bluuebunny/crossref_metadata_2025 huggingface dataset:
Dependence of keV x-ray generation on the temporal and spatial separation of two KrF laser pulses
Authors: Bobkowski R. , Broughton J. N. , Fedosejevs R. , Willis R. J. , Cervenan M. R.
Published: July 2002

The generation of keV x-ray emission from plasma produced by a 15 mJ 90 ps KrF laser pulse focused to an 8 μm spot on a copper target preceded by a similar 8 mJ pulse is studied experimentally. The change in efficiency of generation of keV x rays by the second pulse due to the presence of plasma produced by the preceding laser pulse on target is investigated as a function of the spatial and temporal separation of the laser pulses. When closely overlapped in space and time the pair of pulses behave like a single higher intensity pulse. When separated in space by more than 100 μm the two pulses behave independently generating x rays at the same rate as they would as two separate pulses. However, when overlapped in space and separated in time the x-ray generation by the second pulse is enhanced for small temporal separations and then suppressed for larger temporal separations due to the plasma created by the first pulse. These results can be explained by the initial production of hot plasma which enhances x-ray generation followed by the production of an extended colder plasma from the unloading of the shock heated target surface which suppresses keV x-ray production by the second pulse. The observed time of transition from enhancement to reduction of x-ray generation agrees with the time for the hot initial plasma due to the first laser pulse to expand and cool significantly.
Effect of Al overlayer thickness on the leakage current of radiation detectors using Nb/Al-AlOx/Nb superconducting tunnel junctions
Authors: Matsumura A. , Takahashi T. , Kurakado M.
Published: July 2002

Nb/Al-AlOx/Nb superconducting tunnel junctions for the application of radiation detectors were fabricated using dc magnetron sputtering. The surface morphology of polycrystalline Nb films was observed using an atomic force microscope. Junctions with various Al film thicknesses were fabricated. The leakage current of the junctions is found to decrease as the Al thickness increases, indicating an improvement of the Al film coverage over the Nb surface. X rays were detected with the junctions prepared and the low leakage property across the junction makes possible x-ray detection with various size junctions up to 200×200 μm2.
Further evidence for a ‘‘second universality’’ in alternating-current conductivity relaxation
Authors: Nowick A. S. , Vaysleyb A. V. , Lim B. S.
Published: July 2002

The existence of a regime of constant-loss (independent of frequency) behavior was reported earlier, based on ac conductivity measurements at low temperatures and/or high frequencies, for a wide range of ionically conducting crystalline materials and glasses. Here new results in support of this concept are presented. First, it is shown that the example of NaCl:Zn2+ which Macdonald examined in detail [J. Appl. Phys. 75, 1059 (1994)] is a poor one to serve as a test of constant-loss behavior. Additional cryogenic measurements, and a more detailed analysis of both the real and imaginary parts of the complex dielectric constant for materials such as crystalline CaTiO3:Al3+ and Na2O⋅3SiO2 glass are shown to support the conclusion that constant-loss behavior occurs at low temperatures for a wide range of materials.
Determination of surface roughness of InP (001) wafers by x-ray scattering
Authors: Cui S. F. , Li J. H. , Li M. , Li C. R. , Gu Y. S. , Mai Z. H. , Wang Y. T. , Zhuang Y.
Published: July 2002

The surface roughness of polished InP (001) wafers were examined by x-ray reflectivity and crystal truncation rod (CTR) measurements. The root-mean-square roughness and the lateral correlation scale were obtained by both methods. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model of surface faceting is proposed to explain the experimental data. The sensitivities of the two methods to the surface structure and the role of the resolution functions in the CTR measurements are discussed.
Epitaxial growth and giant magnetoresistance of Co/Cu (111) superlattices on Cr buffer layers
Authors: Tian Zeng-Cheng , Sakaue Kiyoshi , Terauchi Hikaru , Ito Haruo
Published: July 2002

The Co/Cu superlattices have been prepared by molecular-beam epitaxy on Si(001) substrates with a Cr buffer layer. X-ray-diffraction experiments showed that highly (111)-oriented superlattices were obtained. The magnetoresistance was investigated as a function of Cu spacer thickness tCu from 20 to 45 Å. The giant magnetoresistance (GMR) is oscillatory and the period of oscillation is about 8–9 Å. The maximum GMR was found as high as 10% with 300 Oe low field at room temperature. In addition, the resistance versus field curves were found to be clearly hysteretic, and the fields corresponding to the maximum resistence Rmax are oscillatory with Cu layer increasing, as long as the saturated field has oscillatory changes with tCu.
Effects of photoinduced hole doping on transport properties of YBa2Cu3Oy grain boundary junctions
Authors: Tanabe K. , Hosseini Teherani F. , Kubo S. , Asano H. , Suzuki M.
Published: July 2002

The effects of photoinduced hole doping on the electrical characteristics of grain boundary weak link junctions based on high Tc YBa2Cu3Oy thin films are investigated. Visible light illumination induces a substantial enhancement in the critical current Ic of approximately 20%–40% even at temperatures well below Tc. The Ic enhancement is accompanied by a clear decrease in the junction resistance RN which can be explained by the photoinduced hole doping of the degraded regions with oxygen deficiency and/or disorder near the grain boundary. It is also demonstrated that photoinduced hole doping provides a way of improving the IcRN product and also a way of controlling the Ic value in situ in these weak link junctions.
Ablation of metal films by picosecond laser pulses imaged with high-speed electron microscopy
Authors: Bostanjoglo O. , Niedrig R. , Wedel B.
Published: July 2002

