bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characterisation Of Quantum Wells By Auger Analysis On Chemical Bevels
Authors: Olivier Jean , Etienne P. , Razeghi M. , Alnot P. 
Published: March 2011

ABSTRACTIn recent papers, we have described a depth profiling method using chemical beveling and Auger line scan measurement, quick and easy to perform, with a sufficient resolution to check the abruptness of chemical interfaces obtained in epitaxial growth of III-V compounds.The variation law of chemical species concentrations at the interfaces is tighly related to the mecanisms of initial growth. We present here a theoretical analysis applied to chemically beveled interfaces with sharply graded concentrations.We show on two examples of LP-MOCVD quantum wells (InP/GaInAs/InP and GaInP/GaAs/GaInP) the good agreement between experimental and theoretical curves, assuming exponentially varying concentrations. Finally, we discuss the correlations with the dynamics of the first steps of the LP-MOCVD growth.
Optical Reflectance and Rheed Transients During Mbe Growth on (001) GaAs
Authors: Aspnes D. E. , Harbison J. P. , Studna A. A. , Florez L. T. 
Published: March 2011

ABSTRACTWe compare the dynamic responses of reflection high energy electron diffraction (RHEED) and optical reflectance-difference (RD) signals caused by abrupt changes in incident fluences of Ga and As during MBE growth of GaAs on (001) GaAs. Our results reveal that RHEED and RD originate mainly, although not completely, from structural order and chemical bonding, respectively, and thus provide complementary information for real-time studies of MBE growth. We also measure the wavelength dependence of the RD spectrum and show that it can be described approximately from 2 to 4 eV by a single Lorentzian absorption line centered at 2.4 eV.
Stresses in Multilayered Thin Films on a Thick Substrate
Authors: Suhir E. 
Published: March 2011

ABSTRACTThe analysis contains an engineering method for the prediction of thermally induced stresses in single- and multilayered heteroepitaxial structures on a thick substrate. The examined stresses include 1) normal stresses acting in the film layers themselves and responsible for their ultimate and fatigue strength, and 2) interfacial stresses responsible for film blistering and peeling. The developed formulas are simple, visible, easy-to-use, and clearly indicate how material and structural characteristics affect the magnitude and the distribution of stresses and deflections. Some recommendations for smaller stresses in film structures are presented. The obtained results can be utilized as a guidance for an optimal physical design of multilayered heteroepitaxial structures used in Microelectronics.
Effect of Free Surfaces and Interfaces on Dopant Distribution Profiles
Authors: Jagannadham K. , Narayan J. 
Published: March 2011

ABSTRACTThe drift force exerted by a free surface, a coherent or a noncoherent interface on dopant atoms has been analyzed. We have considered a coherent or a noncoherent interface present in an epilayer of finite thickness. The difference in the shear modulus of the matrix and of the second phase is responsible for the configurational energy of a dopant atom in a coherent two-phase medium. On the other hand, the hydrostatic component of stress associated with a misfit dislocation in a noncoherent interface gives rise to a first-order size interaction. The drift forces are responsible for dopant diffusion towards a free surface or an interface. The diffusion equation including the drift term is solved using an eigen function expansion method with appropriate boundary conditions. The concentration profiles obtained from the analysis of the diffusion equation are in qualitative agreement with those obtained experimentally. The attractive drift force gives rise to a peak in the dopant concentration at the interface followed by a minimum near the interface. These calculations and the observed concentration profiles enable us to evaluate the interaction energy term of the dopants with free surfaces and interfaces and the dilatation associated with the point defects.
The Structure of GaAs/Si(211) Heteroepitaxial Layers
Authors: Liliental-Weber Zuzanna , Weber E.R. , Washburn J. , Liu T.Y. , Kroemer H. 
Published: March 2011

ABSTRACTGallium arsenide films grown on (211)Si by molecular-beam epitaxy have been investigated using transmission electron microscopy. The main defects observed in the alloy were of misfit dislocations, stacking faults, and microtwin lamellas. Silicon surface preparation was found to play an important role on the density of defects formed at the Si/GaAs interface.Two different types of strained-layer superlattices, InGaAs/InGaP and InGaAs/GaAs, were applied either directly to the Si substrate, to a graded layer (GaP-InGaP), or to a GaAs buffer layer to stop the defect propagation into the GaAs films. Applying InGaAs/GaAs instead of InGaAs/InGaP was found to be more effective in blocking defect propagation. In all cases of strained-layer superlattices investigated, dislocation propagation was stopped primarily at the top interface between the superlattice package and GaAs. Graded layers and unstrained AlGaAs/GaAs superlattices did not significantly block dislocations propagating from the interface with Si. Growing of a 50 nm GaAs buffer layer at 505°C followed by 10 strained-layer superlattices of InGaAs/GaAs (5 nm each) resulted in the lowest dislocation density in the GaAs layer (∼;5×l07/cm2) among the structures investigated. This value is comparable to the recently reported density of dislocations in the GaAs layers grown on (100)Si substrates [8]. Applying three sets of the same strained layersdecreased the density of dislocations an additional ∼2/3 times.
Physical Properties of Ternary Re-Fe Base Alloys and their Relationship to Permanent Magnet Applications
Authors: Buschow K. H. J. 
Published: March 2011

