bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-dimensional magnetic phase transition of ultrathin iron films on Pd(100)
Authors: Liu C. , Bader S. D.
Published: July 2002

Epitaxial Fe films grown on Pd(100) are used to study monolayer magnetism, critical behavior, and surface magnetic anisotropy, by means of in situ surface magneto-optical Kerr-effect measurements. Auxiliary LEED-Auger observations in 10−11 Torr vacuum are used to characterize the (1×1) epitaxy and the layer-by-layer film-growth mode. Ferromagnetic hysteresis loops were detected for all Fe thicknesses from 0.6–4 monolayers (ML) with the TC monotonically increasing with thickness, independent of the easy-axis orientation. The easy axis is perpendicular to the film plane below a critical thickness of 2.5 ML for 100-K film growth, and reorients in-plane above this thickness, and for all thicknesses for films grown at 300 K. The temperature dependence of the magnetization was obtained from the height of the Kerr loops in the remanent state and used to extract an effective magnetization exponent β for different film thicknesses and spin orientations. A value of β=0.127±0.004 is reported for a 1.2-ML Fe film with perpendicular spin orientation from a log-log plot for T>0.9TC, in agreement with the value of the theoretical two-dimensional Ising critical exponent βc=1/8=0.125.
Laser-induced chemical vapor deposition of AlN films
Authors: Li Xin , Tansley T. L.
Published: July 2002

AlN intended for metal-insulator-semiconductor applications has been prepared by laser activated chemical vapor deposition from ammonia and trimethylaluminum precursors. The films are high-density polycrystalline with 〈100〉 preferential orientation, and there is no evidence of the presence of Al2O3. A consistent band gap of 6.00±0.03 eV is obtained for film thickness in the range 0.3–1.2 μm. Conductivity is below 5×10−14 Ω−1 cm−1 at room temperature and is thermally activated with a range of excitation energies between 0.6 and 0.8 eV. The breakdown electric field is about 3×106 V/cm, while the relative permittivity varies between 7.8 and 8.2 in the frequency range 100 Hz–13 MHz.
Magnetic properties of bcc Co films
Authors: Bland J. A. C. , Bateson R. D. , Riedi P. C. , Graham R. G. , Lauter H. J. , Penfold J. , Shackleton C.
Published: July 2002

The magnetic properties of epitaxial bcc Co films of thickness 10–100 Å have been investigated using the surface magneto-optic Kerr effect (SMOKE), polarized neutron reflection (PNR), and nuclear magnetic resonance (NMR). The in-plane coercivity is found to vary strongly with thickness and a large magnetocrystalline anisotropy develops in-plane at 60 Å which is maintained in thicker films. PNR measurements on a Au-coated 100-Å bcc Co film at 300 K are consistent with a layer averaged magnetic moment per atom of 1.4 μB and a magnetization profile within 50 Å of the GaAs interface. NMR measurements on a 75-Å bcc Co film at 4.2 K yield the center frequency consistent with a moment per atom of 1.4 μB.
Coercivity of domain-wall motion in thin films of amorphous rare-earth–transition-metal alloys
Authors: Mansuripur M. , Giles Roscoe , Patterson George
Published: July 2002

Computer simulations of a two-dimensional lattice of magnetic dipoles are performed on the Connection Machine. The lattice is a discrete model for thin films of amorphous rare-earth–transition-metal alloys with application to erasable optical data-storage systems. Simulated dipoles follow the dynamic equation of Landau, Lifshitz, and Gilbert under the influence of an effective magnetic field arising from local anisotropy, near-neighbor exchange, classical dipole-dipole interactions, and externally applied fields. By introducing several types of defects and inhomogeneities in the lattice, we show that the motion of domain walls can be hampered in various ways and to varying degrees.
Effect of atomic environment on the 57Fe hyperfine field in Fe2MnSi
Authors: Goto T. , Kashiwakura A. , Ido H.
Published: July 2002

Fe2MnSi forms the Heusler-type L21 structure with some degree of preferential DO3 disorder, in which about 10% of the Mn sites (c sites) are occupied by iron atoms. The observed Mössbauer spectrum of Fe2MnSi at 5.5 K is considered to be composed of four kinds of sextets. This spectrum can be well explained by a model based on the effect of atomic environment. The hyperfine field of 57Fe atoms on the Fe sites (b, and d sites) is considered to vary with the number of neighboring iron atoms n. They are 64, 116, and 158 kOe for n=0, 1, and 2, respectively. The field of the 57Fe on the c-sites with about eight neighboring iron atoms is 287 kOe. It may be concluded that the iron atoms without their neighboring iron atoms have a very small, or nearly zero, magnetic moment, from the empirical relation between magnetic moment and magnetic hyperfine field.
Magnetic properties of Pd/Co multilayers
Authors: Bennett W. R. , England C. D. , Person D. C. , Falco C. M.
Published: July 2002

Measurements of the magnetic anisotropy of a series of Pd/Co multilayers grown by magnetically enhanced dc triode sputter deposition revealed an interface contribution of 0.53 erg/cm2 and a net volume anisotropy of −13.2×108 erg/cm3. A characteristic reduction of perpendicular anisotropy for ultrathin Co layers (<4 Å) is explained by using a pair interaction model to evaluate a thickness dependence of the interface anisotropy. For Pd-Co interfaces with perfectly sharp composition modulation an interface anisotropy of 0.79 erg/cm2 is predicted. By modeling the magnetization behavior a domain-wall energy density of 2±0.5 erg/cm2 is obtained for Pd/Co (2 Å) multilayers with Pd thicknesses in the range of 4–20 Å. For perpendicular Pd75Co25 alloy thin films the same model gives domain-wall energy densities in the range of 1–2 erg/cm2. For ultrathin Co layers the interlayer Co–Co coupling range is ∼20 Å. Temperature dependence of saturation magnetization of a Pd(11 Å)/Co(7.8 Å) multilayer demonstrates complete mixing of Pd and Co by 380 °C.
Magnetic hardening of (Sm,Zr)Fe3 alloys
Authors: Wecker J. , Katter M. , Schnitzke K. , Schultz L.
Published: July 2002