The ablation of free-standing metal films by picosecond laser pulses (50 ps, 0.6–8 J/cm2, 532 nm) was visualized by electron microscopy. Triple-frame high-speed transmission electron microscopy was applied, with exposure times below 5 ns and frame repetition times ≳20 ns. Ablation was observed to proceed by hole opening within 5–30 ns, speeding up with increasing laser fluence, and punching out of most of the melt. Once opened, the holes expanded by capillary forces with a velocity of ≊100 m/s for some 10 ns. At large pulse fluences the evaporation was collimated along the film axis. This and the absence of patterned flow of the melt are conspicuous differences to the ablation of films by nanosecond laser pulses. The effects typical for picosecond pulses are attributed to an evaporation that is one-sided because of a thermal gradient along the film axis, and that exerts a large recoil.
Mott metal-insulator transition driven by an external electric field in coupled quantum dot arrays and its application to field effect devices
Authors: Ugajin Ryuichi
Published: July 2002

The Mott metal-insulator transition in coupled quantum dot arrays (CQDAs) can be driven by an external electric field perpendicular to the arrays. By changing the applied electric field, the transfer energy is effectively modulated and quantum states of two electrons in a pair of coupled quantum dots change from uncorrelated states to correlated states. Our numerical results suggest that the Mott transition can be driven by a base electrode, and the effect provides a new method of modulating transport properties in CQDAs. We can modulate only collective excitations in a channel from metallic excitations carrying the current to insulating excitations if we use the effect for transistor operations.
Chemisorption of In and Al on GaAs(110)
Authors: Ortega José , Demkov Alexander A. , Sankey Otto F.
Published: July 2002

A quantum molecular dynamics study of the interface formation of the Al/GaAs(110) and In/GaAs(110) interfaces has been performed. It is found that, at very low coverages, both Al and In atoms chemisorb on threefold coordinated sites, bonded to two As atoms and one Ga atom. The metal atoms attract each other on the GaAs surface and tend to form islands. The electronic structure for one monolayer is found to be semiconducting for both In and Al. The results presented here are consistent with the temperature-dependent evolution of Fermi level from the very low metal coverage limit to the overlayer limit for the deposition of In and Al on GaAs(110).
Energy levels of single and coupled quantum wells embedded in cylindrical buffer barriers affected by an axial magnetic field
Authors: Ping Er-Xuan , Dalal Vikram
Published: July 2002

Energy levels of an electron in single and coupled quantum wells embedded in cylindrical buffer barriers are studied under the influence of an axial magnetic field. An iteration algorithm which matches the boundary conditions of the wave function across interfaces is developed. It is shown that energy levels are tuned continuously as a function of the magnetic field strength. The degeneracy of energy levels occurring in planar coupled quantum wells for large barrier thickness is completely lifted due to the break of the symmetry in the cylindrical coupled quantum wells. Potential applications for tunable semiconductor lasers are proposed.
Hydrogen blisters on β-NbD after laser pulse heating
Authors: Schober T. , Bechthold P. S.
Published: July 2002

Plates of β-NbD0.7 were irradiated with single pulses of a KrF excimer laser having a wavelength of 248.4 nm. The power level was varied from roughly 0.1 to 1 GW/cm2. The pulse length was about 15 ns. The irradiated areas at low and medium power displayed individual hydrogen (deuterium) blisters which started overlapping at higher power levels. Calculation of the thermal diffusivity showed that a surface layer roughly 0.6 μm thick was heated during the pulse. Applying Bechtel’s formula [J. Appl. Phys. 46, 1585 (1975)] for the temperature increase in the heated layer demonstrated that melting of the heated layer must have occurred during the pulse. The formation of hydrogen (deuterium) blisters is understandable in terms of the very high equilibrium vapor pressure of the hydrides near the melting temperature. A model for the formation of the blisters is presented.
Transmission loss characteristics of telluride-based chalcogenide glass optical fibers
Authors: Katsuyama Toshio , Matsumura Hiroyoshi
Published: July 2002

Transmission loss characteristics of Ge-Se-Te ternary chalcogenide glass optical fibers are investigated. It is found that there is a broadband transmission loss at wavelengths between 2 and 10 μm, which greatly affects the loss at 10.6 μm (CO2 laser emission wavelength). This broadband loss increases exponentially with the Te content, and is inversely proportional to the third power of the wavelength. Furthermore, the glass is found to contain hexagonal Te microcrystals. These results indicate that the broadband loss originates from the Mie scattering due to hexagonal Te microcrystals whose diameters are estimated to be 0.2 μm. Therefore, it is found that to obtain low-loss Te-based optical fibers it is essential to eliminate the hexagonal Te microcrystals.
Promotion of the oxidation of silicon carbide by a rubidium overlayer
Authors: Riehl-Chudoba M. , Soukiassian P. , Jaussaud C.
Published: July 2002

We investigate the effect of a Rb overlayer on the cubic SiC(100) surface oxidation by core level photoemission spectroscopy. The presence of Rb monolayer on the β-SiC(100) surface enhances the oxygen uptake by four orders of magnitude at room temperature and results in the selective formation of high silicon oxidation states. Furthermore, the Rb catalyst could be desorbed from the surface by rapid thermal annealing at moderate temperature, leading to the formation of a Si oxide layer on SiC(100) with no carbon atom present on the surface and/or in the oxide layer.
Magnetic properties and magnetic hardening mechanism of Pt-Co-B alloys
Authors: Qiu Ning , Flanagan William F. , Wittig James E.
Published: July 2002

