bluuebunny/crossref_metadata_2025 huggingface dataset:
Epitaxial growth of wurtzite GaN on Si(111) by a vacuum reactive evaporation
Authors: Zhang Haoxiang , Ye Zhizhen , Zhao Binghui
Published: July 2002

A single crystalline GaN film on Si(111) with a GaN buffer layer is grown by a simple reactive evaporation method. Scanning electron microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence measurement (PL), and Hall measurement results indicate that the single crystalline wurtzite GaN was successfully grown on the microcrystalline GaN buffer layers on Si(111) substrate. The surface of the GaN films is mirror-like and crack-free. A pronounced GaN (0002) peak appears in the XRD pattern. The full width at half maximum (FWHM) of the double-crystal x-ray rocking curve for (0002) diffraction from the GaN epilayer is 30 arcmin. The TEM reveals that a 10 nm GaN buffer layer in the microcrystalline state exists between the Si substrate and the epilayer, which dissipates most of the stress energy. The PL spectrum shows that the GaN epilayer emits light at the wavelength of 365 nm with a FWHM of 8 nm (74.6 meV). The unintentionally doped films were n type with a carrier concentration of 1.76×1018/cm3 and an electron mobility of 142 cm3/V s. The growth technique described is simple but very powerful for growing single crystalline GaN films on Si substrates.
Simulation of negative-effective-mass terahertz oscillators
Authors: Cao J. C. , Liu H. C. , Lei X. L.
Published: July 2002

We present a model calculation of hole current oscillations in doped p+pp+ ballistic diodes using the nonparabolic balance-equation theory and a time-dependent drift-diffusion model. Such an oscillation originates from a negative effective mass (NEM) region in the hole dispersion relation. In the present balance-equation calculation, we consider the scatterings by hole-impurity, hole-acoustic phonon, hole-polar-phonon, and hole-nonpolar-phonon–hole interactions, and yield a “N-shape” velocity-field relation, which are quite different from the two-valley results for electrons in bulk GaAs. We provide a detailed analysis of the resulting oscillations as a function of the applied voltage, base length, base doping, and the dispersion relation. Typical frequencies for a 0.2 μm structure NEM oscillator are in the terahertz range. Qualitative agreement is obtained between the present calculations and the existing Boltzmann results.
Micromagnetic studies of read and write process in magnetoresistive random access memory
Authors: Wei Dan , Ong C. K. , Yang Zheng
Published: July 2002

A micromagnetic model is established to analyze the read and write processes in a magnetoresistance random access memories (MRAM) cell. The magnetoresistive curves of NiFeCo/interlayer/NiFeCo cells are analyzed and compared with available experiments. The switching fields of the two magnetic layers A and B in a MRAM cell are studied versus different geometrical parameters of a cell. The word current IwA and IwB corresponding, respectively, to the switching of films A and B, and the related error of IwA and IwB under a given sense current Is, are analyzed and compared with experiment. In a cell with a given width W, proper geometrical parameters such as the thickness of the magnetic layers and the aspect ratio (length over width) are found based on the analysis of the IwA−Is and IwB−Is curves.
Platinum–aluminum nitride–silicon carbide diodes as combustible gas sensors
Authors: Samman A. , Gebremariam S. , Rimai L. , Zhang X. , Hangas J. , Auner G. W.
Published: July 2002

In this article we report on the novel use of a Pt/AlN/SiC structure as a combustible gas sensor. This device structure was fabricated by depositing a 2000 Å thick layer of AlN on the SiC substrate at 900 °C by laser ablation. Catalytic Pt gates were deposited onto the AlN at room temperature either by laser ablation or by rf sputtering. The electrical characteristic of the resultant devices from room temperature to 650 °C revealed current rectifying behavior. Most importantly, their electrical characteristic changed in response to propane, propylene, and CO introduced into their ambient at temperatures as low as 250 °C. They responded to changing gas composition over a wide range of combustible and oxygen concentrations from lean to fuel rich conditions. The response was a function of the ratio of combustible/oxygen concentration rather than to the absolute combustible concentration. However, this relationship exhibited some degree of combustible specificity.
Ultrasonic microspectroscopy characterization of silica glass
Authors: Kushibiki Jun-ichi , Wei Ting-Cun , Ohashi Yuji , Tada Akira
Published: July 2002

Ultrasonic microspectroscopy technology, using the line-focus-beam and plane-wave ultrasonic material characterization system, is applied to characterization of silica glass. Eight types of commercial silica glasses fabricated by different production methods and conditions are used as specimens to measure their acoustic properties accurately, viz., acoustic velocity, density, elastic constant. The variations in those acoustic properties resulting from small amounts of impurities, such as hydroxyl (OH) and chlorine, incorporated during the production processes were quantitatively obtained. The longitudinal velocity, density, and elastic constant c11 of silica glass with 860 ppm OH ions were less by 0.28%, 0.06%, and 0.62%, respectively, than those for OH-free silica glass, while the temperature coefficient of the longitudinal velocity was greater by 7.4%. In contrast, the leaky surface acoustic wave velocity, shear velocity and elastic constant c44 of silica glass containing 1500 ppm residual chlorine were less by 0.23%, 0.30%, and 0.59%, respectively, than those of OH-free and chlorine-free silica glass, while the longitudinal velocity, elastic constant c11, and density increased slightly by 0.04%, 0.10%, and 0.02%, respectively. Further, it was found that the decrease in acoustic velocity due to OH or chlorine is mainly related to the decrease in the elastic constant, which also corresponds to the decrease in the viscosity of the SiO2 material. These results demonstrate that the ultrasonic method is very useful and effective for analyzing and evaluating both the glass properties and the production processes.
Anodic metal matrix removal rate in electrolytic in-process dressing I: Two-dimensional modeling
Authors: Chen Hong , Li James C. M.
Published: July 2002

