, in addition to holograms of distant objects from reference waves.
    Ultramicroscopy 01/2014; 137:7–11. · 2.47 Impact Factor
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    ABSTRACT: In this study, quantitative reciprocal-space analyses of magnetic domain structures in magnetic artificial lattices of patterned elements were performed by means of the small-angle electron scattering (SAES) technique. Using a conventional transmission electron microscope with a LaB(6) thermal-emission electron gun, Lorentz deflection due to magnetic moments in patterned elements and Bragg diffraction due to the lattice periodicity are simultaneously recorded at an angle of the order of less than 1 10(-)(6) rad when using electron waves with high spatial coherency and large camera length. The present SAES technique together with TEM real-space imaging methods such as Lorentz microscopy will be useful in analyzing electromagnetic fields in nano-scaled materials.
    Journal of electron microscopy 10/2012; · 1.31 Impact Factor
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    ABSTRACT: La1-xSrxMnO3 (LSMO) is one of interesting materials with strongly-correlated electrons, wherein a complex variety of ground states are generated depending on the Sr doping concentration x. In this work, we have microscopically investigated changes of the magnetic states by applying magnetic fields in single crystals of LSMO by using Lorentz transmission electron microscopy. In the specimen with x = 0.175, the magnetic stripe domains appear at regular intervals of about 200 nm as a magnetic ground state in zero magnetic field at 110 K. Importantly, we have clarified that magnetic domains as large as 100 nm are generated in the magnetic stripe domains in vertical magnetic fields and take a form of the magnetic vortex with tilted magnetic components. To the best of our knowledge, these magnetic domains are new kinds of magnetic ground states (spin textures) in manganites. In the presentation, we will explain detailed responses of magnetic vortices to various experimental parameters of external magnetic fields and discuss the nucleation and growth mechanism of magnetic vortices in the magnetic stripe domains and the expected functionality of magnetic vortices in manganites.
    02/2012;
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    ABSTRACT: We microscopically investigate the magnetic domain wall motion induced by current pulse application in a small in-plane magnetic field in U-shaped Permalloy wires by means of Lorentz microscopy together with simultaneous transport measurement. An in-plane magnetic field less than 7 Oe parallel to the wire direction in U-shaped geometry effectively works to impede bidirectional motion of the domain wall induced by current pulse application, i.e. to suppress the stochastic nature of the domain wall displacement. The present finding will provide practical and reliable ways of controlling and manipulating the domain wall dynamics, which are widely applicable in spintronic devices, especially when stochastic nature causes serious problems in device operation. Reliable manipulation of the magnetic state is discussed using the current-driven domain wall motion and domain nucleation in the magnetic wire device.
    Journal of Physics D Applied Physics 01/2011; 44(6):064015. · 2.53 Impact Factor
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    ABSTRACT: We investigate crystalline structures of a permalloy nanowire subject to current pulses by using standard transmission and scanning electron microscope techniques, together with the analysis of magnetic states by means of Lorenz microscopy. We find that crystalline grains grow slightly larger in a current region around the Curie temperature, which does not affect significantly the uniformly-magnetized state in the wire because of strong uniaxial shape anisotropy given by the wire geometry. When crystalline grains grow large enough to additionally provide in-plane anisotropy comparable to the shape anisotropy by averaging crystalline anisotropy over the grains, the magnetic ripple will be induced in the wire. Such situations can be found in the case when very large current pulses are applied to the wire.
    Journal of Applied Physics 06/2010; · 2.21 Impact Factor
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    ABSTRACT: We microscopically investigate the dynamics of magnetic domains and domain walls induced by a current pulse in Permalloy narrow wires by means of Lorentz microscopy and simultaneous transport measurement. A variety of magnetic domain and domain wall dynamics are induced as a function of current density flowing into the wire and wire resistance. Important finding is that observed magnetic domain wall displacement and domain nucleation explicitly exhibit stochastic nature, indicating that the magnetic state in the wire is hardly controlled by using solely the current pulse. However, the application of small in-plane magnetic field changes drastically the nature into deterministic, which effectively improves controllability of the magnetic domain and domain wall dynamics using current.
