bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties of carbon ion deposited tetrahedral amorphous carbon films as a function of ion energy
Authors: Xu Shi , Tay B. K. , Tan H. S. , Zhong Li , Tu Y. Q. , Silva S. R. P. , Milne W. I.
Published: July 2002

Ion energy, controlled by the substrate bias, is an important parameter in determining properties of films deposited by the filtered cathodic vacuum arc technique. The substrate bias determines the ion energy distribution of the growth species. The ion energy is varied, while keeping the other deposition conditions constant, in order to study the effect of ion energy on the film properties. The films were characterized by their optical and mechanical parameters using an ellipsometer, surface profilometer, optical spectrometer, and nanoindenter. Electron energy-loss spectroscopy and Raman spectroscopy were used for structural analysis of the films.
Formation of supersaturated solid solutions in the immiscible Ni–Ag system by mechanical alloying
Authors: Xu J. , Herr U. , Klassen T. , Averback R. S.
Published: July 2002

The mechanical alloying process in the immiscible Ni–Ag system with a positive heat of mixing was investigated by x-ray diffraction and differential scanning calorimetry. High energy ball milling of mixed elemental powders, with nominal composition NixAg100−x (x=95, 90, 70, 50, and 30), results in the formation of mixtures of supersaturated, nanocrystalline Ni-rich and Ag-rich solid solutions. The solubilities and final grain sizes of these phases depend on the nominal composition of the powder. The maximum solubilities were determined using Vegard’s law to be 4.3 at. % Ni in Ag and 6.6 at. % Ag in Ni for samples milled at room temperature. The effect of milling temperature on mechanical alloying was examined in the range −195 to 250 °C. Lower temperature milling leads to a larger solubility of Ni in the Ag-rich samples, up to 7.1 at. % for the Ni30Ag70 composition. Indications for the existence of a concentrated solid solution (Ni36Ag64-Ni44Ag56) were also found. Milling at higher temperatures leads to lower solubilities. A study of the thermal stability of supersaturated Ag-rich and Ni-rich phases shows that milling at high temperature can be understood in terms of a competition between mechanical mixing and thermal decomposition. At room temperature, nonequilibrium vacancies are responsible for decomposition. The results give new insight into the general characteristics of the mechanical alloying process in thermodynamically unstable systems.
Micromagnetics of polycrystalline two-dimensional platelets
Authors: Tako K. M. , Wongsam M. A. , Chantrell R. W.
Published: July 2002

A numerical micromagnetic technique is used to simulate magnetization processes in two-dimensional thin metallic platelets. The platelets are modeled as an array of interacting polycrystalline grains. The technique assumes a triangular discretization at the subgrain level with the magnetization varying linearly over each triangle. The coupling between the grains has a profound effect on the magnetic structure of the platelets as does the grain size. For systems with strongly exchange coupled grains, approximately solenoidal magnetization structures exist. A single domain behavior exists for systems with weakly coupled grains. The magnetization pattern of the platelets has been characterized by the vorticity of the magnetization vector field.
The growth of Fe on sulphur passivated Ge(100): A technique for avoiding intermixing
Authors: Anderson G. W. , Ma P. , Norton P. R.
Published: July 2002

The growth of Fe on Ge(100) surfaces results in the intermixing of Ge with the Fe overlayer, producing relatively thick magnetic dead layers. In this paper we describe a new technique, the growth of Fe overlayers on S-passivated Ge(100), which successfully prevents the intermixing of Ge with the Fe overlayer. Bcc Fe(100) is observed to grow epitaxially on this substrate, with the S floating out as an ordered overlayer. The S prevents intermixing by acting as a surfactant, holding the Fe on top of the substrate and preventing interdiffusion. The Fe overlayers are observed to be ferromagnetic in nature, displaying sharp hysteresis loops with easy axes along the Fe[010] directions. A uniaxial in-plane anisotropy is observed, which results in the two easy axes being inequivalent.
The trade-off between large magnetoresistance and small coercivity in symmetric spin valves
Authors: Egelhoff W. F. , Chen P. J. , Powell C. J. , Stiles M. D. , McMichael R. D. , Lin C.-L. , Sivertsen J. M. , Judy J. H. , Takano K. , Berkowitz A. E.
Published: July 2002

We have investigated the use of various alloys as substitutes for pure Co in the center film of symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO. The aim of this work is to identify magnetic materials that exhibit smaller coercivities than pure Co for the center or ‘‘valve’’ film but which retain much of the giant magnetoresistance associated with a pure Co film. The materials investigated include Co95Fe5, Co90Fe10, Ni80Fe20, Co86Fe10.5Ni3.5, and Co85B15. It appears that each of these alloys scatters electrons more strongly than does pure Co as they cross the center film. This scattering degrades the dual spin-valve effect, which is the primary advantage of the symmetric spin valve. As a result, a tradeoff exists between large GMR and small coercivity when using these materials.
Influence of dispersive exciton motion on the recombination dynamics in porous silicon
Authors: Pavesi Lorenzo
Published: July 2002

