bluuebunny/crossref_metadata_2025 huggingface dataset:
Static magnetization orientation under perpendicular surface anisotropy (abstract)
Authors: Woods J. P. , O’Handley R. C.
Published: July 2002

Secondary electron-spin polarization measurements1 on single-crystal Fe surfaces in ultrahigh vacuum suggest a perpendicular magnetization at the surface but in-plane magnetization at 10 Å or more beneath the surface. We consider this problem with a simple model. The depth dependence of magnetization tilt angle θ(z) (measured from the surface normal) is solved analytically for a semi-infinite, exchange-coupled ferromagnet with dipole energy and perpendicular surface anisotropy Ks. The surface magnetization is found to remain in plane for Ks≤(πM2A)1/2=Kc, a critical surface anisotropy. For Fe, Kc is 4 ergs/cm2. The analytic solution of the magnetization rotation θ(z)=tan−1 [sinh(a+bz)] is identical to the bulk domain wall solution but the length scale of the rotation, b−1, is reduced by more than an order of magnitude compared to bulk domain walls in soft magnets (5 nm vs 100 nm). The magnetization configuration is, in fact, a partial domain wall at the surface.
Magnetic ordering in U(M,M′)2Ge2 compounds (M,M′=Co,Ni,Cu)
Authors: Kuznietz Moshe , Pinto Haim , Melamud Mordechai
Published: July 2002

Several U(M,M′)2Ge2 compounds (M,M′=Co,Ni,Cu) have been studied by neutron diffraction. U(Co1−xCux)2Ge2 and UNiCuGe2 exhibit ThCr2Si2-type tetragonal structure, with a=4.02–4.10 Å and c=9.86–10.25 Å, and a magnetically ordered uranium sublattice with moments along c. The ordering is antiferromagnetic with a wave vector k=(0,0,1) for x=0,0.25, and 0.50 and UNiCuGe2 (TN=175, 130, 100, and 140 K, respectively), and ferromagnetic for x=0.75 (TC=102 K) and 1 (TC=107 K). In UCoNiGe2 (a=4.07 Å, c=9.52 Å) no magnetic ordering has been found down to 10 K. Two other samples of UCo2Ge2 (a=4.04 Å, c=9.44 and 9.38 Å) were previously reported not to order magnetically down to 4.2 K. The absence of magnetic ordering is related to smaller c parameters and lower crystal symmetry.
Improvement of the magnetic properties of CoNiCr thin films by annealing
Authors: Duan S. L. , Artman J. O. , Hono K. , Laughlin D. E.
Published: July 2002

The changes in the magnetic properties of CoNiCr/Cr thin films with annealing are found to depend not only on the annealing conditions but also on the Cr underlayer thickness dCr. After vacuum anneal, the in-plane coercivity Hc increased initially with total annealing time τa and then leveled off. The Hc increase was greater at higher annealing temperatures Ta and in samples with thicker Cr underlayers. Hc values as high as 1800 Oe were obtained. The squareness S* decreased with τa, and the saturation magnetization Ms decreased slightly with τa. No significant change was found in the grain size and the crystal texture of the films after annealing, regardless of the Cr underlayer thickness. However, for the sample with a large Hc increase following anneal, it was found that the corresponding Lorentz micrograph image changed dramatically. TEM x-ray energy dispersive data indicated that the Cr concentration in the CoNiCr layer increased after anneal. The changes in Hc, S*, and Ms following air anneal were similar to those found following vacuum anneal. However, at the same Ta, the increase in Hc was less; also Ms was found to decrease noticeably. We believe that the changes in magnetic properties following anneal are associated with increased Cr content in the grain boundaries.
Magnetic and structural properties of CoCrTa films and multilayers with Cr
Authors: Sellmyer D. J. , Wang D. , Christner J. A.
Published: July 2002

We report our studies of epitaxial growth of CoCrTa films on Cr underlayers and the properties of CoCrTa/Cr multilayers. The coercivity, Hc, strongly depends on Ta composition, sputtering conditions, and the thicknesses of the magnetic layer and Cr underlayer. An Hc value of 1300 Oe was obtained for a Ta composition of 2 at. %, a Cr underlayer thickness of 4000 Å, and a magnetic layer thickness of 400 Å. The x-ray data show that the high Hc occurs when crystallites of the Cr underlayer and CoCrTa layer are aligned with the Cr (200) and CoCrTa (110) planes in the film plane. Thus, the c axis of the CoCrTa lies essentially in the plane of the film. When the thickness of the magnetic layer increases above 1000 Å the c axis begins to tip out of the film plane. The basal plane lattice parameter varies roughly linearly with Ta content up to 13 at. %. For the CoCrTa/Cr multilayered films, Hc values up to 1200 Oe were obtained although the c-axis orientation of the magnetic layer becomes somewhat dispersed. Models for the dependence of magnetization reversal on microstructure are discussed.
Two-dimensional time-domain solution of the eddy current problem in rotating electrical machinery
Authors: Chari M. V. K. , Bedrosian G. , Sober T. J. , Scheibel J.
Published: July 2002

A time-domain finite-element solution for evaluating the magnetic field distribution in a two-dimensional rotating structure is presented. The analysis is applied to crack detection in rotating shafts and the numerical results are compared with tests with good agreement.
Boundary element solution of nonlinear induction hardening applications with rotational symmetry
Authors: Ahmed M. Ramadan , Lavers J. D. , Burke P. E.
Published: July 2002

