bluuebunny/crossref_metadata_2025 huggingface dataset:
Deuterium in (Ba,Sr)TiO3 thin films: Kinetics and mechanisms of incorporation and removal during annealing
Authors: McIntyre P. C. , Ahn J-H. , Becker R. J. , Wang R-V. , Gilbert S. R. , Mirkarimi L. Wills , Schulberg M. T.
Published: July 2002

The kinetics of deuterium doping of and removal from polycrystalline (Ba,Sr)TiO3 (BST) thin films during annealing were investigated using secondary ion mass spectrometry depth profiling, and the data were correlated to changes in the electrical behavior of the films. Results for deuterium doping of exposed BST films on a Pt bottom electrode layer are consistent with incorporation of deuterium interstitial defects at the BST/Pt interface and “upward” diffusion toward the film surface. The incorporation kinetics of deuterium in Pt/BST/Pt capacitors are more complex and are greatly enhanced by the presence of the Pt top electrode. Removal of deuterium from D2/N2-exposed Pt/BST/Pt specimens during oxygen recovery anneals appears to be limited by the rate of an interfacial reaction at low temperatures (200–250 °C). The pre-D2 exposure leakage current properties of the BST capacitors were found to be largely recovered when the deuterium concentration in the films was reduced to ∼1019 cm−3 during post-D2 oxygen recovery anneals. Recovery annealing in vacuum, although it removed deuterium from the films, was found to result in an increase in leakage current density for annealing temperatures greater than 300 °C. These results suggest that introduction of large amounts of positive space charge into the BST films has a pronounced effect on the electrical properties of the Pt/BST interface.
Influence of space charge on the performance of the Kelvin probe
Authors: Luo G.-N. , Yamaguchi K. , Terai T. , Yamawaki M.
Published: July 2002

Measurement using a Kelvin probe depends vitally on the acquisition of a zero field between the probe and the sample. The charges in the space significantly affect the measurement of the work function change via the Kelvin probe due to the induced electric field between the probe and the sample, which adds uncertainty to the probe output. A simple model is presented in this article to depict the origin of the measurement errors. Then a solution, error deduction method, is suggested to effectively eliminate the influence due to charging by introducing a reference sample. The solution is also valid to cancel the error due to the change in the work function of the probe. The experimental results verified the feasibility of the method. Also discussed are other factors that affect the state of charging within a vacuum chamber. Careful shielding is necessary even if a stable reference sample is available. In the case where no stable reference samples are available, the use of insulating materials should be kept at a minimum. Once the insulating materials are used, they should be protected from being charged as much as possible. Any charge-generating components should be isolated by appropriate shielding, if possible, to avoid any leakage of charge into the space.
Transient resonance Raman spectroscopy and density functional theory investigation of iso-CHBr2Cl and iso-CCl3Br photoproducts produced following ultraviolet excitation of CHBr2Cl and CCl3Br
Authors: Zheng Xuming , Lee Cheong Wan , Li Yun-Liang , Fang Wei-Hai , Phillips David Lee
Published: July 2002

We report transient resonance Raman spectra and density functional theory calculations for the photoproducts produced after ultraviolet excitation of CHBr2Cl and CCl3Br in cyclohexane solution. Comparison of the computed vibrational frequencies to the experimental Raman frequencies revealed that the iso-CHBrCl–Br and iso-CHClBr–Br species are mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CHBr2Cl. Similar comparisons for CCl3Br showed the iso-CCl2Cl–Br species is mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CCl3Br. Additional density functional theory computations were done to examine the chemical reactions of iso-CH2Br–Cl and iso-CH2Cl–Br with ethylene to give cyclopropane and Br–Cl product. We briefly discuss the possibility for release of reactive halogens into the atmosphere via the photochemical and chemical reactions of iso-polyhalomethane molecules formed after ultraviolet excitation of polyhalomethanes in condensed phase environments.
Effect of radio-frequency bias voltage on the optical and structural properties of hydrogenated amorphous silicon carbide
Authors: Cui J. , Rusli  , Yoon S. F. , Teo E. J. , Yu M. B. , Chew K. , Ahn J. , Zhang Q. , Osipowicz T. , Watt F.
Published: July 2002

Hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films have been deposited using the electron cyclotron resonance chemical vapor deposition process under varying negative rf-bias voltage at the substrate. The optical and structural properties of these films are characterized using Rutherford backscattering spectroscopy, transmittance/reflectance spectrophotometry, photothermal deflection spectroscopy, Fourier transform infrared absorption, Raman scattering, and room temperature photoluminescence (PL). These films deposited using a gas mixture of silane, methane, and hydrogen at a constant gas flow ratio showed a slight increase in the carbon fraction x, but very obvious structural transformation, at increasing rf induced bias voltage from −20 to −120 V. Near stoichiometric a-Si1−xCx:H films with a carbon fraction x of almost 0.5 are achieved at low bias voltage range from −20 to −60 V. Visible PL with relatively low efficiency can be observed from such films at room temperature. For larger bias voltages from −80 to −120 V, slightly C-rich a-Si1−xCx:H films (x>0.5) with larger optical gaps are obtained. These films have relatively higher PL efficiency, and the relative quantum efficiency was also found to depend strongly on the optical gap. Structurally, it was found that there is an increase in the hydrogen content and carbon sp2 bonding in the films at larger bias voltages. The latter leads to an increase in the disorder in the films. The linear relationship observed between the Urbach energy E0 and B factor in the Tauc equation suggests that the local defects related to microstructural disorder resulting from alloying with carbon dominate the overall defect structure of the films. Substrate biasing is noted to be crucial for the formation of Si–C bonds, as deduced from the Raman scattering results.
Radiative recombination process in InGaN active layers of GaN-based light emitting diodes
Authors: Zheng Ruisheng , Taguchi Tsunemasa
Published: July 2002

