bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of n+ polycrystalline-Si/Al2O3/Si metal–oxide– semiconductor structures prepared by atomic layer chemical vapor deposition using Al(CH3)3 and H2O vapor
Authors: Park Dae-Gyu , Cho Heung-Jae , Lim Kwan-Yong , Lim Chan , Yeo In-Seok , Roh Jae-Sung , Park Jin Won
Published: July 2002

We report interface and dielectric reliability characteristics of n+ polycrystalline-silicon (poly-Si)/Al2O3/Si metal–oxide–semiconductor (MOS) capacitors. Al2O3 films were prepared by atomic layer chemical vapor deposition using Al(CH3)3 and H2O vapor. Interface state density (Dit) and dielectric reliability properties of n+ poly-Si/Al2O3/Si MOS structures were examined by capacitance–voltage, conductance, current–voltage, and time-dependent dielectric breakdown measurements. The Dit of the n+ poly-Si/Al2O3/Si MOS system near the Si midgap is approximately 8×1010 eV−1 cm−2 as determined by the conductance method. Frequency dispersion as small as ∼20 mV and hysteresis of ∼15 mV were attained under the electric field of ±8 MV/cm. The gate leakage current of ∼36 Å effective thickness Al2O3 dielectric measured at the gate voltage of −2.5 V is ∼−5 nA/cm2, which is approximately three orders of magnitude lower than that of a controlled oxide (SiO2). Time-dependent dielectric breakdown data of Al2O3/Si MOS capacitors under the constant current/voltage stress reveal excellent charge-to-breakdown characteristics over controlled oxide. Reliable gate oxide integrity of Al2O3 gate dielectric is manifested by the excellent distribution of gate oxide breakdown voltage on 128 million MOS capacitors having isolation edges. Extracted time constant and capture cross section of the Al2O3/Si junction are discussed.
Lattice contraction and cation ordering of ZrTiO4 in the normal-to-incommensurate phase transition
Authors: Kim Young K. , Jang Hyun M.
Published: July 2002

It is known that microwave dielectric ZrTiO4 undergoes a normal-to-incommensurate phase transition (NICPT) near 1125 °C. The role of the cation ordering and its relation to the anomalous lattice contraction observed in the NICPT were studied by x-ray diffraction and Raman scattering of ZrTiO4 specimens prepared employing various cooling rates between 1250 and 1000 °C. It was shown that the lattice parameter b underwent a substantial change even if the long-range cation ordering was absent. Analysis of the Raman spectra indicated that the compositional fluctuation-induced line broadening was mainly responsible for the extrapolated linewidth at 0 K, suggesting that the observed lattice contraction was initiated by the local short-range cation ordering. Based on thermodynamic arguments, the anomalous lattice contraction of ZrTiO4 was attributed to a direct consequence of the gradient coupling between the two order parameters that describe a compositional modulation and a displacive modulation in the NICPT.
Weak hyperfine interaction of E′ centers in gamma and beta irradiated silica
Authors: Agnello S. , Boscaino R. , Gelardi F. M. , Boizot B.
Published: July 2002

We report on the effects of photon (γ) and electron (β) irradiation in a dose range extending from 100 to 5×109 Gy in a variety of silica samples studied by electron paramagnetic resonance. The E′ centers and a weak intensity satellite signal of their resonance line were generated both in γ- and in β-irradiated samples. We investigated the dependence of their intensity on the irradiation dose. Evidence of the existence of a common generation mechanism for the related paramagnetic point defects is found. These defects are induced mainly through the conversion of precursors except at very high doses, where the direct activation from the unperturbed matrix is concurrent. Our data support the model attributing the satellite signal to the weak hyperfine structure of the E′ center arising from interaction with a second nearest neighbor nuclear spin.
Gain limitations of scaled InP/InGaAs heterojunction bipolar transistors
Authors: Rohner M. , Schnyder I. , Huber D. , Jäckel H. , Bergamaschi C.
Published: July 2002

We investigate a current gain degradation mechanism in self-aligned InP/InGaAs heterojunction bipolar transistors. We show that surface level pinning at the emitter sidewall gives rise to a peripheral emitter injection current into the base which increases the base current due to electrons recombining at the base contacts. The surface charge at the extrinsic base surface causes the formation of a conducting channel which further enhances the electron flow from the emitter to the base contacts. Two-dimensional numerical simulations of the emitter region combined with Monte-Carlo simulations of the injection process at the abrupt base-emitter heterojunction are in good agreement with the measurements. This effect will be especially severe for submicron emitter widths where the emitter perimeter to emitter area ratio is large.
Photorefractive InGaAs/GaAs multiple quantum wells in the Franz–Keldysh geometry
Authors: Iwamoto S. , Kageshima H. , Yuasa T. , Nishioka M. , Someya T. , Arakawa Y. , Fukutani K. , Shimura T. , Kuroda K.
Published: July 2002

We fabricate semi-insulating InGaAs/GaAs multiple quantum wells and observe the excitonic enhancement of the photorefractivity in the Franz–Keldysh geometry at wavelengths of 0.92–0.94 μm. A maximum two-wave mixing gain of 138 cm−1 and a maximum diffraction efficiency of 1.5×10−4 are obtained. The saturation intensity and the spatial resolution are also measured by four-wave mixing. The diffraction efficiency is saturated at a high external electric field. The dominant cause of this saturation is the deviation of the excitonic electroabsorption from its quadratic law.
Fractal structure of the magnetic field in the laminar zone of the Dynamic Ergodic Divertor of the Torus Experiment for Technology-Oriented Research (TEXTOR-94)
Authors: Abdullaev S. S. , Eich Th. , Finken K. H.
Published: July 2002

