bluuebunny/crossref_metadata_2025 huggingface dataset:
Piezoelectric properties and phase transitions of 〈001〉-oriented Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals
Authors: Ren W. , Liu S-F. , Mukherjee B. K.
Published: July 2002

Electric-field-induced phase transitions and piezoelectric properties of 〈001〉-oriented Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals for x=4.5% and 8% (PZN–4.5%PT and PZN–8%PT) have been investigated as a function of temperature. It was found that the piezoelectric properties and phase transitions for both crystals are strongly dependent on temperature. The measurements of polarization and strain as a function of unipolar electric field show that the field for the rhombohedral–tetragonal phase transition decreases linearly with temperature in the range between 25 and 105 °C. Thus, raising the temperature can stabilize the tetragonal phase in 〈001〉-oriented PZN–PT crystals. Longitudinal piezoelectric constant d33 in the rhombohedral phase increases with temperature for both crystals; in PZN–4.5%PT the slope itself increases significantly at temperatures over 60 °C. In the field-induced tetragonal phase, there is little change in d33 for PZN–8%PT within the temperature range investigated, while in PZN–4.5%PT a slight increase of d33 with temperature was observed at temperatures above 80 °C.
Effects of energetic electrons on magnetized electrostatic plasma sheaths
Authors: Tskhakaya D. , Kuhn S. , Petržilka V. , Khanal R.
Published: July 2002

The influence of energetic electrons on magnetized plasma sheaths is studied for different current regimes and different secondary-electron emission coefficients. (Here, the term “plasma sheath” denotes the collisionless region consisting of the non-neutral Debye sheath and the quasi-neutral magnetic presheath.) It is shown that the presence of even a small population of energetic electrons can significantly influence the potential drop across the sheath and the energy flux to the wall. For example, for plasma parameters typical of contemporary tokamaks, the presence of a fast-electron population with density smaller than 0.1% (!) can double the potential drop across the sheath and the energy flux to the wall, and the presence of a few percent of fast electrons can enhance these values by up to one order. The effect of fast electrons decreases with increasing secondary-electron emission coefficient and increasing current to the wall. Analytical results obtained are checked against particle-in-cell (PIC) simulations for different current regimes and different secondary-electron emission coefficients, showing good agreement except in some cases where the simulation results exhibit strong fluctuations.
Langevin dynamics simulation of counterion distribution around isolated flexible polyelectrolyte chains
Authors: Liu S. , Muthukumar M.
Published: July 2002

We have investigated systematically the nature of counterion condensation on the uniformly charged flexible polyelectrolyte chain by monitoring the role of counterion valency, chain length, temperature, and dielectric constant through a Coulomb strength parameter, and volume of the system. Radius of gyration of the polymer, radial distribution functions of monomers and counterions, and formation of dipoles are also studied. Our simulations show that the essential features of counterion condensation for a flexible polyelectrolyte are qualitatively different from the Manning theory.
Cellular pattern formation in circular domains
Authors: Palacios Antonio , Gunaratne Gemunu H. , Gorman Michael , Robbins Kay A.
Published: July 2002

An analysis of stationary and nonstationary cellular patterns observed in premixed flames on a circular, porous plug burner is presented. A phenomenological model is introduced, that exhibits patterns similar to the experimental states. The primary modes of the model are combinations of Fourier–Bessel functions, whose radial parts have neighboring zeros. This observation explains several features of patterns, such as the existence of concentric rings of cells and the weak coupling between rings. Properties of rotating rings of cells, including the existence of modulated rotations and heteroclinic cycles can be deduced using mode coupling. For nonstationary patterns, the modal decomposition of experimental data can be carried out using the Karhunen–Loéve (KL) analysis. Experimental states are used to demonstrate the possibility of using KL analysis to differentiate between uniform and nonuniform rotations. The methodology can be extended to study more complicated nonstationary patterns. In particular, it is shown how the complexity of “hopping states” can be unraveled through the analysis.
A fast serial algorithm for the finite temperature quenched Potts model
Authors: Hassold G. N. , Holm Elizabeth A.
Published: July 2002

An efficient serial algorithm for finite temperature, quenched Potts model simulations of domain evolution has been developed. This ‘‘n-fold way’’ algorithm eliminates unsuccessful spin flip attempts a priori by flipping sites with a frequency proportional to their site activity, defined as the sum of the probability of success for every possible spin flip at that site. Finite temperature efficiency for high-spin degeneracy systems is achieved by utilizing a new, analytical expression for the portion of the site activity due to flips to non-neighbor spin values. Hence, to determine the activity of a site, only flips to the nearest neighbor spin values need be considered individually; all other flips are evaluated in a single expression. A complexity analysis of this algorithm gives the dependence of computing time on system parameters and on simulation progress. While a conventional Potts model algorithm has a constant computing time per simulation timestep, the n-fold way algorithm increases in efficiency as domain coarsening progresses. Computer experiments confirm the complexity analysis results and indicate that the n-fold way algorithm is much more efficient than the conventional algorithm even at high fractions of the critical temperature.
Shock viscosity and the prediction of shock wave rise times
Authors: Swegle J. W. , Grady D. E.
Published: July 2002