We present a theoretical analysis of radiative recombination process in active layers of blue/green InGaN-based light emitting diodes (LEDs) in the framework of a quantum disk model. Taking the structural and compositional inhomogeneity and the finite subband-states effects into account we modify the optical absorption and energy relaxation equations for quantum-disk systems. The carrier relaxation dynamic process and related time-dependent photoluminescence spectra are calculated numerically. Our results show that the quantum-disk model can interpret the main optical properties of InGaN-based LEDs reasonably.
Microstructure and growth mechanism of epitaxial SrRuO3 thin films on (001) LaAlO3 substrates
Authors: Jiang J. C. , Pan X. Q.
Published: July 2002

SrRuO 3 thin films deposited on (001) LaAlO3 substrates by 90° off-axis sputtering at 600 °C were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both AFM and cross-section TEM investigations show that the films have a rough surface. Plan-view TEM studies demonstrate that the films are composed of all three different types of orientation domains (twins). These domain structures and surface morphology are different from the SrRuO3 film deposited on the (001) SrTiO3 substrate which has an atomically flat surface and is composed of only the [110]-type domains. The reason for these differences was ascribed as the effect of lattice mismatch across the film/substrate interface. It is proposed that a SrRuO3 thin film grows on a (001) SrTiO3 substrate through a two-dimensional nucleation process, while a film on LaAlO3 grows with three steps: the coherent growth of a few monolayers at the initial stage through a two-dimensional nucleation process; the formation of misfit dislocations when the film reaches a critical thickness; and an island-like growth thereafter due to the nonuniform distribution of stress along the film surface.
Effects of core size and shell thickness on phonon modes in GaAs/AlAs shell quantum dots
Authors: Qin G. , Ren Shang-Fen
Published: July 2002

Phonon modes in spherical GaAs quantum dots (QDs) embedded in spherical AlAs shells with up to 11 855 atoms (about 8.0 nm in diameter) are calculated by using a valence force field model. All the vibration frequencies and vibration amplitudes of these shell QDs are calculated directly from the lattice dynamic matrix by employing the projection operators of the irreducible representations of the group theory. The effects of the size of GaAs cores and the thickness of AlAs shells on the phonon modes are investigated. The quantum confinement effects of both the vibration frequencies and vibration strengths are investigated. The microscopic interface modes, which can only be revealed by the microscopic model, are identified and discussed.
Structure–property relationship of nanocrystalline tin dioxide thin films grown on (1̄012) sapphire
Authors: Pan X. Q. , Fu L. , Dominguez J. E.
Published: July 2002

This work demonstrates the correlation between the microstructure of nanocrystalline SnO2 thin films and their electrical transport properties and sensitivities to reducing gases. SnO2 thin films were deposited on the (1̄012) surface of α-Al2O3 (sapphire) using electron beam evaporation of a pure SnO2 ceramic source, followed by postdeposition annealing in synthetic air. SnO2 thin films with randomly oriented nanosized grains were obtained by annealing an amorphous SnO film deposited at room temperature. Films with nanosized SnO2 laminates were obtained by annealing epitaxial α-SnO films deposited at 600 °C. The laminates are oriented with their (101) planes parallel to the substrate surface and have a high density of coherent twin boundaries. Hall measurements indicate that the electron concentration of the film with laminate grains is much lower than for the film with random grains. It is proposed that the high density twin boundaries inside the laminates trap conducting electrons and significantly reduce the electron concentration. As a result, the sensitivity to reducing gases of the laminar film is higher than that of the corresponding film with randomly oriented SnO2 grains. It was also found that the grain size has strong effects on the sensitivity of SnO2 films.
Integrated neutron Couette system for nonequilibrium studies of polymers under flow
Authors: Baroni P. , Pujolle-Robic C. , Noirez L.
Published: July 2002

A temperature-controlled Couette-type shear system has been designed and constructed allowing in situ the simultaneous neutron scattering and the optical microscopy study of the nonequilibrium states of thermotropic complex fluids subjected to a steady-state shear flow. In contrast to the usual neutron Couette devices, this cell is adapted to the rheology of viscous materials (typically, 500–1000 Pa s), which can be examined in a wide temperature range (from room temperature to 140 °C). An optical magnification lens is coupled to the shear system to allow the observation of the texture evolution of mesophases. A recent experiment carried out at the vicinity of the nematic–isotropic transition of a side-chain liquid-crystalline polymer is presented.
Experimental and theoretical characterization of an inductively coupled plasma source
Authors: Scheubert P. , Fantz U. , Awakowicz P. , Paulin H.
Published: July 2002

A typical planar inductively coupled plasma source was characterized by means of several diagnostic methods. Langmuir probe measurements as well as quantitative spectroscopic methods were used in order to determine electron density, mean electron energy, electron energy distribution functions, and the neutral particle temperature. A model based on hydrodynamic balance equations has been developed and used to predict parameters which cannot be determined by direct measurements. As the agreement of experimental and simulated data is excellent, the overall discharge efficiency, influences of substrate holder, fluxes as well as energies, and incident angle of ions on the substrate were determined.
Near-infrared light transcutaneous telemetry system having an implantable transmitter driven by external laser irradiation
Authors: Goto Kazuya , Nakagawa Tetsuya , Nakamura Osamu , Kawata Satoshi
Published: July 2002

