bluuebunny/crossref_metadata_2025 huggingface dataset:
Saturation magnetostriction of an ultrathin CoFe free-layer on double-layered underlayers
Authors: Fukuzawa Hideaki , Kamiguchi Yuzo , Koi Katsuhiko , Iwasaki Hitoshi , Sahashi Masashi
Published: July 2002

We investigated the saturation magnetostriction (λs) of an ultrathin CoFe free-layer on double-layered underlayers of Ru/Cu. By increasing the Ru underlayer thickness, λs becomes positively large; on the other hand, λs becomes negatively large by increasing the Cu underlayer thickness. This tendency can be explained by the model where λs is changed by the film strain. The λs sensitivity on the Cu underlayer thickness becomes large with increasing the Co concentration of the CoFe free-layer. This indicates that CoFe λs becomes sensitive to strain with increasing Co concentration. In order to realize small magnetostriction spin-valve films with an ultrathin CoFe free-layer, we have to take care of the lattice strain of the film.
A simple molecular thermodynamic theory of hydrophobic hydration
Authors: Ashbaugh Henry S. , Truskett Thomas M. , Debenedetti Pablo G.
Published: July 2002

A recently developed microscopic model for associating fluids that accurately captures the thermodynamics of liquid water [Truskett et al., J. Chem. Phys. 111, 2647 (1999)] is extended to aqueous solutions with nonpolar species. The underlying association model incorporates the highly directional and open nature of water’s hydrogen-bond network, and, as a result, captures a number of the distinguishing properties of liquid water, such as the density anomaly. The model for aqueous mixtures developed herein predicts many of the thermodynamic signatures of hydrophobic hydration without resorting to empirical temperature-dependent parameters. The predicted solubility of nonpolar species is slight over a wide range of temperatures, and exhibits a minimum as a function of temperature, in accord with experiment. Hydration is opposed by a dominant entropy and favored by the enthalpy at low temperatures. At elevated temperatures these roles are reversed. Furthermore, the hydration entropies for hydrophobes of varying size converge over a very narrow temperature range. Comparison with experimental and simulation data for nonpolar solutes in water shows that the theory tends to exaggerate the solute’s transfer heat capacity at low temperature, and hence solubility minima and entropy convergence are predicted to occur at lower temperatures than observed. Our results support the emerging view that hydrophobic effects can be attributed in large part to the equation of state for pure water.
Magnetic behavior of lithographically patterned particle arrays (invited)
Authors: Ross C. A. , Haratani S. , Castaño F. J. , Hao Y. , Hwang M. , Shima M. , Cheng J. Y. , Vögeli B. , Farhoud M. , Walsh M. , Smith Henry I.
Published: July 2002

This article reviews recent progress in the fabrication, characterization, and analysis of large area arrays of sub-100-nm magnetic particles made by lithographic techniques. Particles are made by electrodeposition, evaporation and liftoff, or sputtering and etching, leading to a wide range of shapes, compositions, and microstructures. The remanent states, magnetic hysteresis, and uniformity of the particles and the interparticle interactions will be discussed.
Raman spectroscopy of nanocrystalline GaN synthesized by arc plasma
Authors: Li H. D. , Zhang S. L. , Yang H. B. , Zou G. T. , Yang Y. Y. , Yue K. T. , Wu X. H. , Yan Y.
Published: July 2002

We report on a Raman study of nanocrystalline GaN with the wurtzite structure synthesized by arc plasma method. Resonant Raman scattering is observed using 514.5 nm (2.41 eV) laser excitation, which is near the band gap of the “yellow band” (2.2–2.3 eV). Under such near-resonant excitation, new Raman bands unexpected in an ideal wurtzite GaN crystal were found. The transverse optical modes of A1 (531 cm−1) and E1 (560 cm−1), and the nonpolar modes of E2 [567 cm−1 (high) and 143 cm−1 (low)] normally observed in bulk crystals, were recorded and were observed to be resonantly enhanced. Two new bands (680 and 344 cm−1) were assigned to the inactive optical phonon modes B1 (high) and B1 (low), respectively. A broadband centered at 710 cm−1 was attributed to surface modes of the nanocrystals, providing good agreement with the calculated result based on Fröhlich theory. As a result of this study, Raman scattering of GaN nanocrystals has been characterized.
Dynamics of self-driven microcantilevers
Authors: Passian A. , Muralidharan G. , Kouchekian S. , Mehta A. , Cherian S. , Ferrell T. L. , Thundat T.
Published: July 2002

The small amplitude thermal vibrations of the microcantilever of an atomic force microscope can be enhanced via a delayed feedback system. This is verified experimentally for a triangular cantilever, and modeled theoretically as a boundary value problem resulting in a second order functional differential equation for the temporal behavior of the cantilever. The eigenvalues of the resulting delay differential equation describing the transverse vibrations of the cantilever are calculated and analyzed. These values are compared with the corresponding resonant frequencies predicted by a point mass model and with the experimentally observed values.
Modified genetic algorithm to model crystal structures. I. Benzene, naphthalene and anthracene
Authors: Bazterra Victor E. , Ferraro Marta B. , Facelli Julio C.
Published: July 2002