The electric field or current density distribution around a diamond particle embedded in a metal anode during electrolytic in-processing dressing (ELID) is calculated for the two-dimensional case of a long diamond particle without protrusion and for a periodic array of such long, parallel particles. It is found that there is a field concentration at the diamond/metal boundary so that the metal dissolution rate is the largest at such boundary. For the periodic array, the average current density is independent of the diamond concentration so that the metal dissolution rate increases with increasing diamond concentration. Hence, for the same ELID effect, the current required for a high diamond concentration tool is less than that required for a low diamond concentration tool.
Anodic metal matrix removal rate in electrolytic in-process dressing II: Protrusion effect and three-dimensional modeling
Authors: Chen Hong , Li James C. M.
Published: July 2002

The electric field distribution around a diamond particle embedded in a metal anode during a steady-state electrolytic in-process dressing (ELID) is calculated for a few more simple situations: A disk-shaped diamond particle with some protrusion, a spherical diamond particle with a hemispherical protrusion, and a long diamond particle with a half elliptical protrusion. It is found that the field concentration effect is greatly reduced with protrusion to a value of about 2 when half of the diamond particle is exposed. The effect is the largest at the diamond/metal boundary, decreases sharply to a small value at a distance of the order of the size of the diamond particle. Our analysis suggests that ELID is ideal for exposing diamond in metal-bonded tools.
Multimode transport in a T-shaped quantum transistor
Authors: Appenzeller J. , Schroer Ch.
Published: July 2002

This article presents numerical studies on multimode transport in a T-shaped quantum transistor geometry. Solving the time independent Schrödinger equation with adequate boundary conditions we model the current for up to six one-dimensional modes in the T structure. It is found, that independent of the number of modes periodic features dominate the conductivity as a function of gate voltage. Their origin is explained in terms of mode coupling in the stub region of the transistor for which the electron velocity in the waveguide is essential. The results are compared with experimental data.
Time dependent effects and transport evidence for phase separation in La0.5Ca0.5MnO3
Authors: Roy M. , Mitchell J. F. , Schiffer P.
Published: July 2002

The ground state of La1−xCaxMnO3 changes from a ferromagnetic metallic to an antiferromagnetic charge-ordered state as a function of Ca concentration at x∼0.50. We present evidence from transport measurements on a sample with x=0.50 that the two phases can coexist, in agreement with other observations of phase separation in these materials. We also observe that, by applying and then removing a magnetic field to the mainly charge-ordered state at some temperatures, we can “magnetically anneal” the charge order, resulting in a higher zero-field resistivity. We also observe logarithmic time dependence in both resistivity and magnetization after a field sweep at low temperatures.
Fabrication and magnetic characterization of single domain Co91Fe9 nanostructures
Authors: Moreau C. E. , Caballero J. A. , Loloee R. , Pratt W. P. , Birge Norman O.
Published: July 2002

Linear arrays of 15 nm thick, polycrystalline Co91Fe9 nanoelements have been fabricated using e-beam lithography, collimated sputtering and lift-off. The elements have widths of 0.17 and 0.25 μm, lengths from 0.5 to 4 μm, pointed ends, and center-to-center spacing of 0.5 μm. Characterization by magnetic force microscopy shows that in the virgin state these elements are single-domain and have magnetizations parallel to their long axes. The field at which an element switches its magnetization direction is shown to depend mainly on its width, and only slightly on its length. Fields of about 440 and 680 Oe are needed to switch completely the 0.25 and 0.17 μm structures, respectively. This separation in switching fields is adequate to allow elements of different widths to be switched independently. Also, magnetic force microscopy and SQUID magnetometry analysis show that a capping layer is necessary to protect the films from oxidation. Unprotected films show an altered hysteresis loop and their associated nanostructures are not consistently single-domain.
Temperature dependence of exchange biased thin films
Authors: McGrath B. V. , Camley R. E. , Wee Leonard , Kim Joo-Von , Stamps R. L.
Published: July 2002

A thin ferromagnet exchange coupled to an antiferromagnet often results in an enhanced width and a shift in the center position for the hysteresis curve. Recent calculations have shown how these features could occur in for both compensated and uncompensated antiferromagnet interfaces. These calculations were performed at zero temperature. We explore a model which allows for imperfectly compensated interfaces due to interface roughness and which calculates the spin configurations and hysteresis curves as a function of temperature. We find that the Koon results—ferromagnet spins directed perpendicular to the antiferromagnet spins—is appropriate for low temperature and nearly compensated interfaces. Increasing temperature and noncompensation favors a configuration where the ferromagnetic spins line up closer to the easy axis of the antiferromagnet. A particularly interesting result is that the coercive field decreases much more rapidly than the bias field as temperature is increased. This is in agreement with some recent experimental results, and we speculate that the exchange bias is substantially due to the surface structure of the antiferromagnet while the coercive field depends on the behavior of the spins in the bulk of the antiferromagnet.
Domain structure and magnetotransport in epitaxial colossal magnetoresistance thin films
Authors: Suzuki Y. , Wu Yan , Yu J. , Ruediger U. , Kent A. D. , Nath T. K. , Eom C. B.
Published: July 2002

