bluuebunny/crossref_metadata_2025 huggingface dataset:
Focusing and guiding charged particles by a superconducting tube
Authors: Roth Peter
Published: July 2002

Electrons traveling through a superconducting tube are studied. Two simple models are proposed to describe the interaction of the magnetic self-field of the electron stream with the superconducting tube. In the first model A, a complete expulsion of the magnetic flux from the superconductor due to the Meissner effect is assumed. On the contrary, a complete flux penetration through vortices is supposed in the second model B. Both models indicate an electron focusing when the space-charge effect is compensated. The focusing length is estimated. For electrons traveling in the vicinity of the tube axis, the focusing length determined according to model A is nearly independent on the distance of the electrons from the tube axis, while the focusing length according to model B increases at increasing distance of the electrons from the tube axis. Similar focusing effects are obtained by a superconducting cone.
Complex electro-optic constants of dye-doped polymer films determined with a Mach–Zehnder interferometer
Authors: Ono Hiroshi , Misawa Kazuhiko , Minoshima Kaoru , Ueki Akikatsu , Kobayashi Takayoshi
Published: July 2002

The real and imaginary parts of the linear electro-optic (EO) constants of four dyes, i.e., N,N-dimethylaminonitrostilbene (DANS), 4-[N-(2-hydroxyethyl)-N-ethyl]amino-4′-nitroazo- benzene [disperse red 1 (DR1)], N,N-dimentylindoaniline [phenol blue (PB)], and 1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide (HITCI) in polymethylmethacrylate (PMMA) films were determined using a Mach–Zehnder interferometer. The electronic transitions of DANS, DR1, PB, and HITCI are off-resonant, near-resonant, and resonant on the higher and lower energy sides with a He-Ne laser wavelength (632.8 nm), respectively. The different ratios of the real and imaginary parts of the EO constants of four dyes including their signs are successfully explained in terms of detuning energy.
Study on the formation mechanism of a complex domain structure in LiNbO3
Authors: Zhang Zhi-yong , Zhu Yong-yuan , Zhu Shi-ning , Shu Hong , Wang Hai-feng , Hong Jing-fen , Ge Chuan-zhen , Ming Nai-ben
Published: July 2002

A complex domain structure in LiNbO3 is fabricated by proton exchange followed by heat treatment. Domain inversion at the +c and −c faces as well as reinversion in the inverted region near the +c surface are observed experimentally. The proton and lithium concentrations are measured by secondary ion mass spectroscopy. It is found that the proton profile peaks away from the surface due to indiffusion and outdiffusion. The domain inversion mechanism is discussed. An internal electric field model is proposed which is based on the concentration gradient of proton and lithium deficiency.
Schottky barrier heights of InxAl1−xAs (0≤x≤0.35) epilayers on GaAs
Authors: Chyi J.-I. , Shieh J.-L. , Lin R.-J. , Pan J.-W. , Lin R.-M.
Published: July 2002

The electrical characteristics of Al Schottky diodes on n-type InxAl1−xAs (0≤x≤0.35) were investigated in detail by current-voltage and capacitance-voltage measurements. These high-quality InAlAs epilayers were grown on GaAs using step-graded buffers under proper growth conditions. It was found that the Schottky barrier height of the epilayers increases with Al content as opposed to what was predicted previously. The effect of the interfacial oxide layer on the determination of Schottky barrier height is also presented.
Effect of Ge concentration on the propagation characteristics of SiGe/Si heterojunction waveguides
Authors: Yang Z. , Weiss B. L. , Shao G. , Namavar F.
Published: July 2002

The variation of the modal propagation loss of planar SiGe/Si heterojunction waveguides with Ge concentrations ranging from 1.3% to 10% has been determined for both TE and TM polarizations at wavelengths of 1.15 and 1.523 μm. The results show that at 1.15 μm wavelength the propagation loss increases with increasing Ge concentration due to the band-edge absorption, which dominates the waveguide loss characteristics, while at the wavelength of 1.523 μm it decreases with increasing Ge concentration. The polarization sensitivity is only found at the longer wavelength and is thought to be due to the interaction of the evanescent field at the SiGe/Si interface with the Si substrate.
Two-dimensional numerical simulation of the pulse response of a semi-insulating InGaAs:Fe photodetector
Authors: Hurd C. M. , McKinnon W. R.
Published: July 2002

A calculation is described of the transient pulse response of a planar metal-semiconductor-metal photodetector consisting of Schottky contacts made to an active layer of semi-insulating InGaAs:Fe that is supported on an InP:Fe substrate. The simulation uses a two-dimensional, drift/diffusion calculation and includes external circuit elements. Realistic conditions are considered where the semi-insulating material is represented by a two-level compensation model with Fe as a deep acceptor and hole trap that compensates shallow n-type impurities. The calculated results are compared directly with experimental ones for micron-scale devices described in the literature. The calculation gives a microscopic picture of how trapping controls particularly the falling side of the transient response, and it also shows how the measured performance of the device can reflect the influence of typical external circuit elements.
Slow domains in semi-insulating GaAs in high magnetic fields
Authors: Neumann A. , Willing B. , Jansen A. G. M. , Wyder P. , Deltour R.
Published: July 2002