We present an optical telemetry system using a small implantable transmitter which is both powered and given a carrier wave by transcutaneous near-infrared laser irradiation. The transmitter transcutaneously sends a phase-modulated signal to the receiver by the use of a near-infrared light-emitting diode. The received signal is demodulated with a lock-in amplifier. The operation of the transmitter is stable as long as it is irradiated with enough power. In the irradiation intensity range of 3.2–19 mW/cm2, the variation in the receiver output was only 3% for a baseband signal of 3.0 V, in peak-to-peak amplitude. Using the present system, we have succeeded in optical transmission through chicken several millimeters thick.
An ion gating, bunching, and potential re-referencing unit
Authors: Dedman C. J. , Roberts E. H. , Gibson S. T. , Lewis B. R.
Published: July 2002

A novel design to achieve the gating, bunching, and potential re-referencing of an ion beam, suitable for use in a photofragment spectrometer, is presented. The device simultaneously performs all three functions in a simple, compact, and easily aligned unit. It requires only a single digital signal and one high voltage supply for operation, and provides higher flux density than previous designs. The unit uses lensing to perform beam gating, an approach which has not been reported previously. The design does not require grids, and does not introduce divergence into the ion beam. Experimental results for the combined gating, bunching, and re-referencing unit are presented, and compared with modeled performance.
Calculated paramagnetic resonance parameters of the luminescent Re6S8Cl63− cluster ion
Authors: Alvarez-Thon Luis , Hernández-Acevedo Lucı́a , Arratia-Pérez Ramiro
Published: July 2002

Dirac molecular orbital calculations on the octahedral paramagnetic Re6S8Cl63− cluster ion are reported. As the parent diamagnetic Re6S8Cl64− cluster, the calculated relativistic molecular orbitals indicate that the manifold of closely spaced unoccupied energy levels are mainly localized on the octahedral [Re6S8] core, while the cluster highest occupied molecular orbital is largely centered on the terminal chloride ligand. Thus, the probability distribution of the unpaired electron spin in Re6S8Cl63− is 3.5% on each Re187 nuclei, 0.8% on each capping S33 nuclei, and 12.1% on each terminal Cl35 nuclei. The current calculations predicted an isotropic Zeeman interaction, which is in good agreement with single crystal solid state cluster EPR experiments. We also calculated the paramagnetic hyperfine interactions (Ahfi) of the Re187, Cl35, and S33 nuclei allowing us to describe that the metal and apical ligand hyperfine tensors are anisotropic, while the hyperfine tensors of the capping S ligands are small and isotropic. It is postulated that the reversible redox couple [Re6S8Cl64−/Re6S8Cl63−] could constitute a suitable molecular nanocell for applications in molecular electronics.
Temperature and density dependent solute vibrational relaxation in supercritical fluoroform
Authors: Myers D. J. , Shigeiwa Motoyuki , Cherayil Binny J. , Fayer M. D.
Published: July 2002

Temperature- and density-dependent vibrational relaxation data for the v6 asymmetric stretch of W(CO)6 in supercritical fluoroform (trifluoromethane, CHF3) are presented and compared to a recent theory of solute vibrational relaxation. The theory, which uses thermodynamic and hydrodynamic conditions of the solvent as input parameters, shows very good agreement in reproducing the temperature- and density-dependent trends of the experimental data with a minimum of adjustable parameters. Once a small number of parameters are fixed by fitting the functional form of the density dependence, there are no adjustable parameters in the calculations of the temperature dependence.
Dielectric constant and loss tangent in LiNbO3 crystals from 90 to 147 GHz
Authors: Lee Mark
Published: July 2002

The complex dielectric constant ε(ω) of poled LiNbO3 crystals has been measured from 90 to 147 GHz using a reflectometer bridge. Electric field polarizations in both the ordinary and extraordinary axes were studied. The real part of ε(ω) differs only slightly from its value near 10 GHz, while the imaginary part is significantly larger than reported at lower frequencies. The data are used to model the dissipative losses of a gold co-planar waveguide on LiNbO3.
Real-space computation of dynamic hyperpolarizabilities
Authors: Iwata J.-I. , Yabana K. , Bertsch G. F.
Published: July 2002

A real-space method is developed to calculate molecular hyperpolarizabilities in the time-dependent density functional theory. The method is based on the response function formalism which was developed by Senatore and Subbaswamy for the third harmonic generation of rare-gas atoms [Phys. Rev. A 35, 2440 (1987)]. The response equations are discretized in real space employing a uniform grid representation in the three-dimensional Cartesian coordinate, and are solved with iterative methods such as conjugate-gradient and conjugate-residual methods. The method works efficiently for both small and large molecules, and for any nonlinear optical processes up to third order. The spatial convergence of the calculation can be examined with two intuitive parameters, the grid spacing and the spatial box size. Applications of our method are presented for rare-gas atoms and molecules, N2, H2O, C2H4, C6H6, and C60. Our results agree well with other calculations employing basis functions except for a slight deviation in a large molecule, C60.
Exciton localization in In0.15Ga0.85As/GaAs quantum wire structures grown on (553)B-oriented GaAs substrate
Authors: Liu B. L. , Liu B. , Xu Z. Y. , Ge W. K.
Published: July 2002

