bluuebunny/crossref_metadata_2025 huggingface dataset:
Inverse estimation of parameters for multidomain flow models in soil columns with different macropore densities
Authors: Arora Bhavna , Mohanty Binayak P. , McGuire Jennifer T.
Published: April 2011

Soil and crop management practices have been found to modify soil structure and alter macropore densities. An ability to accurately determine soil hydraulic parameters and their variation with changes in macropore density is crucial for assessing potential contamination from agricultural chemicals. This study investigates the consequences of using consistent matrix and macropore parameters in simulating preferential flow and bromide transport in soil columns with different macropore densities (no macropore, single macropore, and multiple macropores). As used herein, the term “macropore density” is intended to refer to the number of macropores per unit area. A comparison between continuum‐scale models including single‐porosity model (SPM), mobile‐immobile model (MIM), and dual‐permeability model (DPM) that employed these parameters is also conducted. Domain‐specific parameters are obtained from inverse modeling of homogeneous (no macropore) and central macropore columns in a deterministic framework and are validated using forward modeling of both low‐density (3 macropores) and high‐density (19 macropores) multiple‐macropore columns. Results indicate that these inversely modeled parameters are successful in describing preferential flow but not tracer transport in both multiple‐macropore columns. We believe that lateral exchange between matrix and macropore domains needs better accounting to efficiently simulate preferential transport in the case of dense, closely spaced macropores. Increasing model complexity from SPM to MIM to DPM also improved predictions of preferential flow in the multiple‐macropore columns but not in the single‐macropore column. This suggests that the use of a more complex model with resolved domain‐specific parameters is recommended with an increase in macropore density to generate forecasts with higher accuracy.
Magnetic pulse generation for high-speed magneto-optic switching
Authors: Kemmet Sasha , Mina Mani , Weber Robert J.
Published: April 2011

In this article, the magnetic pulse characteristics needed to achieve high-speed magneto-optic (MO) switching are investigated. A fiber-based, MO, low-voltage optical switch capable of 200 ns switching is presented, along with the special circuit characteristics for magnetic field generation for high-speed switching. The switch consists of the optical system, the MO material (bismuth substituted iron garnet [(Bi1.1Tb1.9)(Fe4.25Ga0.75)O12]), and a high-speed magnetic field driving circuit. A Faraday rotator is placed within the interferometric loop of a fiber-optic Sagnac interferometer, and interference at the output ports is controlled by the applied field. The fast switching speed is accomplished via the special design of the magnetic pulse generation circuitry. The applied magnetic field overshoots the field necessary to achieve the desired Faraday rotation and then settles to a steady state field. If the duration of the overshoot is less than the time it takes the material to saturate, a fast optical switching time can be achieved without saturating the material. The effects of the overshoot amplitude and duration and steady-state amplitude on optical rise time (determined by domain wall velocity) are studied and experimental results are presented.
The effect of flake thickness on anisotropic SmCo5 nanoflakes/submicron-flakes with high energy product
Authors: Knutson S. J. , Shen Y. , Horwath J. C. , Barnes P. , Chen C. H.
Published: April 2011

Establishing an effective process to synthesize anisotropic magnetic nanopowders that have both high energy products and high coercivity is highly desirable for many applications. A recent effort using surfactant-assisted high energy milling is proved to be capable of making SmCo5 nanoflakes/submicron-flakes with high performance. Using the cast alloy as a starting powder, a series of the flakes with various thicknesses were made, which had energy product values up to 22 MGOe and coercivity values up to 21 kOe. The highest coercivity corresponds to a typical flake thickness of 180 nm. The key to our technique is retaining the crystallinity, which allows a high degree of anisotropy to be produced. Results of the analyses from the micrograph of SEM, XRD patterns, and magnetic measurements demonstrate the effect of flake thickness on the magnetic properties. More importantly, a relationship of property-morphology correlation in nanoscale is established for rare earth magnetic powder/flakes.
Effect of pressure on microstructure and magnetic properties of FePt:X media
Authors: Yuan Hua , Chernyshov Alex , Mardinly John , Srinivasan Kumar , Acharya Ramamurthy , Bertero Gerardo , Yamashita Tom
Published: April 2011

L10 FePt film exhibits a very large magnetocrystalline anisotropy (7×107 ergs/cc). It therefore becomes a promising material candidate for ultrahigh areal density heat assisted magnetic recording media. This paper discusses the effect of pressure on the microstructure and magnetic properties of L10 FePt:X thin films. It has been found that Ar pressure plays an important role in controlling the properties of the composite polycrystalline thin films. As the pressure increases, FePt:X films show much better segregation in the microstructure of the amorphous materials within the grain boundary, and the FePt grain size decreases accordingly. It was also found that media coercivity increases as the film microstructure changes from a maze type to a granular type, along with the pressure increase. This is likely due to the enhanced exchange decoupling of the FePt grains in the magnetic layer. The microstructure’s evolution with pressure is in agreement with a structural zone model of the composite polycrystalline thin films.
Magnetic and microwave absorption properties of W-type Ba(ZnxCo1−x)2Fe16O27 hexaferrite platelets
Authors: Zi Z. F. , Dai J. M. , Liu Q. C. , Liu H. Y. , Zhu X. B. , Sun Y. P.
Published: April 2011