Our studies of compressively strained La0.7Sr0.3MnO7 (LSMO) thin films reveal the importance of domain structure and strain effects in the magnetization reversal and magnetotransport. Normal and grazing incidence x-ray diffraction indicate that the compressive strain on these LSMO thin films on (100) LaAlO3 is not completely relaxed up to thicknesses on the order of 1000 Å. The effect of the compressive strain is evident in the shape of the magnetization loops and the magnetotransport measurements at various temperatures. Although the domain wall contribution to the magnetoresistance is significantly larger than that predicted from a simple double exchange picture, the contribution is a small fraction of the measured magnetoresistance.
Magnetic and structural properties of CoCrTaNiPt longitudinal thin-film media
Authors: Yoshimura Satoru , Djayaprawira D. D. , Shoji Hiroki , Takahashi Migaku
Published: July 2002

The effects of Ni addition on magnetic and structural properties of CoCrTaNiPt/Cr longitudinal media fabricated under the UC process have been investigated. It is found that the addition of Ni up to 13 at. % increases the magnetocrystalline anisotropy field of grain Hkgrain by 10%, and decreases the saturation magnetization Ms by 20%. As a result, coercivity Hc increases by 10%–15% and the ratio of the saturation flux density to anisotropy field 4πMs/Hkgrain decreases by 30%. Further addition of Ni from 13 to 19 at. % results in a decrease of Hc by 7% and a decrease of Hc/Hkgrain from 0.38 to 0.36. This is due to the degraded grain-boundary structure and the increase of the stacking fault density. The addition of Ni up to 13 at. % is effective in increasing Hkgrain and decreasing 4πMs/Hkgrain, while retaining a high Hc/Hkgrain value of about 0.38. As a result, Hc as high as 3.2 kOe is obtained even when using an isotropic NiP/Al substrate.
Increase in perpendicular coercivity of Co–Ni ferrite-plated films by Zn ferrite underlayers
Authors: Kitamoto Y. , Zhang F. , Abe M. , Naoe M.
Published: July 2002

The present article explains that Co–Ni ferrite-plated films with perpendicular coercivity Hc⊥ of 3.8 kOe were obtained using Zn ferrite underlayers at the substrate temperature of 90 °C without postannealing. Though a thinner recording layer is required for low noise media, perpendicular anisotropy of Co–Ni ferrite single-layer films of 50 nm in thickness is weak due to worse crystallinity in the initial growth region. Their reproduced waveforms were single pulses, as observed in the longitudinal recording media. Thus, the underlayers of a spinel type of Zn ferrite were used to improve the crystallinity in the Co–Ni ferrite layers, in particular, in the initial growth region. The thickness of the Co–Ni ferrite layer tCoNi was 50 nm, and that of the Zn ferrite underlayers tZn was in the range of 25–110 nm. While Hc⊥ of the Co–Ni ferrite single layers with tCoNi of 50 nm was 2.5 kOe, Hc⊥ of the double-layer films increased with increasing tZn, reaching 3.8 kOe at tZn of 110 nm. The reproduced waveforms exhibited dipulses that are typical in the perpendicular recording media. These results will open the way to develop the perpendicular recording disks with high Hc⊥ using plastic substrates at low temperature.
Annealing effects on recording characteristics of CoCr-alloy perpendicular magnetic recording media
Authors: Hirayama Y. , Honda Y. , Kikukawa A. , Futamoto M.
Published: July 2002

Annealing effects of CoCrTa and CoCrPt perpendicular magnetic recording media have been investigated. The coercivity and the squareness increase by postannealing when the annealing temperature is higher than 450 °C and CoCr35/TiCr10 dual underlayer is employed. The CoCr35 underlayer suppresses Ti diffusion to the magnetic layer and provides Cr up to the magnetic layer. The signal-to-noise ratio is improved and the magnetization irregularity diameter observed by magnetic force microscopy (MFM) decreases by the postannealing treatment.
Micromagnetic study of readout process in magneto-optical multilayers
Authors: Sbiaa Rachid , Suzuki Takao
Published: July 2002

The areal storage density of magneto-optical (MO) recording can be increased with the magnetically induced super-resolution (MSR) readout technique. In the present work, the effect of laser energy, disk velocity, and cooling time on the magnetic reversal processes in the MSR technique is discussed. The magnetization distribution and the temperature profile within MO multilayer media are investigated based on a micromagnetic model using a heat conduction theory. The magnetization of the readout layer, initially with in-plane magnetic anisotropy, can be switched perpendicularly to the film plane in a small temperature region. This magnetic reorientation depends on the strength of the exchange coupling between different layers. The correlation between this exchange coupling and dynamic parameters is discussed.
Topology of six degrees of freedom magnetic bearing
Authors: van Acht V. M. G. , Damen A. A. H. , van den Bosch P. P. J. , Kamerbeek E. M. H.
Published: July 2002