    01/2010;
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    ABSTRACT: We investigate the magnetization dynamics induced by a current pulse in Permalloy nanowires by means of Lorentz microscopy and electron holography, together with simultaneous transport measurements. A variety of magnetization dynamics is observed below the Curie temperature. Local transformation, displacement of magnetic domain wall and nucleation and annihilation of magnetic domain, i.e. magnetization reversal are presented as a function of current density flowing into the wire and wire resistance. Shift of threshold current densities for domain wall displacement and magnetization reversal when changing current pulse duration and thermal conductance of the sample supports that observed behavior of magnetic domains and domain walls is associated with the spin transfer torque and thermal excitation. For the well-controlled magnetization reversal, we microscopically demonstrate that applying small in-plane magnetic field is very effective to controllably nucleate and erase the magnetic domain using a current pulse. Stochastic nature of the magnetization reversal due to spin-wave and thermal excitation in the absence of magnetic field completely disappears and turns into deterministic in the presence of small magnetic field, which enables the magnetization reversal control using current.
    Proc SPIE 08/2008;
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    ABSTRACT: We microscopically demonstrate that the magnetic domain is controllably nucleated and erased in the uniformly magnetized wire using a current pulse in small magnetic fields. Lorentz microscopy is performed in Permalloy nanowires with in-plane anisotropy. The stochastic nature of the magnetization reversal due to spin wave and thermal excitations in the absence of magnetic field completely disappears and turns into deterministic in the presence of small magnetic field, which enables the magnetization reversal control. We interpret that the phenomena are associated with Zeeman energy stabilization.
    Applied Physics Letters 01/2008; 92. · 3.79 Impact Factor
  • e-Journal of Surface Science and Nanotechnology 01/2008; 6:29-34.
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    ABSTRACT: We investigate the current-driven magnetization dynamics in narrow Permalloy wires by means of Lorentz microscopy and electron holography. Current pulses are found to transform the magnetic structure in the uniformly magnetized state below the Curie temperature. A variety of magnetic states including reversed magnetic domains are randomly obtained in low probability. The dynamics of vortices found in most of observed magnetic states seems to play a key role in triggering the magnetization reversal.
    Japanese Journal of Applied Physics 01/2007; · 1.07 Impact Factor
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    ABSTRACT: We investigate the current-excited magnetization dynamics in a narrow ferromagnetic Permalloy wire by means of Lorentz microscopy, together with the results of simultaneous transport measurements. A detailed structural evolution of the magnetization is presented as a function of the applied current density. Local structural deformation, bidirectional displacement, and magnetization reversal are found below the Curie temperature with increasing the current density. We discuss probable mechanisms of observed features of the current-excited magnetization dynamics. Comment: 11 pages, 2 figures, 3 videos, accepted in Japanese Journal of Applied Physics (Express Letters)
    Japanese Journal of Applied Physics 06/2006; · 1.07 Impact Factor
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    ABSTRACT: Triple-biprism electron interferometry has been developed to flexibly control all interference parameters, i.e., interference region (size and shape), fringe spacing, and fringe angle. Each filament electrode of the upper and middle electron biprisms is installed in the individual image planes of the objective and first magnifying lenses, respectively. One more filament electrode of the lower biprism is set between the crossover plane and image plane behind the second magnifying lens. The azimuth angle between the upper and middle filaments is set at 90°, and lower filament is set at about 45° for both of the above filaments. Among many possible optical setups, we chose the setup with the easiest and most flexible control of all the interference parameters through theoretical investigation and experimental confirmation. The interferometer was constructed in a 1×106 V field emission electron microscope, and the overall performance was demonstrated and investigated experimentally.
    Journal of Applied Physics 05/2006; 99(11):113502-113502-7. · 2.21 Impact Factor
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    ABSTRACT: We have investigated the vortex dynamics for the ‘ratchet’ operation in a niobium superconductor via a direct imaging of Lorentz microscopy. We directly observe one-directional selective motion of field-gradient-driven vortices along fabricated channels. This results from the rectification of vortices in a spatially asymmetric potential under the oscillating magnetic field in a temporally symmetric manner. Based on the observation of the individual motion of vortices, we clarify the elementary process involved in this rectification.
    Pramana 01/2006; 66(1):279-287. · 0.56 Impact Factor
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    ABSTRACT: A method of confirming information transfer in spatial frequency regions higher than 10 nm <sup>-1</sup> by the objective lens of an electron microscope is introduced. Two electron-diffraction waves, 000 and hkl , from a crystalline specimen are selected by an objective aperture inserted asymmetrically on the back-focal plane, and a lattice image is observed at appropriate defocus values to prove that information corresponding to the spatial frequency of hkl diffraction has been transferred. We confirmed that information for 15.6 nm <sup>-1</sup> ( 0.064- nm -spacing fringes in real space) and less could be transferred in a 1- million - volt field emission electron microscope.
