S.X. Wang

Stanford University, Palo Alto, CA, United States

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Publications (52)148.58 Total impact

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    ABSTRACT: Resonances are observed in the transmission between two coplanar waveguides coupled by ferromagnetic Co90Ta5Zr5 tubes that wrap around their shorted ends. The resonances are assigned to the magnetostatic surface waves that counter propagate along the tube perimeter. We use a model based on an infinite ferromagnetic tube, with elliptical cross section of roughly the same dimensions as the studied structure. Additional theoretical analysis of the fundamental precession mode observed in experiment is carried out. Periodic boundary conditions dictated by the tube perimeter and applied to magnetostatic surface waves quantitatively account for the experimentally observed bandwidth of excited modes, despite the contorted tubular shape. The tubular topology appears to be more important than the shape details.
    Full-text · Article · Jan 2012 · Journal of Applied Physics
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    ABSTRACT: Micron scale ferromagnetic tubes placed on the ends of ferromagnetic CoTaZr spin waveguides are explored in order to enhance the excitation of Backward Volume Magnetostatic Spin Waves. The tubes produce a closed magnetic circuit about the signal line of the coplanar waveguide and are, at the same time, magnetically contiguous with the spin waveguide. This results in a 10 fold increase in spin wave amplitude. However, the tube geometry distorts the magnetic field near the spin waveguide and relatively high biasing magnetic fields are required to establish well defined spin waves. Only the lowest (uniform) spin wave mode is excited.
    Preview · Article · Sep 2010 · Journal of Applied Physics
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    ABSTRACT: Giant magnetoresistive biosensors are becoming more prevalent for sensitive, quantifiable biomolecular detection. However, in order for magnetic biosensing to become competitive with current optical protein microarray technology, there is a need to increase the number of sensors while maintaining the high sensitivity and fast readout time characteristic of smaller arrays (1-8 sensors). In this paper, we present a circuit architecture scalable for larger sensor arrays (64 individually addressable sensors) while maintaining a high readout rate (scanning the entire array in less than 4s). The system utilizes both time domain multiplexing and frequency domain multiplexing in order to achieve this scan rate. For the implementation, we propose a new circuit architecture that does not use a classical Wheatstone bridge to measure the small change in resistance of the sensor. Instead, an architecture designed around a transimpedance amplifier is employed. A detailed analysis of this architecture including the noise, distortion, and potential sources of errors is presented, followed by a global optimization strategy for the entire system comprising the magnetic tags, sensors, and interface electronics. To demonstrate the sensitivity, quantifiable detection of two blindly spiked samples of unknown concentrations has been performed at concentrations below the limit of detection for the enzyme-linked immunosorbent assay. Lastly, the multiplexing capability and reproducibility of the system was demonstrated by simultaneously monitoring sensors functionalized with three unique proteins at different concentrations in real-time.
    Preview · Article · Feb 2010 · Biosensors & Bioelectronics
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    ABSTRACT: Giant magnetoresistive biosensors possess great potential in biomedical applications for quantitatively detecting magnetically tagged biomolecules. Magnetic sensing does not suffer from the high background levels found in optical sensing modalities such as the enzyme linked immunosorbent assay translating into a technology with higher sensitivity. However, to reveal the full potential of these sensors and compensate for non-idealities such as temperature dependence, digital correction and calibration techniques are not only useful but imperative. Using these calibration techniques to correct for process variations and dynamic changes in the sensing environment (such as temperature and magnetic field), we are able to obtain extremely sensitive and, more importantly, reproducible results for quantifiable biomolecular reorganization. The reproducibility of the system was improved by over 3 x using digital correction techniques and the sensors are made temperature independent by using a novel background correction technique.
    Preview · Article · Feb 2010 · Biosensors & Bioelectronics
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    ABSTRACT: Resonant coupling of coplanar waveguides is explored by wrapping proximate shorted ends of the waveguides with micron size ferromagnetic Co90Ta5Zr5 tubes. Ferromagnetic resonance and up to 7 outer surface modes are identified. Experimental results for these contorted rectangular tubes are in good agreement with micromagnetic simulations and model calculations of magnetostatic modes for an elliptical ferromagnetic tube. These results indicate that the modes are largely determined by tube topology and dimensions but less so by the detailed shape. Comment: 3 pages, 5 figures
    Full-text · Article · Oct 2009