We report on the drastic effect of a transverse high magnetic field on the low-frequency current oscillations in semi-insulating GaAs at room temperature, related to the propagation of slow high-electric-field domains. At a critical magnetic field Bc, we find a sharp transition between two ranges of the domain drift velocity, indicating a substantial change in the behavior of the involved deep level. Bc depends linearly on the applied voltage. The amplitude of the current oscillations decreases with increasing magnetic field, which reflects the interaction between the latter and the contact region.
Polarimetric scattering from heterogeneous particulate media
Authors: Jin Ya-Qiu
Published: July 2002

Electromagnetic (EM) scattering in a medium containing discrete scatterers is a problem with broad interdisciplinary interest. The Mueller matrix solution has been applied to the study of EM polarimetric scattering and the vector radiative transfer from a layer of random medium. In this article, we report the methods to polarimetric scattering from a layer of particulate media containing: randomly dense spherical particles, nonuniformly oriented spheroids, or random clusters of spheroids. The complex scattering amplitude functions and phase matrix are explicitly obtained. Numerical results of the Mueller matrix solution show the properties of completely polarimetric scattering of those random media and its functional dependence on the relevant physical parameters.
Holographic recording in photorefractive thin films: Edge effects
Authors: Aguilar María , Carrascosa M. , Agulló-López F. , Magaña L. F.
Published: July 2002

The initial holographic recording rate has been analytically studied for a photorefractive thin slab. The finite thickness is explicitly taken into account by using a two-dimensional formulation (coordinates x and z parallel and perpendicular to the slab faces, respectively). It is shown that for a thickness comparable to the grating period, the solution appreciably departs from the usual one-dimensional result, i.e., edge effects are remarkable. In particular, significant space-charge fields perpendicular to the slab faces are generated. These effects may substantially modify the diffraction properties of the recorded photorefractive gratings, as exemplified for a GaAs film.
Photoreflectance of single buried Si1−xGex epilayers (0.12<x<0.24)
Authors: Carline R. T. , Hosea T. J. C. , Hall D. J.
Published: July 2002

Photoreflectance spectra have been obtained from single strained Si1−xGex epilayers (0.12<x<0.24) buried under a silicon cap. Despite dramatic changes in the shape of photoreflectance spectra obtained at different positions on a sample wafer, which are explained in terms of a strong interference effect as the Si cap thickness varied, spectra were fitted adequately using three Lorentzian oscillators. Critical point transition energies calculated in this way were consistent with E1 and E1+Δ1 in Si1−xGex and E′0 in silicon and allowed for determination of composition to within x±1.5%.
Influence of boron doping and hydrogen dilution on p-type microcrystalline silicon carbide thin films prepared by photochemical vapor deposition
Authors: Ghosh Sukriti , Dasgupta Arup , Ray Swati
Published: July 2002

Boron doped μc-SiC:H films have been prepared by low power (10 mW/cm2) photochemical vapor decomposition of SiH4, C2H2, and B2H6 gases diluted with hydrogen. The effect of boron doping and hydrogen dilution on structural and opto-electronic properties have been studied. The microstructure consists of Si crystallites while carbon remains at the grain boundaries and amorphous parts. Diborane doping beyond an optimum value has been observed to deteriorate the formation of microcrystallites. By optimizing the process parameters, p-type μc-SiC:H having σd∼3.0×10−3 S cm−1 with an E04 of 2.34 eV has been obtained. This film exhibits high optical transmittivity compared to its amorphous counterpart.
Structure and magnetic properties of Fe–N films prepared by ion-beam-assisted deposition
Authors: Jiang H. , Wu Q. L. , Tao K. , Li H. D.
Published: July 2002

Fe–N films were prepared by ion-beam-assisted deposition at different N/Fe atomic arrival ratios to the substrates. Films consisted of nitrogen-rich α-Fe and different phases of iron nitrides including ζ-Fe2N, ε-Fe2-3N, and γ′-Fe4N were formed. The phase composition of Fe–N films was found to depend sensitively on the N/Fe atomic arrival ratio and deposition temperature. The magnetic properties of the films mainly depends on phase composition. It was found that nitrogen-rich α-Fe films exhibited higher Ms than that of the pure iron film, and their Ms could be increased further by vacuum annealing at relatively low temperatures.
Photocurrent transients in almost ideal silicon p-n junctions
Authors: Basso G. , Pellegrini B.
Published: July 2002

Results obtained from measurements and spectroscopic analysis of current transients induced in almost ideal silicon n+-p junctions by infra-red stimulation are reported. In particular, by illuminating a reverse biased junction by means of an infra-red emitting diode for a few seconds, a current transient is generated which, at 0 °C, lasts several hours. This can be decomposed, by means of a proper spectroscopic method, into a sum of four exponential contributions with time constants ranging from tens up to thousands of seconds. Similar dark current transients were already obtained for the same junctions when they were stimulated by a change of the reverse bias voltage, without any optical excitation. The spectroscopy of both optical and voltage induced current transients gives four exponential components with the same time constants. Both the dark current and the photocurrent transients are ascribed to the same SiyOx clusters containing hundreds of Si atoms and four types of single energy level defect centers with different localization. While the voltage induced current transients implicate the activation of such defect centers in the p-region near the n+-p interface only, those due to the photostimulation produce their activation throughout the whole n+ region as well. This fact leads to much greater values for the transient photocurrent in comparison to those induced by voltage changes and, as a consequence, to greater reliability and accuracy in the measurements and in the results obtained from their analysis.
Pyrochlore based oxides with high dielectric constant and low temperature coefficient
Authors: Cava R. J. , Peck W. F. , Krajewski J. J.
Published: July 2002