This paper describes a new computational scheme to model crystal structures of organic compounds employing a modified genetic algorithm. The method uses real-valued Cartesian coordinates and Euler angles between molecules in a crystal block as variables identifying the genetic parameters, i.e., genes. The model does not make any assumption on the crystallographic group at which the compound belongs nor to the number of molecules in the unit cell. The method has been implemented in the computer package MGAC (Modified Genetic Algorithm for Crystal and Cluster structures) that allows for optimizations using any arbitrary selection function. The examples presented here for the crystalline structures of benzene, naphthalene and anthracene, using an empirical potential energy function as the selection function, show excellent agreement with the experimental ones. While these examples use the “rigid molecule approximation,” the method is quite general and can be extended to take into account any number of intramolecular degrees of freedom.
Modified genetic algorithm to model crystal structures. II. Determination of a polymorphic structure of benzene using enthalpy minimization
Authors: Bazterra Victor E. , Ferraro Marta B. , Facelli Julio C.
Published: July 2002

The Modified Genetic Algorithm scheme, presented in Paper I to model crystal structures in organic compounds (MGAC), is applied here to test its performance in the determination of a polymorphic crystalline structure of benzene that has been observed at 25 Kbar. This polymorph, named here benzene II, is not a global minimum of the energy or even close to any of the local minimum determined using the energy evolution of benzene structures described in the previous paper. The benzene II structure corresponds to an enthalpy minimum. This paper shows that it is possible to use the MGAC procedure, modified to use the minimization of the enthalpy instead of the energy in the GA (Genetic Algorithm) selection process, to find this high pressure structure of benzene.
Effects of fluorine on structure, structural relaxation, and absorption edge in silica glass
Authors: Saito K. , Ikushima A. J.
Published: July 2002

The effects of fluorine (F) doping on silica glass structure, structural relaxations, and vacuum ultraviolet (VUV) absorption edge were investigated by the infrared and vacuum ultraviolet absorption measurements. Linewidth of 2260 cm−1 absorption band, which is one of the good indicators of structural disorder, decreases proportionally to the fictive temperature (Tf). On the other hand, it is irrespective of F concentration. Structural relaxations are strongly stimulated by F doping. The transparency near the absorption edge depends on both F concentration and Tf. When Tf is constant, absorption edge shifts to a shorter wavelength proportionally to the F concentration. These results strongly suggest that there are two main factors that control the VUV absorption edge of F-doped silica glass; band gap widening by increasing Si–F bond and reduction of structural disorder. The latter factor is not directly caused by structural change by F doping. F enhances the structural relaxation, and it is easy to obtain samples with lower Tf, resulting in reducing structural disorder. Furthermore, a way to determine Tf of F-doped silica glass is offered.
Trap levels and effect of oxygen in poly[2-methoxy-5- (2′-ethyl-hexyloxy)-1,4-phenylene vinylene] diodes
Authors: Kažukauskas V. , Tzeng H. , Chen S. A.
Published: July 2002

We investigated carrier transport and capture in poly[2-methoxy-5-(2′-ethyl-hexyloxy)1,4-phenylene vinylene] Schottky diodes by thermally stimulated currents and current–voltage characteristics. Two main charge traps were found after the white light excitation. Their effective activation energies were in the ranges 0.207–0.355 eV and 0.75–0.91 eV, and the total charges were up to (2–8)×1014 cm−3 and ∼(1–2)×1016 cm−3 respectively. The activation energy and level filling could be increased significantly by exposing the sample to the air. The deeper trap is likely located near the surface, meanwhile the shallower trap is most probably distributed over the sample depth. As far as these traps are related to the oxygen, both of them could be identified as electron traps. In contrast, none of the traps could be recharged by applied voltage. Instead, the injected carriers created a long-living sample polarization. The nonexponential depolarization lasted for several thousands of seconds and was not thermally activated even above the glass transition temperature. These facts make it necessary to include into analysis other possible physicochemical mechanisms, e.g., reversible chemical reactions or chain structure reorganization induced by electric field.
Time-dependent density functional theory employing optimized effective potentials
Authors: Hirata So , Ivanov Stanislav , Grabowski Ireneusz , Bartlett Rodney J.
Published: July 2002

Exchange-only ab initio (parameter-free) time-dependent density functional calculations for the vertical excitation energies of atoms and polyatomic molecules are performed by employing optimized effective potentials (OEP’s) and their corresponding adiabatic exchange kernels for the first time. Accurate OEP’s are obtained by a novel linear-combination-of-atomic-orbital (LCAO) algorithm [R. Colle and R. K. Nesbet, J. Phys. B 34, 2475 (2001)] in which a potential is represented as a sum of a seed potential having the correct −1/r asymptotic behavior and a small and rapidly decaying correction, the latter being approximated accurately by a linear combination of Gaussian functions. The time-dependent OEP (TDOEP) methods with and without the Tamm–Dancoff approximation are implemented by using a trial-vector algorithm, which allows us to avoid the storage or manipulation of transformed two-electron integrals or the diagonalization of large matrices. No approximation is made to TDOEP, besides the adiabatic approximation to the exchange kernel, the LCAO expansion of the orbitals and potentials, and occasionally the Tamm–Dancoff approximation. The vertical excitation energies of the beryllium atom and the nitrogen and water molecules calculated by TDOEP are compared with those obtained from time-dependent density functional theory (TDDFT) employing conventional local or gradient-corrected functionals, configuration interaction singles (CIS), time-dependent Hartree–Fock (TDHF) theory, similarity-transformed equation-of-motion coupled-cluster with single and double substitutions, and experiments. TDOEP, which neglects electron correlation while treating the exchange contribution rigorously within the Kohn–Sham DFT framework, performs equally well as, or even appreciably better than, CIS or TDHF. The slightly better performance of TDOEP might be attributed to the local nature of the exchange potentials that allows the bare orbital energy differences to approximate excitation energies well. Nevertheless, TDDFT employing local or gradient-corrected functionals outperforms TDOEP for low-lying valence excited states, implying that the former somehow accounts for electron correlation effectively, whereas for high-lying and Rydberg excited states, the latter performs better than the former. By combining the desirable features of OEP and local or gradient-corrected exchange-correlation potentials, we arrive at a simple asymptotic correction scheme to the latter. TDDFT with the asymptotic correction yields uniformly accurate excitation energies for both valence and Rydberg excited states.
Excitation of thermographic phosphors using a blue light emitting diode: Spectral characteristics and instrumentation applications
Authors: Allison S. W. , Cates M. R. , Gillies G. T.
Published: July 2002

