bluuebunny/crossref_metadata_2025 huggingface dataset:
Localized system with a mobility edge in epitaxial compensated AlxGa1−xAs
Authors: Maaref H. , Kraiem S.
Published: July 2002

Optimal experimental liquid phase epitaxy growth conditions have been achieved to study by time-resolved photoluminescence the influence of the compensation ratio and temperature on the decay time associated with radiative transition between carriers localized in spatially separated potential wells formed by fluctuations in the local impurity concentration in silicon-doped AlxGa1−xAs. We find that the decay time temperature dependence is a characteristic of a localized system with a mobility edge and can be attributed to the tunneling recombination of electrons and holes followed by a nonradiative transition at a deep electronic level.
Carbon-doped p-type AlGaAs/InAs superalloy by metalorganic chemical-vapor deposition
Authors: Pan N. , Elliott J. , Hendriks H. , Lee D. , Welch M.
Published: July 2002

C doping in InGaAlAs lattice matched to InP with a p-type carrier concentration as high as 3×1019 cm−3 was demonstrated using trimethylarsenic (C source) and a superalloy approach (16 Å Al0.1Ga0.9As/17 Å InAs). A superalloy approach was necessary to permit C incorporation since the carbon doping was accomplished during the growth of Al0.1Ga0.9As. The carbon concentration was verified by both Hall-effect and secondary-ion-mass-spectrometry measurements. Hydrogen passivation was significant in as-grown samples but the hydrogen concentration decreased after the sample was annealed in a He atmosphere without the presence of hydrogen or arsine. The oxygen concentration in the superalloy needs to be further reduced in order for the superalloy to be effective as a base layer in a heterojunction bipolar transistor.
Effects of In profile on material and device properties of AlGaAs/InGaAs/GaAs high electron mobility transistors
Authors: Look D. C. , Jogai B. , Kaspi R. , Ebel J. L. , Evans K. R. , Jones R. L. , Nakano K. , Sherriff R. E. , Stutz C. E. , DeSalvo G. C. , Ito C.
Published: July 2002

The molecular-beam-epitaxial growth of InxGa1−xAs on GaAs or AlyGa1−yAs leads to a variation of In content with depth, due to In segregation. However, by predepositing In at the beginning of InxGa1−xAs growth, and also thermally removing the excess In at the end, we can produce a layer with the ideal ‘‘square’’ In profile. We find that the performance of AlyGa1−yAs/InxGa1−xAs/GaAs high electron mobility transistors is most enhanced by the predeposition step alone.
Magnetic texturing of bulk samples of the superconductor Bi2Sr2Ca0.8Dy0.2Cu2O8−y
Authors: Stassen S. , Rulmont A. , Vanderbemden Ph. , Vanderschueren A. , Gabelica Z. , Cloots R. , Ausloos M.
Published: July 2002

The texturing process of Bi–Sr–Ca(Dy)–Cu–O (2212) is optimized by sintering under a magnetic field. The precursor crystalline powder is melted and grown under a 1.2 T magnetic field. Textured samples are characterized by x-ray diffraction, scanning electron microscopy, electrical resistivity, and magnetic susceptibility.
Suppression of thermally induced reactions at SiO2/single-crystalline Al interfaces
Authors: Miura Y. , Hirose K.
Published: July 2002

Thermally induced reactions at SiO2/Al interfaces are investigated by cross-sectional transmission electron microscopy. For the SiO2/Al interfaces fabricated by chemical vapor deposition of SiO2 on polycrystalline Al films, precipitates of reduced Si as the reaction products are observed in the Al films and along the SiO2/Al interfaces, after annealing at 500 °C for more than 2 h. On the other hand, the interfaces fabricated on the single-Al(111) films show no precipitation even after 4 h of annealing. The differences in the interfacial reactivities are discussed in relation to the existence of grain boundaries and Al film orientations.
High-frequency capacitance of bipolar resonant tunneling diodes
Authors: Fobelets K. , Van Hoof C. , Genoe J. , Stake J. , Lundgren L. , Borghs G.
Published: July 2002

