bluuebunny/crossref_metadata_2025 huggingface dataset:
Wavelet transform approach to the analysis of specular x-ray reflectivity curves
Authors: Prudnikov I. R. , Matyi R. J. , Deslattes R. D.
Published: July 2002

A method for analyzing x-ray reflectivity curves from multilayered structures with interfacial roughness using a wavelet transform approach has been developed. By using this approach, we have been able (1) to extract the contribution of a particular rough interface to a specular reflectivity curve, and (2) to determine the root-mean-square amplitude of the roughness of a particular interface independently of the other interfaces in the multilayered structure from the specular reflectivity data. Analytical procedures that allow the interpretation of the wavelet coefficients obtained from specular reflectivity curves have been developed. This approach has been successfully applied to experimental reflectivity curves obtained from Cu/Ta, Ta2O5/Ta, and Ta2O5/Ta2N bilayer structures.
Photodissociation dynamics of the CH2Cl radical: Ion imaging studies of the Cl+CH2 channel
Authors: Dribinski V. , Potter A. B. , Demyanenko A. V. , Reisler H.
Published: July 2002

The photodissociation of the chloromethyl radical, CH2Cl, to chlorine atom and methylene is examined following excitation at selected wavelengths in the region 312–214 nm. CH2Cl is produced in a molecular beam by using pulsed pyrolysis. Cl and CH2 products are detected by laser ionization and their velocity and angular distributions are determined by using the ion imaging technique. The spectrum obtained by monitoring the Cl fragment yield as function of photolysis wavelength shows that throughout this wavelength region Cl atoms are major products. With 312–247 nm photolysis, the angular distributions are typical of a perpendicular transition (β=−0.7) and the main products are CH2(X̃ 3B1)+Cl(2P3/2). The available energy is partitioned preferentially into the translational degrees of freedom. “Hot band” transitions are prominent in this region even in the molecular beam indicating that the geometries of the ground and excited states of CH2Cl must be very different. With 240–214 nm photolysis, the angular distributions are typical of a parallel transition (β∼1.2), and the predominant products are Cl(2P3/2) and Cl(2P1/2), with CH2(ã 1A1) as the main cofragment. A large fraction of the available energy is partitioned into internal energy of CH2(ã 1A1). Comparison with the ab initio calculations of Levchenko and Krylov presented in the accompanying paper enables the assignment of the perpendicular and parallel transitions predominantly to 1 2A1←1 2B1 and 2 2B1←1 2B1 excitations, respectively, and both upper states are probably repulsive in the C–Cl coordinate. The electronic states of the products obtained via these two transitions are in agreement with the predictions of a simple diabatic state correlation diagram based on the calculated vertical energies of the upper states.
Study of a fast, high-impedance, high-voltage pulse divider
Authors: He W. , Yin H. , Phelps A. D. R. , Cross A. W. , Spark S. N.
Published: July 2002

A novel two-tube, two-stage, self-compensating resistive voltage divider with minimized stray capacitance using copper sulphate solution was explored to measure a fast high-impulse voltage output from a cable Blumlein generator. Self-compensation of the probe was achieved by matching the RC time constant in the two dividing stages. Stray capacitance of the probe was minimized by electric field contouring and matching methods. Experiments were carried out to investigate the probe responses as affected by self-capacitance, impedance, and stray capacitance. Calibration of the optimized probe showed a rise time of 15 ns at a load impedance of 8.68 kΩ with a hold-off voltage of 600 kV.
Fabrication and characterization of epitaxial NbN/MgO/NbN Josephson tunnel junctions
Authors: Kawakami Akira , Wang Zhen , Miki Shigehito
Published: July 2002

We fabricated epitaxial NbN/MgO/NbN Josephson tunnel junctions with good tunneling characteristics in the range of JC=0.2–70 kA/cm2. The counter and base NbN electrodes of the tunnel junctions had the same TC and 20 K resistivity at about 15.7 K and 60 μΩ-cm, respectively. X-ray analysis showed that all the layers that formed the tunnel junctions grew epitaxially. In the range of JC=0.2–15 kA/cm2, the tunnel junctions fabricated had large gap voltages (5.6–5.9 mV), narrow gap widths (less than 0.1 mV), high ICRN products (2.6–3.8 mV), and small subgap leakage current (Vm=40–96 mV).
Maxwell–Wagner piezoelectric relaxation in ferroelectric heterostructures
Authors: Damjanovic D. , Demartin Maeder M. , Duran Martin P. , Voisard C. , Setter N.
Published: July 2002

Using a serial bilayer model we show that heterogeneous ferroelectric systems may exhibit substantial Maxwell–Wagner piezoelectric relaxation. The relaxation has its origin in the coupling of the dielectric and piezoelectric properties. The model predicts both retardation (positive phase angle and counterclockwise charge–pressure hysteresis) and relaxation (negative phase angle and clockwise charge–pressure hysteresis) of the longitudinal direct piezoelectric coefficient. The theoretical results are confirmed experimentally in ferroelectric ceramic–ceramic bilayers and in a single-phase ferroelectric Aurivillius compound ceramic in which neighboring grains with strongly anisotropic properties may behave as basic Maxwell–Wagner units.
Polyelectrolyte–macroion complexation. II. Effect of chain flexibility
Authors: Jonsson Marie , Linse Per
Published: July 2002