The temperature dependent dielectric constants in the vicinity of room temperature have been measured for bulk ceramics which are phase-mixtures of Pb2(Nb,Mg,Ti)2O6+x pyrochlores and Pb(Nb,Mg,Ti)O3 perovskites. A band of compositions is found in which the negative temperature coefficient of dielectric constant for the pyrochlore is very closely compensated by the positive temperature coefficient of dielectric constant of the perovskite. These compositions have dielectric constants near 200, with measured Q’s near 200 at 100 kHz.
Trapping noise in semiconductor devices: A method for determining the noise spectrum as a function of the trap position
Authors: Longoni A. , Gatti E. , Sacco R.
Published: July 2002

This work deals with the study of the ‘‘trapping noise,’’ that is the low frequency noise caused by the random capture and release of carriers by localized traps. A numerical method, based on the linearization of the semiconductor device equations, is presented which allows one to determine the parameters of the Lorentzian noise spectrum due to a single trap inside a semiconductor device as a function of the position of the trap, of its activation energy and of the biasing conditions. The occupied trap is simulated by adding a single ionized shallow acceptor at the position of the trap. The method allows one to also determine the spectral density of the noise due to a general distribution of noninteracting traps inside the considered device. Numerical tests on elementary semiconductor devices are included to validate the proposed scheme and the trapping noise is then studied on a simple junction field effect transistor.
Selection rule for localized phonon emission in GaAs/AlAs double-barrier structures
Authors: Turley P. J. , Wallis C. R. , Teitsworth S. W.
Published: July 2002

Phonon-assisted tunneling (PAT) has been studied in detail for two similar GaAs/AlAs double-barrier structures. Calculations of the PAT current—including effects of optical-phonon localization—are in good agreement with experimental data, and the emission rate for certain phonon types is found to depend sensitively on GaAs well width. We find that GaAs-like modes clearly dominate in structures with wider wells, while GaAs and AlAs-like modes contribute equivalently in narrower well structures. A simple overlap integral—involving the phonon potential and electronic wave functions—provides an effective selection rule for determining which types of phonons are preferentially emitted.
Strain and alloying effects on the electronic and vibrational properties of InyAl1−yAs on InP
Authors: Pavesi L. , Houdré R. , Giannozzi P.
Published: July 2002

Photoluminescence and Raman measurements have been carried out as a function of In content (y) in relaxed and strained InyAl1−yAs alloys grown by molecular-beam epitaxy on InP. From the analysis of photoluminescence data near the lattice matched condition a reliable energy-gap dependence on In content for the strained material has been derived. From this the Bir–Pinkus electronic deformation potentials at the lattice matched condition have been determined. The Raman results for the relaxed material have been interpreted in the framework of the modified random element isodisplacement theory, yielding the localized Al mode in InAs, the gap In mode in AlAs, and quadratic dependences versus y for the different optical-phonon frequencies (AlAs-like and InAs-like). Optical-phonon deformation potentials have been calculated within ab initio theory for AlAs and successfully used to fit the AlAs-like phonon frequencies in strained InyAl1−yAs. Some problems have been found in interpreting the InAs-like strained phonon frequencies.
A computational study of host effects on Er3+ upconversion and self-quenching efficiency in ten garnets
Authors: Spangler Lee H. , Farris Bruce , Filer Elizabeth D. , Barnes Norman P.
Published: July 2002

A quantum-mechanical model is used to predict the energy levels and transition rates of Er in a variety of garnets in order to optimize laser performance. The existence of near resonances among the various Er manifolds as well as the transition rates are used to estimate upconversion and self-quenching of Er in ten different garnets. Morrison’s quantum-mechanical point charge program for calculating crystal-field effects on rare-earth ions was used to fit experimentally determined Er:YAG energy levels obtained from the literature. Radial factors from this fit were used, along with x-ray data, to calculate theoretical levels and transition rates for Er3+ in other garnets. The calculations suggest that the crystal-field effect on the upper four levels of the ground 4I15/2 manifold can dramatically effect resonances and thus affect upconversion and cross-relaxation processes. Additionally, a method by which they indicate how a large population density can efficiently be built up in the 4S3/2 manifold is proposed, a necessary condition for upconversion lasing from that manifold. Based on a simple summation of cross-relaxation resonances that can contribute, LuGG is predicted to enhance this upconversion process.
Optical response of single-crystal (La,Ca)MnOδ thin films
Authors: Hao J. H. , Zeng X. T. , Wong H. K.
Published: July 2002

(La,Ca)MnOδ thin film can be used as a bolometer operated over a wide temperature range around its metal–semiconductor transition where its resistance (R) changes rapidly. This material has a high optical absorption coefficient over a broad wavelength range and is chemically stable in air. We measured the optical response of single-crystal (La,Ca)MnOδ thin films to a He–Ne laser (632.8 nm) as a function of temperature (T), light modulation frequency (f), bias current (I), and laser power (P). The temperature dependence of the measured signal follows the dR/dT curve and the signal depends linearly on I and P. These results indicate that the response is mainly bolometric. Room temperature operation is possible if the sample composition and oxygen stoichiometry are adjusted appropriately.
Slow states in vacuum ultraviolet irradiated metal–oxide–silicon systems
Authors: Druijf K. G. , de Nijs J. M. M. , van der Drift E. , Granneman E. H. A. , Balk P.
Published: July 2002