Magnetic field properties at the plasma edge in the Dynamic Ergodic Divertor (DED) for the Torus Experiment for Technology Oriented Research (TEXTOR-94) [Fusion Eng. Des. 37, 337 (1997)] have been studied. This is done by using a generalized symplectic mapping method of integration of field line equations. It is shown that by the radial shift of the resonant magnetic surfaces implemented by change of the plasma current (or the toroidal field) one can strongly vary the plasma edge regimes from the ergodic zone dominated one to the laminar zone dominated regimes in which the field lines with short wall to wall connection lengths are predominant. The embedded in the laminar zone narrow ergodic zones have fractal structures which are investigated in detail. The fractal structures of the magnetic footprints on the divertor plate are also studied.
Reactivity of Au with ultrathin Si layers: A photoemission study
Authors: Ivanco J. , Kobayashi H. , Almeida J. , Margaritondo G. , Pincik E.
Published: July 2002

We present a photoemission study on reactivity of the Au–Si system. We studied gold films evaporated atop ultrathin silicon layers previously deposited on GaAs. Following analysis of both the Si 2p core level and the Au 5d valence band spectra related response, we show that the reaction mechanism between Au and Si is affected by structural imperfections of the silicon layer. This is in sharp opposition to some current models of the reactivity mechanism.
Improved vapor–liquid equilibria predictions for Lennard-Jones chains from the statistical associating fluid dimer theory: Comparison with Monte Carlo simulations
Authors: Blas Felipe J. , Vega Lourdes F.
Published: July 2002

The statistical associating fluid theory (SAFT), with monomer and dimer Lennard-Jones (LJ) reference fluids, is used to predict the phase equilibria of pure chains with different lengths. Predictions from the two versions of the theory are compared with Monte Carlo simulation results taken from the literature. We find that the additional structural information from the dimer version of the theory gives predictions in better agreement with simulation values. It is also found that the dimer version provides a much better description of the vapor pressure than the monomer one for long chains for which simulation data are available.
The extended surface forces apparatus. II. Precision temperature control
Authors: Heuberger M. , Vanicek J. , Zäch M.
Published: July 2002

We describe the design and performance of a low-cost, bidirectional temperature control system with very high stability and low thermal gradients in a working range from −10 to 70 °C. System elements such as thermal insulation, thermal pumps, and feedback control as well as safety features are discussed. The performance of the entire system under realistic experimental conditions is documented. Precise temperature control has led to significantly improved instrumental stability. An extension of the instrumental window towards slower measurements has been made possible. This temperature-control system is presented in the framework of the extended surface forces apparatus but is also applicable to other scientific instruments that require subnanometer positioning stability over extended periods of time.
From 1/f noise to multifractal cascades in heartbeat dynamics
Authors: Ivanov Plamen Ch. , Nunes Amaral Luı́s A. , Goldberger Ary L. , Havlin Shlomo , Rosenblum Michael G. , Stanley H. Eugene , Struzik Zbigniew R.
Published: July 2002

We explore the degree to which concepts developed in statistical physics can be usefully applied to physiological signals. We illustrate the problems related to physiologic signal analysis with representative examples of human heartbeat dynamics under healthy and pathologic conditions. We first review recent progress based on two analysis methods, power spectrum and detrended fluctuation analysis, used to quantify long-range power-law correlations in noisy heartbeat fluctuations. The finding of power-law correlations indicates presence of scale-invariant, fractal structures in the human heartbeat. These fractal structures are represented by self-affine cascades of beat-to-beat fluctuations revealed by wavelet decomposition at different time scales. We then describe very recent work that quantifies multifractal features in these cascades, and the discovery that the multifractal structure of healthy dynamics is lost with congestive heart failure. The analytic tools we discuss may be used on a wide range of physiologic signals.
Estimate of the cutoff errors in the Ewald summation for dipolar systems
Authors: Wang Zuowei , Holm Christian
Published: July 2002

Theoretical estimates for the cutoff errors in the Ewald summation method for dipolar systems are derived. Absolute errors in the total energy, forces and torques, both for the real and reciprocal space parts, are considered. The applicability of the estimates is tested and confirmed in several numerical examples. We demonstrate that these estimates can be used easily in determining the optimal parameters of the dipolar Ewald summation in the sense that they minimize the computation time for a predefined, user set, accuracy.
Magnetic properties of self-assembled nanoscale La2/3Ca1/3MnO3 particles in an alumina matrix
Authors: Katiyar Pallavi , Kumar D. , Nath T. K. , Kvit Alex V. , Narayan J. , Chattopadhyay Soma , Gilmore Walter M. , Coleman Steve , Lee Clinton B. , Sankar J. , Singh Rajiv K.
Published: July 2002

We have investigated the processing and properties of La2/3Ca1/3MnO3 self-assembled nanodots formed in a nonmagnetic alumina matrix, which were produced by a pulsed-laser deposition process. The size of the nanodots was found to be in the range of 10–15 nm using high-resolution transmission electron microcopy. The average interlayer separation between two dots has been found to be 2–5 nm, which is sufficient to decouple the magnetic grains. The decoupling of the grains is supported by the zero-field-cooled and field-cooled magnetization (M) data. The coercivity of the La2/3Ca1/3MnO3 nanodots has been measured using magnetization measurements as a function of field (H) at different temperatures above and below the blocking temperature of the samples. The coercivity is found to vary from 600 Oe at 10 K to 400 and 200 Oe at 20 and 50 K, respectively. Above the blocking temperature, the sample is found to transform to a superparamagnetic magnetic state, resulting in the disappearance of any hysteresis in the M–H loops.
Collapse and rebound of a laser-induced cavitation bubble
Authors: Akhatov I. , Lindau O. , Topolnikov A. , Mettin R. , Vakhitova N. , Lauterborn W.
Published: July 2002

