Wen Siang Lew

Kyoto University, Kioto, Kyōto, Japan

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Publications (36)113.74 Total impact

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    ABSTRACT: We report on the magnetization dynamics at a bifurcation in a dual-branch magnetic network structure. When a transverse domain wall (DW) propagates through the network, interaction with an edge defect at the bifurcation leads to the transformation of the DW from transverse to vortex. The topological charge is conserved as the DW moves through the bifurcation, and this charge conservation is intrinsically linked to a −1/2 topological defect in the system. Magnetic force microscopy (MFM) imaging enables the direct observation of defect displacement during DW transformation, which induces a selective switching in the branch of the network structure.
    Applied Physics Express 10/2014; 7(11):113003. · 2.73 Impact Factor
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    ABSTRACT: We demonstrate a fabrication technique to create cylindrical NiFe magnetic nanoparticles (MNPs) with controlled dimensions and composition. MNPs thicker than 200 nm can form a double vortex configuration, which consists of a pair of vortices with opposite chirality. When MNPs thicker than 300 nm are relaxed after saturation, it forms a frustrated triple vortex state which produces a higher net magnetization as verified by light transmissivity measurements. Therefore, a greater magnetic torque can be actuated on a MNP in the triple vortex state.
    Applied Physics Letters 10/2014; 105:152405. · 3.79 Impact Factor
  • Sensors and Actuators B Chemical 02/2014; 191:19-23. · 3.84 Impact Factor
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    ABSTRACT: We report on transverse domain wall (DW) depinning mechanisms at the geometrical modulations in NiFe cylindrical nanowires. The DW depinning field and current density always follow opposite trends with diameter modulation. For current driven DW, the depinning current density decreases with increasing notch depth. This interesting behavior arises due to a combination of DW deformation and rotation at the pinning site. With increasing anti-notch height, two distinct depinning mechanisms are observed for both field and current driven DW. Above a critical height, the DW transformation from transverse to vortex configuration leads to a change in the potential barrier. For field-driven, the barrier is lowered, whereas for current-driven, the barrier increases. The increase in the potential barrier for the current driven DW is due to the appearance of an intrinsic pinning within the anti-notch.
    Applied Physics Letters 01/2014; 115(8). · 3.79 Impact Factor
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    ABSTRACT: We present a method to drive multiple domain walls in the absence of direct current application in a coupled nanowire system. The domain walls were driven by a combination of remote coupling and exchange repulsion force from the domain wall compressions. The domain walls were compressed as they were unable to annihilate each other due to having similar topological charges. The compressions are present between the subsequent domain walls, which allow them to be driven as a group in the coupled nanowire system.
    Applied Physics Letters 01/2014; 104(9):092414-092414-4. · 3.79 Impact Factor
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    ABSTRACT: Spin wave emission due to field-driven domain wall (DW) collision has been investigated numerically and analytically in permalloy nanowires. The spin wave modes generated are diagonally symmetric with respect to the collision point. The non-propagating mode has the highest amplitude along the middle of the width. The frequency of this mode is strongly correlated to the nanowire geometrical dimensions and is independent of the strength of applied field within the range of 0.1 mT to 1 mT. For nanowire with film thickness below 5 nm, a second spin wave harmonic mode is observed. The decay coefficient of the spin wave power suggests that the DWs in a memory device should be at least 300 nm apart for them to be free of interference from the spin waves.
    Journal of Applied Physics 01/2014; 115(24):243908-243908-5. · 2.21 Impact Factor
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    S. Krishnia, I. Purnama, W.S. Lew
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    ABSTRACT: In a multiple nanowire system, we show by micromagnetic simulations that a transverse domain wall in a current-free nanowire can undergo a remote Walker breakdown when it is coupled to a nearby current-driven domain wall. Moreover, for chirality combination with the highest coupling strength, the remote Walker breakdown preceded the current-induced Walker breakdown. The Walker breakdown limit of such coupled systems has also been shifted towards higher current densities, where beyond these, the coupling is shown to be broken.