The present study is focused on viscouslike behavior of solids during large-amplitude compressive stress-wave propagation. Maximum strain rate in the plastic wave has been determined for 30 steady- or near steady-wave profiles obtained with velocity interferometry methods. The materials include six metals, aluminum, beryllium, bismuth, copper, iron, and uranium, and two insulating solids, magnesium oxide and fused silica. A plot of Hugoniot stress versus maximum strain rate for each material is adequately described by η̇=aσmh. The exponent m is approximately 4 for all materials while the coefficient a is material dependent. A model is developed which incorporates the observed trends of the shock viscosity data in a three-dimensional framework. Finite-difference calculations using the model reproduce the experimental wave profile data.
Nonlinear integrated optics
Authors: Stegeman George I. , Seaton Colin T.
Published: July 2002

We review the work done to date in the field of nonlinear integrated optics. Emphasis is placed on (intrinsic) second- and third-order phenomena occurring in planar geometry structures and several applications are discussed. All of the nonlinear interactions are discussed in a single notation.
Stability of chains of permeable spherical beads in an applied magnetic field
Authors: Paranjpe R. S. , Elrod H. G.
Published: July 2002

An analysis is presented which has application to the magnetic brush method of development in an electrophotographic process. In this analysis, the stability of a collection of bead-chains in a nonuniform magnetic field is examined from the point of view of minimizing the potential energy of the collection. The most stable chain length is determined. Then the mechanism of individual chain breakage is examined. A magnetomechanical stability analysis of the bead-chain structure is carried out using small perturbations to investigate the stability of a single chain and to determine the onset of instability as the chain is rotated with respect to the applied field direction. We find that the length of the most stable chain does not depend on the strength of the applied field, but depends on its rate of falloff. We also find that chain breakage occurs at a definite angle between the applied field and the chain.
Effective thermal conductivity of composites with a thermal contact resistance between the constituents: Nondilute case
Authors: Benveniste Y.
Published: July 2002

The effective thermal conductivity of a particulate composite exhibiting a thermal contact resistance at interphase boundaries is considered. Two micromechanical models, the generalized self-consistent scheme, and the Mori–Tanaka theory [Acta Metall. 21, 571 (1973)] previously used in composite media with perfect interfaces, are generalized to allow for the phenomenon of thermal contact resistance between the constituents. Spherical inclusions are considered and their interaction is approximately taken into account, so that the presented treatment is valid at nondilute concentrations. Both methods, distinctly different in their approach, result in the same closed-form simple expression for the effective thermal conductivity.
Thermodynamic theory of PbTiO3
Authors: Haun M. J. , Furman E. , Jang S. J. , McKinstry H. A. , Cross L. E.
Published: July 2002

A phenomenological thermodynamic theory of PbTiO3 was developed using a modified Devonshire form of the elastic Gibbs free energy. The spontaneous strain as a function of temperature was determined from pure sol-gel derived PbTiO3 powder and used with selected data from the literature to determine the coefficients of the energy function. The theoretical prediction of the phase stability, spontaneous polarization and strains, and dielectric and piezoelectric properties agree well with experimental data. This theory provides a way of predicting the intrinsic single domain dielectric and piezoelectric properties of PbTiO3, which have not been completely determined from experimental measurements.
A magnetomechanical model of the magnetic brush
Authors: Paranjpe R. S. , Elrod H. G.
Published: July 2002

A mathematical model based on first principles has been formulated to predict the behavior of the magnetic brush in the development zone of an electrophotographic process. As a first approximation, the brush is modeled as a collection of straight chains whose length distribution is derived from a magnetic energy minimization principle. Results with the model are compared with experimental measurements of the tangential and normal forces exerted by the brush on the photoreceptor. The agreement is sufficiently good to confirm the essential aspects of the theory.
Laser processing of a SiC/Al-alloy metal matrix composite
Authors: Dahotre Narendra B. , McCay T. Dwayne , McCay Mary Helen
Published: July 2002

Preliminary studies were conducted on the laser processing of SiC/A356-Al alloy metal matrix composite (MMC) for such applications as welding/joining and cutting. The SiC/A356-Al MMC was processed using several different laser specific energies. Microstructural observations after laser processing revealed that the extent of reinforced material (SiC)-matrix (A356-Al) reaction is directly proportional to the laser energy input. As energy input increased, SiC particle dissolution became greater and aluminum carbide formation increased in both size and quantity. It appears possible to control substantial change (physical and chemical) in SiC particles during processing by controlling the amount and mode of energy input.
Correlation of interference effects in photoreflectance spectra with GaAs homolayer thickness
Authors: Kallergi N. , Roughani B. , Aubel J. , Sundaram S.
Published: July 2002

Undoped GaAs epilayers grown by metalorganic chemical vapor deposition on Zn- and Si-doped GaAs substrates were studied using the technique of photoreflectance (PR) spectroscopy. Oscillations were observed in the PR spectra of the samples grown on n-type substrates below the fundamental band gap of GaAs. The origin of these oscillations was investigated in terms of interference between the light beam reflected from the air/film interface and the modulated beam reflected from the film/substrate interface. The latter is due to the modulation of the index of refraction of the substrate. The thickness of each film was calculated by a model which was used to fit the experimental spectra of the differential change in reflectance (ΔR/R) versus energy. The observed changes in the period of these oscillations as a function of film thickness were in good agreement with the calculated results. Scanning electron microscope measurements of the thicknesses of the films supported these calculations.
Effect of additive elements on magnetic properties and irreversible loss of hot-worked Nd-Fe-Co-B magnets
Authors: Yoshikawa N. , Kasai Y. , Watanabe T. , Shibata S. , Panchanathan V. , Croat J. J.
Published: July 2002

