S. H. Liou

University of Nebraska at Lincoln, Lincoln, Nebraska, United States

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Publications (230)518.03 Total impact

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    ABSTRACT: The nonlinear temperature dependence of nucleation field (Hn) of [Co/Pt]n multilayer based perpendicular magnetic tunnel junction stacks with wedged Co60Fe20B20 insertion layers is reported. When temperature decreases, the tilted spins away from perpendicular direction near Co60Fe20B20/AlOx interface play dominating role to promote the nucleation process and lower Hn value and even change the sign of Hn. By taking advantage of high temperature annealing under high perpendicular magnetic field, this nonlinear behavior and sign of Hn could be manipulated. The understanding of temperature dependent properties in such system is important for spintronics applications, such as in memory and spin oscillators.
    Journal of Applied Physics 04/2013; 113(17). · 2.21 Impact Factor
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    ABSTRACT: The [Co/Pt]n multilayer based perpendicular magnetic tunnel junction stacks with wedged Co60Fe20B20 insertions up to 2 nm, and corresponding perpendicular magnetic tunnel junctions were magnetically and electrically investigated. The focus is on the influence of CoFeB insertions in the free and reference electrodes on the overall junction magnetization reversal and magnetoresistance response. The exchange spring behavior was revealed as the Co60Fe20B20 spins canting towards the in-plane direction in the [Co/Pt]n/Co60Fe20B20 hard/soft perpendicular magnetic electrodes. The broad range thickness of wedged Co60Fe20B20 insertion enables to reveal the critical transition, in particular, from rigid coupling to exchange spring coupling. With the help of 375°C annealing under 10 kOe magnetic field, the recovery from distinct multi-domain structure to nearly single domain structure was distinctly observed in the unpatterned perpendicular magnetic tunnel junction (p-MTJ) films with CoFeB thickness tCFB ≥ 1.5 nm. Meanwhile, for the corresponding patterned perpendicular magnetic tunnel junctions with AlOx barrier, the tunnel magnetoresistance (TMR) ratio exhibited an intense enhancement over 100%. The TMR results and spin configurations were illustrated using an exchange spring model in both magnetic electrodes. The presented study shows the benefit of using exchange spring magnetic electrodes in perpendicular magnetic tunnel junction on their performance.
    Journal of Applied Physics 04/2013; 113(13). · 2.21 Impact Factor
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    ABSTRACT: Anatase TiO2 is known as a promising host material for a wide-gap ferromagnetic semiconductor as it is a good solvent for numerous transition-metal elements. We report on the structural and magnetic properties of epitaxially grown anatase (Ti1−xVx)O2 layers where x covers the whole range of solubility of V atoms in anatase TiO2 and beyond the solubility limit of 21%. We measured an average magnetic moment per vanadium as high as 1 µB with a magnetic percolation threshold of less than 6% which agrees with recent theoretical predictions. Interestingly, our results show a decrease of the average V magnetic moment as x increases throughout the solubility range. Anatase (Ti1−xVx)O2 is no longer present beyond the solubility limit, where the nonmagnetic VO2 phase forms and destroys the net magnetization.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
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    ABSTRACT: A scheme for molecular sensing using magnetic tracer particles and a microcantilever torsional magnetometer is investigated. The present report deals with the example of explosive 2,4,6-trinitrotoluene (TNT) detection. The sensors consist of silicon microcantilevers functionalized with TNT aptamers that are weakly bonded to magnetic particles via TNT-analog molecules. When exposed to TNT, the magnetic signal, initially maximum, is expected to undergo a steplike decrease as the TNT molecules replace the magnetic particles on the TNT receptors. We demonstrate the feasibility of this detection technique in terms of chemical reactions and our magnetometer sensitivity that reaches the range of 10<sup>-11</sup> emu at room temperature with commercial atomic force microcopy cantilevers.