A strong laser pulse that is focused into a liquid produces a vapor cavity, which first expands and then collapses with subsequent rebounds. In this paper a mathematical model of the spherically symmetric motion of a laser-induced bubble is proposed. It describes gas and liquid dynamics including compressibility, heat, and mass transfer effects and nonequilibrium processes of evaporation and condensation on the bubble wall. It accounts also for the occurrence of supercritical conditions at collapse. Numerical investigations of the collapse and first rebound have been carried out for different bubble sizes. The results show a fairly good agreement with experimental measurements of the bubble radius evolution and the intensity of the outgoing shock wave emitted at collapse. Calculations with a small amount of noncondensable gas inside the bubble show its strong influence on the dynamics.
A critical comparison of equilibrium, non-equilibrium and boundary-driven molecular dynamics techniques for studying transport in microporous materials
Authors: Arya Gaurav , Chang Hsueh-Chia , Maginn Edward J.
Published: July 2002

Transport in an idealized model with variable pore diameter as well as an AlPO4-5 zeolite is examined using three different molecular dynamics techniques: (1) equilibrium molecular dynamics (EMD); (2) external field nonequilibrium molecular dynamics (EF–NEMD); and (3) dual control volume grand canonical molecular dynamics (DCV–GCMD). The EMD and EF–NEMD methods yield identical transport coefficients for all the systems studied. The transport coefficients calculated using the DCV–GCMD method, however, tend to be lower than those obtained from the EMD and EF–NEMD methods unless a large ratio of stochastic to dynamic moves is used for each control volume, and a streaming velocity is added to all inserted molecules. Through development and application of a combined reaction–diffusion–convection model, this discrepancy is shown to be due to spurious mass and momentum transfers caused by the control volume equilibration procedure. This shortcoming can be remedied with a proper choice of streaming velocity in conjunction with a well-maintained external field, but the associated overhead makes it much less efficient than either the EMD or EF–NEMD techniques.
Magnetic nanodot arrays produced by direct laser interference lithography
Authors: Zheng M. , Yu M. , Liu Y. , Skomski R. , Liou S. H. , Sellmyer D. J. , Petryakov V. N. , Verevkin Yu. K. , Polushkin N. I. , Salashchenko N. N.
Published: July 2002

Periodic magnetic nanodot arrays have been produced on an area as large as 1 cm×1 cm by direct nanolithography using interferometric laser radiation. The dots are formed by the local annealing of sputtered amorphous Co–C films in regions where the laser intensity is highest. At room temperature the dots exhibit ferromagnetic order and are embedded in a paramagnetic matrix. The onset of room-temperature ferromagnetism is caused by nanoscale chemical and morphological changes during dot formation and reflects the phase separation of magnetic Co-rich clusters. The present single-step nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays.
Polarization consistent basis sets: Principles
Authors: Jensen Frank
Published: July 2002

The basis set convergence of Hartree–Fock energies for the H2, H3+, C2, N2, N4, O2, O3, F2, HF, and CH4 molecules is analyzed using optimized basis functions. Based on these analysis a sequence of polarization consistent basis sets are proposed which should be suitable for systematically improving Hartree–Fock and density functional energies. Analogous to the correlation consistent basis sets designed for correlation energies, higher angular momentum functions are included based on their energetical importance. In contrast to the correlation consistent basis sets, however, the importance of higher angular momentum functions decreases approximately geometric, rather than arithmetic. It is shown that it is possible to design a systematic sequence of basis sets for which results converge monotonic to the Hartree–Fock limit. The primitive basis sets can be contracted by a general contraction scheme. It is found that polarization consistent basis sets provide a faster convergence than the correlation consistent basis sets. Results obtained with polarization consistent basis sets can be further improved by extrapolation.
Numerical study of the effective surface area of obliquely deposited thin films
Authors: Suzuki Motofumi , Taga Yasunori
Published: July 2002

We have developed a three-dimensional Monte Carlo simulator for thin film growth and applied it to understand the morphologies of thin films prepared by dynamic oblique deposition with a special emphasis on an effective surface area. Columnar shapes zigzag, helix, and cylinder are successfully reproduced, and the packing density of simulated thin films agrees quantitatively with that obtained experimentally. The effective surface area is enhanced by oblique deposition owing to columnar formation and becomes maximum at a deposition angle of 70 °. On the contrary, no significant dependence of the effective surface area on the columnar shapes has been observed. Consequently, morphology-related quantities of obliquely deposited thin films such as the packing density and the effective surface area are understood in terms of the deposition angle and are insensitive to the motion of the azimuthal deposition direction.
On the interface states generated under different stress conditions
Authors: Zhang W. D. , Zhang J. F. , Uren M. J. , Groeseneken G. , Degraeve R. , Lalor M. , Burton D.
Published: July 2002

