bluuebunny/crossref_metadata_2025 huggingface dataset:
CO2 laser light absorption characteristics of metal powders
Authors: Haag M. , Hügel H. , Albright C. E. , Ramasamy S.
Published: July 2002

Absorption characteristics of metal powders for 10.6 μm CO2 laser radiation were examined. Using a calorimetric method, absorptance measurements were performed on four different powder materials, including aluminum, copper, iron, and titanium aluminide. The experimental results showed that laser absorptance depends on powder porosity and material. The measured absorptance values at low laser intensities ranged between 28% and 43%. The titanium aluminide powders showed the highest absorptance values, and aluminum powders the lowest. As laser intensity was increased, the copper and iron powders showed strong signs of oxidation when irradiated in air, resulting in an increase in absorptance. Neither oxidation nor increased absorptance were observed when helium or argon were used as shielding gas.
Investigation of the stress-dependent magnetic easy axis in steel using magnetic Barkhausen noise
Authors: Krause Thomas W. , Clapham L. , Pattantyus Andras , Atherton David L.
Published: July 2002

Angular-dependent magnetic Barkhausen noise (MBN) measurements were performed on a pipeline steel sample for various values of applied uniaxial stress at three angles with respect to the sample’s zero stress magnetic easy axis direction. It was observed that the response of the MBN signal to stress was dependent upon the direction of the stress with respect to the zero stress easy axis. The stress response of the MBN signal was greatest for (i) tensile stresses oriented perpendicular to the zero stress easy axis direction and (ii) compressive stresses applied parallel to the easy axis direction. The modification of the MBN signal under an applied stress was attributed primarily to a change in the 180° domain wall population in the material investigated. Results were described by a model that considered regions of locally correlated domain behavior, termed ‘‘interaction regions,’’ that were typically the size of grains within the steel material. A basic result of the model was the stress required to modify the number of 180° domain walls within an interaction region. Theoretical calculations of these threshold stresses for a typical grain size were found to be in agreement with the range of applied stresses that was observed to modify the angular-dependent MBN signal obtained from the sample.
A theoretical model for the giant magnetoimpedance in ribbons of amorphous soft-ferromagnetic alloys
Authors: Machado F. L. A. , Rezende S. M.
Published: July 2002

Giant magnetoimpedance (GMI) measured in ribbons of the soft ferromagnet Co75−xFexSi15B10 annealed in the presence of a transverse magnetic field exhibits peaks in its field dependence. The GMI is strongly dependent on the magnitude of the longitudinal field and on the frequency of the applied current. We present a theoretical model which explains the existence of the peaks and its frequency dependence. The model is based on the skin depth effect and on the domain-wall motion due to the magnetic field and the ac current.
Giant magnetoresistance and soft magnetic properties of Co90Fe10/Cu spin-valve structures
Authors: Kamiguchi Y. , Saito K. , Iwasaki H. , Sahashi M. , Ouse M. , Nakamura S.
Published: July 2002

Spin-valve structures with a magnetic material buffer layer, CoNbZr/NiFe/Co90Fe10/Cu/Co90Fe10/Fe50Mn50, were prepared on a oxidized Si substrate. From x-ray diffraction analysis, it was shown that the CoNbZr/NiFe buffer layer enhances the fcc (111) orientation of the spin-valve structure on oxidized Si. After annealing, the giant magnetoresistance (GMR) ratio of Δρ/ρ=8% and the soft magnetic properties of Hc=0.1 Oe and Hk=2.8 Oe were obtained. The sensitivity of GMR, Δ(Δρ/ρ)/ΔH, of 1.4%/Oe is achieved.
First-principles calculation of the 3d magnetocrystalline anisotropy energy of YCo5
Authors: Yamaguchi M. , Asano S.
Published: July 2002

The electronic structure of YCo5 was calculated self-consistently by the scalar-relativistic linear muffin–tin orbital method using an atomic sphere approximation within the framework of the local spin density approximation. Using the spin–orbit interaction (SO) and orbital polarization term (OP), magnetocrystalline anisotropy energy due to 3d-electrons (3d-MAE) was calculated as a function of bandfilling q [Ea(q)]. At the Fermi level, the calculated Ea(q), including SO and OP, depended greatly on whether an f basis was used for Co. Just below the Fermi level, however, Ea(q) showed a large and c-axial 3d-MAE comparable with the experimental data. This behavior of Ea(q) is closely related to the calculated anisotropy of orbital magnetic moments and the shape of density of states of the 3d band.
Electronic transport properties of Sr1−xLaxTiO3 ceramics
Authors: Moos Ralf , Härdtl Karl Heinz
Published: July 2002

