H. Miyajima

Keio University, Edo, Tōkyō, Japan

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Publications (191)336.05 Total impact

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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.
    Journal of Magnetism and Magnetic Materials 01/2014; 364:34–38. · 1.83 Impact Factor
  • 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.
    Journal of Physics Condensed Matter 01/2012; 24(2):024201. · 2.22 Impact Factor
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    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.
    08/2011;
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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.
    IEEE Transactions on Magnetics 07/2011; · 1.42 Impact Factor
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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.
    Physical review. B, Condensed matter 06/2011; 84(6). · 3.77 Impact Factor
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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.
    Journal of Physics Conference Series 01/2011; 266(1):012013.
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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.
    Journal of Physics Conference Series 01/2011; 266(1):012113.
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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.
    Journal of Physics Conference Series 01/2011; 266(1):012080.
  • 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.
    Journal of Physics Conference Series 01/2011; 266(1).
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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.
    07/2010;
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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.
    IEEE Transactions on Magnetics 07/2010; · 1.42 Impact Factor
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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.
    Physical review. B, Condensed matter 06/2010; 81(22). · 3.77 Impact Factor
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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.
    Journal of Physics Conference Series 02/2010; 200(4):042028.
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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.
    Applied Physics Express - APPL PHYS EXPRESS. 01/2010;
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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.
    Applied Physics Letters 10/2009; · 3.79 Impact Factor
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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.
    Journal of Applied Physics 05/2009; · 2.21 Impact Factor
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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.
    Physical review. B, Condensed matter 01/2009; 79(22). · 3.77 Impact Factor
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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 (dc) current. In this paper, we propose a phenomenological model, which describes the time-dependent anisotropic magnetoresistance and time-dependent planer Hall effect. The nonlinearity of the spin dynamics accompanied by spin-waves as functions of rf and dc currents is also studied, as well as those of the magnitude and orientation of the external magnetic field.
    12/2008;
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    ABSTRACT: We demonstrate successful operation of a scanning Hall probe microscope with a few micron-size resolution by using a silicon metal-oxide semiconductor field-effect transistor (Si-MOSFET) Hall bar, which is designed to improve not only the mechanical strength but also the temperature stability. The Si-MOSFET micro-Hall probe is cheaper than the current micro-Hall probes and is found to be as sensitive as a micro-Hall probe with GaAs/AlGaAs heterostructure or an epitaxial InSb two-dimensional electron gas. This was used to magnetically image the surface of a Sm(2)Co(17) permanent magnet during the magnetization reversal process as a function of an external magnetic field below 1.5 T. This revealed firm evidence of the presence of the inverse magnetic seed as theoretically predicted earlier. Magnetically pinned centers, with a typical size 80 mum, are observed to persist even under a high magnetic field, clearly indicating the robustness of the Si Hall probe against the field application as well as the repetition of the measurement.
    The Review of scientific instruments 09/2008; 79(8):083703. · 1.52 Impact Factor
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    ABSTRACT: We investigate the impedance and direct-current (DC) electric response induced by ferromagnetic resonance (FMR) in a single layered ferromagnetic wire. The impedance difference in the FMR state rectifies part of the radio-frequency (RF) current and induces the DC voltage. The impedance measurement using a vector network analyzer reveals the characteristics of the rectifying effect. A phenomenological model interpreting the impedance changes derived from the magnetization precession is proposed. The rectifying effect lies in the ability to demodulate the RF signal for wireless mobile communications.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    08/2008;