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    ABSTRACT: Emerging applications in microwave communication, RFIC, and power delivery system are driving the need for the miniaturization, cost-reduction and quality optimization of inductors. In our prior work, high-quality-factor and low-resistance embedded inductors were fabricated on low-cost printed circuit board (PCB) for system-on-package applications. The inductor with a CoFeHfO magnetic core patterned by sand-blasting experimentally produced an inductance gain of ~12% over the air-core inductor. In order to further improve the inductance gain, we looked at alternative processes and materials to use in the fabrication of magnetic cores. In this paper, we investigated and compared the magnetic properties of CoFeHfO magnetic cores on the PCB substrate patterned by both wet-etching and sandblasting. The magnetic domain images by Kerr microscopy, hysteresis loops and permeability spectra clearly showed that the CoFeHfO core patterned by wet-etching had a 3times larger permeability value and softer magnetic properties than the core patterned by sand-blasting. With the implementation of the CoFeHfO core by wet-etching, the theoretic calculation predicted an inductance gain of 32% for our embedded inductor. Besides CoFeHfO, we also explored the magnetic properties of CoTaZr film deposited on the same PCB substrate and measured its permeability value to be ~600, which theoretically indicated a gain of 128% for the inductor we designed.
    No preview · Conference Paper · Sep 2009
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    ABSTRACT: Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3 mu m, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively "tunnel" through air gaps, produced by focused ion beam etching, as large as 1.5 mu m. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3079767]
    Full-text · Article · May 2009 · Journal of Applied Physics
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    ABSTRACT: Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion and up to three different modes are identified. Attenuation lengths of the order of 3 mum are several orders of magnitude shorter than that predicted from eddy currents in these thin wires.
    Full-text · Article · Jan 2009 · Applied Physics Letters
  • A.L. Koh · W. Hu · R.J. Wilson · S.X. Wang · R. Sinclair
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    ABSTRACT: Synthetic anti-ferromagnetic nanoparticles (SAFs) are a novel type of magnetic nanoparticle (MNP) fabricated using nanoimprint lithography, direct deposition of multilayer films and retrieval in liquid phase via an ‘etching’ release process. Such physical fabrication techniques enable accurate control of particle shape, size and composition. We systematically varied the processing conditions to produce different configurations of SAF nanoparticles and performed extensive characterization using transmission electron microscopy (TEM) and alternating gradient magnetometry (AGM) to study their corresponding structural and magnetic behavior. A key focus of this paper is the preparation of TEM cross-section specimens of SAF nanoparticles for their structural characterization. This is not a trivial task, but is useful and revealing in terms of structural features. A major finding from our study shows that the introduction of oxygen during deposition of the copper release layer gives significantly improved flatness of the multilayer structure but no significant change in the magnetic properties. Magnetic measurements show that these nanoparticles have nearly zero magnetic remanence, a linear response of magnetization and more than twice the saturation magnetization compared to iron oxide nanoparticles.
    No preview · Article · Dec 2008 · Philosophical Magazine A
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    S.X. Wang · Guanxiong Li
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    ABSTRACT: We present a review of giant magnetoresistance (GMR) spin valve sensors designed for detection of magnetic nanoparticles as biomolecular labels (nanotags) in magneto-nano biodetection technology. We discuss the intricacy of magneto-nano biosensor design and show that as few as approximately 14 monodisperse 16-nm superparamagnetic nanoparticles can be detected by submicron spin valve sensors at room temperature without resorting to lock-in (narrow band) detection. GMR biosensors and biochips have been successfully applied to the detection of biological events in the form of both protein and DNA assays with great speed, sensitivity, selectivity, and economy. The limit of molecular detection is well below 10 pM in concentration, and the protein or DNA assay time can be under two hours. The technology is highly scalable to deep multiplex detection of biomarkers in a complex disease, and amenable to integration of microfluidics and CMOS electronics for portable applications. On-chip CMOS circuitry makes a sensor density of 0.1-1 million sensors per square centimeter feasible and affordable. The theoretical and experimental results thus far suggest that magneto-nano biochip-based GMR sensor arrays and nanotags hold great promise in biomedicine, particularly for point-of-care molecular diagnostics of cancer, infectious diseases, radiation injury, cardiac diseases, and other diseases.
    Preview · Article · Aug 2008 · IEEE Transactions on Magnetics
  • D. W. Lee · S. X. Wang
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    ABSTRACT: The permeability of the magnetic core is closely related to the inductance improvement for integrated inductors. Simulation data indicate that the effective permeability of the finite-sized magnetic core can be significantly reduced from the relative permeability of the magnetic material due to the demagnetization effect. The observed trend in the permeability of the patterned CoTaZr magnetic cores, however, shows that the demagnetization effect alone is not sufficient to predict the effective permeability and that the magnetic domain patterns need to be considered as well. The measured permeability was found to decrease as the patterned magnetic core was narrowed or shortened.
    No preview · Article · Apr 2008 · Journal of Applied Physics
  • AL Koh · W Hu · RJ Wilson · SX Wang · R Sinclair