The hard magnetic properties of Sm27−xZrxFe73 alloys, having (Sm,Zr)Fe3 as the main phase (PuNi3 crystal structure), are investigated for 0 ≤x≤ 19. The easy magnetization direction is always directed along the c-axis up to the Curie temperature of about 385 °C. The anisotropy field, determined by the singular point detection (SPD) method, is found to be 14 T at room temperature for x<7. Highly coercive samples are prepared by mechanical alloying or rapid quenching, respectively. Binary SmFe3 shows a coercivity, Hci, of 8.5 kA/cm and a remanence, Jr, of 0.43 T. Maximum coercivity, Hci=10.8 kA/cm (Jr=0.43 T), is measured for mechanically alloyed Sm20Zr10Fe70. For a higher Zr content the coercivity decreases like the anisotropy field whereas the remanence can be improved up to 0.53 T.
Time-resolved spectroscopic reflection measurements in shock-compressed materials
Authors: Gustavsen R. , Gupta Y. M.
Published: July 2002

An experimental method has been developed for obtaining time-resolved reflection measurements on shock-compressed materials in the 300–500 nm spectral range. The sample, contained between transparent windows, is shocked and broadband light from a xenon flashlamp is reflected off the sample and collected by optical fibers. One fiber guides the reflected light to a system consisting of a spectrograph, a streak camera, a vidicon detector, and an optical multichannel analyzer. This system records spectra with 50 ns time resolution and 6 nm wavelength resolution. With a more sensitive detector, higher time and spectral resolution can be obtained. Additional fibers transport reflected light through narrow bandpass filters to photomultiplier tubes to provide higher time resolution (<5 ns). Results of an experiment on carbon disulfide are presented which indicate an increase in reflectivity at 300 nm from less than 0.33% at ambient pressure to ∼10% at 105 kbar; at longer wavelengths the changes are smaller. These results are consistent with existing absorption measurements which indicate large increases in the absorption strengths and bandwidths of absorption bands located near 320 nm.
A modified harmonic oscillator approximation scheme for the dielectric constants of AlxGa1−xAs
Authors: Terry Fred L.
Published: July 2002

The dielectric functions of AlxGa1−xAs have recently been measured for several Al mole fractions over the 1.5–6.0 eV wavelength range [D.E. Aspnes, S. M. Kelso, R. A. Logan, and R. Bhat, J. Appl. Phys. 60, 754 (1986)]. To make use of this data to perform optical modeling for spectroscopic ellipsometry analysis of AlxGa1−xAs-containing samples, and for other optical modeling purposes, a reasonable interpolation scheme is required to estimate the dielectric functions of intermediate compounds. In this work, we will present a modified version of the harmonic oscillator approximation (HOA) of Erman et al. [M. Erman, J. B. Theeten, P. Chambon, S. M. Kelso, and D. E. Aspnes, J. Appl. Phys. 56, 2664 (1984)] to model the experimental data and interpolate between the known compositions over the 1.5–5.0 eV range. Our model uses additional harmonic oscillators and allows each oscillator to have an independent phase. These modifications significantly improve the accuracy of the approximation for photon energies at and below the fundamental band-gap energy. This allows much more accurate modeling of reflection problems for multilayer GaAs/AlGaAs structures. We will present test of this approach with simulations of spectroscopic ellipsometry data using known data, and with measured spectroscopic ellipsometer data on AlxGa1−xAs-containing samples grown by molecular-beam epitaxy and organometallic chemical vapor deposition.
Plasmon excitation and self-coupling in a bi-periodically modulated two-dimensional electron gas
Authors: Wilkinson R. J. , Ager C. D. , Duffield T. , Hughes H. P. , Hasko D. G. , Ahmed H. , Frost J. E. F. , Peacock D. C. , Ritchie D. A. , Jones G. A. C. , Whitehouse C. R. , Apsley N.
Published: July 2002

We report far infrared (FIR) studies of plasmons in spatially modulated two-dimensional electron gases (2DEGs) in AlGaAs/GaAs heterostructures using biased overlaid metal gratings, including interdigitated gratings, both as optical couplers and as spatially modulating gates. Comparison of the experimental results with the predictions of scattering matrix calculations of the FIR response of a modulated 2DEG in the presence of a perfectly conducting lamellar grating allow us to deduce the spatial variation of the number density distribution in the 2DEG as a function of grating bias. For the interdigitated grating gates, the 2DEG can be modulated at a period of twice that of the grating fingers by differentially biasing alternate fingers; 2D plasmon resonances have been observed at half-integral values of the grating wave vector G, corresponding to the electrically induced periodicity of the 2DEG modulation itself acting as an optical coupler in addition to the metal grating. The observed G/2 plasmon frequencies decrease with increasing amplitude of the 2DEG number density modulation, in quantitative agreement with those obtained from scattering matrix calculations of the optical response of a modulated 2DEG under a perfectly conducting lamellar grating; calculations of the oscillating charge density profiles show that this occurs because, as the modulation amplitude increases, the oscillation becomes localized in regions of low 2DEG number density which are also under one of the sets of grating fingers, and is therefore better screened.
Direct measurement of polymer temperature during laser ablation using a molecular thermometer
Authors: Sandy Lee I.-Yin , Wen Xiaoning , Tolbert William A. , Dlott Dana D. , Doxtader Mark , Arnold Dana R.
Published: July 2002

