bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetization processes in exchange-biased MnPd∕Fe bilayers studied by polarized neutron reflectivity
Authors: Blomqvist P. , Krishnan Kannan M. , Srinath S. , te Velthuis S. G. E.
Published: November 2004

The magnetization processes in exchange-biased MnPd∕Fe bilayers have been investigated using polarized neutron reflectivity and vibrating sample magnetometry. The measurements show that by breaking the symmetry of the intrinsic cubic fourfold anisotropy of the Fe film the induced unidirectional anisotropy radically changes the magnetization processes. If the exchange bias is large the induced unidirectional anisotropy is able to pull the net magnetization of the sample to the bias direction after saturation along any of the magnetic hard Fe⟨110⟩ directions. However, if the exchange bias is small it will only give rise to a difference in net magnetization along the magnetic easy axes adjacent to the saturation direction. Thus, the net magnetic moment along the bias direction depends on the relative magnitudes of the cubic and the unidirectional anisotropies.
Tribological properties of nanoliquid film under an external electric field
Authors: Luo J. B. , Shen M. W. , Wen S. Z.
Published: November 2004

A technique of relative optical interference intensity and a high-precision force measuring system are used to investigate the formation of ordered layer in the thin film lubrication by applying an external electric field (EEF). Experimental results indicate that the thickness of the lubricant film in a nanogap increases with EEF first, and then trends to a stable value. A strong EEF results in a high friction coefficient. When the electric power is turned off, the friction coefficient reduces again, but it cannot get back to that before the EEF was applied. The EEF effects on the film thickness and the friction coefficient are hardly related to the polarity of the EEF. The reasons for these experimental phenomena have been analyzed in the paper.
The magnetic behavior of iron oxide passivated iron nanoparticles
Authors: Baker C. , Hasanain S. K. , Shah S. Ismat
Published: November 2004

Iron oxide passivated iron nanoparticles were synthesized in an inert gas condensation system. The nanoparticles were single domain but not superparamagnetic. Varying amounts of oxide passivation of the nanoparticles were achieved. The oxide-passivated particles exhibit an exchange bias when cooled below a blocking temperature characteristic of the thickness of the oxide layer present. It was found that the exchange bias and blocking temperature both increase with oxide thickness with the blocking temperature in all cases being much lower than the Néel temperature for Fe oxides. We find that the oxide shell-core spin interaction leaves its imprint on the low field dc and ac magnetizations as well. Furthermore, below a characteristic freezing temperature a sharp increase in the field cooled magnetic moment of the samples is evidenced and is suggestive of a spin freezing process at the surface of the particles.
Simulation of ion generation and breakdown in atmospheric air
Authors: Zhang W. , Fisher T. S. , Garimella S. V.
Published: November 2004

Understanding of ion generation in air provides insights to several applications, such as gas sensors, electrohydrodynamic pumping, and air purification. In this paper, ion generation processes in atmospheric air are simulated using a particle-in-cell and Monte Carlo method with emphasis on the prediction of ion generation and breakdown characteristics in microscale gaps. The simulation results are validated through comparison to Townsend’s discharge theory and experiments. The significance of each relevant electron-molecule reaction is characterized to improve understanding of ion generation dynamics. Self-sustaining discharge and ionization are predicted under sufficient voltage bias, and the predicted trends of breakdown voltage are similar to those obtained from Paschen’s curve. Corrections to Paschen’s curve in microscale gaps also are identified and compare well to experiments. Electron field emission produces stable electron current that suggests a controllable ionization device without external electron injection sources.
Optimization of AlGaN∕GaN current aperture vertical electron transistor (CAVET) fabricated by photoelectrochemical wet etching
Authors: Gao Y. , Ben-Yaacov I. , Mishra U. K. , Hu E. L.
Published: November 2004

AlGaN ∕ GaN current aperture vertical electron transistor (CAVET) was fabricated and optimized for band gap selective photoelectrochemical wet etching. The large polarization induced voltage offset (around 2.5–4eV) observed in the first generation CAVET was reduced to 0.7V in this structure by employing a δ Si doping layer buried 60Å below the In0.03Ga0.97N (60nm thick) and bottom GaN interface to screen the polarization fields. Other sample structures were studied to achieve an aperture with both good undercut etching and a small voltage offset. It was clearly demonstrated that etch selectivity in the GaN∕InGaN∕GaN undercut structures was influenced by hole confinement and the chemical activity of the N-face GaN.
Electronic transport in carbon nanotubes using the transfer-matrix method
Authors: Umegaki Toshihito , Ogawa Matsuto , Makino Yasuyuki , Miyoshi Tanroku
Published: November 2004

We have studied the basic conductive characteristics in carbon nanotubes (CNTs) for the purpose of application to a high-frequency device. In the analysis, the current flow in the CNTs is viewed as the quantum transport of electronic waves. First, we analyzed the dispersion relations of electronic waves in the CNTs based on the linear combination of atomic orbitals expansion method. In addition, we investigated the current-density distributions around the circumference of the CNTs and the current-voltage characteristics by using the transfer-matrix method. As a result, the current distributions were found to be significantly controlled by both the chirality of the CNTs and the position of the current sources around the circumference. Based on these results, we propose herein a ferrite device that acts as a filter in the terahertz frequency domain. In this device, the high-frequency current flowing on the CNTs may excite and receive directly the spin waves in the ferrite film beneath them, and the operating wavelength may be controlled by means of the chirality via the current-density distributions on the CNTs. In addition, we found that the performance of the device can be improved by using the p-type CNTs as the excitation electrodes rather than the intrinsic CNTs.
Lattice dynamics in BaxSr1−xTiO3 thin films studied by Raman spectroscopy
Authors: Tenne D. A. , Soukiassian A. , Xi X. X. , Choosuwan H. , Guo R. , Bhalla A. S.
Published: November 2004