We describe a study of the generation of slow interface states (time constants ≳40 s) by vacuum ultraviolet irradiation (Kr lamp, hν=10 eV) on metal–oxide–silicon samples with approximately 30 nm of thermally grown SiO2 and a transparent aluminum gate. The density of the centers in a given energy range was obtained from the shift of the midgap voltage over this range. A detailed energy level spectrum was determined from the stabilization times of the charging or discharging current observed after each of a series of 0.1 V steps in the bias voltage. Peaks in the energy level spectrum were observed for a position of the Fermi level at the Si–SiO2 interface near the band edges, at ≊0.25 eV above the valence-band edge and at ≊0.65 eV above the valence-band edge. The corresponding defects are identified as the oxygen vacancy, the Pb center (tentatively), and a hydrogen atom trapped at an oxygen atom in a strained Si–O–Si configuration near the Si/SiO2 interface.
Tunneling through a narrow-gap semiconductor with different conduction- and valence-band effective masses
Authors: Hatta E. , Nagao J. , Mukasa K.
Published: July 2002

We have calculated tunneling conductance in metal–narrow-gap-semiconductor (NGS)–metal tunnel junctions. Flietner’s two-band model is used to describe the dispersion relation within the energy gap in an isotropic NGS with different conduction- and valence-band edge effective masses. The results are compared with the tunneling conductance calculated by Kane’s two-band model, which has been commonly used to describe the tunneling characteristics through the energy gap in semiconductors. These results propose that the tunneling conductance in the tunnel junctions in which a narrow gap semiconductor of largely different conduction- and valence-band effective masses is used as a tunneling barrier can exhibit quite a different behavior, especially in the region of the midgap, from the tunneling conductance described by Kane’s two-band model.
Downstream ion drift in an electron cyclotron resonance plasma process
Authors: Beresford R.
Published: July 2002

The collisionless two-fluid model of a plasma is applied to the drift of ions and electrons downstream from an electron cyclotron resonance source. A plasma potential drop of 20 V is found for the system parameters typical of plasma-assisted epitaxy of the group III nitrides. This model provides a simple way to design downstream magnet coils to tune the ion acceleration, a key parameter in growth and etching processes. Compared to previous analyses, the present work makes clear the conditions under which an ion accelerating potential actually appears, in terms of the expansion that the plasma undergoes upon exiting from the source region. Published measurements of the potential profile downstream from a N2 plasma source can be fit assuming an electron temperature kTe=3.2 eV, somewhat greater than half the excitation energy of the long-lived A 3Σ+u metastable state. The analysis is consistent with the plane probe current–voltage characteristics obtained from the growth stage in a plasma-assisted molecular-beam epitaxy process. For a 10 W plasma used in the growth of GaN, the value of ion saturation current implies a plasma density at the source of 1010 cm−3. Considering the suppression of electron current by the magnetic field, the measured ratio of ion to electron saturation current implies an electron temperature of 3.8 eV.
Method of fabricating a free-standing diamond single crystal using growth from the vapor phase
Authors: Posthill J. B. , Malta D. P. , Humphreys T. P. , Hudson G. C. , Thomas R. E. , Rudder R. A. , Markunas R. J.
Published: July 2002

By combining a low temperature (600 °C) chemical vapor deposition process for homoepitaxial diamond and conventional ion implantation, we have made and lifted off a synthetic diamond single crystal plate. Before growth, a type Ia C(100) crystal was exposed to a self implant of 190 keV energy and dose of 1×1016 cm−2. Homoepitaxial diamond growth conditions were used that are based on water-alcohol source chemistries. To achieve layer separation (‘‘lift-off’’), samples were annealed to a temperature sufficient to graphitize the buried implant-damaged region. Contactless electrochemical etching was found to remove the graphite, and a transparent synthetic (100) single crystal diamond plate of 17.5 μm thickness was lifted off. This free-standing diamond single crystal plate was characterized and found to be comparable to homoepitaxial films grown on unimplanted single crystal diamond.
Thermal effects on near-critically biased superconducting thin film particle detectors
Authors: Garzarella A. , Martoff C. J.
Published: July 2002

Measurements of dc critical current as a function of temperature, and spontaneous voltage transients, are reported for superconducting thin films of Nb, Mo, and Ti near their critical temperatures and currents. The form of the temperature dependence is found to reflect the presence of stable and/or metastable normal regions in the films, as well as the different temperature scaling laws for the intrinsic and thermal-runaway critical currents. Voltage transients (pulses) occur in the nominally superconducting samples when the current is below both the intrinsic and thermal-runaway values. Thermal coupling between film and substrate is reported for a wide variety of samples fabricated by different methods. The coupling in all cases follows a thermal boundary resistance law with power flux proportional to the difference of fourth powers of film and substrate temperatures. Applications to minimum-ionizing particle detection above liquid helium temperatures are discussed.
Effects of a new stacking method on characteristics of multilayered BaTiO3 thin film
Authors: Song Man Ho , Lee Yun Hi , Hahn Taek Sang , Oh Myung Hwan , Yoon Ki Hyun
Published: July 2002

A new deposition method used to prepare BaTiO3 thin films resulted in multilayered structure with higher dielectric constant, capacitance per unit area, and breakdown strength than those prepared by a conventional stacking method; the new method continuously decreased the substrate temperature after initial deposition of a polycrystalline BaTiO3 layer. The observed high dielectric constant could be explained only by a multilayered amorphous/microcrystalline/polycrystalline structure, the nature of which was confirmed by scanning electron microscopy and index of refraction measurement. Well-defined ferroelectric hysteresis loops were observed as well with insignificant leakage current effects.
Self-consistent energy levels in p-type delta-doped quantum wells in GaAs
Authors: Gaggero-Sager L. M. , Pérez-Alvarez R.
Published: July 2002