In this paper, the 2D thermal problem with short ferromagnetic billets in induction surface hardening applications is considered. An iterative numerical solution based on the impedance boundary condition formulation is developed. In this solution, the full matrix is generated once and stored. The effects of the nonlinear parameters of the magnetic material and the time-dependence temperature rise on the field calculations are considered in estimating the surface impedance. Throughout the total heating time, the latter is updated to the change that occurred in the values of these parameters. The main advantage of this solution is that in each iteration only the coefficients of the matrix which are dependent on the surface impedance have to be updated. Therefore, the CPU time of the solution does not increase proportionally with the number of iterations. Numerical results for the surface power density distribution of a ferromagnetic cylinder coaxial with an induction coil are presented. Also, results showing the corresponding temperature rise and its effect on the nonlinear magnetic permeability are given. Finally, the effect of the nonlinear parameters on the phase angle of the field distributions is discussed.
Asymmetry in bus bars due to proximity effects
Authors: Cao M. , Biringer P. P.
Published: July 2002

This paper shows the asymmetry in heavy current-carrying bus-bar systems due to proximity effects. Calculations show that current distributions and losses may be different in each phase in spite of the symmetrical system geometry. Both a two-conductor and a three-phase bus system are analyzed. Current densities, power losses, and electromagnetic forces are calculated.
Multiple conductor induction problem: Analytical approach
Authors: Namjoshi K. V. , Biringer P. P.
Published: July 2002

Using an analytical method, this paper calulates the magnetic field when two long, perfectly conducting nonmagnetic cylinders are placed in a uniform transverse magnetic field. The expressions derived here may be used to estimate power loss in cylinders with finite conductivity but small skin depth to indicate proximity effects. The expressions are also shown to describe the field for cylinders with high permeability at large skin depths.
Magnetic properties and microstructures of the Nd-Fe-B magnet powder produced by hydrogen treatment-(II) (abstract)
Authors: Nakayama R. , Takeshita T. , Itakura M. , Kuwano N. , Oki K.
Published: July 2002

The hydrogen treatment of the Nd-Fe-B alloy ingots was found to produce magnet powders with good magnetic properties. Typical magnetic properties of these powders are as follows: 4πIs=9.5 kG, Br=7.7 kG, iHc=9.4 kOe, and (BH)max=12.2 MG Oe. Microstructural studies of these powders showed that they are made of fine crystalline grains of ∼0.3 μm diameter and that these crystalline grains in the individual magnet powder are not necessarily enclosed with boundary phase(s), which is quite different from previously known Nd-Fe-B magnets, i.e., the sintered magnet (the nucleation type magnet) or the amorphous ribbon magnet (the pinning type magnet). It is also noted that the size of these crystalline grains is comparable to that of the single magnetic domain of the tetragonal Nd2Fe14B intermetallic compound and the coercivity force of these powders appears to be related with their fine crystalline grain size.
NMR studies of YB2Cu3O7−δ (T
c=90 K) (invited) (abstract)
Authors: Slichter Charles P.
Published: July 2002

The author and his colleagues report Cu NMR studies of YB2Cu2O7−δ (Tc=90 K) in the normal state (using oriented single crystals) and superconducting state (using oriented powders). In the normal state they measure the electric-field-gradient tensor, the magnetic-shift tensor, the spin-lattice-relaxation tensor, and the time dependence of the transverse decay (spin-echo envelope), and draw conclusions about the charge and spin states of the Cu atoms. In the superconducting state they measure the temperature dependence of the shift tensor, using the 89Y resonance to measure the internal field in the sample. The data are compared with various detailed models of spin pairing.
A Mössbauer spectroscopy study of the structural and magnetic properties of Nd16(Fe0.78Co0.18Mo0.04)76B8 and Nd16(Fe0.80Co0.18W0.02)76B8
Authors: Diao Zhitao , Chen Xinshu , Shen Xufang , Wang Yongqiang
Published: July 2002

The crystallographic and magnetic properties of Nd16(Fe0.78Co0.18Wo0.02)76B8 and Nd16(Fe0.80Co0.18W0.02)76B8, for the sake of contrast analysis, including Nd16(Fe0.82Co0.18)76B8 and Nd16Fe76B8, have been investigated by a Mössbauer effect study with six magnetic Lorentzian sextets and one paramagnetic Lorentzian doublet at room temperature. The hyperfine field changes with the addition of transition-metal molybdenum or tungsten into Nd16(Fe0.82Co0.18)76B8 indicating a preferential occupation of the k2, {e,c} sites for molybdenum atoms and of the j1, k2, and e sites for the tungsten atoms over the six distinct iron sites in the tetragonal phases of these magnetic alloys. The Mössbauer spectral analysis confirms that the transition metals molybdenum and tungsten characterize Nd16(Fe0.78Co0.18Mo0.04)76B8 and Nd16(Fe0.80Co0.18W0.02)76B8 in coercivities, Curie temperatures, and saturation magnetizations, and even in the stabilities of the alloys. The microscopic structure of the paramagnetic phases in these alloys, which is non-negligible in determining the performance of the magnetic alloys, is otherwise shown to be modified by substitution of these transition metals for iron based on the present Mössbauer spectral study.
Lightweight permanent magnet stack for traveling-wave tubes
Authors: Leupold H. A. , Potenziani II E. , Tauber A.
Published: July 2002