The high-frequency characteristics of bipolar resonant tunneling diodes are experimentally investigated at room temperature. The electron accumulation and discharging in these resonant tunneling light-emitting diodes are studied at frequencies up to 35 GHz. The experiments show capacitance peaks due to electron charge disappearing from the quantum well. The measurements are found to be in agreement with our theoretical model for the calculation of the high-frequency characteristics of resonant tunneling devices. The high-frequency characteristics of the bipolar light-emitting resonant tunneling diode are compared to the unipolar resonant tunneling diode and the resonant interband tunneling diode. The comparison shows a similar discharging characteristic of the quantum well, but a different overall variation of the capacitance.
A study of the magnetic structure of the ordered alloys Fe(Pd1−xAux)3
Authors: Kim E. C. , Bykov V. I.
Published: July 2002

The antiferromagnetic structure with the wave vector K=[001/2], corresponding to one prong of a three-pronged star {K10} in a simple cubic lattice ΓC, is realized at atomic ordering in alloys Fe(Pd1−xAux)3 in the range 0.133<x<0.4. The magnetic moments of the Fe atoms are oriented antiparallel to the direction 〈001〉. Their values depend on the Au concentration in the alloy. The magnetic phase diagram of these alloys, where the coexistence of ferro- and antiferromagnetism is discussed in the framework of noncollinear magnetic structures, was determined.
Time-resolved emission studies of GaAs/AlGaAs laser diode arrays on different heat sinks
Authors: Voss M. , Lier C. , Menzel U. , Bärwolff A. , Elsaesser T.
Published: July 2002

Laser radiation from GaAs/AlGaAs laser diode arrays of high output power is studied during nano- to millisecond carrier injection in temporally and spectrally resolved emission measurements. A red shift of the multimode emission spectrum by up to 12 nm and a concomitant increase of the total bandwidth are caused by a transient rise of the device temperature by up to 50 K. Spatially resolved experiments reveal a lateral temperature difference of about 2 K between the center and the edge emitters. Different laser array/heat sink combinations are investigated in order to reduce the transient temperature increase.
Force-sensitive resistor of carbon-filled liquid silicone rubber
Authors: Hu Wangyu , Zhao Lihua , Wu Lijun , Wang Lingling , Zhang Bangwei , Hu Wangyu , Guan Hengrong , Zhang Bangwei
Published: July 2002

The effects of carbon content, tensile force, and temperature on the electrical resistance of carbon-filled liquid silicone rubber composites are studied. The relaxation process of resistance following loading can be described by an exponential function. The force dependence of the equilibrium resistance can be expressed by a second order polynomial, and such a relationship can be derived from the quantum mechanical tunneling conduction mechanism by assuming that the separation distance between carbon aggregates changes as a function of the tensile force with a form of Δw=kF2. Combining with the experimental data and typical values of theoretical parameters, the elastic modulus, the separation distance, and the proportional constant k can be obtained. Finally, the temperature dependence of resistance can be interpreted by the general form of R=R0 exp(const./T) with two different constants at different temperature ranges.
Growth temperature dependent band alignment at the Ga0.51In0.49P to GaAs heterointerfaces
Authors: Liu Q. , Derksen S. , Prost W. , Lindner A. , Tegude F. J.
Published: July 2002

Photoluminescence analysis of Ga0.51In0.49P/GaAs single-quantum well structures grown by metal-organic vapor-phase epitaxy in the temperature range from 570 to 720 °C have been carried out. Besides the GaAs band-edge emissions, all SQW samples studied here exhibit a dominant long-wavelength peak, which is attributed to the spatially indirect transition due to the type-II band alignment of Ga0.51In0.49P/GaAs heterojunctions. The energy of the type-II PL emission has been found to depend strongly on the growth temperature indicating the strong influence of the growth temperature on the band alignment. The shifts of the type-II PL emission have been used to estimate the growth temperature dependent conduction and valence band discontinuity of the Ga0.51In0.49P/GaAs heterojunction. X-ray diffraction measurements and simulations using the dynamical theory were carried out to study the influence of the growth temperature on the unintended interfacial layers.
Effects of dislocations on transport properties of two dimensional electron gas. II. The quantum regime
Authors: Bougrioua Z. , Farvacque J. L. , Ferré D.
Published: July 2002