Photothermal laser ablation is studied using poly-(methyl methacrylate) films doped with a dye, IR-165, which functions as a molecular heater and thermometer. Direct optical measurements of temperature are performed on samples heated by 100 ns near-IR pulses at 1.064 μm, at rates dT/dt≊5×109 deg/s. Below ablation threshold, the heat capacity measured by optical calorimetry is precisely the value obtained by conventional calorimetry. At ablation threshold, the peak surface temperature is Tabl=600 °C. The increase in heat capacity observed above threshold, together with the results of a conventional thermal analysis, is used to determine the weight fraction of material decomposed at ablation time χth=0.02. With increasing pulse energy, the fraction decomposed increases and a more forceful ablation is observed, but the surface temperature does not continue to increase past Tlim=715 °C, which is determined to be the limiting temperature for thermal decomposition.
Deuterium interactions with ion-implanted SiO2 layers in silicon
Authors: Myers S. M. , Brown G. A. , Revesz A. G. , Hughes H. L.
Published: July 2002

The interactions of deuterium gas (D2) with implantation-formed Si-SiO2-Si structures were investigated in the temperature range 500–1000 °C using nuclear-reaction analysis. At 600 °C and above there is substantial permeation of D through the overlying Si layer, and this D becomes stably bound within the buried SiO2. The reactions within the implantation-formed oxide differ significantly from those in thermal oxides and are characterized by two distinct time regimes. In the first regime there is a rapid transient reaction which, at least at 600 °C, is controlled by permeation through the Si overlayer. After the initial transient there is a much slower reaction within the oxide, prominent at 700 °C and higher, which is controlled by a reaction barrier. The results indicate that the implantation-formed oxides contain more numerous hydrogen-reactive sites than do silica or thermal oxides on Si, and further that the number of these sites is substantially increased by Si epitaxial deposition. Deuterium permeation through the Si overlayer was used to determine the permeability of hydrogen in Si at 600 °C for the first time.
Giant magnetoresistance with current perpendicular to the layer planes of Ag/Co and AgSn/Co multilayers (invited)
Authors: Pratt W. P. , Lee S.-F. , Yang Q. , Holody P. , Loloee R. , Schroeder P. A. , Bass J.
Published: July 2002

A two-channel model of electronic conduction is shown to provide a satisfactory description of the magnetoresistances of Ag/Co and AgSn/Co multilayers with the current flowing perpendicular to the layer planes. A combination of data for as-prepared samples at magnetic field H=0 (for which the magnetization M≊0), and for the same samples in the ferromagnetically ordered state, is used to extract ‘‘best’’ spin-orientation-dependent parameters of αCo=2.9(+0.5,−0.3) and αCo/Ag=12(+5,−2) from Ag/Co data and αCo=2.4(+1,−0.7) and αCo/AgSn=11(+7,−4) from AgSn/Co data.
Friction and wear of plasma-deposited diamond films
Authors: Miyoshi Kazuhisa , Wu Richard L. C. , Garscadden Alan , Barnes Paul N. , Jackson Howard E.
Published: July 2002

Reciprocating sliding friction experiments in humid air and in dry nitrogen and unidirectional sliding friction experiments in ultrahigh vacuum were conducted with a natural diamond pin in contact with microwave-plasma-deposited diamond films. Diamond films with a surface roughness (R rms) ranging from 15 to 160 nm were produced by microwave-plasma-assisted chemical vapor deposition. In humid air and in dry nitrogen, abrasion occurred when the diamond pin made grooves in the surfaces of diamond films, and thus, the initial coefficients of friction increased with increasing initial surface roughness. The equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. In vacuum the friction for diamond films contacting a diamond pin arose primarily from adhesion between the sliding surfaces. In these cases, the initial and equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. The equilibrium coefficients of friction were 0.02–0.04 in humid air and in dry nitrogen, but 1.5–1.8 in vacuum. The wear factor of the diamond films depended on the initial surface roughness, regardless of environment; it increased with increasing initial surface roughness. The wear factors were considerably higher in vacuum than in humid air and in dry nitrogen.
Atomic imaging of an InSe single-crystal surface with atomic force microscope
Authors: Uosaki Kohei , Koinuma Michio
Published: July 2002

The atomic force microscope was employed to observed in air the surface atomic structure of InSe, one of III-VI compound semiconductors with layered structures. Atomic arrangements were observed in both n-type and p-type materials. The observed structures are in good agreement with those expected from bulk crystal structures. The atomic images became less clear by repeating the imaging process. Wide area imaging after the imaging of small area clearly showed that a mound was created at the spot previously imaged.
An investigation of as-implanted material formed by high dose 40 keV oxygen implantation into silicon at 550 °C
Authors: Li Y. , Kilner J. A. , Chater R. J. , Nejim A. , Hemment P. L. F. , Tate T. J.
Published: July 2002