ABSTRACTSeveral ternary compounds of tetragonal structure and with the composition R2Fe14B have found an application as starting materials for permanent magnets. Several physical properties of R2Fe14B compounds will be discussed in relation to the modification in properties that can be obtained by substituting other elements for R, Fe or B. Also investigated is the extent to which it will be possible to find alternative materials differing in structure and composition from R2Fe14B. For this purpose the results obtained on 3d-rich ternaries based on well-known structure types will be reviewed. Included in this survey will be novel ternary intermetallics made by crystallization from the amorphous state. These compounds, though stable enough for applications, are actually metastable. They are not found in the corresponding equilibrium diagrams.
Magnetic Anisotropy and Spin Reorientation in Nd2-xPrxCo14B
Authors: Pourarian F. , Sankar S. G. , Pedziwiatr A. T. , Boltich E. B. , Wallace W. E. 
Published: March 2011

ABSTRACTNd2-xPrxCo14B intermetallics crystallize in tetragonal form isostructural to Nd2Fe14B. Structural and magnetic properties of the polycrystalline materials have been determined. Room temperature magnetic measurements indicate that these intermetallics exhibit uniaxial anisotropy throughout the entire composition range. Substitution of praseodymium in Nd2-xPrxCo14B significantly enhances the anisotropy field both at 78 K and at room temperature. Magnetization versus temperature plots show spin reorientation TSR2 at 546 K for Nd2Co14B and at 668 K for Pr2Co14B. TSR2 increases linearly with composition x. The 3d sublattice anisotropy which seeks the tetragonal plane appears to dominate above these temperatures. In addition to the above, low temperature magnetic measurements demonstrate the spin reorientation in Nd2Co14B and Nd1.9Pr0.1Co14B. Further additions of praseodymium strengthen the axial anisotropy at temperatures down to 4.2 K. Effect of competing crystal field interactions and the consequences of praseodymium substitution that result in considerable increase in anisotropy field in these quaternaries are discussed.
High Field Torque Magnetometry Measurements on Y1.8Er0.2Fe14B
Authors: Williams C. M. , Koon N. C. , Das B. N. 
Published: March 2011

ABSTRACTThe magnetocrystalline anisotropy energy has been determined for single crystal Y1.8 Er0.2Fe14B in the (001) and (100) planes between 5 K and 300 K using torque magnetometry techniques. The results are compared with a model based on crystal field theory. Excellent agreement was obtained between the model and experiment. Both experiment and model showed a first order spin reorientation between the [001] and an angle 70 degrees from the [001] in the (100) plane.
Misch-Metal and/or Aluminum Substitutions in ND-FE-B Permanent Magnets
Authors: Ma B. M. , Willzian C. J. 
Published: March 2011

ABSTRACTThe effects of misch-metal–and/or Al substitutions in the Nd-Fe-B alloy system have been investigated. Selected compositions were processed into magnets using the conventional powder metallurgy technique. As expected, misch-metal substitutions act to degrade the intrinsic magnetic characteristics and thus degrade the properties of the sintered magnet. It was observed that at a level of 25% of the total rare earth being comprised of misch-metal, energy products of the order of 25 MGOe were still able to be achieved. Processing of such material is difficult because higher sintering temperatures are required to fully densify the magnet, and the grain growth that occurs at these temperatures dramatically reduces the coercivity. The effects of misch-metal substitutions are quantified in this discussion.Al was found to be beneficial for improving the coercivity in both the Nd-Fe-B and misch-metal-Fe-B alloy systems. Unfortunately Al substitutions reduce both the remanence and the Curie temperature. If the Al content is kept to 1 wt.% or less, magnets exhibiting reasonable properties can be made. Al doping as opposed to doping with expensive heavy rare earth elements such as Th or Dy is a less costly method of improving Hei, in the Nd-Fe-B magnets.
Magnetic Hardening Mechanism in Sintered R-Fe-B Permanent Magnets
Authors: Sagawa M. , Hirosawa S. 
Published: March 2011

