Wen Siang Lew

Nanyang Technological University, Tumasik, Singapore

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Publications (50)158.73 Total impact

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    ABSTRACT: Magnetic fluid is a promising material for sensing applications due to its remarkable magneto-optic properties. An optical fiber magnetic field sensor was developed using a long-period grating (LPG) coated with magnetic fluid. Magnetic fluid undergoes magnetization, aggregation, and phase transitions when it is under an external magnetic field. Optical properties changes that induced by the magnetic field can be sensed by the LPG of which resonant wavelength and transmission minimum are highly sensitive to the change of ambient medium. We demonstrate that the proposed sensor can maintain a high sensitivity of ∼0.154 dB/Gauss at field strength of as low as ∼7.4 Gauss.
    Journal of optics 06/2015; 17(6). DOI:10.1088/2040-8978/17/6/065402 · 2.01 Impact Factor
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    Indra Purnama · Gan Wei Liang · De Wei Wong · Wen Siang Lew
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    ABSTRACT: Magnetic skyrmions are particle-like magnetization configurations which can be found in materials with broken inversion symmetry. Their topological nature allows them to circumvent around random pinning sites or impurities as they move within the magnetic layer, which makes them interesting as information carriers in memory devices. However, when the skyrmion is driven by a current, a Magnus force is generated which leads to the skyrmion moving away from the direction of the conduction electron flow. The deflection poses a serious problem to the realization of skyrmion-based devices, as it leads to skyrmion annihilation at the film edges. Here, we show that it is possible to guide the movement of the skyrmion and prevent it from annihilating by surrounding and compressing the skyrmion with strong local potential barriers. The compressed skyrmion receives higher contribution from the spin transfer torque, which results in the significant increase of the skyrmion speed.
    Scientific Reports 05/2015; 5:10620. DOI:10.1038/srep10620 · 5.58 Impact Factor
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    Hiu Tung Fook · Wei Liang Gan · Indra Purnama · Wen Siang Lew
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    ABSTRACT: Current-driven skyrmions drift from the intended direction of motion in a thin magnetic film due to the presence of the Magnus force and are annihilated upon reaching the film edge. This paper proposes two methods to engineer a 1D potential well to confine the skyrmion motion in the center region of nanowires and thus preventing annihilation. By patterning the magnetic anisotropy of the film or by adding a layer of magnetic material at the edges, the barrier height and width of the potential well can be controlled. Magnetic skyrmions in such nanowires can then be guided to traverse only along the axis of the nanowire, even in nanowires with steep bends. In addition, we also report a compression mechanism where the skyrmion size and separation distance can be reduced by modifying the potential well, thus increasing the skyrmion packing density in a nanowire. The guided motion and high skyrmion density made possible by our proposed methods will allow for the realization of high density skyrmion based memory.
    IEEE Transactions on Magnetics 05/2015; PP:1-1. DOI:10.1109/TMAG.2015.2433677 · 1.21 Impact Factor
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    ABSTRACT: For heat assisted magnetic recording (HAMR) media development, measurement of erasure temperature (Te) is interesting and important for practical HAMR testing and applications. Here, we present an investigation on Te measurements of L10 ordered FePt granular HAMR media made using a Laser Heating (LH) method on a home-built HAMR write test system versus that from a bulk heating approach. The HAMR write test system provides HAMR writing, micro-MOKE (magneto-optical Kerr effect) signal detection, and MOKE imaging functions at the same testing spot in one single system. Magnetic force microscopy (MFM) and magnetic Kerr microscopy observations of the scanning laser induced degradation/erasure/demagnetization of the pre-recorded magnetic patterns on disk media (over a wide area of a few hundreds of μm2) show that the magnetic (MFM and Kerr signal) amplitude of the pre-recorded magnetic patterns decreases slowly with increasing laser power (Pw) (/temperature rise) for Pw ≲ 66 mW and then drops sharply to nearly zero for Pw ≥ ∼72 mW (the laser power corresponding to complete thermal erasure when the media temperature is ∼Te). It was further found that this trend of magnetic amplitude reduction with increased Pw is similar to that from magnetic amplitude decrease of pre-recorded magnetic patterns with increased bulk heating temperature. The temperature for complete erasure at laser power, Pw = 72 mW for the LH method, corresponds therefore to ∼650 K (≈Te) for the bulk heating methods. Besides fast measurement, LH (as a comparable and viable approach for erasure measurement) is dynamic, localized, and has time scales closer to practical HAMR situation.