A simple model with focus on the electrostatic interaction has been used to examine the complexation of a linear polyelectrolyte possessing variable flexibility with one or several oppositely charged macroions. Composition, structure, and thermodynamic properties of the complexes were obtained by using Monte Carlo simulations. Binding isotherms obtained were Langmuir-type with a quantitative binding up to a neutral complex with a plateau value corresponding to ∼50% overcharging, the largest overcharging appearing for the stiffest chain. Free energy calculations demonstrated that the complexation of the first macroion becomes less favorable as the chain stiffness is increased, whereas the opposite was found for a complexation of a macroion to a neutral complex. For a neutral complex and with a flexible chain, the repulsion between complexed macroions is strongly screened and the complexed macroions are located near each other. However, for a very stiff chain, the macroions are sequentially positioned along the chain with much fewer polyelectrolyte segments near them and with a considerable effective repulsion between the macroions. Furthermore, our results are discussed using the concept of an overcharged/undercharged complex with respect to both the polyelectrolyte and a macroion as the central species. The location of the small ions was also considered and in particular for a neutral system they are nearly homogeneously distributed in the solutions.
Vibrational relaxation in liquid chloroform following ultrafast excitation of the CH stretch fundamental
Authors: Sibert Edwin L. , Rey Rossend
Published: July 2002

Vibrational energy flow in liquid chloroform that follows the ultrafast excitation of the CH stretch fundamental is modeled using semiclassical methods. Relaxation rates are calculated using Landau-Teller theory and a time-dependent method both of which consider a quantum mechanical CHCl3 solute molecule coupled to a classical bath of CHCl3 solvent molecules. Probability flow is examined for several potentials to determine the sensitivity of calculated relaxation rates to the parameters that describe the model potentials. Three stages of relaxation are obtained. Probability is calculated to decay initially to a single acceptor state, a combination state of the solute molecule with two quanta of excitation in the CH bend and one in the CCl stretch, in 13–23 ps depending on the potential model employed. This is followed by rapid and complex intramolecular energy flow into the remaining vibrational degrees of freedom. During this second stage the lowest frequency Cl–C–Cl bend is found to serve as a conduit for energy loss to the solvent. The bottleneck for relaxation back to the ground state is predicted to be the slow 100–200 ps relaxation of the CH bend and CCl stretch fundamentals. Several aspects of the incoherent anti-Stokes scattering that follows strong infrared excitation of the CH fundamental as observed by Graener, Zürl, and Hoffman [J. Phys. Chem. B 101, 1745 (1997)] are elucidated in the present study.
Extending the epitaxial thickness limit in low-substrate-temperature- grown GaAs
Authors: Apostolopoulos G. , Boukos N. , Travlos A. , Herfort J. , Ploog K. H.
Published: July 2002

A method for extending the epitaxial thickness limit in low-temperature-grown GaAs (LT-GaAs) is presented. It is shown that the use of vicinal GaAs(001) substrates with a high misorientation angle reduces the surface roughness of LT-GaAs and inhibits the nucleation of defects which cause the breakdown of perfect epitaxial growth. Kinetic Monte Carlo simulations are used to describe the influence of the vicinal substrate on the growth mode and to estimate the appropriate misorientation angle.
Reaction of the Si/Ta/Ti system: C40 TiSi2 phase formation and in situ kinetics
Authors: Via F. La , Mammoliti F. , Grimaldi M. G.
Published: July 2002

The effect of a thin Ta layer at the Si/Ti interface on the intermediate phase formation has been studied in detail by in situ sheet resistance, X-ray diffraction and transmission electron microscopy of partially reacted samples. When a Ta layer is deposited at the Si/Ti interface, a new intermediate phase has been detected, i.e., the hexagonal TiSi2 C40. This phase grows on the C40 TaSi2 that is formed at the interface with silicon. The activation energies of the C40 formation (1.9±0.3 eV) and the C40–C54 phase transition (3.7±0.5 eV) have been determined and compared to the activation energies for the C49 (1.7±0.1 eV) formation and the C49–C54 (5.1±0.9 eV) transition. Both the transformation kinetics and the film morphology are consistent with an increase of the nucleation density with respect to the C49–C54 transition.
Design and simulation of terahertz quantum cascade lasers
Authors: Köhler Rüdeger , Iotti Rita C. , Tredicucci Alessandro , Rossi Fausto
Published: July 2002

Strategies and concepts for the design of THz emitters based on the quantum cascade scheme are analyzed and modeled in terms of a fully three-dimensional Monte Carlo approach; this allows for a proper inclusion of both carrier–carrier and carrier–phonon scattering mechanisms. Starting from the simulation of previously published far-infrared emitters, where no population inversion is achieved, two designs are proposed. The first one follows the well-established chirped-superlattice scheme whereas the second one employs a double-quantum well superlattice to allow energy relaxation through optical phonon emission. For both cases a significant population inversion is predicted at temperatures up to 80 K.
Electron microscopy and electron energy loss spectroscopy studies of carbon fiber formation at Fe catalysts
Authors: Blank V. D. , Kulnitskiy B. A. , Batov D. V. , Bangert U. , Gutiérrez-Sosa A. , Harvey A. J.
Published: July 2002

Nanofibers were produced by the chemical vapor deposition method in the presence of Fe-catalyst particles at temperatures of 700–850 K by disproportionation of CO. Electron diffraction and high resolution electron imaging as well as electron energy loss spectroscopy and x-ray analysis techniques were used to study the nanofiber formation in different places at the catalyst surface. The particles enclosed by the fibers were found to be Fe7C3 and Fe3C. Crystallographic relationships between deposit and particle were established. The structural properties of the deposit were found to be dependent on the position at the particle surface. Graphitic growth was favored at certain facets. At the particle tip graphene sheet formation competed with Fe2O3 oxide formation. We also report the occurrence of a low loss feature between 3 and 4 eV, concurrent with the dispersion of an interface plasmon at the graphite/Fe2O3 interface.
Optimization of external coupling and light emission in organic light-emitting devices: modeling and experiment
Authors: Lu M.-H. , Sturm J. C.
Published: July 2002

