C. K. Ong

National University of Singapore, Tumasik, Singapore

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Publications (486)1097.96 Total impact

  • Source
    Wee Tee Soh · Yasmin Yeow · Xiaoxi Zhong · C. K. Ong
    Journal of Physics D Applied Physics 09/2015; 48(34-34):345002. DOI:10.1088/0022-3727/48/34/345002 · 2.72 Impact Factor
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    ABSTRACT: A hybrid ferroelectric thin film varactor device architecture is presented that allows for integrating the features of both structures of coplanar and parallel-plate varactors. In the hybrid varactor structure, a high-resistivity ZnO ultra-thin film is used as dc bias bottom electrode and at the same time the electrode does not contribute in the electric field distribution of microwave signal. The frequency response of the proposed hybrid varactor with the outer dc bias structure is characterized and its analysis shows that the high-resistivity thin film layer is serving as a dc bias path for high tunability. The fabricated hybrid barium strontium titanate thin film varactor exhibited low capacitance and improved tunability compared with the conventional coplanar varactor.
    Thin Solid Films 06/2015; 585. DOI:10.1016/j.tsf.2015.04.002 · 2.13 Impact Factor
  • Nguyen N. Phuoc · C. K. Ong
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    ABSTRACT: A detailed investigation of electrical tuning of dynamic magnetization of the FeCo magnetic thin film grown onto a PMN-PT piezoelectric substrate was carried out based on the measurement of the zero-field permeability spectra under the application of a voltage across the thickness of the substrate. The resonance frequency can be tuned from 2.95 GHz to 5.9 GHz upon the application of a voltage on the sample in unpoled state. After poling, the resonance frequency of the sample can be tuned in the range from 4.75 GHz to 5.9 GHz. In addition, it was found that after poling the peak of the permeability spectra is broadened compared to before poling, which can be tentatively attributed to the magnetic anisotropy dispersion arising from the presence of the stress-induced anisotropy. The result is also discussed in conjunction with the angular measurement of the static hysteresis loops of the sample before and after poling.
    Journal of Applied Physics 06/2015; 117(21):213905. DOI:10.1063/1.4921909 · 2.19 Impact Factor
  • Nguyen N Phuoc · C K Ong
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    ABSTRACT: The permeability spectra of a multiferroic heterostructure composed of a FeCoZr thin film grown onto a [Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) (PMN-PT) substrate are characterized as a function of an electrical field applied through the thickness of the substrate. When the sample is in an unpoled state and the applied electrical field is increased from 0 kV cm−1 to 2 kV cm−1, the resonance frequency remains relatively the same. However, as the electrical field is increased beyond 2 kV cm−1, the resonance frequency is drastically increased from 2.17 GHz to 3.28 GHz and the peak of the permeability spectra becomes much broader. When the electrical field is further increased from 2 kV cm−1 to 6 kV cm−1, the resonance frequency is gradually increased and finally reaches 4 GHz. As the electrical field is reduced from 6 kV cm−1 back to 2 kV cm−1, the resonance frequency is reduced in the same manner, and the peak disappears when the electrical field is reduced to less than 2 kV cm−1. These behaviors are discussed in terms of the magnetization rotation and magnetic anisotropy dispersion based on the stress distribution of the piezoelectric substrate as a function of the applied electrical field. This argument is consistent with the hysteresis loops measured before and after poling. The result suggests that the electrical tunability of the magnetization rotation in multiferroic heterostructures can be employed to electrically turn on and off the microwave operation of the materials, which is promising for applications.
    05/2015; 2(5):055006. DOI:10.1088/2053-1591/2/5/055006
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    Wee Tee Soh · Bin Peng · C. K. Ong
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    ABSTRACT: We demonstrate the localized excitation and dc electrical detection of magnetostatic surface spin waves (MSSWs) in yttrium iron garnet (YIG) by a shorted coaxial probe. Thin films of NiFe and Pt are patterned at different regions onto a common bulk YIG substrate. A shorted coaxial probe is used to excite spin precession locally near various patterned regions. The dc voltages across the corresponding regions are recorded. For excitation of the Pt regions, the dc voltage spectra are dominated by the spin pumping of MSSWs from YIG, where various modes can be clearly distinguished. For the NiFe region, it is also found that spin pumping from MSSWs generated in YIG dominated the spectra, indicating that the spin pumped currents are dissipated into charge currents via the inverse Spin Hall effect (ISHE) in NiFe. For all regions, dc signals from YIG MSSWs are observed to be much stronger than the ferromagnetic resonance (FMR) uniform mode, likely due to the nature of the microwave excitation. The results indicate the potential of this probe for microwave imaging via dc detection of spin dynamics in continuous and patterned films.