    Journal of Applied Physics 05/2011; · 2.21 Impact Factor
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    ABSTRACT: The magnetization reversal in patterned thin-film arrays of elliptical submicron permalloy elements has been investigated by magnetic-force microscopy and micro-magneto-optic Kerr effect. Three different spatial arrangements of chains are considered, namely chains aligned parallel to the long axis of the ellipse, chains aligned parallel to the short axis of the ellipse, and arrays with roughly equal element-to-element spacings in both directions. Comparison of the hysteresis loops in an in-plane field perpendicular to the ellipses' long axes shows that the magnetization reversibility is highest for chains along the long axis. This is due to the nearly coherent magnetization rotation in the applied magnetic field and to the formation of a head-to-tail domain arrangement. Other arrangements, such as chains of ellipses aligned parallel to short axis, yield flux-closure domains as the applied magnetic field is changed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3567180]
    Journal of Applied Physics 01/2011; 109(7). · 2.21 Impact Factor
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    ABSTRACT: We demonstrate a simple low-power, magnetic sensor system suitable for high-sensitivity magnetic-field mapping, based on solid-state magnetic tunnel junction devices with minimum detectable fields in a 100 pT range at room temperature. In this paper, we discuss a method that uses multilayer thin films to improve the performance of the soft ferromagnetic layer in magnetoresistive sensor applications, by reducing the coercivity and/or improving the reversibility. We have used it in the design of our new magnetic sensor. This sensor has a sensitivity as high as 750%/mT. The magnetic sensor only dissipates 1 mW of power while operating under an applied voltage of 1V . Index Terms—Magnetic concentrators, magnetic noise, magnetic sensor, magnetic tunnel junction (MTJ). HE measurement of magnetic field in the picotesla range is important for a wide range of homeland security, industrial, and biomedical applications. Many of these applications require sensors with field noise of less than 1 nT/Hz , as well as low-frequency operation, low maintenance, and low power consumption [1]‐[12]. The minimum detectable field (the field noise times the measurement bandwidth) of magnetic tunnel junction (MTJ) sensors is limited by thermal Johnson, shot, electronics, and magnetic noise, in which magnetic noise dominates at low frequencies and nonreversible (hysteretic) behavior, current state-of-the-art MTJ devices have not demonstrated the desired performance. In this paper, we discuss a method that uses multilayer thin films to improve the performance of the soft ferromagnetic layer in magnetoresistive (MR) sensor applications, by reducing the coercivity and/or improving the reversibility. In this study, a layer of very soft Fe Ni is added onto the Co Fe B transition-metal layers that show a moderate improvement of the soft-magnetic performance. We demonstrate a simple low-power, magnetic sensor system suitable for high-sensitivity magnetic field mapping, based on solid-state MTJ devices with minimum detectable fields in the picotesla range at room temperature. The key features of this design are: 1) decreasing the noise by the use of a 64-element MTJ bridge, 2) reducing the magnetic hysteresis by using a coupled soft magnetic layer, and 3) increasing the signal by using an external flux-to-field magnetic flux concentrators (MFCs). This sensor has a sensitivity as high as 750%/mT. The magnetic sensor only dissipates 1 mW of power, while operating under a applied voltage of 1 V.
    IEEE Transactions on Magnetics 01/2011; 47(10):3740-3743. · 1.42 Impact Factor
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    ABSTRACT: The mesoscopic Kondo effect in metallic nanoparticles containing a magnetic impurity is investigated by model calculations. A Maxwell–Garnett approach is used to approximately determine the resistivity of doped nanoparticles in a matrix, and the magnetic susceptibility is estimated from the confinement of the conduction electrons. Conductivity measurements of nanoparticles embedded in a matrix are difficult to realize, because metallic matrices distort the Kondo cloud, whereas insulating or semiconducting matrices yield a very weak signal. By comparison, susceptibility measurements do not suffer from these shortcomings. The Kondo effect survives in nanoparticles even if the cluster size is much smaller than the Kondo screening cloud, but the effective Curie constant becomes constant below a particle-size dependent transition temperature and the temperature dependence of the susceptibility is no longer universal.