An extended photoluminescence (PL) study of porous silicon is presented. Different PL techniques have been used: continuous wave excited (cw) PL, selectively excited PL, excitation spectroscopy of the PL, time decay of the PL, and time resolved PL. These measurements have been performed on a set of samples of various porosities and at various temperatures. Strong experimental evidence is found for the influence of disorder and of dispersive motion of excitons on the recombination dynamics. The data are interpreted in the framework of the trap-controlled hopping mechanism for the dispersive motion of excitons in a disordered array of Si nanocrystals.
Valence-band mixing effects on exciton dipole terahertz emission from asymmetric triple quantum wells
Authors: Hernández-Cabrera A. , Ramos A.
Published: July 2002

We analyze the influence of band mixing on the dynamics of hole tunneling in asymmetric GaAs–Ga1−xAlxAs triple quantum wells. A combination of the time-dependent Schrödinger equation and the Luttinger Hamiltonian is used to calculate the terahertz radiation from excitons. When the electric field required for resonance of electrons between the central and right well coincides with the field needed for hole resonance between the central and left well the radiation is strongly increased. If this field coincides with the hole mixing field at k∥≠0, a new modulation of the dipole emission appears, being drastically affected in frequency and amplitude. The calculated effect is greater than previous results for asymmetric double quantum wells.
Quantitative in situ x-ray diffraction analysis of magnetic multilayers during annealing
Authors: Chládek M. , Dorner C. , Buchal A. , Valvoda V. , Hoffmann H.
Published: July 2002

In situ high-angle x-ray diffraction (XRD) measurements during an annealing process and extensive low- and high-angle XRD analysis before and after annealing have been performed to study the influence of annealing on the microstructure of a Ni81Fe19/Ag multilayer film. We concentrate on a detailed description and quantification of induced changes in microstructural properties and on changes in the crystalline structure of sublayers. The observed effects obtained from the high-angle XRD, where superlattice structural refinement and standard powder diffraction methods were used, are utilized to model the changes from microscopic point of view. The low-angle XRD reflectivity measurements were analyzed using a new distorted-wave Born approximation approach which enables to get information about interlayer structure parameters such as interface roughnesses, degree of vertical correlation and lateral correlation length. The presented low-angle and high-angle analysis demonstrates general applicability of x-ray diffraction for complex and quantitative in-situ investigation of structural changes in metallic multilayers during annealing. An effect of lateral scaling for different types of roughness is discussed and a general approach is shown for the case of metallic multilayers.
Electro-optical properties of polymer dispersed liquid crystal materials
Authors: Cheng Si-Xue , Bai Ru-Ke , Zou Ying-Fang , Pan Cai-Yuan
Published: July 2002

Polymer dispersed liquid crystal (PDLC) materials based on poly(polyethyleneglycol methacrylate) are prepared by ultraviolet light induced polymerization. Microphase structure of PDLC films is observed by scanning electron microscopy. It reveals that reversed morphology type PDLC is obtained. The liquid crystal (LC) is a continuous phase and fills up the irregular space of the polymer network. A comprehensive investigation is performed on the electro-optical properties of PDLC films. The light scattering and transmittance properties of PDLC films are studied. The effects of LC domain size, light wavelength, applied voltage, frequency, temperature, incident angle, and film thickness on electro-optical properties of PDLC films are discussed. The results show that size and shape of LC domain seriously affect the electro-optical properties of PDLC film.
Defect structure of metal-organic chemical vapor deposition-grown epitaxial (0001) GaN/Al2O3
Authors: Wu X. H. , Brown L. M. , Kapolnek D. , Keller S. , Keller B. , DenBaars S. P. , Speck J. S.
Published: July 2002

Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal-organic chemical vapor deposition using a two-step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high-temperature growth at 1080 °C. The near-interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near-interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in these films is associated with cubic to hexagonal transformation of the nucleation layer region during high temperature growth.
Oxidation characteristics and magnetic properties of iron carbide and iron ultrafine particles
Authors: Zhao X. Q. , Liang Y. , Hu Z. Q. , Liu B. X.
Published: July 2002

Magnetic Fe3C and α-Fe ultrafine particles were prepared by laser-induced pyrolysis of Fe(CO)5 and C2H4. It is found that after passivation, the Fe3C particles exhibit a high saturation magnetization of 132 emu/g compared to that of the α-Fe particle, 95 emu/g. By determining the oxygen content and the present states of oxygen the particles contained, it is found that not only oxygen content of the α-Fe particles is much higher than that of the Fe3C particles, but the oxygen is in differnt states for the two ultrafine particles. The oxygen present on the Fe3C particles is primarily in absorbed form, compared to chemically combined oxygen as in the α-Fe particles. Thin amorphous carbon layers, formed on the surfaces of the Fe3C particles, inhibit oxidation of the Fe3C and therefore result in higher saturation magnetization achieved by Fe3C particles, relative to the α-Fe particles on which no carbon layer was present.
Temperature dependence of the energy gap of zinc-blende CdSe and Cd1−xZnxSe epitaxial layers
Authors: Lunz U. , Kuhn J. , Goschenhofer F. , Schüssler U. , Einfeldt S. , Becker C. R. , Landwehr G.
Published: July 2002