There has been limited work on the effects of alloying elements on the properties of hot-worked anisotropic magnets. The coercivity mechanisms for sintered and rapidly solidified Nd-Fe-B magnets are different, and additives can have different effects on magnetic properties. The present study was conducted to clarify the effects of various alloying elements on the properties of hot-worked magnets. Ribbons of composition (Nd13.6−xRx)(Fe77.6−yTyCo2.8)B6.0 (R=Ce and Dy; T=Al, V, Cu, Ga, Zr, Nb, Mo, and Sn; x=0, 1, and 2; y=0, 0.5, and 1.0) were prepared by melt spinning. Anisotropic magnets were made at 1073 K with 55.6% reduction in height by die upsetting. Room-temperature magnetic properties and irreversible losses after exposure at 323–473 K were determined. The results showed that small amounts of Al, V, Cu, and Ga enhanced the remanence of the magnets, while Dy, Al, Ga, Zr, Nb, and Mo improved the coercivity. Dy, Ga, and Mo additions were effective in increasing the heat resistance temperature (HRT), defined as the temperature where the irreversible loss is 3%. HRT is dependent on initial intrinsic coercivity and the squareness ratio (Hk/iHc) of the demagnetization curve. An equation relating these parameters is given. An HRT of 453 K was obtained with a suitable alloy composition.
Microstructure and physical properties of Mn-Zn ferrites for high-frequency power supplies
Authors: Otsuki E. , Yamada S. , Otsuka T. , Shoji K. , Sato T.
Published: July 2002

The power loss of Mn-Zn ferrites and its dependency on the microstructural factors have been investigated. The power loss was found to be lowered due to the formation of a high-electrical-resistive boundary layers, which is accomplished by an addition of specific refractory oxides, which are concentrated on the grain boundary and form a boundary layer with CaO. The new material, having a low power loss at high frequency (∼1 MHz), has been developed with the HfO2 addition.
New methods for preparing mercury-based ferrofluids
Authors: Linderoth S. , Rasmussen L. H. , Mo/rup S.
Published: July 2002

Metallic ferrofluids based on magnetic particles in mercury have been produced by two new methods. Alloy particles of Fe-B, Fe-Co-B, Fe-Ni-B, and Co-B were prepared by reduction of the transition metal ions in aqueous solutions by NaBH4 and subsequently suspended in mercury. In one preparation, the metallic mercury was added to the solution with precipitates and in another method produced simultaneously with the precipitates by reduction of mercury ions to metallic mercury. The samples were characterized by magnetization and Mössbauer spectroscopy measurements.
Universal dispersion and power curves for transverse magnetic waves propagating in slab waveguides with a nonlinear self-focusing substrate
Authors: Fontaine Marie
Published: July 2002

For TM-polarized waves, a mode-power measure is applied to characterize optical waveguides with a nonlinear self-focusing substrate, producing a concise overview of the waveguiding properties at a given power for various designs. We also show how birefringence varies between the TE and TM modes as a function of the total power carried by the waveguide. Since our description is based on universal parameters, our results are applicable to different geometries of waveguide through simple scaling rules.
Polytype-controlled single-crystal growth of silicon carbide using 3C→6H solid-state phase transformation
Authors: Yoo Woo Sik , Matsunami Hiroyuki
Published: July 2002

The phase transformation from 3C-SiC to 6H-SiC at high temperature has been applied to crystal growth of SiC. Single-crystal growth of controlled polytype has been carried out on chemical-vapor-deposition-grown 3C-SiC(001) films by using a sublimation method. The polytype of grown layers was controlled as either cubic 3C-SiC or hexagonal 6H-SiC. The growth temperature was varied in the range of 1800–2400 °C. A growth rate up to several hundred micrometers per hour was achieved. The polytypes, crystallographic planes, and crystallinity of the grown layers were examined using photoluminescence, Raman spectroscopy, x-ray diffraction, reflection high-energy electron diffraction, and molten KOH (potassium hydroxide) etching. The polytype of the grown layers was changed from 3C-SiC to 6H-SiC with an increase in substrate temperature, and 6H-SiC(011̄4) planes were grown on 3C-SiC(001) at high temperatures. The crystallographic relationship between 6H-SiC(0114) and 3C-SiC(001) is described. The growth mechanism of 6H-SiC(011̄4) on 3C-SiC(001) is discussed based on experimental results on the thermal stability of 3C-SiC.
Effective dielectric and elastic constants of piezoelectric polycrystals
Authors: Olson Tamara , Avellaneda Marco
Published: July 2002

The effective dielectric constant, bulk modulus, and shear modulus of isotropic polycrystals with piezoelectric grains are studied using an effective medium approximation (EMA) and generalized Hashin–Shtrikman bounds. The EMA determines self-consistently the electromechanical interaction of grains with the surrounding composite. Numerical values for the moduli are computed for barium titanate and compared with available experimental data, as well as with classical estimates for the moduli. Further assessment of the EMA is made by computing numerical values of the effective moduli for ideal polycrystals, based on numerical data for crystals with strong piezoelectric coupling and comparing the resulting values with classical estimates. Similar comparisons are made for the generalized Hashin–Shtrikman bounds. On ‘‘ideal’’ polycrystals the gap between the upper and lower bounds can be 30% narrower than the corresponding gap if piezoelectric coupling is neglected.
Domain wall clamping in ferroelectrics by orientation of defects
Authors: Robels U. , Arlt G.
Published: July 2002