    Journal of Applied Physics 06/2010; · 2.21 Impact Factor
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    ABSTRACT: Soft bilayer alloys were produced by depositing granular Fe: SiO2 onto transition-metal thin films. We have prepared series of samples using different soft-magnetic materials including permalloy (Ni80Fe20), hcp Co90Fe10, and amorphous Co60Fe20B20, all covered by a layer of superparamagnetic Fe particles in a silicon-oxide matrix. The thickness of transition-metal layer was fixed and the magnetic properties of the bilayer system were studied by varying the Fe: SiO2 layers thicknesses from 0 to 70 nm. The bilayer yields a moderate improvement of the soft-magnetic performance. The hysteresis loops show a pronounced twofold anisotropy in the film plane for permalloy and amorphous CoFeB, which is consistent with the presence of external magnetic field during processing. This indicates that the coercivity is controlled by the pinning of 180 degrees domain walls and that the effect of the Fe particles on the hysteresis is mediated by the interaction with the domain-wall stray fields. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3360768]
    Journal of Applied Physics. 01/2010; 107(9).
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    ABSTRACT: We present a design of a low power, compact, magnetoresistive sensor. The key features of the design are (1) decreasing the noise by the use of a 64 element magnetic tunnel junction (MTJ) bridge, (2) reducing the magnetic noise by annealing of the MTJ sensors in high magnetic field and a hydrogen environment, and (3) increasing the signal by the use of external low-noise magnetic flux concentrators. The field noise of our prototype magnetic sensor is approximately 4.5 pT/Hz<sup>1/2</sup> at 1 kHz, and 222 pT/Hz<sup>1/2</sup> at 1 Hz at room temperature. The magnetic sensor dissipates about 2 mW of power while operating at an applied voltage of 2 V.
    Sensors, 2009 IEEE; 11/2009
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    ABSTRACT: Self-aligned growth of ultra-short single-walled carbon nanotubes (SWNTs) was realized by utilizing optical near-field effects in a laser-assisted chemical vapor deposition (LCVD) process. By introducing the optical near-field effects, bridge structures containing single suspended SWNT channels were successfully fabricated through the LCVD process at a relatively low substrate temperature. Raman spectroscopy and I-V analyses have been carried out to characterize the SWNT-bridge structures. Numerical simulations using a high-frequency structure simulator revealed that significant enhancement of local heating occurs at metallic electrode tips under laser irradiation; it is about one order of magnitude higher than that in the rest of the electrodes. This technique suggests a novel approach to in situ low-temperature fabrication of SWNT-based devices in a precisely controlled manner, based on the nanoscale heating enhancement induced by the optical near-field effects.
    Nanotechnology 02/2009; 20(2):025601. · 3.84 Impact Factor
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    ABSTRACT: The image contrast enhancement in scanning electron microscopy of single-walled carbon nanotubes (SWNTs) on SiO2 surfaces was experimentally investigated using a field-emission scanning electron microscope (FESEM) using a wide range of primary electron (PE) voltages. SWNT images of different contrasts were obtained at different PE voltages. Image contrast enhancement of SWNTs was investigated by charging SiO2 surfaces at different PE voltages. The phenomena are ascribed to the surface potential difference and charge injection between SWNTs and SiO2 substrates induced by the electron-beam irradiation.
    Applied Surface Science 01/2009; 255(7):4341-4346. · 2.54 Impact Factor
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    ABSTRACT: The minimum detectable field of magnetoresistive sensors is limited by their intrinsic noise. Magnetization fluctuations are one of the crucial noise sources and are related to the magnetization alignment at the antiferromagnetic-ferromagnetic interface. In this study, we investigated the low frequency noise of magnetic tunnel junctions (MTJs) annealed in the temperature range from 265 to 305 °C and magnetic fields up to 7 T, either in helium or hydrogen environments. Our results indicate that the magnetic fluctuators in these MTJs changed their frequency based on annealing field and temperature. The noise of the MTJs at low frequency can be reduced by annealing in high magnetic field (7 T) and further improved by annealing in a hydrogen environment.