W-type barium hexaferrite platelets Ba(ZnxCo1−x)2Fe16O27 (0.1 ≤ x ≤ 0.9) were prepared by a chemical coprecipitation route. The results of field-emission scanning electronic microscopy showed that the grains were regular hexagonal platelets with sizes from 0.5 to 1.5 μm. With increasing x, the coercivity decreased, whereas the saturation magnetization increased. Microwave absorption properties were investigated in the frequency range of 8–12 GHz. An optimal reflection loss (−24.8 dB) reached at 10 GHz for x = 0.5 with a thickness of 2 mm. These results indicated that our samples might be a potential candidate for permanent magnets and microwave absorption materials.
Formation mechanisms of precursors of radiation-induced color centers during fabrication of silica optical fiber preform
Authors: Tomashuk A. L. , Zabezhailov M. O.
Published: April 2011

Samples in the form of transverse slices of rods and optical fiber preforms made from the high-hydroxyl KU-1 and low-hydroxyl KS-4V silica by the plasma outside deposition (POD) method are γ-irradiated to a dose of ∼1 MGy (SiO2). Next, the radial dependences of the radiation-induced nonbridging oxygen hole center (NBOHC) and E′-center (three-coordinated silicon) in the samples are constructed by measuring the amplitudes of their 4.8 and 5.8 eV absorption bands, respectively. Based on the analysis of these radial dependences and considering the temperature and duration of the preirradiation heat treatment of the rods and preforms at the POD-installation, we determine the ratio of the oscillator strengths of the above bands and the microscopic thermoinduced processes occurring during preform fabrication and producing precursors of the radiation-induced NBOHC and E′-center. These processes are found to be associated with the escape of either H2 or H2O from neighboring hydroxyl groups, and, therefore, can occur in high-hydroxyl silica only. It is concluded that enhancement of the radiation resistance of high-hydroxyl silica optical fibers requires decreasing the temperature and duration of the preform fabrication process, in particular, changing from the POD-technology to the low-temperature plasmachemical vapor deposition (PCVD) or surface PCVD (SPCVD)-technology.
Local orientation analysis by electron backscatter diffraction in highly textured sintered, die-upset, and hydrogenation disproportionation desorption and recombination Nd–Fe–B magnets
Authors: Güth K. , Woodcock T. G. , Thielsch J. , Schultz L. , Gutfleisch O.
Published: April 2011

Local texture in polycrystalline Nd–Fe–B powders produced by hydrogenation disproportionation desorption and recombination (HDDR) and by pulverization of hot deformed material (MQA-F) has been studied by electron backscatter diffraction and compared with the ideal case of a powder consisting of single-crystalline particles. The HDDR powder particles exhibited a biaxial {001}, 〈100〉 local texture, whereas the MQA-F particles showed a local 〈001〉 fiber texture. This was explained by differences in the texture mechanisms of the two materials. Resin bonded compacts were prepared from the two polycrystalline powders following alignment in an external magnetic field. The degree of texture on the global scale was determined from magnetic measurements of these compacts and the results were compared with those from a commercial Nd–Fe–B sintered magnet and hot deformed magnet.
Magnetization configurations and reversal of magnetic nanotubes with opposite chiralities of the end domains
Authors: Chen Ai-Ping , Gonzalez Julian M. , Guslienko Konstantin Y.
Published: April 2011

For thick soft magnetic nanotubes with an anisotropy axis directed along the nanotube length the equilibrium energy ground states present magnetization configurations with opposite rotating senses in two tube ends (B-state), referring as antiparallel chiralities of the end vortex domains. For nanotubes with outer radius R of 50 nm, 100 nm and 150 nm, and length L = (2.5–20)R the B-state remanent magnetization and the reversal field dependence on tube thickness and anisotropy strength are studied by using both two-dimensional simulation and analytic methods. The equilibrium states, the hysteresis loops and the switching field values calculated numerically are presented as the functions of tube size and material parameters. For the short nanotubes the domain walls patterns, such as transverse walls and vortex walls, nucleating in the tube center, as well as the hysteresis loops of the nanotubes with transverse walls are presented. The numerical results are interpreted by a simple analytical model in which the equilibrium state of nanotube is described by θ, the angle of the magnetization M deviation from the intrinsic tube easy axis. θ as a function of the tube aspect ratio L/R, tube thickness ΔR, and uniaxial anisotropy constant Ku, obtained by minimizing the total magnetic energy, well describes dependences of the shape of hysteresis loops and switching field values on the tube geometric and material parameters in the B-state.
Off-track error probability due to track squeeze in shingled writing
Authors: Miura Kenji , Yamamoto Eiji , Aoi Hajime , Muraoka Hiroaki
Published: April 2011

A statistical investigation of off-track errors for shingled and conventional writing was carried out to reveal the behavior at high track densities. The remanent data width in shingled writing was wider than that in conventional writing for the same average track pitch. Comparison of the distributions of the calculated remanent data width showed that for conventional writing erroneous tracks were generated when the track was squeezed by both neighboring tracks, i.e., double-squeezing. Shingled writing has an advantage in that this double-squeezing does not occur and therefore a higher track density is achievable.
FastMag: Fast micromagnetic simulator for complex magnetic structures (invited)
Authors: Chang R. , Li S. , Lubarda M. V. , Livshitz B. , Lomakin V.
Published: April 2011