ABSTRACTThe state-of-the-art description of the magnetic hardening mechanism in the sintered Nd-Fe-B permanent magnet is given. Recent experimental results concerning the coercivity-anisotropy (Hc -HA) correlation in B-rich Pr-Fe-B and Nd-Fe-B sintered magnets and the inhluence of the surface conditions of the sintered Nd-Fe-B magnets on the coercivity are reported. These results are interpreted in terms of the μ0Hc versus cμ0 HA -NIS plot, where I is the spontaneous magnetization of R2 Fe14 (R=Pr or Nd) and N the effective demagnetization field coefficient.
Microstructural Studies of High Coercivity Nd15Fe77B8 Sintered Magnets
Authors: Ghandehari M. H. , Fidler J. 
Published: March 2011

ABSTRACTMicrostructures of Nd15−xDyxFe77B8 prepared by alloying with Dy, and by using Dy2O3 as a sinl'ken adidive, have been determined using electron microprobe and transmission electron microscopy. The results have shown a higher Dy concentration near the grain boundaries of the 2–14–1 phase for magnets doped with Dy2O 3, as compared to the Dy-alloyed magnets. A two-step post sintering heat treatment was also studied for the two systems. The resultant concentration gradient of Dy in the 2–14–1 phase of the oxide-doped magnets is explained by the reaction of Dy2O3 with the Nd-rich grain boundary phase and its slow diffusion into thg 4–14–1 phase. Increased Dy concentration near the grain boundary is more effective in improving the coercivity, as domain reversal nucleation originates at or near this region.
Aluminum Substitutions in Nd-Fe-B Sintered Magnets
Authors: Chen J. K. , Thomas G. 
Published: March 2011

ABSTRACTA microstructural and microanalytical study of aluminum substituted Nd-Fe-B sintered permanent magnets were carried out to determine the effect, if any, of aluminum on structure, composition, and magnetic properties, particularly on the observed increase in coercivity. It was found that Al enrichment occurred in the Nd-rich phase at the grain boundaries. The possible role(s) of this enrichment on the observed coercivity increase are discussed.
Effect of Hydrogen on Magnetic Properties of Quaternary Pr2Fe14−xCoxB Compounds
Authors: Pourarian F. , Jiang S. Y. , Wallace W. E. 
Published: March 2011

The effect of absorbed hydrogen on the structural and magnetic behavior of Pr2Fe14−xCoxB has been investigated. Introduction of hydrogen in the host material reduces the average magnetic moment of the 3d element by ˜ 4 to 7%. Anisotropy fields for all systems studied are significantly reduced upon hydrogenation. Evidence of spin reorientation at temperatures ranging between ˜ 295 K and 496 K for some of the hydrides is observed. Pr2Co14B and its hydride exhibit uniaxial anisotropy at room temperature and at 77 K.The role of hydrogen in expanding the lattices of these intermetallics and modifying their magnetic properties is discussed.
Neutron Diffraction Studies of Magnetic Materials
Authors: James William J. 
Published: March 2011

ABSTRACTThe ability of neutron diffraction in determining the nature and extent of magnetic ordering is illustrated for the intermetallic compounds, Y6(Fe,Mn)23, and ErFe3. Substitution with other 3d transition metals influences the Fe-Fe exchange forces such as to alter, sometimes considerably, the magnetic properties, e.g., local site magnetic anisotropies in Er(Fe,Ni)3 and thermal expansion anomalies in the R2 (Fe,Co)14B compounds. When the 3d atoms are near neighbors in the periodic chart, their nuclear scattering lengths for neutrons are sufficiently different to permit the detection of preferential occupation of the several nonequivalent crystallographic 3d metal sites, i.e., atomic ordering, present in the R6M23, and R2Fe14B structures.
Synthesis of R2 Fe14 B Single Crystals
Authors: Das B. N. , Koon N. C. 
Published: March 2011

ABSTRACTSingle crystals of R2 Fe14 B weighing 5 to 15 g were grown from a liquid melt by tri-arc and levitation Czochralski methods. The arc method was used for the growth of smaller size crystals from 5 to 7 g, and the levitation method was used for the growth of larger sizes, from 10 to 15 g. Crystals of (R1)2 –x(R2)xFe14 B could also be grown with isomorphous replacement of rare earth atoms. The starting alloy composition for the crystal growth process was chosen based on solidification microstructure. In this paper we discuss the solidification microstructure, isomorphous replacement, seeding and their interactions with the crystal growth processes and crystal quality.
Microstruoture-Property Relationships in Magenqubnch Magnets
Authors: Rishra Raja K. 
Published: March 2011

