C. K. Ong

University of Electronic Science and Technology of China, Hua-yang, Sichuan, China

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Publications (450)911.1 Total impact

  • 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. · 2.53 Impact Factor
  • 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. · 2.73 Impact Factor
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    ABSTRACT: In this manuscript, we describe how the map of high frequency conductivity distribution of an oxide-doped anti-ferromagnetic 200 nm thin film can be obtained from the quality factor (Q) measured by a near-field scanning microwave microscope (NSMM). Finite element analysis (FEA) is employed to simulate the NSMM tip-sample interaction and obtain a curve related between the simulated quality factor (Q) and conductivity. The curve is calibrated by a standard Cu thin film with thickness of 200 nm, together with NSMM measured Q of Ag, Au, Fe, Cr and Ti thin films. The experimental conductivity obtained by the NSMM for IrMn thin films with various doped concentrations of Al2O3 is found consistent with conventional voltammetry measurement in the same tendency. That conductivity decreases as the content of doped Al2O3 increases. The results and images obtained demonstrate that NSMM can be employed in thin film analysis for characterization of local electrical properties of materials in a non-destructive manner and for obtaining a map of conductivity distribution on the same film.
    03/2014; 4(4).
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    ABSTRACT: We describe a shorted microstrip method for the sensitive quantification of Spin Rectification Effect (SRE). SRE for a Permalloy (Ni80Fe20) thin film strip sputtered onto SiO2 substrate is demonstrated. Our method obviates the need for simultaneous lithographic patterning of the sample and transmission line, therefore greatly simplifying the SRE measurement process. Such a shorted microstrip method can allow different contributions to SRE (anisotropic magnetoresistance, Hall effect, and anomalous Hall effect) to be simultaneously determined. Furthermore, SRE signals from unpatterned 50 nm thick Permalloy films of area dimensions 5 mm × 10 mm can even be detected.
    The Review of scientific instruments 02/2014; 85(2):026109. · 1.52 Impact Factor
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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 01/2014; 602:87–93. · 2.73 Impact Factor
  • Nguyen N. Phuoc, C.K. Ong
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    ABSTRACT: We experimentally demonstrate that the stress-induced magnetic anisotropy in composition-graded ferromagnetic NiFeTa layers can be employed to trigger exchange bias in NiFeTa/IrMn bilayers without using any deposition field or field cooling procedure. In particular, we found that the NiFeTa/IrMn bilayers exhibit double-shifted loops being composed of both negative and positive exchange biases when the deposition angle is low. As the deposition angle is increased, the magnetization curves change into a single-shifted loop with spontaneous exchange bias aligned along negative or positive direction. The results can be explained by the formation of magnetic domain state under the presence of a strong stress-induced magnetic anisotropy and this domain state is pinned upon the deposition of an antiferromagnetic layer. We also studied the dynamic permeability spectra of the films with respect to temperature and discussed the results in relation with the static magnetic properties.
    Journal of Applied Physics 01/2014; 115(14):143901-143901-6. · 2.21 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.
    Journal of Magnetism and Magnetic Materials 01/2014; 365:8–13. · 1.83 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. 01/2014; 610:126–131.
  • N.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 01/2014; 50(7):1-6. · 1.42 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 01/2014; 105(2):022905-022905-5. · 3.79 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 01/2014; 105(3):032901-032901-5. · 3.79 Impact Factor
  • Nguyen N. Phuoc, C. K. Ong
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    ABSTRACT: An experimental study of the magnetic and microwave properties together with the thermal stability of FeCoHf films fabricated by a novel hybrid oblique gradient-composition sputtering technique is presented. It was found that both ferromagnetic resonance frequency and thermal stability can be significantly enhanced by changing the oblique deposition angle. The result is tentatively explained by separating the role of each contribution of the magnetic anisotropy such as stress-induced anisotropy, shape anisotropy and pair-ordering anisotropy. On the basis of this experimental result, it is suggestive that this method is an effective way to fabricate thin films with good microwave performance oriented for high-frequency applications.
    Journal of Physics D Applied Physics 12/2013; 46(48):5002-. · 2.53 Impact Factor
  • Yungui Ma, Tiantian Wu, C. K. Ong
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    ABSTRACT: Mirror-closed dielectric lenses such as Maxwell’s fisheye lens have been recently discussed to have unique light focusing capacity. The key issue for this lens is to make a perfect deep subwavelength drain which is able to absorb the entire wavefronts emitted by the source. In this work the authors proposed two methods to solve this problem and designed small size drains using time reversed scattering and transformation optics. At the image position, deep subwavelength focusing spots were achieved in a closed dielectric-embedded lens by these drains. The transformed drain could work at arbitrary wavelength and polarization. Our results showed the critical role of an attenuating drain in the imaging process of a Maxwell fisheye lens and gave technical approaches for their implementation. The general methods developed here may serve to design other interesting electromagnetic devices.
    Journal of Optics 12/2013; 15(12):5705-.
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    ABSTRACT: Invisibility cloaking based on optical transformation involves materials singularity at the branch cut points. Many interesting optical devices, such as the Eaton lens, also require planar media index singularities in their implementation. We show a method to transmute two singularities simultaneously into harmless topological defects formed by anisotropic permittivity and permeability tensors. Numerical simulation is performed to verify the functionality of the transmuted conformal cloak consisting of two kissing Maxwell fish eyes.
    Journal of the Optical Society of America A 11/2013; 30(11):2280-5. · 1.67 Impact Factor
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    ABSTRACT: In this paper, we report the fabrication of a homogeneous anisotropic invisibility carpet cloak that works for transverse magnetic (TM) wave in free space. Linear coordinate transformation is employed for the design of the dielectric tensors, and the cloak is made of Al2O3 slabs with a Teflon background. The triangular invisible region has a height of 32.4 mm (one third of the height of the whole device) and a width of 200 mm. The cloaking effect is examined in far-field free-space transmission experiments by measuring the scattering parameters. The results show our device has very good cloaking performance in a wide frequency range from 8 to 12 GHz.
    EPL (Europhysics Letters) 10/2013; 104(1):14003-. · 2.26 Impact Factor
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    ABSTRACT: The permeability μ and resonant frequency fr of stripe domain ferrite doped CoFe thin films are enhanced from 14 to 22 and 6.0 GHz to 6.4 GHz, respectively, by using Ta buffer layer. The observed enhancement of both permeability and resonant frequency seems to deviate from Snoek's law which expects the value of μ decrease as the fr increases. Compared to the case of using Permalloy (Ni80Fe20) buffer layer where μ is enhanced to 25 and fr is reduced to 4.1 GHz, the present result suggests that the employment of Ta buffer layer may be useful in the quest for high permeability magnetic thin films at GHz frequency range.
    Journal of Physics D Applied Physics 10/2013; 46(41):5001-. · 2.53 Impact Factor
  • Source
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    ABSTRACT: Electromagnetic invisible devices usually designed by transformation optics are rather complicated in material parameters and not suitable for general applications. Recently a topology optimized cloak based on level-set method was proposed to realize nearly perfect cloaking by Fujii et al [Appl. Phys. Lett. 102, 251106 (2013)]. In this work we experimentally implemented this idea and fabricated a unidirectional cloak with a relative large invisible region made of single dielectric material. Good cloaking performance was verified through measurement which consists very well with numerical simulation. The advantages and disadvantages of this optimization method are also discussed.
    Applied Physics Letters 08/2013; 103(12). · 3.79 Impact Factor
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    ABSTRACT: Through a systematic investigation of the field-dependent dynamic magnetization of a series of NiFe films with and without stripe domains in conjunction with the static magnetization process, we demonstrate that the experimental rotatable anisotropy field is not a fixed value but strongly varied with the external in-plane magnetic field, being qualitatively associated with the emergence of stripe domains. Moreover, the frequency linewidth spectra of the films with stripe domains show an abnormal behavior with three distinct regimes which are strongly correlated with both the static magnetization process and the competition between external magnetic field and dynamic anisotropy field. The results are discussed in terms of the effect of inhomogeneous magnetization associated with the formation of stripe domains and the field-dependent dynamic anisotropy that cause the broadening of frequency linewidth.
    Journal of Applied Physics 08/2013; 114(5). · 2.21 Impact Factor
  • Source
    Nguyen N. Phuoc, C. K. Ong
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    ABSTRACT: A systematic investigation of the non-linear interplay between the uniaxial magnetic anisotropy induced by oblique deposition and the unidirectional magnetic anisotropy induced by exchange bias coupling in NiFe/MnIr bilayers was performed. It was found that depending on the oblique angle, the uniaxial anisotropy induced by oblique deposition may play a vital role in defining the orientation of exchange bias. In particular, when the oblique angle is in the range from 31° to 45°, the oblique-deposition-induced anisotropy is large enough to reorient the easy axis of exchange bias to the direction perpendicular to the deposition field leading to the appearance of double-shifted loops. When the oblique angle is less than 30° or higher than 45°, normal exchange bias is observed with typical behavior. The dynamic permeability spectra are also presented and discussed in conjunction with the static magnetic properties.
    Journal of Applied Physics 07/2013; 114(4). · 2.21 Impact Factor
  • Source
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    ABSTRACT: In the past years quasi-conformal mapping has been generally used to design broadband electromagnetic cloaks. However, this technique has some inherit practical limitations such as the lateral beam shift, rendering the device visible or difficult to hide a large object. In this work we circumvent these issues by using strict conformal mapping to build the first isotropic cloak. Microwave near-field measurement shows that our device (with dielectric constant larger than unity everywhere) has a very good cloaking performance and a broad frequency response. The present dielectric approach could be technically extended to the fabrication of other conformal devices at higher frequencies.
    Scientific Reports 07/2013; 3:2182. · 5.08 Impact Factor

