Wen Siang Lew

Nanyang Technological University, Tumasik, Singapore

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Publications (74)201.94 Total impact

  • Source
    C. Murapaka · P. Sethi · S. Goolaup · W. S. Lew
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    ABSTRACT: An all-magnetic logic scheme has the advantages of being non-volatile and energy efficient over the conventional transistor based logic devices. In this work, we present a reconfigurable magnetic logic device which is capable of performing all basic logic operations in a single device. The device exploits the deterministic trajectory of domain wall (DW) in ferromagnetic asymmetric branch structure for obtaining different output combinations. The programmability of the device is achieved by using a current-controlled magnetic gate, which generates a local Oersted field. The field generated at the magnetic gate influences the trajectory of the DW within the structure by exploiting its inherent transverse charge distribution. DW transformation from vortex to transverse configuration close to the output branch plays a pivotal role in governing the DW chirality and hence the output. By simply switching the current direction through the magnetic gate, two universal logic gate functionalities can be obtained in this device. Using magnetic force microscopy imaging and magnetoresistance measurements, all basic logic functionalities are demonstrated.
    Full-text · Article · Feb 2016 · Scientific Reports
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    P Sethi · C Murapaka · S Goolaup · Y J Chen · S H Leong · W S Lew
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    ABSTRACT: Controlling the domain wall (DW) trajectory in magnetic network structures is crucial for spin-based device related applications. The understanding of DW dynamics in network structures is also important for study of fundamental properties like observation of magnetic monopoles at room temperature in artificial spin ice lattice. The trajectory of DW in magnetic network structures has been shown to be chirality dependent. However, the DW chirality periodically oscillates as it propagates a distance longer than its fidelity length due to Walker breakdown phenomenon. This leads to a stochastic behavior in the DW propagation through the network structure. In this study, we show that the DW trajectory can be deterministically controlled in the magnetic network structures irrespective of its chirality by introducing a potential barrier. The DW propagation in the network structure is governed by the geometrically induced potential barrier and pinning strength against the propagation. This technique can be extended for controlling the trajectory of magnetic charge carriers in an artificial spin ice lattice.
    Full-text · Article · Jan 2016 · Scientific Reports
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    Y.P. Liu · W.S. Lew · S. Goolaup · Z.X. Shen · L. Sun · T.J. Zhou · S.K. Wong

    Full-text · Dataset · Dec 2015
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    Yanping Liu · Wen Siang Lew · Li Sun

    Full-text · Dataset · Dec 2015
  • Source
    Yanping Liu · Wen Siang Lew · Li Sun

    Full-text · Dataset · Dec 2015
  • Source
    Y.P. Liu · W.S. Lew · S. Goolaup · Z.X. Shen · L. Sun · T.J. Zhou · S.K. Wong