The electrical conductivity of Sr1−xLaxTiO3 ceramics (x≤0.4) and single crystals (x≤0.1) was investigated in the temperature range between 19 and 1673 K. The mobility was calculated from the carrier concentration, n, which was determined by Hall measurements as well as by a chemical Ti3+ titration. The mobility of the single crystals agrees well with a model originally developed for undoped strontium titanate. At low T and high n scattering by ionized donor centers predominates. Above room-temperature phonon scattering becomes predominant. Sr1−xLaxTiO3 ceramics follow the same model, but only if they had been previously reduced in water-free and oxygen-free hydrogen atmospheres. The behavior during the reducing and cooling process can be explained by a defect chemical model, using a set of constants developed in a former work. However, reduced in water saturated hydrogen the conductivity at low T decreases by decades compared to single-crystal data and cannot be explained anymore by the above-mentioned models. In conjunction with impedance spectroscopy experiments it was found, that in this case the electrical behavior of the ceramics is completely governed by grain-boundary phenomena. These high-ohmic grain boundaries may be depletion layers, which are formed or annihilated depending on the conditions of the preceding high-temperature process.
Effect of argon dilution on the structure of microcrystalline silicon deposited from silane
Authors: Das U. K. , Chaudhuri P. , Kshirsagar S. T.
Published: July 2002

The structure of the hydrogenated amorphous and microcrystalline silicon films deposited by radio-frequency (rf) glow discharge decomposition of silane diluted in argon has been studied by transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy techniques. It has been observed that argon acts not only as a passive diluent gas but also plays an important role in the growth of the amorphous or microcrystalline network. Calculation of the variation of equilibrium concentration of Ar*(3P0,2) and Ar+ with argon dilution shows that at high argon dilution (≳90%) equilibrium concentration of Ar* rises sharply with increase in argon dilution. Variation of structural properties with argon dilution suggests involvement of these excited states of argon in the growth process. A model has been proposed based on the energy exchange between the Ar* states and the growth zone of the materials to explain the structural changes observed in the presence of argon in the plasma.
Hole transport in strained Si
Authors: Dijkstra J. E. , Wenckebach W. Th.
Published: July 2002

In this article Monte Carlo simulation of hole transport in uniaxially strained Si is reported. The results are obtained using an exact analytical diagonalization of a six band k⋅p model. Thus we include the spin-orbit, light hole and heavy hole subbands, anisotropy, non-parabolicity and strain. Optical phonon and acoustical phonon scattering are implemented. In weak electric fields the drift mobility is found to increase with strain. The increase depends on the strain being tensile or compressive. The difference found in the two cases can be explained by the larger scattering rate in the tensile case which is caused by the greater density of states in the heavy hole band. At higher fields the drift mobility drops rapidly in strained Si as a function of the electric field.
Magnetic properties of partially-inverted zinc ferrite aerogel powders
Authors: Hamdeh H. H. , Ho J. C. , Oliver S. A. , Willey R. J. , Oliveri G. , Busca G.
Published: July 2002

Fine powders of ZnFe2O4 with an average particle size of 10 nm and inversion parameter of 0.21 were synthesized by the aerogel procedure. Portions of the powders were calcined in air at 500 and 800 °C and other portions were ball-milled for 10 h. The materials were characterized by x-ray diffractometry, vibrating sample, and SQUID magnetometry, Mössbauer spectrometry, and low temperature calorimetry. Upon calcination the powders underwent significant changes in grain size, inversion parameter, and hence magnetic properties. The magnetic state of the as-produced and calcined samples is best described as disordered and highly dependent on temperature. Upon ball-milling the grain size varied widely and the inversion parameter attained a value of 0.55. The magnetic properties of the ball-milled sample are similar to those of ferrimagnetic MgFe2O4 powders having comparable grain size and inversion parameters.
Dielectric and microstructure properties of polymer carbon black composites
Authors: Brosseau C. , Boulic F. , Queffelec P. , Bourbigot C. , Le Mest Y. , Loaec J. , Beroual A.
Published: July 2002

Dielectric and physicochemical properties of a composite material prepared by incorporating carbon black particles into a polymer matrix were investigated. Two types of carbon blacks, having very different structures of aggregates, were used. The volume fraction of the carbon blacks ranged from 0.2% to 7%, i.e. below and above the percolation threshold concentration observed from the measurements of dc conductivity. The composite samples were characterized in terms of: swelling by a compatible solvent, electron paramagnetic resonance (EPR) response, and frequency variation of permittivity. First, the article attempts to evaluate the diffusion coefficient of an appropriate solvent in these materials. Sorption kinetics experiments with toluene indicate that the initial uptake of solvent exhibits a square root dependence in time as a consequence of Fick’s law and permit to evaluate the effective diffusion coefficient in the range 10−11–10−12 m2 s−1 depending on the volume fraction of the carbon black in the sample. Second, the analysis of the carbon black concentration dependence of the intensity and linewidth of the EPR signals indicates that EPR is an important experimental probe of the structure of the elasticity network. The most notable feature of the present work is that we find a correlation of the percolation threshold concentration which is detected from the dc electrical conductivity with moments of the EPR lines. The conclusions on the elasticity networks deduced from swelling measurements are confirmed by EPR data carried out on swollen samples. On qualitative grounds the role of the specific surface of carbon black is further analyzed. It is suggested that the elasticity network is mainly controlled by secondary (respectively primary) aggregates for samples containing low (respectively high) specific surface carbon blacks. Last, the article reports precise experimental data on the permittivity of these composite materials as a function of frequency. Thanks to a sensitive measurement technique using an impedance analyzer, we are able to measure the complex permittivity and permeability values of the samples in the frequency range from 108 to 1010 Hz. It is found that the real part of the permittivity is a function of frequency f, via a power law expression ε′=af−b, where a and b are two parameters depending upon carbon black concentration, in the range of frequency investigated. The data analysis reaffirms the result that percolation threshold is a key parameter for characterizing the topological arrangement in these structures.
Ion energy distributions in a dc biased rf discharge
Authors: Zeuner Michael , Neumann Horst , Meichsner Jürgen
Published: July 2002