A fast micromagnetic simulator (FastMag) for general problems is presented. FastMag solves the Landau–Lifshitz–Gilbert equation and can handle multiscale problems with a high computational efficiency. The simulator derives its high performance from efficient methods for evaluating the effective field and from implementations on massively parallel graphics processing unit (GPU) architectures. FastMag discretizes the computational domain into tetrahedral elements and therefore is highly flexible for general problems. The magnetostatic field is computed via the superposition principle for both volume and surface parts of the computational domain. This is accomplished by implementing efficient quadrature rules and analytical integration for overlapping elements in which the integral kernel is singular. Thus, discretized superposition integrals are computed using a nonuniform grid interpolation method, which evaluates the field from N sources at N collocated observers in O(N) operations. This approach allows handling objects of arbitrary shape, allows easily calculating of the field outside the magnetized domains, does not require solving a linear system of equations, and requires little memory. FastMag is implemented on GPUs with ?>GPU–central processing unit speed-ups of 2 orders of magnitude. Simulations are shown of a large array of magnetic dots and a recording head fully discretized down to the exchange length, with over a hundred million tetrahedral elements on an inexpensive desktop computer.
Investigation on microstructure and magnetic properties of Sm2Co17 magnets aged at high temperature
Authors: Feng Haibo , Chen Hongsheng , Guo Zhaohui , Pan Wei , Zhu Minggang , Li Wei
Published: April 2011

The Sm2Co17 magnet is the most promising candidate for high temperature applications. The microstructure evolutions and losses in the magnetic properties of the magnet in high temperature aging status have been investigated. The Sm(CobalFe0.22Cu0.068Zr0.025)7.75 magnets were prepared using the conventional powder sintering method. The magnet samples were isothermally aged at 500°C, 600 °C, and 700 °C for 72 h, respectively. The magnetic properties and the demagnetization curve were kept invariable for the magnet samples aged at 500 °C. The coercivity Hcj of the magnet samples decreased with increasing aging temperature. The Hcj decreased from 29.2 kOe for the original status samples to 10.8 kOe for the samples aged at 700 °C. The cell structure in the magnet is not destroyed after aging at 700 °C for 72 h. The deterioration of the magnetic properties and the demagnetization-curve squareness was caused by an increasing lattice mismatch between the 2:17R cell phase and the cell-boundary 1:5H phase, and by an increasing cell diameter for the magnet sample aged at a high temperature.
The influence of microstructure on magnetoresistive properties of Cu80Fe5Ni15 ribbons
Authors: Cazottes S. , Fnidiki A. , Lemarchand D. , Danoix F. , Ochin P. , Lardé R. , Coisson M.
Published: April 2011

The influence of microstructure on magnetoresistive properties of Cu80Fe5Ni15 (at. %) melt-spun and annealed ribbons was studied. The microstructure was characterized by means of energy filtered transmission electron microscopy, small angle neutron scattering and atom probe tomography. Giant magnetoresistance (GMR) measurements were made at low temperature. The samples were annealed for 2 h at different temperatures from 400 °C to 600 °C in order to modify their microstructure and thus their GMR properties. The magnetic behavior was correlated to the structural data to explain the GMR properties that are observed. A precise structural characterization, using atom probe tomography, small angle neutron scattering and energy filtered transmission electron microscopy, is presented. It reveals that no precipitates or clusters are present into the core grains of the as spun state; thus the sample is a solid solution. The magnetic properties measurements indicate that the ribbon exhibits a superparamagnetic behavior and presents relatively high GMR properties. These results imply that GMR can be obtained from a solid solution. Therefore, interface scattering is not the only precursor for GMR. The presence of diluted magnetic atoms leads to the formation of magnetic clusters that behave like superparamagnetic particles which are interacting and creating GMR.
Effect of disorder on spin-transfer torque in magnetic tunnel junctions
Authors: Tang Y.-H. , Kioussis Nicholas , Kalitsov Alan , Car Roberto
Published: April 2011

We have generalized the nonequilibrium Green’s functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, 〈T∥〉, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average 〈T∥〉 as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average 〈T∥〉 in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of 〈T∥〉 in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.
Equation-of-motion approach of spin-motive force
Authors: Yamane Yuta , Ieda Jun’ichi , Ohe Jun-ichiro , Barnes Stewart E. , Maekawa Sadamichi
Published: April 2011

We formulate a quantitative theory of an electromotive force of spin origin, i.e., spin-motive force, by the equation-of-motion approach. In a ferromagnetic metal, electrons couple to the local magnetization via the exchange interaction. The electrons are affected by spin dependent forces due to this interaction and the spin-motive force and the anomalous Hall effect appears. We have revealed that the origin of these phenomena is a misalignment between the conduction electron spin and the local magnetization.
Exploring the limits of soft x-ray magnetic holography: Imaging magnetization reversal of buried interfaces (invited)
Authors: Camarero J. , Jiménez E. , Vogel J. , Tieg C. , Perna P. , Bollero A. , Yakhou-Harris F. , Arm C. , Rodmacq B. , Gautier E. , Auffret S. , Delaup B. , Gaudin G. , Dieny B. , Miranda R.
Published: April 2011