We present a Raman spectroscopy study of the lattice-dynamical properties of the BaxSr1−xTiO3 thin films with x=0.05, 0.1, 0.2, 0.35, and 0.5 in the temperature range of 5–300K. Soft phonon modes were observed, and the temperature dependence of their frequencies and Raman intensities were investigated. The Raman spectra of the BaxSr1−xTiO3 films are compared with the single crystals of the same compositions. The essential differences in the lattice-dynamical properties of the thin films and single crystals are observed, such as the forbidden first-order Raman scattering in the films well above the phase-transition temperature in bulk, hardening of the soft phonon modes in films compared to crystals, and significantly larger range of the soft-mode overdamping in the films. On the other hand, the lattice-dynamics behavior in the BaxSr1−xTiO3 films is found similar to that of the relaxor ferroelectrics. Analogous to the relaxors, the presence of the polar nanoregions existing in the films at the temperatures above the bulk ferroelectric phase transition explains the specific lattice-dynamical properties of the BaxSr1−xTiO3 thin films.
Low loss striated YBa2Cu3O7−d coated conductor with filamentary current sharing
Authors: Barnes Paul N. , Sumption Michael D.
Published: November 2004

A simple investigation into the properties of weakly linked filaments in striated YBa2Cu3O7−d (YBCO) coated conductors has been performed. Such weak-link connections may arise in two different contexts within the coated conductors, the one case being the occurrence of unintentional connections. However, it may be desirable to intentionally induce the weak-link connections in order to allow a current sharing between the filaments. This allows the localized degradation of a given filament to be circumvented at a small cost in terms of the added losses and magnetization. This problem has been treated mathematically with a comparison of the superconductor weak linking in contrast to the normal-metal-induced current sharing. The estimates for target values of filament coupling are deduced, and the results in terms of magnetization are then determined. The YBCO samples created, using the pulsed laser deposition, are described that have these properties. The samples consist of one control and one striated sample. The magnetization response of these samples is then considered in terms of the expressions developed.
Structural characterization and electron-energy-loss spectroscopic study of pulsed laser deposited LiNbO3 films on a-sapphire
Authors: Dai J. Y. , Lam H. K. , Li Quan , Wang J. , Chan H. L. W. , Choy C. L.
Published: November 2004

Highly c- and a-oriented LiNbO3 films were deposited on a-sapphire substrates by pulsed laser deposition. The film microstructure and crystal orientation were studied by transmission electron microscopy, and the mechanism forming the different film orientations was interpreted in terms of the adatom energy and oxygen pressures. The electron-energy-loss functions derived from the electron-energy-loss spectra exhibit characteristic energy-loss peaks at about 7.0, 11.0, and 14.0eV. These peaks correspond to electron transitions from the maximum density of the states in the valence band of O 2p to the split conduction band. The dielectric functions of the LiNbO3 films were also derived from the energy-loss function.
Zinc-embedded silica nanoparticle layer in a multilayer coating on a glass substrate achieves broadband antireflection and high transparency
Authors: Kim Sang Woo , Bae Dong-Sik , Shin Hyunho
Published: November 2004

A zinc-embedded silica (Zn-SiO2) nanoparticle layer has been applied as the outermost layer over the three-layer coating system, Zn-SiO2∕SiO2∕ITO (indium tin oxide), coated on a soda-lime glass substrate. The additional coating of the zinc-embedded nanoparticle layer over the 2-layer/glass, i.e., SiO2∕ITO∕glass system, yielded a significant diminution in reflectance, as well as an improved transmittance as compared to the 2-layer/glass system. Plausible mechanisms responsible for such phenomena are discussed. The application of the zinc-embedded silica nanoparticle layer to the multilayer coating system is shown to provide a flexible way to achieve a broadband antireflection and a high transmission.
Optical diffraction gratings from polymer-dispersed liquid crystals switched by interdigitated electrodes
Authors: Drevenšek-Olenik I. , Sousa M. E. , Crawford G. P. , Čopič M.
Published: November 2004

A spatial dependence of the refractive index perturbation induced in a 5-μm-thick polymer-dispersed liquid-crystal layer, by the use of a periodic electric field from the system of 50μm separated interdigitated electrodes, was investigated by measuring the induced in-plane birefringence of the sample. The efficiency of various diffraction orders was measured as a function of the applied voltage amplitude, and the dependence achieved for the 1st diffraction order was compared to the values calculated on the basis of the refractive index profile. The switching properties in response to the amplitude modulation of the driving voltage of various modulation depths were investigated. Our results demonstrate that a very effective phase modulation with response times considerably below 10ms can be achieved at the expense of slightly higher driving fields than needed for the bulk nematic liquid-crystal gratings.
Comparison of GaNAsSb and GaNAs as quantum-well barriers for GaInNAsSb optoelectronic devices operating at 1.3–1.55μm
Authors: Yuen Homan B. , Bank Seth R. , Wistey Mark A. , Harris James S. , Moto Akihiro
Published: November 2004