After dry oxidation and when annealing in a hydrogen environment is carefully avoided, the energy distribution of interface states has two peaks, one in the lower and one in the upper half of the silicon band gap. Following hydrogen passivation, however, the interface states created by irradiation or electron injection typically have only one peak in the upper half of the gap. This letter investigates how to generate a double peak distribution by electrical stress and we attempt to link the distribution with the generation mechanism.
Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. II. Generalizations based on mass-weighting, idempotency, energy conservation and choice of initial conditions
Authors: Iyengar Srinivasan S. , Schlegel H. Bernhard , Millam John M. , A. Voth Gregory , Scuseria Gustavo E. , Frisch Michael J.
Published: July 2002

A generalization is presented here for a newly developed approach to ab initio molecular dynamics, where the density matrix is propagated with Gaussian orbitals. Including a tensorial fictitious mass facilitates the use of larger time steps for the dynamics process. A rigorous analysis of energy conservation is presented and used to control the deviation of the fictitious dynamics trajectory from the corresponding Born–Oppenheimer dynamics trajectory. These generalizations are tested for the case of the Cl−(H2O)25 cluster. It is found that, even with hydrogen atoms present in the system, no thermostats are necessary to control the exchange of energy between the nuclear and the fictitious electronic degrees of freedom.
Surface-mediated light-controlled Friedericksz transition in a nematic liquid crystal cell
Authors: Boichuk Valeriy , Kucheev Sergey , Parka Janusz , Reshetnyak Victor , Reznikov Yuriy , Shiyanovskaya Irina , Singer Kenneth D. , Slussarenko Sergey
Published: July 2002

Surface-mediated director reorientation in a pure nematic liquid crystal (LC) cell in the presence of both a dc and low-power optical field has been observed. A surface-dependent lowering of the Friedericksz transition voltage in a planar cell upon light irradiation was found. It is believed that this effect is due to a light-induced change in the near-surface ion concentration in the presence of a dc field and a photosensitive surface. Enrichment in the ion concentration near the surface causes a redistribution of the electric field in the cell and its localization near the surface. As a consequence, the energy of interaction between the LC and the dc field near the surface overcomes the anchoring energy and results in the director reorientation.
The final stage of the collapse of a cavitation bubble close to a rigid boundary
Authors: Brujan E. A. , Keen G. S. , Vogel A. , Blake J. R.
Published: July 2002

The final stage of the collapse of a laser-produced cavitation bubble close to a rigid boundary is studied both experimentally and theoretically. The temporal evolution of the liquid jet developed during bubble collapse, shock wave emission and the behavior of the “splash” effect are investigated by using high-speed photography with up to 5 million frames/second. For a full understanding of the bubble–boundary interaction, numerical simulations are conducted by using a boundary integral method with an incompressible liquid impact model. The results of the numerical calculations provided the pressure contours and the velocity vectors in the liquid surrounding the bubble as well as the bubble profiles. The comparisons between experimental and numerical data are favorable with regard to both bubble shape history and translational motion of the bubble. The results are discussed with respect to the mechanism of cavitation erosion.
Simulation of charge injection enhancements in organic light-emitting diodes
Authors: Masenelli B. , Berner D. , Bussac M. N. , Nüesch F. , Zuppiroli L.
Published: July 2002

We have investigated by numerical simulation of real devices the reasons for electron injection enhancement due to lithium fluoride (LiF) and for hole injection enhancement due to copper phtalocyanine (CuPc) in organic light-emitting diodes (OLEDs). The reference data introduced in the code were obtained from Kelvin probe and charge transport measurements. In the case of LiF, the reduction of the injection barrier is mainly due to a static dipolar charge distribution across the ionic layer, while in CuPc the space charge which lowers the barrier results from a large hole accumulation at the CuPc/hole-transmitting layer interface, during injection.
Thermal decomposition of ZrO2/SiO2 bilayer on Si(001) caused by void nucleation and its lateral growth
Authors: Watanabe Heiji , Ikarashi Nobuyuki
Published: July 2002

The stability of a ZrO2/SiO2 bilayer on a Si(001) substrate was investigated in terms of thermal decomposition during ultrahigh-vacuum annealing. In spite of the intrinsic thermal stability of the ZrO2/SiO2 system, void nucleation initiated by local defects and the following lateral growth of the voids proceed at temperatures over 900 °C. The remaining Zr atoms accumulate and react with the Si substrate to form silicide (ZrSi2) islands within the voids. It was found that the decomposition temperature of the ZrO2/SiO2 bilayer is lower than that of a SiO2 single layer, which suggests defect generation in the SiO2 underlayer by ZrO2 deposition.
Domain polarity and temperature induced potential inversion on the BaTiO3(100) surface
Authors: Kalinin Sergei V. , Johnson C. Y. , Bonnell Dawn A.
Published: July 2002

Variable temperature scanning surface potential microscopy is used to determine thermodynamic and kinetic parameters associated with polarization screening on BaTiO3(100) surfaces. The temperature dependence of the surface potential is indicative of the interplay between the fast dynamics of atomic polarization and slower dynamics of screening charge. The screening charge relaxation kinetics are found to be weakly dependent on temperature with activation energy Ea∼4 kJ/mole. Equilibrium domain potential difference depends linearly on temperature; the zero potential contrast is observed at ∼110 °C. At room temperature the sign of domain potential is determined by the screening charges rather than polarization charge. A thermodynamic model for screening of ferroelectric surfaces based on Ginzburg–Devonshire theory is developed so that the enthalpy and entropy of charge compensation can be derived from the temperature dependence of surface potential contrast. In the case of BaTiO3 in air, the charge compensation mechanism is surface adsorption.
Magnetoelectronic characteristics of a GMR transpinnor and a magnetic random access memory using a closed-flux NiFe/Cu/CoFe/Cu/NiFe pseudo spin-valve
Authors: Bae Seongtae , Judy Jack H. , Chen P. J. , Egelhoff W. F. , Zurn Shayne , Sheppard Larry , Torok Edward J.
Published: July 2002