The emission of light and external coupling after the appropriate excitons have been formed in the organic light-emitting devices (OLEDs) has been investigated. The internally emitted light can be classified into three modes: externally emitted, substrate waveguided, and indium–tin–oxide (ITO)/organic waveguided. A combined classical and quantum mechanical microcavity model is used to calculate the distribution of light emission into these three modes in an OLED on planar substrates. The ITO/organic modes maybe suppressed due to the thinness of the ITO/organic layers. Consequently, as much as over 50% of the internally generated light is emitted externally in some structures, much greater than the ∼20% figure given by classical ray optics. This model is used to examine how this distribution varies with exciton to cathode distance, the thickness of the ITO layer, and the index of refraction of the substrate. It can also be applied to OLEDs on shaped substrates where an increase in the total external emission up to a factor of 2.3 has been demonstrated. The numerical results agree well with experimentally measured far-field intensity profiles, edge emissions, and increase in external emission due to shaped substrates. Finally, based on these results, we discuss different approaches to device optimization, depending on the fluorescence efficiency of the emitter and whether a shaped substrate is used.
Simulation of photonic band gaps in metal rod lattices for microwave applications
Authors: Smirnova E. I. , Chen C. , Shapiro M. A. , Sirigiri J. R. , Temkin R. J.
Published: July 2002

We have derived the global band gaps for general two-dimensional (2D) photonic band gap (PBG) structures formed by square or triangular arrays of metal posts. Such PBG structures have many promising applications in active and passive devices at microwave, millimeter wave, and higher frequencies. A coordinate-space, finite-difference code, called the photonic band gap structure simulator (PBGSS), was developed to calculate complete dispersion curves for lattices for a series of values of the ratio of the post radius (r) to the post spacing (a). The fundamental and higher frequency global photonic band gaps were determined numerically. These universal curves should prove useful in PBG cavity design. In addition, for very long wavelengths, where the numerical methods of the PBGSS code are difficult, dispersion curves were derived for the transverse-magnetic (TM) mode by an approximate, quasi-static approach. Results of this approach agree well with the PBGSS code for r/a<0.1. The present results are compared with experimental data for transverse-electric (TE) and TM mode PBG resonators built at Massachusetts Institute of Technology (MIT) and the agreement is found to be very good.
Structural and optical properties of ZnO films grown on R–Al2O3 substrates
Authors: Zhang B. P. , Segawa Y. , Wakatsuki K. , Kashiwaba Y. , Haga K.
Published: July 2002

Structural and optical properties of ZnO films grown on R–Al2O3 substrates by atmospheric pressure chemical-vapor deposition were investigated using x-ray diffraction and photoluminescence. The (112̄0) plane of the ZnO film tilted 0.3° with respect to the (11̄02) plane of the substrate and rotated about 7° around the normal of the sample surface. Symmetric (112̄0) and asymmetric (202̄2) x-ray reflection on ZnO films with different thicknesses were carried out. Comparison with photoluminescence measurements allowed us to conclude that the optical properties of the ZnO films are predominately determined by the in-plane, rather than out-of-plane, structural features.
Equation of state of bulk metallic glasses studied by an ultrasonic method
Authors: Wang Wei Hua , Wen Ping , Wang Li Min , Zhang Yong , Pan Ming Xiang , Zhao De Qian , Wang Ru Ju
Published: July 2002

The acoustic velocities and their pressure dependence of various Zr- and Pd-based bulk metallic glasses (BMGs) have been measured by using a pulse echo overlap method. The elastic constants as well as their pressure dependence of the BMGs have been determined, and the equation of state of these BMGs was obtained and compared to that of other glasses and crystalline solids. The structural characteristic of the BMGs is discussed.
Two-body zeroth order Hamiltonians in multireference perturbation theory: The APSG reference state
Authors: Rosta Edina , Surján Péter R.
Published: July 2002

A special version of multi-reference perturbation theory is investigated which differs from standard ones by using a zeroth order Hamiltonian that contains two-electron terms explicitly. The method is applicable to reference states that can be written as an antisymmetrized product of two or more electron functions. In that case the zeroth order Hamiltonian has a well defined physical meaning and the matrix elements that come about can be evaluated in an efficient manner. We implemented the theory for the antisymmetrized product of strongly orthogonal geminals wave function and, as a special case, for the generalized valence bond. Illustrative calculations on sample molecules show the reliability of the approach, as well as a significant improvement in many cases compared to MRPT versions based on one-body zeroth order Hamiltonians.
Effect of free surface heat loss and rotation on transition to oscillatory thermocapillary convection
Authors: Sim Bok-Cheol , Zebib Abdelfattah
Published: July 2002