Using time-resolved photoluminescence (PL) measurements, we have studied the exciton localization effect in InGaAs/GaAs quantum wire (QWR) structures formed in corrugated narrow InGaAs/GaAs quantum wells (QWs) grown on (553)B GaAs substrate. The PL decay time in the QWR structure was found to be independent of the temperature for T<70 K, showing a typical dynamical behavior of the localized excitons. This result is in striking contrast to the corresponding quantum well structures, where a linear increase of the PL decay time was observed. In addition, an increase of the exciton lifetime was observed at low temperature for the QWR structure as compared to a reference InGaAs/GaAs quantum well sample (1200 vs 400 ps). The observed longer decay time was attributed to the reduction in the spatial coherence of excitons in the QWR-like structure. In PL measurements, a significant polarization anisotropy was also found in our narrow InGaAs/GaAs QWs grown on (553)B GaAs.
Excitation of elastic surface pulses by fiber optics and near-field optical devices
Authors: Frass A. , Hess P.
Published: July 2002

Elastic surface pulses or surface acoustic wave (SAW) pulses were launched at the surface by confining pulsed laser radiation with single-mode fibers and tapered fiber tips, produced by the focused ion beam technique, in near-field configuration. The propagation of the elastic surface pulses was studied in a point-source point-probe experiment. The transient surface displacements of the propagating SAW pulses were detected by an actively stabilized Michelson interferometer with sub-angstrom resolution. For tip aperture sizes in the micrometer range confinement effects could be observed, extending the frequency spectrum of the launched SAW pulses to approximately 800 MHz. The actual frequency spectrum was limited by the applied detection method. An inverse square root dependence of the SAW amplitude with propagation distance was confirmed experimentally, as expected for the excitation geometry employed. Results are presented for layered systems with dispersion of the ultrasonic surface pulses. The relevance of the frequency spectrum of the SAW pulse for the determination of elastic and mechanical film properties in dispersive media is discussed.
Temperature dependence of gate currents in thin Ta2O5 and TiO2 films
Authors: Luo Z. J. , Guo Xin , Ma T. P. , Tamagawa T.
Published: July 2002

This letter reports our study of the temperature dependence of gate currents in thin Ta2O5 and TiO2 films. The study was conducted (1) to study the conduction mechanisms and band alignments, and (2) to determine whether the gate leakage current is tolerable at high temperatures for either of these high-dielectric-constant (high-k) oxides. The I–V characteristics of these oxides were measured and analyzed over a wide temperature range from 25 to 400 °C. Currents in Ta2O5 samples exhibited stronger temperature dependence than those in TiO2 samples, especially at high fields, mainly due to a much smaller electron barrier height of Ta2O5 over Si (0.28 eV) than that of TiO2 over Si (0.9 eV).
Equilibrium thermodynamic theory explicitly including heat transport for evaluation of temperature distributions in steady plane-wave fronts
Authors: Sano Tomokazu , Sano Yukio
Published: July 2002

Previous theories that implicitly included heat transport predicted that temperatures were reduced in proportion to the viscous stresses in the whole region of an overdriven steady wave front in 2024 Al shocked at 80 GPa. Although there was a decrease in temperature caused by the shear stress, the decrease was very slight. A more justifiable theory that includes a heat transport term, the coefficient of which depends on the form and thickness of the effective part of a specific volume wave in the steady wave front, was then developed. The range of appropriate thicknesses of the effective part of the linear and sine forms for shocks up to 80 GPa in 2024 Al were determined. In these ranges, strain increments were sufficiently small and effective temperature rise-times were sufficiently long on the electron–phonon relaxation time scale to justify using equilibrium thermodynamics. In addition, the efficacy of the inside temperature method [Sano and Abe, J. Appl. Phys. 89, 105 (2001)] was illustrated by evaluating heat transport. The inside temperature distribution can be specified if the thickness is measured because the distribution is not influenced to a great extent by the wave form.
Quantitative imaging of magnetic domain walls in thin films using Lorentz and magnetic force microscopies
Authors: McVitie Stephen , White Gordon S. , Scott Jamie , Warin Patrick , Chapman John N.
Published: July 2002

Images of a thin film permalloy element taken with Lorentz and magnetic force microscopies are compared with those from a simulation of the expected magnetic structure of the element. Measurements taken from the domain walls present in the element allow a quantitative comparison to be made. In the case of magnetic force microscopy, quantification is made possible by using a nonperturbative approach based on an extended charge model for the magnetic probe. Excellent agreement between experiment and simulation is observed for both imaging techniques.
Bidirectional photorefractive ring resonator pumped by a single beam
Authors: Kaczmarek Malgosia , Yang Changxi , Tatarkova Svetlana A. , Eason Robert W.
Published: July 2002

Bidirectional oscillations can be supported in a single photorefractive ring resonator using Ce:BaTiO3. We show that two oscillation beams can simultaneously propagate in opposite directions under different geometries. The difference in power between the two oscillation beams can reach two orders of magnitude and their stability depends on the pump beam angle of incidence and the crystal’s configuration.
In situ observation of epitaxial growth of [Au/Co/Cu] and [Cu/Co/Au] superlattices and their magnetic interface anisotropies
Authors: Kamada Yasuhiro , Hitomi Yoshitaka , Kingetsu Toshiki , Yamamoto Masahiko
Published: July 2002