    Journal of Applied Physics 03/2008; 103(7):07E920-07E920-3. · 2.21 Impact Factor
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    L. Yuan, Y.S. Lin, Dexin Wang, S.H. Liou
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    ABSTRACT: Magnetic microstructures in the reference layer in magnetic tunneling junctions (MTJs) are tuned by a reversal field under ambient conditions to investigate their effects on the magnetoresistance (MR) and the exchange coupling field (H<sub>E</sub>) between the reference layer and the free layer. Magnetization changes in the reference layer can be probed by measuring minor MR loops. The results show the H<sub>E </sub> of the minor MR loops versus the applied reversal field changes from negative to positive and crosses zero. These results can be explained by the magnetic inhomogeneities at the interface between anti-ferromagnetic/pinned-ferromagnetic layers, which causes the partial magnetization reversal in the reference layer
    IEEE Transactions on Magnetics 07/2007; · 1.42 Impact Factor
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    ABSTRACT: Recent prominent progresses in synthesizing and manipulating single-walled carbon nanotubes (SWNTs) stimulated extensive interests in developing SWNT-based devices for nanoelectronics and nanoelectromechanical systems (NEMS). Thermal chemical vapor deposition (CVD) is one of the most widely accepted technique for growing SWNTs by heating the whole chamber and substrate to required reaction temperatures. In this study, we demonstrated a process for position-controllable synthesis of SWNT-FET by bridging the SWNT across pre-defined electrodes using the laser chemical vapor deposition (LCVD) technique. The SWNT-FET was back-gate modulated, showing p-type semiconducting characteristics. The process is very fast and can be conducted using both far-infrared CO2 laser (10.6 mum) and near-infrared Nd:YAG laser (1064 nm). We have also demonstrated localized synthesis of SWNTs by a focused laser beam. Due to the unique advantages of LCVD process, such as fast and local heating, as well as its potential to select chiralities during the growing process, it may provide new features and versatilities in the device fabrication.
    Proc SPIE 03/2007;
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    ABSTRACT: Antiferromagnetically coupled magnetic thin films with perpendicular anisotropy exhibit domain overlap regions originating from magnetostatic stray fields localized in the vicinity of the domain walls. Using high resolution magnetic force microscopy, the authors investigate these overlap regions in [ Co / Pt ]/ Ni O /[ Co / Pt ] multilayers with various strengths of the interlayer exchange coupling. They develop a simple model that provides a quantitative explanation of the formation of these regions and the relationship between the domain overlap width and the coupling strength. Their results are important for application of magnetic layered structures with perpendicular anisotropy in advanced magnetoresistive devices.
    Applied Physics Letters 12/2006; · 3.79 Impact Factor
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    ABSTRACT: Microcantilever torque magnetometry (MTM) is a sensitive tool to measure small magnetization changes in the sample. In this paper, we investigated a process for preparing patterned magnetic films on cantilevers and studied the magnetic interactions of a single pair of micrometer-sized Ni<sub>80</sub>Fe<sub>20</sub> bars (7 mum times 3.5 mum times 30 nm) separated by 50 nm using MTM. The bars were prepared with focused ion-beam milling. The magnetic hysteresis loops show that the switching field of a single bar is larger than the reversing field of only one of the paired bars and less than that of both paired bars. This clearly indicates that the magnetostatic interaction exists between the bars
    IEEE Transactions on Magnetics 11/2006; · 1.42 Impact Factor
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    ABSTRACT: Direct synthesis of single-walled carbon nanotubes (SWNTs) bridging prepatterned Mo electrodes has been achieved using laser-assisted chemical vapor deposition (LCVD). The synthesized SWNTs are found predominantly semiconducting. By controlling the spot size of the focused laser beam, synthesis of SWNTs can be achieved in a localized manner, which is governed by the thermal and optical properties of materials as well as the laser parameters. The synthesis process is fast and can be achieved in both far- and near-infrared laser wavelength regions. LCVD method provides a potential approach to in situ remove SWNTs with specific chiralities during the growth.