A blue light emitting diode (LED) has been used as the excitation source in a series of sensor-related characterization studies carried out on thermographic phosphors. The motivation for the described effort is the potential utility of LEDs for fluorescence based thermometry applications. The phosphors that were evaluated included La2O2S:Tb, Gd2O2S:Tb, Y2O2S:Pr, and various other rare-earth activated ceramics. Periodic and pulsed excitation of the phosphors was demonstrated, with LED gate-on times of 10–12 μs at operating levels of ≈25 V. The spectral response of the phosphors under these conditions is described, and the implications of such devices for the design of electro-optic instrumentation systems (including remote thermometry applications) are discussed. A beneficial finding of this work is that a good match between the LED emission spectrum and the phosphor excitation spectrum is not required in order to produce useful, detectable fluorescence.
Comparison of the ultrafast to slow time scale dynamics of three liquid crystals in the isotropic phase
Authors: Gottke S. D. , Cang Hu , Bagchi Biman , Fayer M. D.
Published: July 2002

The dynamics of three liquid crystals, 4′(pentyloxy)-4-biphenylcarbonitrile (5-OCB), 4′-pentyl-4-biphenylcarbonitrile (5-CB), and 1-isothiocyanato-(4-propylcyclohexyl)benzene (3-CHBT), are investigated from very short time (∼1 ps) to very long time (>100 ns) as a function of temperature using optical heterodyne detected optical Kerr effect experiments. For all three liquid crystals, the data decay exponentially only on the longest time scale (> several ns). The temperature dependence of the long time scale exponential decays is described well by the Landau–de Gennes theory of the randomization of pseudonematic domains that exist in the isotropic phase of liquid crystals near the isotropic to nematic phase transition. At short time, all three liquid crystals display power law decays. Over the full range of times, the data for all three liquid crystals are fit with a model function that contains a short time power law. The power law exponents for the three liquid crystals range between 0.63 and 0.76, and the power law exponents are temperature independent over a wide range of temperatures. Integration of the fitting function gives the empirical polarizability–polarizability (orientational) correlation function. A preliminary theoretical treatment of collective motions yields a correlation function that indicates that the data can decay as a power law at short times. The power law component of the decay reflects intradomain dynamics.
Recursive construction for a class of radial functions. I. Ordinary space
Authors: Guhr Thomas , Kohler Heiner
Published: July 2002

A class of spherical functions is studied which can be viewed as the matrix generalization of Bessel functions. We derive a recursive structure for these functions. We show that they are only special cases of more general radial functions which also have a properly generalized, recursive structure. Some explicit results are worked out. For the first time, we identify a subclass of such radial functions which consist of a finite number of terms only.
Molecular dynamics study of the vibrational relaxation of OClO in bulk liquids
Authors: Chorny Ilya , Vieceli John , Benjamin Ilan
Published: July 2002

The vibrational relaxation of OClO in bulk water, acetonitrile, and ethanol is studied using classical and semiclassical molecular dynamics computer simulations. Nonequilibrium classical trajectory calculations provide insight into the early stages of vibrational energy relaxation of highly excited states. Equilibrium force autocorrelation functions are used to determine the relaxation rate for the v=1→v=0 transition. Good agreement with experiments is found. The calculations suggest that the hydrogen bonding in water, as reflected by the high density of librational modes, is the reason for the fast relaxation in this liquid compared with that in ethanol and acetonitrile.
Structure-sensitive oxidation of the indium phosphide (001) surface
Authors: Chen G. , Visbeck S. B. , Law D. C. , Hicks R. F.
Published: July 2002

The oxidation of anion- and cation-rich indium phosphide (001) has been investigated by exposure to unexcited molecular oxygen. Indium phosphide oxidation is an activated process and strongly structure sensitive. The In-rich δ(2×4) surface reacts with oxygen at 300 K and above. Core level x-ray photoemission spectra have revealed that the O2 dissociatively chemisorbs onto the δ(2×4), inserting into the In–In dimer and In–P back bonds. By contrast, the P-rich (2×1) reconstruction does not absorb oxygen up to 5×105 Langmuir at 300 K, as judged by the unperturbed reflectance difference spectrum and low energy electron diffraction pattern. Above 455 K, oxygen reacts with the (2×1) inserting preferentially into the In–P back bonds and to a lesser extent into the phosphorus dimer bonds.
Piezoelectric and electrostrictive strain behavior of Ce-doped BaTiO3 ceramics
Authors: Ang Chen , Yu Zhi , Jing Zhi , Guo Ruyan , Bhalla A. S. , Cross L. E.
Published: July 2002

