Wen Siang Lew

Nanyang Technological University, Tumasik, Singapore

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Publications (44)141.52 Total impact

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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.
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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 · 5.08 Impact Factor
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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 01/2015; 5:9603. DOI:10.1038/srep09603 · 5.08 Impact Factor
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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 · 5.08 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: 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 · 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). DOI:10.1063/1.4867004 · 3.52 Impact Factor
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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 · 4.84 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
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    ABSTRACT: A set of CrAlSiN coatings was synthesized in homogenous and graded composition via magnetron sputtering in mixed Ar and N2 ambient. The microstructures are investigated using glancing angle X-ray diffractometry, field emission scanning electron microscopy and transmission electron microscopy. With compositional grading, CrAlSiN coatings exhibit much improved scratch adhesion strength and better crack propagation resistance at a little expense of hardness.
    Surface and Coatings Technology 09/2013; 231:346-352. DOI:10.1016/j.surfcoat.2012.03.036 · 2.20 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. DOI:10.1186/1556-276X-8-335 · 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). DOI:10.1063/1.4804579 · 3.52 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). DOI:10.1063/1.4792318 · 3.52 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. DOI:10.1063/1.4776699 · 3.52 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. DOI:10.1016/j.apsusc.2012.11.022 · 2.54 Impact Factor