The temperature dependence of the energy gap of zinc-blende CdSe and Cd1−xZnxSe has been determined over the entire range of composition from optical transmission and reflection measurements at temperatures between 5 and 300 K. The experimental results can be expressed by the following modified empirical Varshni formula, whose parameters are functions of the composition x: Eg(x,T)=Eg(x,0)−β(x)T2/[T+γ(x)]. Eg(x,0) exhibits a nonlinear dependence on composition, according to Eg=Eg(0,0)(1−x)+Eg(1,0)x−ax(1−x). The parameters β(x) and γ(x) can be expressed by β(x)=β(0)(1−x)+β(1)x+bx(1−x) and γ(x)=γ(0)(1−x)+γ(1)x.
Mechanism of the visible electroluminescence from metal/porous silicon/n-Si devices
Authors: Oguro Tsuyoshi , Koyama Hideki , Ozaki Tsuyoshi , Koshida Nobuyoshi
Published: July 2002

The excitation and radiative recombination mechanisms of carriers in electroluminescent porous silicon (PS) have been studied for the device with the structure of Au/PS/n-type Si. Experiments focus on the electroluminescence (EL) and photoluminescence (PL) spectra, the current-voltage-EL intensity relationship and its temperature dependence, and the excitation-wavelength dependence of the electric-field-induced PL quenching. The results of these experiments suggest the following points: (1) the EL occurs mainly near the Au/PS contact; (2) there exists an extremely high electric field at the Au/PS contact; (3) the EL originates from radiative recombination of strongly localized excitons; and (4) the radiative recombination rate is in proportion to the diode current. Based on these observations, an operation model is proposed. In our model, a large number of electrons and holes are generated in the PS layer by a field-assisted mechanism. Light emission occurs by radiative recombination of these electrons and holes via localized states. Because of field-enhanced carrier separation, however, the EL efficiency of this device is limited to a relatively low value of about 0.05%. Possible ways to improve the EL characteristics are discussed.
Controlled cluster condensation into preformed nanometer-sized pits
Authors: Hövel H. , Becker Th. , Bettac A. , Reihl B. , Tschudy M. , Williams E. J.
Published: July 2002

We have performed scanning tunneling microscopy (STM) in ultrahigh vacuum and transmission electron microscopy (TEM) on silver and gold clusters grown in preformed nanometer-sized pits on the surface of highly oriented pyrolytic graphite. We describe the preparation method and evaluate the three-dimensional shape of the clusters using a combination of STM and TEM applied to the same cluster sample. The nanometer-sized pits were essential to fix the clusters in position when using STM. The influence of the tip shape on the STM imaging of nanometer-sized clusters is discussed.
Thermodynamic properties of perovskite ARuO3 (A=Ca, Sr, and Ba) single crystals
Authors: Shepard M. , McCall S. , Cao G. , Crow J. E.
Published: July 2002

Magnetic susceptibility, transport properties (including magnetoresistivity and the Hall effect), and specific heat of perovskite ARuO3 (A=Ca, Sr, and Ba) single crystals were measured. These compounds, particularly CaRuO3 (paramagnetic conductor) and SrRuO3 (ferromagnetic conductor) are believed to have a narrow π* band and their thermodynamic properties depend in an extremely sensitive way on the degree of the band filling and band width. This study reveals that the π* bandwidth of these compounds is largely determined by the ionic radii of the alkaline-earth A cations, i.e., the interaction between A s and O 2p orbitals and the electron correlation becomes progressively stronger in the series BaRuO3, SrRuO3, and CaRuO3.
First principles investigations of 4d magnetism on C(0001)
Authors: Chen Lujun , Wu Ruqian , Kioussis N. , Blanco J. R.
Published: July 2002

Possible 4d magnetism in X/C(0001) (X=Pd, Rh, and Ru) is investigated using the full potential linearized augmented plane wave method. We find that while small magnetic moments of 0.24 and 0.28 μB can survive for Rh and Ru on C(0001), the magnetism of Pd is completely diminished. Due to the unexpectedly strong interfacial interaction between the overlayer and the flat C(0001) substrate, the magnetic moment is very sensitive to changes in the interlayer distances.
The influence of ultraviolet illumination on OH formation in dielectric barrier discharges of Ar/O2/H2O: The Joshi effect
Authors: Falkenstein Zoran
Published: July 2002

The influence of ultraviolet (UV) illumination on the formation of hydroxyl radicals in dielectric barrier discharges (DBD) in mixtures of Ar/O2/H2O (79/18/3) will be presented. The UV is provided from a spectrally calibrated, commercial low pressure mercury lamp while the irradiance power of the discharge cell has been previously determined. First, the filamentary character of the DBD is shown by measurement of the relative intensity of the transition OH(A2Σ→X2Π, 0-0) with integrated emission spectroscopy as a function of the absolute water concentration at different energy densities coupled into the gas. Then, the influence of UV illumination on the hydroxyl emission is shown by time-gated and time-resolved emission spectroscopy of OH(A2Σ→X2Π). The results of the combined process is an increased total emission but reduced emission for each microdischarge. This result corresponds to an increase in the number of microdischarges which is supported by the measurement of the microdischarge current.
Refractive indices of rutile as a function of temperature and wavelength
Authors: Rams J. , Tejeda A. , Cabrera J. M.
Published: July 2002