A novel magnetic topology has been designed for a six degrees of freedom, magnetically levitated and driven mirror, to be used in a three dimensional (3D) measurement system based on laser interferometry. The translations of the mirror are to be kept small, whereas the rotations are to be controlled over a large range with a high bandwidth and high accuracy. Finite element modelling (FEM) is used to analyze the proposed topology. For computational load reduction, a 2D FEM model has been derived from the actual 3D topology, which incorporates most of the magnetic subsystems. Simulations show that cross-influence between the actuators is small, that the forces and torques are proportional to the applied currents and that the angle of the rotor is of little influence. This allows the multiple in multiple out system to be regarded as multiple linear single in single out systems.
Rotational and domain wall motion aftereffect in a patterned array of small particles
Authors: Pardavi-Horvath M. , Vertesy G. , Keszei B. , Vertesy Z.
Published: July 2002

Aftereffect for magnetization processes by rotation and by domain wall motion was investigated on the same, single domain, two-state system of a square 2D (two-dimensional) array of garnet particles. Aftereffect measurements were performed magnetooptically. The particles are thermally stable, the particle energy is 10−6 erg compared to the thermal energy of 10−12 erg. No aftereffect of rotation switching of the system of “up” and “down” magnetized particles could be observed at room temperature. At increased temperatures thermally activated switching, very weakly depending on magnetic field, is observed. Each individual particle can be demagnetized into a metastable stripe domain structure. The barrier for DW (domain wall) motion is much lower than the barrier for rotation, and a significant aftereffect was measured on the same particles, demagnetized into a domain structure. The observed time dependence for DW aftereffect is exponential, M(t)/Ms=d* exp(−et), where d=0.04, and e=1/τ follows the increase of the magnetization with field, de/dH=0.021 /s/Oe.
Potts model with algebraically decaying interactions in two dimensions
Authors: Bayong E. , Diep H. T.
Published: July 2002

We study the 2d Potts model with long-range interaction algebraically decayed as 1/rd+σ using extensive Monte Carlo simulations to determine the nature of the phase transition with histogram technique. The model is shown to exhibit either a first-order transition or a second-order phase transition with critical exponents depending on σ and q. For q=3 there is a critical value of σ below which a first-order transition occurs. For q⩾5 the transition is of first order for any σ.
Ground state properties of the S=1/2 Heisenberg antiferromagnet on the triangular lattice
Authors: Capriotti Luca , Trumper Adolfo E. , Sorella Sandro
Published: July 2002

We investigate the ground state properties of the S=1/2 Heisenberg antiferromagnet on the triangular lattice using mainly exact diagonalization (for lattices up to N=36 sites) and the recently developed Green Function Monte Carlo with stochastic reconfiguration (up to N=144). A systematic size scaling of the order parameter and of the spin gap gives strong evidences for the presence of long-range Néel order in the ground state of this highly frustrated model.
A new magnetic displacement sensor and linear actuator device
Authors: Brandão Faria J. A.
Published: July 2002

A magnetic device that can be used either as a displacement sensor or as a linear actuator is described and analyzed, the analysis being conducted by making use of Maxwell’s equations in integral form together with energy considerations. The magnetic circuit, with coaxial symmetry, includes an n-turn winding and a movable conducting ring, both them placed around the inner cylinder rod of the magnetic circuit; the magnetic field lines are radial at the air-gap between the inner and outer cylinders. It is shown that by monitoring the equivalent self-inductance of the winding at rest any change in the position of the conducting ring can be linearly sensed, the sensitivity of the sensor being inversely proportional to the air-gap size and proportional to the radius of the inner cylinder rod. In addition, it is shown that once a time-varying current is made to flow in the winding, a driving force is exerted on the ring, making it move away from the winding; the actuating force is independent of the ring position and can be controlled by controlling the current.
Recent advances in the development of (Fe,Co)88M7B4Cu1 magnets (invited)
Authors: Willard M. A. , Laughlin D. E. , McHenry M. E.
Published: July 2002

Annealing of amorphous precursor alloys, with compositions (Fe,Co)88M7B4Cu1 (M=Zr, Nb, Hf), above their primary crystallization temperature results in the nanocrystallization of the ferromagnetic α′-FeCo phase. This work describes results of the characterization of these alloys, including morphological and chemical stability of the α′-FeCo phase, examination of alloy compositions, and development of a pseudo-Slater–Pauling curve for the amorphous precursor alloys. Samples with the composition Fe44Co44Zr7B4Cu1 were annealed at 600 °C for 10, 31, 100, 308, 1000, and 3072 h in Ar and examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Scherrer analysis of x-ray peak breadths was used to infer only a slight increase in the grain size of the sample annealed for 3072 h (∼60 nm) compared to the samples annealed for short times (∼40 nm). TEM studies revealed a distribution of grain sizes in the material with an average grain size of 42 nm for the 3072 h annealed sample. Samples annealed at higher temperatures exhibited the additional minority phases (Fe,Co)3Zr and ZrO. XRD analysis of the samples annealed for extended times at 600 °C do not indicate any formation of these phases, even in the sample annealed for 3072 h. TEM indicates the formation of minority phases, probably resulting from the devitrification of the amorphous grain-boundary phase and/or oxidation upon extended annealing. Differential scanning calorimetry results on alloys of composition (Fe0.5Co0.5)89Zr7B4 and (Fe0.65Co0.35)88Zr7B4Cu1 show primary crystallization temperatures of 495 and 480 °C, respectively.
Nanostructured soft magnetic material with low loss and low permeability
Authors: Fukunaga H. , Furukawa N. , Tanaka H. , Nakano M.
Published: July 2002

