H. Miyajima

Keio University, Edo, Tokyo, Japan

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Publications (236)361.17 Total impact

  • Akinobu Yamaguchi · Keiichi Motoi · Hideki Miyajima
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    ABSTRACT: This study shows that broadband magnetic noise in a ferromagnetic wire can be detected over a wide frequency range between 500 MHz and 8 GHz using a lock-in detection technique. The magnetic noise spectrum from a 20 nm-thick single-layered Fe19Ni81 wire biased with a dc current is measured as functions of an external field and dc current. This noise is caused by thermal agitation in magnetization due to ambient temperature and Joule heating. The noise behaviors are well reproduced by a stochastic model. Thus, this paper presents a stochastic analysis of magnetic noise behaviors induced by thermal agitation using a highly sensitive technique for detecting the magnetic noise in a single layered ferromagnetic wire.
    No preview · Article · Mar 2016 · Journal of Magnetism and Magnetic Materials
  • A. Yamaguchi · K. Motoi · H. Miyajima · Y. Utsumi
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    ABSTRACT: We measure the broadband spectra of magnetic response in a single layered ferromagnetic nano-scale wire in order to investigate the size effect on the ferromagnetic resonance. We found that the resonance frequency difference between 300-nm- and 5-μm-wide wires was varied by about 5 GHz due to the shape anisotropy. Furthermore, we experimentally detected the magnetization precession induced by the thermal fluctuation via the rectification of a radio-frequency (rf) current by incorporating an additional direct current (dc) by using Wheatstone bridge circuit. Our investigation renders that the shape anisotropy is of great importance to control the resonance frequency and to provide thermal stability of the microwave devices.
    No preview · Article · Sep 2014 · Journal of Magnetism and Magnetic Materials
  • A Yamaguchi · A Hirohata · T Ono · H Miyajima
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    ABSTRACT: We observed a magnetic domain wall (DW) motion induced by the spin-polarized pulsed current in a nanoscale Fe(19)Ni(81) wire using a magnetic force microscope. High current density, which is of the order of 10(11) A m(-2), was required for the DW motion. A simple method to estimate the temperature of the wire was developed by comparing the wire resistance measured during the DW motion with the temperature dependence of the wire resistance. Using this method, we found the temperature of the wire was proportional to the square of the current density and became just beneath at the threshold Curie temperature. Our experimental data qualitatively support this analytical model that the temperature is proportional to the resistivity, thickness, width of the wire and the square of the current density, and also inversely proportional to the thermal conductivity.
    No preview · Article · Jan 2012 · Journal of Physics Condensed Matter
  • Source
    Akinobu Yamaguchi · Keiichi Motoi · Hideki Miyajima
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    ABSTRACT: The magnetic noise spectra induced by direct-current (dc) current flowing through a micron-scale ferromagnetic wire have been investigated. We have observed the noise spectra with a resonance frequency. Under the application of the magnetic field in the plane, the magnetic field dependences of the resonance frequency and amplitude were well interpreted by the analytical calculation based on the stochastic model. The noise spectra are attributable to the resistance oscillation reflecting the uniform magnetization precession which is produced by the Joule heating due to the dc current.
    Full-text · Article · Aug 2011
  • Source
    Y. Kasatani · A. Yamaguchi · H. Miyajima · Y. Nozaki
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    ABSTRACT: The broadband ferromagnetic resonance study on both the single crystalline and polycrystalline Fe wires were performed using the rectifying effect. The effective Gilbert damping in the polycrystalline Fe wire was about three times larger than that in the single crystalline wire. This is attributed to the enhancement of the energy dissipation due to the incoherent rotation of the magnetization at the grains and grain boundaries in the polycrystalline wire. The difference between the experimental data and analytical calculation can be explained by the strong magnetic shape anisotropy that overcomes the external static magnetic field and forces the magnetization to be directed along the wire axis.
    Full-text · Article · Jul 2011 · IEEE Transactions on Magnetics
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    ABSTRACT: Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.
    Full-text · Article · Jun 2011 · Physical review. B, Condensed matter
  • A Yamaguchi · K Motoi · H Miyajima · T Uchiyama · Y Utsumi
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    ABSTRACT: The broadband spin dynamics of patterned ferromagnetic Fe19Ni81 microwire with thickness of 80 nm has been investigated experimentally using broadband rectifying method. The rectifying effect provides a highly sensitive method to detect the high-order perpendicular standing spin wave (PSSW) mode. Present analytical calculation reproduces the observed relation between resonance frequency and applied magnetic field. The effective thickness is explained by the pinning condition of magnetic moment at the surface of the wire.
    No preview · Article · Jan 2011 · Journal of Physics Conference Series
  • Y Kasatani · A Yamaguchi · H Miyajima · Y Nozaki