We studied crystal growth of [Au/Co/Cu](111) and [Cu/Co/Au](111) superlattices during molecular-beam epitaxy and their magnetic anisotropies, and discussed the relationships between the interface structures and the perpendicular magnetic anisotropies. To study the structure at or near the interface of the superlattices, we continuously observed the change of surface in-plane lattice constant during growth using reflection high-energy electron diffraction (RHEED) on a real-time basis. From the RHEED observations, we deduce that gradually decreasing strain in the thickness direction exists in the Co layers at the Co/Au interfaces in the [Cu/Co/Au] superlattices, in which the Co layers are grown on the Au layers, and that coherency strain due to the Cu underlayers and strain due to the Au overlayers coexist in the Co layers in the [Au/Co/Cu] superlattices. From the magnetic measurements and detailed considerations, we conclude that both the magnetocrystalline interface anisotropy (or the Néel-type magnetic surface anisotropy) and the magnetoelastic interface anisotropy contribute to the total interface anisotropy in both types of superlattices. However, we find that the magnetoelastic interface anisotropy originating from the interface regions of the Co layers on the Au underlayers is larger than that under the Au overlayers. This leads to the perpendicular magnetic anisotropy in the [Cu/Co/Au] superlattice but not in the [Au/Co/Cu] superlattice when the Co layers are five monolayers thick. We also find that the magnetoelastic interface anisotropy originating from the interface regions of the Co layers under the Au overlayers strongly depends on the underlayer material of the Co layers, and is much larger in the [Au/Co/Cu] superlattices than in the [Au/Co/Ag] and the [Co/Au] superlattices. Moreover, we demonstrate that the above two contributions to the interface anisotropy can be separately evaluated in the [Cu/Co/Au] and the [Au/Co/Cu] superlattices. The present result is consistent with that of earlier work on [Au/Co/Ag](111) and [Ag/Co/Au](111) superlattices [T. Kingetsu and K. Sakai, Phys. Rev. B 48, 4140 (1993)].
IUPAC-NIST Solubility Data Series. 77. C2+ Nitroalkanes With Water or Organic Solvents: Binary and Multicomponent Systems
Authors: Sazonov Valerii P. , Shaw David G. , Marsh Kenneth N. , Sazonov Valerii P. , Sazonov Nikolai V. , Lisov Nikolai I. , Chernysheva Mariya F.
Published: July 2002

The mutual solubilities and liquid-liquid equilibria of binary and multicomponent systems composed of C2+ nitroalkanes with solvents are reviewed. The solvents (mainly in liquid phase) include water, inorganic compounds and a variety of organic compounds such as hydrocarbons, halogenated hydrocarbons, alcohols, acids, esters, and nitrogen compounds. A total 81 binary, 21 ternary, 2 quaternary and 1 five component systems whose properties were described in the chemical literature through 1998 are compiled. For 14 systems sufficient data were available to allow critical evaluation. All data are expressed as mass and mole fractions as well as the originally reported units. Similar reviews of gas, liquid and solid solubilities for other systems have been within the Solubility Data Series. This is volume 77 of this series.
Statistical thermodynamics of polydisperse polymer systems in the framework of lattice fluid model: Effect of molecular weight and its distribution on the spinodal in polymer solution
Authors: Yang Jian , Sun Zhaoyan , Jiang Wei , An Lijia
Published: July 2002

With the aid of thermodynamics of Gibbs, the expression of the spinodal was derived for the polydisperse polymer-solvent system in the framework of Sanchez–Lacombe Lattice Fluid Theory (SLLFT). For convenience, we considered that a model polydisperse polymer contains three sub-components. According to our calculation, the spinodal depends on both weight-average (M̄w) and number-average (M̄n) molecular weights of the polydisperse polymer, but the z-average molecular weight (M̄z) dependence on the spinodal is invisible. The dependence of free volume on composition, temperature, molecular weight, and its distribution results in the effect of M̄n on the spinodal. Moreover, it has been found that the effect of changing M̄w on the spinodal is much bigger than that of changing M̄n and the extrema of the spinodal increases with the rise of the weight-average molecular weight of the polymer in the solutions with upper critical solution temperature (UCST). However, the effect of polydispersity on the spinodal can be neglected for the polymer with a considerably high weight-average molecular weight. A more simple expression of the spinodal for the polydisperse polymer solution in the framework of SLLFT was also derived under the assumption of υ*=υ1*=υ2* and (1/r10)−(1/r2i0)→(1/r10).
Rotational alignment in the photodesorption of CO from Cr2O3(0001): A systematic three-dimensional ab initio study
Authors: Thiel S. , Pykavy M. , Klüner T. , Freund H.-J. , Kosloff R. , Staemmler V.
Published: July 2002