Oscillatory thermocapillary convection in an open cylindrical annulus heated from the inside wall is investigated numerically. Results for aspect ratio of 1 and Prandtl number of 30 are obtained to compare the simulations with available space experiments. The influence of surface heat loss as quantified by a Biot number (Bi) or spacecraft or orbital rotation through the Coriolis force, as quantified by a vector Taylor number of magnitude Ta, on the critical Reynolds number (Rec) and pattern of convection is investigated. The flow is steady and axisymmetric at sufficiently low values of Re. In the absence of rotation, we observe a two-lobed, isotherm pattern travelling clockwise at the free surface near Rec. A two- or three-lobed pulsating or a three-lobed rotating pattern is observed with increasing Re beyond Rec. Isotherm patterns on the free surface are in good agreement with experimental results. With Ta less than 1, which is relevant to the experiments, the effect of rotation on Rec and flow field is small. Here Rec is found to decrease with increasing Ta. Heat loss from the free surface stabilizes the flow. Increasing Bi leads to an increase in Rec and a small decrease in the critical frequency. The critical parameters and their dependence on the container size are in good agreement with the experiments, assuming Bi⩽20.
Optimization of plasma grid material in cesium-seeded volume negative-ion sources
Authors: Kashiwagi M. , Morishita T. , Okumura Y. , Taniguchi M. , Hanada M. , Watanabe K. , Krylov A.
Published: July 2002

In cesium-seeded hydrogen negative-ion sources, surface production on the plasma grid plays an important role in negative ion production. To enhance the surface, it is required to use material that would give a lower work function when Cs is absorbed on the surface. In a semicylindrical and cesium-seeded volume negative-ion source, eight materials (W, Cu, Mo, V, Cr, Ni, Ag, and Au) were tested as candidates for the plasma grid material. To avoid deposition of the cathode material on these materials, a filament-free plasma source was used, to fire the microwave (2.45 GHz) discharge in the Kamaboko source. The material surface was examined by measuring the photoelectron current by laser irradiation. It was observed that the discharge enhanced the photoelectron current when the material was biased negatively to the plasma potential during discharge. In the present experiment, Ni, Au, and Ag surfaces with a Cs layer showed a higher photoelectron current than the others. This was 1.5 times larger than that of Cu and W used as a plasma grid and filament in conventional high current negative ion sources. It is expected that higher negative-ion production efficiency would be obtained by using Ni, Ag, and Au as the plasma grid material, if deposition of filament materials is avoided on the surface.
Charge state breeding applications with the ECR PHOENIX source: From low to high current production
Authors: Lamy T. , Bouly J. L. , Curdy J. C. , Geller R. , Lacoste A. , Sole P. , Sortais P. , Thuillier T. , Vieux-Rochaz J. L. , Jayamanna K. , Olivo M. , Schmor P. , Yuan D.
Published: July 2002

The electron cyclotron resonance (ECR) charge state breeding method based on the PHOENIX source is first dedicated to the continuous high charge state radioactive ion production for cyclotrons, due to their q/m requirements, and to the high charge state production efficiency. It can be used as an injector for RFQ, linacs, and synchrotrons either in cw or pulsed operation. The efficiency of the 1+ to the n+ ion production and the charge breeding time are the fundamental parameters of the method, their dependence with regard to the 1+ ion beam emittance injected has been measured and will be discussed with regard to the radioactive ion beam acceleration. The efficiency of the process slightly decreases when injecting ion beam emittances higher than 30 π mm mrad, and remains constant in the 1+ radioactive intensity range available today and in the future. The ECR ion trap tuning associated with the afterglow mode permits the ionization, the accumulation, and the extraction of the injected primary beam keeping interesting efficiencies suitable to pulsed accelerators.
Combining multideterminantal wave functions with density functionals to handle near-degeneracy in atoms and molecules
Authors: Pollet R. , Savin A. , Leininger T. , Stoll H.
Published: July 2002

Control of near-degeneracy effects and dynamical correlation in atoms and molecules is within sight, thanks to an economical method that mixes configuration interaction (CI) and density functional theory (DFT). The influence of the size of the configuration-space has been studied for light systems including elements of the first and second period of the Periodic Table.
Ruthenium-induced enhanced magnetization and metal–insulator transition in two-dimensional layered manganites
Authors: Sahu Ranjan K. , Mohammad Qureshi , Rao Manju L. , Sundar Manoharan S. , Nigam A. K.
Published: July 2002

A strong correlation between magnetism and transport behavior is observed in ruthenium (Ru) substituted two-dimensional layered manganites, La1.2Ca1.8Mn2O7, which show an increase in the Curie temperature and the metal–insulator transition (MIT) by 15–20 K up to x=0.2, due to a dominant double-exchange mediated transport, aided by the presence of a Ru (V) ion, which is metrically and chemically similar to the Mn (IV) ion. Beyond x=0.225, Tc and the metal–insulator transition decrease marginally, predominantly controlled by (i) hole carrier concentration and (ii) spin-wave excitation with a T2 dependence. The magnetoresistance ratio is not affected significantly by Ru doping in the vicinity of the MIT, indicating the double-exchange interaction between Ru and Mn.
Numerical investigation of thermophoretic effects on cluster transport during thermal plasma deposition process
Authors: Han P. , Yoshida T.
Published: July 2002

Cluster transport and deposition processes under the effects of thermophoresis are numerically investigated in terms of thermal plasma deposition processes. The cluster transport equation is solved within a prescribed flow field, where it is assumed that clusters with a given uniform size are generated within the boundary layer. Cluster concentration and deposition flux are calculated for clusters in the size range of 1–6 nm. Results are also given for comparative cases without thermophoresis effects. It is found that the thickness of the concentration boundary layer is significantly suppressed by the thermophoretical force. The effect of thermophoresis plays a more dominant role than that of diffusion, thus an almost uniform deposition efficiency is achieved for clusters of different sizes.
Extraction of edge radiation within a straight section of Aladdin
Authors: Bosch R. A.
Published: July 2002