Only a very few experimental techniques can address the microscopic magnetization reversal behavior of the different magnetic layers in a multilayered system with element selectivity. We present an element-selective study of ferromagnetic (FM) [Co/Pt]n multilayers with perpendicular anisotropy exchange-coupled to antiferromagnetic (AFM) FeMn and IrMn films performed with a new experimental set-up developed for both soft x-ray spectroscopy and holography imaging purposes. The spectroscopy analysis allows the quantification of the unpinned (pinned) uncompensated AFM moments, providing direct evidence of its parallel (antiparallel) alignment with respect to the FM moments. The holography experiments give a direct view of both FM and uncompensated AFM magnetic structures, showing that they replicate to each other during magnetization reversal. Remarkably, we show magnetic images for effective thicknesses as small as one monolayer. Our results provide new microscopic insights into the exchange coupling phenomena and explore the sensitivity limits of these techniques. Future trends are also discussed.
Steady streaming: A key mixing mechanism in low-Reynolds-number acinar flows
Authors: Kumar Haribalan , Tawhai Merryn H. , Hoffman Eric A. , Lin Ching-Long
Published: April 2011

Study of mixing is important in understanding transport of submicron sized particles in the acinar region of the lung. In this article, we investigate transport in view of advective mixing utilizing Lagrangian particle tracking techniques: tracer advection, stretch rate and dispersion analysis. The phenomenon of steady streaming in an oscillatory flow is found to hold the key to the origin of kinematic mixing in the alveolus, the alveolar mouth and the alveolated duct. This mechanism provides the common route to folding of material lines and surfaces in any region of the acinar flow, and has no bearing on whether the geometry is expanding or if flow separates within the cavity or not. All analyses consistently indicate a significant decrease in mixing with decreasing Reynolds number (Re). For a given Re, dispersion is found to increase with degree of alveolation, indicating that geometry effects are important. These effects of Re and geometry can also be explained by the streaming mechanism. Based on flow conditions and resultant convective mixing measures, we conclude that significant convective mixing in the duct and within an alveolus could originate only in the first few generations of the acinar tree as a result of nonzero inertia, flow asymmetry, and large Keulegan–Carpenter (KC) number.
Polyethylene nanocomposite dielectrics: Implications of nanofiller orientation on high field properties and energy storage
Authors: Tomer V. , Polizos G. , Randall C. A. , Manias E.
Published: April 2011

Nanocomposite formation, through the incorporation of high aspect ratio nanoparticles, has been proven to enhance the dielectric properties of thermoplastic polymers, when the mitigation of internal charges and the nature of the interfacial regions are properly adjusted. Here, we explore polyethylene/montmorillonite nanocomposites, and we specifically investigate how to impart desirable dielectric behavior through controlled nanoscale texturing, i.e., through control of the spatial arrangement of the high aspect ratio nanofiller platelets. In particular, it is shown that filler alignment can be used to improve the high electric-field breakdown strength and the recoverable energy density. The origins of the improved high field performance were traced to improved charge-trapping by a synergy of nanofillers and polar maleic anhydride (MAH) groups—introduced via polyethylene-MAH copolymers—as templated by the inorganic nanofillers. Further, it is conclusively demonstrated that the alignment of the two-dimensional nanoparticles has a measurable positive effect on the breakdown strength of the materials and, consequently, on the maximum recoverable energy density.
First-principles study of the noble metal-doped BN layer
Authors: Zhou Y. G. , Yang P. , Sun X. , Wang Z. G. , Zu X. T. , Gao F.
Published: April 2011

Intriguing electronic and magnetic properties of boron nitride (BN) layer with noble metal (Pd, Pt, Ag and Au) doping are obtained by first-principles calculations. Adsorbed Pd (or Pt) reduces the bandgap of BN sheet owing to the induction of impurity states. The unpaired electrons in the Ag (or Au)-adsorbed and the Pd (or Pt)-substituted BN layers are polarized, and thus, exhibit a magnetic moment of 1.0 μB, leading to these BN configurations to be magnetic semiconductors. The half-metallic feature of the Ag-substituted BN layer, along with the delocalization of spin states, renders this configuration an excellent spin filter material. Thus, these findings offer a unique opportunity for developing BN-based nanoscale devices.
Structural and magnetic properties of a core-shell type L1 FePt/Fe exchange coupled nanocomposite with tilted easy axis
Authors: Ma Bin , Wang Hao , Zhao Haibao , Sun Chengjun , Acharya Ramamurthy , Wang Jian-Ping
Published: April 2011

Structural and magnetic properties of core-shell type L10 FePt/Fe exchange coupled nanocomposites are studied systematically. Core-shell nanocomposites with FePt core and Fe shell are obtained by depositing Fe cap layers on granular L10 FePt films. Epitaxial growth is disclosed by x-ray diffraction. Coercivity decreases drastically for FePt/Fe with the thickness increase of Fe cap layers. The coercivity reduction is due to the much increased domain wall area pinned and compressed at the soft-hard interface, and the tilted effective easy axis because of the presence of demagnetized energy. L10 FePt/Fe with a 3 nm Fe layer has high thermal stability and gain factor for media applications.
The induction of a graphite-like phase on diamond films by a Fe-coating/post-annealing process to improve their electron field emission properties
Authors: Huang Pin-Chang , Shih Wen-Ching , Chen Huang-Chin , Lin I-Nan
Published: April 2011