The ferroelectric and strain behavior of the Ba(Ti1−yCey)O3 solid solutions was studied. Three phase transition temperatures were shifted with increasing Ce concentration, and pinched at y=0.06. The samples could be structurally indexed by x-ray diffraction in a tetragonal symmetry at y⩽0.06; and in a pseudo-cubic symmetry at y>0.06. The strain levels in the range of 0.15–0.19% at ∼60 kV/cm with small hysteresis were obtained for the samples with low content Ce. With increasing Ce concentration, the tetragonality decreases, the strain level decreases, and becomes less hysteretic. The physical mechanism of the electrostrictive, piezoelectric, and ferroelectric-relaxor behavior is briefly discussed.
An inhomogeneous model of protein dielectric properties: Intrinsic polarizabilities of amino acids
Authors: Song Xueyu
Published: July 2002

A simple inhomogeneous model of protein dielectric properties is discussed. A protein in solution is modeled as a collection of polarizable dipoles in a cavity embedded inside a dielectric medium. The intrinsic polarizabilities of 20 amino acids are assumed to be portable to all proteins in nature. A reasonable set of these polarizability values has been obtained by comparing dielectric fluctuations from molecular dynamics simulations with model calculations. The results are consistent within a data set of three small proteins.
Flexible constraints: An adiabatic treatment of quantum degrees of freedom, with application to the flexible and polarizable mobile charge densities in harmonic oscillators model for water
Authors: Hess Berk , Saint-Martin Humberto , Berendsen Herman J. C.
Published: July 2002

In classical molecular simulations chemical bonds and bond angles have been modeled either as rigid constraints, or as nearly harmonic oscillators. However, neither model is a good description of a chemical bond, which is a quantum oscillator that in most cases occupies the ground state only. A quantum oscillator in the ground state can be represented more faithfully by a flexible constraint. This means that the constraint length adapts itself, in time, to the environment, such that the rotational and potential forces on the constraint cancel out. An accurate algorithm for flexible constraints is presented in this work and applied to study liquid water with the flexible and the polarizable “mobile charge densities in harmonic oscillators” model. The iterations for the flexible constraints are done simultaneously with those for the electronic polarization, resulting in negligible additional computational costs. A comparison with fully flexible and rigidly constrained simulations shows little effect on structure and energetics of the liquid, while the dynamics is somewhat faster with flexible constraints.
Enhanced nitrogen incorporation by pulsed laser annealing of GaNxAs1−x formed by N ion implantation
Authors: Yu K. M. , Walukiewicz W. , Beeman J. W. , Scarpulla M. A. , Dubon O. D. , Pillai M. R. , Aziz M. J.
Published: July 2002

We demonstrate that pulsed laser annealing followed by rapid thermal annealing greatly enhances incorporation of substitutional N in N+-implanted GaAs. Films implanted to 1.8% N exhibit a fundamental band gap of 1.26 eV (a band gap reduction of 160 meV), corresponding to an N activation efficiency of 50%. The optical and crystalline quality of the synthesized film is comparable to GaNxAs1−x thin films of similar composition grown by epitaxial growth techniques. Compared to films produced by N+ implantation and rapid thermal annealing only, the introduction of pulsed laser annealing improves N incorporation by a factor of 5. Moreover, we find that the synthesized films are thermally stable up to an annealing temperature of 950 °C.
Fermi level unpinning in ex situ Schottky contacts on n-GaAs capped with low-temperature-grown GaAs
Authors: Lodha Saurabh , Janes David B. , Chen Nien-Po
Published: July 2002

The Schottky barrier behavior of a GaAs layer structure consisting of a thick n-GaAs layer, capped by a thin (3.5 nm) layer of as-grown unintentionally or Be-doped low-temperature-grown GaAs (LTG:GaAs), both grown by molecular beam epitaxy, has been studied. Nonalloyed, ex situ Schottky contacts using three different metals were fabricated on the LTG:GaAs-capped layers and on n-GaAs control samples, in order to study the interface barrier height (φb) versus the metal work function (φm). High frequency capacitance–voltage measurements, along with simulations that incorporate a complete description of the defect states in LTG:GaAs, were used to extract the φb values. The variation in φb with the metal work function is nearly six times greater in the LTG:GaAs capped contacts than in uncapped n-GaAs samples. This direct observation of Fermi level unpinning is consistent with earlier measurements that indicated the inhibited oxidation of LTG:GaAs layers in comparable structures for short air exposure times.
Electrical properties of highly conducting and transparent thin films of magnetron sputtered SnO2
Authors: Goodchild R. G. , Webb J. B. , Williams D. F.
Published: July 2002

Conducting and transparent thin films (60 and 120 nm) of tin oxide were prepared by reactive rf planar magnetron sputtering of an undoped hot-pressed polycrystalline tin oxide target onto unheated substrates. The effects of the oxygen partial pressure on the room-temperature electrical properties of the films were studied. The properties vary markedly with, for example, the value of resistivity ranging from a high of ∼8×10−1 Ω cm to a minimum value of about 3×10−3 Ω cm. This minimum is the lowest value of resistivity reported for undoped films prepared on unheated substrates. These films have optical transmittances greater than 80% averaged between 850 and 550 nm.
Common occurrence of distortion in semiconductor luminescence spectra
Authors: Warwick C. A.
Published: July 2002