Edge radiation may be extracted by a slotted mirror located within a straight section of the Aladdin electron storage ring. For wavelengths exceeding 1 mm, this method is expected to provide a greater flux than a large aperture located downstream of a straight section or bending magnet source. The edge radiation within an Aladdin straight section may find application as a broadband source for wavelengths of 1 μm–60 mm, or as a diagnostic of the bunch length through coherent emission.
Rotationally resolved quenching and relaxation of CH(A2Δ,v=0,N) in the presence of CO
Authors: Meden P. , Kind M. , Stuhl F.
Published: July 2002

Kinetic properties of the single rotational states 2⩽N⩽8 of the electronically excited CH(A2Δ,v=0) radical have been studied in the gas phase at room temperature in the presence of CO. Rate constants of the state-to-state relaxation are presented. Further, rate constants were determined for the electronic quenching of single N states and compared with data recently reported by Cerezo and Martin [J. Photochem. Photobiol., A 134, 127 (2000)]. The radiative lifetimes of the rotational levels are given, too.
Dynamics of the axisymmetric core-annular flow. II. The less viscous fluid in the core, saw tooth waves
Authors: Kouris Charalampos , Tsamopoulos John
Published: July 2002

The nonlinear dynamics of the concentric, two-phase flow of two immiscible fluids in a circular tube is studied when the viscosity ratio of the fluid in the annulus to that in the core of the tube, μ, is larger than or equal to unity. For these values of the viscosity ratio the perfect core-annular flow (CAF) is linearly unstable and it is necessary to keep the ratio of the thickness of the annulus to the radius of the tube small so that the solutions remain uniformly bounded. The simulations are based on a pseudospectral numerical method while special care has been taken in order to minimize as far as possible the effect of the boundary conditions imposed in the axial direction allowing for multiple waves of different lengths to develop and interact. The time integration originates with the analytical solution for the pressure driven, perfect CAF or the perfect CAF seeded with either the most unstable mode or random disturbances. Quite regular wave patterns are predicted in the first two cases, whereas multiple unstable modes grow and remain even after saturation of the instability in the last case. The resulting waves generally travel in the same direction and faster than the undisturbed interface, except for the case with μ=1 for which they are stationary with respect to it. Depending on parameter values, waves move with the same velocity or interact with each other exchanging their amplitudes or merge and split giving rise to either chaotic or organized solutions. For fluids of equal viscosities and densities (μ=ρ=1) and for a Reynolds number, Re(≡Λρ̂1R̂2Ŵ0/μ̂1)=0.0275 and an inverse Weber number, W(≡T̂/(ρ̂1Ŵ02R̂2))=145.4, both based on the properties of the inner fluid, the tube radius, R̂2, and the average flow velocity, Ŵ0, small amplitude waves are predicted. The increase of μ by almost two orders of magnitude does not affect their amplitudes, but increases their temporal period linearly. Varying W by more than three orders of magnitude increases their amplitudes proportionately, while their period increases with the logarithm of W. Similar to that is the effect of increasing Re. The present analysis confirms and extends results based on long wave expansions, which lead to the Kuramoto–Sivashinsky equation and modifications of it.
Scattering mechanisms and electronic behavior in transparent conducting ZnxIn2Ox+3 indium–zinc oxide thin films
Authors: Marcel C. , Naghavi N. , Couturier G. , Salardenne J. , Tarascon J. M.
Published: July 2002

Thin films of ZnxIn2Ox+3 (x=2, 3, 4, 5, 7, 9, 11, 13, and 15) were grown by pulsed laser deposition and their electronic properties characterized. Through resistivity, Hall mobility and carrier concentration measurements as a function of temperature, we found that whatever the composition, these films behave as n-type degenerate semiconductors exhibiting a metal-like behavior. Transport properties are driven by two types of electronic scattering, namely ionized impurities (oxygen vacancies) at low temperature and optical phonons at high temperature. Modeling of optical transmittance spectra in the visible-infrared region yields two different trends among the different compositions: for (x=2, 3, 4, 5, 7, and 9) values the electronic behavior can be described by applying the single Drude model on a plasma of nearly free carriers, whereas for zinc-rich (x=11, 13, and 15) compounds a supplementary Lorentz oscillator is necessary to simulate the spectroscopic data, revealing the presence of bound optically active electrons. Good agreement is found in every case between the experimental Hall mobility and the “optical” mobility calculated from relaxation time and effective mass fitted parameters. We show that the mobility drop can be explained primarily by a drastic increase in the effective mass for the highest x values.
Mössbauer spectroscopic and x-ray diffraction studies of Fe/SiO2 nanocomposite soft magnetic materials
Authors: Yang D. P. , Zhang Y. D. , Hui S.
Published: July 2002

Nanocomposite high resistive soft magnetic materials Fe/SiO2 (Fe volume fraction 50%) have been synthesized using a wet chemical reaction method. A series of metal-ceramic Fe/SiO2 nanocomposite powder samples were obtained by annealing the precursor at temperatures from 400 to 900 °C in the presence of a reducing agent. The compositions of the precursor and the successive heat-treated samples have been investigated by Fe57 Mössbauer spectroscopy and x-ray diffraction, which revealed that the annealing process reduces nanosize granular ferrihydrite to α-Fe, and indicated that 800 °C is the optimum annealing temperature.
Microstructures and surface step-induced antiphase boundaries in epitaxial ferroelectric Ba0.6Sr0.4TiO3 thin film on MgO
Authors: Jiang J. C. , Lin Y. , Chen C. L. , Chu C. W. , Meletis E. I.
Published: July 2002