    Applied Physics Letters 08/2006; 89(8). · 3.79 Impact Factor
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    ABSTRACT: The origin of the oscillatory interlayer exchange coupling in [Co/Pt]/NiO/[Co/Pt] multilayers is investigated using advanced microscopy and spectroscopy techniques and micromagnetic modeling. X-ray magnetic circular dichroism (XMCD) measurements show the presence of the canting of Ni spins in the NiO film being greater for antiferromagnetically coupled multilayers than for ferromagnetically coupled ones. This behavior is consistent with the model, which assumes a different sign of the exchange coupling at the two interfaces and the antiferromagnetic layer-by-layer coupling in the NiO film. An unexpectedly short attenuation length of 4Å for secondary electrons in NiO is measured, which has implications for the interpretation of XMCD data. Domain images obtained using XMCD-photoemission electron microscopy at the Co and Ni resonances indicate that the canting of the Ni spins occurs on both a microscopic and macroscopic scale. The average size of the domains is shown to increase with exchange coupling strength. In antiferromagnetically coupled samples, the competition between magnetostatic and interlayer exchange effects gives rise to a region of overlapping domains. The size of this region scales inversely with coupling strength. Finally, the temperature dependence of the interlayer coupling shows both reversible and irreversible effects. The irreversible effects stem from oxidation/reduction reactions at the Co/NiO interface. The reversible effects stem from the temperature dependences of the many factors that play a role in the interlayer coupling and exhibit nonmonotonic temperature dependence.
    Physical Review B 01/2006; 74:054419-13. · 3.66 Impact Factor
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    ABSTRACT: Suspended single-walled carbon nanotubes (SWNTs) have been synthesized on Si inverse-opal structures by laser-assisted chemical vapour deposition (LCVD). A CW CO2 laser at 10.6 µm was used to directly irradiate the substrates during the LCVD process. At a laser power density of 14.3 MW m-2, suspended SWNT networks were found predominantly rooted at the sharp edges in the Si inverse-opal structures. Raman spectroscopy indicated that the SWNT networks were composed of high-quality defect-free SWNTs with an average diameter of 1.3 nm. At a lower laser power density (6.4 MW m-2), multi-walled carbon nanotubes (MWNTs) were grown on the entire surface of the substrates. The preference for the synthesis of SWNTs or MWNTs was attributed to the difference in the catalyst sizes as well as the growth temperature in the LCVD process.
    Nanotechnology 01/2006; 17(15):3822-3826. · 3.84 Impact Factor
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    ABSTRACT: This paper compares the images obtained by superparamagnetic and permanent magnetic force microscopy (MFM) tips, that allows us to explain the issues related to the frequency double in some of domain images of the recording media. This paper also investigates the domain structures of high density recording bits (up to 1100 kfci) written on ac and dc-erased CoPtCrO perpendicular magnetic recording (PMR) media with permanent magnet MFM tips under ambient conditions.
    Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 01/2006

Publication Stats

2k Citations
518.03 Total Impact Points

Institutions

  • 1994–2013
    • University of Nebraska at Lincoln
      • • Department of Physics and Astronomy
      • • Nebraska Center for Materials and Nanoscience
      Lincoln, Nebraska, United States
  • 2001
    • National Changhua University of Education
      • Department of Physics
      Taipei, Taipei, Taiwan
    • Icahn School of Medicine at Mount Sinai
      Manhattan, New York, United States
  • 1996–2001
    • Tri-Service General Hospital
      T’ai-pei, Taipei, Taiwan
    • National Institute of Standards and Technology
      Maryland, United States
    • University of Colorado at Boulder
      Boulder, Colorado, United States
  • 2000
    • Institute of Occupational Safety and Health, Taiwan
      T’ai-pei, Taipei, Taiwan
    • University of Tennessee
      Knoxville, Tennessee, United States
    • Chang Gung University
      • Department of Public Health
      Hsin-chu-hsien, Taiwan, Taiwan
  • 1994–1999
    • National Defense Medical Center
      • Department of Public Health
      Taipei, Taipei, Taiwan
  • 1998
    • Purdue University
      • School of Materials Engineering
      West Lafayette, Indiana, United States
    • National Defense University, Taiwan
      Taoyuan City, Taiwan, Taiwan
  • 1996–1998
    • University of Nebraska at Omaha
      Omaha, Nebraska, United States
  • 1982–1988
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 1987
    • Martin Marietta Laboratories
      Baltimore, Maryland, United States