The ordinary and extraordinary refractive indices of rutile (TiO2) have been measured as a function of temperature and wavelength within the ranges 30–160 °C and 454–1330 nm, respectively, by using a prism coupling reflectivity setup. The measurements have been done with regard to the accurately known indices of a congruent LiNbO3 substrate. The index values are given with the same accuracy as that of the substrate taken as reference, i.e., ±0.001 at room temperature and ±0.003 at 160 °C. The room temperature values are coincident with those previously published by other workers. Sellmeier-like equations which include wavelength and temperature as variables within the studied ranges are given for both indices.
Influence of mobile space charges on the ferroelectric properties of (K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30 ceramics
Authors: Bu Sangdon , Chun Dukhyun , Park Gwangseo
Published: July 2002

We have measured ac complex impedance and thermal depolarization current for aged (K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30 ceramic samples to investigate the influence of mobile space charges on the ferroelectric properties. The impedance behavior was analyzed using an equivalent circuit composed of a capacitor connected in series with a parallel resistor-capacitor circuit. The dielectric relaxation is found to involve two mechanisms; one is attributed to the hopping process of the space charges bound inside grains and the other is associated both with the intrinsic polarization of the ferroelectric material and with the space charges at the grain boundary layer. The experimental results are also discussed in terms of the aging mechanism. It is suggested that the space charges bound inside grains play an important role in the aging process of the ferroelectric ceramics.
Electron density and temperature measurements in a laser produced carbon plasma
Authors: Harilal S. S. , Bindhu C. V. , Issac Riju C. , Nampoori V. P. N. , Vallabhan C. P. G.
Published: July 2002

Plasma generated by fundamental radiation from a Nd:YAG laser focused onto a graphite target is studied spectroscopically. Measured line profiles of several ionic species were used to infer electron temperature and density at several sections located in front of the target surface. Line intensities of successive ionization states of carbon were used for electron temperature calculations. Stark broadened profiles of singly ionized species have been utilized for electron density measurements. Electron density as well as electron temperature were studied as functions of laser irradiance and time elapsed after the incidence of laser pulse. The validity of the assumption of local thermodynamic equilibrium is discussed in light of the results obtained.
Quantum efficiency of Pr3+ doped transparent glass ceramics
Authors: Quimby R. S. , Tick P. A. , Borrelli N. F. , Cornelius L. K.
Published: July 2002

The quantum efficiency of the 1.3 μm transition in Pr3+ doped oxyfluoride transparent glass ceramics is measured using a self-calibrating fluorescence ratio technique. The measured value of 8%±1% is much higher than expected for Pr3+ in an oxide glass, and is consistent with the incorporation of the Pr3+ ions into the fluoride nanocrystals of the glass ceramic. Measured quantum efficiencies are compared with values calculated using the Judd–Ofelt theory, and it is found that the best agreement is obtained when fractional differences between calculated and measured oscillator strengths are minimized, rather than absolute differences. The Judd–Ofelt analysis is used along with a spectral analysis to show that excited-state absorption should not significantly reduce the gain around 1.3 μm in these materials. The fluorescence ratio technique used to measure the quantum efficiency also provides information about ion–ion interactions at higher concentrations. It is found that the addition of the monovalent dopant Ag+ reduces the tendency of the Pr3+ ions to cluster, allowing higher concentrations of Pr3+ without significant concentration quenching.
Nonequilibrium solidification of hypercooled Co–Pd melts
Authors: Volkmann T. , Wilde G. , Willnecker R. , Herlach D. M.
Published: July 2002

Co–Pd alloy melts are characterized by a low heat of fusion and, as a consequence, by a comparably small critical undercooling for the hypercooling limit ΔThyp of the order of 300 K. It is shown that containerless processing of bulk melts by electromagnetic levitation offers undercooling levels of ΔT≈350 K thus exceeding the hypercooling limit considerably. Solidification of undercooled melts from the hypercooling regime leads to rapid crystallization of the entire sample under nonequilibrium conditions. The electromagnetic levitation technique in combination with time-resolved recalescence detection was used to measure the growth velocity of Co–Pd alloy melts as a function of undercooling prior to solidification. The growth velocities of undercooled metallic melts were measured in an undercooling range exceeding the hypercooling limit. The experimental results are discussed within current theory of dendritic growth.
Resonance splitting effect and wave-vector filtering effect in magnetic superlattices
Authors: Guo Yong , Gu Bing-Lin , Li Zhi-Qiang , Yu Jing-Zhi , Kawazoe Yoshiyuki
Published: July 2002

The resonance splitting and wave-vector filtering for electron tunneling through magnetic superlattices are investigated theoretically. Two kinds of magnetic superlattices are examined. One is a periodic arrangement of identical magnetic barriers while the other is periodically juxtaposed with two different magnetic barriers. In general, one resonant domain in the former splits into two resonant domains in the latter. It is confirmed that both the resonance splitting and wave-vector filtering strongly depend on the structure of the magnetic superlattices. The numerical results indicate that the magnetic superlattice, which is a periodic arrangement of two different magnetic barriers, possesses stronger wave-vector filtering.
Transport and magnetic properties of epitaxial and polycrystalline magnetite thin films
Authors: Li X. W. , Gupta A. , Xiao Gang , Gong G. Q.
Published: July 2002