    Journal of Applied Physics 04/2015; 117(17). DOI:10.1063/1.4914362 · 2.19 Impact Factor
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    ABSTRACT: We report on a micromagnetic study on the dynamics of current-driven helical domain wall (DW) in cylindrical NiFe nanowires. The helical DW is a three-dimensional transition region between magnetizations with clockwise and anticlockwise vortex orientations. A minimum current density is needed to overcome an intrinsic pinning to drive the helical DW, and the propagation along the nanowire is accompanied by a rotational motion. As the driving current strength is increased, the rotation ceases while the DW propagates at an increased velocity. However, a velocity barrier is experienced which results in the decrease of the DW mobility. Throughout its motion, the propagated helical DW maintains a stable profile without showing any sign of structural breakdown even at relatively high driving current
    Journal of Applied Physics 04/2015; 117:17A747. DOI:10.1063/1.4919045 · 2.19 Impact Factor
  • S Goolaup · M Ramu · C Murapaka · W S Lew
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    ABSTRACT: Domain wall (DW) based logic and memory devices require precise control and manipulation of DW in nanowire conduits. The topological defects of Transverse DWs (TDW) are of paramount importance as regards to the deterministic pinning and movement of DW within complex networks of conduits. In-situ control of the DW topological defects in nanowire conduits may pave the way for novel DW logic applications. In this work, we present a geometrical modulation along a nanowire conduit, which allows for the topological rectification/inversion of TDW in nanowires. This is achieved by exploiting the controlled relaxation of the TDW within an angled rectangle. Direct evidence of the logical operation is obtained via magnetic force microscopy measurement.
    Scientific Reports 04/2015; 5:9603. DOI:10.1038/srep09603 · 5.58 Impact Factor
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    ABSTRACT: The operating performance of a domain wall-based magnetic device relies on the controlled motion of the domain walls within the ferromagnetic nanowires. Here, we report on the dynamics of coupled Ne´el domain wall in perpendicular magnetic anisotropy (PMA) nanowires via micromagnetic simulations. The coupled Ne´el domain wall is obtained in a sandwich structure, where two PMA nanowires that are separated by an insulating layer are stacked vertically. Under the application of high current density, we found that the Walker breakdown phenomenon is suppressed in the sandwich structure. Consequently, the coupled Ne´el domain wall of the sandwich structure is able to move faster as compared to individual domain walls in a single PMA nanowire.
    Scientific Reports 03/2015; 5:8754. DOI:10.1038/srep08754 · 5.58 Impact Factor
  • M. Chandra Sekhar · M. Tran · L. Wang · G. C. Han · W. S. Lew
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    ABSTRACT: We present a detailed study of spin-transfer torque induced noise in self-biased differential dual spin valves (DDSV) which could be potentially used as magnetic read-heads for hard-disk drives. Micromagnetics studies of DDSV were performed in all the major magnetic configurations experienced by read-heads and we show that in every case, self-biased DDSV provide a much stronger stability against spin-transfer torque noise than conventional spin valves. Provided are also insights on the influence of the dipolar interlayer coupling, shape anisotropy, exchange bias and relative orientation between the 2 free layers. Our results demonstrate the viability of DDSV read-heads for future hard disk drives generations.
    Journal of Magnetism and Magnetic Materials 12/2014; 374. DOI:10.1016/j.jmmm.2014.08.102 · 2.00 Impact Factor
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    ABSTRACT: Controlling domain wall (DW) generation and dynamics behaviour in ferromagnetic nanowire is critical to the engineering of domain wall-based non-volatile logic and magnetic memory devices. Previous research showed that DW generation suffered from a random or stochastic nature and that makes the realization of DW based device a challenging task. Conventionally, stabilizing a Néel DW requires a long pulsed current and the assistance of an external magnetic field. Here, we demonstrate a method to deterministically produce single DW without having to compromise the pulse duration. No external field is required to stabilize the DW. This is achieved by controlling the stray field magnetostatic interaction between a current-carrying strip line generated DW and the edge of the nanowire. The natural edge-field assisted domain wall generation process was found to be twice as fast as the conventional methods and requires less current density. Such deterministic DW generation method could potentially bring DW device technology, a step closer to on-chip application.