The magnetoelectronic device characteristics of a GMR “transpinnor,” a new multifunctional solid-state device, and a magnetic random access memory (MRAM) have been investigated using the structure of closed-flux NiFe/Cu/CoFe/Cu/NiFe pseudo spin-valve (PSV) for a new current driver-one MRAM cell architecture on the same substrate. The electrical and magnetic device characteristics of a PSV-MRAM cell and single or dual-input “transpinnors” are a GMR ratio of 3%–6% and a sheet resistivity of 4–5Ω/□. The switching characteristics of PSV MRAM cells were measured using patterned Si/Si3N4/PSV with a NiFe(30 nm)/Cu(90 nm) word or input line. The writing field was set at ±40 Oe and a 1–3 mV output signal was obtained when a 10.2 Oe sensing field was induced by a word line with a current density of 2.7×106 A/cm2. The output characteristics of the GMR “transpinnor” were measured using a PSV fabricated on the same wafer with the MRAM cell. Similar to standard transistor characteristics, the transpinnor exhibits a very high output signal between 2 and 3 mA (output load R=1 Ω) at a low 8 Oe switching field, which should be potentially useful as a current driver for activating PVS-MRAM circuits and for realizing various new logic gates such as OR, NAND, and other multivalue logic states.
Characteristic correlations between the Δm* interaction curves and the Preisach diagram in particulate media
Authors: Bottoni G. , Candolfo D. , Cecchetti A.
Published: July 2002

Magnetic interparticle interactions are analyzed by Δm* curves calculated from the initial magnetization curve and the hysteresis loop, instead of the usual deviation Δm based on remanence curves. This study is made on tapes of acicular CrO2 particles and on tapes of CoTi-doped Ba ferrite platelet-shaped particles. The standard Preisach map of the samples is experimentally constructed and the Δm* values, computed from this map, and the values agree fairly well with the Δm* curves measured for the ac demagnetized state and various dc demagnetized states. We describe a procedure for remagnetizing the dc demagnetized samples. For the acicular particles, the peak of the remagnetized curves is lower than that computed from the Preisach map. The opposite occurs for the platelet-shaped particles. This different behavior is ascribed to the different influence of the statistical interaction field and the mean interaction field.
Structure and magnetic properties of RFe6−xGa6+x (R=rare earth)
Authors: Guo Yongquan , Ali Naushad , Sardela Mauro R.
Published: July 2002

The crystal structures and magnetic properties of RFe6−xGa6+x (R=Ce, Pr, Nd, Sm, and Gd) alloys have been investigated by x-ray powder diffraction and magnetic measurement. A ternary intermetallic compound with an orthorhombic ScFe6Ga6 type structure is found in these alloys. The lattice parameter contraction is observed when the higher atomic number lanthanides are substituted into the intermetallic. In each unit cell, there are six nonequivalent crystal positions, i.e., 2a, 4e, 4f, 4g, 4h, and 8k, which are occupied by 2R, 4GaI, 4(FeI,Ga), 4GaII, 4GaIII, and 8FeII, respectively. The RFe6−xGa6+x alloys have ferromagnetic ordering but the magnetic transitions have been found in Nd based alloy. The RFe6−xGa6+x alloys show a soft magnetic behavior but the magnetization hysteresis loops have been observed in the Nd and Sm alloys at 5 K. The saturated magnetic moments per molecule tend to increase from Ce to Nd. However, a drastic decrease of magnetic moment in GdFe6Ga6 alloy is observed and is likely due to the Gd spin alignment being antiparallel to Fe spin. The saturation magnetic moments of these alloys are calculated and agree with the experimental values.
Ferromagnetic nano-dot array fabricated by electron beam radiation induced nano-scale phase transition
Authors: Zhou T. J. , Zhao Y. , Wang J. P. , Thong J. T. L. , Chong T. C.
Published: July 2002

We present a method of direct magnetic patterning of a nonmagnetic Co–C amorphous film by electron beam radiation induced nano-scale phase transition. Co–C alloy films with C concentration from 30 to 50 at % and thickness of 30–60 nm were prepared by alternately sputtering Co and C films onto C-buffered glass substrates. The films are amorphous and nonmagnetic with C concentration up to 40 at %. Due to their negative mixture entropy, as-deposited amorphous Co–C alloy films are metastable. Focused electron-beam irradiation causes localized phase segregation of the immiscible magnetic (Co-rich) and nonmagnetic (C-rich) phases. Ferromagnetic Co(C) nano-dot array was fabricated by subjecting the as-deposited Co60C40 films to electron beam radiation using a beam current of 16 nA, a beam energy of 20 keV and a dwell time of 5 s per dot. Magnetic force microscopy images and magnetic measurements confirm the formation of the ferromagnetic phase. The present single-step nanolithography eliminates the cumbersome traditional processes and is potentially a new and flexible alternative for fabricating patterned magnetic nanostructures for submicron magnetic devices.
Unconventional critical behavior of magnetic susceptibility as a consequence of phase separation and cluster formation in La0.7Ca0.3MnO3 thin films
Authors: Huhtinen H. , Laiho R. , Lähderanta E. , Salminen J. , Lisunov K. G. , Zakhvalinskii V. S.
Published: July 2002