    No preview · Article · Aug 2007 · Microscopy and Microanalysis
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    ABSTRACT: A high density GMR sensor array was integrated with a standard CMOS chip for DNA hybridization detection. Absorption of magnetic nanoparticles by the hybridized DNA alters the sensor resistance, and generated electrical signals are directly measured with the on-die circuitry. The proposed biochip can be applied to other bio-reaction detection, e.g. protein assay, through different surface modifications
    No preview · Conference Paper · Jan 2007
  • S.-J. Han · L. Xu · R.J. Wilson · S.X. Wang
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    ABSTRACT: In this paper, we present a novel drift compensation mechanism for highly sensitive biodetection based on giant magnetoresistive (GMR) sensors and magnetic nanoparticles. Micromagnetic simulations showed the quantitative detection ability of this new method. The proposed detection scheme uses both ac current sources and ac magnetic fields along with two dc bias states. Experiments were carried out to detect Miltenyi Biotec (MACS) magnetic nanoparticles on the spin-valve sensor (0.2 mumtimes4 mum). The experimental results show that sub-microvolt drifts can be achieved by this dual bias-double modulation (DBDM) method
    No preview · Article · Nov 2006 · IEEE Transactions on Magnetics
  • L Li · D Lee · S.X. Wang · M. Mao · T Schneider · R. Bubber · K. Hwang · Y. Min
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    ABSTRACT: This paper compares the permeability spectra of granular CFHO and amorphous CoZrTa (CZT) thin films with different thickness and laminated structures, and show that granular CFHO has much better high-frequency response and can be an excellent candidate for high-frequency applications.
    No preview · Conference Paper · Jun 2006
  • S Han · L Xu · S.X. Wang · J Xie · S Sun
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    ABSTRACT: New bio-detection methods based on giant magnetoresistance (GMR) or tunneling magnetoresis-tance (TMR) sensors and magnetic nanoparticles are attracting much attention due to their high sensitivity and low cost. In literature, signal to noise ratio (SNR) is often used to estimate detection limit. However, because the actual bio-signal is obtained by differentiating signals before and after biological reactions, large temperature coefficient (>1000PPM/degC) of these devices can result into significant temperature drifts during the biological reaction, which may limit the resolution of the system. This will further degrade when considering the integration of biosensors and integrated circuit (IC) since analog components are also exposed to chemical solutions and contribute to drift. In this paper, a drift free detection scheme called "Double Modulation-Dual Bias" (DMDB) method is proposed as a solution to this issue.
    No preview · Conference Paper · Jun 2006
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    D. Lee · S. X. Wang · Y. Tang · J. Hong · A. Berkowitz
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    ABSTRACT: Amorphous or granular magnetic coatings formed with magnetic particles have high resistivities, which are desirable to reduce the eddy current t losses greatly. However, since magnetic particles are separated from one another, the permeabilities of magnetic coatings tend to be low, and obtaining accurate permeability spectra can be not so trivial. The measured relative permeability of FeSiB coating at 30 MHz was 21.5, while the expected value based on 4piMs and Hk values was 5.4. The modifications in calibration procedure allow more accurate measurement of permeability of magnetic films with intermediate or low permeability and hence enable the permeameter to cover the wider range of the permeability values.
    Full-text · Article · May 2006
  • D. W. Lee · S. X. Wang
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    ABSTRACT: Multiple resonance peaks were observed below the ferromagnetic resonance in the permeability spectra of CoTaZr films with thickness up to 3 μm. Analysis shows the power-law-like dependence of peak frequencies on the film thickness and the peak order, and the observed behavior was identical for both blanket and patterned CoTaZr samples. Experimental findings suggest that the observed modes corresponding to the multiple resonance peaks may not be related to the in-plane domain structure but rather are likely associated with a mechanism in the out-of-plane direction. Multiple resonance peaks disappeared when the lamination structures were used.
    No preview · Article · Apr 2006 · Journal of Applied Physics
  • JT Kemp · RW Davis · RL White · SX Wang · CD Webb
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    ABSTRACT: We describe a novel method for rapidly identifying and distinguishing between different DNA sequences using short tandem repeat (STR) analysis and DNA microarrays. The method can be used to deduce identity, length, and number of STRs of the target molecule. We refer to this technique as the "variable-length probe array" method for STR profiling (VLPA). The method involves hybridization of the unknown STR target sequence to a DNA microarray displaying complementary probes that vary in length to cover the range of possible STRs. A post-hybridization enzymatic digestion of the DNA hybrids is then used to selectively remove labeled single-stranded regions of DNA from the microarray surface. The number of repeats in the unknown target is then deduced based on the pattern of target DNA that remains hybridized to the array. This DNA profiling technique is useful for performing forensic analysis to uniquely identify individual humans or other species.
    No preview · Article · Sep 2005 · Journal of Forensic Sciences
  • D. W. Lee · A. M. Crawford · S. X. Wang
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    ABSTRACT: In-plane biaxial anisotropy in permeability, desired for integrated inductor cores, was achieved using a composite-anisotropy sandwiched structure. With Tb-doped CoTaZr alloy as the material for magnetic layers, the permeability of the composite film was close to 50% of that of blanket film along the easy axes of both top and bottom magnetic layers, and the ferromagnetic-resonance frequencies were about 2 GHz. Simulations show that the integrated inductors with the composite film as their magnetic cores have inductances near a half of that for the ideal isotropic permeability case.
    No preview · Article · May 2005 · Journal of Applied Physics

Publication Stats

818 Citations
148.58 Total Impact Points

Institutions

  • 1995-2012
    • Stanford University
      • • Department of Materials Science and Engineering
      • • Department of Electrical Engineering
      • • Geballe Laboratory for Advanced Materials
      Palo Alto, CA, United States