We present a self-consistent calculation of the electronic structure of p-type delta-doped quantum wells in GaAs. We examine the dependence of the energy levels, Fermi level and the depth of the well with impurity concentration. We show that in this system the Fermi level is very close to the valence band; the heavy-hole subbands, as expected, contain many more states than the light-hole ones. Our results agree quite well with the experimental results available for this system.
Electron beam fabrication and characterization of high-resolution magnetic force microscopy tips
Authors: Rührig M. , Porthun S. , Lodder J. C. , McVitie S. , Heyderman L. J. , Johnston A. B. , Chapman J. N.
Published: July 2002

The stray field, magnetic microstructure, and switching behavior of high-resolution electron beam fabricated thin film tips for magnetic force microscopy (MFM) are investigated with different imaging modes in a transmission electron microscope (TEM). As the tiny smooth carbon needles covered with a thermally evaporated magnetic thin film are transparent to the electron energies used in these TEMs it is possible to observe both the external stray field emanating from the tips as well as their internal domain structure. The experiments confirm the basic features of electron beam fabricated thin film tips concluded from various MFM observations using these tips. Only a weak but highly concentrated stray field is observed emanating from the immediate apex region of the tip, consistent with their capability for high resolution. It also supports the negligible perturbation of the magnetization sample due to the tip stray field observed in MFM experiments. Investigation of the magnetization distributions within the tips, as well as preliminary magnetizing experiments, confirm a preferred single domain state of the high aspect ratio tips. To exclude artefacts of the observation techniques both nonmagnetic tips and those supporting different magnetization states are used for comparison.
Magnetic and microstructural properties of CoCrTa/Cr/Al thin films (abstract)
Authors: Chang H. S. , Lee T. D. , Park J. K. , Shin K. H. , Park K.
Published: July 2002

CoCrTa/Cr/Al and CoCrTa/Cr thin films were deposited on glass substrate at 280 °C using dc magnetron sputtering. The coercivity of CoCrTa/Cr films increased by introducing an Al underlayer. In 500 Å CoCrTa/Cr films, the coercivity increased with increasing Cr thickness and saturated above 700 Å Cr thickness (Hc=1050 Oe). On the other hand, in 500 Å CoCrTa/Cr/700 Å Al films, the coercivity increased sharply with increasing Cr thickness and showed a maximum value at 700 Å Cr thickness (Hc=1380 Oe). The coercivity squareness of the CoCrTa/Cr/Al and CoCrTa/Cr films was increased with increasing Cr thickness. The coercivity squareness of CoCrTa/Cr/Al films was lower than that of CoCrTa/Cr films. To study the origin of coercivity enhancement in CoCrTa/Cr films when deposited on an Al underlayer, microstructures of the two films were investigated using transmission electron microscopy (TEM). We controlled grain sizes of CoCrTa films by changing the thickness of the Cr layer. The grain size of CoCrTa films increased with increasing Cr thickness. The coercivity of CoCrTa/Cr/Al films showed a maximum value at a grain size of 430 Å. These results suggest that the transition from single domain to multidomain could occur at a grain size of ∼400 Å. To understand the coercivity squareness changes by the introduction of an Al underlayer, the angular variation of coercivity was measured. The magnetization of CoCrTa/Cr films was reversed by domain wall motion.

On the other hand, by the incorporation of an Al underlayer, the angular variation of coercivity deviated from the domain wall motion mode. Cross-sectional TEM studies of CoCrTa/Cr/Al films showed that Cr atoms in Cr layers diffused into an Al underlayer and Al atoms were detected in both Cr and CoCrTa layers, but magnetization of the magnetic layer was not decreased. The decrease in coercivity squareness of CoCrTa/Cr/Al films would be attributed to magnetic decoupling by segregation of Al atoms in grain boundaries of CoCrTa films.
Hall effect and giant magnetoresistance in lanthanum manganite thin films
Authors: Núñez-Regueiro J. E. , Gupta D. , Kadin A. M.
Published: July 2002

Magnetoresistance and Hall coefficient have been investigated as a function of temperature in patterned thin films of La–Ca–Mn–O. Our data on magnetoresistance show a maximum magnetoresistance ratio Δρ/ρ(0) of ≊−60% at 1.5 T and 240 K. The Hall coefficient indicates that the carriers are predominantly holes, with apparent density that varies as a function of temperature. Although changes in mobility were also observed, our results indicate that changes in carrier density become more important than changes in mobility at temperatures lower than the Curie temperature. We interpret our data in terms of a simple picture that includes two parallel contributions to the carrier density and the conductivity.
Magnetic domain percolation in granular Co-Ag
Authors: Gavrin A. , Kelley M. H. , Xiao J. Q. , Chien C. L.
Published: July 2002

The most dramatic changes in the properties of granular metals occur near the percolation threshold xc, which generally occurs at a volume fraction of approximately 0.50. There has been much speculation concerning the evolution of magnetic domains at xc, however, no direct observations have been produced. Using scanning electron microscopy with polarization analysis, we investigate the formation of domains in a series of granular Co-Ag samples. We find xc to lie in the range [0.50, 0.55], and we report on the size and morphology of the observed domains. Below xc, a domain pattern appears which may be due either to interparticle correlations or to residual Co in the silver matrix; we have previously discussed the ramifications of this observation on an understanding of GMR. In this paper, we place limits on the latter of these two models. Above xc, the domains resemble ‘‘stripe domains’’ which have been observed in amorphous materials. This observation suggests the presence of anisotropy perpendicular to the film plane.
Near-neighbor structure of rare-earth elements in fluorozirconate glasses
Authors: Wang Wen-cai , Chen Yu , Hu Tian-dou
Published: July 2002