The intrinsic coercivity is found to be maximized in the Pt42Co45B13 ternary alloy which is undercooled and rapidly solidified (quenched using a 70 m/s wheel speed after undercooling), and then annealed (800 °C for 2400 min). The same alloy, processed at slower cooling rates and annealed in the same way, has a much larger scale microstructure and a much lower resulting magnetic coercivity. The microstructure which would optimize the coercivity of this ternary alloy is a completely ordered L10 Pt-Co matrix with a submicron magnetic single-domain Co-boride precipitate. The L10 phase is highly anisotropic magnetically while the Co-boride precipitate is somewhat less so. Annealing treatments designed to produce single-domain Co-boride precipitates enhance the coercivity. This suggests that the refined microstructure is responsible for the high coercivities found in the rapidly solidified and annealed alloy. The magnetic domain wall thickness for a Co-boride precipitate is determined from both experimental observation and theoretical calculation in order to evaluate its influence on the coercivity of the alloy. The effects of the pinning of domain walls and the barrier to the nucleation of reverse domains on the coercivity are discussed. Both microstructural analysis and theoretical calculation indicate that the high coercivities in the Pt42Co45B13 alloy are due to the difficult nucleation of reverse magnetic domains.
On the relationship between magnetic viscosity and coercivity of perpendicular media
Authors: te Lintelo J. G. Th. , Lodder J. C.
Published: July 2002

The dependent and dc-demagnetization measurements were performed on two types of perpendicular thin film media, Co-Cr and Fe-alumite. From these the parameters for magnetic viscosity and irreversible susceptibility were derived. Their ratio, resulting in the fluctuation field, is shown to relate logarithmically to the coercivity. This so-called ‘‘Barbier plot for perpendicular media’’ is found to exhibit a slope of approximately one and a half instead of one, as is found for in-plane media. The activation volumes were also derived and related logarithmically to the coercivity. Here a slope of approximately minus one and a half was found, instead of minus 0.73 as is found for in-plane media.
Procedure for fitting Monte Carlo calculated impact ionization coefficient to experiment
Authors: Ershov M. , Ryzhii V.
Published: July 2002

A procedure for fitting of Monte Carlo calculated impact ionization coefficient to experimental data has been proposed. This procedure has been applied for optimization and sensitivity analysis of fitting parameters of impact ionization model for electrons in Si. Strong correlation between threshold energy and preexponential factor of the impact ionization model and redundancy of power exponent have been found. A wide range of data on impact ionization coefficient can be fitted by adjusting the parameters of the microscopic impact ionization model.
Charge-defect thermodynamic equilibrium and ‘‘metastable’’ defects in amorphous silicon
Authors: Fortmann C. M. , Dawson R. M. , Liu H. Y. , Wronski C. R.
Published: July 2002

A thermodynamic equilibrium description of the high-temperature (150–250 °C) steady-state behavior of light-induced defects in amorphous silicon is presented. The entropy and enthalpy of dangling-bond formation are quantified. In contrast to the behavior of vacancies in single-crystalline silicon the creation of the dangling-bond defect in amorphous silicon produces negative entropy and enthalpy changes indicating that lattice relaxations contribute to the free-energy changes. Over the temperature range examined, the creation of dangling bonds lowers the free energy due to the relatively large negative enthalpy change. Practical issues such as the estimation of the saturated dangling-bond density resulting from given illumination level at temperatures too low to experimentally observe true saturation are also considered.
Decomposition products of superconducting (Bi,Pb)2Sr2Ca2Cu3O10+δ
Authors: Xu Ming , Finnemore D. K.
Published: July 2002

High-temperature in situ x-ray studies of Pb-doped Bi2Sr2Ca2Cu3O10+δ have been carried out in a variety of different environments in order to determine the range of stability and the decomposition products that appear as this material transforms into other phases. In air at 840 °C, a combination of Bi2Sr2Ca0Cu1O6+δ, Ca2PbO4, (Sr,Ca)2Cu1O3, and some liquid phase coexists with Bi2Sr2Ca2Cu3O10+δ as it gradually decomposes. When the Bi2Sr2Ca2Cu3O10+δ is completely gone, the Bi2Sr2Ca0Cu1O6+δ, Ca2PbO4, and (Sr,Ca)2Cu1O3 transform to a liquid with CaO and other phases floating on the liquid. If Ag is micromilled with the Bi2Sr2Ca2Cu3O10+δ, the decomposition process is similar, but all of the temperatures are lowered substantially. If the partial pressure of O2 is reduced to 10 kPa over the Bi2Sr2Ca2Cu3O10+δ/Ag sample, the decomposition temperatures and products are similar to the 20 kPa case.
Redistribution of local current densities in high-T
c ceramics in external magnetic fields
Authors: Schatteburg S. , Kliem H. , Bauhofer W.
Published: July 2002

Using a model with Lorentz force induced fluxon motion to describe transport current properties in high-Tc ceramics, a locally varying critical current density jc(r) has been predicted. An external magnetic field Ba applied perpendicular to the transport current shifts the maximum of jc(r) from the center position of the sample to that side of the cross section where the self-field Bs and the external field Ba have opposite directions. The calculated redistribution of the critical current density obtained with the force model is verified experimentally by measuring the asymmetric change of the magnetic self-field close to the sample boundaries. A nearly quantitative agreement between model calculation and experiment is found by adjusting a single parameter.
Thermal activation in KrF laser ablation of CuCl
Authors: Küper S. , Brannon K. , Brannon J.
Published: July 2002