Device grade 〈100〉 single crystal silicon wafers have been implanted with 40 keV oxygen ions (16O+) over the dose range of 1×1017–8×1017/cm2 at a temperature of 550±10 °C. Transmission electron microscopy, ion channeling, and secondary ion mass spectroscopy studies show that during implantation the critical dose required to form a buried oxygen-rich amorphous (SiOx, x<2) layer is lower than 1×1017 O+/cm2. As the dose increases from 1×1017 to 4×1017/cm2 the thickness of the buried SiOx layer increases and there is a corresponding decrease in the thickness of the single crystal silicon top layer, with the oxygen concentration and residual radiation damage playing important roles in determining its position and thickness. A dose of 5×1017/cm2 results in a continuous surface amorphous layer, with a buried SiO2 sublayer being formed in the region corresponding to the implanted oxygen peak. With further increasing dose, the buried SiO2 sublayer grows primarily towards the surface. The results for the sample implanted at a dose of 1×1017/cm2 show that surface defects can be attributed to agglomeration of interstitial silicon atoms created by the internal oxidation process.
On analytic modeling of casing and liner thickness variations in a shaped charge
Authors: Kelly R. J. , Curtis J. P. , Bremer M.
Published: July 2002

The jet from a shaped charge is formed by the collapse of its typically conical liner under the high pressures resulting from a detonation front. Nominally the charge is axisymmetric and the jet travels along the axis of symmetry of the charge. However, when asymmetries are present in the shaped charge the velocities imparted to liner elements at a given axial distance from the cone tip will be different. Thus the jet formation process becomes asymmetrical and a nonzero off-axis component of the jet velocity is generally produced. Such an off-axis component can cause considerable degradation in the penetration achieved by the jet. In this article a recent novel approach to the modeling of shaped charge liner collapse with constant liner projection velocities in the presence of asymmetries is generalized to include Randers–Pehrson acceleration of the liner elements. The Gurney formula for an asymmetric closed sandwich is used for the limiting liner element speed in the Randers–Pehrson model and it is shown how the required mass elements may be calculated in a shaped charge application. Simple illustrative models are used to describe thickness variations in the shaped charge casing and liner.
A complete characterization of trapping levels in red mercuric iodide single crystals
Authors: Sharma S. L. , Pal T. , Acharya H. N.
Published: July 2002

Photon stimulated current measurements have been performed on red mercuric iodide single crystals in order to determine the trap energy, trap density, and capture cross section for different trapping levels that may be present in the platelet-shaped single crystals of red mercuric iodide grown by polymer controlled growth technique in vapor phase. The procedure of analysis of these measurements is outlined in brief. The analysis has shown that these crystals in general possess four trapping levels: two electron trapping levels with energies 0.10 and 0.31 eV and two hole trapping levels with energies 0.15 and 0.59 eV. The analysis has further shown that the total trap density is only of the order of 1013/cm3, indicating the superiority of this growth technique over other techniques. The analysis has also shown that all four trapping levels are of retrapping type. Finally, the possible mechanisms of formation of these trapping levels and the ways in which these trapping levels will influence the detector operation at room temperature are discussed.
Magnetic properties of heterotrophic bacteria (abstract)
Authors: Verkhovceva Nadezda V. , Glebova Irina N. , Romanuk Anatoly V.
Published: July 2002

The magnetic properties (magnetic susceptibility and saturation magnetization) of six species of heterotrophic bacteria were studied: alcaligenes faecalis 81, arthrobacter globiformis BKM 685, bacillus cereus 8, leptothrix pseudo-ochracea D-405, proteus vulgaris 14, and seliberia stellata. It has been shown that the magnetic properties of bacteria depend on (1) the peculiarity of the micro-organism (species-specific and connected with cultivation conditions); (2) the source of the iron in the media. Most of the bacteria are diamagnetic in media with a minimum of iron (χ∞=−7.2−0.3×10−6 sm3/g). The spore forming species (bacillus cereus) has increased diamagnetism. Usually the bacteria are paramagnetic in iron-containing media because they concentrate into Fe compounds. The paramagnetism of the iron-concentrating species (anthrobacter globiformis −χpar=2.4×10−6, leptothrix pseudo-ochtracea χpar=11.0×10−6 and seliberia stellata χpar=3.2×10−6 sm3/g) depends, in general, on magnetically ordered compounds. Iron compounds not accumulated by proteus vulgaris and these species are always diamagnetic .
Role of defects on electron transport through semiconductor barriers
Authors: Chaabane H. , Bourgoin J. C.
Published: July 2002

We describe the effect of electron irradiation on the current-voltage characteristics of nonintentionally doped GaAlAs barriers imbedded in n-type doped GaAs. It is found that below typically 100 K where the Fowler–Nordheim tunneling applies, tunneling switches from the lowest (Γ) band to a higher (X) one after a fluence of few 1016 electron cm−2. At high temperature a defect assisted process involving tunneling into defect sites followed by the thermal ionization of electrons into the barrier conduction band, dominates. This process is also modified by the irradiation, the native defects being replaced by the electron induced defects.
70 K hysteretic Josephson effect in mesas patterned on (Bi,Pb)2Sr2CaCu2Oy single crystals
Authors: Régi F. X. , Schneck J. , Palmier J. F. , Savary H.
Published: July 2002

We report the engineering of sets of mesas on lead-substituted single crystals of Bi2Sr2CaCu2Oy. Each mesa behaves as a series array of intrinsic Josephson junctions with, up to 70 K, ‘‘two states’’ hysteretic I(V) characteristics, free of branching, whose RnIc products can be tuned through geometrical patterning and adjustment of the lead contents. RnIc values as high as 550 mV at 4.2 K have been reached. The tuning through lead substitution is attributed to the decrease of the anisotropy with increasing lead content.
Theory of magnetostriction with application to Terfenol-D
Authors: James R. D. , Kinderlehrer D.
Published: July 2002