A quantitative model which describes the aging of the material properties in acceptor doped ferroelectric ceramics as a result of the reorientation of defects in time is proposed. By this reorientation the domain walls are clamped by a time dependent force leading to the observed decrease of the material properties during aging. The model is experimentally confirmed with aging and deaging measurements on barium titanate and lead zirconate titanate ceramics.
Generation and annihilation of traps in metal-oxide-semiconductor devices after negative air corona charging
Authors: Prasad Ila , Srivastava R. S.
Published: July 2002

Surface and bulk traps along with positive oxide charge accumulation have been found to be generated in metal-oxide-semiconductor capacitors, when subjected to negative air corona discharge at slightly reduced pressure (≂10−1 Torr). The effects are neutralized and device quality improved when annealed at 200 °C in air. The bulk traps and a fraction of oxide charges were annealable when kept at room temperature for several months. The results have been analyzed by Nicollian–Goetzberger’s conductance technique and a plausible explanation is given.
Silicon carbide diode operating at avalanche breakdown current density of 60 kA/cm2
Authors: Vassilevski K. V. , Dmitriev V. A. , Zorenko A. V.
Published: July 2002

SiC diode operating at high avalanche breakdown current has been fabricated. The 6H-SiC p-n structure was grown by liquid phase epitaxy. The diodes displayed avalanche breakdown at ∼80 V. The breakdown current density of 60 kA/cm2 and the input power density of 9 MW/cm2 were achieved at a current pulse duration of 60 ns. For the first time, the temperature coefficient of avalanche breakdown voltage of SiC pn junction was studied at high current density. It was found that the avalanche breakdown voltage of the diodes decreases with temperature from 300 to 750 K and increases from 750 to 900 K.
Dispersion observed in electrical properties of titanium-substituted lithium ferrites
Authors: Kuanr Bijoy Kumar , Srivastava G. P.
Published: July 2002

The current-voltage characteristic of polycrystalline titanium-substituted lithium ferrites has been measured at temperatures above room temperature. At voltages below a critical value an Ohmic relationship is observed, while it obeys a power-law relationship (i∝Vα, where α is a temperature-dependent constant) above the critical voltage. At low voltages conduction is determined by the boundary layer, whereas at higher voltages, the current in the solid is mainly due to the space-charge-limited carriers. ac conductivity (σac) and dielectric permittivity (ε′) show a large temperature-dependent dispersion in the 5–10-MHz range. The experimental results indicate that titanium substitution suppresses all these parameters. The dielectric relaxation has a very large relaxation intensity. In the theoretical model the temperature- and frequency-dependent layer properties are inserted into Koop’s model and the theoretical dispersion curves are obtained. The agreement of these data with the experimental findings is remarkable.
Co-Sn substituted barium ferrite particles
Authors: Zhou X. Z. , Morrish A. H. , Yang Zheng , Zeng Hua-Xian
Published: July 2002

Co-Sn substituted barium ferrite particles, BaFe12−2xCoxSnxO19 (0≤x≤1.4), have been investigated using x-ray, VSM, and Mössbauer spectroscopy. The magnetocrystalline-anisotropy constant has been determined as a function of x from an analysis of the approach to magnetic saturation. Mössbauer spectra indicate that Co2+ and Sn4+ ions have a strong preference for the 2b, 4fVI, and 12k sites. The reduced anisotropy constants k=K1(x)/K1(0) have been calculated based on the single-ion model and the site preferences and agree well with the measured values. The origin of the magnetic changes caused by Co2+ and Sn4+ substitution can be attributed to the site preferences.
Deep level defects and carrier removal due to proton and alpha particle irradiation of InP
Authors: Rybicki George C. , Zorman Christian A.
Published: July 2002

Deep level transient spectroscopy and capacitance voltage measurements of the defect introduction rates and carrier removal rates for 2 MeV proton and 2 MeV alpha particle irradiations in p-type InP have been performed. The carrier removal rate for 2 MeV protons was 6500 cm−1 and for 2 MeV alpha particles was 73 400 cm−1, or 12 times higher. The defect introduction rates for H4, the predominant radiation induced defect in p-type InP, were 229 per 2 MeV proton and 9000 per 2 MeV alpha particle, or 39 times higher for alpha irradiation. The very large rates of carrier removal and high defect introduction rates in the latter case may limit the usefulness of InP in a radiation environment containing high energy alpha particles.
Structure analysis of coprecipitated ZnFe2O4 by extended x-ray-absorption fine structure
Authors: Jeyadevan B. , Tohji K. , Nakatsuka K.
Published: July 2002