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    ABSTRACT: The broadband ferromagnetic resonance (FMR) of a single-crystalline Fe microwire was measured using a rectifying effect. The experimental result indicates the uniform mode dominates the system. The effective Gilbert damping factor of the Fe wire was estimated by the rectifying effect. A rectifying spectrum is broadened because of the spin wave excitation and the thermal fluctuation as the input power increases.
    No preview · Article · Jan 2011 · Journal of Physics Conference Series
  • M Goto · H Hata · A Yamaguchi · H Miyajima · Y Nakatani · T Yamaoka · Y Nozaki
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    ABSTRACT: A magnetic vortex core confined in a micron-scale magnetic disk is resonantly excited by spin-polarized radio-frequency (rf) current and rf field. We show that rectifying voltage spectra caused by the vortex core resonance is dependent on the core polarity. Rectifying voltage spectra are given by the superposition of the polarity-dependent term and the polarity-independent term. The sign of the polarity-dependent rectifying voltage reverses when the sign of polarity P or external field H is reversed. This experimental result can be explained by the anisotropic magnetoresistance effect caused by the vortex core motion.
    No preview · Article · Jan 2011 · Journal of Physics Conference Series
  • T. Yoda · H. Miyajima · M. Shimada · R. Nakata · H. Hashimoto
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    ABSTRACT: The XAFS measurement of the MSQ type low-k dielectrics (LKD™) was conducted to clarify the structure change with and without the EB cure. Furthermore, three different types of other MSQ films, the ladder structure, the random structure and the CVD film, have been investigated as references. We have determined Si-O-Si bond angle and Si-O(Si-C) bond length by fitting the Fourier transformed EXAFS spectra.The ladder structure and the random structure have Si-O-Si bond angle of 133 and 146, respectively.Si-O-Si bond angle of LKD™ film is among that of the ladder and the random structure, and the XANES spectrum of LKD™ film displays two broad features, corresponding to the mixture of both structures. In contrast, Si-O-Si angles of the EB cured LKD™ film and the CVD film resemble each other, and the XANES features of both films are almost identical with that of the random structure.We have confirmed that the EB cure process for LKD™ film makes drastic structure change from the mixture of ladder and random structure to the random network structure.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • T. Yoda · H. Miyajima · K. Fujita · R. Nakata · Y. Nishiyama
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    ABSTRACT: An advanced FSG film of k=3.4 was developed, which exhibited excellent resistance for moisture absorption. Physical and chemical properties of this advanced FSG film were compared by typical FSG films deposited in both high density plasma (HDP) and PE-CVD reactors, for the same k value. The advanced FSG film appears to be superior to the HDP-FSG film by a wide margin in the following tests. The moisture absorption rate by TDS (after 4 days of air exposure) is about 5 times lower, the hardness was 1.8 times more, and the hygroscopicity (after 1 hr. boiling) was 2.6 times lower. We conclude that these differences are mainly due to the unique film structure of the advanced FSG film.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • A. Yamaguchi · Y. Kasatani · H. Miyajima
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    ABSTRACT: The characterization of magnetization reversal in a magnetic domain wall (DW) in a single crystalline Fe wire with crystalline anisotropy and shape anisotropy has been investigated by the giant magnetoresistance (GMR) measurement. The DW propagation velocity reached about 1 km/s in 110 Oe field at 77 K. The broad distribution of the propagation velocity versus the switching field in the Fe wire with longitudinal axis parallel to the langle110rangle direction was found. The magnetic anisotropy energy including the crystalline and shape anisotropies play an important role in the nucleation and propagation of DW.
    No preview · Article · Jan 2011 · Journal of Physics Conference Series
  • Akinobu Yamaguchi · Tomoaki Kishimoto · Hideki Miyajima
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    ABSTRACT: Quantum interference incorporating spatially asymmetric potential profiles is realized experimentally to manipulate a magnetic domain wall (DW) into a single giant magnetoresistance (GMR)-type wire whose spacer has a thickness gradient for generating asymmetrical interlayer exchange coupling from side to side. We demonstrate experimentally how to guide a DW in a microscale ferromagnetic wire without reflection symmetry of the interlayer exchange coupling. This is the first observation of asymmetric DW propagation in the wire with oscillating interlayer exchange coupling. The experimental results can be described well by numerical simulations considering spatially asymmetric potential profiles due to quantum interference.
    No preview · Article · Sep 2010 · Applied Physics Express
  • Source
    Akinobu Yamaguchi · Tomoaki Kishimoto · Hideki Miyajima
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    ABSTRACT: Quantum interference incorporating spatially asymmetric potential profiles is realized experimentally to manipulate a magnetic domain wall (DW) into a single multilayered wire whose spacer has a thickness gradient for generating asymmetrical interlayer exchange coupling from side to side. We demonstrate experimentally how to guide a DW in a micron-scale ferromagnetic wire without reflection symmetry of the interlayer exchange coupling. This is the ratcheting of a DW in a form of ratchet potential using quantum interference. The experimental results can be described well by numerical simulations considering spatially asymmetric potential profiles due to quantum interference.