The transport and magnetic properties of magnetite (Fe3O4) thin films grown epitaxially on single crystal MgO(100) and SrTiO3(100) substrates, and with multiple grain orientations on polycrystalline SrTiO3 substrates, have been investigated. The films are grown using pulsed laser deposition and their epitaxial quality determined using ion channeling measurements. Transport and magnetic studies of Fe3O4 films as a function of thickness and morphology suggest that epitaxial strain and growth defects affect the width and temperature of the Verwey transition. In addition, these factors also significantly influence the magnetic coercivity of the films. The low-field magnetoresistance (MR) behaviors of epitaxial and polycrystalline films as a function of temperature have been compared and they were found to be quite similar, suggesting very small contribution to the MR from grain boundaries.
Tunneling evidence of half-metallicity in epitaxial films of ferromagnetic perovskite manganites and ferrimagnetic magnetite
Authors: Wei J. Y. T. , Yeh N.-C. , Vasquez R. P. , Gupta A.
Published: July 2002

Direct evidence of half-metallic density of states is observed by scanning tunneling spectroscopy of ferromagnetic La0.7Ca0.3MnO3 and La1−xSrxMnO3 (x=0.3, 0.33) epitaxial films which exhibit colossal magnetoresistance (CMR). At 77 K, well below the Curie temperatures, the normalized tunneling conductance (dI/dV)/〈I/V〉 for all samples exhibits similar pronounced peak structures, consistent with the spin-split density of states spectra for the itinerant bands in the ferromagnetic state. The exchange energy splitting between the majority and minority spins, as well as an apparent energy gap near the Fermi level, show variations with the chemical composition and the temperature. For comparison, the tunneling spectrum of a half-metallic ferrimagnet Fe3O4 is also studied. The characteristic spin-split density of states spectrum is observed, and the similarities and differences of Fe3O4 compared with the perovskite manganites are discussed.
Semiquantitative analysis of magnetic phase transitions in the MnFeP1−xAsx series of compounds
Authors: Zach R. , Guillot M. , Toboła J.
Published: July 2002

On the basis of the relation T(σ,H) derived within the framework of the Bean–Rodbell model a semiquantitative analysis of (B,T) magnetic phase diagrams in the hexagonal region of the MnFeP1−xAsx (0.15⩽x⩽0.66) series of compounds was carried out. It was found that the first order transition line in the (B,T) plane is terminated by the isolated critical point for x=0.33 and x=0.5 contents. The positive value of dBC/dT and the saturation of the magnetization was found to be in agreement with the experiment. Thus the Bean–Rodbell model and the molecular field approximation can be applied to describe the discontinuous Ferro–Para magnetoelastic phase transition. Semiquantitative agreement between the theoretical and the experimental (B,T) magnetic phase diagrams was obtained. For the MnFeP0.67As0.33 compound the electronic structure calculations were performed by the Korringa–Kohn–Rostoker with the coherent potential approximation method. The density of states (DOS) curve calculated in the nonmagnetic state shows that the Fermi level falls in a deep minimum of DOS. It indicates that the metamagnetic B-field induced phase transition between the paramagnetic and the ferromagnetic states may be observed (metamagnetism of itinerant electrons) which remains in good agreement with our experimental results.
Improvement of protective coating on Nd–Fe–B magnet by pulse nickel plating
Authors: Cheng C. W. , Cheng F. T. , Man H. C.
Published: July 2002

Sintered Nd–Fe–B magnets were coated by pulse nickel plating at different plating conditions. Optimal pulse plating condition was established (average current density=1 A/dm2, peak current density=6 A/dm2 with Ton:Toff=1:2). In order to make a comparison, magnets with similar nickel coating thickness plated by dc were also prepared. The corrosion resistance of the coated magnets was evaluated by (i) Normal Salt Spray Test (5% NaCl, 35 °C) and (ii) potentiodynamic polarization measurement (3.5% NaCl solution). It was found that the corrosion resistance of the pulse nickel plated magnet was significantly improved as compared with that of the conventional dc plated ones, with negligible deterioration in magnetism. The microstructure of the coating was examined by optical microscopy and scanning electron microscopy. It was found that the porosity was much lower, and the grains much finer in the pulse-plated layer as compared with the dc plated ones.
Effects of deposition temperature on polycrystalline silicon films using plasma-enhanced chemical vapor deposition
Authors: Hasegawa S. , Sakata M. , Inokuma T. , Kurata Y.
Published: July 2002

The 200-nm-thick polycrystalline Si films were deposited by changing the deposition temperature (Td=150–750 °C) using plasma-enhanced chemical vapor deposition of monosilane–hydrogen mixtures. The structural and bonding properties were examined using techniques of Raman scattering, x-ray diffraction, infrared (IR) absorption, and electron spin resonance. Except for Td at 150 and 650 °C, crystallization of the films was observed, and the occurrence of two IR absorption bands around 850 and 1000 cm−1 and an increase in the density of Si dangling bonds were observed in the range of Td higher than 500 °C. These origins were discussed in connection with the mechanisms of disappearance of crystalline phases from the film at Td=650 °C.
Young’s modulus of single-walled carbon nanotubes
Authors: Yao Nan , Lordi Vincenzo
Published: July 2002

We report in detail that unlike other materials, carbon nanotubes are so small that changes in structure can affect the Young’s modulus. The variation in modulus is attributed to differences in torsional strain, which is the dominant component of the total strain energy. Torsional strain, and correspondingly Young’s modulus, increases significantly with decreasing tube diameter and increases slightly with decreasing tube helicity.
Frustration effect in a quantum Heisenberg spin system
Authors: Santamaria C. , Quartu R. , Diep H. T.
Published: July 2002