Though the anomalous antiferromagnetism of zinc ferrite was a subject of intensive studies in the past, the unusually high magnetization of coprecipitated zinc ferrite at low temperatures has drawn renewed interest among researchers. The local structures of zinc ferrites around Fe and Zn atoms are measured using extended x-ray-absorption fine structure and the results are discussed in correlation with their magnetic properties. The structure around the Zn atom was found to differ between zinc ferrite produced by ceramic and coprecipitation techniques. The position of the second-nearest neighbor of the Zn atom for coprecipitated zinc ferrite was shorter than the one produced by the ceramic method. This suggests the possible occupation of the Zn atoms in the octahedral sites and the cause for the unusually high magnetization in coprecipitated zinc ferrite. Furthermore, the intensity of the peak is weak compared to the one produced by the ceramic method. This is thought to be due to the deformation induced by the occupation of zinc ions in the octahedral sites, causing a decrease in the structural periodicity.
Formation of epitaxial Fe3−xSi1+x (0≤x≤1) silicides on Si(111)
Authors: Hong S. , Wetzel P. , Gewinner G. , Bolmont D. , Pirri C.
Published: July 2002

Epitaxial Fe3−xSi1+x films have been grown on Si(111) by codeposition at room temperature. Their structural and electronic properties have been investigated by means of low-energy electron diffraction (LEED), x-ray photoelectron diffraction (XPD), and x-ray photoemission spectroscopy (XPS). These films, with compositions ranging from Fe3Si to FeSi, exhibit a (1×1) LEED pattern. Both XPD and core level binding energy measurements indicate that single Fe3−xSi1+x phases (with 0<x<1), without bulk counterpart, can be stabilized by epitaxy on Si(111). The XPD experiment clearly shows that these Fe3−xSi1+x (0≤x≤1) films adopt the same cubic structure. Furthermore, the Si 2p, Fe 2p3/2, and Fe 3s core levels are slightly shifted to higher binding energies resulting from chemical effects and differences in local coordination when going from Fe3Si (DO3) to FeSi (CsCl). Multiplet splittings ΔE3s are observed in Fe 3s core-level XPS spectra for all Fe3−xSi1+x compounds except the FeSi (CsCl) one.
Measurement of surface displacements in 1-3 and 1-1-3 piezocomposites
Authors: Li L. , Sottos N. R.
Published: July 2002

In a piezoelectric ceramic–polymer composite, the piezoceramic and the nonpiezoelectric polymer will have different deformations when subjected to an external electrical or mechanical stimulus. This local, nonuniform deformation eventually determines the overall electromechanical behavior of the composite. In the current investigation, static surface displacements of piezocomposites with 1-3 and 1-1-3 connectivity were measured using a scanning heterodyne laser interferometer. The influence of a thin interlayer surrounding the piezoceramic rods on the resulting displacement profiles was investigated and correlated with rod/polymer adhesion. The elastic stiffness of the interlayer as well as the interfacial adhesion had a significant effect on the rod displacement. Rods coated with a very compliant interlayer had a much larger displacement than untreated rods, while rods treated with a silane coupling agent to enhance adhesion had a much lower displacement. Experimental displacement profiles were found to be in good agreement with theoretical predictions.
NiO structure–exchange anisotropy relation in the Ni81Fe19/NiO films and thermal stability of its NiO film
Authors: Soeya Susumu , Fuyama Moriaki , Tadokoro Shigeru , Imagawa Takao
Published: July 2002

We studied an antiferromagnetic (AF) NiO film for an exchange-biased layer, focusing especially on the relationships between the exchange coupling properties of the Ni81Fe19(top)/NiO(bottom) films and the character of its NiO film. Among the variable sputtering conditions, our experimental data showed that the dominant factor determining the exchange coupling properties was the Ar pressure during the NiO film preparation. Better exchange coupling properties resulted when the NiO film was deposited at low Ar pressure which was attributed to: (i) the smooth surface of the NiO film and (ii) the presence of relatively large particle sizes within it. The former was thought to bring about not only an increase in the number of unidirectional exchange coupled Ni81Fe19/NiO spins, but also the appearance of exchange paths having large local exchange anisotropies. The latter was thought to produce an increase in the AF clusters with a particle volume larger than KeiA/KAFi, where Kei, A, and KAFi are local unidirectional anisotropy, interfacial area of the NiO cluster in contact with the Ni81Fe19 film, and local magnetocrystalline anisotropy of the NiO cluster, respectively. Moreover, the NiO film was thermally stable up to 250 °C, although the AF anisotropy of the NiO film was weakened on increasing the annealing temperature above 250 °C.
Field dependent resonance frequency of hysteresis loops in a few monolayer thick Co/Cu(001) films
Authors: Jiang Q. , Yang H.-N. , Wang G.-C.
Published: July 2002

Dynamic responses of magnetic hysteresis loops in a few monolayer (ML) thick Co/Cu(001) films were studied using surface magneto-optic Kerr effect (SMOKE). For a fixed external field strength H0, the hysteresis loop area increases as a function of frequency with a power law and reaches a maximum at a resonance frequency Ω0. This Ω0 depends on the external periodic field strength as well as the thickness and roughness of the films. The thickness and roughness parameters were measured quantitatively using high-resolution low-energy electron diffraction. For a fixed film thickness, the Ω0 in the low field region is highly dependent on H0, which is consistent with the prediction from the mean field model. For two Co films with an equivalent thickness but different degrees of film roughness, the resonance frequency Ω0 is lower for the rougher films in all the field strengths studied. For a fixed field strength, the value of Ω0 decreases as Co film roughness increases in a few ML regime. The roughness dependency in Ω0 indicates that the slowing down in the magnetization reversal process is due to the increased film roughness.
Magnetic properties of nanosized wires
Authors: Meier J. , Doudin B. , Ansermet J.-Ph.
Published: July 2002