The distortion of semiconductor luminescence spectra by selective absorption of totally internally reflected rays in the specimen is investigated by varying the effective field of view of the collection optics and by local excitation from a focused electron beam. For example, in undoped InP, the results show that an artifactual enhancement of the acceptor related luminescence by a factor of ∼50 occurs if the field of view commonly used in luminescence experiments (∼1 mm) is employed. In addition, a shoulder on a spectral peak close to the absorption edge, which could be interpreted as being due to a separate recombination mechanism, is shown to be merely an artifact arising from this distortion. This communication shows for the first time that the effect can be both detected and eliminated by a novel field-of-view masking technique.
Anisotropic aggregates as the origin of magnetically induced dichroism in ferrofluids
Authors: Reed Wayne , Fendler Janos H.
Published: July 2002

Static and dynamic polarized and depolarized light scattering, static, and time-resolved dichroic anisotropy, as well as conventional magnetization versus applied magnetic field determinations have been carried out on aqueous commercial ferrofluids and on surfactant aggregate stabilized Fe3O4 in aqueous solution. Over a dilution range of more than three orders of magnitude there is no evidence for field-induced cooperative effects. The shape of the dichroic anisotropy versus applied field curve superimposes virtually exactly onto the magnetization curve. Rotational and translational diffusion coefficients indicate ellipsoidal magnetic aggregates with average minor to major axial ratios around 0.33 and major axis of 285 nm, which are insensitive to dilution, and far above the expected value of around 10 nm. Electron micrographs have revealed polydisperse clusters of around 150 nm composed of particles with sizes on the order of 10 nm. The scattered intensity autocorrelation curve shows no appreciable change upon application of a magnetic field to the ferrofluids. Evidence for the shape anisotropy of the presumed 140-nm clusters is apparent in the depolarized light scattering autocorrelation decay curves. In the absence of field-induced particle chaining, aggregation, or shape deformation, the origin of the field-induced dichroism was attributed to the permanent shape anisotropy of the clusters. Subtle field-induced alteration of the spatial arrangement of particles within the stable clusters or an unexpected anisotropic polarizability of the magnetite crystals do not seem to be likely origins of the dichroic effect.
Particle size distribution effects on sintering rates
Authors: Chappell J. S. , Ring T. A. , Birchall J. D.
Published: July 2002

Recent interest in the use of near-monosize powders for ceramic processing has provoked questions concerning the dependence of the processing on the particle size distribution. One such factor is the sintering rate of the compact produced from a powder with a narrow particle size distribution. This issue is addressed theoretically by considering a log-normal particle size distribution which is characterized by a mean particle size and a width parameter. The sintering rates derived for model systems are examined for both the initial and intermediate stages, and these rates are found to be a strong function of the distribution width parameter. The sintering ability of a compact decreases with a broader particle size distribution and this effect becomes dramatic in the intermediate stage when grain growth is considered.
Mean excitation energies for ions in gases and plasmas
Authors: Garbet Xavier , Deutsch Claude , Maynard Gilles
Published: July 2002

A variational approach yields accurate upper and lower bounds for mean excitation energies and other related parameters describing the stopping of nonrelativistic point charges by isolated species and ions embedded in dense and hot matter of relevance to particle-driven inertial fusion. The resulting I compares nicely with previous ones by Hartree–Fock–Slater and with experimental data when available. An efficient pseudo-analytic formula based on the Thomas–Fermi method is obtained, together with a cubic spline interpolation variationally improved. It is shown that in high temperature plasmas (kBT≥10 eV) mean excitation energies are significantly smaller than their cold homologue.
Raman and x-ray scattering from ultrafine semiconductor particles
Authors: Kanata T. , Murai H. , Kubota K.
Published: July 2002

We prepared ultrafine germanium particles by gas-evaporation techniques. The particle composition was studied by Raman and x-ray scattering. We take an analytical model to explain the Raman profile: the particle is composed of a single-crystalline core and a coating amorphous shell. In this single-crystalline part, the phonon mode is completely localized, and as a result the red shift and the broadening of the Raman signal are induced. We introduce a localization factor to describe the size effect. According to this model we determined the mean diameter of a crystalline particle. The result is compared with that from x-ray diffraction. Our results indicate that the coating amorphous shell thickness is nearly constant, although the crystalline particle size varies by the evaporation-gas pressure. Furthermore, we found that the Raman profile changes by the wavelength of the exciting laser.
A plasma etching model based on a generalized transport approach
Authors: Zawaideh Emad , Kim N. S.
Published: July 2002

A plasma etching model for rf-discharge plasma reactors has been developed. The model considers the linear and nonlinear effects on plasma kinetics of atomic effects, ion chemistry, space-charge effects, and the plasma/surface interactions. The linear effects of physical and chemical etching and the nonlinear effects of the ‘‘enhanced’’ physical and chemical etching, which are due to the plasma/surface interactions, are also discussed. New generalized plasma transport equations are introduced. These equations are valid for collisional to weakly collisional plasmas [λ/L≲O(1)], where λ is the ion mean free path and L is the smallest of the scale lengths of the gradient in the electric field or in the macroscopic plasma parameters. The transport equations are used to calculate the magnitude and profile of the plasma particle and energy fluxes for the etching model. The model has shown good agreement with experimental etch rates of silicon dioxide, borophosphosilicate glass, and photoresist.
On the perturbation and break up of a high-speed, elongating metal jet
Authors: Pack D. C.
Published: July 2002