This article deals with the influence of dislocations on the conductivity of two dimensional electron gas measured at helium temperature and at high magnetic field. Two kinds of dislocation substructures have been experimentally studied: they correspond either to a family of parallel dislocation segments introduced by plastic bending or to a family of parallel and long misfit dislocations created during the epitaxial growth by the control of the epilayer thickness. It is shown that both types of dislocation substructure lead to a strong anisotropic behavior of the longitudinal conductivity while the quantum Hall effect is not significantly affected by the presence of dislocations. A theoretical treatment of the case of anisotropic potentials is proposed as a generalization of the simpler case, addressed in the literature, of isotropic (short range) potentials. With the help of this analysis, we can explain, in the case of the dislocation segments, why although anisotropic, the normalized conductivity curves obtained in the principal axis are identical and we show that regular Dingle plots may be used to check the extra broadening of the Landau levels due to dislocations. The second kind of dislocation substructure is shown to be equivalent to quasi one dimensional systems issuing from a truncation of the two dimensional electron gas into quite parallel ribbons localized between the dislocation potential barriers, but no ballistic effects could be observed.
The transformation structure of zirconia-alumina nanolaminates studied by high resolution electron microscopy
Authors: Gajdardziska-Josifovska M. , Aita C. R.
Published: July 2002

Quantitative high resolution electron microscopy (HREM) was employed to study the crystallography of a zirconia-alumina transformation-toughening nanolaminate. The nanolaminate consisted of alternating layers of polycrystalline zirconia and amorphous alumina. The zirconia layer thickness was scaled to insure unity volume fraction of the metastable tetragonal phase at the growth temperature, as predicted by an end-point thermodynamics model and verified by x-ray diffraction. In the microscopy sample, phase identification was achieved from precise measurements of lattice spacings using digital diffractograms of individual nanocrystallites. Of the nanocrystallites analyzed, (22±6)% were monoclinic in a distinct crystallographic relationship with their tetragonal neighbors. The following plane and direction relationships were identified: m(100)//t(100) and m[001]//t[001]. The observed structure is the result of a stress-induced transformation from the tetragonal phase. This transformation was localized to nanosized regions within the individual zirconia layers.
A model for the current instabilities in GaAs-AlGaAs heterojunction
Authors: van Hall P. J. , Kökten H.
Published: July 2002

A model is proposed for the description of the current instabilities in GaAs-AlGaAs heterojunctions. It consists of three parts: the injection of electrons via the contact into the AlGaAs layer, the partial capture of these electrons in deep centers, and the change with time of the band structure. This last ingredient is crucial, since due to the increase of the total number of electrons in the AlGaAs layer the band bending decreases making real-space transfer from the AlGaAs layer to the two-dimensional electron gas possible. We have performed quasistationary simulations of the time dependence of the current. The velocities, average energies, capture rates, etc. were taken from Monte Carlo simulations. It turned out, that the parameters for the modeling of the contact, which are to a high degree unknown, play an essential role.
Noise characteristics and detectivity of YBa2Cu3O7 superconducting bolometers: Bias current, frequency, and temperature dependence
Authors: Fardmanesh M. , Rothwarf A. , Scoles K. J.
Published: July 2002

Meander line patterned infrared detectors, with values of resistance Ronset at the onset temperature Tc onset of 3–5 kΩ, were fabricated from YBa2Cu3O7−x superconducting material on MgO and SrTiO3 crystalline substrates. Noise voltages from the samples were measured versus bias current, radiation modulation frequency, and temperature, in both the normal and superconducting states. Four major types of voltage noise were identified according to where they occurred in temperature relative to Tconset and the zero resistance temperature Tc zero, and their dependence on frequency and bias current. They were also associated with the granularity of the superconducting film, which is related to the substrate material used. From these observations a specific cause for each type of noise is suggested. The results are as follows. (i) In the normal state with temperature T≳Tc onset, noise with a magnitude that is consistent with thermal (Johnson) noise is seen, but it depends linearly on bias current above a threshold value, at low frequencies.