ABSTRACTTransmission electron microscopy has been used to characterize the microstructure of Nd-Fe-B magnets produced by melt-spinning and subsequent hot-pressing/die-upsetting. For a material of starting composition Nd.135Fe.815B.05, the basic microstructure od melt-spun, hot-pressed and dieupset magnets consists of two phases. In the optimally processed melt-spun ribbons and hot-pressed samples, small and randomly oriented Nd2Fe14B grains are surrounded by a thin noncrystalline Nd-rlch phase. The die-upset material consists of closely stacked flat Nd2Fe14B grains surrounded by a second phase of approximate composition Nd7Fe3. No Nd11Fe4B4 phase is observed in these materials, but it can form if the chemical composition and/or processing parameters are varied. In all these materials, Lorentz microscopy reveals that magnetic domain walls are pinned by the second phase. The differences in the hard magnetic properties of the three kinds of MAGNEQUENCH magnets closely correlate with the differences in the distribution of Nd2Fe14B crystallites and the pinning sites.
Optimization of Liquid Dynamic Compaction for Fe-Nd-B Magnet Alloys
Authors: Veistinen Mauri K. , Hara Y. , Lavernia E. J. , O'Handley R. C. , Grant N. J. 
Published: March 2011

ABSTRACTPermanent magnets from the Fe77Nd15B8 alloy were produced using liquid dynamic compaction (LDC). Process parameters such as gas and metal mass flow rates were carefully controlled to refine the microstructure. Accordingly, grain sizes between 0.1 and 5 microns were achieved in the LDC compact. In contrast to our earlier LDC results high intrinsic coercivity in excess of 7500 Oe was obtained for thick (> 7 nm) as-LDC'd samples with lower neodymium content.
Optical Properties of Dopant Induced States in La2−xSrxCuO4-δ Compounds
Authors: Etemad S. , Aspnes D. E. , Barboux P. , Hull G. W. , Kelly M. K. , Tarascon J. M. , Thompson R. , Herr S. L. , Kamaras K. , Porter C. D. , Tanner D. B. 
Published: March 2011

ABSTRACTOptical data over a broad energy range (∼ 0.01 eV to ∼ 6 eV) for a series of La2−xSrxCuO4-δ compounds are obtained in parallel with the Meissner effect and the superconductivity transition temperature, Tc. Two noteworthy trends in the optical spectra are observed as the Sr concentration is increased in small steps from x = 0 to x = 0.3. First is the appearance of a low frequency band in the reflectivity, R, whose strength follows closely the Meissner effect and Tc measured on the same set of samples. The position of the onset of this band is pinned at ∼0.9 eV for all values of x. The origin of this band in R is identified as an absorption process due to an electronic transition with a large oscillator strength. Second is the appearance and disappearance of an intense vibrational mode whose strength also tracks “superconductivity”. This sharp structure in the far infrared is a characteristic vibrational mode associated with the dopant induced electronic state.
Observation of Hexagonally Ordered Flux Quanta in YBa2Cu3O7
Authors: Gammel P. L. , Bishop D. J. , Dolan G. J. , Kwo J. R. , Murray C. A. , Schneemeyer L. F. , Waszczak J. V. 
Published: March 2011

ABSTRACTThe high resolution Bitter Pattern technique has been used to reveal the structure of the array of flux lines which is present when single crystal samples of the high Tc superconductor YBa2Cu3O7 are placed in a magnetic field. At 4.2K with the magnetic field parallel to the c axis, the patterns formed are strongly reminiscent of the analogous structures in ordinary type II superconductors. The structures typically consist of flux spots with short range hexagonal correlations. We interpret the patterns to consist of singly quantized vortices. Very uniform patterns are observed in thin, flat samples cooled in a constant field. Sampling the spot density yields a vortex density consistent with a flux quantum of hc/2e.Experiments at 77K, the only other temperature studied, showed no spatially varying magnetic structure. Recent mechanical measurements which are sensitive to the bulk modulus and dissipation within the vortex lattice show evidence for a mobility transition near 75K, which would be consistent with this observation. We believe this stems from materials properties rather than some more fundamental property of the new superconductors.
Phase Equilibria Studies Near the Superconducting YBa2Cu3Ox Composition by a Solid-State Ionic Technique
Authors: Ahn Byung Tae , Gür Turgut M. , Huggins Robert A. , Beyers Robert , Engler Edward M. 
Published: March 2011