    Scientific Reports 12/2014; 4:7459. DOI:10.1038/srep07459 · 5.58 Impact Factor
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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. DOI:10.7567/APEX.7.113003 · 2.57 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(15):152405. DOI:10.1063/1.4898349 · 3.52 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 07/2014; 105(4):042404-042404-4. DOI:10.1063/1.4891502 · 3.52 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 06/2014; 115(24):243908-243908-5. DOI:10.1063/1.4885453 · 2.19 Impact Factor
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    ABSTRACT: The present disclosure relates to a non-volatile logic device, comprising: a first input element which is magnetizable along a first direction of magnetization to impart or change a chirality of a domain wall traversing the first input element, the chirality representing a first logical input; a second input element joined to the first input element to transport the domain wall, the second input element being magnetizable along a second direction of magnetization which is orthogonal to the first direction of magnetization, a magnetization of the second input element along the second direction of magnetization representing a second logical input; a bifurcated output section connected to the second input element at a junction, the output section comprising a pair of output elements for selectively receiving the domain wall from the second input element, a magnetization of at least part of the respective output elements being changeable by propagation of the domain wall along the respective output elements; and a non-magnetic conductive element disposed in partially overlapping relationship with the second input element at or near the junction; wherein the magnetization in an output element after propagation of the domain wall represents a value of a logical function of the first logical input and/or the second logical input, or a logical complement of the value; and wherein the logical function is selectable by passing an electrical current through the non-magnetic conductive element to induce a magnetic field of a desired magnitude and direction in the second input element.
    Ref. No: WO2014185866, Year: 05/2014
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    I. Purnama · C.S. Murapaka · W.S. Lew · T. Ono
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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 03/2014; 104(9):092414-092414-4. DOI:10.1063/1.4867468 · 3.52 Impact Factor
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    ABSTRACT: Magnetic field sensor by combing magnetic fluid and optical fiber Loyt-Sagnac interferometer is proposed. The sensor takes advantage of the birefringence effect of magnetic fluid. The relative small birefringence of the magnetic fluid is 'magnified' by the properly designed optical fiber Loyt-Sagnac interferometric structure. As compared to the reported MF-based sensors, the achieved sensitivity of the proposed sensor is 592.8 pm/Oe, which is enhanced by 1-3 orders of magnitude. (C) 2013 Published by Elsevier B.V.
    Sensors and Actuators B Chemical 02/2014; 191:19-23. DOI:10.1016/j.snb.2013.09.085 · 4.29 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). DOI:10.1063/1.4867004 · 3.52 Impact Factor
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    ABSTRACT: A memory device comprising a ferromagnetic data nanowire, a ferromagnetic driver nanowire, read element and/or a spaced write element positioned about the data nanowire, wherein driving a domain wall in the driver nanowire remotely drives a domain wall in the data nanowire past the read element and/or the write element.
    Ref. No: WO2014107140 A1, Year: 01/2014
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    ABSTRACT: A recent report proved that polymer residues become grafted to graphene despite the efforts to clean the surface by thermal annealing [Lin, Y.-C., et al. Nano Lett. 2012, 12, 414]. Such residues inevitably originate from the photoresist (e.g., poly(methyl methacrylate) (PMMA)) used for graphene transfer and device processing. Here, through spatially resolved transient absorption spectroscopy and transient photoluminescence spectroscopy, we investigate the effects of such polymer-grafted residues on the carrier dynamics of CVD graphene. The presence of these polymer-grafted residues is validated by both X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Unlike the ultrafast nonradiative recombination at the pristine graphene, these regions exhibit distinct long-lived carrier dynamics that undergo radiative recombination, which is characteristic of the opening of the graphene bandgap. Understanding the influence of such defects on the carrier dynamics and relaxation pathways is key to modifying the optoelectronic properties of graphene-based devices.
    The Journal of Physical Chemistry C 12/2013; 118(1):708–713. DOI:10.1021/jp406675r · 4.77 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 10/2013; 103(16-16):-. DOI:10.1063/1.4825263 · 3.52 Impact Factor