We measured quasisimultaneously the energy distributions of positive ions at the powered rf and grounded electrode of a parallel plate 13.56 MHz discharge using an energy selective mass spectrometer. The resulting ion energy distributions reflect the discharge potential conditions expected from a capacitive plasma sheath model. By means of an externally supplied dc bias of the powered electrode we are able to influence the potential structure and to control ion energy and ion flux independently. The ratio between mean ion energy and mean sheath thickness reflects the effect of collisions on the ion energy distributions and enables estimates of sheath thickness and bulk plasma parameters to be made which are compared with values obtained by Langmuir probe measurements. We are able to demonstrate that changes in sheath potential also affect, via secondary electrons, the ionization regime in the discharge and this can be utilized to control the species composition in the discharge.
Magnetic property of layered compound NbFeTe2
Authors: Zhang Jian H. , Chen Feng , Li Jing , O’Connor Charles J.
Published: July 2002

NbFeTe 2 is a layered cluster compound in which intra Fe–Fe bond distance is 2.488 Å. The magnetic susceptibilities between 100 and 300 K are fit to Curie–Weiss law with C=1.29 emu K/mol, and θ=+69.2 K. The effective magnetic moment of 3.67 μB per Fe at 300 K corresponding to three unpaired electrons may suggest the formation of Fe–Fe single bond. Mössbauer spectrum at 77 K shows a paramagnetic doublet with δ=0.36 mm/s and ΔE=0.62 mm/s. The χac measurement reveals the occurrence of a spin glass transition at Tf=44.0 K. The irreversibility of magnetic properties below Tf has been studied with the measurements of the dc susceptibility, remanent magnetization, and hysteresis loop for both zero field cooled and field cooled samples. The competition between antiferromagnetic and ferromagnetic interactions probably cause the spin glass behavior.
Effect of plastic strain on magnetic and mechanical properties of ultralow carbon sheet steel
Authors: Swartzendruber L. J. , Hicho G. E. , Chopra Harsh Deep , Leigh S. D. , Adam G. , Tsory E.
Published: July 2002

We have investigated how plastic deformation changes the yield stress and the magnetic properties of an ultralow carbon sheet steel. Plastic strains up to 10% nearly doubled the yield stress from 150 to 290 MPa (22–42 ksi), and the coercive field from 0.17 to 0.31 kA/m (2.1–3.9 Oe). The effects on other magnetic properties, such as the Barkhausen signal and permeability, were also determined. It was found that a single magnetic parameter could be related to the yield stress with residual standard deviations as low as 1 MPa (0.2 ksi). Observations using a high-resolution interference–contrast colloid technique revealed a fine intragrain magnetic domain structure with the walls more effectively pinned in the 10% strained material.
Optical and electrical properties of PbO-TiO2, PbO-TeO2, and PbO-CdO glass systems
Authors: Vithal M. , Nachimuthu P. , Banu T. , Jagannathan R.
Published: July 2002

The optical and electrical properties of the lead bearing glasses in the PbO-TiO2, PbO-TeO2, and PbO-CdO systems have been investigated. The density, refractive index, optical band-gap, and Urbach energies of these glasses are reported. It is shown that the additive nature of oxide ion polarizabilities reported for the simple oxides can be extended to glass systems as well. A modified linear relationship between (Eopt)1/2 and 1−(Rm/Vm) holds good for glasses. The oxide ion polarizabilities deduced from two different experimental quantities viz., refractive index and optical band-gap energy agree well. The dc electrical conductivities have been measured in the temperature range 350–700 K. The activation energies are close to 1 eV. The low melting 70PbO-30TeO2 glass shows higher isothermal conductivity compared to other glasses of similar composition. In PbO-TiO2 glass the PbO content contributes significantly to conductivity and composition dependence of pre-exponential factor (σ0) supports tunneling mechanism.
Grain refinement and the stability of dendrites growing into undercooled pure metals and alloys
Authors: Mullis A. M. , Cochrane R. F.
Published: July 2002