The near-neighbor structures of several different rare-earth (RE) elements in fluorozirconate glasses doped with these RE fluorides are studied by extended x-ray absorption fine structure. The RE elements include praseodymium, neodymium, europium, and erbium. Their near-neighbor structures in ternary and multicomponent fluorozirconate glasses are determined and compared. The structural parameters of near neighbors around RE in these glasses are also compared with those of corresponding fluorides. The changes in distance and coordination number of near neighbors in the glasses containing different RE fluorides are obtained and discussed. It is observed that the intensities of the ‘‘white lines’’ at RE LII and LIII absorption edges in these glasses differ from those in corresponding fluorides.
Magnetic force microscopy images of ultrahigh-density bit patterns recorded on high-coercivity longitudinal and perpendicular thin-film media
Authors: Song Xing , Sivertsen John , Judy Jack
Published: July 2002

Magnetic bit patterns recorded on high-coercivity longitudinal thin-film media (L1: Hc=2500 Oe, L2: 2850 Oe), and perpendicular thin-film media (P1: Hc=2830 Oe, P2: Hc=3780 Oe) were investigated using magnetic force microscope (MFM). For the longitudinal media, insufficient writing is responsible for limiting the detectable density. By reducing the head fly height from 3 to 2 microinches, the maximum detectable density of L1 is increased from 7000 to 10 500 fr/mm. At this density the well-defined track edge disappears and a complete bit collapse occurs as the bit length becomes much smaller than the average domain size observed in the dc-saturation remanent state. In the case of perpendicular media, contact recording provides sufficient writing and a well-defined track edge at densities above 10 000 fr/mm for both media. A maximum detectable density about 12 000 fr/mm is obtained in both media. At higher recording densities, domain refinement takes place but magnetic interactions within the track also cause the formation of large domains which eventually destroys the bit periodicity.
Galvanomagnetic properties and magnetic domain structure of epitaxial MnAs films on GaAs(001)
Authors: Park M. C. , Park Y. , Shin T. , Rothberg G. M. , Tanaka M. , Harbison J. P.
Published: July 2002

Epitaxial MnAs films on GaAs(001) in the thickness range 20–200 nm were studied. Using ordinary and extraordinary Hall effect data to determine the field required for perpendicular saturation and saturation magnetizations reported elsewhere, we determined the shape anisotropy constant in the basal plane of the hexagonal structure to be 3.7(0.6)×105 erg/cm3 and the surface anisotropy constant to be −1.3(0.4) erg/cm2. The negative sign indicates thin enough films will be perpendicularly magnetized. By magnetic force microscopy of a 100 nm film we found stripe domains with 180° Bloch walls, thereby avoiding the hard c axis. The widths of the domains and the walls are 4.0(0.3) μm and 95(6) nm, respectively. In magnetoresistance, we observed behavior similar to other ferromagnets, namely peaks centered around the positive and negative coercive fields, and at fields beyond the coercive field a linear dependence on magnetic field. The electrical resistance showed rapid increase with temperature beginning about 5° below the Curie temperature (40 °C) caused by the change in crystal structure from hexagonal to orthorhombic. The resistivities are, respectively, 300(24) and 375(30) μΩ cm. Comparison with bulk values indicates the large lower temperature value is partly due to the presence of some orthorhombic phase observed in x-ray studies.
Magnetic viscosity and switching volumes of annealed Fe/Pt multilayers
Authors: Luo C. P. , Shan Z. S. , Sellmyer D. J.
Published: July 2002

Magnetic viscosity and the phenomenon of the sweep-rate dependence of the coercivity are important for the characterization of magnetic recording media. The activation volume of magnetization reversal or switching volume is directly related to medium noise. The magnetic viscosity behavior of annealed Fe/Pt multilayers was studied and the switching volumes were obtained by both measurements of the sweep-rate dependence of coercivity and time dependence of magnetization. It is found that samples with larger coercivities have smaller switching volumes, and an estimate of the magnetic grain diameter is about 9 nm. The coercivity mechanisms are also discussed.
The field-annealing effect on magnetoimpedance of a zero magnetostrictive metallic glass
Authors: Jen S. U. , Chao Y. D.
Published: July 2002

A commercially made metallic glass, VAC6030, was used for the magnetoimpedance (MI) measurement. Some samples were annealed at a temperature (Ta=340 °C) slightly below its Curie point, and cooled in a transverse or longitudinal field. The frequency of the probe current was from 10 kHz to 10 MHz. MI is defined as ΔR/R≡[R(0)/R(Hs)]−1 and ΔX/X≡[X(0)/X(Hs)]−1, where Hs is the saturating field along sample length, R is the resistance, and X is the reactance. At relatively low frequencies MI is mostly inductive, at relatively high frequencies it is resistive, and a crossover could be defined at a characteristic frequency f0. When the sample is in the as-cast state, f0=890 kHz. If samples have been field annealed, f0 could be shifted either downward or upward; f0 becomes 30 kHz after a transverse anneal, and 8 MHz after a longitudinal anneal. The field annealing would also change the magnitudes of MI. In theory, if f<f0, ΔR∝f2 and ΔX∝f; however, if f≳f0, both ΔR and ΔX are dominated by the skin-depth effect, and therefore proportional to f1/2 instead.
Giant magnetoresistance effect in multilayered wire arrays
Authors: Kume Minoru , Maeda Atsushi , Tanuma Toshio , Kuroki Kazuhiko
Published: July 2002