We study a quantum frustrated Heisenberg model on the two-sublattice bcc crystal taking into account nearest-neighbor and next-nearest-neighbor interactions on one sublattice, J1 and J2 respectively. The classical ground-state spin configuration shows a noncollinear phase. The method used is an improved self-consistent Green function method: we calculate the free energy F of this system and determine the angle at each temperature (T) at the minimum of F, instead of the minimum of the internal energy as previously done. The properties at finite T are studied for collinear and noncollinear phases. The phase diagram in the space (J2/J1,T) is shown. We confirm here by our analytical method the existence of the partial disorder in a frustrated vector spin system discovered recently by MC simulation. The present method opens an efficient way to study noncollinear spin systems over the whole range of temperature and might help to discover new phenomena in more complicated spin systems.
Influence of nanoscale substrate curvature on growth kinetics and morphology of surface nuclei
Authors: Louchev Oleg A. , Sato Yoichiro
Published: July 2002

Results on numerical modeling of the influence of a substrate’s nanoscale curvature on nucleation, growth kinetics, and morphological stability of nuclei during thin film vapor deposition are given. The problem is considered within the framework of the continuum surface diffusion equation taking into account the dependence of the adsorption energy, chemical potential, and the relevant surface mass fluxes on the surface curvature, together with the effect of surface self-shadowing from direct collisions from the gas phase and the contribution of readsorption fluxes. The nuclei growth kinetics are described by an Arrhenius type equation. Linear perturbation analysis of the growth model shows that positive nanoscale curvature (a concave surface) is able (i) to considerably increase the prenucleation concentration and thereby to increase the probability of nucleation and even to enable nucleation on substrates which do not have an affinity for nucleation under the given operating conditions, (ii) to increase considerably the growth rate of surface nuclei, and (iii) to lead to dramatic changes in morphology.
Field emission microscopy of carbon nanotube caps
Authors: Dean Kenneth A. , Chalamala Babu R.
Published: July 2002

We report field emission microscopy images from single walled carbon nanotubes showing resolvable fine structure with five- and six-fold symmetry. We present evidence indicating that these images have atomic resolution and that they depict the electronic structure of individual nanotube caps under high-field conditions. Fine structure is observed only after removal of surface adsorbates. Prior to adsorbate removal, these images show symmetrical lobed patterns and several other behaviors characteristic of chemisorbed resonant tunneling states.
Giant magnetostrictive multilayers (invited)
Authors: Quandt Eckhard , Ludwig Alfred
Published: July 2002

Multilayers combining exchange-coupled giant magnetostrictive materials (amorphous TbFe) and materials with large magnetic polarizations (FeCo or FeCoBSi) exhibit significant improvements in terms of low field magnetostriction and magnetostrictive susceptibility compared to state-of-the-art giant magnetostrictive single layer materials, due to the polarization enhancement and the anisotropy reduction in such multilayers. The magnetic, magnetostrictive, and mechanical properties of these multilayers can be engineered by varying their layer thickness ratios. With TbFe/FeCo multilayers saturation magnetoelastic coupling coefficients exceeding 25 MPa in combination with a magnetic saturation field of 20 mT and a coercive field of 2 mT can be obtained, while in TbFe/FeCoBSi multilayers the hysteresis almost vanishes but at the expense of the magnetostriction. Furthermore these multilayered materials show ΔE and magnetoresistance effect, which should allow the realization of smart thin film actuators.
Quantum computing and single-qubit measurements using the spin-filter effect (invited)
Authors: DiVincenzo David P.
Published: July 2002

Many things will have to go right for quantum computation to become a reality in the lab. For any of the presently proposed approaches involving spin states in solids, an essential requirement is that these spins should be measured at the single-Bohr-magneton level. Fortunately, quantum computing provides a suggestion for a new approach to this seemingly almost impossible task: convert the magnetization into a charge, and measure the charge. I show how this might be done by exploiting the spin-filter effect provided by ferromagnetic tunnel barriers, used in conjunction with one-electron quantum dots.
Near-edge x-ray absorption fine structure and Raman characterization of amorphous and nanostructured carbon films
Authors: Lenardi C. , Piseri P. , Briois V. , Bottani C. E. , Bassi A. Li , Milani P.
Published: July 2002

Amorphous and nanostructured carbon films were grown by using two different techniques: ion sputtering and cluster beam deposition. The films were studied by near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. Depending on the precursors, atoms, or clusters, the films are characterized by a different sp2/sp3 ratio which influences the mechanical and the electronic properties. Due to the sensitivities of NEXAFS (local order) and Raman (medium-range order), we have characterized and compared the structure of the films over different length scales. The complementarity of NEXAFS and Raman techniques for the characterization of disordered forms of carbon is here presented and discussed. We also present an original method of NEXAFS spectra calibration allowing a better determination of peak positions.
Numerical predictions of oscillatory combustion in reactive multilayers
Authors: Jayaraman S. , Knio O. M. , Mann A. B. , Weihs T. P.
Published: July 2002