    Full-text · Article · Jul 2010
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    ABSTRACT: The translational mode of a vortex confined in a soft magnetic Fe<sub>19</sub>Ni<sub>81</sub> circular disk and an elliptically shaped dot causes an asymmetric response in the rectifying planar Hall voltage and dc voltage output spectra. The symmetry breaking of these spectra has been experimentally observed, and is controlled by the simultaneous application of radio-frequency current and direct current. We can qualitatively explain the asymmetry of the rectifying spectra by the nonlinear resonator model, with gyration within a large radius around the vortex equilibrium positions causing the asymmetry.
    Full-text · Article · Jul 2010 · IEEE Transactions on Magnetics
  • Y. Kasatani · A. Yamaguchi · H. Yamamoto · H. Miyajima
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    ABSTRACT: The propagation velocities of a magnetic domain wall (DW) in polycrystalline and single-crystalline layers of Fe(001)/Au/Fe19Ni81 wires were measured using the giant magnetoresistance effect. In a single-crystalline Fe wire, the DW propagation velocity (1 km/s in a 110 Oe field at 77 K) was about three times faster than that in a polycrystalline Fe wire. This is attributable to the increase in an effective damping derived from incoherent rotation of the magnetization, which reduced the DW’s mobility in the polycrystalline wire. Magnetocrystalline anisotropy plays an important role in the nucleation of DW and the switching process. Analytic solutions for the DW’s motion have been found on the basis of an approximation suggested by the Slonczewski-Walker and Becker-Döring models; these solutions can explain our experimental results.
    No preview · Article · Jun 2010 · Physical review. B, Condensed matter
  • H Yamamoto · Y Kasatani · A Yamaguchi · H Miyajima
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    ABSTRACT: The controllable and rapid magnetization reversal in nano-scale wires is fundamental to the operation of new magnetic logic and data storage devices. A lot of previous investigations for the single domain wall (DW) dynamics in nano-scale wires have been performed by soft ferromagnetic material such as polycrystalline permalloy with negligible magnetic crystalline anisotropy. In fact, it is vital to understand the DW dynamics within the crystalline anisotropy for not only the fundamental magnetism but also potential applications. The aim of this study is to present the experimental result of magnetization reversal in epitaxial single crystalline nanowires by using giant magnetoresistance effect. The DW at the edge of the wire, and the switching field strongly depends on the crystalline anisotropy.
    No preview · Article · Feb 2010 · Journal of Physics Conference Series
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    ABSTRACT: By measuring a rectifying planer Hall effect, we have manipulated a vortex core trapped in a single layered Fe <sub>19</sub> Ni <sub>81</sub> disk dependent upon the magnitude of a dc current simultaneously applied with an rf current and a magnetic field. The observed behavior is attributed to a single vortex translational mode. The resonance frequency of the translational mode is found to be almost proportional to the magnitude of the dc current and to be governed by the shape of the energy potential well defined by the disk shape.
    No preview · Article · Oct 2009 · Applied Physics Letters
  • A. Yamaguchi · K. Motoi · A. Hirohata · H. Miyajima
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    ABSTRACT: An anomalous Hall effect and rectification of a Hall voltage are observed by applying a radio-frequency (rf) current through a single-layered ferromagnetic wire located on a coplanar waveguide. The components of the magnetization precession, both in and perpendicular to the plane, can be detected via the Hall voltage rectification of the rf current by incorporating an additional direct current (dc). In this paper, we propose a phenomenological model, which describes the time-dependent anisotropic magnetoresistance and the time-dependent planer Hall effect. The nonlinearity of the spin dynamics accompanied by spin waves as functions of rf currents and dc is also studied, as well as those of the magnitude and orientation of the external magnetic field. .
    No preview · Article · Jun 2009 · Physical review. B, Condensed matter
  • A. Yamaguchi · K. Motoi · H. Miyajima · Y. Nakatani
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    ABSTRACT: The spin and electric charge of electrons in nanoscale artificial magnetic systems exhibit peculiar behaviors especially in a radio-frequency (rf) region via spin-wave excitations. One of the interesting effects is the rectification of the rf current in nanomagnets. We present a detailed experimental magnetic field dependence of the rectifying spectrum induced by a rf current flowing through the ferromagnetic wire and discuss in terms of the quantized spin-wave modes excited by the microwave injection.
    No preview · Article · May 2009 · Journal of Applied Physics

Publication Stats

3k Citations
361.17 Total Impact Points

Institutions

  • 1985-2016
    • Keio University
      • • Department of Physics
      • • Faculty of Science and Technology
      Edo, Tokyo, Japan
  • 2002
    • Kinki University
      Ōsaka, Ōsaka, Japan
    • Olympus
      Manda, Aichi, Japan
    • Kyoto University
      • Department of Orthopaedic Surgery
      Kyoto, Kyoto-fu, Japan
  • 1999
    • Kyushu University
      • Graduate School of Information Science and Electrical Engineering
      Hukuoka, Fukuoka, Japan
  • 1990
    • Foxconn Electronics Inc.
      Shanghai, Shanghai Shi, China
  • 1979-1984
    • The University of Tokyo
      • Institute for Solid State Physics
      白山, Tōkyō, Japan