Three wire arrays, each comprised of a different type of giant magnetoresistive (GMR) multilayer, were fabricated by high-resolution electron-beam lithography and lift-off techniques. With a GMR multilayer of the biquadratic-coupling type, a magnetic field applied parallel to the wires produced a larger MR change and higher response than in the corresponding unpatterned multilayered film; however, the wire structures worsened the MR characteristics of two other types of GMR multilayers.
Effects of discharge pressure on the properties of Ag/Ni superlattices prepared by facing-target sputtering
Authors: Zeng X. T. , Wong H. K.
Published: July 2002

Giant magnetoresistance (GMR) effect in magnetic multilayered thin films is strongly affected by interface roughness. We prepared [Ag/Ni]60 superlattices on Corning’s 7059 glass substrates using a novel facing-target sputtering method and observed that the interface roughness can be controlled by varying the discharge pressure (PAr). The samples prepared at PAr≊10 mTorr exhibit sharpest interfaces, good (111) texture, and largest GMR effects (Δρ/ρs=9% at room temperature). Lower PAr promotes interface mixing while higher PAr results in much rougher interfaces and emergence of structural defects. Both cases weaken the antiferromagnetic coupling strength and thus reduce the MR effect. When PAr≥30 mTorr, we observed a further degradation of the multilayered structure. The GMR effects in these samples disappeared. Low-temperature annealing can improve the flatness of the interfaces and film structure, while higher-temperature annealing (≳300 °C) decomposes the multilayer structures.
Bragg diffraction of laser light by magnetostatic forward volume waves in a layered yttrium–iron–garnet film geometry
Authors: Peng Liang-Ping , Parekh J. P. , Tuan H. S.
Published: July 2002

The present article reports a theoretical study of Bragg diffraction of laser light by magnetostatic forward volume waves (MSFVWs) in a double-yttrium–iron–garnet(YIG)-film-layered structure. The layering provides a means of achieving a significant enhancement in bandwidth for nondispersive MSFVW propagation, leading in turn to a significant enhancement in bandwidth for magneto-optic (MO) coupling. Numerical computations comparing bandwidth for MO coupling between a single-YIG-film geometry and a double-YIG-film geometry are presented.
Factors suppressing transport critical current in Ag/Bi2223 tapes
Authors: Osamura K. , Nonaka S. , Matsui M. , Oku T. , Ochiai S. , Hampshire D. P.
Published: July 2002

The microstructural factors limiting the critical current density Jc have been investigated for the silver-sheathed Bi2223 tapes prepared by the powder-in-tube technique. In a hierarchy among them, which factor is the most effective in determining Jc depends on how extensively each of them occurs. The Jc was found to increase when the volume fraction of 2223 phase increases, suggesting the importance of the connectivity among 2223 grains. The Jc also increased with decreasing amount of residual carbon. The temperature and magnetic field dependences of Jc indicated characteristics of Josephson currents through superconducting–normal–superconducting barriers. After a discussion based on the alternative current limiting mechanisms proposed up to now, it is suggested that the homophase grain boundary limits the Jc.
AuGePt ohmic contact to n-type InP
Authors: Huang Wen Chang , Lei Tan Fu , Lee Chung Len
Published: July 2002

This work studied the AuGe to n-InP/ohmic contact system with the addition of Pt. It studied two contact schemes, i.e., Au/Ge/Pt/n-InP and Au/Pt/Au/Ge/n-InP. The specific contact resistances, the thermal stability, as well as the microstructure of the system, were systematically studied with the aid of scanning electron microscopy, Auger electron spectroscopy, Rutherford backscattering analysis, and x-ray diffraction. The processing condition to achieve the optimum contact resistance was also sought and it was found that a specific contact resistance of 2.15×10−6 Ω cm2 could be obtained if the Au/Ge/Pt/n-InP contact system was rapid thermal annealed at 550 °C for 30 s. The system was also found to be able to achieve low specific contact resistances for a wide range of anneal temperature from 400 to 550 °C. It was found that the Pt existence in-between AuGe and InP improved the surface morphology, the contact interface uniformity, and the thermal stability. The system could withstand a thermal aging at 400 °C for 80 h with only a minimal increase on the specific contact resistance.
Neutron diffraction studies of NdnFem−x−yVxAly [(n,m)=(1,12),  (2,17),  (3,29)]
Authors: Hu Z. , Yelon W. B. , James W. J.
Published: July 2002

Several NdnFem−x−yVxAly [(n,m)=(1,12), (2,17), (3,29)] samples were prepared and analyzed using neutron powder diffraction. Rietveld analysis of the neutron diffraction data indicates that the V and Al substituents take those sites with similar environments in all three phases, as observed in our previous study of Ti-substituted compounds. It was confirmed that the diffraction data of the 3:29 compound can be better refined using the A2/m space group than using the P21/c space group. The SQUID measurements show that all samples have Curie temperatures well above room temperature. The neutron diffraction results show that the easy direction is along the a axis for the 3:29 compounds, along the c axis for the 1:12 compounds and in the a-b plane for 2:17 compounds, respectively. The average site magnetic moments, the metal–metal bond lengths and the unit cell parameters of these compounds are compared with those of the Ti-substituted compounds.
Pulsed laser deposition of hydrogenated amorphous diamond-like carbon films from a polymer target
Authors: Li Z. F. , Yang Z. Y. , Xiao R. F.
Published: July 2002