High-speed elongating metal jets, such as are produced by shaped charges, are known to break up into a number of fragments after some stretching. Attempts to explain the lengths of the segments have usually related to Rayleigh’s work on the separation of a jet of water into droplets as a result of surface tension and on extensions of that work, although more recently research based on the ideas of plastic flow has had some success in showing the growth of instabilities of approximately correct wavelength. In this paper, the full equations of axially symmetric plastic flow are used, along with the von Mises condition, and a series solution sought to first order in a small parameter Y/ρW2, where Y is the yield strength of the metal, ρ its density, and W the fall in velocity along the jet. The disturbances to velocity, shear stresses, and boundary oscillation arising from an initial small disturbance in the axial component of velocity have been found analytically. The results show that the amplitudes of the disturbances grow towards equality with the radius of the elongating jet for times and wavelengths compatible with observations of break up. Two possible criteria for break up are examined: namely, ‘‘necking’’ and the creation of voids in the jet when all principal stresses become tensile. There is radiographic evidence that both kinds of break up can occur. The results obtained here confirm the significance of these mechanisms, but are not able to distinguish sufficiently between them as the prime cause of break up in a given instance.
Crystal structure of R(Ti,Fe)12(R=Nd,Sm) compound
Authors: Liu N. C. , Kamprath N. , Wickramasekara L. , Cadieu F. J. , Stadelmaier H. H.
Published: July 2002

Ferromagnetic phases in Fe-rich Nd-Ti-Fe and Sm-Ti-Fe systems have been synthesized by rf sputtering. Samples have been directly crystallized onto heated substrates with in-the-film-plane magnetic fields applied, as well as first deposited, in an amorphous form and then subsequently crystallized. For the Sm-Ti-Fe system only, a hard magnetic phase has been observed in subsequently crystallized amorphous deposits. The in-plane intrinsic coercivity iHc of this phase was 31 kOe at 293 K and 58 kOe at 5 K. Both these systems exhibited a relatively soft magnetic phase near 85 at. % Fe with a coercivity of 1–2 kOe. The x-ray diffraction patterns of this phase were identified as tetragonal TnMn12 type and its composition was R(TiFe11). Lattice parameters are a=8.579±0.002 and c=4.799±0.002 Å for Sm(TiFe11). These parameters were fitted from 14 x-ray lines from the pattern with 2 θ between 25° and 55°. It has also been possible to directly crystallized Nd-Ti-Fe and Sm-Ti-Fe films that exhibited an extreme (300) texture of the R2Fe17 phase.
Asymmetries in dislocation densities, surface morphology, and strain of GaInAs/GaAs single heterolayers
Authors: Kavanagh K. L. , Capano M. A. , Hobbs L. W. , Barbour J. C. , Marée P. M. J. , Schaff W. , Mayer J. W. , Pettit D. , Woodall J. M. , Stroscio J. A. , Feenstra R. M.
Published: July 2002

The dislocation densities, surface morphology, and strain of Ga1−xInxAs/GaAs epitaxial interfaces as a function of indium composition and layer thickness have been investigated by transmission electron microscopy, medium energy ion blocking, and double-crystal x-ray diffractometry. The electron microscopy shows that in the thinnest dislocated films (90 and 160 nm, x=0.07) 60° α dislocations form first in one 〈110〉 direction at the interface. Surprisingly, however, an asymmetry in residual layer strain is not detected in these samples, suggesting that the dislocations have the same Burgers vector or are evenly distributed between two Burgers vectors. Orthogonal arrays of dislocations are observed in films thicker than 300 nm (60° and edge-type, x=0.07). In this case, dislocation densities in each 〈110〉 direction are equal to within experimental error while an asymmetry in in-plane strain is measured (18% and 30% for x=0.07, 300, and 580 nm thick, respectively). An unequal distribution of Burgers vectors of 60° or edge-type dislocations is considered responsible for the strain asymmetry in these thicker samples.
Effect of large boron additions to magnetically hard Fe-Pt alloys
Authors: Inomata K. , Sawa T. , Hashimoto S.
Published: July 2002

Magnetic properties were investigated for the ternary (Fe0.6Pt0.4)100−xBx and (Fe1−yPty)83B17 alloy ribbons prepared by the twin roll technique. Boron changes the compositions of the ordered FePt (γ1) phase, which coexists with Fe2B at various alloy compositions so that the actual Pt content of γ1 in the ternary alloy goes up with increasing boron content. Coercive force increase due to boron addition is remarkable, in particular at low Pt concentration. Annealing time to get high coercivity becomes much shorter by boron addition. iHc=5.6 kOe and 4πMs=7.8 kG were obtained in the (Fe0.65Pt0.35)83B17 ribbon.
Resonant decay instability of a Langmuir wave in a plasma slab
Authors: Jain V. K. , Sharma A. K. , Tripathi V. K.
Published: July 2002

The parametric decay of a Langmuir wave into a Langmuir wave and an ion acoustic wave is studied in a plasma slab. The coupled mode equations are solved using the perturbation technique which yields a dispersion relation for the parametric instability. The growth rate of the instability is found to increase with the width of the plasma slab and approaches the value for an infinite uniform plasma.
Magnetic anisotropy and crystal structure of intermetallic compounds of the ThMn12 structure
Authors: Solzi M. , Pareti L. , Moze O. , David W. I. F.
Published: July 2002