Nonuniversal critical behavior of the dc magnetic susceptibility χ−1(T)−χ−1(TC)∼(T/TC−1)γ≡τγ is observed near the paramagnetic (PM) to ferromagnetic (FM) transition in La0.7Ca0.3MnO3 film samples. The value of γ≈1.4 obtained for 0<τ<τ1 with τ1≈0.05–0.09 corresponds to a three-dimensional Heisenberg spin system and that of γ≈2.4 for τ1<τ<τ2 with τ2≈0.2–0.3 to a two-dimensional percolation system. This behavior is attributed to phase separation by presence of FM clusters with temperature dependent correlation length in the PM matrix of the film.
Simulation of the charge transport across grain boundaries in p-type SrTiO3 ceramics under dc load: Debye relaxation and dc bias dependence of long-term conductivity
Authors: Hölbling Th. , Waser R.
Published: July 2002

A mathematical-physical model to describe the charge transport across grain boundaries in p-type SrTiO3 ceramics in the low-temperature regime for arbitrary dc voltage steps has been developed. The finite element model structure consists of a one-dimensional cross section through a ceramic scenario. Mathematical formulation comprises a coupled system of continuity equations (utilizing Maxwell-Boltzmann transport equations) and Poisson’s equation, with the appropriate boundary conditions for a potentiostatic simulation approach. The edges of the model are assumed to be blocking for ionic transport, and penetrable for electronic transport. The model was implemented exploiting routines from the numerical class library DIFFPACK™. After an initial electrostatic simulation a dc bias voltage step is applied. The evolution of the spatial profiles of electric potential, defect concentrations, space-charge density, and electric conductivity, and the current response are calculated. The results for the ceramic model structure confirm the experimentally observed Debye relaxation, and the characteristic dependence of long-term conductivity on the dc bias after space-charge polarization, before the onset of resistance degradation.
Effects of groove depth and patterned permalloy film on magnetization switching of LPE-garnet pixels for use in magneto-optic spatial light modulators
Authors: Park J. H. , Nishmura K. , Inoue M. , Lee D. H. , Cho J. K.
Published: July 2002

Effects of pixel groove depth and patterned permalloy assist film for the magnetization switching of LPE-garnet film were studied experimentally. The results showed that the magnetization field to switch a structured pixel could be reduced by controlling the pixel groove depth. It was also found that the permalloy assist film creates a unique pair-domain pattern within a single garnet pixel, which is useful for realizing a new type of magneto-optic spatial light modulator with high resistivity to noise.
Structure and magnetic properties of sputtered (Nd,Dy)(Fe,Co,Nb,B)5.5/M (M=FeCo,Co) multilayer magnets
Authors: Liu W. , Zhang Z. D. , Liu J. P. , Li X. Z. , Sun X. K. , Sellmyer D. J.
Published: July 2002

The magnetic properties of nanocomposite multilayer magnets of the (Nd,Dy)(Fe,Co,Nb,B)5.5/M (M=Co,FeCo) on Ti-buffered Si substrates have been investigated. X-ray diffraction results reveal that the Nd2Fe14B-type phase in almost all the films is randomly oriented. Different thickness x (nm) for Co layers and y (nm) for FeCo layers were adopted in the multilayers. It is found that high remanence is achieved in the nanocomposite multilayer films consisting of the Nd2Fe14B-type phase and soft magnetic phase for Co with 6⩾x⩾4 and for FeCo with 10⩾y⩾6. The enhancement of the remanence in the nanocomposite multilayer films is attributed to the exchange coupling between the magnetically soft and hard phases. Increasing the soft magnetic components results in a continuously decreasing coercivity.
Magnetomechanical damping by polycrystalline TbDy
Authors: Good N. , Dooley J. , Fultz B.
Published: July 2002

Vibration damping in polycrystalline TbDy alloys was studied at cryogenic temperatures. Mechanical hysteretic losses were measured at various strains, frequencies, and loading configurations at 77 K. Some textured TbDy materials demonstrated 22.6% energy dissipation in mechanical measurements at low frequency (0.01 Hz) and a mean logarithmic decrement of 0.23 at a higher frequency (25 kHz). Ultrasonic velocities of longitudinal and shear elastic waves were measured on single and polycrystalline TbDy; little variation in ultrasonic velocities was found even for samples with large variation in crystallographic texture and magnetomechanical properties.
Ordered Fe–Pt(001) thin films by two temperature step depositions for recording media
Authors: Suzuki Toshio , Ouchi Kazuhiro
Published: July 2002

Two-step temperature depositions method has been proposed to reduce the medium noise of ordered Fe–Pt perpendicular double-layered media. Preforming a disordered Fe–Pt thin layer at a temperature of 50 °C before deposition of an ordered Fe–Pt layer at 375 °C leads to reducing a lateral exchange coupling of the total Fe–Pt layer and the domain sizes. Cross sectional transmission electron microscopy observations revealed that the postdeposited ordered Fe–Pt layer was homoepitaxially grown on the preformed Fe–Pt layer and structural defect density in the total Fe–Pt layer was increased. The medium exhibited 4 dB higher signal-to-noise ratio for the output at 500 kfrpi (frequency band width: 0.5–50 MHz), than that of a conventional Fe–Pt double-layered medium.
Enhanced performance of CoFe/AlOx/CoFe magnetic tunnel junctions prepared by an off-axis rf remote plasma oxidation method
Authors: Yoon K. S. , Park J. H. , Yang J. Y. , Kim C. O. , Hong J. P.
Published: July 2002