We present an analysis of the stability of a dendrite against a small perturbation to the tip velocity. We find that dendritic growth in pure metals and alloys will become unstable above some upper critical undercooling ΔT2*. In alloys above a critical concentration, dendritic growth may also become unstable below a lower critical undercooling, ΔT1*. In the example systems studied, Ni–Cu and Ag–O, the location of these unstable regions shows remarkably close agreement with the onset of spontaneous grain refinement. We obtain values of ΔT2* for Ni and Ag of 195 K and 160 K, respectively, in good agreement with the observed values of 170 K and 133–153 K.
Experimental and theoretical investigations of laser-induced crystallization and amorphization in phase-change optical recording media
Authors: Peng Chubing , Cheng Lu , Mansuripur M.
Published: July 2002

We describe the numerical procedure for calculating three-dimensional profiles of temperature in a multilayer stack illuminated by a laser beam, and model the crystallization and amorphization kinetics for phase-change rewritable media. Experimental methods have been used to determine indirectly the probabilities of nucleation and growth for Ge2Sb2Te5 alloy. Some of the fundamental behaviors of phase-change erasable media, such as the crystallization of as-deposited amorphous phase, amorphization of supercooled liquid, and recrystallization of quenched amorphous phase, have been illustrated based on our three-dimensional temperature calculations and the model kinetics. The calculated transient reflectance behavior of as-deposited Ge2Sb2Te5 amorphous films in a single layer and in a quadrilayer stack, as well as the erasure behavior of Ge2Sb2Te5 alloy in a quadrilayer disk are in good agreement with experimental observations.
Characterization of residual defects in cubic silicon carbide subjected to hot-implantation and subsequent annealing
Authors: Itoh Hisayoshi , Ohshima Takeshi , Aoki Yasushi , Abe Koji , Yoshikawa Masahito , Nashiyama Isamu , Okumura Hajime , Yoshida Sadafumi , Uedono Akira , Tanigawa Shoichiro
Published: July 2002

Defects introduced in epitaxially grown cubic silicon carbide (3C-SiC) by implantation of nitrogen (N2+) and aluminum ions (Al+) at a wide temperature range from room temperature to 1200 °C were studied using electron spin resonance (ESR), photoluminescence (PL), and positron annihilation spectroscopy (PAS). It is found that while hot-implantation reduces paramagnetic defects and improves the crystallinity of implanted layers, it causes the simultaneous formation of vacancy clusters. These results can be explained in terms of the migration and combination of point defects during hot-implantation. The formation and reduction of defects by hot-implantation are discussed in connection with implantation temperature, dose, and ion species. Postimplantation annealing of the defects in hot-implanted 3C-SiC was also examined by the ESR, PL, and PAS technique. The influence of residual defects on the electrical properties of implanted 3C-SiC layers is also discussed.
A kinetic Monte Carlo method for the atomic-scale simulation of chemical vapor deposition: Application to diamond
Authors: Battaile C. C. , Srolovitz D. J. , Butler J. E.
Published: July 2002

We present a method for simulating the chemical vapor deposition (CVD) of thin films. The model is based upon a three-dimensional representation of film growth on the atomic scale that incorporates the effects of surface atomic structure and morphology. Film growth is simulated on lattice. The temporal evolution of the film during growth is examined on the atomic scale by a Monte Carlo technique parameterized by the rates of the important surface chemical reactions. The approach is similar to the N-fold way in that one reaction occurs at each simulation step, and the time increment between reaction events is variable. As an example of the application of the simulation technique, the growth of {111}-oriented diamond films was simulated for fifteen substrate temperatures ranging from 800 to 1500 K. Film growth rates and incorporated vacancy and H atom concentrations were computed at each temperature. Under typical CVD conditions, the simulated growth rates vary from about 0.1 to 0.8 μm/hr between 800 and 1500 K and the activation energy for growth on the {111}: H surface between 800 and 1100 K is 11.3 kcal/mol. The simulations predict that the concentrations of incorporated point defects are low at substrate temperatures below 1300 K, but become significant above this temperature. If the ratio between growth rate and point defect concentration is used as a measure of growth efficiency, ideal substrate temperatures for the growth of {111}-oriented diamond films are in the vicinity of 1100 to 1200 K.
Electronic and optical properties of fluorine-doped tin oxide films
Authors: Rakhshani A. E. , Makdisi Y. , Ramazaniyan H. A.
Published: July 2002

Thin films of fluorine-doped SnO2 have been prepared by deposition on borosilicate glass using the spray-pyrolysis technique. The effect of doping on the concentration and mobility of the charge carriers (electrons) as well as the resistivity of the films has been studied. The undoped films had a resistivity of a few m Ω cm; this could be reduced by a factor of 10 by doping. The electron mobility in undoped films was about 3 cm2/Vs but could be improved by a factor of 5 to 6 by doping. The doping yield was about 2.3%. The high quality films which were deposited for photovoltaic applications had a sheet resistance of R□=2 Ω/sq and an average transmittance, in the visible region, of T=85% for a thickness of 1.1 μm. Their figure of merit is one of the highest values reported: φ=T10/R□≈0.1 S. The optical dispersion of our films can be explained perfectly by classical models. In the wavelength region of λ<0.580 μm, the refractive index, N, for undoped and doped films can be given by N=[1+λ2/(0.370λ2−0.0105)]1/2, where λ is in μm. From the study of dispersion and the plasma resonance frequency, the numerical values at optical frequencies of the dielectric constant, electron mobility, and electron effective mass were determined as 3.70, 9.3–11.8 cm2/Vs, and (0.26–0.45)m0, respectively, where m0 is the mass of free electrons. From the variation of direct and indirect optical transition energies with the carrier concentration, the density-of-states effective masses for electrons and holes were obtained as 0.85 m0 and 0.78 m0, respectively. These studies revealed a direct energy bandgap of 4.11 eV for SnO2 in addition to a defect band situated 0.45 eV above the valence band edge.
Mechanical properties of nickel ion-implanted with titanium and carbon and their relation to microstructure
Authors: Myers S. M. , Knapp J. A. , Follstaedt D. M. , Dugger M. T.
Published: July 2002