Assemblies of ferromagnetic cylinders made of Ni with diameters ranging from 35 to 250 nm were produced by electrodeposition in nanoporous membranes. The large coercive fields of Ni nanowires at low temperature could be accounted for by the curling mode of magnetization reversal, taking into account the distributions of wire diameters and orientations. The coercive field of the nanowires of the smaller diameter range decreased from 1500 Oe at 20 K to 200 Oe at 300 K nearly linearly.
Optical properties of boron carbide (B5C) thin films fabricated by plasma-enhanced chemical-vapor deposition
Authors: Ahmad Ahmad A. , Ianno N. J. , Snyder P. G. , Welipitiya D. , Byun D. , Dowben P. A.
Published: July 2002

Variable angle of incidence spectroscopic ellipsometry was used to determine the optical constants near the band edge of boron carbide (B5C) thin films deposited on glass and n-type Si(111) via plasma-enhanced chemical-vapor deposition. The index of refraction n, the extinction coefficient k, and the absorption coefficient are reported in the photon energy spectrum between 1.24 and 4 eV. Ellipsometry analysis of B5C films on silicon indicates a graded material, while the optical constants of B5C on glass are homogeneous. Line shape analyses of absorption data for the films on glass indicate an indirect transition at approximately 0.75 eV and a direct transition at about 1.5 eV.
Complete acoustic stop bands for cubic arrays of spherical liquid balloons
Authors: Kushwaha M. S. , Djafari-Rouhani B.
Published: July 2002

We present theoretical results for acoustic band structures for cubic arrays of spherical water balloons surrounded by a mercury host. Multiple, complete acoustic band gaps (i.e., stop bands) are found for the face-centered cubic (fcc), body-centered cubic (bcc), and simple-cubic (sc) lattices. These stop bands are widest for a volume fraction of ∼24% and the corresponding gap/midgap ratios are about 0.83, 0.77, and 0.62, respectively, for fcc, bcc, and sc lattices. We stress that such a simple three-dimensional (3D) inhomogeneous system of liquids exhibits the largest stop bands ever reported for 3D elastic as well as for dielectric composites. For mercury balloons surrounded by water the gaps obtained are surprisingly small.
Transmission electron microscopy study of stacking faults and the associated partial dislocations in pseudomorphic epilayers of ZnSe/GaAs(001)
Authors: Wang N. , Sou I. K. , Fung K. K.
Published: July 2002

V-shaped double triangular faults in ZnSe/GaAs epilayers have been identified to be incomplete stacking fault pyramids and single stacking faults identified as stacking fault trapezoids. It is confirmed by transmission electron microscopy that 〈101〉 open boundaries of incomplete stacking fault pyramids are 30° Shockley partial dislocations. The closely spaced parallel acute and obtuse stair-rod dislocation pair in stacking fault trapezoids form a dislocation dipole.
The second Stekloff eigenvalue of an inclusion and new size effects for composites with interface thermal barriers
Authors: Lipton Robert
Published: July 2002

Two-phase particle reinforced composites are considered. We treat the case when there is an interfacial thermal barrier between phases. We obtain the geometric criterion that determines when the effects of the interfacial thermal barrier overcome the benefits of a highly conducting reinforcement. This criterion is general and applies to any reinforcement with Lipschitz continuous boundary. It is given in terms of the second Stekloff eigenvalue of the reinforcement. This result is used to predict size effects for a general class of inclusion shapes.
Direct current resistivity studies of various polycrystalline Ni–Zn–Sc ferrites
Authors: Rao B. Parvatheeswara , Rao K. H.
Published: July 2002

Direct current (dc) resistivity studies have been made on two series of Ni–Zn–Sc ferrites with varying concentrations of zinc and scandium to estimate the effects of composition, temperature, and applied electric field. The resistivity has been found to increase with the increase in scandium/zinc content in both the series. From the observed temperature variations of resistivity, activation energies for each sample are calculated. Possible mechanisms contributing to conduction processes in ferrites are discussed in explaining the obtained results. Conclusions have been drawn towards the site preferences of scandium ions in the nickel–zinc ferrite system.
Size dependence of microwave permeability of spherical ferromagnetic particles
Authors: Viau G. , Fiévet-Vincent F. , Fiévet F. , Toneguzzo P. , Ravel F. , Acher O.
Published: July 2002

Spherical, monodispersed, ferromagnetic, metallic particles of different compositions were obtained by the polyol process with a mean radius ranging from 30 nm to 1 μm. The microwave permeability of metallic particles-dielectric matrix composites were studied in the range of 0.1–18 GHz. In the wide particle size range investigated, a size dependence of the dynamic permeability was observed. Whereas the permeability of micrometer-sized particles shows a single resonance band, the permeability of submicrometer-sized particles exhibits several narrow resonance bands which are shifted to high frequencies with decreasing particle size. This latter behavior was found to be in qualitative agreement with the exchange resonance modes calculated by Aharoni. That theory, however, gives an R−2 dependence on particle radius for the resonance frequency instead of the R−0.66 dependence observed experimentally.
Texture and surface/interface topological effects on the exchange and coercive fields of NiFe/NiO bilayers
Authors: Han De-Hua , Zhu Jian-Gang , Judy Jack H. , Sivertsen John M.
Published: July 2002