The electron field emission (EFE) process for diamond films was tremendously enhanced by Fe-coating and post-annealing processes. Microstructural analysis indicates that the mechanism for the improvement in the EFE process is the formation of nanographites with good crystallinity that surround the Fe (or Fe3C) nanoclusters. Presumably the nanographites were formed via the reaction of Fe clusters with diamond films, viz. by the dissolution of carbons into Fe (or Fe3C) clusters and the reprecipitation of carbon species to the surface of the clusters, a process similar to the growth of carbon nanotubes via Fe clusters as catalyst. Not only is a sufficiently high post-annealing temperature (900°C) required but also a highly active reducing atmosphere (NH3) is needed to give a proper microstructure for enhancing the EFE process. The best EFE properties are obtained by post-annealing the Fe-coated diamond films at 900°C in an NH3 environment for 5 min. The EFE behavior of the films can be turned on at E0 = 1.9 V/μm, attaining a large EFE current density of 315 μA/cm2 at an applied field of 8.8 V/μm (extrapolation using the Fowler–Nordheim model leads to Je = 40.7 mA/cm2 at a 20 V/μm applied field).
Nonlinear current-voltage behavior of the isolated resistive switching filamentary channels in CuC nanolayer
Authors: Kim Doo-In , Yoon Jaesik , Park Ju-Bong , Hwang Hyunsang , Kim Young Moon , Kwon Se Hun , Kim Kwang Ho
Published: April 2011

Copper-doped amorphous carbon film was prepared by radio frequency reactive magnetron sputtering and their resistive switching behaviors were studied under a conductive atomic force microscope (cAFM). The repetitive scanning over the same area using cAFM with various bias voltages revealed that most of the isolated conductive paths were involved in resistive switching with asymmetric nonlinear I-V characteristics. The observed I-V behavior of nanoscale filamentary channels indicates that electron transfer mechanism of resistive switching filamentary channel in Pt/CuC/Pt is a tunneling between Cu filamentary channel and electrode through the solid electrolyte rather than conduction through fully connected Cu filamentary channel.
Carrier tunneling effects on the temperature dependent photoluminescence of InAs/GaAs quantum dot: Simulation and experiment
Authors: Zhou X. L. , Chen Y. H. , Zhang H. Y. , Zhou G. Y. , Li T. F. , Liu J. Q. , Ye X. L. , Xu Bo , Wang Z. G.
Published: April 2011

Considering the direct quantum tunneling of carrier, we propose a new carrier rate equation model to simulate the temperature dependent photoluminescence (TDPL) of InAs/GaAs quantum dots (QDs). The TDPL showed abnormal variations: the peak rapid redshift, linewidth shrinkage, and thermal activation energy all decreased with increasing tunneling strength. A criterion, which could be used to evaluate the tunneling strength, has been developed. That is, smaller tunneling strength coefficient α indicates higher carrier tunneling strength. Meanwhile, the criterion is also demonstrated via comparative experimental results of InAs QDs grown on different patterned GaAs substrates. It is found that, to some extent, the tunneling strength would be enhanced by decreasing the dot-dot distance for closely arranged QDs ensembles.
A chemical test of the principle of critical point universality: The solubility of nickel (II) oxide in isobutyric acid + water near the consolute point
Authors: Hu Baichuan , Baird James K. , Richey Randi D. , Reddy Ramana G.
Published: April 2011

A mixture of isobutyric acid + water has an upper consolute point at 38.8 mass % isobutyric acid and temperature near 26 °C. Nickel (II) oxide dissolves in this mixture by reacting with the acid to produce water and nickel isobutyrate. The solubility of nickel (II) oxide in isobutyric acid + water has been measured as a function of temperature at compositions, 25, 38.8, and 60 mass % isobutyric acid. For values of the temperature, T, which were at least 2 K in excess of the liquid–liquid phase transition temperature, the measured values of the solubility, s, lie on a straight line when plotted in van't Hoff form with ln s versus 1/T. The slope, (∂ln s/∂(1/T)), of the line is negative indicating that the dissolution reaction is endothermic. When the temperature was within 2 K of the phase transition temperature, however, (∂ln s/∂(1/T)) diverged toward negative infinity. The principle of critical point universality predicts that when excess solid nickel (II) oxide is in dissolution equilibrium with liquid isobutyric acid + water, (∂ln s/∂(1/T)) should diverge upon approaching the consolute point along the critical isopleth at 38.8 mass % isobutyric acid. As determined by the sign of the enthalpy of solution, the sign of this divergence is expected to be negative. Not only do our experiments confirm these predictions, but they also show that identical behavior can be observed at both 25 and 60 mass % isobustyric acid, compositions which lie substantially to either side of the critical composition.
Optimized low-loss multilayers for imaging with sub-wavelength resolution in the visible wavelength range
Authors: Pastuszczak Anna , Kotyński Rafał
Published: April 2011