A prototype of an iron-free periodic permanent magnet stack was designed and built to serve as the electron beam focusing element in a traveling-wave tube. The stack is composed entirely of permanent magnet material and hence has no iron pole pieces that are subject to saturation. For the bore dimensions and period of interest, this design affords an order of magnitude size and weight reduction below conventional structures for field amplitudes in the technologically interesting 4 to 5 kOe range. A comparison with other novel and conventional structures is made with regard to attainable field amplitude, mass, bulk, and general practicality. The measured field of the prototype agrees with the theoretical values to within a few percent.
Temperature-compensated Pr1−x−ySmxRyCo5−δ permanent magnets (R=Er, Dy, Ho, Gd, and Tb; x=0.24; y=0.2, 0.3, and 0.4)
Authors: Velu E. M. T. , Obermyer R. T. , Sankar S. G. , Wallace W. E.
Published: July 2002

Application of permanent magnets in devices such as traveling wave tubes requires a high degree of flux stability over a wide temperature range (−50 to 150 °C). PrCo5-based permanent magnets with near zero temperature coefficient of remanence (α) can be fabricated by the substitution of a suitable heavy rare-earth element for Pr in Pr1−x−ySmxRyCo5 (R=Er, Dy, Ho, Gd, or Tb). Pr0.76−ySm0.24RyCo5 magnets with y=0.2, 0.3, and 0.4 and R=Er, Dy, Ho, Gd, and Tb were fabricated and the temperature coefficient of remanence (α) and the temperature coefficient of coercivity (β) were determined. Among all the heavy rare-earth elements examined, the substitution of Er resulted in magnets with maximum Br and (BH)max at all temperatures (22–150 °C). Magnets with the substitution of Gd exhibited the highest Hci at all temperatures. The substitution of Dy and Tb gave magnets with low values for Br and (BH)max. Optimally processed magnets with a typical composition Pr0.46Sm0.24Er0.3Co4.6 exhibited Br =8000 G, Hci =15 800 Oe, (BH)max =16 MG Oe at room temperature, and α=−0.015 (%/ °C) and β=−0.34 (%/ °C) over the temperature range 22–150 °C.
Mechanism of metallic ferromagnetism
Authors: Hirsch J. E.
Published: July 2002

Recent results on a new model proposed to describe metallic ferromagnetism are summarized. In addition, the condition on the parameters in the model to give rise to full spin polarization is found exactly for a d-dimensional half-filled band system on a hypercubic lattice. Mean-field theory reproduces this condition over an increasing parameter range as the dimensionality increases.
Room-temperature susceptibility of Cr-Mn alloy films
Authors: Hejase H. , Schröder K. , Miller A.
Published: July 2002

Very thin chromium-rich Cr-Mn alloy films were prepared in situ in a high-vacuum torque magnetometer and their magnetic moments were measured as a function of field at room temperature. It was found that the susceptibility of most of the alloys with 15 at. % Mn was close to zero, whereas alloys with a higher Mn concentration were paramagnetic, with susceptibility values larger than those found for bulk manganese.
Transient laser-induced voltages in room-temperature films of YBa2Cu3O7−x
Authors: Tate K. L. , Johnson R. D. , Chang C. L. , Hilinski E. F. , Foster S. C.
Published: July 2002

Transient laser-induced electrical signals with peak voltages of ∼50 mV and <20 ns duration (full width half-maximum) have been observed in room-temperature thin films of YBa2Cu3O7−x in the absence of an applied current. It is demonstrated that the signal polarity is reversed when the films are illuminated through the substrate rather than at the air/film interface. The mechanism for this effect has yet to be elucidated.
The influence of thermal preanneals on the hydrogen passivation efficiency in liquid encapsulated Czochralski-grown GaAs
Authors: Stievenard D. , Boddaert X. , von Bardeleben H. J.
Published: July 2002

The introduction rate of a hydrogen-related bistable electron trap, formed by a 150 °C plasma annealing, depends strongly on thermal preannealing. A pure thermal 850 °C preanneal completely anneals the native defects EL6 and EL3, reduces the native defect concentration EL2 by a factor of 10, and decreases equally the introduction rate of the bistable defect by a factor of 10. From a comparison between the profiles of these defects, it is possible to get informations on the microscopic structure of this hydrogen related defect as well as on the EL2 defect.
A new mode of thermomagnetic direct overwrite with a fixed bias field
Authors: Suzuki Yoshio , Niihara Toshio , Ohta Norio
Published: July 2002

The magneto-optical write and overwrite process on Tb24Fe65Co11 films with a fixed bias magnetic field was investigated using a static recording system. It was found that with increasing the write laser pulse energy the recorded domain size first increases, then decreases, and finally increases again for films thicker than 100 nm. At certain conditions domains written by irradiation with a light pulse can be annihilated by application of a light pulse with higher energy. Based on this observation a new mode of direct overwrite with fixed bias magnetic field is proposed in which domains are written with a light pulse, and erased with a pulse of larger energy.
Theorems relating to space-charge retardation and enhancement of current transport in solids
Authors: Fromhold A. T.
Published: July 2002

The following theorem regarding charge transport in solids is proved: The current of a single mobile species given by the linear diffusion equation under constant voltage and fixed values for the conductivity at the interfaces is retarded by the space charge of that mobile species. Two related theorems, likewise proved, show that dominant space charge of opposite sign to a given mobile species enhances the current of the mobile species, while dominant space charge of the same sign as a given mobile species retards the current of the mobile species.
Effect of metallization on crystalline perfection and level of stress in semi-insulating and n-type gallium arsenide single-crystal wafers
Authors: Lal Krishan , Niranjana S. , Goswami N. , Würfl Joachim , Hartnagel H. L.
Published: July 2002