The texture and surface/interface topological effects on the exchange field Hex and coercivity Hc in rf-sputtered NiO/NiFe bilayers were studied. The NiO/NiFe bilayers with NiO crystalline textures of (111), (200), and (220), and different surface/interface roughness, were fabricated by changing the sputtering conditions. The Hex of the NiO/NiFe bilayers was inversely proportional to the thickness of NiFe. The NiO/NiFe bilayers with different NiO textures of (111) dominant, (200) dominant, or combinations of (111), (200), and (220), respectively, showed almost the same dependence of Hex on NiFe thickness. Thus, Hex was insensitive to the texture of NiO films. The surface/interface roughness has a significant effect on Hc of the NiO/NiFe bilayers. By controlling the sputtering process, NiO (45 nm)/NiFe (10 nm) bilayers with a Hex of 39 Oe, a Hc of 4.1 Oe, and an Hex/Hc ratio of 9.51 could be obtained. A new parameter of Hex/Hc was introduced to describe the exchange coupling and coercive properties in ferromagnet/antiferromagnet coupled bilayers. A large Hex/Hc ratio is very desirable for spin-valve structures.
Light-induced ferromagnetism in III-V-based diluted magnetic semiconductor heterostructures
Authors: Munekata H. , Abe T. , Koshihara S. , Oiwa A. , Hirasawa M. , Katsumoto S. , Iye Y. , Urano C. , Takagi H.
Published: July 2002

We report for the first time the occurrence of light-induced ferromagnetic order in the III-V-based diluted magnetic semiconductor heterostructures (In,Mn)As/GaSb. We believe that the phenomenon is based on the generation of excess carriers (holes) in the (In,Mn)As layer by the irradiation of light, which enhances a carrier-mediated ferromagnetic interaction between Mn ions.
Coercivity and magnetic anisotropy of sintered Sm2Co17-type permanent magnets
Authors: Téllez-Blanco J. C. , Kou X. C. , Grössinger R. , Estévez-Rams E. , Fidler J. , Ma B. M.
Published: July 2002

Anisotropic and isotropic Sm2Co17-type permanent magnets have been prepared by using the conventional sintering technique. Transmission electron microscopy was used to characterize the cellular structure within the Sm2(Fe,Co)17 grains. Hysteresis loops (M∼H) were measured in the temperature range from 4.2 to about 1000 K by using a pulsed-field magnetometer with a maximum field strength up to 24 MA/m. The magnetocrystalline anisotropy field HA has been measured up to 1000 K by using the singular point detection technique on anisotropic samples with external fields applied perpendicular to the magnetic alignment direction. The saturation magnetization Ms has been measured on anisotropic samples with external fields applied parallel to the magnetic alignment direction. From studies of the coercivity mechanism by using a micromagnetic analysis of the temperature dependence of the coercivity field, it follows that the coercivity of Sm(Co,Fe,Cu,Zr)z is controlled at elevated temperature (above 520 K) by a nucleation process of reversal domains.
The enthalpy of formation of thin film titanium disilicide
Authors: Kasica R. J. , Cotts E. J.
Published: July 2002

Solid state reactions between thin films of titanium and amorphous silicon (a-Si) were studied using differential scanning calorimetry. Multilayered diffusion couples were heated from just above room temperature to 900 K while monitoring the heat flow from the sample during the reaction. From analysis of our data we have calculated the enthalpy of formation to be ΔHf=−62±5 kJ/mol for the reaction, a-Si+Ti→C49−TiSi2 and have subsequently estimated the heat evolved during the reaction c-Si+Ti→C54−TiSi2 as ΔHf=−56±5 kJ/mol. Based upon this estimate, we find the enthalpy as determined from thin film samples for this system agrees with previous studies which made use of bulk sampling techniques.
Growth kinetics and morphology of high quality AlN grown on Si(111) by plasma-assisted molecular beam epitaxy
Authors: Calleja E. , Sánchez-Garcı́a M. A. , Monroy E. , Sánchez F. J. , Muñoz E. , Sanz-Hervás A. , Villar C. , Aguilar M.
Published: July 2002

AlN layers were grown on Si(111) substrates by plasma-assisted molecular beam epitaxy. Crystal quality was assessed by atomic force microscopy and high resolution x-ray diffraction. The III/V ratio and the growth temperature, rather than thickness and growth rate, are found to be critical parameters to achieve good quality AlN layers. III/V ratios close to stoichiometry, and high growth temperatures (⩾900 °C) lead to optimal AlN layers. The growth rate is barely modified when growth temperature changes from 780 to 920 °C, but the growth mode and surface roughness are strongly affected. Optimal AlN layers have full-widths at half-maximum values of 10 arcmin, and an average surface roughness of 48 Å.
Coupling of chemical kinetics, gas dynamics, and charged particle kinetics models for the analysis of NO reduction from flue gases
Authors: Eichwald O. , Yousfi M. , Hennad A. , Benabdessadok M. D.
Published: July 2002