    Applied Physics Letters 01/2014; 105(4):042404-042404-4. · 3.79 Impact Factor
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    ABSTRACT: We report a systematic investigation of the temperature dependence of electrical resistance behaviours in tri- and four-layer graphene interconnects. Nonlinear current--voltage characteristics were observed at different temperatures, which are attributed to the heating effect. With the resistance curve derivative analysis method, our experimental results suggest that Coulomb interactions play an essential role in our devices. The room temperature measurements further indicate that the graphene layers exhibit the characteristics of semiconductors mainly due to the Coulomb scattering effects. By combining the Coulomb and short-range scattering theory, we derive an analytical model to explain the temperature dependence of the resistance, which agrees well with the experimental results.
    Nanoscale Research Letters 07/2013; 8(1):335. · 2.52 Impact Factor
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    ABSTRACT: The phenomenon of laser self-induced tunable birefringence of magnetic fluid is investigated. This phenomenon exists in magnetic fluid, no matter whether it is under an external magnetic field or not. The variation trend of the laser self-induced birefringence with the laser power follows a linear relationship. Besides, dichroism is not observed in accompany with the laser self-induced birefringence.
    Applied Physics Letters 05/2013; 102(18). · 3.79 Impact Factor
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    ABSTRACT: We report on the electrical injection and detection of spin accumulation in trilayer-graphene/MgO/Permalloy lateral spin-valve (LSV) structure. Non-local spin valve signal is clearly observed in the LSV, indicating that spin coherence extends underneath all ferromagnetic contacts. We also show that low-resistivity graphene/MgO/Py junctions enable efficient spin injection and detection in LSV with high applied current density, which leads to large spin accumulation of 120 mu V at room temperature. A spin diffusion length of 1.5 mu m was obtained for the injector-detector separation dependence of spin valve signal measurements carried out at room temperature, while at T = 10 K, the diffusion length increases to 2.3 mu m. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4776699]
    Applied Physics Letters 02/2013; 102(7). · 3.79 Impact Factor
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    ABSTRACT: To combat the brittleness of hard nc-CrAlN/a-SiNx nanocomposite (nc-: nanocrystalline, a-: X-ray amorphous), different Ni content (from 0 to 39.8 at.%) is doped via magnetron sputtering. Glancing Angle X-ray Diffractometry, X-ray photoelectron spectroscopy, Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy are employed to investigate the microstructural evolution. With increased Ni, the grain size decreases accompanied with morphology change, from dense glassy to coarse columns. With 4.2 at.% Ni, scratch toughness of nc-CrAlN/a-SiNx hard nanocomposite (28 GPa) is improved by around 200% at expense of only 18% hardness.
    Applied Surface Science 01/2013; 265:418–423. · 2.54 Impact Factor
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    ABSTRACT: Different crystallographic stacking configurations in graphene provide an additional degree of freedom in the electronic structure. We have conducted systematic investigations of the transport properties of ABAB- and ABCA-stacked four-layer graphene. Our results reveal that ABAB and ABCA graphene exhibit markedly different properties as functions of both temperature and magnetic field. The temperature-dependant resistance measurement reveals that the excitonic gap of ABCA stacked graphene increases as a function of temperature, while for ABAB, a shrinking excitonic gap configuration is observed.
    Applied Physics Letters 01/2013; 103(16):-. · 3.79 Impact Factor
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    ABSTRACT: We report on the electrical injection and detection of spin accumulation in trilayer-graphene/MgO/Permalloy lateral spin-valve (LSV) structure. Non-local spin valve signal is clearly observed in the LSV, indicating that spin coherence extends underneath all ferromagnetic contacts. We also show that low-resistivity graphene/MgO/Py junctions enable efficient spin injection and detection in LSV with high applied current density, which leads to large spin accumulation of 120 μV at room temperature. A spin diffusion length of 1.5 μm was obtained for the injector-detector separation dependence of spin valve signal measurements carried out at room temperature, while at T = 10 K, the diffusion length increases to 2.3 μm.