Results obtained by x-ray diffractometry, topography, and curvature measurements made with a quadruple-crystal x-ray diffractometer used in (+,−,+) setting and as a Lang system are reported. About 0.1-mm-thick semi-insulating (SI) (Cr-compensated) and n-type wafers with dimensions ∼10×10 mm2 or less were used. The metallizations were (i) Ge-Ni-WSi2-Au and (ii) Ge-Au-Ni-WSi2-Au, with layer thicknesses as follows: Ge: 20 nm; Au: 5 nm; Ni: 10 nm; WSi2: 100 nm; and Au: 100 nm. Both types of metallizations were deposited on n-type crystals, whereas only the second type was used with SI wafers. Diffraction curve half-widths of the wafers without deposits were in the range 15–22 arcsec (SI) and 18–30 arcsec (n type). Radii of curvature (R) were around 6 and 3 m for SI and n-type crystals, respectively. Topographs showed low defect density. Metallization decreased the level of perfection and enhanced curvature. Diffraction curve half-widths increased up to 155 arcsec (SI) and 180 arcsec (n type). High-strain field and bandlike features were observed in topographs, indicating nonhomogeneous distribution of strain. R values decreased to 0.8 m (SI) and 0.3 m (n type). These correspond to stress values of 1.7×1010 and 4.5×1010 dyn cm−2, respectively. Anisotropy in R values was studied by changing diffracting planes. Lattice planes parallel to the metallization were also curved, but in small regions the degree of perfection was high. The five-layered metallization produced more stress in n-type crystals in comparison to the other deposit.
Guided-wave optical bistability and limiting in zinc sulfide thin films
Authors: Svensson B. , Assanto G. , Stegeman G. I.
Published: July 2002

All-optical limiting and bistability are experimentally demonstrated in zinc sulfide waveguides excited by distributed grating couplers. The results are consistent with the presence of a diffusive nonlinearity through the optothermal effect, and a simplified model of the nonlocal traveling-wave interaction between radiation and guided modes is in agreement with the experimental data.
Drift time limited hydrogenated amorphous silicon detectors with picosecond response times
Authors: Beinstingl W. , Sawadcitang P. , Höpfel R. A. , Gornik E.
Published: July 2002

We have utilized rf glow discharge hydrogenated amorphous silicon (a-Si:H) to build detectors in sandwich geometry with response times in the ps region. Analyzing the shape of the pulses recorded with a sampling head, carrier relaxation times and drift times were obtained. A transition from relaxation time limited to drift time limited response was observed by increasing the bias voltage. Carrier relaxation times and drift mobilities were determined as 460 ps and 0.2 cm2/V s, respectively, the shortest drift time limited response was below 80 ps.
Nature of epitaxial growth of high-T
c laser-deposited Y-Ba-Cu-O films on (100) strontium titanate substrates
Authors: Singh R. K. , Narayan J.
Published: July 2002

Epitaxial YBa2Cu3O7 films were fabricated on (100) SrTiO3 substrates by the pulsed laser (λ =308 nm, τ=45×10−9 s) evaporation of bulk targets in a controlled oxygen atmosphere (partial pressure 100–600 mTorr). The nature of epitaxial growth and orientation of the film was controlled by the oxygen partial pressure in the chamber and the substrate temperature during the deposition process. These parameters were also important in determining the superconducting properties of the thin films. The best films exhibited zero electrical resistance at about 90 K with very high critical current densities. Thin single-crystal films were grown in situ at substrate temperatures greater than 550 °C with the c axis perpendicular to the substrate, whereas films deposited at lower substrate temperatures showed a mixture of c- and a axes perpendicular to the substrate. The films deposited in high vacuum or low oxygen partial pressure (<1 mTorr) and further annealed at high temperatures (∼900 °C) in oxygen showed rectangular grains with the c axis parallel to the substrate. Higher-temperature (>920 °C) annealed films showed different surface morphology with the c axis perpendicular to the substrate. The epitaxial growth of YBa2Cu3O7 films on (100) SrTiO3 substrates is examined as a function of lattice mismatch between the thin film and the substrate. As the a′ and c axes of the tetragonal unit cell (above transformation temperature) of YBa2Cu3Ox varies both with the temperature and the oxygen content, a new method is proposed to calculate the lattice parameters of the superconducting phase. This method is based on the lattice parameters calculation at the tetragonal to orthorhombic transformation temperature which always occurs approximately at the same oxygen content regardless of the processing oxygen partial pressure.
Optical emission spectroscopy during sputtering of Y-Ba-CU-oxide targets
Authors: Fleddermann C. B.
Published: July 2002

The sputter deposition of high-temperature superconducting thin films was studied using optical emission spectroscopy. Argon or oxygen ions generated by a Kaufman ion gun were used to sputter material from a composite target containing yttrium, barium, and copper which had been oxygen annealed. The impact of ions onto the target generates a plume of sputtered material which includes various excited-state atoms and molecules. In these studies, optical emission is detected for all the metallic components of the film as well as for metallic oxides ejected from the target. No emission due to atomic or molecular oxygen was detected, however. Variations in sputter conditions such as changes in sputter ion energy, oxygen content of the beam, and target temperature are shown to greatly affect the emission intensity, which may correlate to the characteristics of the sputtering and the quality of the films deposited. The results suggest that optical emission from the sputtered material may be useful for real-time monitoring and control of the sputter deposition process.
The properties of Y1Ba2Cu3O7−δ thin films with silver doping prepared by spray pyrolysis
Authors: Cukauskas E. J. , Allen L. H. , Newman H. S. , Henry R. L. , Van Damme P. K.
Published: July 2002