Nanostructured Fe–Cu–Nb–Si–B ribbons with relative permeability of a few hundred were prepared by using creep-induced anisotropy and their soft magnetic properties were evaluated. The observed domain pattern for an obtained ribbon suggests that magnetization process is due to the magnetization rotation mode. The variation of magnetization by its rotation results in frequency-independent behavior of the permeability up to 1 MHz with magnetic loss comparable to that of ferrite cores. A large saturation magnetization of the studied ribbons enables us to obtain the figure of merit for applications to the storage of magnetic energy being much larger than that of ferrite cores with air gaps. These results suggest that the Fe–Cu–Nb–Si–B ribbons with creep-induced anisotropy are suitable for a magnetic material for the storage of magnetic energy.
Furnace and current annealing of the amorphous Fe72Cu1Nb4.5Si13.5B9 alloy
Authors: Principi G. , Maddalena A. , Gupta A. , Bhagat N. , Malhotra N. , Dasannacharya B. A. , Amenitsch H. , Bernstorff S.
Published: July 2002

Specimens of the amorphous Fe72Cu1Nb4.5Si13.5B9 alloy have been nanocrystallized by furnace (FA) and current (CA) annealing and analyzed by simultaneous small-angle (SAXS) and wide-angle (WAXS) x-ray scattering using the beam-line 5.2L at Elettra Synchrotron Source, Trieste. The analysis has been carried out also dynamically on untreated samples current heated in situ. Significant differences in the SAXS regime have been observed between the FA and CA samples having the same amount of nanocrystallization as determined from previous WAXS and Mössbauer measurements. These differences can be attributed to a lower contrast of the nanocrystallites in the CA samples as compared to FA samples. The Mössbauer spectra show that in FA and CA samples the nanocrystalline grains consist of a nonstoichiometric partially disordered Fe3Si phase, and indicate the presence of boron atoms in the nanocrystals of CA samples. This is in agreement with the lower contrast found by SAXS and implies a lower segregation of boron atoms and/or of borides at the boundaries of nanocrystallites of CA samples, accounting for their lower brittleness.
Temperature dependence of exchange field and coercivity in polycrystalline NiO/NiFe film with thin antiferromagnetic layer: Role of antiferromagnet grain size distribution
Authors: Khapikov A. F. , Harrell J. W. , Fujiwara H. , Hou C.
Published: July 2002

The temperature dependence of the exchange bias Heb and coercivity Hc, has been measured for a NiO(6.5 nm)/Ni81Fe19(12 nm) bilayer which demonstrated at room temperature zero exchange bias, but an enhanced coercivity (Hc=65 Oe). Upon cooling the sample in a magnetic field (H=300 Oe), the exchange bias remains zero down to T=250 K, whereas the coercivity increases roughly linearly with decreasing temperature. Below this critical temperature an exchange shift of the hysteresis loop occurs that is accompanied by a decrease in the coercivity. Decreasing the temperature further results in an increase in both Heb and Hc. A thermal fluctuation model, where both fluctuating and stable AF grains contribute to the coercivity, with an additional assumption of increasing the density of NiO interfacial uncompensated spins at low temperature, predicts the correct temperature behavior of Heb and Hc.
Magneto-optical imaging of the first order spin-flop transition in the layered manganite La1.4Sr1.6Mn2O7
Authors: Welp U. , Berger A. , Miller D. J. , Vlasko-Vlasov V. K. , Gray K. E. , Mitchell J. F.
Published: July 2002

The spin-flop transition in the antiferromagnetic layered manganite La1.4Sr1.6Mn2O7 was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.
Linear variable differential transformer sensor using Fe-rich amorphous wires as an active core
Authors: Chiriac H. , Hristoforou E. , Neagu Maria , Peptanariu M. , Castano F. J.
Published: July 2002

Results concerning linear variable differential transformer (LVDT) displacement measurements using Fe77.5Si7.5B15 amorphous wires as movable core are presented. Taking advantage of the large Barkhausen (LBE) effect in amorphous wire, a small exciting field and a small number of windings in the secondary coils are necessary to get an large output signal of the LVDT. The thermal noise of the output becomes low, due to the small number of coil turns, increasing thus the sensitivity of the LVDT. The results obtained show a linear dependence of the LVDT response on the displacement of the moving core for displacements up to about 14 mm, with accuracy of 1 μm.
Rotationally symmetric solutions of the Landau–Lifshitz and diffusion equations
Authors: Mayergoyz I. D. , Bertotti G. , Serpico C.
Published: July 2002

The problem of isotropic conducting ferromagnetic film subject to in-plane circular polarized magnetic fields is discussed. This problem requires simultaneous solution of diffusion and Landau–Lifshitz equations. It is observed that the mathematical formulation of the problem is invariant with respect to rotations in the film plane. By exploiting this invariance, the rotationally symmetric solutions of the Landau–Lifshitz equation coupled with the diffusion equation are obtained and examined.
Electron and negative ion densities in C2F6 and CHF3 containing inductively coupled discharges
Authors: Hebner G. A. , Miller P. A.
Published: July 2002