The suggested noise source is conductivity fluctuations due to Cooper pairs. (ii) Noise was found to occur below Tc zero in granular films. With increasing bias current its magnitude increases, and it shifts to a lower temperature range; however, the noise magnitude becomes constant as the current goes to zero. It is weakly dependent on frequency above 400 Hz. Suggested cause is voltage fluctuations in superconductor–normal–superconductor junctions at grain boundaries. (iii) This noise also occurs below Tc zero with peaks at various temperatures. With increasing bias current the peaked noise spreads to lower temperatures, but the noise goes to zero as the bias current goes to zero. Its suggested cause is magnetic flux tube motion. (iv) This noise occurs between Tc onset and Tc zero and is present in all samples, but lowest on samples prepared on SrTiO3 substrates. Its suggested cause is fluctuations in the volume fraction of the superconducting phase along the current path. While the measured detectivity D* of our samples at a wavelength of 20 μm was only 106 cm Hz1/2/W, engineering changes can be expected to raise the value to above 1010 cm Hz1/2/W.
A numerical study of the use of magnetometers to detect hidden flaws in conducting objects
Authors: Sepúlveda Néstor G. , Wikswo John P.
Published: July 2002

Superconducting quantum interference device magnetometers can be used to detect flaws in conducting objects by measuring the magnetic fields produced by flaw-induced perturbations in an otherwise uniform applied electric current. We examine the electric-potential and magnetic-field distributions from prolate spheroidal flaws in a homogeneous conducting plate. Beginning with Laplace’s equation in prolate spherical coordinates, we derive analytical expressions for the potential distribution outside of prolate spheroidal flaws exposed to either a uniform axial or transverse applied current. The influence of the flaw over the uniform field is practically negligible for radial distances three times that of the boundary surface of the flaw. Because the magnetic field due to the perturbation of the uniform currents by the flaw arises from the discontinuity of the component of the current tangential to the surface of the flaw, the magnetic field can be evaluated numerically using the analytical expressions for the current at the flaw surface. In the far field, B falls off as the inverse square of the distance from the flaw, consistent with the flaw acting as a current dipole. The dipole strength is determined by flaw volume and orientation, and the dependence of signal strength upon the direction of the current can be used to infer the shape of the flaw.
Nanometer-scale modification and characterization of lead-telluride surface by scanning tunneling microscope at 4.2 K
Authors: Davydov D. N. , Lyanda-Geller Yu. B. , Rykov S. A. , Hancotte H. , Deltour R. , Jansen A. G. M. , Wyder P.
Published: July 2002

Nanometer scale features on the semiconductor p-PbTe single crystal surface were created and studied at 4.2 K by means of a scanning tunneling microscope. Local tunneling spectra were obtained simultaneously with the surface topographic image. The local tunneling density of states and the magnetotransport data reveal the amorphous state of the modified regions on the surface.
Thermal waves in materials with inhomogeneous thermal conductivity: An analytical approach
Authors: Fivez J. , Thoen J.
Published: July 2002

An exact asymptotic expression for thermal waves in an inhomogeneous semi-infinite condensed matter sample at high frequencies is derived. The theory also allows for a fast and accurate retrieval of subsurface thermal conductivity profiles from surface temperature values in response to incident modulated radiation.
Electron trapping in α-alumina observed by electron-induced x-ray emission
Authors: Jonnard P. , Vergand F. , Kefi M. , Bonnelle C.
Published: July 2002