Ba 0.6 Sr 0.4 TiO 3 thin films were epitaxially grown on (001) MgO substrates using pulsed laser ablation. Cross-sectional and plan-view transmission electron microscopy have been employed to study the microstructures and the interface behavior of the as-grown thin films. The 110-nm-thick Ba0.6Sr0.4TiO3 thin films have a flat surface and sharp interface. The entire thin film has a single-crystal cubic structure with an interface relationship of (001)Ba0.6Sr0.4TiO3//(001)MgO and 〈100〉Ba0.6Sr0.4TiO3//〈100〉MgO with respect to the substrate. The 6.4% lattice mismatch between Ba0.6Sr0.4TiO3 and MgO was completely released at the interface by forming equally spaced misfit dislocations with a distance of ∼3.2 nm. High-resolution transmission electron microscopy investigation shows that the initial grown layer of the film is the TiO2 monolayer. The growth models of “substrate surface-terrace induced defects” for perovskite on a rock-salt system have been developed to understand the as-grown features where the conservative and nonconservative antiphase boundaries can be formed.
Ru
x
Cr
1−x
/
Ta
underlayer for Co-alloy perpendicular magnetic recording
Authors: Wierman K. W. , Klemmer T. J. , Lu B. , Ju G. , Howard K. J. , Roy A. G. , Laughlin D. E.
Published: July 2002

The effects of the RuxCr1−x/Ta underlayer on the microstructural and magnetic properties of CoCrPtB perpendicular films were investigated. The hcp RuxCr1−x (0002) texture was observed to grow perpendicular to the film plane with narrow rocking curves of 2°–3°. High-resolution transmission electron microscopy indicated epitaxial growth of the CoCrPtB on top of the RuxCr1−x/Ta underlayer. In-plane x-ray diffraction scans indicate that as the Cr atomic composition increases in the RuxCr1−x underlayer, the a-lattice parameter was found to contract more closely matching the CoCrPtB a-lattice parameter [measured with (112̄0)]. In addition, the rocking curves for the CoCrPtB (0002)-texture induced by the (0002) textured RuxCr1−x buffer showed narrowing peaks (4.5°–3.5°) as the Cr concentration increased from 0% to 40%.
High-performance nanocrystalline PrFeB-based magnets produced by intensive milling
Authors: Bollero A. , Gutfleisch O. , Müller K.-H. , Schultz L. , Drazic G.
Published: July 2002

The intensive milling technique has been used to produce nanocrystalline isotropic PrFeB-based powders. Highly coercive powders were obtained using the base composition R16T76B8 (R: rare earth, T: transition metal) with Dy and Zr additions achieving a high value of μ0Hc=2.66 T for Pr15Dy1Fe75.9B8Zr0.1. The influence of Dy and Zr, and the substitution of Pr by Nd, on the microstructural and magnetic properties have been studied. Reduction of the R content, increase of the T content, and the presence of Co in the starting composition gave rise to an improved performance with values of Jr=0.92 T, μ0Hc=1.25 T, and (BH)max=140 kJ/m3 for Pr9Nd3Dy1Fe72Co8B6.9Zr0.1, originating from a very fine microstructure with a mean grain size of 20 nm. Furthermore, intensive milling has been shown to be a very versatile technique to produce high-performance nanocomposite magnets by blending this latter alloy with different fractions of soft magnetic α-Fe (x=5–35 wt % Fe). A 25 wt % Fe addition resulted in an optimum combination of magnetic properties with a very high (BH)max value of 178 kJ/m3 due to an effective exchange coupling between the hard- and soft-magnetic phases. A Curie temperature of about 370 °C was observed for this magnet. Demagnetization recoil loops of both single-phase and nanocomposite magnets showed clear differences with relatively open minor loops in the case of the latter due to the exchange-spring mechanism present. δJ plots for the nanocomposite magnets showed a negative deviation of the demagnetizing remanence from the Wohlfarth model, indicative of exchange-coupling interactions being dominant.
Evolution of magnetic coupling in ferromagnetic tunnel junctions by annealing
Authors: Schmalhorst J. , Brückl H. , Reiss G. , Gieres G. , Wecker J.
Published: July 2002

In spin valve and magnetic tunnel junction systems, the soft sensing layer should be sensitive to small fields of a few hundred A/m and should not be influenced by the adjacent magnetic hard layer. In polycrystalline films, however, congruent interface roughness invokes a ferromagnetic coupling, which can be compensated by an antiferromagnetic magnetostatic dipole coupling in patterned elements. In this paper, we demonstrate the evolution of the magnetic interlayer interaction by annealing in tunnel junctions with Al2O3 barrier. Whereas the minor loop shifting field of the soft magnetic NiFe sense layer is constant or varies only slightly from 2 kA/m to 1.15 kA/m for 1–1.5 nm thick barriers, an eightfold increase of the coercivity is found after annealing above 350 °C. Microstructural investigations give evidence that small ferromagnetic clusters arise in the hard layer of the junctions (Cu/CoFe/Ru/CoFe) by diffusion of Cu, Co, and Fe. The density and the stray field of these clusters are determined by magnetic force measurements and can be correlated to the increased switching field strength.
Electron field emission from diamond-like carbon films and a patterned array by using a Ti interfacial layer
Authors: Mao D. S. , Liu X. H. , Wang X. , Zhu W.
Published: July 2002