Electron and negative ion densities have been measured in inductively coupled discharges containing C2F6 and CHF3. Line integrated electron density was determined using a microwave interferometer, negative ion densities were inferred using laser photodetachment spectroscopy, and electron temperature was determined using a Langmuir probe. For the range of induction powers, pressures and bias power investigated, the electron density peaked at 9×1012 cm−2 (line-integrated) or approximately 9×1011 cm−3. The negative ion density peaked at approximately 1.3×1011 cm−3. A maximum in the negative ion density as a function of induction coil power was observed. The maximum is attributed to a power dependent change in the density of one or more of the potential negative ion precursor species since the electron temperature did not depend strongly on power. The variation of photodetachment with laser wavelength indicated that the dominant negative ion was F−. Measurement of the decay of the negative ion density in the afterglow of a pulse modulated discharge was used to determine the ion–ion recombination rate for CF4, C2F6, and CHF3 discharges.
Cavity detuning effects in semiconductor microcavity light emitting diodes
Authors: Pratt A. R. , Takamori T. , Kamijoh T.
Published: July 2002

Light output power versus current, emission spectroscopy and far-field emission patterns have been used to characterize microcavity light emitting diodes (MC-LEDs). Evidence that microcavity effects lead to enhanced emission properties is provided by changes in the total emitted light output power, as well as the electroluminescence spectra of the MC-LEDs. Compared to a conventional noncavity type LED structure, enhanced efficiency and narrow spectral linewidths have been observed for the MC-LEDs over a wide range of cavity detunings and cavity Q values. Evidence that control of the cavity detuning leads to temperature insensitive output characteristics is provided by changes in the temperature dependence of the slope efficiencies extracted from the light output versus current characteristics. Variations in the emitted radiation patterns as a function of current injection are also reported demonstrating the important role of the cavity detuning on the emission properties of MC-LEDs.
Thermally based light modulation through free-standing porous silicon film
Authors: Gaburro Z. , Babić D.
Published: July 2002

A large laser induced increase of light absorption has been observed in free-standing porous silicon films, in the wavelength range 500–700 nm. Thermal origin of the observed modulation is suggested by two hitherto unreported observations: the spectral dependence and the comparison between the time decay of modulation with photoluminescence. A simple thermal model, taking into account the porosity of the porous silicon film, provides a good fit over the probed spectrum.
Phase fields of nickel silicides obtained by mechanical alloying in the nanocrystalline state
Authors: Datta M. K. , Pabi S. K. , Murty B. S.
Published: July 2002

Solid state reactions induced by mechanical alloying (MA) of elemental blends of Ni and Si have been studied over the entire composition range of the Ni–Si system. A monotonous increase of the lattice parameter of the Ni rich solid solution, Ni(Si), is observed with refinement of crystallite size. Nanocrystalline phase/phase mixtures of Ni(Si), Ni(Si)+Ni31Si12, Ni31Si12+Ni2Si, Ni2Si+NiSi and NiSi+Si, have been obtained during MA, over the composition ranges of 0–10, 10–28, 28–33, 33–50, and >50 at. % Si, respectively. The results clearly suggest that only congruent melting phases, Ni31Si12, Ni2Si, and NiSi form, while the formation of noncongruent melting phases, Ni3Si, Ni3Si2, and NiSi2, is bypassed in the nanocrystalline state. The phase formation during MA has been discussed based on thermodynamic arguments. The predicted phase fields obtained from effective free energy calculations are quite consistent with those obtained during MA.
Correlation between the green photoluminescence band and the peroxy radical in γ-irradiated silica glass
Authors: Sakurai Yuryo , Nagasawa Kaya
Published: July 2002

We studied the origin of a green photoluminescence (PL) in silica glass at about 2.25 eV using electron spin resonance. The 2.25 eV PL band (lifetime: ∼300 ns, full width at half maximum: ∼0.2 eV) appears strong in γ-irradiated high-oxygen content silica glass suggesting that this PL is associated with a large surplus of oxygen states in silica glass. We investigated the correlation between the 2.25 eV PL band and the peroxy radical by the isochronal annealing method. Peroxy radicals are formed by the O3≡Si–O–O• bond (•: unpaired electron) and the small “peroxy radical” (SPR) bond in silica glass. Based on the annealing characteristics of the 2.25 eV PL band and the peroxy radical, we suggest that the PL band is associated with a surplus of oxygen defects formed by the SPR rather than the O3≡Si–O–O• bond.
Plasma diagnostics and low-temperature deposition of microcrystalline silicon films in ultrahigh-frequency silane plasma
Authors: Sumiya Shigeaki , Mizutani Yuko , Yoshida Ryohei , Hori Masaru , Goto Toshio , Ito Masafumi , Tsukada Tsutomu , Samukawa Seiji
Published: July 2002