Intermetallic compounds of the type RE(TM1−xMx)12 with RE=Y, Er; TM=Fe, Co; M=Si, Ti, V of the ThMn12 tetragonal crystal structure have been studied by the singular point detection technique and high-resolution neutron powder diffraction in order to (a) evaluate both the rare-earth and transition-metal sublattice contributions to the magnetocrystalline anisotropy, (b) compare the Fe and Co anisotropies in the ThMn12 structure, and (c) to verify the existence of any preferential entrance in the transition metal sublattice for various stabilizing ions. The transition metal anisotropy is of the same type (Fe axial, Co planar) and magnitude as in RE2Fe14B compounds. However, there is no anomalous temperature behavior of the anisotropy field Ha. With decreasing temperature, the value of Ha increases from 21 kOe at 293 K to 37 kOe at 78 K in YFe11Ti. Similar values are obtained for other Fe-based compounds. The contribution of Er to the anisotropy is found to be surprisingly low. The anisotropy field of ErFe11Ti varies from 24 kOe at 293 K to 63 kOe at 100 K. The observed low values of the Curie temperatures in these compounds appears once again to limit their use in possible applications. Neutron powder diffraction shows that the 8j and 8f sites of the ThMn12 structure are virtually fully occupied by Fe, while the 8i site is partially occupied by Fe and V in YFe10V2.
Simple method for the simulation of multiple elastic scattering of electrons
Authors: Liljequist D. , Salvat F. , Mayol R. , Martinez J. D.
Published: July 2002

A screened Rutherford cross section is modified by means of a correction factor to obtain the proper transport cross section computed by partial-wave analysis. The correction factor is tabulated for electron energies in the range 0–100 keV and for elements in the range from Z=4 to 82. The modified screened Rutherford cross section is shown to be useful as an approximation for the simulation of plural and multiple scattering. Its performance and limitations are exemplified for electrons scattered in Al and Au.
The influence of electron-hole density profile on the picosecond time-resolved reflectivity measurement in silicon
Authors: Ma H. M. , Liu Y. X. , Fei Y. , Li F. M.
Published: July 2002

The influence of the spatial distribution of electron-hole plasma on reflectivity measurement is studied in this paper. The plasma density deduced from the reflectivity measurement represents the surface density satisfactorily, if the pump penetration depth is not too short compared with probe wavelength. Besides, when plasma resonance does not take place, the probe incident angle should not be close to Brewster’s angle [see, for example, M. Born and E. Wolf, Principles of Optics, 6th ed. (Pergamon, New York, 1980), p. 43] for exponential-like plasma density distribution; otherwise, when the phasma resonance takes place, strong nonlinear recombination and diffusion should be present. In other cases, a general applicable method for data processing is suggested.
Pyroelectric detection of ultraviolet laser ablation products from polymers
Authors: Dyer P. E. , Srinivasan R.
Published: July 2002

A simple pyroelectric detector has been constructed and used to measure the ablation velocity for various polymers as a function of laser (248 nm) fluence. The advantage of this approach over other techniques is that a full sampling of the products in the ablation plume is obtained. Results for polymethyl methacrylate (PMMA) ablation can be interpreted in terms of a single explosive decomposition process in which numerous small fragments such as C2, CO, and CO2 propel small segments of the polymer at velocities which are scaled to the square root of their molecular weights. Data for polymide suggest that this polymer is decomposed by laser photons principally into small molecules. The solid carbon that is obtained on ablation in air is probably derived by reactions between carbon clusters. This technique is suitable for the study of the angular distribution of the ejected material which is useful for modeling and in schemes for the deposition of material by laser ablation.
Anomalous demagnetization process at very low temperature in Nd-Fe-B magnets
Authors: Otani Yoshichika , Coey J. M. D. , Barbara B. , Miyajima Hideki , Chikazumi Soshin , Uehara M.
Published: July 2002

Both Nd15Fe77B8 and Sm(CoCu)5 sintered magnets have been reported to exhibit staircase hysteresis loops with giant Barkhausen jumps below about 10 K. The pattern is reproducible at a fixed temperature. Here the observation of heat pulses associated with the magnetization jumps in Nd15Fe77B8 magnets is reported. The origin of these Barkhausen jumps in sintered magnets is discussed as a two-stage trigger/propagation process. The first stage may be dominated by magnetization reversal in volumes of δ3, where δ is the domain-wall thickness, as in the magnetic after effect, whereas the second stage reflects thermally induced domain-wall motion facilitated by the small lattice specific heat below about 10 K.
Surface mechanisms in O2 and SF6 microwave plasma etching of polymers
Authors: Joubert O. , Pelletier J. , Fiori C. , Nguyen Tan T. A.
Published: July 2002

Photoresist etching mechanisms in O2 abd SF6 microwave plasmas are investigated using x-ray photoelectron spectroscopy (XPS) and etch rate measurements. Experiments are performed in a microwave multipolar plasma using an electron cyclotron resonance at 2.45 GHz and independent rf biasing at 13.56 MHz. The photoresist etch rates are studied as a function of the parameters of the plasma polymer interaction. As in an O2 plasma, the etch rate in SF6 exhibits a two-step evolution with ion energy as well as a monolayerlike adsorption of atomic fluorine on photoresist. The relationship between the surface mechanisms deduced from the etch kinetics and the surface compositions analyzed by XPS is explored. The effect of reactive species concentration, intensity of ion bombardment, and surface temperature on etching and/or degradation of the photoresist is investigated. In particular, the phenomenon of resist damage, described as a graphitization of the polymer layer, is shown to appear when the mechanical effects of ion bombardment become significant with respect to the chemical effects.
Detecting two magnetization components by the magneto-optical Kerr effect
Authors: Florczak J. M. , Dahlberg E. Dan
Published: July 2002