In recent experiments, the rotational alignment of the laser induced nonthermal desorption of CO adsorbed on an epitaxially grown film of Cr2O3(0001) has been studied [Beauport, Al-Shamery, and Freund, Chem. Phys. Lett. 256, 641 (1996)]. At low-rotational quantum numbers J, the molecules desorb like helicopters (J-vector perpendicular to the surface) while at high J-values cartwheel motion is preferred (J-vector parallel to the surface). These stereodynamic effects and the experimental state resolved velocity distributions of the desorbing species are simulated by means of an exact time-dependent wave packet method in three dimensions. As a basis for this quantum-mechanical treatment of the CO desorption ab initio potential energy surfaces (PES) were used. The PES for the electronic ground state of the CO–Cr2O3(0001) system has been calculated previously by Pykavy et al. [Surf. Sci. 479, 11 (2001)] in an embedded cluster approach. As the intermediate state, generated by the laser irradiation, an internal CO excited state (5σ→2π*) was considered. The PES of this a 3Π-like state of CO adsorbed on Cr2O3(0001) was calculated at the ab initio CASSCF-level. Our key findings in the subsequent wave packet calculations are (1) a high-dimensional treatment of the photodesorption process is very important in this system, essentially the angular coordinates, very often neglected in similar studies, are responsible for a “successful” desorption event; (2) the change from the strongly tilted equilibrium geometry in the electronic ground state to the preferred upright position in the electronically excited state after laser irradiation is essential for the mechanistic picture of the desorption process; (3) the experimental phenomemon of rotational alignment can only be explained if the topologies of the PES of both the electronically excited and the ground state are accounted for; (4) the lifetime of the CO*-intermediate is in the order of 10 fs; (5) the molecule–surface vibrations in the electronic ground state do not much influence the asymptotic results at the experimental temperature of T=100 K. However, the inclusion of excited levels of the hindered rotation helps to gain insight into the desorption mechanism on a microscopic level.
Reversal modes of exchange-spring magnets revealed by torque magnetometry
Authors: Platt C. L. , Berkowitz A. E. , David S. , Fullerton Eric E. , Jiang J. S. , Bader S. D.
Published: July 2002

Torque magnetometry measurements and related micromagnetic simulations are reported for SmCo/Fe exchange-spring films with uniaxial in-plane easy axis of magnetization. Rotating an applied magnetic field in plane, away from the easy direction, gives rise to an irreversible jump in the torque curves at a critical angle. The jump is associated with a switch in chirality of the magnetization spiral structure that develops in the magnetically soft Fe layer, pinned at the interface with the hard SmCo layer. Two reversal modes of the chirality are identified, one at low fields due to an in-plane untwisting of the spiral, and the other at high fields, due to an out-of-plane fanning of the spiral.
Variation in γ-ray count rates as a monitor of precipitation rates, radon concentrations, and tectonic activity
Authors: Greenfield M. B. , Domondon A. T. , Okamoto N. , Watanabe I.
Published: July 2002

Precise and reproducible correlations between environmental γ-ray count rates (GRR) with rain precipitation rates (RPR) and snow precipitation rates (SPR) have been measured to be 0.18±0.02 and 4.3±0.04 (counts/s)/(mm/h), respectively. The GRR, observed with two 12.3 cm3 NaI detectors, also exhibit a diurnal cycle (period of 1.01±0.03 day, maximum around sunrise and location dependent amplitude) and a seasonal cycle (smaller amplitude and maximum in mid-February). Increased GRR during precipitation have been observed at ground level, but the effects of the diurnal cycle and other noise are reduced by more than 20 at 15 m above ground. An improved signal-to-noise ratio, response time of less than one hour and precise correlations between the peaks in GRR and precipitation rates enable real-time determinations of RPR and SPR to better than 1 mm/h. The relative insensitivity of these correlations to other meteorological conditions enables GRR to serve as remote and environmentally stable sensors of RPR and SPR. The time-dependence of GRR is consistent with that expected from variations in the presence of radon and radon progeny in the atmosphere. Change in the amplitude of the diurnal cycle may serve as a monitor of radon concentration. A γ-ray detector integrates over a large volume with good time resolution and thus may be responsive to real-time changes in radon concentrations due to increased exhalation from tectonic activity.
Electrical fracture toughness for electrically conductive deep notches driven by electric fields in depoled lead zirconate titanate ceramics
Authors: Wang Tianhong , Zhang Tong-Yi
Published: July 2002

This study demonstrates that electric fields can fracture depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to measure the electrical fracture toughness. The electrical fracture toughness (GCE=263±35 N/m) is about nine times higher than the mechanical fracture toughness (GICM=30.4±3.9 N/m). The highly electrical fracture toughness arises from the greater energy dissipation around the conductive sharp notch tip under purely electric loading, which is impossible under mechanical loading in brittle depoled ceramics.
Extraction and profile analysis of hydrogen-like helium ions in a magnetized sheet plasma
Authors: Blantocas Gene Q. , Ramos Henry J. , Wada Motoi
Published: July 2002

The extraction of He+ ions from a magnetized sheet plasma source is reported. Optimum detection and extraction of He+ ions were conducted using an E×B probe as a mass analyzer. The effect of the probe’s extraction potential, its position relative to the center of the core plasma, gas filling pressure, and discharge conditions in the production and extraction of He+ ions were investigated. The He+ ion current density yield of 8 μA/cm2 was optimum at plasma discharge current of 1.5 A within the vicinity of 5–7 cm from the sheet plasma core. Investigations also show the predominance of detected He++ species. At higher pressures, the formation of molecular helium ions He2+ were observed. The He+ current yield decreases as the plasma current is increased according to the modified Saha population density equation of the collisional-radiative model.
Computationally efficient method to calculate the Coulomb interactions in three-dimensional systems with two-dimensional periodicity
Authors: Kawata Masaaki , Mikami Masuhiro , Nagashima Umpei
Published: July 2002