Microcrystalline silicon thin films were formed on quartz substrates by ultrahigh-frequency (UHF) plasma enhanced chemical vapor deposition from a mixture of silane (SiH4) and hydrogen (H2) gases at low substrate temperatures (Ts). The UHF plasma was excited at a frequency of 500 MHz. The deposition rate and the crystallinity of the films were investigated as a function of H2 dilution, total pressure, mixture ratio of SiH4 to H2 and Ts. A crystalline fraction of 63% with a high deposition rate of 7.7 Å/s was obtained even at a Ts of 100 °C. At a temperature of 300 °C, a crystalline fraction of approximately 86% was achieved at a deposition rate of 1.4 Å/s. Diagnostics of the UHF plasma have been carried out using a Langmuir probe, ultraviolet absorption spectroscopy, and optical emission spectroscopy. Good crystallinity was explained by the balance of the sheath voltage and atomic hydrogen densities in the UHF plasma. Namely, the UHF plasma source achieving a high density plasma with a low electron temperature enabled us to reduce the ion bombardment energy incident on the substrates while maintaining a high density of hydrogen atoms, and which improved the crystallinity at low Ts.
Thermal cross-track cross talk in phase-change optical disk data storage
Authors: Peng Chubing , Mansuripur M.
Published: July 2002

We have investigated the temperature distribution in land/groove phase-change optical disks. The incident beam is linearly polarized either parallel to track (E∥ polarization) or tangential to track (E⊥ polarization). Calculations have shown that temperature profiles in the medium are dependent on the wavelength of light, the state of polarization, the geometry of the grooved structure, and the multilayer stack. The temperature profiles are quite different between the land track and the groove track. Thermal cross-track cross talk from a land track to its neighboring groove tracks is higher than that from a groove track to its neighboring land tracks. The interaction between the E⊥ electric field and the grooved structure is mainly responsible for thermal cross-track cross talk.
Product distributions in the reactions of excited noble-gas atoms with halogen-containing compounds
Authors: Balamuta John , Golde Michael F. , Moyle Alfred M.
Published: July 2002

The products of the reactions of electronically excited Ar(3P0, 3P2), Kr(3P2), and Xe(3P2) atoms with the chloromethanes, chlorofluoromethanes, CF3H, CF2HCl, CFHCl2, CF2Cl⋅CF2Cl, CF3⋅CCl3, and HCl have been investigated by the discharge-flow technique, using atomic resonance fluorescence to probe dissociation channels and emission spectroscopy. Energy transfer leading to cleavage of C–H or C–Cl bonds is extremely efficient, particularly in the reactions of Xe(3P2) with CF2Cl2 and CFCl3 and of Ar(3P0, 2) with CFCl3, in which more than two Cl atoms are released per primary reactive event. The observation of contrasting behavior in the reactions of Xe(3P2) with CF3H and CF4 has led to the discovery of a qualitative correlation between the rate constants, the products of the energy transfer process, and the accessibility of dissociative or ionizing states of the reagent molecule, as revealed by its vacuum UV absorption spectrum.
Artifacts in the adiabatic approximation
Authors: Wagner M.
Published: July 2002

Generally, the adiabatic approximation is defined via a projection onto an adiabatically moving electronic base. The Hamiltonian then is separated into three constituents (‘‘adiabatic,’’ ‘‘diagonal-nonadiabatic,’’ ‘‘nondiagonal-nonadiabatic’’). A model system is considered for which the correct zero-order eigenfunctions are known. It is shown that with these any single of the three Hamiltonian constituents yields diverging expectation values; only their sum yields convergence. From this the conclusion seems stringent that any result pertaining to electronic transitions, which is based on the adiabatic approximation, is an artifact.
Quantum mechanical study of the D+H2→HD+H reaction
Authors: AbuSalbi N. , Kouri D. J. , Shima Y. , Baer Michael
Published: July 2002

A quantum mechanical study is made of the D+H2(vi=0,1)→ HD(vf=0,1,2)+H reactions within the infinite order sudden approximation (IOSA) for the total energy interval 0.28≤Et≤1.28 eV. Results at various stages of the calculation are given ranging from most detailed reactive transition probabilities through opacity functions and γ-dependent cross sections to total and state-to-state integral and differential cross sections, as well as rate constants. The cross sections and rate constants are compared with other available theoretical results and experiments. It is found that the IOSA total cross sections for vi=0,1 overlap very nicely with the corresponding quasiclassical trajectory cross sections, except for the tunneling region. A less satisfactory fit is obtained with the distorted wave born approximation results. The calculated rate constants are compared with experiment and a rather good fit is obtained, in particular for rate constants from the ground state.
Homogeneous nucleation of xenon in supersonic shock tube flow
Authors: Zahoransky R. A.
Published: July 2002

The vapor nucleation of xenon, diluted in helium or argon, was investigated in the supersonic expansion wave of a shock tube. The thermodynamic states for the homogeneous condensation onset were varied between 78 and 115 K with corresponding partial pressures of xenon in the range 110–2100 Pa. It was found that the starting temperature of the expansion affects the onset state. Furthermore, the xenon/argon gas mixture nucleated at lower undercoolings than the xenon/helium system.
Resonant Raman spectra of soluble t
r
a
n
s polyacetylenea)
Authors: Cataliotti R. S. , Paliani G. , Dellepiane G. , Fuso S. , Destri S. , Piseri L. , Tubino R.
Published: July 2002

Resonant Raman spectra of a soluble form of trans polyacetylene are here reported. Accurate excitation profiles have been obtained by using the 1196 and the 1370 cm−1 bands of toluene as an internal standard. Both the integrated and sliced excitation profiles exhibit a resolved vibronic structure, whose shape is interpreted in terms of the magnitude of the electron–phonon coupling and of the inhomogenous distribution of the electronic energies due to the interaction with the solvent. The present data are compared with those obtained from solid samples.
Study of two-body and three-body channels for the reaction of metastable helium atoms with nitrogen
Authors: Pouvesle J. M. , Stevefelt J. , Collins C. B.
Published: July 2002