A mechanism for magnetostriction in the highly magnetostrictive material Terfenol-D is explained in detail. This mechanism is based on a theory of magnetostriction [R. D. James and D. Kinderlehrer, Philos. Mag. B 68, 237 (1993)] that is particularly suited to predictions of the macroscopic behavior of materials that exhibit large magnetostriction. Some experiments that test these predictions are proposed.
Simple analysis of transient photoconductivity for determination of localized-state distributions in amorphous semiconductors using Laplace transform
Authors: Naito Hiroyoshi , Okuda Masahiro
Published: July 2002

A method has been proposed for the determination of localized-state distributions in amorphous semiconductors from transient photoconductivity using Laplace transforms [H. Naito, J. Ding, and M. Okuda, Appl. Phys. Lett. 64, 1830 (1994)]. The method is applicable to both pre- and postrecombination regimes of transient photoconductivity, and to amorphous semiconductors exhibiting either nondispersive or dispersive transport. To simplify the analysis of the method, a new approach with the same advantages of the method is presented.
Laser induced damage in zinc tris(thiourea) sulfate and bis(thiourea) cadmium chloride
Authors: Venkataramanan V. , Subramanian C. K. , Bhat H. L.
Published: July 2002

Laser induced damage threshold values are reported for two metal-organic complex crystals, zinc tris(thiourea) sulfate, and bis(thiourea) cadmium chloride. These crystals have single shot and multiple shot damage thresholds which are the highest among the solution grown crystals. The damage morphology, obtained by irradiating with fluences close to the threshold, reflects the symmetry of the face of the test site. At higher fluences the damage leads to stress-induced fracture or thermo-chemical degradation.
Application of quadrupole ion trap for the accurate mass determination of submicron size charged particles
Authors: Hars Gyorgy , Tass Zoltan
Published: July 2002

An optical method is developed for the precise mass determination of submicron size charged particles. The measurement is carried out in a hyperbolic quadrupole ion trap electrode system in vacuum by adjusting particle trajectory star shape patterns. This method does not involve gravity so this may be suitable to use in space applications in weightlessness. Due to the imperfect electrode alignment of the ion trap the mass/charge ratio can now be calculated by means of the corresponding electrical parameters with a relative accuracy of 103. Since the measured parameter is frequency the relative accuracy of the mass measurement could later be increased to 106. Combined with stepwise electron loss events induced by UV radiation it is possible to determine the mass of a single particle in the 109–1012 amu range with similar precision. These findings would seem to indicate the feasibility of a novel optical method for high precision mass determination of a single particle in the submicron diameter range.
Shift and deformation of the hysteresis curve of ferroelectrics by defects: An electrostatic model
Authors: Robels U. , Calderwood J. H. , Arlt G.
Published: July 2002

Lattice cells containing point defects in ferroelectric single crystals can behave as a second phase. In addition to the role these defects have in the variation of several chemical and physical properties of the ferroelectric, they modify those physical properties of the ferroelectric that are manifested in the hysteresis curve. These effects are: the shearing of the loop by all kinds of defects, the shifting of the hysteresis curve along the field axis (internal bias) by oriented polar defects, and the changing (mostly the increasing) of the coercive field strength. An electrostatic model is used to calculate the strength of these effects.
The growth of GaF3 films on GaAs(110) at elevated temperatures studied with soft x-ray photoelectron spectroscopy
Authors: Simpson W. C. , Durbin T. D. , Varekamp P. R. , Yarmoff J. A.
Published: July 2002

The growth of GaF3 films on GaAs(110) wafers via exposure to XeF2 is studied as a function of substrate temperature with soft x-ray photoelectron spectroscopy. For temperatures between 300 and 550 K, a GaF3 film forms with an interface to the substrate consisting of GaF, elemental As and possibly some AsF. In this temperature range, the films thicken with exposure up to a limit of ∼15 Å. The initial F uptake rate increases with elevated substrate temperature without altering the limiting thickness. In addition, there is little variation in the thickness or composition of the film–substrate interface over this temperature range. Above 550 K, XeF2 etches GaAs, leaving a GaAs surface covered with ∼1 monolayer of elemental As. A film growth mechanism is discussed to explain these results.
Finite size effects in nanoscale Tb particles
Authors: Johnson D. , Perera P. , O’Shea M. J.
Published: July 2002

Tb nanoscale particles are prepared in a matrix of Ti by sputtering. A range of samples are made with average particle diameter ranging from 1.5 to 21 nm. The magnetic transition temperature is depressed for smaller particles due to finite size effects. The coercivity at 4.5 K shows a maximum of 22 kOe at approximately 7 nm being a factor of 2.4 larger than a thick film of Tb. We compare the coercivity of our particle samples to multilayers of the form Tb/Mo and find larger coercivity in the particle systems due to the more confined geometry of the particles.
Gas-phase composition measurements during chlorine assisted chemical vapor deposition of diamond: A molecular beam mass spectrometric study
Authors: Rego C. A. , Tsang R. S. , May P. W. , Ashfold M. N. R. , Rosser K. N.
Published: July 2002