Current analytical models and recent experimental studies assume that exothermic formation reactions self-propagate in multilayer foils with steady temperature and composition profiles and uniform velocities. The numerical study presented here shows that in addition to steady propagation, oscillatory combustion can also exist in certain parameter regimes. The oscillatory regimes are characterized by superadiabatic temperature excursions and large variations in the reaction zone width, the flame thermal width, and the instantaneous flame velocity. The magnitudes of the oscillations are shown to increase as bilayer period decreases, as premixing in the as-deposited state increases, and as ambient temperature decreases.
Resistivity and oxygen content of indium tin oxide films deposited at room temperature by pulsed-laser ablation
Authors: Wu Y. , Marée C. H. M. , Haglund R. F. , Hamilton J. D. , Morales Paliza M. A. , Huang M. B. , Feldman L. C. , Weller R. A.
Published: July 2002

Transparent, conductive indium tin oxide (ITO) films were fabricated by pulsed-laser deposition on substrates held at room temperature. We investigated the relationship between electrical/optical properties of the films and the material stoichiometry, measured by Rutherford backscattering. The lowest resistivity films (∼4×10−4 Ω cm) have excessive oxygen compared with the stoichiometric composition ITO. After annealing in argon at 400 °C for 1.5 h, the oxygen-to-(indium+tin) ratio approaches the stoichiometric composition of 1.5 and resistivities of annealed samples are ∼2.5×10−4 Ω cm. The room-temperature ITO resistivity dependence on chamber gas pressure is explained in terms of a gas-dynamic model and oxygen content of the film.
Fast polarization switching with memory effect in a vertical cavity surface emitting laser subject to modulated optical injection
Authors: Boiko D. L. , Stéphan G. M. , Besnard P.
Published: July 2002

The polarization state of a vertical cavity surface emitting laser (VCSEL) output was driven by means of optical injection from another VCSEL. The bistability inherent to polarization locking allowed us to generate a memory effect. The control parameter was the modulated bias current of the maser laser and effects were shown to exist at frequencies up to 1 GHz.
Automated indexing for texture and strain measurement with broad-bandpass x-ray microbeams
Authors: Chung Jin-Seok , Ice Gene E.
Published: July 2002

Methods are derived for measuring local strain, stress, and crystallographic texture (orientation) in polycrystalline samples when 1–10 grains are simultaneously illuminated by an energy scanable or broad-bandpass x-ray beam. The orientation and unit-cell shape for each illuminated grain can be determined from the diffracted directions of four Bragg reflections. The unit-cell volume is determined by measuring the energy (wavelength) of one reflection. The methods derived include an algorithm for simultaneously indexing the reflections from overlapping crystal Laue patterns and for determining the average strain and stress tensor of each grain. This approach allows measurements of the local strain and stress tensors which are impractical with traditional techniques.
Structural properties of fluorinated amorphous carbon films
Authors: Wang X. , Harris H. R. , Bouldin K. , Temkin H. , Gangopadhyay S. , Strathman M. D. , West M.
Published: July 2002

Fluorinated amorphous carbon films have been deposited in a plasma-enhanced chemical vapor deposition system, and the optical properties examined by Fourier transform infrared and ultraviolet-visible absorption spectroscopy. The infrared absorption spectra in the region from 1000 to 1800 cm−1 were resolved into ten peaks, which were assigned to various carbon–fluorine and carbon–carbon vibration modes. A relationship between the optical band gap and the aromatic carbon (sp2) concentration is demonstrated.
Structure property relations in BiFeO3/BaTiO3 solid solutions
Authors: Kumar M. Mahesh , Srinivas A. , Suryanarayana S. V.
Published: July 2002

BiFeO 3 , when forming a solid solution with BaTiO3, shows structural transformations over the entire compositional range. Above 70 mole % of BiFeO3 the structure is rhombohedral and below 4 mole %, it is tetragonal. In between the structure is cubic. The ferroelectric TC decreases with increasing composition of BaTiO3 and a relatively small relaxation is observed. Impedance measurements showed a structural dependence and analysis of which has clearly shown that the capacitance observed is from the bulk of the sample. Relaxation time (τ) of BiFeO3 was estimated using the relaxation times of the intermediate compositions. Magnetization measurements showed field induced ferromagnetism. As the structure becomes cubic with increasing concentration of BaTiO3, paramagnetism sets in, as evidenced by the electron spin resonance spectra.
Simple model for interface stresses with application to misfit dislocation generation in epitaxial thin films
Authors: Cammarata R. C. , Sieradzki K. , Spaepen F.
Published: July 2002

A simple model for the interfacial free energy of a semicoherent interface is used to develop expressions for interface stresses, which are surface thermodynamic quantities associated with solid–solid interfaces. An analysis of the thermodynamics of thin film epitaxy is presented that incorporates the effects of free surface and interface stresses, and an expression for the critical thickness for thin film epitaxy is obtained. Based on this analysis, the concept of effective pressures exerted by the thin film free surface and film–substrate interface is introduced. If it is assumed that misfit dislocations are generated at the film–substrate interface as a result of glide of threading dislocations, the thermodynamics and kinetics of stress relaxation can be discussed in terms of a balance of Peach–Koehler forces acting on the threading dislocations owing to the surface and interface pressures as well as to the coherency stress. An example is given that shows that, if the film has a relatively large surface pressure that opposes lattice matching, the dependence of the coherency strain on film thickness can be very different from that obtained from conventional analyses which ignore the effect of the free surface; specifically, the largest equilibrium coherency strain of the same sign as the misfit can be much smaller than the total misfit, and an “anomalous” coherency strain of sign opposite that of the misfit can be thermodynamically favorable at small film thicknesses. The analysis used to obtain the critical thickness for thin film epitaxy is extended to give an expression for the critical thickness for misfit dislocation generation at the interface between a substrate and a superlattice thin film. It is shown that this critical thickness depends on a superlattice pressure associated with the interlayer interface stress in addition to the free surface and film–substrate interface pressures.
Well-width dependence of the ground level emission of GaN/AlGaN quantum wells
Authors: Bonfiglio A. , Lomascolo M. , Traetta G. , Cingolani R. , Di Carlo A. , Della Sala F. , Lugli P. , Botchkarev A. , Morkoc H.
Published: July 2002