Ga N As Sb ∕ Ga As quantum wells were grown by solid-source molecular-beam epitaxy utilizing a radio-frequency nitrogen plasma source. The GaNAsSb layers, originally the quantum well barrier materials for GaInNAs(Sb) devices, were studied for their general growth characteristics as well as their structural and optical properties, which give an indication of its quality as a quantum well barrier material. Reflection high-energy electron diffraction, high-resolution x-ray diffraction, secondary-ion mass spectroscopy, and photoluminescence (PL) measurements were used to study those properties. The growth parameters including arsenic overpressure and substrate temperature were changed systematically to determine the properties during deposition and to optimize these conditions. It was found that the addition of antimony to GaNAs did not improve the material as it did for GaInNAs. PL measurements indicated a decreasing optical quality with an increasing substrate temperature and no change with the arsenic overpressure. In addition, the addition of antimony had eliminated the lattice-strain compensation provided by the GaNAs barriers. Using GaNAs rather than GaNAsSb barriers in the GaInNAs(Sb) devices was found to be advantageous and has dramatically improved the performance of long-wavelength GaAs-based lasers.
Optical-injection-induced polarization switching in polarization-bistable vertical-cavity surface-emitting lasers
Authors: Ryvkin B. S. , Panajotov K. , Avrutin E. A. , Veretennicoff I. , Thienpont H.
Published: November 2004

We present a simple quasianalytical formalism for static polarization-resolved light-current characteristics of dual polarization mode vertical-cavity surface-emitting lasers (VCSELs) under optical injection. Dynamics of optical-injection-induced polarization switching are analyzed and simple analytical estimates for the polarization switching time obtained in particular cases. The results suggest the possibility of ultrafast polarization switching, implying that optical-injection-induced polarization switching in VCSELs can operate as a fast flip-flop operation.
Bias dependence and electrical breakdown of small diameter single-walled carbon nanotubes
Authors: Seidel R. V. , Graham A. P. , Rajasekharan B. , Unger E. , Liebau M. , Duesberg G. S. , Kreupl F. , Hoenlein W.
Published: November 2004

The electronic breakdown and the bias dependence of the conductance have been investigated for a large number of catalytic chemical vapor deposition grown single-walled carbon nanotubes (SWCNTs) with very small diameters. The convenient fabrication of thousands of properly contacted SWCNTs was possible by growth on electrode structures and subsequent electroless palladium deposition. Almost all of the measured SWCNTs showed at least weak gate dependence at room temperature. Large differences in the conductance and breakdown behavior have been found for “normal” semiconducting SWCNTs and small band-gap semiconducting SWCNTs.
Structure and simulation of hydrogenated nanocrystalline silicon
Authors: Pan B. C. , Biswas R.
Published: November 2004

We simulated hydrogenated nanocrystalline silicon with molecular dynamics calculations using embedding and melt-quenching approaches. The embedding approach generates a well-defined crystallite residing in an amorphous matrix and a structure free of coordination defects. The H-distribution is inhomogeneous with an excess hydrogen density at the strained grain boundary between the nanocrystallite and the amorphous matrix. The amorphous matrix is better ordered in hydrogenated nanocrystalline-Si than in the homogenous amorphous silicon networks. Nanocrystals have been annealed at higher temperature where the nanocrystalline regions shrink in size. Nucleation and growth theory of these nanocrystals is developed. Melt-quenching simulations generate nc-Si structures which do not show a disordered grain boundary but generate a very high density of defects in the amorphous region, which cannot be annealed. The embedding approach may favor structures closer to experimental H-diluted growth conditions. Melt-quenching may better represent structures formed by laser induced recrystallization.
Mechanism of low-temperature θ-CuGa2 phase formation in Cu-Ga alloys by mechanical alloying
Authors: Hong Soon-Jik , Suryanarayana C.
Published: November 2004

The mechanism on the formation of the θ-CuGa2 phase in binary Cu–Ga alloys has been investigated through mechanical alloying (MA) of blended elemental powders by varying process variables such as milling time and milling temperature. The particle size distribution was very broad at the beginning of milling but became narrower as the milling time increased and steady-state equilibrium was reached. The average powder particle size reached a peak value of 270μm at 30min of milling and then continued to decrease gradually to 6μm on milling for 20h. Formation of the θ-CuGa2 phase started to occur even after milling for 2min and was completed after 1h of milling. Melting of Ga was noted in the early stages of milling, probably due to the rise in powder temperature. To discount the possibility that the melting of Ga was responsible for the θ-phase formation, milling was conducted at lower temperatures by dripping liquid nitrogen on to the container. The θ-phase still formed, suggesting that its formation was not related to the melting of Ga and that it was formed by a solid-state reaction even at low temperatures. The compositional homogeneity range of the θ-phase was also extended under the MA conditions. Details of the mechanism of phase formation, as observed by x-ray diffraction and scanning electron microscopy methods, are presented.
Mapping orientations of easy axes in nanodot arrays by magnetic force microscopy
Authors: Bai J. , Takahoshi H. , Ito H. , Saito H. , Ishio S.
Published: November 2004