    Journal of Applied Physics 04/2015; 117(15-15):153903. DOI:10.1063/1.4918668 · 2.19 Impact Factor
  • Nguyen N. Phuoc · C. K. Ong
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    ABSTRACT: The studied magnetoelectric heterostructure consisting of a NiFeTa thin film grown onto a [Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) (PMN-PT) substrate was prepared by using gradient-composition sputtering technique. A shorted micro-strip transmission-line perturbation method using a vector network analyzer was employed to study the electrical field modulation of microwave properties of the NiFeTa/PMN-PT heterostructure. It was found that the resonance frequency of the sample can be tuned from 1.72 GHz to 2.05 GHz when the applied electrical field is varied from −6 kV/cm to 6 kV/cm. Moreover, we experimentally observed a quasi-linear relationship between the resonance frequency and the electrical field in a wide range of electrical field from 0 kV/cm to 6 kV/cm in the heterostructure, which is suggested to be useful for applications. All the results are discussed taking into account the reverse magnetostrictive effect and the reverse piezoelectric effect.
    Journal of Applied Physics 02/2015; 117(6):064108. DOI:10.1063/1.4908548 · 2.19 Impact Factor
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    ABSTRACT: Under zero external magnetic fields, single-layer FeCo thin films exhibit no ferromagnetic resonance (FMR) peaks, while multiple FMR peaks were obtained by growing FeCo thin films on NiFe underlayers with various thicknesses up to 50 nm. Comprehensive investigations of the dynamic magnetic properties and origin of the peaks were conducted through measurements of microwave permeability via a shorted microstrip perturbation technique. Through fitted values of saturation magnetization Ms, uniaxial anisotropy HK sta, and rotatable anisotropy HK rot extracted from the FMR experiments, it was found that two of the three resonance peaks originate from FeCo, and the third from NiFe. The two magnetic phases of FeCo grains are found to have different values of HK rot and explained by the exchange interaction between FeCo and NiFe grains.
    Journal of Applied Physics 01/2015; 117(1-1):013906. DOI:10.1063/1.4905315 · 2.19 Impact Factor
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    ABSTRACT: A series of Al substituted MgCuZn ferrite powders with composition Mg0.3Cu0.2Zn0.52AlxFe1.98−xO3.99 (0.00≤x≤0.06) have been synthesized with nano-sized precursor powders through the nitrate–citrate auto-combustion route. These powders were calcined, compacted and sintered at 900 °C for 4 h. X-ray diffraction patterns show the formation of cubic spinel structure. Infrared spectra indicate two fundamental absorption bands corresponding to the tetrahedral and octahedral complexes, respectively. A significant increase in density and grain size is observed with increasing Al content. The room temperature saturation magnetization increases for x=0.015 and then decreases for further increase in Al substitution. The initial permeability increases with the Al content attributed to the increase in the grain size and density. Curie temperature is found to be dependent on the Al concentration and it decreases due to decrease in the number of super-exchange interactions between Fe3+ ions in the tetrahedral and octahedral sites.
    Journal of Magnetism and Magnetic Materials 12/2014; 371:29–34. DOI:10.1016/j.jmmm.2014.07.003 · 2.00 Impact Factor
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    ABSTRACT: In this study, the temperature dependence of the permeability spectra in range from 303 K to 423 K of FeSiAl thin films fabricated by oblique deposition are carried out. The resonance frequency is found to increase from 1.6 GHz to 2.9 GHz at room temperature with the oblique deposition angle changed from 18.4° to 45°. It is attributed to the increase of the magnetic anisotropy. The thermal stability of the resonance frequency and dynamic anisotropy are found to be within 15% in the temperature range from 303 K to 423 K. In addition, the behavior of the static and dynamic permeabilities, effective Gilbert damping factor, and frequency line width with temperature for FeSiAl films with different oblique deposition angles are presented and discussed in detail.