Superconducting, silver-doped films of Y1Ba2Cu3O7−δ have been deposited by spray pyrolysis of aqueous nitrate solutions onto MgO substrates. The superconducting transport properties, microstructure, and microwave losses have been characterized for various amounts of AgNO3 added to the spraying solution. These films had resistive transition temperatures between 79 and 85 K with widths from 3 to 7 K. The room-temperature resistivity was a strong function of the silver doping, dropping by a factor of 50 for the heavily doped films. Critical current densities at 4 K were typically several 103 A/cm2 with little correlation to the silver doping. Lattice constants also were not significantly affected by the silver doping level, however, lightly doped films were denser, had the strongest c axis preferred orientation, and a smoother surface. Rf surface resistance was measured at 18 GHz, and for the best films dropped a factor of 10 below copper by 40 K. The London penetration depth was estimated to be approximately 1 μm for the best films.
Structure and superconducting properties of YBa2Cu3O7−x films prepared by nitrogen laser evaporation and CO2 laser annealing in oxygen
Authors: Serbezov V. , Benacka St. , Hadgiev D. , Atanasov P. , Elektronov N. , Smatko V. , Stribik V. , Vassilev N.
Published: July 2002

Superconducting YBa2Cu3O7−x thin films were obtained under high vacuum (10−5 Torr) on substrates of polycrystalline Al2O3 sapphire, SrTiO3, and Si, having zero resistance at 81, 85, 87, and 79 K, respectively. A N2 laser of 3.5 J cm−2 energy density was used for the evaporation. The substrates were heated by a cw single-mode CO2 laser and the annealing was performed by the same laser in O2 atmosphere. Local planar superconducting regions were obtained by focusing the radiation of the cw CO2 laser upon the films. The films were investigated by scanning electron microscope, x-ray microanalysis, and x-ray diffraction.
Influence of mechanical grinding and polishing operations of brittle polycrystalline alumina on the pulsed surface flashover performance
Authors: Bommakanti R. G. , Sudarshan T. S.
Published: July 2002

The degradation of the pulsed surface flashover performance of 99.9% pure polycrystalline alumina in an insulator-bridged vacuum gap, due to the mechanical grinding and finishing of the insulator is reported. The flashover characteristics of three different surface finishes—as-fired (0.8 μm), a ground surface (0.25 μm), and a polished surface (0.05 μm)—were studied using time-coordinated voltage, current, and luminosity diagnostics with a temporal resolution of ∼1.5 ns, for 0.5/15-μs pulsed voltage excitations. The flashover strengths of the ground samples are lower than the as-fired sample by about 50% while the strengths of the polished surfaces are intermediate between the two. These results contradict earlier reports which attribute the lower holdoff of smooth finishes to the enhanced secondary emission yield as compared to the rougher surface finishes. The lower flashover strengths for the ground surface as compared to the as-fired surface are attributed by us to the surface defects consisting of intergranular and transgranular fracture features induced by the mechanical grinding operation. The improved performance of the polished surface over the ground surface is proposed to be due to the partial removal of the damage created during the grinding operations. The suggested increase in the defect density in the surface layers of ground and polished surfaces relative to the as-fired finish is substantiated by characteristic defect-dominated signatures in the luminosity-current profiles. These results are in agreement with an earlier report describing the role of trapped carriers in the flashover process. It is concluded that the insulating properties of the surface as influenced by the microstructural features are strongly dependent on the precise mode of material removal during grinding and polishing operations and hence cannot simply be correlated to the gross surface topography in terms of surface roughness, particularly for hard and brittle ceramic materials.
Anomaly in the Ga-Si phase diagram: Nonretrograde solubility of Ga in Si layers grown by liquid-phase epitaxy
Authors: Linnebach R. N.
Published: July 2002

Silicon layers have been grown on Si substrates from Ga-rich solutions by liquid-phase epitaxy at temperatures between 615 and 1220 K. The diffusion coefficient of Si in liquid Ga is estimated from layer thickness data to be DGaSi=0.0235 exp(−4016 K/T) cm2/s. The incorporation of Ga in the Si layers was investigated by means of neutron-irradiation, double-crystal x-ray rocking curve, Auger recombination, infrared reflection, Rutherford backscattering, and Hall effect experiments. Below 850 K the normal retrograde solid solubility of Ga in Si changes to an anomalous nonretrograde solubility. The doping level increases from p=5×1018 cm−3 to 5×1020 cm−3 at 615 K. The concentrations of net carriers, ionized Ga impurities, and total Ga atoms are identical over the entire temperature range, showing that Ga atoms are incorporated on lattice sites. Imperfections such as dislocations, stacking faults, or precipitates are not responsible for such high levels. The occurrence of Si vacancies or microsegregation due to lateral growth are possible driving forces causing this anomalous behavior. Excess free-energy contributions resulting from internal stress produced by lattice mismatch may also provide a mechanism for producing this effect.
Spatially resolved space-charge density in the hydrogenated amorphous silicon Schottky barrier from surface photovoltage measurements
Authors: Ukah C. I. , Perz J. M. , Zukotynski S.
Published: July 2002

A procedure is developed to derive the differential capacitance of the hydrogenated amorphous silicon (a-Si:H) Schottky barrier from surface photovoltage measurements. An algorithm for deducing the spatial variation of the space-charge density, and of the band-gap density of states, from the frequency-dependent capacitance spectrum, is proposed and implemented for undoped a-Si:H. The space-charge density and the density of states near the Fermi level are found to span three orders of magnitude, falling below 1015 cm−3 and 1016 cm−3 eV−1, respectively, into the bulk of the Pt/a-Si:H Schottky barrier.
A temperature-independent capacitance in semiconductor-insulator-semiconductor capacitors
Authors: Hickmott T. W. , Solomon P. M.
Published: July 2002