Hydrogenated amorphous diamond-like carbon (a-DLC:H) films have been prepared by pulsed laser ablation of a polymer target in the presence of hydrogen (H2) gas. The growth rate of these films was found to be highly dependent on the deposition temperature and the amount of H2 gas used during the deposition process. The optical band gap of these a-DLC:H films changed in a wide range (0.25–2.5 eV) with substrate temperature and H2 pressure, and was found to be highly correlated to the ratio of sp3/sp2 in the film. The gap reached a maximum in a temperature range between 100 and 200 °C and had small values at either lower or higher temperatures. With increasing H2 pressure, the band gap increased monotonically and saturated around 0.5 mbar. It was found that the refractive index and the imaginary part of dielectric constant of these a-DLC:H films monotonically increased with decreases in the wavelength of the probing beam. It was also found that these laser derived a-DLC:H films could be made either completely insulating [∼10−11 (Ω cm)−1] or conducting [∼1 (Ω cm)−1] by simply varying the deposition temperature and H2 pressure. Very little photoconduction has been observed in these a-DLC:H films.
The effect of excess gallium vacancies in low-temperature GaAs/AlAs/GaAs:Si heterostructures
Authors: Kisielowski C. , Calawa A. R. , Liliental-Weber Z.
Published: July 2002

This article shows that the presence of low-temperature-grown GaAs (LT-GaAs) in LT-GaAs/AlAs/GaAs:Si heterostructures increases the Al/Ga interdiffusion at the heterostructure interfaces. The interdiffusion enhancement is attributed to the presence of Ga vacancies (VGa) in the As-rich LT-GaAs, which diffuses from a supersaturation of VGa frozen-in during sample growth. Chemical mapping, which distinguishes between the AlAs and GaAs lattices at an atomic scale, is used to measure the Al concentration gradient in adjacent GaAs:Si layers. A correlation is observed between the Al/Ga interdiffusion and the gate breakdown voltage in metal-insulator field-effect transistor structures containing LT-GaAs.
Numerical investigation of one-dimensional tunnel junction arrays at temperatures above the Coulomb blockade regime
Authors: Hirvi K. P. , Paalanen M. A. , Pekola J. P.
Published: July 2002

Arrays of tunnel junctions provide simple thermometric parameters in the limit where thermal excitations dominate over charging effects. We present numerical simulations for calculating the current versus voltage characteristics of an arbitrary one-dimensional array at arbitrary temperatures on the premise of the ‘‘orthodox theory.’’ The purpose of the computer simulations is to investigate the suitability of tunnel junction arrays for thermometry at low temperatures when the analytical formulas do not hold and, specifically, to see the effect of background charges in this regime.
Electron interaction and spin accumulation in spin polarized tunneling
Authors: Chui S. T.
Published: July 2002

We discuss the effect of electron interaction and spin accumulation on spin polarized tunneling. Charges of opposite magnitude are induced on opposite sides of the insulator. There is a splitting between the spin up and spin down bands which produces an additional contribution to the magnetoresistance. The ratio between the splitting and the current is an effective resistance, Rs. The difference between Rs on opposite sides of the junction is of the order of (much less than) the interfacial resistance when the magnetization of the ferromagnets are parallel (antiparallel) to each other. The signal to background ratio for the change in Rs between the parallel and the antiparallel configuration is much bigger than that for the resistance. This Rs (kΩ) is much larger in magnitude than that (≊10−5 Ω) observed in metallic trilayers and thus maybe of practical interest for applications.
Structural analysis of infinite layer superlattices grown by pulsed laser deposition
Authors: Del Vecchio A. , Tapfer L. , Aruta C. , Balestrino G. , Petrocelli G.
Published: July 2002

In this work we investigate the structural properties of SrCuO2/CaCuO2 infinite layer superlattices by high-resolution x-ray diffraction and x-ray specular reflectivity measurements. The infinite layer superlattices are grown by pulsed laser deposition on slightly misoriented (001) SrTiO3 substrates. We demonstrate that good quality superlattices with few monolayers thick constituent SrCuO2 and CaCuO2 layers can be grown having an interface roughness of less than 3–4 Å. A strain analysis of the epitaxial film shows that the SrCuO2 layers are completely relaxed with respect to the substrate. However, the CaCuO2 layers are elastically strained with respect to the SrCuO2 layer. The Poisson ratio of the CaCuO2 is estimated to be 0.40±0.08.
InP/GaAs self-assembled nanostructures: Modelization and experiment
Authors: Berti M. , Drigo A. V. , Giuliani A. , Mazzer M. , Camporese A. , Rossetto G. , Torzo G.
Published: July 2002

The initial stages of the three-dimensional metal organic vapor phase epitaxy growth of InP/GaAs have been studied by atomic force microscopy and Rutherford backscattering. The results are compared with the predictions of an equilibrium model that predicts an in-plane critical size for island formation. At low growth rates the model fits well the experiments while it needs to be further developed to include kinetic effects at higher growth rates. The experiments indicate a Stranski–Krastanow growth mode with a critical thickness of 2.1 ML.