    Applied Physics Letters 01/2013; 102(3):033105-033105-4. · 3.79 Impact Factor
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    ABSTRACT: Reducing the magnetic shape anisotropy of a cylindrical NiFe nanowire allows the formation of two vortices with opposite chirality at the two ends. At relatively low aspect ratio these two vortices are connected via a gradual rotation of the magnetization over a short region, which forms a three-dimensional helical domain wall. Micromagnetic simulations reveal that it is possible to control the number of helical domain walls in the cylindrical nanowire by geometrical constrictions engineering. A technique to create constricted Ni95Fe5/Ni87Fe13 multilayered nanowires is demonstrated, and magnetic force microscopy imaging was carried out to confirm the prediction of simulated helical domain walls.
    Applied Physics Letters 10/2012; · 3.79 Impact Factor
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    ABSTRACT: Chromium aluminum nitride (CrAlN) coatings were prepared via magnetron sputtering in a mixed Ar and N2 ambient. The effect of negative bias voltage (Vb) on the microstructure was investigated using a spectrum of characterizing techniques in terms of Glancing Angle X-ray Diffractometry, Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy. As Vb increased from 50 to 260 V, hardness was improved from 10 to 26 GPa. Toughness grew at the same time and maximized at around 2 MPa·m1/2 when Vb = 210 V. Simultaneous improvements in hardness and toughness were attributed to the densified microstructure with refined grains and increased compressive stress.
    Surface and Coatings Technology 08/2012; 206(24):5103–5107. · 1.94 Impact Factor
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    ABSTRACT: We report on the magnetization reversal behavior of sub 100-nm triangular shaped Ni 80 Fe 20 dot array fabri-cated by nanosphere lithography. Hysteresis loops measured by magneto-optical Kerr effect magnetometry are classified into single and double-switched loops in 45 nm, 80 nm and 100 nm triangular nanomagnets. Micromagnetic simulations show that a plateau observed in the double-switched loop in the 100-nm trian-gular nanomagnet is due to the formation of a metastable mediating V state.
    Thin Solid Films 06/2012; · 1.87 Impact Factor
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    ABSTRACT: We report on the magnetization reversal process in sub-100 nm Ni80Fe20 asymmetric ring. The switching mechanism involves a stable vortex state due to the strong anisotropy imposed on the narrow arm of the ring. Experimental demonstration shows that such vortex configuration does not annihilate until a large reversal field of 1200 Oe. The asymmetry of the structure promotes a unique reversal process, which gives us control over the chirality of the vortex configuration. Micromagnetic simulations reveal that the highly stable vortex configuration is sustainable in the asymmetric ring structures with a diameter as small as 30 nm.
    Applied Physics Express 05/2012; 5(5):3001-. · 2.73 Impact Factor
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    Carbon. 05/2012;
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    ABSTRACT: Differential dual spin valve (DDSV) is a potential read sensor for ultrahigh density magnetic recording. Recently, it has been found that self-biased DDSVs in which the two free layers (FLs) align in flux closure configuration could potentially remove permanent hard bias. In the present work, we have carried out a systematic study of the effect of the interlayer coupling through gap layer on the reversal process of a self-biased DDSV. From the uniform field response of the DDSV, it was found that the competition among magnetostatic coupling, interlayer coupling and the shape anisotropy of the FLs gives rise to distinctive reversal behaviors, and the magnetostatic coupling could not be perfectly compensated by the interlayer exchange coupling. The down track response of the DDSV reveals that interlayer coupling could play a dominant role on the performance of the DDSV.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
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    ABSTRACT: A novel magnetic field fiber sensor based on magnetic fluid is proposed. The sensor is configured as a Sagnac interferometer structure with a magnetic fluid film and a section of polarization maintaining fiber inserted into the fiber loop to produce a sinusoidal interference spectrum for measurement. The output interference spectrum is shifted as the change of the applied magnetic field strength with a sensitivity of 16.7 pm/Oe and a resolution of 0.60 Oe. The output optical power is varied with the change of the applied magnetic field strength with a sensitivity of 0.3998 dB/Oe.
    Optics Letters 02/2012; 37(3):398-400. · 3.39 Impact Factor