A computationally efficient method was developed for calculating Coulomb interactions in three-dimensional (3D) systems with two-dimensional (2D) periodicity; the 2D particle-mesh Ewald (2D-PME) method we previously developed was extended. The formulation and numerical algorithms are described in detail for calculating the Coulomb potential energy, the Coulomb force, and the Coulomb component of the pressure tensor. Computational efficiency and accuracy of the 2D-PME method were evaluated for two water systems with 2D periodicity in the x and y directions and with non-periodicity in the z direction. Compared with exact results calculated by using the original 2D Ewald summations, the 2D-PME method yielded significantly accurate calculations, similar to the computationally efficient method we previously developed for calculating 2D Ewald summations (2D-EW method). For a given accuracy, the 2D-PME method was faster than the 2D-EW method for the water systems we examined. The computational effort of the 2D-PME method decreases as the computationally efficiency of the Fourier transforms used in the 2D-PME method increases. The 2D-PME method is therefore promising for accelerating molecular dynamics and Monte Carlo simulations for 3D systems with 2D periodicity.
Mössbauer spectroscopy and magnetoresistivity of Fe57 substituted Mn in La0.7−xYxCa0.3MnO3 manganites
Authors: Goya G. F. , Souza J. A. , Jardim R. F.
Published: July 2002

We have studied the general physical properties of polycrystalline samples of La0.7−xYxCa0.3Mn0.9957Fe0.01O3; x=0 and 0.1; manganites prepared through sol–gel precursors. Characterization of these samples through electrical resistivity, ρ(T), and magnetization, M(T), measurements indicated a transition to a ferromagnetic state at TC∼240 and 100 K for x=0 and 0.1, respectively. Mössbauer spectra taken from room temperature down to 4.2 K revealed the presence of two paramagnetic doublets from 300 to TC having temperature independent hyperfine parameters and, that below TC, the appearance of two ordered components that coexist with one paramagnetic phase. The coexistence between these phases is preserved from TC down to ∼20 K. The highest magnetoresistance effect was found in the temperature range in which the change of volumetric fraction of these phases is maximum.
Microstructural observation on effect of oxidation method of AlOx in magnetic tunnel junction by high resolution transmission electron microscopy
Authors: Bae Jun Soo , Shin Kyung Ho , Lee Hyuck Mo
Published: July 2002

The formation of the insulating AlOx tunnel barrier is a critical and sensitive process in magnetic tunnel junctions. Both the natural oxidation and the plasma oxidation methods were employed to fabricate the AlOx insulating layer in this study. In the natural oxidation, the Al layer was exposed to pure oxygen gas at 20 Torr for up to 50 min to produce the AlOx tunnel barrier. It was revealed by high resolution transmission electron microscopy that the oxidation occurred preferentially through the grain boundary of Al grains. Also, the AlOx grains expanded isotropically when fully oxidized, thereby making the surface of the AlOx layer modulated. In plasma oxidation, the oxygen plasma was used at 20 mTorr of pure oxygen gas for up to 30 s and the flat AlOx layer formed uniformly on the Al layer. It had sharp interfaces with the underlying metallic Al and the rate of oxide layer growth decreased as the oxidation proceeded.
Microstructural evolution and phase transformation characteristics of Zr-doped FePt films
Authors: Lee Seong-Rae , Yang Sanghyun , Kim Young Keun , Na Jong Gab
Published: July 2002

In the present study, an emphasis was given on elucidating the details of microstructural changes in both undoped FePt and Zr-doped FePt films upon exposure to postdeposition annealing. Recently, the present authors have discovered [S. R. Lee, S. Yang, Y. K. Kim, and J. G. Na, Appl. Phys. Lett. 78, 4001 (2001)] that 3 at. % Zr doping into Fe59Pt41 films accelerated the ordering kinetics (10 min at 500 °C), accompanying high coercivity Hc of 7.3 kOe. Our transmission electron microscopy study on both Fe59Pt41 and [Fe59Pt41]97Zr3 films annealed at 500 °C for 60 and 10 min, respectively (both exhibited Hc=7300 Oe) revealed that the FePtZr film displayed excellent microstructural features: smaller average grain size (D) with narrower distribution (σ) (D=6.0 nm,σ=2.3 nm) compared with FePt (D=33.6 nm, σ=25.0 nm). For the FePtZr system, we have found that an ordered phase decomposed into disordered phases (thereby losing the high Hc) after 15 min annealing. By evaluating the long-range-order parameter S changes, we speculate that the cause of the disordering was primarily due to the formation of a Pt–Zr compound.
Systematic study of in-plane magnetic anisotropy in CoCrPtB/Cr media
Authors: Yu Mingjun , Choe Geon , Johnson Kenneth E.
Published: July 2002

Circumferential texturing in longitudinal recording media induces in-plane magnetic anisotropy. It is characterized by orientation ratio (OR), which is an important parameter affecting media performance. Our work shows that among the media with different OR values (including isotropic media with OR=1), the ones with higher OR have both better thermal stability and better recording performance. Therefore, it is important to have a better understanding of the mechanism for in-plane anisotropy so that OR can be further increased. Several mechanisms for in-plane anisotropy have been proposed through micromagnetic analysis or empirical methods. A systematic study of OR in CoCrPtB/Cr media was performed to explore the mechanism of in-plane magnetic anisotropy. Circumferential texture roughness has the most effect on OR, but OR is also strongly affected by magnetic alloy composition and sputtering process parameters such as substrate temperature, underlayer thickness, and magnetic-layer thickness. Higher substrate temperature, thinner underlayer thickness, and thinner magnetic-layer thickness all lead to higher OR. These results suggest that the in-plane magnetic anisotropy comes from stress anisotropy via the inverse magnetostriction effect. The stress is induced at the magnetic-layer/underlayer and underlayer/substrate interfaces, while the anisotropy is suggested to arise from the topology of the circumferential texture lines.
Microstructure and magnetic properties of hexagonal barium ferrite thin films with various underlayers
Authors: Kim D. H. , Nam I. T. , Hong Y. K. , Gee S. H. , Park M. H.
Published: July 2002