248 nm excimer laser ablation of carefully prepared CuCl samples is reported, and shown to occur by a predominantly thermal mechanism. Using a quartz-crystal microbalance (QCM) to monitor ablation, a precise and detailed plot of single-pulse mass removal versus incident fluence was obtained for fluences up to 150 mJ/cm2. A two-parameter Arrhenius exponential function was found to fit the experimental ablation data. Calculations of laser-induced surface heating were carried out by use of a finite-difference heating code, formulated in terms of enthalpy. Ablation was observed to commence at a fluence of 25 mJ/cm2, where the calculated surface temperature is approximately 910 K—some 200 K above the melting point. Dynamic ablation was included in the finite-difference calculation by allowing the position of the CuCl surface ξ to vary in time. The best data fit is provided by the zeroth-order kinetic equation: dξ(t)/dt=(16 Å/ns)exp[(−38 kJ/mole)/RTξ], where Tξ is the surface temperature. A thermodynamic calculation shows the average heat of CuCl vaporization in the temperature range from 900 to 2000 K to be near the fit value of 38 kJ/mole. From plots of the ablation depth versus time, the CuCl surface was estimated to recede during the ablation at rates up to 10 cm/s.
Ultrahigh-vacuum scanning electron microscopy characterization of the growth of Fe on CaF2/Si(111): Selective nucleation on electron-beam modified surfaces
Authors: Heim K. R. , Hembree G. G. , Scheinfein M. R.
Published: July 2002

The initial stages of Fe island growth on electron-beam modified and unmodified CaF2/Si(111) surfaces were studied with a nanometer lateral spatial resolution ultrahigh-vacuum scanning electron microscope. Fe coverages between 7 and 8 ML (deposition rates from 0.12 to 0.19 ML/min, 1 ML=7.7×1014 atom/cm2) grown on room temperature through 300 °C CaF2/Si(111) relaxed and unrelaxed substrates produced a relatively uniform distribution of islands that cover 23% of the substrate with an island density of 7.4×1012 island/cm2. Chemical or defect dominated Fe growth on the CaF2/Si(111) substrates is indicated by the temperature independence of the Fe island distributions for 20 °C≤T≤300 °C. Substrate temperatures near 400 °C yielded mottled surfaces and an altered island distribution relative to those formed during growth at temperatures between 20 and 300 °C. Parallel step edges separated by 25–75 nm were observed for unrelaxed films of CaF2 on Si(111), while relaxed CaF2 films exhibited a saw-toothed step pattern. Fe coverages of Θ=21.4 ML produced a percolation network of connected islands rather than a continuous film covering the CaF2 substrate. The production of nanometer-sized surface structures was evaluated for electron-beam modified growth of Fe on CaF2/Si(111) substrates. Pregrowth (100 keV, 8.2–140 pA) electron irradiation doses as low as 1.14 C/cm2 altered the Fe film morphology on the selectively irradiated regions. Areas dosed with electron irradiation prior to Fe growth were more stable to the damaging effects of post-growth electron irradiation as compared to regions that had not been exposed.
Study of diamond thin-film growth mechanism in a filament-assisted excimer laser ablation system
Authors: Chen H. , Maffei N. , Prince R. H.
Published: July 2002

Diamond and amorphous carbon films have been deposited on silicon (100) substrates by filament-assisted XeCl excimer laser ablation of graphite targets. The influence of process parameters on the growth mechanism of diamond were studied by depositing films over a wide range of gas pressures and substrate temperatures. The surface morphology and bonding of the deposited films were characterized by scanning electron microscopy, and Raman and electron-energy-loss spectroscopy. In these experiments, microcrystalline diamond films with growth rates comparable to conventional chemical-vapor-deposition and plasma-enhanced chemical-vapor-deposition techniques could only be obtained at substrate temperatures and hydrogen pressures greater than 700 °C and 1.3 mbar, respectively. Conversely, the best conditions for growing carbon films with predominately sp3-type bonding structures were at temperatures and pressures less than 300 °C and 0.3 mbar, respectively. These results suggest that carbon-hydrogen gas phase reactions as well as gas-surface reactions are both necessary for the formation of diamond.
Preparation and characterization of CeO2-based superconducting field effect transistors
Authors: Petersen K. , Walkenhorst A. , Schmitt M. , Becherer T. , Adrian H.
Published: July 2002

High-Tc superconducting field effect transistors using CeO2 as the dielectric layer in combination with ultrathin YBa2Cu3O7−δ layers have been prepared and investigated. The preparation process of these devices is described in detail. CeO2 is very compatible with ultrathin YBa2Cu3O7−δ layers, resulting in Tc values consistent with SrTiO3-based superconducting field effect transistors. The dielectric constant and the breakdown field of the CeO2 layer are reproducible in all investigated samples, yielding values for εr≊20 and Ebd≊2×108 V/m. This results in a charge transfer of ΔN=±2 μC/cm2. In the charge carrier enhancement mode this value is close to SrTiO3-based superconducting field effect transistors. So far, the maximum achieved modulation of the charge carrier density by the use of CeO2 as dielectric is 1.5%.
Qualitative estimation of optical gain in wide-band-gap semiconductor quantum wells
Authors: Ahn Doyeol
Published: July 2002