An experimental method has been developed for mapping the three-dimensional (3D) orientations of easy axes in nanodot arrays by means of magnetic force microscopy. This method was applied to a CoCrPt-patterned medium to measure 3D easy-axes orientations of individual cylindrical dots of an average of 36 nm in diameter and 40 nm in height. A main easy-axes orientation was found according to the planar number distribution of dots being in the same range of easy-axes orientations. The dot-number dependence on inclining angles of magnetocrystalline easy axes from the perpendicular direction agreed well with the x-ray diffraction result. The magnetization reversal mechanism in the medium was also discussed.
Effect of underdoping on the microwave surface resistance of YBa2Cu3Oz thin films
Authors: Murugesan M. , Obara H. , Kosaka S. , Yamasaki H. , Prijamboedi B.
Published: November 2004

The effect of underdoping on the microwave surface resistance (Rs) and the Hall number (nH) in the Co-substituted YBa2Cu3Oz (YBCO) films grown on LaAlO3 substrates has been investigated, and the results are compared with oxygen-deficient YBCO films. From the analysis of Hall data, it is inferred that the nH is suppressed upon the Co substitution. As the Co concentration increases, the in-plane resistivity (ρ) increases and ρ(T) develops a downward curvature below room temperature, whereas the nH decays exponentially. Microwave studies on both the Co-substituted YBCO as well as the oxygen-deficient YBCO reveal that the reduction in carrier concentration of YBCO, by any means, leads to an undesirable effect on Rs. The Rs(T) value of Co-substituted films increases as the Co concentration is increased. Near the superconducting transition temperature, Tc, the observed high Rs value and a broad transition in the Rs(T) curve in the Co-substituted films are presumed to be due to an increase in the penetration depth (λ), which is caused by the reduced carrier concentration. Moreover, the Rs value of 0.76 mΩ (at 20 K; 22 GHz) for the Co concentration of x=0.15 is very close to the value of oxygen-depleted YBCO examined in the present work. Therefore, the observed high residual Rs value at low temperatures (well below Tc), in both the Co-substituted as well as the oxygen-depleted YBCO systems, is presumably due to (i) the increased penetration depth, λ, and (ii) the poor crystallinity in combination with the deteriorated surface morphology, as evidenced from the results of x-ray diffraction and atomic force microscopy.
Hydrogen analysis of zircaloy tube used in nuclear power station using laser plasma technique
Authors: Kurniawan Koo Hendrik , Lie Tjung Jie , Idris Nasrullah , Kobayashi Takao , Maruyama Tadashi , Kagawa Kiichiro , Tjia May On , Chumakov Alexander Nikitich
Published: November 2004

It is shown that remarkable improvements essential to a quantitative spectrochemical analysis of hydrogen emissions from the zircaloy samples were achieved when the low-pressure surrounding air used in the previous experiment of Nd-YAG laser-induced shockwave plasma was replaced by an inert gas. Using the high-purity (99.999%) nitrogen gas at 1.5 Torr, a linear calibration curve of the HI 656.2 nm emission line was obtained with a zero intercept from the zircaloy samples prepared with various hydrogen concentrations. Further, when the surrounding nitrogen gas was replaced by a helium gas, more than an order of magnitude enhancement was obtained on the signal-to-noise ratio, yielding a detection limit of less than 5 ppm.
Explosion of thin aluminum foils in air
Authors: Baksht R. , Pokryvailo A. , Yankelevich Y. , Ziv I.
Published: November 2004

An inductive-based power supply (240μH, 50kA) was used for the investigation of the foil explosion process in the time range of 0.05ms<texpl<0.2ms. The foil current for these experiments was set between 10kA and 27kA. The foil thickness was 10μm and its length was 5cm. Images of the exploding foil captured by a fast framing camera show that the radiation of the exploding foil discharge starts on the voltage rise. The peak voltage Vpeak decreases with an increasing time to explosion and a decreasing current density: Vpeak=2.8kV at j=9.1MA∕cm2 and Vpeak=1.75kV at j=5.3MA∕cm2. A phenomenological model was suggested for the experimental data interpretations. Contrary to the previous model, we assume that the breakdown initiates with cathode spots instead of a breakdown by a bulk discharge. We suggest that the plasma appearance is related to the ejection of a metal vapor during the metal boiling. The high temperature of the vapor permits ignition of the cathode spots even at a low electric field. It results in the arcing in the metal vapor and bypassing the liquid pieces of the foil.
Ion-beam-induced amorphization and recrystallization in silicon
Authors: Pelaz Lourdes , Marqués Luis A. , Barbolla Juan
Published: November 2004