Publication Stats

3k Citations
911.10 Total Impact Points

Institutions

  • 2014
    • University of Electronic Science and Technology of China
      • State Key Laboratory of Electronic Thin Films and Integrated Devices
      Hua-yang, Sichuan, China
  • 1987–2014
    • National University of Singapore
      • • Department of Physics
      • • Temasek Laboratories
      • • Centre for Superconducting and magnetic materials (CSMM)
      Tumasik, Singapore
  • 2012
    • Hospital Pulau Pinang
      Penang, Pulau Pinang, Malaysia
  • 2011
    • Zhejiang University
      • State Key Lab of Modern Optical Instrumentation
      Hangzhou, Zhejiang Sheng, China
  • 2002–2009
    • Northeast Institute of Geography and Agroecology
      • • Institute of Physics
      • • Shenyang National Laboratory for Materials Science
      Beijing, Beijing Shi, China
  • 2008
    • Penang General Hospital
      Penang, Pulau Pinang, Malaysia
  • 2002–2007
    • Nanjing University
      • Department of Physics
      Nanjing, Jiangsu Sheng, China
  • 2005
    • Sichuan Normal University
      Hua-yang, Sichuan, China
    • Nanyang Technological University
      Tumasik, Singapore
  • 2001
    • Huazhong University of Science and Technology
      Wu-han-shih, Hubei, China
  • 2000
    • Lanzhou University
      Kao-lan-hsien, Gansu Sheng, China
    • Nanjing University of Posts and Telecommunications
      Nan-ching, Jiangsu Sheng, China