Room-temperature linear optical gain of a GaN quantum well is estimated qualitatively and is compared with optical gains of the ZnSe and GaAs quantum wells in order to examine the feasibility of GaN quantum-well lasers. Our analysis is based on the simple model using the density matrix method with parabolic band approximation. For 100 Å quantum wells, the calculated transparency level of a GaN quantum well (8.3×1018 cm−3) is slightly higher than that of a ZnSe structure (7.4×1018 cm−3). But the transparency levels of GaN and ZnSe quantum wells are much higher than the transparency level of a GaAs quantum well (2.3×1018 cm−3). It is expected that gain spectra of a GaN quantum well be narrower than those of a ZnSe quantum well.
Hole subband in p-type channel of semiconductor heterostructures
Authors: Liu Kun , Chu J. H. , Ou H. J. , Tang D. Y.
Published: July 2002

Based on the characteristics of semiconductor surface capacitance, an experimental model is presented for evaluating the hole subband structures in the p-type channel of semiconductor heterostructures. For an n-type InSb metal-oxide-semiconductor sample, the capacitance-voltage spectroscopy is measured and the hole subband structure is derived with using the model presented. The result shows that the Fermi level is always pinned near the bottom of the hole subband, which is attributed to the large density of states of the hole subband. Relevant parameters are also obtained for the hole subband including the subband energy, Fermi level, inversion layer width, and depletion layer width, etc.
Saturation of generation-recombination current for very small recombination times
Authors: Pelaz Lourdes , Orantes J. L. , Enríquez L. , Bailón L. , Barbolla J.
Published: July 2002

Analysis of the generation-recombination processes by numerical simulation allows consideration of a very small recombination time constant in order to study the behavior of p-n junctions in this extreme situation. The generation-recombination current does not increase indefinitely as the generation or recombination mechanism is more and more active. Instead, current saturation occurs as a result of the continuity equation along with the generation-recombination expression. This generation-recombination current saturation takes place in any semiconductor and establishes the maximum current limit that can be yielded by this mechanism.
Scaling of the volume pinning force in superconducting wires and tapes based on Bi2Sr2CaCu2Ox
Authors: Löhle J. , Mattenberger K. , Vogt O. , Wachter P.
Published: July 2002

Silver-sheathed Bi2Sr2CaCu2Ox wires and tapes were fabricated by the powder-in-tube method. The wires show critical current densities of about 18 000 A/cm2 in zero magnetic field and 5300 A/cm2 in a field of 23 T at 4.2 K, respectively. A detailed study of the critical current density as a function of the temperature and the magnetic field resulted in a scaling behavior of the derived volume pinning force FV, which approximately obeys the formula FV≊H*h0.75(1−h)2.75. Here H* represents the irreversible field and h=H/H* is the scaling field. Further, the critical current densities of our textured tapes were measured. In zero magnetic field they reach critical current densities of about 43 000 A/cm2. In a magnetic field applied perpendicularly to the current direction the critical current density shows an anisotropic behavior with respect to the direction of the magnetic field. For a magnetic field of 23 T applied parallel to the tape plane the tapes reached critical current densities of about 15 000 A/cm2 at 4.2 K. In the unfavorable direction, the magnetic field is applied perpendicularly to the tape plane, the current density was reduced to 10 000 A/cm2. The volume pinning force in the tapes was investigated versus the magnetic field at 4.2 K and a power-law behavior FV≊Hp with p=0.75 for H parallel and p=0.55 for H perpendicular to the tape plane was found.
Influence of Ni additions on the low-temperature magnetic properties of a Cu–1% Mn alloy
Authors: Sumption M. D. , Collings E. W.
Published: July 2002

dc susceptibility and magnetization measurements were made on three alloys with the nominal compositions Cu–1% Mn, Cu–15% Ni, and Cu–1% Mn–15% Ni. dc susceptibility [χdc=C/(T+θW)] measurements were made with a Curie-type balance from room temperature down to 77.2 K. Magnetization measurements consisted of M-H loops at 4.2 and 1.37 K, and initial magnetization runs from room temperature down to 1.37 K. The results were analyzed in terms of an addition of Ni to Cu-Mn. High-temperature χdc data were used to find θW, C, and μeff (from C and the concentration), and it is found that these properties are influenced only to a minor extent by the Ni addition. At low temperatures, however, a suppression of the freezing temperature and an enhancement in the superparamagnetic signal are noticed for Cu-Mn-Ni as compared to Cu-Mn. Finally, hysteretic loss and magnetization for the ternary at 4.2 K were compared to that of the Cu-Ni and Cu-Mn alloys in order to determine its suitability for use as matrix material in multifilamentary superconductive wire.
Effect of bias magnetic field on the magnetostrictive vibration of amorphous ribbons
Authors: Asuke Naoshi , Namikawa Tatsuru , Yamazaki Yohtaro
Published: July 2002

Electromagnetic waves generated from the magnetostrictive vibrations of amorphous ribbons under nonuniform bias fields were investigated. The sample ribbon 80 mm in length was placed on a plastic support board where both edges of the ribbon were free. In a uniform bias field generated by a Helmholtz coil, four resonance peaks of 27, 54, 81, and 107 kHz were detected. The nonuniform magnetic fields were generated by single-pole magnetic heads. Two single-pole magnetic heads with 3 mm spacing between the ribbon were moved along the ribbon. When the heads were located at 10 and 70 mm from an edge of the ribbon, the frequency of the detected response was 54 kHz, i.e., the detected response was the second resonance peak. The other resonance peaks (81, 107, and 129 kHz) could also be separated by adjusting the positions of the heads. Therefore, the electromagnetic response of the magnetostrictive vibrations of amorphous ribbons could be controlled by applying appropriate nonuniform bias magnetic fields. From the results, it is suggested that a magnetostrictive amorphous ribbon not only sends one bit data but also sends multiple bit data with the aid of bias magnetic fields.
‘‘Effective radius’’ of the 4f electrons in REBa2Cu3O7, RE=Dy, Ho, Er (abstract)
Authors: Koksharov Yu. A. , Silaev P. K.
Published: July 2002