Ion-beam-induced amorphization in Si has attracted significant interest since the beginning of the use of ion implantation for the fabrication of Si devices. A number of theoretical calculations and experiments were designed to provide a better understanding of the mechanisms behind the crystal-to-amorphous transition in Si. Nowadays, a renewed interest in the modeling of amorphization mechanisms at atomic level has arisen due to the use of preamorphizing implants and high dopant implantation doses for the fabrication of nanometric-scale Si devices. In this paper we will describe the most significant experimental observations related to the ion-beam-induced amorphization in Si and the models that have been developed to describe the process. Amorphous Si formation by ion implantation is the result of a critical balance between the damage generation and its annihilation. Implantation cascades generate different damage configurations going from isolated point defects and point defect clusters in essentially crystalline Si to amorphous pockets and continuous amorphous layers. The superlinear trend in the damage accumulation with dose and the existence of an ion mass depending critical temperature above which it is not possible to amorphize are some of the intriguing features of the ion-beam-induced amorphization in Si. Phenomenological models were developed in an attempt to explain the experimental observations, as well as other more recent atomistic models based on particular defects. Under traditional models, amorphization is envisaged to occur through the overlap of isolated damaged regions created by individual ions (heterogeneous amorphization) or via the buildup of simple defects (homogeneous amorphization). The development of atomistic amorphization models requires the identification of the lattice defects involved in the amorphization process and the characterization of their annealing behavior. Recently, the amorphization model based on the accumulation and interaction of bond defects or IV pairs has been shown to quantitatively reproduce the experimental observations. Current understanding of amorphous Si formation and its recrystallization, predictive capabilities of amorphization models, and residual damage after regrowth are analyzed.
Thermal annealing of junctions with amorphous and polycrystalline ferromagnetic electrodes
Authors: Dimopoulos T. , Gieres G. , Wecker J. , Wiese N. , Sacher M. D.
Published: November 2004

In this work we study Al-oxide-based tunnel junctions with amorphous Co60Fe20B20 and polycrystalline Co90Fe10 ferromagnetic (FM) electrodes. Focus is given on the evolution of the tunnel magnetoresistance and barrier characteristics (resistance-area product, effective thickness, height, and asymmetry) as a function of the annealing temperature up to 400°C. Junctions with two CoFeB electrodes show the largest thermal stability of the tunnel magnetoresistance. Substituting firstly one and then both CoFeB electrodes with CoFe leads to an increasingly faster degradation of the spin-dependent transport upon annealing. The observed differences suggest an improved interface quality between the amorphous FM and the Al oxide.
Phase-plane analysis and classification of transient regimes for high-field electron transport in nitride semiconductors
Authors: Sokolov V. N. , Kim K. W. , Kochelap V. A. , Woolard D. L.
Published: November 2004

We present a detailed theoretical analysis of steady-state, transient time-dependent, and spatially dependent electron transport in the group-III nitrides at high and ultrahigh electric fields. To develop an analytical model, we derive time-dependent differential equations describing the hot-electron rates of momentum and energy relaxation in electron–polar-optical-phonon scattering and analyze them by employing phase-plane analysis. From the structure of the phase-plane partitioning based on the phase trajectories, the transient regimes are investigated and classified depending on various initial conditions. We have studied different subpicosecond regimes and found a considerable velocity overshoot effect. One of our findings is that when the velocity reaches the maximum, the electron temperature is of a moderate magnitude but increases considerably in the subsequent stage. Dynamic regimes with high electron temperature overshoot have been revealed. For the dominant electron–polar-optical-phonon scattering, the observed overshoot can be treated as a rudiment of the runaway effect typical for that mechanism. In nanoscale nitride diodes with space-charge limited transport, the transient processes are extended to sufficiently larger distances, the overshoot is weaker and the electron heating in the region of the peak velocity is greater than that found for time-dependent problem with a constant electric field throughout a homogeneous sample.
The role of N-related point defects in the degradation process of ZnSe-based white light-emitting diodes
Authors: Katayama Koji , Adachi Masahiro , Abe Tomoki , Urata Akihiro , Tsutsumi Sueyuki , Inoue Noboru , Nakamura Takao , Ando Koshi
Published: November 2004

The role of N-related point defects in the degradation process of ZnSe-based white light-emitting diodes under operation was investigated. The generation rate of microscopic dark spots, which do not correspond to the original stacking faults or threading dislocations in the epilayer, was found to correspond to the initial concentration of the N-related deep-level H0 (ET−Ev=0.8±0.2eV) in the p-ZnMgSSe layer. Devices with high initial H0 concentrations resulting from thermal treatments, at the same time, showed a drastic reduction in their net acceptor concentrations. The origin of this effect is believed to be due to the deactivation of the N acceptor, rather than the hole-trapping effect of the H0 center.
Microstructure and electrical properties of sputter-deposited Zn0.87Fe2.13O4 thin layers
Authors: Capdeville Stéphanie , Alphonse Pierre , Bonningue Corine , Presmanes Lionel , Tailhades Philippe
Published: November 2004

Sputtering deposition of spinel ferrites leads to a combination of attractive physical properties (optical, magnetic, and semiconducting), which can be adjusted by carefully selecting the preparation conditions and stoichiometry during controlled postdeposition annealing. Thin-sputtered ferrite films are of great interest for integration as functional parts in microsystems. In this paper, we report the effect on electrical properties of the microstructure of sputtered zinc-ferrite films. Transmission electron microscopy and atomic force microscopy were used to observe these nanostructured layers. The measured surface area to projected surface area ratio was calculated on the basis of Brunauer-Emmett-Teller measurements with krypton at 77K. The contribution of grains and boundaries to the global resistivity of the ferrite films was studied by spectroscopic impedance measurements. Impedance variations were in agreement with the microstructure evolution when the argon deposition pressure was increased. The increase of film dc resistivity versus annealing temperatures was explained by the influence of the microstructure on Fe2+ cation oxidation.
Molecular volume and electronic and vibrational polarizibilities for amorphous LaAlO3
Authors: Busani T. , Devine R. A. B.
Published: November 2004