    Applied Physics Letters 11/2005; · 3.79 Impact Factor
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    ABSTRACT: Using Lorentz microscopy to directly image vortices, we investigate vortex motion control and rectification in a niobium superconductor. We directly observe a net motion of vortices along microfabricated channels with a spatially asymmetric potential, even though the vortices were driven by an oscillatory field. By observing the individual motion of vortices, we clarify elementary processes involved in this rectification. To further demonstrate the ability to control the motion of vortices, we created a tiny vortex "racetrack" to monitor the motion of vortices in a closed circuit channel.
    Physical Review Letters 09/2005; 95(8):087002. · 7.73 Impact Factor
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    ABSTRACT: Double-biprism electron interferometry has been extended by enabling the control of azimuth rotation between the two filament electrodes of the biprisms. Varying the azimuth angle between the filaments induces two parameters not found in conventional electron interferometry: the azimuth angle of the interference fringes and the vertical length of the interference region. This extended interferometry enables us to obtain the phase profile of one-dimensionally structured materials widely along their elongated direction without reconstruction. Evaluation of the overall performance of the interferometry theoretically and experimentally demonstrated that it provides additional flexibility for interferometry.
    Japanese Journal of Applied Physics 04/2005; 44:L636-L639. · 1.07 Impact Factor
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    ABSTRACT: A novel system of electron interferometry and holography using two electron biprisms has been developed. The first biprism is installed in the image plane of the objective lens and the second one is set behind the first magnifying lens, inside the shadow area of the first biprism. The system can independently control two important parameters for interferograms and holograms, the fringe spacing and interference width. Thus, it gives us more flexibility on performing electron interferometry and holography. The good performance of the system was demonstrated using a 1MV field-emission electron microscope. We introduce a variety of optical set-ups for the system and explain the advantages of each set-up in detail, with experimental results.
    Journal of Electron Microscopy 02/2005; 54(1):19-27. · 1.44 Impact Factor
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    ABSTRACT: We investigated vortex dynamics in the ratchet system in a Niobium superconductor by using a direct imaging technique of Lorentz microscopy. We directly observed one-directional selective motion in fabricated channels, which resulted from the rectification of vortices in a spatially asymmetric potential under the oscillating magnetic field in a temporally symmetric manner. Based on observation of the individual motion of vortices, elementary process involved in this rectification was clarified.
    Physica C Superconductivity 01/2005; 426:141-146. · 0.72 Impact Factor
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    ABSTRACT: High-resolution electron holography has been achieved by using a double-biprism interferometer implemented on a 1 MV field emission electron microscope. The interferometer was installed behind the first magnifying lens to narrow carrier fringes and thus enabled complete separation of sideband Fourier spectrum from center band in reconstruction process. Holograms of Au fine particles and single-crystalline thin films with the finest fringe spacing of 4.2 pm were recorded and reconstructed. The overall holography system including the reconstruction process performed well for holograms in which carrier fringes had a spacing of around 10 pm. High-resolution lattice images of the amplitude and phase were clearly reconstructed without mixing of the center band and sideband information. Additionally, entire holograms were recorded without Fresnel fringes normally generated by the filament electrode of the biprism, and the holograms were thus reconstructed without the artifacts caused by Fresnel fringes.
    Journal of Applied Physics 11/2004; 96(11):6097-6102. · 2.21 Impact Factor
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    ABSTRACT: Electron holography based on two electron biprisms was developed. The upper biprism was installed just on the image plane of the objective lens, and the lower one was set between the crossover point and image plane of the magnifying lens. This system was able to control two important parameters of the hologram—fringe space and width of interference region—independently. The system enabled us to perform electron holography and interferometry more flexibly. We confirmed the good performance of the system and did preliminary applications using a 1-MV field-emission electron microscope. © 2004 American Institute of Physics.
    Applied Physics Letters 04/2004; 84(17):3229-3231. · 3.79 Impact Factor

Publication Stats

288 Citations
86.99 Total Impact Points

Institutions

  • 2010–2012
    • Osaka Prefecture University
      • Nanoscience and Nanotechnology Research Center
      Sakai, Osaka-fu, Japan
  • 2008–2010
    • RIKEN
      Вако, Saitama, Japan
  • 2006
    • The University of Tokyo
      Edo, Tōkyō, Japan
  • 2004–2005
    • Toyota Physical and Chemical Institute
      Seto, Aichi, Japan