It is well known that in most high-temperature superconductors rare earth (RE) ions and superconducting copper-oxygen planes coexist as rather isolated subsystems. It would be expected that the ‘‘magnetic’’ 4f electrons are to be strongly localized as it takes place in many oxides. On the other hand, significant crystal-field (CF) splitting of RE ground states is evidence that the ‘‘effective radius’’ of the 4f electron wave functions (WF) could be large. This matter could be responsible for the disappearance of the superconductivity in Pr123 since the ‘‘effective radius’’ of the 4f electron WF increases from Yb to Ce. There is the adequate complete information about CF splitting in HoBa2Cu3O7 (Ho123) from inelastic neutron scattering experiments. The energies of low-lying levels are known also for Dy123 and Er123. To compute CF splitting in RE123 compounds the hydrogen-like one-electron radial WF R(r)=(rN)exp(−alpha r) are used. Only five parameters are taken into account to accord the calculated and experimental data: alpha, N, and effective charges of nearest oxygen, copper, and barium ions. The minimal discrepancy for Ho123 is small enough for such simple approximation. The minimum is observed with enough magnitude of alpha than the one in the case of free Ho3+ ion. The same is true for Dy123 and Er123. This fact points out the strong localization of the 4f one-electron radial wave function and probably on the small covalent mixture of 4f (‘‘magnetic’’) and 3d-2p (‘‘superconducting’’) orbitals.
Electromigration failure in a finite conductor with a single blocking boundary
Authors: Dwyer V. M. , Wang F.-S. , Donaldson P.
Published: July 2002

The (one-dimensional) electromigration boundary-value problem is considered for the case of a single blocking boundary with a constant vacancy supply at the other boundary. Using the drift/diffusion model expressed by the Fokker–Planck equation, we find that the saturation time (tsat) increases exponentially with current density (j) and not as j−2, as has been suggested. However, it is not the saturation time which determines the lifetime (tbd); it is the time to reach some critical vacancy concentration (c*). In agreement with experimental results and numerical calculations, we find that tbd∼j−2. We also find that tbd∼c*.
Statistical distribution functions in 2D foams
Authors: de Icaza M. , Jiménez-Ceniceros A. , Castaño V. M.
Published: July 2002

The statistical distribution of bubbles in a 2D foam has been determined according to their area. A 2D foam chamber was designed and constructed, which allows long-term video recordings to be made. The bubbles corresponding to the last six frames, when the so-called scaling state is attained, were digitized at equally spaced time intervals, to measure their surface and count their number of neighbors. From the numerical results the empirical distribution functions are determined. Kolmogorov’s compatibility criterion was used to measure the differences between these empirical and three theoretical distribution functions: the normal distribution (bad fit, but a favorite of earlier reports), the power regression (good fit), and the log normal (the best fit).
Magnetization processes in remanence enhanced materials (invited) (abstract)
Authors: Street R. , Allen P. , Ding J. , Feutrill E. , Folks L. , Smith P. A. I. , Woodward R. C.
Published: July 2002

Isotropic assemblies of hard and soft magnetic phases in nanocrystalline form have been produced by high energy ball milling and melt spinning. These materials exhibit remanence enhancement, i.e., the remanence exceeds 0.5Msat, the value expected of an isotropic material. The phenomenon has been investigated using models in which interaction energy across the interfaces between magnetically hard and soft components is taken into account. An example of such a two-dimensional model will be discussed. Measurements have been made of the magnetic properties of SmFeN, Sm(Fe,Co), and NdFeB two phase nanocrystalline materials. All of them exhibit remanence enhancement. The time dependent behavior of materials exhibiting remanence enhancement is unusual. Magnetic viscosity occurs on both branches of recoil loops as well as on the initial magnetizing and demagnetizing curves.
Magnetic properties of the one-dimensional Heisenberg compounds (3-X-anilinium)8[CuCl6]Cl4; X=Br, I
Authors: Vyas Gayatri , ter Haar Leonard W.
Published: July 2002

Recent x-ray diffraction studies have suggested that (3-Cl-anilinium)8[CuCl6]Cl4 exhibits compressed [CuCl6]4− octahedra. We report here the synthesis of the (3-Br-anilinium)8[CuCl6]Cl4 and (3-I-anilinium)8[CuCl6]Cl4 analogs. Their magnetic behavior is quite similar to one another, and in comparison to the chloro compound, the only similarity is that they all exhibit chain magnetism, as evidenced by their broad maxima in the region of 9–12 K. The bromo and iodo compounds do not exhibit the field dependence of the chloro compound, and in this respect, the chloro compound remains unique. For all three compounds, magnetic susceptibility data can be analyzed by the antiferromagnetic one-dimensional Heisenberg model.
Stress effect on the magnetization of Dy in the Dy/DyFe2 eutectic
Authors: Teter J. P. , Cheng S. F. , Cullen J. R.
Published: July 2002