Grazing incidence x-ray reflectivity measurements are used to determine the density of sputter-deposited LaAlO3 and anodized LaAl films. Together with refractive index and dielectric constant measurements, it is demonstrated that a coherent picture emerges explaining the low dielectric constant of the amorphous films (∼13) as compared to the single-crystal value (∼26). The importance of molecular volume dependence of the electronic and vibrational molecular polarizabilities is underlined.
Abnormal phase transitions for tetragonal (1−x)Pb(Mg1∕3Nb2∕3)O3-xPbTiO3 single crystals at low temperature
Authors: Wan Xinming , Zheng R. K. , Chan H. L. W. , Choy C. L. , Zhao Xiangyong , Luo Haosu
Published: November 2004

Dielectric spectroscopy and ultrasonic sound velocity were investigated for tetragonal PMN-xPT single crystals over a wide range of temperature. In addition to the expected phase transition from the tetragonal ferroelectric phase to the cubic paraelectric phase at a high temperature, an abnormal ferroelectric phase transition below room temperature is observed. The low-temperature phase transition shows a relaxor behavior though tetragonal PMN-xPT single crystal is considered to be typical normal ferroelectrics. By comparing the dielectric constants of poled and depoled conditions and by analyzing [BO6] oxygen-octahedra units of perovskite structure, it is suggested that the low-temperature phase transition in poled tetragonal PMN-xPT single crystal could be tetragonal to rhombohedral ferroelectric phase transition.
Molecular-dynamics simulations of organic polymer etching by hydrocarbon beams
Authors: Yamada Hideaki , Hamaguchi Satoshi
Published: November 2004

Molecular-dynamics simulations of hydrocarbon beam injections into a poly (1,4-phenylene) substrate surface are carried out with the use of classical potential functions for covalent bonds of carbon and hydrogen atoms. Van der Waals interactions among carbon atoms are also taken into account. In the low injection energy (50eV) regime, we have observed that injected carbon atoms tend to be deposited on the surface, whereas hydrogen atoms tend to chemically break carbon bonds in the substrate. With the combination of chemical effects by hydrogen with large momenta carried by the injected carbon atoms, hydrogen-rich carbon clusters can etch organic polymer surfaces with relatively high efficiency. Implications of our simulation results on etching processes of low-dielectric-constant organic polymers by hydrogen-nitrogen plasmas are also discussed.
Plasma deposition of low dielectric constant (k=2.2∼2.4) Boron Nitride on methylsilsesquioxane-based nanoporous films
Authors: Liu Jun , Loh Kian Ping , Lin Ming , Foo Yong Lim , Wang Wei De , Chi Dong Zhi
Published: November 2004

The growth of low dielectric constant (k) Boron Nitride (BN) film on silicon as wel1 as methylsilsesquioxane-based nanoporous films (LKD-5109) using plasma-discharged borazine was investigated. BN films were grown using microwave plasma (2.45GHz) or radio-frequency (rf) atom beam deposition (13.56MHz) on LKD-5109 in order to evaluate the compatibility of the two plasma processes with the physical integrity of the nanoporous films. Capacitance–voltage measurements were used to characterize the dielectric constants of the films on silicon and BN-integrated LKD (keff≈2.4). The composition and phases of the films were studied using cross-section transmission electron microscopy and electron-energy-loss spectroscopy. Although the microwave plasma process could produce BCxN films with a k value of 2.2, the process was not compatible with the nanoporous LKD substrate due to the ion-induced damage of the films. We found that only the rf atom beam deposition process, which was characterized by low-energy neutral fluxes, maintained the dielectric property of the BN-integrated LKD stack at an overall value of 2.4. In addition, the deposited BN films can act as an effective copper diffusion barrier on the LKD and can be lithographically processed to form trench patterns.
Numerical simulation of metal plasma-immersion ion implantation and deposition on a cone
Authors: Cornet Christophe , Kwok Dixon T. K. , Bilek M. M. M. , McKenzie D. R.
Published: November 2004

A two-dimensional particle-in-cell simulation in r-z cylindrical co-ordinates is used to model metal plasma-immersion ion implantation and deposition on a cone. We show that a sharp cone mounted on a plane or stage exhibits an ion-focusing effect, such that an increased ion dose at the sides of conical features will occur during the application of high negative voltage plasma-immersion ion implantation pulses. This focusing effect is due to the shape of the equilibrium sheath and is strongly enhanced by sharper cones. The focusing effect increases for sharper cones and the ion trajectories bend more sharply. However, this deflection of the trajectories is not strong enough to direct the ions normal to the cone surface. Consequently, sharper cones exhibit a more oblique incident angle for the implanted ions.
Formation of orthorhombic tin dioxide from mechanically milled monoxide powders
Authors: Lamelas F. J.
Published: November 2004