Dual ion implantation of titanium and carbon into nickel was shown to produce an amorphous layer with exceptionally high strength and hardness and substantially improved tribological properties. Indentation testing at submicrometer penetrations combined with finite-element modeling permitted quantification of the intrinsic elastic and plastic properties of the amorphous layer, which was found to have a yield strength near 5 GPa. During unlubricated sliding contact with a steel pin, the implantation treatment reduced friction, suppressed adhesion-and-fracture wear, and reduced wear depth. These tribological effects may enhance the performance and lifetime of microelectromechanical systems constructed from nickel.
Tin doped indium oxide thin films: Electrical properties
Authors: Bel Hadj Tahar Radhouane , Ban Takayuki , Ohya Yutaka , Takahashi Yasutaka
Published: July 2002

Tin doped indium oxide (ITO) films are highly transparent in the visible region, exhibiting high reflectance in the infrared region, and having nearly metallic conductivity. Owing to this unusual combination of electrical and optical properties, this material is widely applied in optoelectronic devices. The association of these properties in a single material explains the vast domain of its applicability and the diverse production methods which have emerged. Although the different properties of tin doped indium oxide in the film form are interdependent, this article mainly focuses on the electrical aspects. Detailed description of the conduction mechanism and the main parameters that control the conductivity is presented. On account of the large varieties and differences in the fabrication techniques, the electrical properties of ITO films are discussed and compared within each technique.
Processing and cation redistribution of MnZn ferrites via high-energy ball milling
Authors: Fatemi D. J. , Harris V. G. , Browning V. M. , Kirkland J. P.
Published: July 2002

We have produced MnZn ferrites via high-energy ball milling (HEBM) of elemental oxides MnO, ZnO, and Fe2O3. X-ray diffraction (XRD) indicates a pure phase spinel forms after 21 h of HEBM. Extended x-ray absorption fine structure (EXAFS) analysis shows a nonequilibrium cation distribution, with an unusually high population of Zn cations on the octahedral sublattice. We then used EXAFS modeling to study cation site occupancy in an equilibrium MnZn-ferrite standard subjected to HEBM. We found that HEBM produces an increased preference for octahedral-site occupation among all cations for milling durations up to 300 min. After an initial improvement in magnetic properties, the magnetization diminishes steadily throughout this interval. With further increases in milling duration these structural and magnetic trends reverse, possibly due to annealing effects evidenced by XRD.
Magneto-optical properties of one-dimensional photonic crystals composed of magnetic and dielectric layers
Authors: Inoue Mitsuteru , Arai Ken’ichi , Fujii Toshitaka , Abe Masanori
Published: July 2002

The magneto-optical (MO) Faraday effect of one-dimensional photonic crystals (1D-PCs) composed of Bi-substituted yttrium–iron–garnet films and dielectric films such as SiO2 and TiO2 films were studied theoretically. Because of considerable localization of light, these media exhibit a very large MO effect. For instance, when the film structure is chosen to be appropriate for supporting the localization of light, the 1D-PC films can possess a huge Faraday rotation angle reaching to −28 deg/μm at λ=1.15 μm. The analysis reveals that the MO characteristics of the 1D-PC films are almost governed by the degree of localization of light, which can be controlled by varying the number of reflection layers in the films.
Searching for new nonlinear optical materials on the basis of the anionic group theory
Authors: Ye Ning , Chen Qixi , Wu Baichang , Chen Chuangtian
Published: July 2002

Based on the anionic group theory of nonlinear optical crystals and the relationship between the microscopic nonlinear coefficients of the anionic group and the macroscopic coefficients of the crystal, an evaluation system has been established to analyze the spatial arrangement of the anionic groups in a crystal lattice. A structural criterion is given to find which crystal has a favorable spatial arrangement of the anionic groups in terms of its nonlinear effects. Some of the currently available nonlinear optical crystals have been analyzed in this way with a computer program to test the evaluation method, and results demonstrate that the above structural criterion is quite reliable. Several compounds with crystallographic data from an inorganic crystal structure database were evaluated and found to be possible good candidates for nonlinear applications.
Chemical composition and coercivity of SmCo5 magnets
Authors: de Campos M. F. , Landgraf F. J. G. , Saito N. H. , Romero S. A. , Neiva A. C. , Missell F. P. , de Morais E. , Gama S. , Obrucheva E. V. , Jalnin B. V.
Published: July 2002