This work reports the characterization of late time periods in the decay of metastable He(2 3S) atoms produced in 1–4 atm of helium containing up to 1400 mTorr of N2. Results have been obtained that exploit the large dynamic range and high data acquisition rates available from time resolved spectroscopic studies of preionized stripline discharges. A new value was found for the termolecular reaction of (2 3S) with N2 of 1.5×10−31 cm6 s−1 that is in good agreement with a model of collisionally induced capture. An indirect channel of reaction was also found that seems to explain the anomalously rapid and conflicting termolecular rates that were previously reported.
Ab initio studies on the vacuum ultraviolet (VUV) excimer emission spectra of NeB+ and ArB+
Authors: Iwata Suehiro , Sato Nobuyuki
Published: July 2002

In the ab initio calculations the NeB+ and ArB+ diatomic ions are found to emit the excimer-type bound-free broad bands in the vacuum ultraviolet region. The emission band from the v′=0 of the lowest singlet excited state 1Π of NeB+ has a peak at 72 000 cm−1 (139 nm), while the spectrum from the v′=1 state exhibits two peaks at 73 500 and 70 200 cm−1, which reflects the node of the vibrational wave function of the initial state. The calculated lifetime of the v′=0 state is 1 ns. The corresponding band of the v′=0 of ArB+ peaks at 49 000 cm−1 (204 nm) and the lifetime is 6 ns.
Empirical potential for the He+CO2 interaction: Multiproperty fitting in the infinite-order sudden approximation
Authors: Keil Mark , Parker Gregory A.
Published: July 2002

An empirical intermolecular potential for the interaction of He with CO2 is obtained via data reduction of phenomenological cross sections. The infinite order sudden approximation is used to calculate the total differential, total integral, and spectral line broadening cross sections, and diffusion, viscosity, thermal conductivity, thermal diffusion factors, and second virial coefficients. Second order Chapman–Cowling corrections were used to determine some of the transport coefficients, and quantum corrections to the classical virial coefficients were included. The empirical potential obtained simultaneously fits all nine different types of experimental data to within their experimental error and the computational accuracy.
The electronic spectra of matrix-isolated nickel atoms: The coexistence of the 3D3 and 3F4 ground states in Ar, Kr, and Xe
Authors: Schroeder W. , Grinter R. , Schrittenlacher W. , Rotermund H. H. , Kolb D. M.
Published: July 2002

The electronic spectra of nickel atoms isolated in matrices of the four noble gases have been assigned in considerable detail. The associated experimental and theoretical study has unequivocally revealed that nickel atoms can simultaneously exist in two electronic ground states, 3d94s1:3D3 and 3d84s2:3F4, in argon, krypton, and xenon matrices where they also occupy at least three different trapping sites. In neon they appear to exist in one site and one state, 3d84s2:3F4, only. In argon, krypton, and xenon matrices the 3D3 is the dominant ground state. Some sites occupied by atoms in the 3F4 ground state can be annealed away, but others are found to be stable to this procedure. The effects of the matrix upon the guest atoms are briefly discussed. It is interesting that only one possible case of band splitting can be identified in the spectra though the observed blue shifts are of the usual magnitude.
Infrared intensities of amide modes in N-methylacetamide and poly(glycine I) from a
b i
n
i
t
i
o calculations of dipole moment derivatives of N-methylacetamide
Authors: Cheam T. C. , Krimm S.
Published: July 2002

The infrared intensities of the amide modes in N-methylacetamide (NMA) and poly(glycine I) (PGI) have been studied using ab initio dipole moment derivatives obtained for the peptide group in NMA and an empirical force field refined for PGI. Good agreement is found between the calculated transition moment magnitudes and directions of the amide I and II modes and experimental intensity and dichroism data. By analyzing the separate contributions of each internal coordinate to the total intensity, we are able to understand in detail the origins of the IR intensities of the amide modes. Besides demonstrating one approach by which IR intensities can be studied in complex molecules and polymers, our results also provide a basis for using IR intensities in structural studies of peptides and polypeptides.
Quantized semiclassical trajectory approach for evaluation of vibronic transitions in anharmonic molecules
Authors: Warshel Arieh , Hwang Jenn-Kang
Published: July 2002

Significant progress has been made in recent years in simulation of molecular spectra. It appears, however, that the available methods do not offer a reliable and practical way for simulation of the vibronic line shapes of large anharmonic molecules. As a step in the effort of attacking this problem we develop a semiclassical approach that generates quantized vibronic transitions between electronic surfaces of anharmonic molecules. The method evaluates vibronic transition probabilities by using the energy representation (Green’s functions) of the corresponding semiclassical propagators. The resulting expression generates automatically interference effects for nonquantized actions and gives significant intensity only for quantized vibronic transitions. The method is demonstrated in several test cases including the simulation of the line shape for the X→B transition in I2. The agreement between the quantum mechanical and classical results is excellent. The powerful physical insight provided by the method and its potential use in fundamental studies of line broadening in anharmonic molecules is outlined.