    Journal of Alloys and Compounds 10/2014; 610:126–131. DOI:10.1016/j.jallcom.2014.04.169 · 2.73 Impact Factor
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    Wee Tee Soh · Xiaoxi Zhong · C K Ong
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    ABSTRACT: FeAlSi (Sendust) is known to possess excellent soft magnetic properties comparable to traditional soft magnetic alloys such as NiFe (Permalloy), while having a relatively higher resistance for lower eddy current losses. However, their dynamic magnetic and magneto-transport properties are not well-studied. Via the spin rectification effect, we electrically characterize a series of obliquely sputtered FeAlSi films at ferromagnetic resonance. The variations of the anisotropy fields and damping with oblique angle are extracted and discussed. In particular, two-magnon scattering is found to dominate the damping behavior at high oblique angles. An analysis of the results shows large anomalous Hall effect and anisotropic magneto-resistance across all samples, which decreases sharply with increasing oblique incidence.
    Applied Physics Letters 09/2014; 105(112401):112401-43905. DOI:10.1063/1.4895721 · 3.52 Impact Factor
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    ABSTRACT: We propose a novel approach using Co underlayer to grow FeCo thin films by sputtering oblique deposition technique to achieve desirable performance for high frequency applications working in an environment where in the temperature may fluctuate. In particular, the films show a very high resonance frequency tunable up to 6.2 GHz with the permeability of 391 and an excellent thermal stability with the variation less than 1% in the temperature range from 300 K to 420 K. (C) 2014 Elsevier B.V. All rights reserved
    Journal of Magnetism and Magnetic Materials 09/2014; 365:8–13. DOI:10.1016/j.jmmm.2014.04.046 · 2.00 Impact Factor
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    ABSTRACT: Microwaves have the capability of penetrating through materials with low permittivity. By means of a near-field scanning microwave microscope system, we obtained two-dimensional maps of the incident microwave's reflection coefficient intensity and frequency shift, which correspond well to the spatial distribution and electrical conductance of fluids and metallic thin film structures hidden underneath 15–170 μm thick dielectric covers. The lateral resolution and sensitivity to conductivity for the target samples were found closely related to the thickness of the cover layer. The technique offers a real-time, in-situ, and a non-invasive approach to monitor the local chemical reactions, the motion of fluids, and the distribution or concentrations of ions or bio-materials in lab-on-a-chip systems. This technique also has the potential to be developed for the detection of live cells and tissues.
    Journal of Applied Physics 07/2014; 116(4):044904-044904-9. DOI:10.1063/1.4891215 · 2.19 Impact Factor
  • Nguyen N. Phuoc · C.K. Ong
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    ABSTRACT: The magnetic and microwave properties of a heterostructure composing of a composition-graded FeCoTa film and a [Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) substrate were investigated with respect to a varied applied electric field. For the as-deposited sample in the initial unpoled state, the resonance frequency is 7.2 GHz and it is suddenly increased to 8.4 GHz when the applied electric field reaches 2 kV/cm. Then upon the changing of the electric field from 6 kV/cm to −6 kV/cm, the resonance frequency is varied in the range from 8.4 GHz to 9.2 GHz. The resonance frequency can only be reset back to the lower level with the value of 7.6 GHz through the depolarization process. The results are interpreted in terms of the change in ferroelectric domain structures leading to the variation of the stress-induced magnetic anisotropy, which is experimentally verified by the magnetic and microwave behaviors after depolarization.
    Applied Physics Letters 07/2014; 105(3):032901-032901-5. DOI:10.1063/1.4891048 · 3.52 Impact Factor
  • Nguyen N. Phuoc · C.K. Ong
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    ABSTRACT: The multiferroic heterostructure of FeCoZr/[Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) (PMN-PT) prepared by oblique sputtering deposition technique shows a large electrical tunability of ultra-high ferromagnetic resonance frequency from 7.4 GHz to 12.3 GHz. Moreover, we experimentally demonstrate the possibility of realizing electrically reconfigurable magnetoelectric microwave devices with ultra-low power consumption by employing the heterostructure under different resetting electric fields through a reconfiguration process. In particular, the tunability of the FeCoZr/PMN-PT heterostructure from 8.2 GHz to 11.6 GHz can retain in a remanent state after releasing the resetting electric field. This suggests that the tunable microwave devices based on such heterostructures are permanently reconfigurable by simply using a trigger electric field double-pulse which requires much less energy than that of the conventional ones wherein an electric field needs to be constantly applied during operation.