A chemical kinetics model is developed to analyze the time evolution of the different main species involved in a flue gas initially stressed by a pulsed corona discharge at the atmospheric pressure and including N2, O2, H2O, and CO2 with a few ppm of NO. The present chemical kinetics model is coupled to a gas dynamics model used to analyze the radial expansion of the gas in the ionized channel created during the discharge phase. It is also meant to analyze the gas heating due to the Joule effect. This chemical kinetics model is also coupled to charged particle kinetics models based on a Boltzmann equation model to calculate the electron-molecule reaction coefficients in the flue gas and on a Monte Carlo code to estimate the energy and momentum transfer terms relative to ion-molecule collisions which are the input data for the gas dynamics model. It is shown, in particular, that the evolution of the radicals and the oxides is substantially affected by the gas temperature rise (from the initial value of 300 K up to 750 K near the anode) thus emphasizing the present coupling between gas dynamics, charged particle, and chemical kinetics models.
Magnetic characterization of γ-Fe2O3 nanoparticles fabricated by aerosol pyrolysis
Authors: Martı́nez B. , Roig A. , Molins E. , González-Carreño T. , Serna C. J.
Published: July 2002

The magnetic properties of γ-Fe2O3 nanoparticles prepared by spray pyrolysis of dilute solutions of Fe(III) and Fe(II) salts have been investigated using both magnetic measurements and Mössbauer spectroscopy. The most outstanding magnetic features are the strong reduction of the saturation magnetization compared with that of the bulk material, and the existence of magnetic hysteresis up to the highest field used (55 kOe) in both the M(H) loops and the zero-field cooling-field cooling M(T) curves. The magnetic particles exhibit superparamagnetic behavior at room temperature. From the evolution of the ferromagnetic ratio, MR/MS, with temperature we have determined the distribution of the blocking temperatures that can be described properly with a log–normal distribution, which allows the determination of mean blocking temperatures 〈TB〉 and mean particle volumes.
Interaction of laser-generated surface acoustic pulses with fine particles: Surface cleaning and adhesion studies
Authors: Kolomenskii Al. A. , Schuessler H. A. , Mikhalevich V. G. , Maznev A. A.
Published: July 2002

The mechanical forces associated with the surface acceleration in high-amplitude surface acoustic waves (SAWs) detach the particles from the surface. The removal of micron sized particles with a nanosecond SAW pulse excited by a focused laser beam in a silicon wafer was quantitatively investigated. Both vertical and horizontal particle displacements have been observed. It is shown that for nanosecond SAW pulses the limit of the surface acceleration of about 1010 m/s2 is set by the fracture of the material and corresponds to the removal of particles larger than about 0.05 μm. In addition, the nonlinear transformation of the excited SAW pulses results in an increase of the surface acceleration and contributes to the cleaning process extending it to even smaller particle dimensions. The technique is applicable in vacuum and improves the energetic effectiveness of the cleaning due to the removal of particles not only in the irradiated region, but also in the wider area covered by the SAW pulse propagation. It can be also used for the determination of the Hamaker constant of the adhesion force.
Observation of the effects of tip magnetization states on magnetic force microscopy images
Authors: Rice Paul , Russek Stephen E.
Published: July 2002

We have investigated the effect of the orientation between the tip and the tip-magnetizing field on the subsequent magnetic force microscopy images. We have observed that commonly used CoCr coated tips have a component of the remanent magnetization pinned parallel to the tip axis. In order to study the pinned component of the tip, saturating fields were applied at various angles relative to the tip axis. Each time the tip was magnetized, we imaged a hard disk sample that had lithographically patterned landmarks designed to allow us to find, image, and compare images of the same nanometer-sized magnetic feature. We have also observed a component of the tip magnetization that is free to rotate dynamically due to the stray magnetic fields of the sample being imaged. To study this effect, an external bias field was applied parallel to the tip axis while imaging. This field effectively pinned the tip magnetization. With these techniques, we are thus able to compare two effects of tip magnetization on magnetic force microscope images.
Influence of the cubic anisotropy constants on the hysteresis loops of single-domain particles: A Monte Carlo study
Authors: Garcı́a-Otero J. , Porto M. , Rivas J. , Bunde A.
Published: July 2002

The influence of the first and second cubic anisotropy constants on the hysteresis loops of noninteracting single-domain magnetic particles is studied by Monte Carlo simulation, which turns out to be a very powerful method for studying simple magnetic models. Both signs in the anisotropy constants are taken into account. Relevant properties such as coercivity and remanence are studied as a function of temperature when the second anisotropy constant is negligible. The influence of the second term of the anisotropy energy is studied in detail for T=0 K. It is concluded that this term has a big influence on the static magnetic behavior when the first anisotropy constant is negative.
Gate voltage dependent mobility of oligothiophene field-effect transistors
Authors: Horowitz Gilles , Hajlaoui Riadh , Fichou Denis , El Kassmi Ahmed
Published: July 2002

Organic field-effect transistors, in which the active semiconductor is made of oligothiophenes of various lengths, have been fabricated and characterized. A method is developed to estimate the field-effect mobility μ corrected for the contact series resistance. The mobility is found to increase by a factor of nearly 100 from quaterthiophene (4T) to octithiophene (8T). More importantly, μ increases quasilinearly with gate voltage. The origin of this gate bias dependence is discussed. One explanation could be the presence of traps that limit charge transport. Alternatively, the gate-voltage dependence is tentatively attributed to a dependence of the mobility with the concentration of carriers in the accumulation layer.