The magnetization of the alloy DyFe1.5 was studied as a function of temperature from 50 to 300 K in a range of magnetic field up to 10 kOe. Metallographic studies showed the existence of a eutectic phase containing both DyFe2 and elemental Dy embedded in the DyFe2 matrix. The presence of elemental Dy was observed in the magnetization data through a cusp in the temperature dependence of the magnetization at 178 K and an increase of the magnetization starting at 130 K and continuing to 80 K. While the first feature occurs at precisely the Néel temperature of bulk Dy, the second contrasts with the jump at the Curie temperature of 89 K observed in low fields in bulk single-crystal Dy. The more gradual increase in magnetization observed in the DyFe1.5 alloy is attributed to variations in stress to which the Dy is subjected. The temperature dependence of the magnetic moment from 80 to 130 K is modeled as arising from a collection of Dy particles of varying Curie temperatures. This variation is in turn caused by the stress distribution. Curie temperature distributions are found for a range of magnetic fields and then are extrapolated to zero field in order to eliminate the effect of field. The calculations indicate that a significant amount of the Dy remains in the helimagnetic phase down to zero temperature.
Proximity effect in MBE-grown superconducting/spin-glass multilayers
Authors: Wilks Carlos W. , Engel Brad N. , Falco Charles M.
Published: July 2002

We have grown epitaxial superconductor/spin-glass multilayers, Nb/CuMn, as well as complimentary nonmagnetic Nb/Cu multilayers by molecular beam epitaxy. To probe the interaction of superconductivity and magnetism, we measured the resistivity and ac susceptibility as a function of temperature for multilayers of nominally constant Nb thickness and varying normal-metal thickness. The reduction of the transition temperature of the Nb/Cu multilayers with increasing Cu thickness is in excellent agreement with the de Gennes–Werthamer proximity effect theory. The inclusion of Mn in the Cu causes a significant additional suppression of the transition temperatures relative to the Nb/Cu multilayers. The extension of the de Gennes–Werthamer theory to include the effects of random magnetic impurities agrees well with the data from the Nb/CuMn multilayers for small CuMn layer thicknesses. However, deviations occur at the largest CuMn thicknesses studied. These deviations between the data and theory may be due to a decoupling of the Nb layers, as a result of the spin-glass ordering, causing a three-dimensional to two-dimensional crossover.
Effects of field orientation on field uniformity in permanent magnet structures
Authors: Jensen J. H. , Abele M. G.
Published: July 2002

The effects of magnetic-field orientation are investigated for permanent magnet structures designed for the generation of highly uniform fields. The structures considered are based on closed cavities that produce exactly uniform internal fields. An open structure with an approximately uniform field is obtained by removing part of the wall from such a cavity. This paper explores how the magnetic-field uniformity depends on the field orientation relative to the opening. A yokeless, cylindrical cavity that is opened at both ends is considered in detail with the field oriented both perpendicular to and parallel to the cylinder’s axis. The field uniformity is shown to be substantially better for the perpendicular orientation. The leakage field outside the magnet is also contrasted for the two cases.
Laminar construction of spheroidal field sources with distortion-free access
Authors: Leupold H. A. , Tilak A. S. , Potenziani E.
Published: July 2002

Certain high-field permanent magnet structures in the forms of spheroidal and cylindrical shells can be altered so that access ports through specified areas of the shells have minimal detrimental effect on the field integrity in the internal working space. This can be effected either parametrically or geometrically. Both approaches result in an increase of fabricational complexity; the former because it entails components with a variety of magnetizations and the latter because it requires a variety of geometric dimensions. This article describes how simplifying approximations to either type of structure can be obtained by an assembly of laminar pieces cut from sheets or slabs of permanent magnet materials with their magnetization oriented parallel to their principal faces.
A magnetic coupling without parasitic force for measuring devices
Authors: Yonnet Jean-Paul , Delamare Jérôme
Published: July 2002

Original structures of permanent magnet couplings have been studied. They can transmit a torque without creating forces or stiffnesses between the driver and the driven part. Except for torque, the two parts are in indifferent equilibrium. These couplings have been designed for measuring devices in which they exert no force on the bearings but they can be used in other domains.
An extended magnet in a passive dI/dt limiter
Authors: Young S. J. , Dawson F. P. , Konrad A.
Published: July 2002

This article examines a composite magnetic device which has the potential to function as a rate of current change (dI/dt) limiter. A possible application of such a device is to protect voltage-source-fed systems, either ac or dc, from overcurrents during fault situations. The operation of this dI/dt limiter is discussed, and performance equations are derived. Results obtained from this analysis are compared to a set of finite element simulations. An improvement to the basic design is suggested, in which the permanent magnet slices are extended in order to decrease the amount of leakage flux surrounding the dI/dt limiter.
Study of spin wave resonance in a superconductor with paramagnetic impurities
Authors: Garifullin I. A. , Goryunov Yu. V. , Khaliullin G. G.
Published: July 2002

The spin dynamics of paramagnetic impurities in a superconductor in the presence of external magnetic field has been studied both theoretically and experimentally. Long-wave excitations (q≤ξ−1, where ξ is the coherence length) are shown to have coherent spin-wave nature even in a paramagnetic phase. This effect is caused by the long-range (R∼ξ) indirect exchange interaction between localized spins in a superconductor. The spin-wave effects in EPR of Er3+ ions in the superconducting (SC) La1−xErx thin films have been observed. An additional absorption at the high field side of the main resonance line which leads to its distortion has been found. Satisfactory description of all features of observed spectra was obtained by simulating the spectra with account for the microwave absorption by spin-wave excitations. The spin stiffness coefficient and the temperature dependence of the spin susceptibility of the SC lanthanum has been determined.
Nuclear secondary echo in ferromagnets caused by quadrupole and Suhl–Nakamura interactions
Authors: Tsifrinovich V. I.
Published: July 2002