X-ray scattering measurements are used to show that the metastable orthorhombic phase of tin dioxide is produced by the oxidation of mechanically milled litharge-phase tin monoxide. After milling to a grain size of approximately 20nm, followed by heating to 575°C, the fraction of the orthorhombic phase is approximately 80%. The orthorhombic phase was originally observed in high-pressure experiments, but more recently, it has been produced in a wide variety of thin-film and nanoparticle samples. The data presented here demonstrate the importance of small-grain-size tin monoxide as a precursor in the ambient-pressure synthesis of the orthorhombic phase. This result has practical importance in the production of tin dioxide gas sensors. A more fundamental observation is that the particle size of a precursor phase can have a marked effect on subsequent phases produced during oxidation. Lastly, a formula for determining the orthorhombic fraction in two-phase tin dioxide samples is developed using the method of standard additions.
The photosensitivity and ultraviolet absorption change of Sn-doped silica film fabricated by modified chemical vapor deposition
Authors: Chen Guanghui , Li Yigang , Liu Liying , He Yaoji , Xu Lei , Wang Wencheng
Published: November 2004

10.5 μ m thick Sn-doped silica films were prepared by the modified chemical vapor deposition followed by the solution-doping method. The films were exposed to 248 and 266nm laser light, respectively. Positive refractive index change up to 2×10−4 at 1550nm was observed by measuring the reflectivity based on Fresnel formulas. The data of UV absorption spectra suggest that the photosensitivity of the Sn-doped silica film under high energy density laser irradiation should be mainly due to the bond breaking of oxygen deficient defects, while under relatively low energy density laser irradiation, the refractive index change probably originates from photoconversion of optically active defects.
Micromagnetism and the microstructure of high-temperature permanent magnets
Authors: Goll D. , Kronmüller H. , Stadelmaier H. H.
Published: November 2004

Sm 2 ( Co , Cu , Fe , Zr ) 17 permanent magnets with their three-phase precipitation structure (cells, cell walls, and lamellae) show two characteristic features which so far are difficult to interpret but which are the prerequisites for high-temperature applications: (1) The hard magnetic properties only develop during the final step of the three-step annealing procedure consisting of homogenization, isothermal aging, and cooling. (2) Depending on the composition and on the annealing parameters, the temperature dependence of the coercivity can be easily changed from the conventional monotonic to the recent nonmonotonic behavior showing coercivities up to 1T even at 500K. The magnetic hardening during cooling is due to the fact that the cell walls order chemically and structurally during the cooling process. From an analysis of electron diffraction patterns of the superimposed structures existing before and after cooling it could be proven that a phase transition from a phase mixture of defective phases 2:17, 2:7, and 5:19 to the ordered 1:5 phase takes place in the cell walls during cooling. The nonmonotonic temperature dependence of the coercivity is narrowly related to the magnetic hardening mechanism which can be either pinning or nucleation and results from the magnetic and microstructural properties of the cell walls. These properties have been determined quantitatively from hysteresis loop measurements and from high-resolution transmission electron microscopy and energy dispersive x-ray analysis. Due to the temperature dependence of the intrinsic magnetic properties, the nonmonotonic temperature dependence of the coercivity is found to be determined by repulsive pinning of domain walls at the cell walls at low temperatures, by attractive pinning of domain walls in the cell walls at intermediate temperatures, and by nucleation at high temperatures. This complex temperature behavior is also reflected in characteristic changes of the angular dependence of the coercivity and can be described quantitatively on the basis of micromagnetism.
Mixed finite element-tight-binding electromechanical analysis of carbon nanotubes
Authors: Pantano A. , Parks D. M. , Boyce M. C. , Buongiorno Nardelli M.
Published: November 2004

Electrical transport properties of carbon nanotubes can be dramatically changed by mechanical deformations that alter tube shape and the corresponding positions of the atoms comprising the tube wall. In principle, detailed atomic/electronic calculations can provide both the deformed configuration and the resulting electrical transport behavior of the tube. Here we simplify the process by refining a previously-developed nonlinear structural mechanics finite-element-based procedure for modeling mechanical behavior of carbon nanotubes to account explicitly for tube chirality. A quadrilateral element overlay procedure provides an isotropic finite element model of hexagonal cells within a graphene sheet, with the only nodal positions coincident with those of the atoms. Mechanical deformation of the nanotube structure is simulated with finite elements, and the evolving atomic [nodal] coordinates are processed within the finite element (FE) program by using a tight-binding (TB) code to calculate deformation-induced changes in electrical transport properties of the nanotube. Results of the mixed FE/TB calculations compare favorably with existing atomistic simulations of single-walled nanotubes subjected to torsion, intense lateral squeezing, and large, kink-producing bending.
Dielectric and Magnetoelectric Characterization of CoFe2O4∕Sr0.5Ba0.5Nb2O6 Composites
Authors: Chen X. M. , Tang Y. H. , Chen I.-W. , Xu Z. C. , Wu S. Y.
Published: November 2004

Magnetoelectric composites were created through incorporating dispersed CoFe2O4 ferromagnetic grains into a Sr0.5Ba0.5Nb2O6 relaxor ferroelectric matrix. The two phases are chemically compatible and coexist in the dense, sintered composites. Unlike other relaxors, the Sr0.5Ba0.5Nb2O6-containing composites can be electrically poled. The apparent dielectric constant is enhanced by the conductivity in the ferrite phase and the space charge at the phase interface. A significant magnetoelectric effect was observed in all compositions investigated, with a maximum dE∕dH value of 24.8 mV/cm/Oe obtained for 0.4CoFe2O4∕0.6Sr0.5Ba0.5Nb2O6, which is much greater than that of magnetoelectric compounds and solid solutions. These results demonstrate that the Pb-free (Sr,Ba)Nb2O6 relaxor can be advantageously used as the matrix for ferromagnetic/ferroelectric composites in the future development of magnetoelectric materials.
Single-crystal elastic constants of disordered and ordered FePd
Authors: Ichitsubo Tetsu , Tanaka Katsushi
Published: November 2004