    Full-text · Dataset · Nov 2015
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    ABSTRACT: Artificial magnetic spin-ice nanostructures provide an ideal platform for the observation of magnetic monopoles. The formation of a magnetic monopole is governed by the motion of a magnetic charge carrier via the propagation of domain walls (DWs) in a lattice. To date, most experiments have been on the static visualization of DW propagation in the lattice. In this paper, we report on the low field dynamics of DW in a unit spin-ice structure measured by magnetoresistance changes. Our results show that reversible DW propagation can be initiated within the spin-ice basis. The initial magnetization configuration of the unit structure strongly influences the direction of DW motion in the branches. Single or multiple domain wall nucleation can be induced in the respective branches of the unit spin ice by the direction of the applied field.
    No preview · Article · Oct 2015 · Journal of Applied Physics
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    Fusheng Ma · yan zhou · Wen Siang Lew
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    ABSTRACT: We present a numerical investigation of the magnonic band structure of spin waves (SWs) in a novel magnonic crystal consisting of a waveguide with a linear array of periodically spaced skyrmions created along its center by micromagnetic simulations. The interfacial Dzyaloshinskii–Moriya interaction (DMI) induced the presence of skyrmions causes a periodical magnetization modulation of the waveguide, which can be dynamically controlled by changing either the strength of the external magnetic field or the period of the skyrmion lattice. The diameters of the skyrmions are highly dependent on the strength of the applied magnetic field, and they can exist for a wide magnetic field range depending on the density of the DMI. The calculated dispersion relation of SWs in the proposed skyrmion magnonic crystal, frequency versus wave vector, exhibits a periodical property, which is the characteristic feature of the band structure of conventional magnonic crystals. Similarly, the calculated magnonic spectra exhibits allowed frequency band and forbidden frequency bandgaps. Our findings could stimulate further exploration on multiple functionalities provided by magnonic crystals based on periodic skyrmion lattices.
    Full-text · Article · Oct 2015 · IEEE Transactions on Magnetics
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    M.B.A. Jalil · S.G. Tan · Z.B. Siu · W. Gan · I. Purnama · W.S. Lew
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    ABSTRACT: We analyze the topological charge of a skyrmion qs, and the corresponding Hall conductivity σxy, which can serve as an electrical read-out for skyrmion-based memory. We derived the general form of the Dzyaloshinskii-Moriya (DM) interaction for any arbitrary orientation of the DM vector D. Based on the DM interaction energy, we obtained the dependence the skyrmion helicity angle γ on the orientation of D. We showed via general mathematical arguments, the topological nature of the skyrmionic charge qs, and its independence of γ and specific details of the interior of the skyrmion (e.g., its core size). Finally, we showed via numerical micromagnetics the stability of qs under varying applied B-fields till the annihilation field, despite the drastic reduction in the skyrmion core size.
    Full-text · Article · Sep 2015 · Journal of Magnetism and Magnetic Materials
  • H. Teoh · S. Goolaup · C. Engel · W. Lew
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    ABSTRACT: In recent years, magnetic tunnel junctions (MTJs) have attracted great interests in the development of next generation high density non-volatile memory and logic chips. Though several MTJ-based logic structures have been proposed and demonstrated, these designs necessitate the inclusion of other electronic components to read the spintronic data as an electronic signal. Additionally, different levels of operations are required to achieve the desired logical function. In this work, we present a novel scheme for logic operation that is able to perform logical operations in two steps and requires no intermediate circuitry to sense the spintronic data. The device works by exploiting the magnetization dynamics of a spatial dependent distribution of current density of a magnetic tunnel junction free layer.
    No preview · Article · Jul 2015
  • D. Loy · S. Goolaup · W. Lew
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    ABSTRACT: In the array of 3D stackable non-volatile logic and memory devices, magnetic tunnel junction (MTJ) devices are most important due to their virtually unlimited endurance, fast read/write speed and low power consumption. These features render MTJs as an ideal candidate for realizing logic structures [1]. An MTJ structure consists of two ferromagnetic layers separated by a thin, insulating spacer layer. The resistance of this MTJ structure can be remarkably tuned in changing the magnetic layers from a parallel to an anti-parallel state which can be used as logic '0' and logic '1'. Different approaches for switching one ferromagnetic layer have been introduced, namely the 2-termi-nal and 3-terminal MTJs. While 2-terminal cells can exploit spin transfer torque (STT) switching and have the advantage of scalability, 3-terminal MTJs employ domain wall motion (DWM) for writing operations and switching. In this work, we present a simplified circuit configuration based on a 5-terminal MTJ structure that is capable of NAND logic operations. This design greatly enhances the scalability by using only two MTJ structures to carry out logic operations.
    No preview · Article · Jul 2015
  • P. Sethi · C. Murapaka · S. Goolaup · W. Lew
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    ABSTRACT: Fundamental properties like observation of magnetic monopoles in artificial spin ice lattice [1] depends on the underlying dynamics of magnetic domain walls (DWs) in geometrically frustrated structures. It has been reported previously that the DW follows a chirality dependent path in magnetic network structures [2-4]. This concept has been applied to generate 1D Dirac strings in artificial Kagome type lattice [3]. Thus the deterministic trajectory of magnetic charge carriers in spinice structure and observation of magnetic monopoles depends on the robustness of the chirality of the injected DW. However above a critical field (Walker field), the DW changes its chirality after propagating a distance larger that its fidelity length [5]. This leads to inherent stochasticity in the trajectory of the DW. To overcome this issue, we introduce an asymmetry in the magnetic network structure. We demonstrate with the aid of magnetic force microscopy (MFM) imaging that the DW is constrained to move along one of the two branches irrespective of its chirality. This study can be extended to define trajectory of magnetic charge carriers in artificial spin ice lattice.
    No preview · Article · Jul 2015