    Applied Physics Letters 07/2014; 105(2):022905-022905-5. DOI:10.1063/1.4890411 · 3.52 Impact Factor
  • Bin Peng · Nguyen N. Phuoc · C.K. Ong
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    ABSTRACT: In this work, we prepare and characterize FeCo/IrMn–Al2O3 multilayers using magnetron sputtering method with various doping concentrations in order to study the doping effect of the antiferromagnetic layer in exchange-biased films on their high frequency magnetic properties and their thermal stability for fundamental research as well as for application. The influences of the nonmagnetic Al2O3 doping in the antiferromagnetic layers on the high frequency magnetic properties from 1 GHz up to 8 GHz and their thermal stability of the FeCo/IrMn exchange-biased multilayer have been investigated in the temperature range from room temperature up to 420 K. The temperature behavior of exchange bias, static magnetic anisotropy field, dynamic magnetic anisotropy field, rotatable magnetic anisotropy field, resonant frequency and damping of the samples are presented and interpreted by classifying the antiferromagnet grains into random spin grains, frozen spin grains and rotatable spin grains. In particular, the roles and changes of each grain type with the doping amount and with temperature are discussed in conjunction with the variations of the magnetic and microwave parameters.
    Journal of Alloys and Compounds 07/2014; 602:87–93. DOI:10.1016/j.jallcom.2014.03.009 · 2.73 Impact Factor
  • Nguyen N. Phuoc · C.K. Ong
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    ABSTRACT: Magnetic anisotropy is always known to decrease with temperature and so far there has been no work in the literature reporting about any experimental observation of the increment of magnetic anisotropy with temperature in any thin films other than the one reported in our previous papers. In this paper, we demonstrate that gradient-composition sputtering technique is a method to fabricate magnetic thin films having an in-plane magnetic anisotropy increased with temperature. In particular, our composition-graded magnetic thin films of FeCoX (X (=) Ta, Zr, Hf, Lu) show an increase of ferromagnetic resonance frequency when the temperature is elevated, which evidently indicates the increase of magnetic anisotropy with temperature. This peculiar behavior is ascribed to the stress-induced origin of the magnetic anisotropy present in the films grown by gradient-composition sputtering in which the stress of the films is increased with temperature. In order to gain a deeper understanding of the mechanism of this unusual thermal behavior, an additional set of composition-graded FeCoTa thin films were fabricated onto different substrates showing that the increase of magnetic anisotropy with temperature is present in all the films regardless of the substrates used. This experimental result unambiguously implies that the increasing of stress with temperature is not only due to the difference between the thermal expansion coefficients between the substrates and the films, but also owing to the intrinsic nature of the composition gradient of the films.
    IEEE Transactions on Magnetics 07/2014; 50(7):1-6. DOI:10.1109/TMAG.2013.2296936 · 1.21 Impact Factor
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    Wee Tee Soh · Bin Peng · C K Ong
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    ABSTRACT: Through the inverse spin Hall effect, spin-pumped currents from a Py(Ni80Fe20) layer are converted to direct current (dc) voltages in an adjacent platinum (Pt) layer, which adds to dc voltages generated by the spin rectification effect originating from anisotropic magnetoresistance, ordinary and anomalous Hall effect in the permalloy (Py) layer. Based on a shorted microstrip fixture, we perform angular measurements of rectified dc voltages in a Py(Ni80Fe20)/Pt bilayer to simultaneously separate spin pumping-induced inverse spin Hall voltages from spin-rectified voltages. A systematic approach to accurately determine the spin-Hall angle of Pt is presented.