We have constructed a molecular beam mass spectrometer designed specifically to sample gases from a diamond chemical vapor deposition (CVD) process chamber thereby enabling characterization of the gas-phase CVD environment. With this in situ diagnostic technique we have obtained quantitative measurements of the composition of the gas-phase species as a function of filament temperature for a variety of C/H/Cl gas mixtures. The precursor mixtures used were 1% of a chloromethane (CH4−nCln, n=1-4) in hydrogen and 1% CH4 in hydrogen with added Cl2 varying from 1% to 4%. At filament temperatures optimum for diamond growth (∼2300°C) the relative CH4/C2H2/C2H4 product distribution measured in the gas mixture is remarkably similar to that established when CH4 is the carbon precursor species. At these filament temperatures almost all the chlorine is reduced to HCl, its concentration being proportional to the Cl fraction in the source gas, regardless of the form of the chlorine in the source gas mixture. Compositional analysis of the as-grown diamond films indicated that no chlorine was present in the bulk of the films, though trace amounts of chlorine were detected on the film surface. From these observations we surmise that chlorine atoms are involved in the gas-surface reactions which produce active growth sites on the diamond surface.
Specific heat of R3Co (R: La,Pr, and Nd) (abstract)
Authors: Umehara I. , Nakano K. , Lu Q. F. , Sato K.
Published: July 2002

It was reported that La3Co is an interesting superconductor which has a critical temperature Tc of 4.5 K. Pr3Co and Nd3Co are ferromagnetic materials, which have Curie temperatures of 10 and 25 K, respectively. It was claimed that their magnetic structures are associated with the canting moment reflected to the symmetry of their crystal structure. In this paper we present the results of the specific heat measurement in these compounds in the temperature range from 2 to 30 K by a standard adiabatic heat pulse method. Two phase transitions are observed at 8 and 25 K in Nd3Co. One of them at 25 K corresponds to the Curie temperature mentioned above. Another is the transition corresponds to the moment reorientation, which was observed in the thermomagnetic curves and electrical resistivity measurement. This results claim that the moments are canted in this compounds. Magnetic entropy reaches 90% of R ln 2 at 25 K. However, it increases above the Curie temperature because the crystalline electric field effects plays an important role in this compounds. There are two peaks of 10 and 12 K in Pr3Co. The above temperature corresponds the Curie temperature but another was unknown. Thus we performed the specific heat measurement for Pr7Co3, which is very close to the Pr3Co in the phase diagram. We could observe the phase transition at 12 K for Pr7Co3 in the specific heat measurement and electric resistivity measurement. It is clear that the phase transition at 12 K comes from impurity of Pr7Co3 in our Pr3Co compound. We prepare the good quality sample of Pr3Co and try to examine the specific heat measurement.
Electronic and related properties of crystalline semiconducting iron disilicide
Authors: Filonov A. B. , Migas D. B. , Shaposhnikov V. L. , Dorozhkin N. N. , Petrov G. V. , Borisenko V. E. , Henrion W. , Lange H.
Published: July 2002

Band structure calculations for β-FeSi2 have been performed by the linear muffin-tin orbital method within the local density approximation scheme including exchange and correlation effects. A detailed analysis of the conduction and valence band structure around high-symmetry points has shown the existence of a quasidirect band gap structure in the material. It is experimentally confirmed that between the threshold energy of optical interband transition of 0.73 eV and the first direct gap transition with appreciable oscillator strength at about 0.87 eV there is a region in which direct transition of low oscillator strength and indirect transitions overlap. That explains the tricky behavior of β-FeSi2 in experimental investigations demonstrating it to be either a direct or indirect gap semiconductor.
Defect states responsible for leakage current in Ta2O5 films on Si due to Si contamination from the substrate
Authors: Lau W. S. , Khaw K. K. , Qian P. W. , Sandler N. P. , Chu P. K.
Published: July 2002

A defect state, defect B, was found in Ta2O5 after postdeposition annealing in O2 by a novel zero-bias thermally stimulated current spectroscopy technique. The activation energy ET was estimated to be about 0.3 eV. Evidence is given that defect B is a hole trap. We believe that defect B is an acceptor level in Ta2O5 due to Si substituting for Ta. The presence of Si contamination in Ta2O5 due to diffusion of Si from the Si substrate into Ta2O5 was confirmed by secondary-ion-mass spectrometry.
On the existence of a distribution of barrier heights in Pd2Si/Si Schottky diodes
Authors: Chand Subhash , Kumar Jitendra
Published: July 2002

The current–voltage characteristics of Pd2Si based Schottky diodes on both n- and p-type silicon measured over a wide temperature range (52–295 K) have been interpreted on the basis of thermionic emission-diffusion mechanism and the assumption of a Gaussian distribution of barrier heights. It is shown that while the occurrence of a distribution of barrier heights is responsible for the apparent decrease of the zero-bias barrier height (φb0) and nonlinearity in the activation energy plot, the voltage dependence of the standard deviation causes the unusual increase of ideality factor (η) at low temperatures. Also, it is demonstrated that the forward bias shifts the mean barrier height towards the higher side and causes narrowing of the distribution as well. A simple method, involving the use of φb0 vs 1/T data, is suggested to gather evidence for the occurrence of a Gaussian distribution of barrier heights and obtain values of mean barrier height and standard deviation. The experimental results correspond to a mean barrier height of 0.80 V, standard deviation 0.05 V, and ideality factor 1.21 for Pd2Si based Schottky barriers on n-type silicon; these values for p-type silicon are 0.38 V, 0.03 V, and 1.07, respectively.
Effect of the large magnetostriction of Terfenol-D on microwave transmission
Authors: Dewar G.
Published: July 2002