We optimize the effective skin-depth and resolution of Ag-TiO2, Ag-SrTiO3, and Ag-GaP multilayers for imaging with sub-wavelength resolution. In terms of transmission and resolution, the optimized multilayers outperform simple designs based on combined use of effective medium theory, impedance matching and Fabry–Perot resonances. For instance, an optimized Ag-GaP multilayer consisting of only 17 layers, operating at the wavelength of 490 nm and having a total thickness equal to one wavelength, combines 78% intensity transmission with a resolution of 60 nm. It is also shown that use of the effective medium theory leads to sub-optimal multilayer designs with respect to the trade-off between the skin depth and resolution already when the period of the structure is on the order of 40 nm or larger.
Random magnetic anisotropy in isotropic nanocrystalline composite permanent magnets
Authors: Sato Suguru , Lee S. J. , Mitsumata Chiharu , Yanagihara Hideto , Kita Eiji
Published: April 2011

In this study, the random magnetic anisotropy in isotropic nanocrystalline composite permanent magnets was investigated by means of numerical simulations. The magnetization reversal of randomly oriented hard-soft exchange-coupled grains was simulated using the Landau–Lifshitz–Gilbert equation in which the magnetization in a particular grain is assumed to align in the same direction (single-spin model). The calculations show that the energy product (BH)max of nanocomposite magnets has a peak value 50 MGOe at 6 nm. It is about 80% of that of single-crystal Nd2Fe14B. The coercivity HC exhibits a steep decrease toward smaller grain size, which is the result of the suppression of the random magnetic anisotropy by exchange interaction. Therefore, in isotropic nanocomposite magnets, the enhancement of energy product is limited by the suppression of the random magnetic anisotropy by exchange interaction.
Influence of compressive strain on oxygen distribution in La0.7Ba0.3MnO3 thin films
Authors: Li Bing , Yang Lei , Tian JinZeng , Wang XiaoPing , Zhu Hong , Endo Tamio
Published: April 2011

The effects of annealing ambient on the structural and transport properties of epitaxial La0.7Ba0.3MnO3 films suffering large compressive strain on LaAlO3 substrates (LBMO/LAO) have been investigated. Independent of oxygen or argon ambient, a high temperature (900 °C) post-annealing process leads to a c-axis contraction, which can be attributed to the oxygen transfer from the in-plane to the out-of-plane direction. Meanwhile, the transport properties are similar with each other for both the Ar-annealed and O2-annealed films. The comparison of the ambient annealing effects between the highly strained LBMO/LAO films and negligibly strained LBMO films on SrTiO3 substrates indicates that a large in-plane compressive strain favors decreasing oxygen deficiency in the films, especially in the ab-plane.
Electronic structure of chromium-doped lead telluride-based diluted magnetic semiconductors
Authors: Skipetrov E. P. , Pichugin N. A. , Slyn’ko E. I. , Slyn’ko V. E.
Published: April 2011

The crystal structure, composition, galvanomagnetic, and oscillatory properties of Pb1−x−ySnxCryTe (x=0,0.05–0.30; y⩽0.01) alloys are studied for different matrices and chromium impurity concentrations. It is shown that impurity chromium ions dissolve in the lattice in amounts below 1mol.%, while higher chromium concentrations lead to the appearance of microscopic regions with elevated chromium contents and inclusions of chromium-tellurium compounds. Reductions in the hole concentration, p−n conversion of the conductivity type, and stabilization of the Fermi level by the resonance level of chromium are observed with increasing chromium content. The initial rates of change of the charge carrier concentrations during doping are determined. A two-band Kane dispersion relation is used to calculate the electron concentration and Fermi level as functions of the tin content. A diagram showing the rearrangement of the electronic structure of the chromium doped alloys with varying matrix composition is constructed.
Effect of hydrogenation on transport and magnetic properties in homogeneous amorphous MnxGe1−x:H films
Authors: Qin Yu-Feng , Yan Shi-Shen , Xiao Shu-Qin , Li Qiang , Dai Zheng-Kun , Shen Ting-Ting , Kang Shi-Shou , Dai You-Yong , Liu Guo-Lei , Chen Yan-Xue , Mei Liang-Mo
Published: April 2011

Homogeneous amorphous MnxGe1−x:H films were synthesized under thermal nonequilibrium condition by magnetron co-sputtering technology with hydrogen in Ar atmosphere. Compared to the MnxGe1−x films without hydrogen, the MnxGe1−x:H films with hydrogen show higher concentration of hole carriers, larger conductivity, and higher saturation magnetization. Moreover, it was found that the anomalous Hall resistivity is proportional to the perpendicular magnetization. These electrical and magnetic properties indicate that the ferromagnetism of the MnxGe1−x:H films is intrinsic ferromagnetism mediated by the spin-polarized hole carriers.
Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field
Authors: Sladkov Maksym , Bakker M. P. , Chaubal A. U. , Reuter D. , Wieck A. D. , van der Wal C. H.
Published: April 2011