Both measurements of capacitance-voltage (C-V) curves of n− GaAs-undoped AlxGa1− xAs- n+ GaAs (AlGaAs) capacitors at different temperatures, and calculations of C-V curves of semiconductor-insulator-semiconductor (SIS) capacitors at different temperatures, show that there is a temperature-invariant capacitance CC and voltage VC at which C-V curves at different temperatures intersect. We show that this is a general property of SIS capacitors having a degenerate gate and nondegenerate substrate of the same doping type, and that qVC, where q is the electron charge, is approximately equal to the Fermi energy of the degenerate GaAs gate. VC provides a good estimate for the voltage required to establish an accumulation layer on n− GaAs at low temperatures, which is determined from magnetotunneling measurements on AlGaAs capacitors.
Resonant optical absorption by discontinuous transparent overlayers on gold surfaces
Authors: Truong Vo-Van , Ashrit P. V. , Bader G. , Girouard Fernand E. , Yamaguchi Tomuo
Published: July 2002

Discontinuous MgF2 film deposited on Au surfaces shows enhanced optical absorption. This absorption may result from the coupling of light with the dielectric particles, making possible the excitation of surface plasmons. A simple ellipsoidal particle mode is used to account for the observed absorption.
Trap emission rates in GaAs in the presence of surface acoustic waves
Authors: Janes D. , Hoskins M. J.
Published: July 2002

Experimental and theoretical analyses of the effects of surface acoustic waves (SAW) on trap emission rates in GaAs are presented. Measurements of electron trap emission rates using deep level transient spectroscopy measurements performed in the presence of SAW are discussed. For the three trapping levels investigated, the observed increases in the trap emission rates can be described by trap activation energies which decrease in direct proportion to the SAW power. A theoretical analysis of the effects of the acoustic strain, electric field, and heating associated with the SAW is also presented. This analysis is used to predict the magnitude and functional dependence of the emission rate enhancement. The theoretical predictions follow the same functional dependence as the experimental data but the predicted magnitudes of the shifts in trap energies are about a factor of 2 lower than the measured values.
WSiN/SiO2 capped annealing for Si-implanted GaAs
Authors: Tamura Akiyoshi , Ikeda Yoshito , Yokoyama Takahiro , Inoue Kaoru
Published: July 2002

A new cap annealing method using WSiN/SiO2 multilayer film for Si-implanted GaAs has been developed. The post-implant annealing was performed by varying the SiO2 thickness in Ar/H2 atmosphere without As overpressure. The annealed layers were characterized by Hall effect, capacitance-voltage, secondary ion mass spectrometry, and 77-K photoluminescence measurements. The WSiN/SiO2 capped annealing method was found to offer high-activation efficiency without the deep tailing of carrier concentration profiles. Enhanced activation efficiency with the increase in SiO2 thickness was also observed for this annealing method.
Formation of copper silicides from Cu(100)/Si(100) and Cu(111)/Si(111) structures
Authors: Chang Chin-An
Published: July 2002

The reaction between copper and silicon is studied using differently oriented Si and Cu. Copper films, deposited by electron-beam evaporation at room temperature without intentional heating of the substrates, show an epitaxial relation with the Si substrates, resulting in (100)- and (111)-oriented Cu films on the (100) and (111) Si, respectively. An orientation dependence is observed for the Cu-Si reaction, with different phases of the silicides formed. At 200 °C, for example, the silicide formation rate is about five times faster for the (100)-oriented structures than for the (111) ones. An interface bonding model is used to relate to the orientation dependence observed, and is compared with previous work on the Au-Si and Au-Cu systems. The diamond structure of Si favors the (100) surface for reaction, as is observed for the Au-Si reaction. The face-centered-cubic structure of Cu, however, favors the (111) orientation for reaction. The latter is observed for the Au-Cu reaction using epitaxial Cu films on Si. Combined bonding consideration for both Cu and Si indicates a dominant role of Si release at these temperatures.
Effects of finite radial geometry on free-electron-laser instability with a longitudinal wiggler field
Authors: Mishra G. , Tripathi V. K.
Published: July 2002

The theory of free-electron-laser instability for a tenuous relativistic electron beam propagating parallel to a longitudinal wiggler field has been extended to include the influence of finite radial geometry on its stability properties. The growth rate of the instability shows a sensitive dependence on (ν/γ)1/3, where ν is the Budker parameter and γ is the beam energy. The growth rate increases with increasing beam intensity. However, the instability bandwidth with respect to the beam location inside the waveguide decreases for beams with somewhat higher ν/γ values. The instability is broadband for several harmonics excited.
Induced emission cross section of Nd:Y3Al5O12 ceramics
Authors: Sekita M. , Haneda H. , Yanagitani T. , Shirasaki S.
Published: July 2002