ABSTRACTThis paper reports the findings of a phase-equilibrium study of the Y-Ba-Cu-O quaternary system in which the oxygen activity was monitored and controlled by a solid-state ionic technique. A stabilized zirconia electrolyte was used to titrate oxygen in or out of the sample and to monitor the oxygen activity inside a sealed sample chamber.Compositions at or near YBa2Cu3Ox were studied at elevated temperatures in a wide range of oxygen pressures. All compositions indicated the presence of an invariant polyphase reaction- e.g., at 1 × 10-3 atm of oxygen pressure at 850 °C-- below which the superconducting phase becomes thermodynamically unstable and undergoes a decomposition reaction. Complementary x-ray diffraction analysis of samples prepared under well-defined oxygen pressures identified the phases present and, along with the oxygen coulometric titration data, allowed the construction of a portion of the relevant quarternary phase diagram.
X-Ray Absorption Studies of La2−xSrxCuO4 and Y1−xPrxBa2Cu3O7-δ
Authors: Alp E. E. , Shenoy G. K. , Soderholm L. , Goodman G. L. , Hinks D. G. , Veal B. W. , Montano P. A. , Ellis D. E. 
Published: March 2011

ABSTRACTThe question of valence of Cu and rare-earth atoms in the newly discovered oxide superconductors with high transition temperatures is crucial to the understanding of their electronic structure. We have measured the X-Ray Absorption Near Edge Structure (XANES) of Cu K transition and Pr Ljjj transition to obtain information on the valence of Cu, and Pr in La2−xSrxCuO4 and Y1−xPrxBa2Cu3O7-δ. We will present the experimental results, compare them with calculations of absorption cross-section for model atomic clusters, and discuss the valence of Cu as a function of oxygen concentration.
Developments of Tape and Wire Fabrication Processes for High Tc Ba2YCu3O7-y Oxide Superconductors
Authors: Togano K. , Kumakura H. , Shimizu H. 
Published: March 2011

ABSTRACTThe application of superconducting material to a magnet requires the winding tape or wire conductor with sufficient flexibility into a solenoid. Tape conductors of Ba2YCu3O7-y were successfully prepared by a new technique of surface reaction. Cu tape or Cu-electroplated Ag tape was coated with a fine mixture of Ba2CO3, and Y2O3 and then heat treated at about 900°C to form Ba2YCu3O7-y oxide layer with high density on both surfaces of the tape. Zero resistance state has been achieved at the temperature above liquid nitrogen temperature for Ag-substrate tape. The critical current density is about 40A/cm at 77K and zero magnetic field. Although, at present, the superconducting properties of these tapes are somewhat inferior to those of conventional powder metallurgy processed conductors, the surface reaction process has the potential feasibility of processing Ba2YCu3O7-y into a long-length ductile tape desirable for practical application.
The Deposition, Fabrication and Characteristics of High Critical Temperature Devices
Authors: Morris G. W. , Blamire M. G. , Somekh R. E. , Tomlinson E. J. , Evetts J. E. 
Published: March 2011

ABSTRACTIn the first section of this paper the sputter deposition of thin films of YBa2Cu3O7−x is considered. In the second section low and high temperature heat treatments are discussed in the light of the thermodynamics of the material and in the third the development, fabrication and current-voltage (IV) characteristics of a variety of device structures are described, including the hysterctic characteristics of certain YBa2Cu3O7−x / YBa2Cu3O7−x devices that reveal Josephson supcrcurrcnt tunnelling.
Field Dependence of Meissner Effect in a Single Crystal of YBa2Cu3Oxin the Ultra-Low Magnetic Field Regime
Authors: Krusin-Elbaum L. , Malozemoff A. P. , Yeshurun Y. 
Published: March 2011

ABSTRACTWe report a strong field-dependence (from 0.1% to 71%) of flux expulsion (Meissner effect) in a single crystal of YBa2Cu3Oxin magnetic fields from 36 kOe down to 50 mOe. The data is discussed in the context of flux pinning forces and glassy behavior.
Application of Gel Technology in Preparation of High-Tc Perovskite Superconductors
Authors: Liu R. S. , Lin G. C. , Sung H. M. , Chen Y. C. , Lin O. C. C. 
Published: March 2011

ABSTRACTLa-Ba-Cu-O and Y-Ba-Cu-O superconducting systems have been successfully prepared by gel techniques with high degree of chemical homogeneity. The precursor gel was synthesized from mixture of the corresponding metallic nitrates and di-carboxylic acids. The sintered oxides prepared from the different dicarboxylic acids were all Tc = 90K perovskite superconductors. However differences in electrical and magnetic properties were also observed. Effects due to the different acids elucidated by magnetic susceptibility measurements and X-ray diffraction analysis will be discussed.