    Journal of Physics D Applied Physics 06/2014; 47(28):285001. DOI:10.1088/0022-3727/47/28/285001 · 2.72 Impact Factor
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    ABSTRACT: ZnFe2O4 and MgxCu0.2Zn0.82-xFe1.98O4 (where x = 0.20, 0.25, 0.30, 0.35 and 0.40) nanoparticles were synthesized by sol-gel assisted combustion method. X-Ray Diffraction (XRD), FTIR spectroscopy, Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area studies were used to characterize the synthesized compounds. ZnFe2O4 and the doped compounds crystallize in Fd3m space group. The lattice parameter of ZnFe2O4 is calculated to be a = 8.448(3) Å, while the doped compounds show slight decrease in the lattice parameter with increase in the Mg content. The particle size of all the compositions are in the range of ~50-80 nm and the surface area of the compounds are in the range of 11-12 m2 g-1. Cyclic voltammetry (CV), galvanostatic cycling and electrochemical impedance spectroscopy (EIS) studies were used to investigate the electrochemical properties of the different compositions. The as-synthesized samples at 600 oC shows large capacity fading while the samples reheated at 800 oC shows better cycling stability. ZnFe2O4 shows a high reversible capacity of 575 mAh g-1 after 60 cycles at a current density of 100 mA g-1. Mg0.2Cu0.2Zn0.62Fe1.98O4 shows a similar capacity of 576 mAh g-1 after 60 cycles with better capacity retention.
    ACS Applied Materials & Interfaces 06/2014; 6(13). DOI:10.1021/am502605s · 6.72 Impact Factor
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    Paul Kumar Upputuri · Zhe Wu · Li Gong · Chong Kim Ong · Haifeng Wang
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    ABSTRACT: We demonstrate far-field super-resolution coherent anti-Stokes Raman scattering (CARS) microscopy by exciting the sample with photonic nanojets. The sub-diffraction photonic nanojets were formed on the surface of the sample by microspheres illuminated by laser beams, and images were acquired by a standard laser-scanning CARS microscope. When the laser beams were focused on the microspheres, the photonic nanojets determined the excitation volume instead of the diffraction-limited laser foci, leading to super-resolution. We imaged the sub-diffraction features of a Blu-ray disc using glass spheres with a refractive index of 1.46 and diameters in the 1-6 µm range. The microspheres provided a lateral magnification factor up to 5.0X and a lateral resolution of at least 200 nm at 796 nm laser wavelength, allowing us to resolve the features on the disc which were invisible under normal CARS imaging. The magnification factor depended on both the microsphere size and the focal plane position of the incident beams. To explain the magnification factor we performed theoretical simulations which showed excellent agreement with experimental results. This super-resolution technique could be very useful for the vibrational imaging of nano-scale objects on films and surfaces.
    Optics Express 06/2014; 22(11):12890-12899. DOI:10.1364/OE.22.012890 · 3.49 Impact Factor
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    Guozhi Chai · Nguyen N. Phuoc · Chong Kim Ong
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    ABSTRACT: We investigated the high-frequency magnetic properties of oblique-sputtered CoFeBSm thin films. We found that the zero-field ferromagnetic resonance frequency fr can be tuned from 3.8 to 6.4 GHz by rotating the samples within the plane. The angular tunable fr is tentatively explained with the competition of uniaxial anisotropy (arising from the oblique deposition) the rotatable anisotropy (originating from the rotatable stripe domain). These results may have great implications for tunable microwave magnetic devices.
    Applied Physics Express 05/2014; 7(6):063001. DOI:10.7567/APEX.7.063001 · 2.57 Impact Factor

Publication Stats

5k Citations
1,097.96 Total Impact Points

Institutions

  • 2008–2015
    • National University of Singapore
      • Department of Physics
      Tumasik, Singapore
    • Penang General Hospital
      Penang, Pulau Pinang, Malaysia
  • 2014
    • University of Electronic Science and Technology of China
      • State Key Laboratory of Electronic Thin Films and Integrated Devices
      Hua-yang, Sichuan, China
  • 2012
    • Hospital Pulau Pinang
      Penang, Pulau Pinang, Malaysia
  • 2007
    • Nanyang Technological University
      • School of Materials Science and Engineering
      Tumasik, Singapore
  • 2002
    • Nanjing University
      • Department of Physics
      Nanjing, Jiangsu Sheng, China