The effects of alloy composition and heat treatment on the intrinsic coercivity Hci of SmCo5 magnets were studied. Alloys having six chemical compositions near that of stoichiometric SmCo5 were used to produce magnets via the usual powder metallurgy techniques. Magnets were either as sintered (1150 °C) or sintered (1150 °C) and treated at 850 °C. The substantial increase in Hci due to the 850 °C heat treatment occurs reversibly and with a negligible change in lattice parameters, Curie temperature Tc and anisotropy field HA. Quantitative metallography and thermomagnetic measurements showed that the microstructural constituent present in addition to the SmCo5 phase in hyperstoichiometric alloys is composed of both the Sm2Co7 phase and the Sm5Co19 phase. Measurements of magnetization in dc magnetic fields up to 33 T suggest that HA is around 53 T, considerably higher than previously reported values. The coercivity of the magnets is discussed in terms of thermal equilibrium populations of lattice defects.
Measurement of the Hall scattering factor in 4H and 6H SiC epilayers from 40 to 290 K and in magnetic fields up to 9 T
Authors: Rutsch G. , Devaty R. P. , Choyke W. J. , Langer D. W. , Rowland L. B.
Published: July 2002

The Hall scattering coefficient rH determines the relationship between the measurable Hall coefficient RH and the free carrier concentration. Reliable knowledge of rH is necessary for the precise interpretation of Hall measurements and to validate theoretical transport calculations. We have measured the Hall scattering factor in nitrogen doped 4H and 6H epitaxial layers from 35 to 290 K in magnetic fields up to 9 T. The effective Hall scattering factor varies between 0.91 and 1.21 in 4H SiC and 0.84 and 1.02 in 6H SiC. The effect of the Hall scattering factor on dopant activation energies obtained from Hall measurements is not large enough to explain the difference between dopant activation energies obtained from Hall effect and infrared absorption measurements.
Magnetic properties and microstructure of amorphous Co100−xTbx thin films
Authors: Kuo P. C. , Kuo Chih-Ming
Published: July 2002

Amorphous Co100−xTbx alloy thin films with the composition of x=7–60 at. % were prepared by dc magnetron sputtering at various powers and argon pressures then annealing in vacuum. Effects of the composition, sputtering power, argon pressure, and annealing temperature on the parallel and normal to the film plane magnetic properties have been investigated. The analysis of transmission electron microscopy diffraction patterns and magnetic measurement data indicate that an amorphous film with isotropic magnetic properties can be produced after low temperature annealing. The maximum observed perpendicular coercivity was as high as 6000 Oe for the as-deposited Co–Tb amorphous films. A nearly magnetically isotropic amorphous Co–Tb film with in-plane coercivity of about 2080 Oe was obtained after annealing.
Structure and magnetic properties of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys
Authors: Willard M. A. , Laughlin D. E. , McHenry M. E. , Thoma D. , Sickafus K. , Cross J. O. , Harris V. G.
Published: July 2002

The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the FINEMET alloys, with the D03 structure, and NANOPERM alloys, with the A2 structure. Thermomagnetic data shows high magnetization to persist to the α→γ phase transformation at 980 °C. The room temperature ac permeability has been found to maintain a high value of 1800 up to a frequency of ∼2 kHz. The room temperature core loss has also been shown to be competitive with that of commercial high temperature alloys with a value of 1 W/g at BS=10 kG and f=1 kHz.
Detonation wave velocity and curvature of a plastic-bonded, nonideal explosive PBXN-111 as a function of diameter and confinement
Authors: Forbes J. W. , Lemar E. R.
Published: July 2002

Detonation velocities and wave front curvatures are measured on various diameter cylinders of PBXN-111 (RDX/AP/Al/HTPB binder with 20/43/25/12 weight percent). The cylinders are either unconfined or encased in 5-mm-thick brass tubes. In all experiments with brass tubes (diameters from 19 to 100 mm) the detonation velocity of PBXN-111 was affected by the confinement. Steady detonation waves propagated in brass encased charges with diameters as small as 19 mm, which is about half of the 37.1 mm failure diameter for unconfined PBXN-111. The radii of curvature at the center of the detonation wave fronts ranged from 52 to 480 mm for charge diameters from 25 to 100 mm, respectively. Detonation velocity as a function of radius of curvature at the wave’s center is represented by a single curve for both cased and uncased cylindrical charges. The difference in the axial position of the detonation wave at the center of the charge and at the edge of the charge (i.e., lag distance) are between 1.6 and 6.7 mm. The angles between the detonation wave fronts and the brass/charge interfaces are between 73° and 82° while the angles at the cylindrical free surface for the uncased charges are between 61° and 64°. Calculation of this angle for brass encased charges using oblique shock equations and assuming no reaction in the shock front resulted in angles 8° higher than measured except for the 100-mm-diam charge which was in agreement. The calculated angles for the uncased charge are in agreement with the measured values.
X-ray absorption, neutron diffraction, and Mössbauer effect studies of MnZn–ferrite processed through high-energy ball milling
Authors: Fatemi D. J. , Harris V. G. , Chen M. X. , Malik S. K. , Yelon W. B. , Long G. J. , Mohan A.
Published: July 2002