We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.
Properties of zinc oxide at low and moderate temperatures
Authors: Lashkarev G. V. , Karpyna V. A. , Lazorenko V. I. , Ievtushenko A. I. , Shtepliuk I. I. , Khranovskyy V. D.
Published: April 2011

The properties of zinc oxide are examined as an analog of gallium nitride over a wide range of temperatures and possible applications. Its economic and environmental advantages are noted, as well as its radiation hardness, compared to group III nitrides. Methods for growing films and nanostructures with high crystal perfection are proposed. In particular, a magnetron technique for layer-by-layer growth of films is implemented which makes it possible to obtain high structural perfection and substantial thicknesses unattainable by several other methods. The feasibility of producing monochromatic UV radiation from films excited by short-wavelength radiation and electrons is demonstrated; this means that they may be useable as short-wavelength radiation sources. Efficient field emission by ZnO nanostructures and films is demonstrated and opens up the prospect of their use in vacuum microelectronics equipment. Nitrogen-doped ZnO films, in particular, have been used to fabricate a phototransistor with a sensitivity two orders of magnitude higher than conventional detectors. The physical basis for creating LEDs for different colors based on ZnO films and solid solutions with CdO is discussed. The importance of studying the physics and technology of zinc oxide-based devices is emphasized.
Balanced terahertz wave air-biased-coherent-detection
Authors: Lu Xiaofei , Zhang X.-C.
Published: April 2011

A balanced heterodyne air-biased-coherent-detection scheme for broadband terahertz waves is achieved using third-order nonlinear susceptibility tensor elements to produce field-induced optical second harmonic photons, which have two orthogonal polarizations. The differential signal between two orthogonally polarized second harmonic photons reduces the common noise in a balanced detection geometry and improves the signal-to-noise ratio by a factor of 2.
pH-dependent x-ray absorption spectra of aqueous boron oxides
Authors: Duffin Andrew M. , Schwartz Craig P. , England Alice H. , Uejio Janel S. , Prendergast David , Saykally Richard J.
Published: April 2011

Near edge x-ray absorption fine structure (NEXAFS) spectra at the boron K-edge were measured for aqueous boric acid, borate, and polyborate ions, using liquid microjet technology, and compared with simulated spectra calculated from first principles density functional theory in the excited electron and core hole (XCH) approximation. Thermal motion in both hydrated and isolated molecules was incorporated into the calculations by sampling trajectories from quantum mechanics/molecular mechanics simulations at the experimental temperature. The boron oxide molecules exhibit little spectral change upon hydration, relative to mineral samples. Simulations reveal that water is arranged nearly isotropically around boric acid and sodium borate, but the calculations also indicate that the boron K-edge NEXAFS spectra are insensitive to hydrogen bonding, molecular environment, or salt interactions.
Interfacial properties of statistical copolymer brushes in contact with homopolymer melts
Authors: Trombly David M. , Pryamitsyn Victor , Ganesan Venkat
Published: April 2011

We use polymer self-consistent field theory to quantify the interfacial properties of random copolymer brushes (AB) in contact with a homopolymer melt chemically identical to one of the blocks (A). We calculate the interfacial widths and interfacial energies between the melt and the brush as a function of the relative chain sizes, grafting densities, compositions of the random copolymer in the brush, and degree of chemical incompatibility between the A and B species. Our results indicate that the interfacial energies between the melt and the brush increase (signifying expulsion of the free chains from the brush) with increasing grafting density, chemical incompatibility between A and B components, and size of the free chains relative to the grafted chains. We also compare the interfacial energies of random copolymers of different sequence characteristics and find that, except for the case of very blocky or proteinlike chains, blockiness of the copolymer has only little effect on interfacial properties. Our results for interfacial energies are rationalized based on the concept of an “effective volume fraction” of the brush copolymers, \documentclass[12pt]{minimal}\begin{document}$f_{\text{eff}}$\end{document}feff, which quantifies the chemical composition of the brush segments in the interfacial zone between the brush and melt copolymers. Using this concept, we modify the strong-stretching theory of brush–melt interfaces to arrive at a simple model whose results qualitatively agree with our results from self-consistent field theory. We discuss the ramifications of our results for the design of neutral surfaces.
Decomposition of unitary matrices for finding quantum circuits: Application to molecular Hamiltonians
Authors: Daskin Anmer , Kais Sabre
Published: April 2011

Constructing appropriate unitary matrix operators for new quantum algorithms and finding the minimum cost gate sequences for the implementation of these unitary operators is of fundamental importance in the field of quantum information and quantum computation. Evolution of quantum circuits faces two major challenges: complex and huge search space and the high costs of simulating quantum circuits on classical computers. Here, we use the group leaders optimization algorithm to decompose a given unitary matrix into a proper-minimum cost quantum gate sequence. We test the method on the known decompositions of Toffoli gate, the amplification step of the Grover search algorithm, the quantum Fourier transform, and the sender part of the quantum teleportation. Using this procedure, we present the circuit designs for the simulation of the unitary propagators of the Hamiltonians for the hydrogen and the water molecules. The approach is general and can be applied to generate the sequence of quantum gates for larger molecular systems.
Accurate band gaps of AlGaN, InGaN, and AlInN alloys calculations based on LDA-1/2 approach
Authors: Pelá R. R. , Caetano C. , Marques M. , Ferreira L. G. , Furthmüller J. , Teles L. K.
Published: April 2011