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    ABSTRACT: Domain walls (DWs) in magnetic nanowire have been recognised to have the potential to be used in ultra-high density storage and non-volatile logic devices. For actual applications, the creation of DW on the nanosecond timescale and perfect control of its dynamics is critical. The existing method for creating DW in which local Oersted field is used, two DWs are always nucleated; a head-to-head (HH) and a tail-to-tail (TT) DWs. Due to the intrinsic characteristics of these DWs, they either annihilate or form a coupled DW(1-3). This leads to the stochastic behaviour of DW generation(1). It has been previously reported that the minimum separation of two DWs for their stable existence is approximately 2.6μm(4). Therefore, for strip line width less than 2.6μm there is a very high probability for mutual annihilation of the two generated DWs if no external magnetic field is applied to drive them apart.
    No preview · Article · Jul 2015
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    ABSTRACT: Recently, magnetic nanoparticles are gaining interest for use in magnetic biomedical applications, such as magnetomechanical cell destruction and magnetic hyperthermia. Biofunctionalized NiFe microdiscs, with the application of a low-frequency alternating magnetic field, have been used to demonstrate magnetomechanical cancer-cell destruction by generating an oscillatory motion that transmits a mechanical force to the cell [1]. A magnetization reversal can also occur in magnetic nanoparticles due to a high-frequency alternating magnetic field resulting in the production of thermal energy, which is expressed by the specific absorption rate (SAR) [2]. The heating ability of magnetic nanoparticles shows great potential for a non-invasive and powerful therapy technique for biomedical applications, such as magnetic hyperthermia. By focusing the magnetic nanoparticles at the tumor site, the temperature at the targeted region can be raised to 42-46 °C, which will greatly lower the viability of cancer cells. The advantage of these methods over the conventional cancer therapy is the localization of treatment of the cancer tumor, which minimizes the detrimental side effects experienced by the patient.
    No preview · Article · Jul 2015
  • J. Chen · W. Gan · W. Lew
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    ABSTRACT: Surface-functionalized superparamagnetic (SPM) beads have been widely used to detect and manipulate chemical and biological agents in lab-on-a-chip systems. Recently, it has been shown that by exploiting the stray field generated by domain walls in magnetic nanostructures, it is possible to capture and couple a SPM bead to a domain wall [1]. To store the captured bead, the domain walls can be pinned by fabricating geometrical defects on the nanotracks. Furthermore, the position of the coupled SPM bead can be pinpointed by measuring the magnetoresistance across nanotrack sections. However, studies on such systems have so far been limited to 1D transport.
    No preview · Article · Jul 2015
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    ABSTRACT: The anisotropic magnetoresistance of symmetrical Py (Ni81Fe19) nanowire network reflects the local magnetization dynamics via the bifurcated dual-branch.[1] The magnetoresistance (MR) would be useful for the development of logic devices utilizing magneto-electric binary bits.[2] Furthermore, the unique magnetization dynamics is able to assist in the understanding of artificial spin ice structures to be a complex integrated magneto-resistive circuit comprising of the bifurcated dual-branch bases.[3]
    No preview · Article · Jul 2015
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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.
    Full-text · Article · Jun 2015 · Journal of optics
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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.
    Full-text · Article · May 2015 · Scientific Reports
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    Fusheng Ma · Yan Zhou · Hans-Benjamin Braun · Wen Siang Lew
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    ABSTRACT: A linear array of periodically spaced and individually controllable skyrmions is introduced as a magnonic crystal. It is numerically demonstrated that skyrmion nucleation and annihilation can be accurately controlled by a nanosecond spin polarized current pulse through a nanocontact. Arranged in a periodic array, such nanocontacts allow the creation of a skyrmion lattice that causes a periodic modulation of the waveguide’s magnetization, which can be dynamically controlled by changing either the strength of an applied external magnetic field or the density of the injected spin current through the nanocontacts. The skyrmion diameter is highly dependent on both the applied field and the injected current. This implies tunability of the lowest band gap as the skyrmion diameter directly affects the strength of the pinning potential. The calculated magnonic spectra thus exhibit tunable allowed frequency bands and forbidden frequency bandgaps analogous to that of conventional magnonic crystals where, in contrast, the periodicity is structurally induced and static. In the dynamic magnetic crystal studied here, it is possible to dynamically turn on and off the artificial periodic structure, which allows switching between full rejection and full transmission of spin waves in the waveguide. These findings should stimulate further research activities on multiple functionalities offered by magnonic crystals based on periodic skyrmion lattices.
    Full-text · Article · May 2015 · Nano Letters
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    Fusheng Ma · Yan Zhou · H. B. Braun · W. S. Lew
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    ABSTRACT: A linear array of periodically spaced and individually controllable skyrmions is intro- duced as a magnonic crystal. It is numerically demonstrated that skyrmion nucleation and annihilation can be accurately controlled by a nanosecond spin polarized current pulse through a nanocontact. Arranged in a periodic array, such nanocontacts allow the creation of a skyrmion lattice that causes a periodic modulation of the waveguide's magnetization, which can be dynamically controlled by changing either the strength of an applied external magnetic �eld or the density of the injected spin current through the nanocontacts. The skyrmion diameter is highly dependent on both the applied �field and the injected current. This implies tunability of the lowest band gap as the skyrmion diameter directly a�ect- s the strength of the pinning potential. The calculated magnonic spectra thus exhibit tunable allowed frequency bands and forbidden frequency bandgaps analogous to that of conventional magnonic crystals where, in contrast, the periodicity is structurally induced and static. In the dynamic magnetic crystal studied here it is possible to dynamically turn on and off� the arti�cial periodic structure, which allows switching between full rejection and full transmission of spin waves in the waveguide. These �findings should stimulate further research activities on multiple functionalities o�ered by magnonic crystals based on periodic skyrmion lattices.
    Full-text · Article · May 2015 · Nano Letters