Electron field emission from diamond-like carbon (DLC) films deposited on Si, Ti/Si, and Au/Si substrates by a filtered arc deposition technique was studied. As compared to DLC/Si and DLC/Au/Si, electron field emission from DLC/Ti/Si was enhanced, showing an increased emission current density and emission site density (∼1.2×103/cm2). An emission site density up to 2.2∼2.2×103/cm2 was obtained after the DLC/Ti/Si had been annealed at 430 °C for 0.5 h. A patterned DLC/Ti/Si array fabricated by the oxygen reactive ion beam etching technique showed further field emission enhancement. An emission site density up to 3.2∼3.5×103/cm2 and a threshold field as low as 2.1 V/μm were achieved. It was shown that the low potential barrier at the interface and high local geometric electric field enhancement around the edges produced by reactive ion beam etching were possible causes of the enhancing effects. It could also be explained by Geis’ metal-diamond-vacuum triple junction emission mechanism.
Stress dependence of soft, high moment and nanocrystalline FeCoB films
Authors: Minor M. Kevin , Crawford T. M. , Klemmer Timothy J. , Peng Yingguo , Laughlin David E.
Published: July 2002

Soft, high moment materials are crucial for the magnetic data recording industry in applications such as write poles and soft underlayers. Ever increasing areal densities have pushed the requirements for materials exhibiting saturation magnetizations above 2 T. In this work we investigate nanocrystalline FeCoB which exhibits a saturation magnetization of ∼2.2 T. FeCoB exhibits a magnetostriction of >50×10−6 which suggests that the magnetoelastic anisotropy is important. Nanocrystalline FeCoB films are rf diode sputtered as a function of Ar pressure. As the pressure is increased, film stress increases from compressive to tensile with a critical pressure of ∼12 mTorr. TEM and XRD show that pressure has no significant effect on microstructure, however, at pressures above a critical pressure, the FeCoB films change from magnetically isotropic to magnetically uniaxial with low coercivity. This is due entirely to the magnetoelastic anisotropy which must be considered as the industry begins to utilize magnetic alloys with nonzero magnetostriction.
Hydrogen response mechanism of Pt–GaN Schottky diodes
Authors: Schalwig J. , Müller G. , Karrer U. , Eickhoff M. , Ambacher O. , Stutzmann M. , Görgens L. , Dollinger G.
Published: July 2002

Besides silicon carbide, group-III nitrides are also suitable large-band-gap semiconductor materials for high-temperature gas sensor devices. Exposing GaN-based Schottky diodes with catalytically active platinum electrodes to hydrogen, we observed a decrease of the rectifying characteristics which we attribute to a decrease in Schottky barrier height. Current–voltage and elastic recoil detection measurements were used to investigate the H-sensing behavior of such devices. Our results indicate an interfacial effect as the origin of the sensor response to hydrogen.
Nonlinear dynamics of spin-injected magnons in magnetic nanostructures
Authors: de Aguiar F. M. , Azevedo A. , Rezende S. M.
Published: July 2002

It has recently been proposed that spin waves can be critically excited in a very thin unbounded single magnetic metal film by a bounded spin-polarized dc current. Here it is shown that, beyond threshold, the interaction between the driven modes may lead to high-frequency oscillatory instabilities and universal chaotic phenomena. Numerical solutions are presented for the resulting equations of motion that describe the dynamics of two coupled modes in a spin-injection field, showing cascades of period-doubling bifurcations, intermittency, and the breakup of a two torus, for a realistic set of phenomenological parameters.
Inhibition of abnormal acicular grain growth of nanostructured Ba–ferrite films by secondary α-Fe2O3 phase
Authors: Doh Seok Joo , Choi Seung Dueg , Je Jung Ho , Cho Tae Sik
Published: July 2002

We have studied the inhibition of abnormal acicular grain growth of the Ba–ferrite phase in the crystallization of Ba–ferrite films using real time synchrotron x-ray scattering, field emission scanning electron microscope, and vibrating sample magnetometer. Amorphous Ba–ferrite film on a SiO2 substrate is naturally crystallized into the abnormal acicular Ba–ferrite grains (major axis of ∼1000 nm in the in-plane direction). However, on α-Al2O3 substrate, fine grain sizes of the primary Ba–ferrite phase (∼70 nm) are obtained during crystallization. Nucleation of the α-Fe2O3 phase on the α-Al2O3 surface at an early stage induces the inhibition of the abnormal acicular grain growth of the Ba–ferrite phase. Because of the grain size refinement of the magnetic Ba–ferrite phase, the intrinsic coercivity of Ba–ferrite/α-Al2O3 film enhances 4.4 kOe, much larger than that of Ba–ferrite/SiO2 film (1.9 kOe). We suggest that the secondary α-Fe2O3 phase act as a useful inhibitor to abnormal grain growth in Ba–ferrite films for high-density recording media.
Effects of two in-plane fields on the magnetization reversal mechanism in magnetic tunnel junction elements
Authors: Shang Ping , Petford-Long Amanda K. , Anthony Thomas C.
Published: July 2002

One of the potential applications of the spin tunnel junction is as magnetic random access memory (MRAM) elements. For MRAM application purposes, two in-plane perpendicularly applied fields are required during magnetization reversal. In this article, Lorentz transmission electron microscopy has been used to study the magnetization reversal mechanism of tunnel junction elements under the influence of two in-plane perpendicular fields. Four hundred NiFe/MnFe/NiFe/Al2O3/NiFeCo elements with differing shapes and sizes have been observed. When an in-plane Y field was applied to the hard axis followed by sweeping the X field along the easy axis, it was found that the number of elements in which 360° domain walls are observed to form decreases as the strength of the Y field increases, and that the magnetization reversal mechanism in the tunnel junction elements gradually changes from a domain wall motion mechanism to a moment rotation mechanism as the Y field increases. As expected, the reversal field (X field) reduces as the Y field increases.
High thermal stability in CoPrTb/CoCrTa composite perpendicular media
Authors: Shimatsu T. , Muraoka H. , Nakamura Y. , Sonobe Y. , Satodate Y. , Muramatsu K. , Watanabe I.
Published: July 2002