We present parameter-free calculations of electronic properties of InGaN, InAlN, and AlGaN alloys. The calculations are based on a generalized quasichemical approach, to account for disorder and composition effects, and first-principles calculations within the density functional theory with the LDA-1/2 approach, to accurately determine the band gaps. We provide precise results for AlGaN, InGaN, and AlInN band gaps for the entire range of compositions, and their respective bowing parameters.
Spin-torque switching window, thermal stability, and material parameters of MgO tunnel junctions
Authors: Devolder T. , Bianchini L. , Miura K. , Ito K. , Kim Joo-Von , Crozat P. , Morin V. , Helmer A. , Chappert C. , Ikeda S. , Ohno H.
Published: April 2011

We study the use of in-plane magnetized free layers with artificially lowered effective magnetization, in the context of spin-torque random access memories with magnetic tunnel junctions. We determine the field-voltage window for direct overwrite switching and thermal stability. We relate them to the magnetic constants extracted from the telegraph noise and high frequency noise exhibited by the junctions under specific conditions.
Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres
Authors: Zhu Jinfeng , Xue Mei , Shen Huajun , Wu Zhe , Kim Seongku , Ho Jyh-Jier , Hassani-Afshar Aram , Zeng Baoqing , Wang Kang L.
Published: April 2011

We present a plasmonic nanostructure design by embedding a layer of hexagonal periodic metallic nanospheres between the active layer and transparent anode for bulk heterojunction organic solar cells. The hybrid structure shows broadband optical absorption enhancement from localized surface plasmon resonance with a weak dependence on polarization of incident light. We also theoretically study the optimization of the design to enhance the absorption up to 1.90 times for a typical hybrid active layer based on a low band gap material.
Mirror symmetry in magnetization reversal and coexistence of positive and negative exchange bias in Ni/FeF2
Authors: Kovylina M. , Erekhinsky M. , Morales R. , Schuller I. K. , Labarta A. , Batlle X.
Published: April 2011

Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 ferromagnetic/antiferromagnetic (AF) heterostructures proceed through the same reversal mechanisms. The MR curves exhibit mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop, suggesting that the interfacial areal density of pinned uncompensated AF spins responsible for PEB and NEB is similar. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a reversal mechanism not previously reported.
Nonlinear variations in the electronic structure of II–VI and III–V wurtzite semiconductors with biaxial strain
Authors: Yang Shenyuan , Prendergast David , Neaton Jeffrey B.
Published: April 2011

Using first-principles calculations within many-body perturbation theory, we predict effects of biaxial strain on electronic band gaps and band edges of wurtzite III–V and II–VI semiconductor compounds. We find strain-induced changes in band gaps are large and highly nonlinear. Under both compressive and tensile biaxial strains, II–VI chalcogenide band gaps are predicted to decrease by as much as 0.6 eV for 10% strain; in contrast, III–V nitrides attain maximum gaps for compressive strains near 4%. Whereas nitrides tend to preserve covalent bond angle, more ionic chalcogenides tend to preserve bond length and volume, leading to qualitatively different trends in electronic structure.
Effects of porosity on dielectric and piezoelectric properties of porous lead zirconate titanate ceramics
Authors: Yang An-Kun , Wang Chang-An , Guo Rui , Huang Yong
Published: April 2011

We report porous lead zirconate titanate ceramics fabricated by tert-butyl alcohol-based gel-casting process which show a very high thickness electromechanical coupling coefficient (0.77), high hydrostatic figure of merit (9594×10−15 m2/N), and low acoustic impedance (3.7 Mrayls). We show that the porosity effectively affects the performance of the samples in two ways: (1) a higher porosity simplifies the resonance behavior, leading to more efficient energy transduction; (2) its replacement of active ceramic phase leads to low relative permittivity, high hydrostatic figure of merit, and low acoustic impedance. It was confirmed the properties could be tailored by controlling the porosity.
Effect of temperature and pressure on the structure, dynamics, and hydrogen bond properties of liquid N-methylacetamide: A molecular dynamics study
Authors: Pattanayak Subrat Kumar , Prashar Nidhi , Chowdhuri Snehasis
Published: April 2011

The structure and dynamical properties of liquid N-methylacetamides (NMA) are calculated at five different temperatures and at four different pressures using classical molecular dynamics simulations. Our results are analyzed in terms of pressure-induced changes in structural properties by investigating the radial distribution functions of different atoms in NMA molecule. It is found that the first peak and also the second peak of C–O and N–H are well defined even at higher temperature and pressure. It is also observed that the number of hydrogen bonds increase with application of pressure at a given temperature. On the other hand, the calculated hydrogen bond energy (EHB) shows that the stability of hydrogen bond decreases with increasing of pressure and temperature. Various dynamical properties associated with translational and rotational motion of neat NMA are calculated and the self-diffusion coefficient of NMA is found to be in excellent agreement with the experiment and the behavior is non-Arrhenius at low temperatures with application of pressures. The single particle orientational relaxation time for dipole vector and N–C vector are also calculated and it is found that the orientational relaxation time follows Arrhenius behavior with a variation of temperature and pressure.