The formation of the secondary nuclear spin echo signals in ferromagnets has been investigated taking into consideration the quadrupole and Suhl–Nakamura interactions. It is assumed that the inhomogeneous distribution of the quadrupole frequency ωQ+δωQ is independent of the distribution of nuclear magnetic resonance frequency ωn+δωn, and a characteristic size of these inhomogeneities is small in comparison to the correlation radius of the exchange interaction in the electron ferromagnetic system. It is shown that the quadrupole interaction causes the formation of the secondary echo signals at t=(2k+1)τ, 1≤k≤I−1/2, where I is the nuclear spin, the time t is measured from the second rf pulse, and τ is the time interval between the rf pulses. The spectrum of the spin echo signal with maximum at t=tE contains (2I+1−tE/τ) frequencies. The Suhl–Nakamura interaction causes the additional echo signals with maximum at even values t/τ.
Hyperfine fields of mercury in single-crystalline cobalt
Authors: Marques J. G. , Correia J. G. , Melo A. A. , Soares J. C. , Alves E. , da Silva M. F.
Published: July 2002

The combined hyperfine interaction of mercury in a hexagonal close-packed cobalt single crystal was measured using the e−−γ perturbed angular correlation technique with the 197Hg and 199Hg probes, without applied magnetic field. The magnetic and quadrupole coupling constants of the 5/2− levels in both 197Hg and 199Hg were measured with extremely high precision, ωL( 197Hg)=951(9) Mrad/s, ωL (199Hg)=1039(10) Mrad/s, vQ (197Hg)=3.7(5) MHz and vQ (199Hg)=32(2) MHz. Using the well-known g factors and quadrupole moments of the respective levels, the magnetic hyperfine field, ‖Hhf(HgCo)‖=581(12) kG, and the electric field gradient, ‖Vzz(HgCo)‖ =2.0(3)×1017 V/cm2, were derived. The importance of using these hyperfine probes for studying the microscopic structure of cobalt-based multilayers is introduced and discussed.
Giant magnetoresistance in f-electron systems (invited) (abstract)
Authors: Kasuya Tadao , Suzuki Takashi
Published: July 2002

Various kinds of giant magnetoresistance have been observed. In the present report, we concentrate on the f-electron systems, particularly on rare earth compounds, and summarize the results by classifying according to mechanisms. One important category is concerning the magnetic impurity state in a doped magnetic semiconductor with a fairly wide band gap. A typical classical example is EuO with O vacancies, in which two conduction electrons with opposite spin directions are trapped at each vacancy. The applied field aligns the spin direction, induces magnetic polaron, expands the extension of the impurity state, and causes a large decrease in resistivity. Another typical example is Gd3-xS4, with x near 1/3 namely, near Gd2S3, in which only a small amount of conduction electrons exists below or around the mobility edge. This system is characteristic in the sense that a strong band tail exists nearly independently on the number of doped carrier. A strong tendency to form magnetic polaron exists and various anomalous properties including giant magnetoresistance is observed. Recently, we performed an extensive study on this material to reveal the fundamental mechanism. Some details will be reviewed. Another category is the intrinsic character, even though it is modified substantially by defects in the real materials. A typical prototype is EuB6, a typical narrow gap magnetic semiconductor. Applied field causes a ferromagnetic alignment of 4f spins, which causes overlapping between the up spin conduction band and the up spin valence band through the d-f exchange and p-f mixing interaction causing the system into a semimetallic state. This further accelerates the 4f spin alignment. To see the real intrinsic mechanism, the sample should be very pure and the most suitable samples for this purpose are Ce and Yb monopnictides. Recently, we performed an extensive investigation on a series of the above materials and found various novel characteristic properties, including both giant positive and negative magnetroresistances. A detailed summary is planned.
Evolution of recombination in a solid HDDR processed Nd14Fe79B7 alloy
Authors: Martinez N. , Jones D. G. R. , Gutfleisch O. , Lavielle D. , Peré D. , Harris I. R.
Published: July 2002

A high-resolution scanning electron microscope was used to study the microstructural changes occurring during the desorption and recombination stages in a cast Nd14Fe79B7 alloy HDDR processed at 780 °C. Phase identification was based on backscattered electron contrast. In the early stages of desorption, the disproportionated structure in the matrix coarsens to produce a mixture of Nd-rich (NdH2 and/or Nd) and Fe-rich (αFe and/or Fe2B) clusters. At the center of the specimen, regions of recombined multicrystalline Nd2Fe14B phase are observed associated with Nd- and Fe-rich clusters. When desorption of hydrogen is almost complete, the microstructure consists of a Nd2Fe14B matrix with a few remaining Nd- and Fe-rich clusters. The subsequent completion of recombination results in a multigrained Nd2Fe14B structure, some grains being separated by Nd-rich material. The magnetic properties of the Nd14Fe79B7 alloy have been measured at significant stages during the desorption/recombination process at 600 °C, the stages being deduced from electrical resistivity measurements. Initially, the material is noncoercive with a high magnetization at 1100 kA/m, reflecting the high proportion of αFe in the sample. As recombination proceeds, the magnetization drops and the coercivity increases as the fine-grained Nd2Fe14B develops and the αFe disappears.