Optical absorption and emission spectra have been measured for Nd-doped Y3Al5O12 ceramics obtained by a urea precipitation method. The optical properties of the ceramics are almost the same as those of single crystals grown by the Czochralski method and floating-zone method, except for a higher background absorption of 2.5–3 cm−1. The energy-level structure for Nd3+ in the Y3Al5O12 ceramics is determined for a 1-at. % Nd concentration. The induced emission cross section for the 1-at. %-Nd-doped ceramic is calculated to be 4.9×10−19 cm2, which falls in the range reported for the Nd:Y3Al5O12 single crystals. The Nd concentration dependence of the emission decay time at the 1064-nm laser wavelength was also measured, and typical concentration quenching was observed. The decay time of the 1-at. %-Nd-doped ceramic is 219 μs, which is slightly smaller than the reported values for single crystals.
Monte Carlo simulation of the transport process in the growth of a-Si:H prepared by cathodic reactive sputtering
Authors: Vidal M. A. , Asomoza R.
Published: July 2002

The transport process of sputtered Si atoms from the target to the substrate, crossing an argon-hydrogen plasma, is modeled by using Monte Carlo techniques. The hydrogen and argon partial pressures, the dark zone voltage, and the target-substrate distance are varied in the calculations. The effect of the above-mentioned parameters upon the thermalization and energy distribution of atoms arriving at the substrate is calculated, allowing the determination of the growth conditions that minimize the damage produced on the films by highly energetic atoms, while maintaining a reasonable growth rate.
Deep level transient spectroscopy analysis of spatially dependent doping profiles
Authors: Schary Alison , Lee Charles A.
Published: July 2002

The study of deep level semiconductor defects frequently employs deep level transient spectroscopy (DLTS), a technique developed for structures with constant doping concentration profiles. In this paper, the theory of the DLTS measurement is extended to the more general case in which both the doping profile and the trap profile vary with depth. This analysis yields a method of calculating exact DLTS transients for the general case, and by its application to several examples, demonstrates that the doping profile shape and the measurement conditions are both factors which determine the degree to which the DLTS transient can be approximated by an exponential. The structures which do exhibit approximately exponential DLTS transients include several which arise in the course of device processing. This study shows that in such structures, a knowledge of the doping profile and the general form of the trap profile can be combined with the standard DLTS boxcar sampling method to obtain trap profile information in addition to the trap activation energy and capture cross-section data.
Quasilinear theory of radiation saturation in a free-electron laser
Authors: Marable W. P. , Sprangle P. A.
Published: July 2002

A quasilinear theory of the self-consistent evolution of the electron-beam distribution and radiation field within a free-electron laser is presented. The relativistic electron beam propagates through an ideal helical wiggler B0=−Bw(cos kwzêx+sin dwzêy), for which the transverse gradients ∂/∂x=0=∂/∂y have been neglected. The transverse variation of both the transverse-polarized radiation field and the electron-beam distribution have also been neglected. Additionally, the electron beam is assumed to be sufficiently tenuous that the longitudinal space-charge potential δφ is negligible and the Compton approximations are applicable. The resulting asymptotic expression for the spectral energy density is used to evaluate the efficiency and saturated radiation power in a variety of parameter regimes relevant to experiments.
Bi(Pb)-Sr-Ca-Cu-O superconducting composite tapes prepared by the powder method using an Ag sheath
Authors: Kumakura H. , Togano K. , Maeda H. , Mimura M.
Published: July 2002

The Bi-based oxide superconducting tapes containing high-Tc (Tc=110 K) and low-Tc(Tc=85 K) phases were prepared by the powder metallurgy method using an Ag sheath, and Jc was measured at 77 and 4.2 K in magnetic fields up to 30 T. A grain-oriented microstructure with the c axes perpendicular to the tape surface was obtained for the tapes with a high-Tc phase by applying a combination of cold rolling and sintering. Jc of the high-Tc tape was significantly improved by this grain orientation. However, large anisotropy in Jc with respect to the magnetic field direction was observed for these textured tapes. For the low-Tc tapes, it was difficult to obtain a grain-oriented microstructure by the cold rolling and sintering process. However, Jc at 4.2 K of the nontextured low-Tc tape is as high as that of the grain-oriented high-Tc tape. Typical Jc values at 4.2 K were above 104A/cm2 at 12 T for both high- and low-Tc tapes. Jc at 4.2 K of the Bi-based tapes is much less sensitive to the magnetic field than the conventional metallic superconductors, which indicates that the Bi-based oxide is a good candidate for the high-field superconducting magnet.
Analysis of in-plane bit structure by magnetic force microscopy
Authors: Wadas A. , Grütter P. , Güntherodt H.-J.
Published: July 2002

This article uses a fully quantitative approach to describe the behavior of the minute tip of the magnetic force microscope (MFM) over longitudinally directed magnetic bits. Three possible magnetic structures are taken into consideration together with a realistic tip model. The magnetic and van der Waals interactions beween the tip and the sample are derived analytically to simulate a force gradient in MFM. The contours of the constant force gradient are presented and related to the known experimental data. The important role of the van der Waals force gradient in constant force gradient measurements is shown. The force gradient from a magnetic interaction is partly negative and partly positive and is balanced or increased by the (always positive) van der Waals force gradient. Force gradient asymmetry above the transition area is revealed when the MFM tip is tilted. By tilting the tip, the detection of two components of a stray field is accomplished. We found that the widths and heights of observed bumps of a force gradient depend on the width of the transition area and the deviation of the magnetization vector from the recording direction. These factors are especially important in determining what tolerance can be put on a read/write head’s performance. A lateral resolution of 90 nm could be achieved with our tip model.
Electrical and optical characterization of GdSi2 and ErSi2 alloy thin films
Authors: Guizzetti G. , Mazzega E. , Michelini M. , Nava F. , Borghesi A. , Piaggi A.
Published: July 2002