A novel technique was detecting two orthogonal in-plane magnetization components is presented. This technique utilizes the magneto-optical Kerr effects to sense the two components. These components of magnetization are parallel and perpendicular to the plane of incidence of the light beam. The ability to sense two components, individually or simultaneously, is a result of the disparity in the longitudinal and transverse Kerr effects. Based on the Frensel reflection coefficients of these two effects, an anlysis is presented describing this dual component sensitivity. The physical conditions are given for simultaneous and individual detection of the two in-plane magnetization components. To substantiate this analysis, magneto-optical measurements are made on single-crystal Fe films. The results are discussed in the context of dual component sensitivity.
1- and 10-V series array Josephson voltage standards in Nb/Al2O3/Nb technology
Authors: Pöpel R. , Niemeyer J. , Fromknecht R. , Meier W. , Grimm L.
Published: July 2002

Josephson arrays containing 2000 and 20 160 Nb/Al2O3/Nb tunnel junctions produce stable quantized voltage steps up to 1.5 and 14.5 V, respectively, with current amplitudes on the order of 100 μA. In precision measurements these voltage steps proved to be stable for typically 30 min. Fabricating the voltage standards with the help of the Nb/Al2O3/Nb technology results in a long durability.
The influence of oxygen on SiO2 sputtering
Authors: Holmén G. , Jacobsson Harald
Published: July 2002

Argon-ion sputtering of thin silicon dioxide films has been studied both in ultrahigh vacuum and in a low pressure oxygen ambient. The variation in ion-induced secondary electron yield with decreasing oxide thickness was used to determine the sputtering yield. The method was found to be a simple and accurate way of in situ measurement of thin-film sputtering. A 30% decrease in sputtering yield was observed for 200-keV Ar ion bombardment when the oxygen pressure was increased from 1×10−8 to 1×10−6 mbar. Secondary electron yield variations were found to give information also on the surface modification process. A mechanism is proposed in which oxygen is incorporated into the oxide due to the ion bombardment. Comparison with ion beam induced oxidation is also made.
Characterization of instability in amorphous silicon thin-film transistors
Authors: Kaneko Yoshiyuki , Sasano Akira , Tsukada Toshihisa
Published: July 2002

Instability mechanism of amorphous silicon-silicon nitride thin-film transistors (TFTs) is examined. By investigating double-layer insulator TFTs, it is demonstrated that the instability is caused by an electrical charge stored at the interface between amorphous silicon and silicon nitride. The amount of stored charge at the interface (Q) does not depend on either drain voltage or drain current. Study on TFTs with several insulator thicknesses has shown that Q strongly depends on the band bending in the amorphous silicon that is related to the gate electric field (E) through the gate insulator. The Q-E relationship is found to be a more general expression of the dependence of threshold voltage shift on gate voltage, and is incorporated into a formula suitable for examining the interface quality.
Evidence of the c-axis magnetic moment in single-crystal Dy
Authors: Willis Frank , Ali Naushad
Published: July 2002

We have studied the magnetic properties of single-crystal Dy and Tb in the temperature range from 4.2 to 250 K. We observe the Néel transition for Dy and Tb at 180 and 230 K, respectively. The Curie temperature was found to be 90 and 220 K for Dy and Tb, respectively. These values for TN and TC agree well with earlier measurements by Behrendt (1958) and Koehler (1963). In addition to these transitions, we observe a new anomaly in single-crystal Dy near 4.5 K manifested by a reduction of the magnetization in the basal plane with a simultaneous rise in the magnetization along the c axis. This suggests the magnetic moments may rise out of the basal plane of Dy and form a component along the c axis at low temperatures. We also observe a small broad peak in the ac susceptibility of Dy near 167 K which is likely due to the vortex state of Dy. The ac susceptibility along the b axis of single-crystal Tb shows a rapid decrease below 30 K while the c-axis susceptibility shows a corresponding increase.
Phase analysis and magnetic properties of RTiFe11−xCox (R=Y,Gd) (x=0–11)
Authors: Cheng S. F. , Sinha V. K. , Ma B. M. , Sankar S. G. , Wallace W. E.
Published: July 2002

Both as-cast and 1000 °C annealed RTiFe11 samples are nearly single phase with a small amount of Fe2Ti. Small amounts of Co additions do not change the microstructure, but the minor phase changed to a Fe,Co,Ti solid solution. At high Co concentration, as-cast samples are multiphase [1-12, 2-17, (Fe,Co,Ti)] and high-temperature (∼1200 °C) annealing is needed in order to produce a single-phase microstructure. Results of x-ray analysis and magnetic measurements show that YTiFe11, YTiCo11, GdTiFe11, and GdTiCo11 are uniaxial and their anisotropy fields at room temperature are 22, 18, 33, and 25 kOe, respectively. Therefore, Fe and Co sublattices are uniaxial with Fe sublattice anisotropy stronger than that of Co. In the intermediate range of composition, there is a conical-planar region for both Y and Gd systems and spin reorientations with temperature were observed. In general, the anisotropy behavior of the RTiFe11−xCox systems (R=Y,Gd) is similar but opposite to their counterpart of R2Fe17−xCox. This is due to the similarity of their crystal structure, as both are derivatives of 1-5. However, the strong contribution from the K2 anisotropy constant makes the anisotropy phase diagrams of 1-12 more complicated than those of 2-17.