BaM (barium ferrite) thin films and underlayers (Fe, Cr, Al2O3, Fe2O3, ZnFe2O4, TiO2) were prepared by rf/dc magnetron sputtering on (100) oriented bare Si substrates. The effects of the underlayer on grain orientation, magnetic properties, and microtexture of BaM film were studied. All the BaM films, except BaM/Fe/Si film, attained nearly the same perpendicular and in-plane coercivities. The BaM/TiO2/Si exhibits the highest coercivity. However, regardless of the underlayer, BaM grains are randomly oriented. By adopting ZnFe2O4 as an underlayer, the interdiffusion of Si from substrate was prohibited to some degree. Elongated grains from the extinction of small platelet grains were grown with an increase in the BaM film thickness of BaM/Si. The microstructure of BaM in BaM/TiO2/Si was strongly dependent on both the microstructure of TiO2 underlayer and the total sputtering gas pressure.
Coulomb blockade effects on magnetoresistance of granular magnetic media
Authors: Jalil M. B. A.
Published: July 2002

A full Monte Carlo simulation is performed of the magnetotransport across a granular system of Co nanoparticles, embedded in an insulating Al2O3 matrix. The interplay between spin-dependent tunneling and Coulomb blockade effects is modeled using the so-called “orthodox” theory of single-charge tunneling. The tunneling magnetoresistance ratio (TMR) shows a strong temperature dependence, especially for the case of nonmagnetic contact electrodes. This behavior contradicts previous predictions by Inoue and Maekawa [Phys. Rev. B 53, 11927 (1996)], and can be explained by considering the crucial role of the initial rate-determining tunneling step.
Recording performance and magnetization switching of CoTb/CoCrPt composite perpendicular media
Authors: Uwazumi H. , Shimatsu T. , Sakai Y. , Enomoto K. , Takenoiri S. , Watanabe S. , Muraoka H. , Nakamura Y.
Published: July 2002

Magnetic properties, recording performance, and thermal stability of CoTb amorphous/CoCrPt granular composite media are discussed. Thermal stability at low recording density of the composite media with CoCrPtB improves significantly as CoTb thickness increases, in accord with the increase of loop squareness, Mr/Ms. The media having CoTb thickness of more than 6 nm show almost no signal decay. Moreover, the signal-to-medium-noise ratio, SNR, improves with increasing CoTb thickness up to 6 nm. CoTb(6 nm)/CoCrPtB(20 nm) medium shows the best SNR performance under the present experimental conditions with no thermal decay. Media noise of the composite media with CoCrPt increases super linearly from ∼300 kfci, which is caused by the percolation of the recorded bits, probably due to the strong exchange coupling in the CoTb layer. The media with CoCrPtB shows less media noise than that with CoCrPt, and the super-linear increase of this media noise starts at ∼400 kfci which is higher than that of the media with CoCrPt. It is thought that the well-segregated grain structure of CoCrPtB layer, having the enhanced and fine distribution of Ms, reduces the magnetic cluster size effectively, resulting in the reduction of the media noise.
Improvement of preferred orientation of NiAl/CrMn underlayers deposited on prebaked tape substrates
Authors: Lee Hwan-Soo , Laughlin David E. , Bain James A.
Published: July 2002

It was observed that the coercivity of CoCrPt films deposited on a polyimide tape was much lower than the coercivity deposited on a rigid glass substrate under identical conditions. X-ray diffraction showed that the desirable (112) and (110) texture of the NiAl/CrMn underlayers on the tape substrate was severely degraded during the deposition process. This resulted in the destruction of (10.0) growth texture of CoCrPt films and a significant reduction of magnetic coercivity. However, degassing of the tape substrate under vacuum before deposition significantly enhanced the magnetic properties. The coercivity of the film reached 170 kA/m which is near that achieved on a glass substrate. Auger analysis also revealed that oxygen was present in the NiAl/CrMn underlayers deposited without degassing the tape substrate.
Site occupancy of Ga and Nb in Nd12.5FebalGa0.3Nb0.2B6.2 during the hydrogenation, disproportionation, desorption, and recombination process
Authors: Konno K. , Matsuura M. , Ikuta N. , Sakurai M. , Mishima C.
Published: July 2002

The local structure changes around Ga and Nb in Nd12.5FebalGa0.3Nb0.2B6.2 during homogenization, disproportionation, and recombination processes are studied by fluorescence XAFS in order to understand the roles of Ga and Nb on the improvement of (BH)max. Comparing the observed XAFS of the Ga K edge with the calculated ones, Ga is proved to occupy preferentially the Fe(4c) site in Nd2Fe14B structure after homogenization and recombination, while it dissolves in α-Fe after the disproportionation. On the other hand, XAFS results of the Nb K edge does not change in the whole hydrogenation, disproportionation, desorption, and recombination process; the XAFS results of Nb indicate the formation of a stable NbFeB phase in the homogenized state.