A new off-axis rf remote oxidation technique for the insulating barrier was carried out to enhance performance of CoFe/AlOx/CoFe magnetic tunneling junctions. The rf remote plasma method was designed to reduce self-bias voltage effect on the barrier during the rf oxidation process and to increase atomic oxygen concentration at high power operation, compared with a conventional rf plasma oxidation method. In addition, the off-axis geometry was chosen to give rise to high uniformity of the insulating layer. Experimentally observed root mean square of the barrier was decreased from 5 Å to 1.5 Å in our method. Electrical breakdown voltage and magnetoresistance of our MTJ devices increased from 0.8 V up to 1.4 V and from 20.2% up to 33.6%, respectively, correlated with the improved structural information of the barrier.
Characterization of domain states in submicron sized permalloy particles with perpendicular anisotropy
Authors: Eames Peter , Dahlberg E. Dan
Published: July 2002

Magnetic force microscopy (MFM) with in situ magnetic fields was used to classify and determine the distributions of magnetic domain structures in 190-nm-thick Ni81Fe19 particles with diameters from 850 to 250 nm. A deposition induced perpendicular anisotropy energy resulted in a simple stripe domain pattern observed in MFM images on unpatterned witness films. The domain structures in the particles are not stripes but consist of a small number of distinct magnetization states for each diameter of particle. Repeated in-plane magnetic field saturations were applied to effectively anneal the magnetic states of an array of same diameter particles. For some of the states Boltzmann statistics were used to calculate an “energy ratio” of the particles magnetization states based on the observed distribution for each diameter.
Neutron diffraction analysis of melt spun 2:14:1 type (NdPr)–Fe–B compounds with Ti and Zr additions
Authors: Chu Z. , Yelon W. B. , Ma B. M. , Chen Z. , Brown D. N.
Published: July 2002

There has been continued technological and research interest in Nd2Fe14B-based compounds due to their excellent intrinsic magnetic properties [(BH)max*∼64 MGOe,Ha∼73 kOe]. It is found that Ti and Zr substitution in Nd2Fe14B leaves its magnetic properties largely unaffected. We have carried out neutron diffraction studies on Zr and Ti substituted 2:14:1 compounds. Three specimens of melt spun alloys of compositions (Nd0.75Pr0.25)12Fe80B8, (Nd0.75Pr0.25)8.4Fe79.7Ti4.7B7.2 and (Nd0.75Pr0.25)9.2Fe79.2Ti2Zr2.2B7.4 were studied. Rietveld refinements reveal that the 4f site is equally occupied by Nd and Pr while the 4g site is occupied only by Nd. In the Ti-containing sample, the lattice expands and Ti is found on the j1, j2 sites normally occupied by Fe, (4% and 8%, respectively). This accounts for about 30% of the added Ti. In the third sample, the lattice contracts, which could occur only if Zr replaces the larger rare-earth atoms. However, due to the possibility of four different species occupying the 4f site (i.e., Nd, Pr, Ti, and Zr), it is not possible to determine their relative concentrations from neutron diffraction data alone. The balance of Ti and Zr in this sample must be locked in the grain boundaries. Despite peak broadening associated with the fine grain structure, robust refinements were achieved.
Magnetoelastic effects in epitaxial Co80Pt20 films
Authors: Xu B. , Doyle W. D. , Klemmer T. J.
Published: July 2002

The lattice strain in sputtered epitaxial Co80Pt20 films has been investigated using an in-plane x-ray diffraction method. For films with a Cr underlayer, Co80Pt20 is compressed in the film plane, while for films with a W underlayer, the Co80Pt20 is stretched in the film plane. The measured strain magnitude is much smaller than the calculated mismatch between the Co80Pt20 and the underlayer even at a thickness of 1 nm. The uniaxial anisotropy constants were found to have little dependence on the strains, which suggests very small magnetoelastic coupling constants for Co80Pt20.
Oxidation of iron in iron/gold core/shell nanoparticles
Authors: Ravel B. , Carpenter E. E. , Harris V. G.
Published: July 2002

Recently, reverse micelles were used to produce core/shell nanoparticles of iron and gold of highly uniform size distribution and reported to contain metallic iron. A composite structure of this sort is attractive in biosensor and bioactuator applications; the metallic iron core provides high magnetic sensitivity and the gold coating is easily functionalized. With iron/gold nanoparticles prepared by this method, we directly measure the chemical state of the iron component using x-ray absorption spectroscopy (XAS) and interpret our results along with the measured magnetization of the particles. In earlier reports, metallicity of the iron core was inferred from magnetization measurements rather than directly measured. In contrast, XAS directly measures the structural environment of the iron atoms. Because XAS interpretation is independent of symmetry or periodicity, it is an ideal tool for characterizing nanoparticles. Using XAS, we demonstrate that the iron component of the iron/gold nanoparticles is fully oxidized. We suggest that oxidation is a persistent problem in core/shell nanoparticles of this variety and recommend that characterization by XAS be a standard part of nanoparticle preparation.
Analysis of interactions for magnetic particles assembling on magnetic templates
Authors: Yellen B. , Friedman G. , Feinerman A.
Published: July 2002

Dipole approximations are used to model up to three superparamagnetic particle systems as they assemble on magnetic island templates. It is shown that small variations in the ratio of particle-to-island diameter as well as the particle positions above the island can change magnetic forces from attractive to repulsive. This observation could potentially permit a template to be designed to promote single particle coverage of individual magnetic islands.
Electron traps in Ga(As,N) layers grown by molecular-beam epitaxy
Authors: Krispin P. , Spruytte S. G. , Harris J. S. , Ploog K. H.
Published: July 2002