Single-crystal elastic constants of A1 (fcc disordered) and single-variant L10-ordered FePd alloy are presented. The ordered phase consisting of single-variant domains was formed using the compression method proposed by our previous paper [T. Ichitsubo, M. Nakamoto, K. Tanaka, and M. Koiwa, Mater. Trans. JIM 39, 24 (1998)]. A complete set of elastic constants has been determined with the resonance ultrasound spectroscopy technique. The disordered alloy exhibits common features of usual fcc metals, and the ordered alloy clearly shows a tetragonal elastic anisotropy, c11<c33 and c44<c66. The temperature dependence of the moduli has also been investigated using a quenched disordered sample. The anisotropy factor 2c44∕(c11−c12) is fairly large (about 3.0) at any temperatures, which may be related with the tweed pattern formation observed experimentally in the early stage of ordering.
Self-focused electron beams from pyroelectric LiNbO3 crystals and their pattern-replication capability
Authors: Kim Dong-Wook , Moon C. W. , Jeong Soo-Hwan , Yoo In K.
Published: November 2004

We investigated the characteristics of electrons emitted from LiNbO3 single crystals using a current detector and a 1:1 electron projection system. Exposure to a poor vacuum of 10−1Torr restored the screening charges to the crystal during discharge and enabled reproducible electron emission within 1min. We fabricated a patterned Si thin-film mask layer on a LiNbO3 crystal. As conducting thin films suppressed the pyroelectric electron emission from LiNbO3, we could transfer the mask pattern of an array of 10-μm dots to an electron-beam resist using the projection system. The electron beam from the crystal was highly self-focused, and the pattern replication needed neither an electric nor a magnetic field.
Effects of micro- and macro-plasma-sheath electric fields on carbon nanotube growth in a cross-field radio-frequency discharge
Authors: Hatakeyama R. , Jeong G.-H. , Kato T. , Hirata T.
Published: November 2004

Effects of magnetic-field introduction and micro∕macroelectric fields in a plasma sheath on the carbon nanotube growth are investigated by employing a cylindrical magnetron-type radio-frequency (rf) plasma-enhanced chemical-vapor deposition method. The cross-field magnetic-field application is accompanied by the high-density plasma generation and the reduction of direct impinge of high-energy ions to a rf electrode substrate, achieving the carbon nanotube formation without harmful sputtering phenomenon. It is found that microelectric fields in the plasma sheath are useful in the sense of substrate pretreatment and macroelectric fields have to be optimized in order to obtain well-aligned and refined nanotube structures in a large area. These experimental results lead to putting into practice of quite simple methods for the site-selected carbon nanotube growth in a relatively large area, so-called substrate-scratching method and mesh-masking method.
A physical model describing the electro-optic behavior of switchable optical elements based on electrowetting
Authors: Roques-Carmes Thibault , Hayes Robert A. , Schlangen Luc J. M.
Published: November 2004

Recently, we have demonstrated reflective display pixels based on the voltage-controlled two-dimensional movement of an oil/water interface across a hydrophobic fluoropolymer insulator [R. A. Hayes and B. J. Feenstra, Nature (London) 425, 383 (2003)]. Here a physical model is reported that describes the basic electro-optic behavior of these electrowetting-based optical elements. The model extends the classical electrowetting theory developed for a free water droplet on an insulator surface to the controlled two-dimensional movement of a confined oil/water interface. The model takes into account the spatial extent of the oil layer (<5mm), where the oil film is well approximated by a spherical cap. The calculated results are in very good agreement with experimental data without employing fitting parameters. The model can be used to predict and optimize the electro-optic behavior of devices as a function of a range of well-defined parameters including the oil film thickness, the thickness of the insulating layer, the size of the optical element, and the oil/water interfacial tension.
Improvement of coupling-out efficiency in organic electroluminescent devices by addition of a diffusive layer
Authors: Nakamura Toshitaka , Tsutsumi Naoto , Juni Noriyuki , Fujii Hironaka
Published: November 2004

We demonstrated that the diffusive layer laminated to glass substrate surface increased the light output of organic electroluminescent devices by extracting and reemitting light trapped in the substrate of such devices. Lamination of the diffusive layer improved the efficiency of the coupling-out factor, which was also changed by the thickness of the electron transporting layer (ETL). High total emitting flux for the sample with ETL thickness around quarter wavelength optical thickness (QWOT) was not improved significantly by lamination of the diffusive layer. Conversely, low total emitting flux for the sample with ETL thickness around 2QWOT without the diffusive layer was largely increased by the lamination of the diffusive layer, which was due to the extraction of waveguided light from the substrate mode. As the results, large dependence of total emitting flux on ETL thickness was significantly minimized by the lamination of diffusive layer. In addition, lamination of the diffusive layer also significantly reduced the variation of emission color and luminous intensity, which were strongly dependent on viewing angle and ETL thickness for the sample without the diffusive layer.