GdSi2 and ErSi2 polycrystalline thin films were studied using electrical resistivity in the temperature range 10–900 K, Hall effect from 10–300 K and reflectivity spectra from 0.2–100 μm at room temperature. Composition and structure in these films were investigated by Rutherford backscattering spectroscopy and x-ray diffraction techniques. These silicides are metallic with (i) a remarkable difference in their residual resistivity, (ii) a phonon contribution to the resistivity which showed a negative deviation linearity, and (iii) low energy interband transitions. Resistivity data indicated that GdSi2 and ErSi2 have a Debye temperature of 328 and 300 K respectively and a limiting resistivity value much higher than that observed in other transition metal disilicides. The charge carrier concentration was estimated to be 4×1021 cm−3 at room temperature according to Hall measurements, and the mean free path was 63 Å and 320 Å for GdSi2 and ErSi2, respectively, at 10 K. The parameters obtained by the optical analysis are in good agreement with those extracted from the transport measurements, thus permitting one to obtain a reasonable value for the Fermi velocity.
Sound emission during crystallization of undercooled liquid germanium
Authors: Lau C. F. , Kui H. W.
Published: July 2002

Liquid Ge which had been fluxed in dehydrated B2O3 was allowed to cool below Tm in a well controlled environment. The cooling rate was 6 °C/min. It always crystallized to the diamond cubic phase and the resulting grain size decreased with increasing undercooling. A maximum undercooling of 342 °C was recorded. It was found that sound was emitted during crystallization whenever the undercooling was ≳258 °C for a ∼7-mm-diam specimen. It was also determined that cracks were the source of the sound.
The pinning force between a Bloch wall and a planar pinning site in MnAlC
Authors: Chen Xinhe , Gaunt P.
Published: July 2002

The maximum pinning force between a Bloch wall and a planar pinning site is calculated to depend linearly on K3/21/A1/21, where K1 and A1 are the anisotropy and exchange constants of the ferromagnet, and as a result the pinning force is, in general, a function of the temperature. The temperature dependence of the coercive field Hc(T) of a MnAlC permanent magnet is well described in a model that considers two factors; one is the temperature dependence of the height of the energy barrier that a domain wall has to overcome on breaking away from a pinning site, and the other is thermal energy which assists a domain wall to cross the energy barrier.
Picosecond transient thermoreflectance: A new probe of two-dimensional and quasi-two-dimensional structures
Authors: Lörincz András
Published: July 2002

A nondestructive type of measurement, picosecond transient thermoreflectance, is proposed to determine cluster sizes, surface coverage, or the fractal dimension of light absorbing species of submonolayer coverage on surfaces and interfaces. The idea is to use a high-repetition-rate pump-and-probe technique of high dynamic range to excite the molecules on the surface selectively and to monitor the transient change of temperature through the change of reflectivity. It is shown that the range of transient reflectivity measurements is not limited by the wavelength of the probe beam, but the relaxation time of the species, and it may be in the nanometer range. An expression for the determination of the dimension of fractal structures is derived.
Stoichiometry-dependent deep levels in n-type GaAs
Authors: Nishizawa Jun-ichi , Oyama Yutaka , Dezaki Kazushi
Published: July 2002

The photocapacitance (PHCAP) method in the constant capacitance condition has been applied to determine the energy and density of deep levels within the limits of a certain depletion layer thickness as a function of the photon energy and the applied voltage. Samples used were various n-type GaAs crystals annealed for a long time under excess As vapor pressure and rapidly cooled. The present PHCAP investigations have revealed several stoichiometry-dependent deep levels including some photoquenching levels. These are Ec−0.66 and −0.72 eV deep donors, the photoquenching level at 1.03, 1.25, and 1.41 eV. The As-vapor-pressure dependence of the density of these deep levels in quasithermal equilibrium were also clarified. The 1.25-eV photoquenching level density increased monotonically with increasing applied As vapor pressure and finally saturated at extremely high As vapor pressures. The density of Ec −0.72 eV deep donors showed a similar dependence on the As vapor pressure, but showed less saturation under higher As vapor pressures. The Ec −0.72 eV deep donor is quite different from the so-called EL2 level. But the temperature dependence of the PHCAP responses (ΔVph -T) revealed strong interaction effects among these defect structures. From the photoluminescence results and conventional deep-level transient spectroscopy measurements,it was clarified that both Ec −0.66 and −0.72 eV deep donors had larger Frank–Condon shifts, 0.28 and 0.24 eV, respectively, compared with that of the so-called EL2 level (0.12 eV). The PHCAP measurements after photoexcitation were carried out to detect de-ionization of the photoionized deep levels. Additional de-ionization of the photoionized state of the 1.41-eV photoquenching level was detected adjacent to those of the EL2 level. These de-ionization could be detected only in the course of the photoquenching phenomenon in n-type GaAs. The As-vapor-pressure dependence of these deep levels shows that interstitial As atoms play a vital role in forming deep levels in annealed GaAs crystals.
Combinations of the electroelastic and electrostrictive constants of quartz determined using the thickness modes of plates
Authors: Hruska Carl K. , Brendel Remi
Published: July 2002

Ten linear combinations of the fundamental material constants of electroelasticity and electrostriction of quartz have been determined. They were obtained by the least-squares fit to 134 observations of the change in the resonance frequency of the thickness modes of quartz plates caused by a dc field. With one exception, there is very good agreement between our results and their counterparts obtained independently by the transit-time method (pulse propagation in bulk quartz). The differences between comparable constants are no longer out of proportion with their standard errors. This work supersedes earlier results [J. Appl. Phys. 65, 715 (1989)].