We have performed a systematic investigation of GaN/AlGaN quantum wells grown on different buffer layers (either GaN or AlGaN) in order to clarify the role of strain, structural parameters, and built-in field in determining the well-width dependence of the ground level emission energy. We find that identical quantum wells grown on different buffer layers exhibit strong variation of the ground level energy but similar well-width dependence. The data are quantitatively explained by an analytic model based on the envelope function formalism which accounts for screening and built-in field, and by a full self-consistent tight binding model.
Nanotribology of polymer blends
Authors: Terada Yasuhiko , Harada Masashi , Ikehara Takayuki , Nishi Toshio
Published: July 2002

We investigated rheological and tribological properties of polystyrene (PS) and poly (vinyl methyl ether) blends by atomic force microscopy. Glass transition temperature measurements showed that the blends whose PS contents were 80% (PS80) and 60% (PS60) were in the glassy state and in the glass-rubber transition state at room temperature, respectively. The force-distance curves were consistent with these results. The dependence of the frictional force Ff of mica on the load indicated that only adhesional component contributed to Ff. In contrast, the viscoelastic deformation of the surface of the blends contributed to the additional component of Ff. PS60, which was in the transition state, exhibited the largest value of Ff because of the large deformation. The value of Ff of PS60 had a hysteresis in the loading and unloading processes because the tip penetration depth was larger in the unloading process. The large penetration depth of PS60 in the negative load region in the unloading process is ascribed to the pull-off effect of the polymer chains.
Terahertz surface resistance of high temperature superconducting thin films
Authors: Wilke I. , Khazan M. , Rieck C. T. , Kuzel P. , Kaiser T. , Jaekel C. , Kurz H.
Published: July 2002

We report on measurements of the surface resistance of YBa2Cu3Ox thin films at frequencies between 0.087 and 2 THz and temperatures between 50 and 120 K by time-domain terahertz-transmission spectroscopy (TDTTS) and resonant microwave spectroscopy. The determination of the surface resistance of superconducting thin films by TDTTS is extended to higher frequencies and thicker films than previously by numerically solving the complex transmission coefficient. The numerical solution also provides the dielectric function of the YBa2Cu3Ox thin films. The temperature and frequency dependence of the surface resistance of YBa2Cu3Ox thin films in the THz range is successfully explained by a weak coupling model of d-wave superconductivity which incorporates inelastic and elastic scattering. The surface resistance of YBa2Cu3Ox thin films at THz frequencies is compared to the surface resistance of gold and niobium. The advantages of YBa2Cu3Ox thin films for superconducting THz electronic devices are discussed.
High-temperature shrinkage of single-walled carbon nanotube bundles up to 1600 K
Authors: Yosida Y.
Published: July 2002

An experimental study of x-ray diffraction of single-walled carbon nanotube (SWNT) bundles in the temperature range from 290 to 1600 K is reported. The lattice constant (L0) of a close-packed trigonal arrangement of SWNTs in the bundles shrank over the entire temperature range. The hysteretic temperature dependence of L0 was observed and discussed in terms of desorption of molecules from the bundle surface and of a magnetic phase transition of ferromagnetic metal catalyst particles. The coefficient of volume thermal expansion for the bundles is suggested to be negative over the measured temperature range by assuming that the tube-axis expansion is similar to the in-plane expansion for graphite.
Carrier induced ferromagnetic interactions and transport properties of p-Zn(1−x)MnxTe epilayers
Authors: Ferrand D. , Cibert J. , Wasiela A. , Bourgognon C. , Tatarenko S. , Fishman G. , Koleśnik S. , Jaroszyński J. , Dietl T. , Barbara B. , Dufeu D.
Published: July 2002

The conductivity and Hall effect of p-type doped Zn(1−x)MnxTe layers are described and analyzed in terms of a two-fluid model where part of the carriers are weakly localized as bound magnetic polarons. At low temperature, the Hall effect is dominated by the spin-dependent component (extraordinary Hall effect), in good agreement with a crude estimate taking into account the properties of the valence band. The long range ferromagnetic interaction appears to be induced by the weakly localized holes.
Domain configurations in domain engineered 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 single crystals
Authors: Yin Jianhua , Cao Wenwu
Published: July 2002

We report an optical microscopy study on the domain structures of 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 rhombohedral phase single crystals poled along [001] of the cubic coordinates. Both charged and uncharged domain walls are observed in domain engineered samples and they could join together to form “L” and “T” shaped domain walls, which are very unique. The observed domains are between 10 and 100 μm in size, which are much bigger than expected. Most of the samples have only two of the four possible degenerate domain states, making the macroscopic symmetry to be mm2 or lower rather than 4mm previously assumed.