The calculated transmission of microwave power at GHz frequencies through Terfenol-D (Tb0.27Dy0.73Fe2) is strongly influenced by the enormous magnetostriction parameter of this material. For conditions satisfying ferromagnetic resonance there is a peak in the transmission corresponding to sound and spin waves transporting energy through a slab-shaped sample. The magnetostriction mixes the character of these waves; the calculated transmission also depends on the exchange stiffness parameter, the elastic constants, and the radio-frequency magnetic properties. With thin (<40 μm) samples for which the static magnetization is along the magnetically soft (111) direction the calculated transmission peak is augmented by a fringe pattern with more than 20 secondary peaks. This arises from interference between spin and sound waves. For the configuration with the static magnetization oriented along the magnetically hard (100) direction the fringe pattern is absent. In this configuration the spin wave is a soft mode and the sample should develop a static distortion of the magnetization and lattice. The periodicity of this static distortion is tunable with an external magnetic field, although lattice imperfections may introduce hysteresis by pinning the distortion.
Search for point-contact giant magnetoresistance in Co/Cu multilayers
Authors: Tsoi M. V. , Jansen A. G. M. , Bass J.
Published: July 2002

We report the first measurements of the point-contact magnetoresistance (MR) of magnetic multilayers. These measurements were intended to see if the additional local resistance associated with a small contact area between a sharpened tip and a magnetic multilayer shows a giant magnetoresistance (GMR) such as that seen in bulk multilayers. Point-contact measurements on two independent multilayers of Co/Cu revealed MRs dominated simply by that of the square resistance of the multilayer, with little or no contribution from the contact area itself. Further studies are still needed to determine if this result means that a small contact has little or no local GMR, or if the needle point-contact technique used destroyed the multilayer around the contact, thereby suppressing the GMR.
Preparation of ferrimagnetic Mn4N compound films at low substrate temperature by reactive facing target sputtering (abstract)
Authors: Nakagawa Shigeki , Li Yueng , Naoe Masahiko
Published: July 2002

It has become attractive to prepare films of transition metal nitride for both the studies of ferrimagnetism and their practical applications. Mn4N is well known as one of the typical ferrimagnetic materials with an effective moment of Bohr magnetons of 1.0–1.2 μB per unit cell. However, high temperature processing is required to synthesis such materials. We have succeeded in depositing Mn4N films at a relatively low substrate temperature Ts of around 300°C by reactive facing target sputtering (r-FTS).1 In this study, Al and Cu underlayers were deposited to enhance the epitaxial effect between (111) preferred texture in such layers and the (111) texture of Mn4N crystallites in order to deposit Mn4N films with better crystallinity at lower Ts. Mn4N films were deposited on Si wafer substrates by sputtering a pair of sintered Mn disks in a mixture of Ar and N2 gases. Total gas pressure was set at 2 mTorr and the ratio in pressure of N2 gas to the RN2 mixture ranged from 0% to 40%. Ts was varied in the range of 40–400°C. Al and Cu layers with thicknesses of about 500 Å were used as underlayers. Mn–N films with (111) oriented Mn4N crystallites could be prepared at RN2 of 10% and Ts above 270°C. Those films revealed the saturation magnetization 4πMs of about k40 emu/cc. Excessive RN2 and Ts above 400°C caused the formation of another phase nitride, such as Mn3N2, which caused the degradation of 4πMs. Basically, Mn atoms in Mn4N crystallites form fcc lattices. Since Al and Cu atoms form fcc lattices and their lattice constant is almost the same as that of Mn4N, Al and Cu thin layers were used as underlayers. Substrate biasing of about −160 and −70 V during deposition of Al and Cu layers, respectively, was very effective in attaining (111) orientation in those films. Mn nitride films deposited at Ts as low as about 200°C on (111) oriented Al underlayers were composed of (111) oriented Mn4N crystallites which revealed 4πMs of about 24 emu/cc at room temperature. (111) oriented Cu layers were not as effective in depositing Mn4N films at low Ts. Consequently, Al underlayers were suitable to deposit films composed of (111) oriented Mn4N crystallites at low Ts of around 200°C.
Interfacial reactions in oligo(arylene-vinylene)–metal systems
Authors: Nguyen T. P. , de Vos S. , Tran V. H.
Published: July 2002

X-ray photoelectron spectra of 4,4′-bis(4 styryl-styryl) benzene oligomer (4PV) films metallized with chromium and aluminum films were used to investigate the nature of 4PV–metal interfacial reactions. The changes in the core level spectra of carbon, metal, and oxygen indicated the formation of a carbide in a chromium covered oligomer and a metal–oxide carbon and/or carbide in aluminum covered films. Yet no apparent reaction was observed in the interfacial layer when depositing the oligomer film on the chromium layer. These reactions are used as a possible explanation for the electrical characteristics of diodes made from these materials.
Inverse CPP-GMR in (A/Cu/Co/Cu) multilayers (A=NiCr, FeCr, FeV) and discussion of the spin asymmetry induced by impurities
Authors: Vouille C. , Fert A. , Barthélémy A. , Hsu S. Y. , Loloee R. , Schroeder P. A.
Published: July 2002

We present CPP-GMR experiments on a series of multilayers of the type (A/Cu/Co/Cu)×N where A is a layer of NiCr, FeCr, or FeV alloys with concentrations of Cr or V between 2.5 and 30 at. %. The inverse GMR effects we observe can be accounted for by the opposite scattering spin asymmetries in Co (positive spin asymmetry) and A (negative spin asymmetry). As the spin asymmetry in A is positive for interface scattering and negative for bulk scattering, the inverse GMR is observed only for thicknesses greater than one at which interface and bulk scattering compensate. We determine the magnitude and sign of the spin asymmetry coefficients, and we notice the agreement with previous data on dilute alloys and with theoretical calculations.
Spatial texture distribution in thermomechanically deformed 2–14–1-based magnets
Authors: Lewis L. H. , Thurston T. R. , Panchanathan V. , Wildgruber U. , Welch D. O.
Published: July 2002