MnZn–ferrite has been prepared via high-energy ball milling of elemental oxides MnO, ZnO, and α-Fe2O3. Neutron diffraction measurements suggest a high density of vacancies in a spinel structure. The spinel phase appears to comprise 99.8 wt % of the material in the sample milled for 40 h, with the remainder attributable to unreacted α-Fe2O3. The x-ray absorption near-edge structure was analyzed to provide an understanding of the charge state of the constituent Fe ions. This analysis reveals about 2/3 of Fe cations to be trivalent, increasing to about 3/4 after a 5 h anneal at 450 °C. The heat treatment is also observed to induce a cation redistribution in the ball-milled ferrite toward that of a standard processed via ceramics methods. Results from Mössbauer spectroscopy determine the average hyperfine fields in the sample milled 40 h to be 289 and 487 kOe at 295 and 78 K, respectively. The average isomer shift is 0.32 mm/s at 295 K and 0.46 mm/s at 78 K, values which are typical of iron (III) in a spinel oxide lattice. As expected for a cubic-like environment, the quadrupole shifts are very small, ranging from 0.07 mm/s at 295 K to 0.00 mm/s at 78 K.
High frequency giant magnetoresistance in evaporated Co/Cu multilayers deposited on Si(111) and Si(100)
Authors: Rausch T. , Szczurek T. , Schlesinger M.
Published: July 2002

We report giant magnetoresistance (GMR) in Co/Cu multilayers deposited using high vacuum electron beam evaporation on Si(111) and Si(100). Samples grown on Si(111) show enhanced magnetoresistance and coercivity, which may be attributed to ill-defined interfacial boundaries between successive layers. Previous work exploring the frequency dependency of GMR has been done using microwave techniques to indirectly determine GMR. Our magnetoresistance data was obtained by replacing one side of a square loop antenna with a Co/Cu multilayer and measuring the impedance. No effect on the magnitude of the GMR, due to high frequency test signals, was observed for a frequency range from dc to 5.5 GHz.
Cumulative second-harmonic generation of Lamb-mode propagation in a solid plate
Authors: Deng Mingxi
Published: July 2002

This article describes the physical process of cumulative second-harmonic generation of Lamb-mode propagation in a solid plate. In general, cumulative second-harmonic generation of a dispersive guided wave propagation does not occur. However, the present results show that the second harmonic of Lamb-mode propagation arising from the nonlinear interaction of partial bulk waves and the restriction of the two boundaries of the solid plate does have a cumulative effect once some conditions are satisfied. Through boundary and initial conditions of excitation, the analytical expression of the cumulative second harmonic of Lamb-mode propagation has been determined. Numerical results reveal that the cumulative second-harmonic fields are symmetrical regardless of whether Lamb-mode propagation is symmetrical or antisymmetrical, and that the cumulative second-harmonic field patterns of Lamb-mode propagation are associated with the position of excitation source.
EuSe
2
:
A novel antiferromagnetic rare-earth polychalcogenide
Authors: Cowen J. A. , Michlin P. , Kraus J. , Mahanti S. D. , Aitken J. A. , Kanatzidis M. G.
Published: July 2002

The reaction of Eu with a molten mixture of Li2Se and excess Se has produced a new rare earth dichalcogenide, EuSe2, with a type “a” antiferromagnetic structure. The compound crystallizes in the tetragonal space group I4/mcm forming into small single crystal rectangular blocks with their long axis along c. The high temperature magnetic susceptibility exhibits Curie–Weiss behavior with a ferromagnetic Θ=15±1 K and an effective moment μeff=7.9±0.1 Bohr magnetons consistent with the theoretical value of 7.94 for the Eu2+ free ion. There is a peak in χc at Tn=8.0±0.5 K which is attributed to an antiferromagnetic transition. At T=2.5 K with the magnetic field, H, applied along c, M(H) exhibits a sharp increase near 1.5 T attributed to a metamagnetic transition. With H along a the moments saturate at about 5 T. A simple mean field model in which the Eu ions in the ab plane are coupled into ferromagnetic sheets, with alternate sheets along c coupled antiferromagnetically to form a bulk antiferromagnet is used to interpret the data.
First-principles exchange interactions in Fe, Ni, and Co
Authors: van Schilfgaarde M. , Antropov V. P.
Published: July 2002

Using a multiple-scattering approach and the local-density and rigid-spin approximations, the exchange interactions are calculated for Fe, Ni, and Co, and the spin-wave spectrum obtained. The interactions contributing to the spin-wave spectrum are found to be rather long-ranged. The exchange parameters are calculated as a function of band filling, and predict transition to antiferromagnetic behavior for electron-deficient alloys of bcc Fe and fcc Co.
Inductive measurement of ultrafast magnetization dynamics in thin-film Permalloy
Authors: Silva T. J. , Lee C. S. , Crawford T. M. , Rogers C. T.
Published: July 2002

