C. K. Ong

National University of Singapore, Tumasik, Singapore

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Publications (477)1063.78 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We develop a method for universally resolving the important issue of separating the inverse spin Hall effect (ISHE) from spin rectification effect (SRE) signal. This method is based on the consideration that the two effects depend on the spin injection direction: The ISHE is an odd function of the spin injection direction while the SRE is independent on it. Thus, inversion of the spin injection direction changes the ISHE voltage signal, while SRE voltage remains. It applies generally to analyzing the different voltage contributions without fitting them to special line shapes. This fast and simple method can be used in a wide frequency range, and has the flexibility of sample preparation.
    No preview · Article · Dec 2015
  • Nguyen N. Phuoc · C. K. Ong
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    ABSTRACT: By using oblique deposition technique to fabricate FeCo/MnIr/[Pb(Mg1 / 3Nb2 / 3)O3]0.68-[PbTiO3]0.32 (011) multiferroic heterostructures, we experimentally demonstrate that it is feasible to dually control the ferromagnetic resonance frequency by changing an oblique deposition angle and an applied electrical field. In particular, by changing the oblique angle, the resonance frequency of the samples in unpoled state can be fixed from 2.39 GHz to 9 GHz. Upon the application of the electrical field varied from 0 kV/cm to 6 kV/cm, the resonance frequency of each sample shows different tunability range depending on the oblique angle and it can be tailored beyond 10 GHz. The result can be interpreted in terms of the enhancement of the magnetic anisotropy arising from the inverse magnetostriction effect. Our demonstration suggests a great implication for tunable microwave applications with ultra-wide band requirement.
    No preview · Article · Sep 2015 · Journal of Applied Physics
  • Source
    Wee Tee Soh · Yasmin Yeow · Xiaoxi Zhong · C. K. Ong
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    ABSTRACT: Antiferromagnetic Mn3Ir, which is widely employed in exchange-biased applications, has attracted much attention recently due to its predicted and subsequently observed large spin Hall effect, therefore increasing its potential for spintronic devices in place of conventional paramagnetic 5d spin Hall metals. (Pt, Ta and W) Via the electrical detection of ferromagnetic resonance, we study a series of exchange biased NiFe/MnIr films for various MnIr thicknesses. In these systems, spin-pumped spin currents from NiFe are converted into dc voltages within MnIr via the inverse spin Hall effect (ISHE), which mixes with spin rectification voltages generated from NiFe. Through angular measurements, we separate these different voltage contributions to qualitatively detect non-zero ISHE in MnIr, which coexists with a non-zero unidirectional anisotropy. We find significant extrinsic damping contributions which prevent the accurate quantification of spin pumping-induced ISHE in MnIr films. The results show that spin currents may propagate and dissipate in MnIr films through ISHE in the presence of exchange bias.
    Full-text · Article · Sep 2015 · Journal of Physics D Applied Physics
  • Source
    Wee Tee Soh · Bin Peng · C. K. Ong
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    ABSTRACT: The spin rectification effect (SRE), a phenomenon that generates dc voltages from ac microwave fields incident onto a conducting ferromagnet, has attracted widespread attention due to its high sensitivity to ferromagnetic resonance (FMR) as well as its relevance to spintronics. Here, we report the non-local detection of yttrium iron garnet (YIG) spin dynamics by measuring SRE voltages from an adjacent conducting NiFe layer up to 200 nm thick. In particular, we detect, within the NiFe layer, SRE voltages stemming from magnetostatic surface spin waves (MSSWs) of the adjacent bulk YIG which are excited by a shorted coaxial probe. These non-local SRE voltages within the NiFe layer that originates from YIG MSSWs are present even in 200 nm-thick NiFe films with a 50 nm thick SiO2 spacer between NiFe and YIG, thus strongly ruling out the mechanism of spin-pumping induced inverse spin Hall effect in NiFe as the source of these voltages. This long-range influence of YIG dynamics is suggested to be mediated by dynamic fields generated from YIG spin precession near YIG/NiFe interface, which interacts with NiFe spins near the simultaneous resonance of both spins, to generate a non-local SRE voltage within the NiFe layer.
    Preview · Article · Aug 2015 · AIP Advances
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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.
    No preview · Article · Jun 2015 · Thin Solid Films
  • 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.
    No preview · Article · Jun 2015 · Journal of Applied Physics
  • 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.
    No preview · Article · May 2015
  • Source
    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.
    Full-text · Article · Apr 2015 · Journal of Applied Physics
  • 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.
    No preview · Article · Feb 2015 · Journal of Applied Physics
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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.
    Full-text · Article · Jan 2015 · Journal of Applied Physics
  • Nguyen Nguyen Phuoc · Chong Kim Ong
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    ABSTRACT: We report our experimental observation of a large electrical tuning of ferromagnetic resonance frequency in FeCo/MnIr/[Pb(Mg1/3Nb2/3)O3]0.68–[PbTiO3 ]0.32(0 1 1) multiferroic heterostructures. Specifically, by varying an applied electrical field across the thickness of the substrate, the ferromagnetic resonance frequency can be tuned in a wide band from 3.6 to 10.5 GHz. The results are discussed in terms of the strain-mediated magneto-electric coupling between the ferromagnetic thin films and the piezoelectric substrates.
    No preview · Article · Jan 2015 · IEEE Magnetics Letters
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    ABSTRACT: In this manuscript, a near-field scanning microwave microscope (NSMM) of our own design is introduced while using a multi-walled carbon nanotube (MWCNT) bundle as the tip (referred to as ‘CNT tip’). Clear images of gold-patterned numbers, photoresist stripes and corneal endothelial cells (cell line B4G12) were obtained by mapping the resonant frequency fr and S11 amplitude of a given area while the NSMM is operating in tapping mode. The CNT tip helps to improve image quality and reveals more information about the sample as compared to a traditional metallic tip. The CNT tip is flexible and does not scratch the surface of the sample during the scan, which is useful for imaging soft material in biological science. In the imaging of the B4G12 endothelial cells, the nuclei and cytoplasm can be clearly distinguished from the rest of the cell and its surrounding medium.
    No preview · Article · Dec 2014 · Ultramicroscopy
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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.
    Full-text · Article · Dec 2014 · Journal of Magnetism and Magnetic Materials
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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.
    Full-text · Article · Oct 2014 · Journal of Alloys and Compounds
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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.
    Full-text · Article · Sep 2014 · Applied Physics Letters
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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
    No preview · Article · Sep 2014 · Journal of Magnetism and Magnetic Materials
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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.
    Full-text · Article · Jul 2014 · Journal of Applied Physics
  • 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.
    No preview · Article · Jul 2014 · Applied Physics Letters
  • 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.
    No preview · Article · Jul 2014 · Applied Physics Letters
  • 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.
    No preview · Article · Jul 2014 · Journal of Alloys and Compounds

Publication Stats

6k Citations
1,063.78 Total Impact Points

Institutions

  • 2008-2015
    • National University of Singapore
      • Department of Physics
      Tumasik, Singapore
    • Penang General Hospital
      Penang, Penang, Malaysia
  • 2014
    • University of Electronic Science and Technology of China
      • State Key Laboratory of Electronic Thin Films and Integrated Devices
      Hua-yang, Sichuan, China
  • 2007
    • Nanyang Technological University
      • School of Materials Science and Engineering
      Tumasik, Singapore
  • 2002
    • Nanjing University
      • Department of Physics
      Nanjing, Jiangsu Sheng, China
    • Chinese Academy of Sciences
      • Institute of Metal Research
      Peping, Beijing, China
  • 2001
    • Tohoku University
      Sendai-shi, Miyagi, Japan