Quantifying the relationship between crystallographic texture and magnetic properties is highly desirable for engineering high-energy-product magnets. Such an undertaking is a challenge for melt-quenched magnets formed from Nd2Fe14B-based alloys because they possess a nanoscale microstructure complicated by the presence of multiple phases, both ferromagnetic and paramagnetic. Procedures for the evaluation of texture in permanent magnets often rely upon magnetic remanence measurements; however, such determinations are based on the use of the Stoner–Wohlfarth model, and are applicable only to assemblies of noninteracting particles, which nullifies their use in texture determinations of “exchange-spring” magnets. To overcome these inherent experimental difficulties, alternative techniques for the measurement of the bulk texture of permanent magnets are explored in this work. Crystallographic alignment was investigated in both neutron and hard x-ray diffraction experiments. Transmission synchrotron x-ray diffraction measurements of the rocking curve width as a function of both the vertical and horizontal positions were performed on a series of thermomechanically processed, melt-quenched magnets based on the Nd2Fe14B composition. The results of the hard x-ray diffraction experiments were verified by neutron diffraction work. A simple model based on a squared-Lorentzian distribution is used to obtain a quantitative estimation of the population of misoriented grains for a given rocking curve half-width. It is deduced that even the most deformed magnets still possess a significant degree of particle misalignment; over 50% of the constituent grains have a misorientation of at least 15° from the overall axial direction at the center of the magnet. The spatial distribution of the rocking curve widths indicates that the development of the c-axis texture is initiated in the center region of the magnet and progresses to the perimeter with increasing deformation. Samples with lower deformation levels possess more uneven texture distributions than do those with higher deformation levels, and the degree of orientation seems to develop rapidly near a deformation level of 50%. The results of this work are utilized to produce a phenomenological model, consistent with the findings of past researchers, of deformation based on mass transport under applied stress via a liquid grain-boundary phase. It is proposed that the micromagnetic structure consists primarily of relatively large clusters of exchange-coupled grains that result in lower-coercivity areas within the magnet. Recommendations are included for improving the production and processing of thermomechanically deformed melt-quenched magnets.
Interface roughness effects in the giant magnetoresistance in magnetic multilayers
Authors: Barnaś J. , Palasantzas G.
Published: July 2002

In-plane electronic transport in thin layered magnetic structures composed of two ferromagnetic films separated by a nonmagnetic spacer is analyzed theoretically in the Born approximation. Particular attention is paid to the role of interface roughness in the giant magnetoresistance (GMR) effect. The analysis applies to self-affine interfaces described by the k-correlation model. Our results show that GMR is sensitive to the roughness exponent H (0⩽H⩽1) in a manner that depends on spin asymmetries for bulk and interfacial scattering. The limit of low electron concentration is also considered.
Transient tunneling current in laser-assisted scanning tunneling microscopy
Authors: Lyubinetsky I. , Dohnálek Z. , Ukraintsev V. A. , Yates J. T.
Published: July 2002

The transient tunneling current induced by pulsed laser irradiation of a scanning tunneling microscope (STM) tunneling gap was observed to occur over a 100 μs time scale range in response to a 20 ns duration of the laser pulse. The amplitude of the transient current varies exponentially with laser power, confirming our previous suggestion that thermal expansion of the STM tip is the main source of the transient increase of tunneling current. This thermal expansion mechanism is also supported by the observation of a qualitatively similar variation of the tunneling current during the piezo-driven decrease of the tip-sample separation.
Electron transport in a starburst oxadiazole
Authors: Bettenhausen J. , Strohriegl P. , Brütting W. , Tokuhisa H. , Tsutsui T.
Published: July 2002

In this article we report on the electron mobility of a starburst oxadiazole, 1,3,5 -[(4-tert.-butylphenyl)-2-oxadiazolyl] benzene. For direct evidence of the electron transport capability the charge drift mobility and its field dependence have been measured by the time-of-flight technique. The electron mobility showed a square root dependence of the electric field and was somewhat larger than that for PBD. In addition, the oxadiazole compound has been used as the electron transport layer (ETL) in light-emitting diodes (LEDs) together with poly(1,4-phenylene vinylene). The characteristics of these devices are compared to analogous LEDs with Alq3 as the ETL.
Hot carrier effects in AlGaAs/InGaAs high electron mobility transistors: Failure mechanisms induced by hot carrier testing
Authors: Meneghesso Gaudenzio , Mion Alvise , Haddab Youcef , Pavesi Maura , Manfredi Manfredo , Canali Claudio , Zanoni Enrico
Published: July 2002

A new failure mechanism of AlGaAs/InGaAs pseudomorphic high electron mobility transistors has been observed after hot carrier dc accelerated testing. Hot carrier effects have been characterized by means of gate current measurements and electroluminescence spectroscopy. After accelerated testing, a permanent degradation has been found, consisting of the decrease of drain current ID, and of the absolute value of the pinch-off voltage Vp, at low drain-source voltage VDS, resulting in the development of a remarkable “kink” in the output characteristics. Direct current, pulsed, and low-frequency ac measurements demonstrate that the failure mechanism consists of the creation of deep levels under the gate which act as electron traps at low gate-to-drain electric fields. Deep level transient spectroscopy and photoinjection measurements reveal the presence of two levels at 0.77 eV and 1.22 eV. The intensity of the 1.22 eV peak is correlated with the degradation observed in stressed devices.