An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni81Fe19). Data are presented for impulse- and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are clearly observed. The oscillation frequency increases as the dc bias field parallel to the anisotropy axis increases as predicted by classical gyromagnetic theory. The data are fitted to the Landau–Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. Damping for step excitations is characterized by an anomalous transient damping which rapidly increases at low dc bias field. Transformation of the data to the frequency domain reveals a higher order precessional mode which is also preferentially excited at low dc bias fields. A possible source for both phenomena is precessional mode saturation for large peak rotations. The technique has the potential for 20 ps resolution, although only 120 ps resolution is demonstrated due to the limited bandwidth of the waveguides used.
Current transport mechanism in trapped oxides: A generalized trap-assisted tunneling model
Authors: Houng Mau Phon , Wang Yeong Her , Chang Wai Jyh
Published: July 2002

A generalized trap-assisted tunneling (GTAT) model is proposed in this work, where an effective tunneling barrier of trapezoidal shape is considered, instead of the triangular barrier utilized in the conventional trap-assisted tunneling (TAT) model. It is demonstrated that trapezoidal barrier tunneling dominates at low electric fields (E<4 MV/cm), while triangular barrier tunneling contributes the main part of the tunneling current at high electric fields (E=6–8 MV/cm). The comparisons of this improved model and the results of the conventional TAT model at high and low electric fields are discussed. It is concluded that GTAT can more accurately model the current density-electric field (J–E) curves for the conduction enhancement of a trapped oxide film under various deposition conditions over a wider range of electric fields. This is confirmed by the comparative use of both TAT and GTAT models on experimental data obtained from existing reports. Furthermore, a simple method for determining the trap energy level is derived from the J–E relationship. This method provides a convenient way to characterize the trap levels inside the oxide layers, without the need of other complicated measurements. The developed GTAT model can be applied to the investigations of gate oxide reliability, especially the stress-related effects and impurity incorporated oxide films (i.e., SiOF or SiON).
Electronic properties of light-induced recombination centers in boron-doped Czochralski silicon
Authors: Schmidt Jan , Cuevas Andrés
Published: July 2002

In order to study the electronic properties of the recombination centers responsible for the light-induced carrier lifetime degradation commonly observed in high-purity boron-doped Czochralski (Cz) silicon, injection-level dependent carrier lifetime measurements are performed on a large number of boron-doped p-type Cz silicon wafers of various resistivities (1–31 Ω cm) prior to and after light degradation. The measurement technique used is the contactless quasi-steady-state photoconductance method, allowing carrier lifetime measurements over a very broad injection range between 1012 and 1017 cm−3. To eliminate all recombination channels not related to the degradation effect, the difference of the inverse lifetimes measured after and before light degradation is evaluated. A detailed analysis of the injection level dependence of the carrier lifetime change using the Shockley–Read–Hall theory shows that the fundamental recombination center created during illumination has an energy level between Ev+0.35 and Ec−0.45 eV and an electron/hole capture time constant ratio between 0.1 and 0.2. This deep-level center is observed in all samples and is attributed to a new type of boron–oxygen complex. Besides this fundamental defect, in some samples an additional shallow-level recombination center at 0.15 eV below Ec or above Ev is found to be activated during light exposure. This second center dominates the light-degraded carrier lifetime only under high-injection conditions and is hence only of minor importance for low-injection operated devices.
Low-resistance ohmic contacts to p-type GaN achieved by the oxidation of Ni/Au films
Authors: Ho Jin-Kuo , Jong Charng-Shyang , Chiu Chien C. , Huang Chao-Nien , Shih Kwang-Kuo , Chen Li-Chien , Chen Fu-Rong , Kai Ji-Jung
Published: July 2002

A contact has been developed to achieve a low specific contact resistance to p-type GaN. The contact consisted of a bi-layer Ni/Au film deposited on p-type GaN followed by heat treatment in air to transform the metallic Ni into NiO along with an amorphous Ni–Ga–O phase and large Au grains. A specific contact resistance as low as 4.0×10−6 Ω cm2 was obtained at 500 °C. This low value was obtained by the optimization of Ni/Au film thickness and heat treatment temperatures. Below about 400 °C, Ni was not completely oxidized. On the other hand, at temperatures higher than about 600 °C, the specific contact resistance increased because the NiO detached from p-GaN and the amount of amorphous Ni–Ga–O phase formed was more than that of the sample annealed at 500 °C. The mechanism of obtaining low-resistance ohmic contacts for the oxidized Ni/Au films was explained with a model using energy band diagrams of the Au/p-NiO/p-GaN structure.
On the relationship of high coercivity and L1 ordered phase in CoPt and FePt thin films
Authors: Ristau R. A. , Barmak K. , Lewis L. H. , Coffey K. R. , Howard J. K.
Published: July 2002

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed for both CoPt and FePt. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.