Deep levels in the upper half of the band gap of strained Ga(As,N) with a GaN mole fraction of 3% are examined by deep-level transient Fourier spectroscopy on GaAs/Ga(As,N)/GaAs heterojunctions grown by molecular-beam epitaxy. In as-grown structures, we find a dominant electron trap at 0.25 eV below the conduction bandedge with a concentration above 1017 cm−3. Its capture cross section of about 10−17 cm2 for electrons is too small for an efficient nonradiative recombination center in Ga(As,N). According to theoretical predictions, this level is most likely connected with a nitrogen-split interstitial defect (N–N)As. The giant concentration of this trap can be strongly reduced by rapid thermal annealing.
Magnetization process of a single chain of nanomagnets
Authors: Adeyeye A. O. , Welland M. E.
Published: July 2002

We have investigated the magnetization reversal process of a single chain of identical nanomagnetic dots fabricated from 30 nm thick Ni80Fe20. The structures consist of two 5 μm wide support wires bridged with a single chain of identical dots of diameter δ in the range 100–250 nm. For fields applied perpendicular to the single chain, we observed an unusual size dependent hysteretic behavior in the magnetoresistance curve at high field. This is due to the magnetostatic interaction arising from the proximity of the magnetic charges. We are able to deduce from a simple micromagnetic simulation that the reversal process in the chain of dots with δ=100 nm is mediated by a collective process of nearly coherent spin rotation. The magnetotransport measurements along the chain reveal a complex magnetization reversal process in the chain of nanomagnets.
Surface structure of aqueous 2-butoxyethanol mixtures studied by x-ray reflection
Authors: Yano Yohko F.
Published: July 2002

The surface structure of 2-butoxyethanol (BE)–water mixtures at 300 K was studied by x-ray reflection and surface tension measurements. It is striking that a finite intrinsic surface width attributed to the surface excess BE was observed only below BE mole fraction xBE=0.017 where the pseudophase transition occurs in the bulk. The monolayer formation at xBE≈0.005 was found from the result of a curve fitting to the measured reflectivities by using the density profile of a single-layer “box” model in which interfaces were smeared by thermal capillary waves.
Nonequilibrium photocurrent modeling in resonant tunneling photodetectors
Authors: Henrickson Lindor E.
Published: July 2002

An efficient and versatile many-body nonequilibrium approach is formulated for computation of photocurrent and photoexcited properties of device structures where quantum effects dominate. This method, based on nonequilibrium Green’s function quantum transport equations, makes it possible to consider open systems of arbitrary dimensionality having complex potentials, complex geometries, and multiple terminals. In contrast to other approximate computational approaches, no a priori assumptions regarding the particular nature of the phototransitions are required (i.e., bound-to-bound, bound-to-continuum, or continuum-to-continuum). Furthermore, if desired, electron–phonon and electron–electron interactions can also be rigorously accounted for within the same formalism. In this article, the method is applied to two typical resonant-tunneling infrared detector heterostructures as examples: (1) a single-quantum-well structure, and (2) a multiperiod superlattice structure.
Electric field-induced second harmonic generation studies of chromophore orientational dynamics in photorefractive polymers
Authors: Ostroverkhova Oksana , Stickrath Andrew , Singer Kenneth D.
Published: July 2002

Photorefractive (PR) polymers are promising for use in various applications that require fast response times. The main factors that determine the PR speed in polymers are photoconductivity and chromophore reorientation in an electric field. In this article, we investigate the chromophore reorientational dynamics in various PR composites using the electric field second harmonic generation (EFISHG) technique, and then relate it to the PR dynamics observed in a four-wave mixing holographic experiment. We also report on the enhancement of the EFISHG signal in the presence of HeNe light, which we attribute to a photoinduced internal electric field that formed in the PR polymer.
Excellent thermal stability of Al2O3/ZrO2/Al2O3 stack structure for metal–oxide–semiconductor gate dielectrics application
Authors: Chang Hyo Sik , Jeon Sanghun , Hwang Hyunsang , Moon Dae Won
Published: July 2002

The thermal stability of a nanolaminate (Al2O3/ZrO2/Al2O3) gate stack prepared by atomic layer chemical vapor deposition was investigated using medium-energy ion scattering spectroscopy, and x-ray photoelectron spectroscopy. We observed that the structure was stable up to 1000 °C under ultrahigh vacuum conditions. However, annealing in a nitrogen or oxygen ambient at 1 atm yielded the formation of an interfacial Zr–Al silicate layer at much lower temperatures. The growth of the interfacial silicate layer could be significantly reduced during furnace annealing via the use of plasma nitridation.
In-doped SrTiO3 ceramic thin films
Authors: Dai Shouyu , Lu Huibin , Chen Fan , Chen Zhenghao , Ren Z. Y. , Ng D. H. L.
Published: July 2002

We report the characterization of the ceramic SrIn0.1Ti0.9O3 thin film grown by laser molecular-beam epitaxy. The lattice constant is determined to be 0.3948 nm, slightly larger than that of the SrTiO3 substrate. Hall measurement confirms that this film is a p-type semiconductor either below 92 K or above 158 K. X-ray photoemission spectroscopy study shows that the width of the valence band of the p-type SrIn0.1Ti0.9O3 film is narrower than that of the n-type SrNb0.1Ti0.9O3 film. There is a 0.35 eV difference in the Fermi energy level of the two films. The electronic state of the surface layer of the SrIn0.1Ti0.9O3 film is found to be different from that of its interior.