Thermal stability and media noise performance of the CoPrTb/CoCrTa composite perpendicular media are discussed. Angular dependence of remanance coercivity and the ΔM plot indicate that magnetization reversal in the CoPrTb/CoCrTa composite media is mainly proceeded by local switching of magnetization, while the magnetization loop of the composite media show very steep slope and large squareness Mr/Ms compared to those of CoCrTa media. Recording performance of (CoPrTb/CoCrTa)/Ti/CoZrNb double-layered perpendicular media was examined using a single-pole-type write head and a GMR read head. The recording resolution, D50, of a CoPrTb(10 nm)/CoCrTa(20 nm) composite medium is about 50 kfrpi higher than that of CoCrTa medium, and the signal to medium noise ratio, S/Nm, was about 10 dB higher than that of the CoCrTa medium (at 300 kfrpi). The CoPrTb/CoCrTa composite media show very high thermal stability of read back signals even at low recording density, although the value of Mr/Ms is smaller than 0.9. The continuous CoPrTb layer is assumed to prevent the thermal reversal of individual grains of the CoCrTa layer by the interfacial exchange coupling.
Micromagnetics of perpendicular write heads with extremely small pole tip dimensions
Authors: Bai Daniel Z. , Zhu Jian-Gang
Published: July 2002

A three-dimensional micromagnetic model has been developed to study single pole write heads with soft magnetic underlayer (SUL) for perpendicular recording at extremely small track widths. The study points out that at any practical ABS-to-SUL spacing, the maximum head field within the recording media at deep submicron track width and pole-tip thickness becomes significantly smaller than the ideal value of 4πMs. Head field gradient in the medium also degrades with decreasing track width. Maintaining the conventional design, a significantly large remanent head field develops for track width below 100 nm, presenting danger of erasing previously recorded bits. The origin of the large remanent head field is the failure for a pole tip to reach a completely demagnetized state due to the overwhelming ferromagnetic exchange energy at the deep submicron pole-tip dimensions.
Microwave, structural, and magnetic properties of Cu/Fe/CoFe/Cu
Authors: Lubitz P. , Cheng S. F. , Rachford F. J. , Miller M. M. , Harris V. G.
Published: July 2002

The structure and the static and dynamic magnetic properties of pure Fe films with a surface overlayer of Co9Fe1 were studied. These structures are potential components of spin-valve or tunneling devices in which small magnetic damping, large moment, low anisotropy and high spin polarization may be advantageous. The films are polycrystalline and have Cu under and over layers. The Fe layers studied are from 3 to 20 nm thick and the CoFe layer was usually 1 nm. With a CoFe overlayer we found a range of Fe thicknesses from below 4 to near 6 nm in which low coercivity and narrow ferromagnetic resonance (FMR) linewidth resulted. Both below and above this range the properties degraded, apparently because the 2 nm Cu top layer was inadequate to protect the Fe against oxidation. Using extended x-ray absorption fine structure, we found only bcc Fe; atomic force microscopy shows a systematic decrease in roughness with increasing thickness of Fe, which may explain the magnetic hardness for the thinnest films. Fe (5 nm)/CoFe (1 nm), with a Gilbert α∼0.004, has FMR linewidths about 2/3 those of Permalloy films of comparable thickness. For some applications investigated, distinct advantages can be obtained using the high Q of the ferromagnetic system.
Nondestructive evaluation of reactor pressure vessel steels using the giant magnetoimpedance sensor
Authors: Kim D. J. , Park D. G. , Hong J. H.
Published: July 2002

Recently, the giant magnetoimpedance (GMI) effect found in amorphous wires has been noticed as a method for sensing a magnetic field. The GMI sensor was applied to nondestructive evaluation of microstructural changes for reactor pressure vessel steels passing through the refining process. They were measured by using a GMI sensor and the measured GMI signals were strongly influenced by the microstructural features. The signals are closely related to the grain size, carbide morphology, lath width, and lath boundary that act as a barrier to irreversible domain wall motion.
Laser induced fluorescence spectra of the D 1∑u+→B′ 1∑g+ and C 1Πg→A 1∏u systems of C2 in solid Ne
Authors: Wakabayashi Tomonari , Ong Aik-Loong , Krätschmer Wolfgang
Published: July 2002

In our study of carbon vapor molecules trapped in Ne matrices at 6 K, we observed laser induced fluorescence spectra of the D 1∑u+→B′ 1∑g+ system of C2 upon excitation of the Mulliken transition D 1∑u+←X 1∑g+ of C2 at 232 nm. A vibrational progression was clearly observed going from the upper vibrational v′=0 level of the electronic D state into several lower v″=0–6 levels of the B′ state. We found that the progression spans from 359 to 517 nm with a large Franck–Condon shift showing the intensity maximum for the 0–2 or 0–3 transition. The vibrational constants for the B′ state were derived as ωe=1427 cm−1 and ωexe=2.1 cm−1. We also observed the C→A and the Swan band emissions, showing that from the excited D state also additional states were populated by internal conversion and intersystem crossing. The constants for the A state were derived as ωe=1613.5 cm−1 and ωexe=18.5 cm−1. Fluorescence excitation spectra of the D→B′ vibronic transitions were found to reproduce well the relatively narrow absorption feature of the Mulliken D←X system. We discuss the entirely different excitation spectra occurring in argon matrices in terms of matrix-site effects. Each vibronic band of the D→B′ progression shows fine structures probably due to translations and librations of C2 molecules coupled with the surrounding lattice of Ne atoms.