Radiative recombination from states located in the band gap of single-crystal α-alumina has been observed by electron-induced x-ray emission spectrometry. The variation of intensity as a function of the incident electron beam current has been determined. From this variation, we show that trapping of thermalized incident electrons takes place in the defect sites associated with the observed states. From the binding energies of the states, the sites have been identified as oxygen vacancies.
Microstructure and magnetoelastic coupling coefficients in thin NiFe and Ni films
Authors: Song O. S. , Kim C. K. , O’Handley R. C.
Published: July 2002

The effective magnetoelastic (ME) coupling coefficients Beff of polycrystalline Ni79Fe21 and Ni films have been reported as functions of thickness t. Beff was observed to have a thickness-independent bulk term, Bb, and a thickness-dependent surface term, Bs/t consistent with the Néel model. For thickness below 50 Å the surface term is important; it can change the sign of the effective ME coupling constant and dramatically increase its magnitude. We expect that the surface ME coupling we observe in polycrystalline thin films may be a combination of the intrinsic Néel surface effect and extrinsic microstructure effects. Our transmission electron microscopy, Auger depth profiling, and atomic force microscopy rule out film discontinuity and compositional gradients as significant sources of the surface ME effect.
An asymmetric quantum well infrared photodetector with voltage-tunable narrow and broad-band response
Authors: Lenchyshyn L. C. , Liu H. C. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

We describe a 9 μm AlGaAs/GaAs asymmetric quantum well infrared photodetector with voltage tunable spectral bandwidth. A very narrow spectral response of 9.2 meV (0.6 μm) full width half maximum is observed for an applied electric field of 28 kV/cm. The linewidth quadruples when the bias polarity is reversed, with very little shift in the peak detection wavelength. This structure is based on a conventional intersubband photodetector modified by using AlGaAs barriers that are graded in Al content and by adding a thin AlGaAs confinement layer on one side of the well. The asymmetry in the barriers is shown to give rise to the dependence of the spectral linewidth on applied bias. As well, a series of unusually well-resolved and intense bound-to-continuum transitions are observed at low bias, that may indicate that the unique barrier shape also leads to enhanced electron interference effects at the well/barrier interfaces.
Epitaxial growth of Y-doped SrZrO3 films on MgO by pulsed laser deposition
Authors: Beckers L. , Sanchez F. , Schubert J. , Zander W. , Buchal Ch.
Published: July 2002

Epitaxial thin films of Y-doped SrZrO3 have been grown on MgO(001) by pulsed laser deposition. The deposition process has been performed at temperatures of 1000–1200 °C and at an oxygen pressure of 1.5×10−1 mbar. The samples are characterized by Rutherford backscattering spectrometry/channeling (RBS/C) and x-ray diffraction (XRD). We found an epitaxial relationship of SrZrO3 (0k0) [101]∥MgO (001) [100]. Good crystalline quality is confirmed by RBS/C minimum yield values of 9% and a FWHM of 0.35° of the XRD rocking curve.
Attenuation of picosecond electrical pulses by two-dimensional electron gases integrated in coplanar striplines
Authors: Gerritsen H. J. , Keil U. D. , Reijnders G. L. A. , Haverkort J. E. M. , Wolter J. H.
Published: July 2002

We have measured the attenuation of picosecond electrical pulses by two-dimensional electron gases that are monolithically integrated in coplanar transmission lines. By integrating the two-dimensional electron gas structure into the transmission line, we avoid impedance mismatches that give rise to spurious reflection. The attenuated transients are simulated by an equivalent circuit transmission line model, where the conductivity of the two-dimensional electron gas underneath the transmission line is taken into account. The measured pulses are fitted with a specific conductivity of the layers corrected with a mode overlap factor, which describes the overlap of the electrical field lines with the two-dimensional layer. We obtain an overlap factor of 0.8–1.1.
Investigation of switching processes in laminar domain structure in triglycine sulfate crystal doped with L- and D-alanine by the liquid-crystal method
Authors: Qi M. , Tikhomirova N. A. , Shuvalov L. A.
Published: July 2002

It was found that the spontaneous polarization Ps in some area of triglycine sulfate (DL-alanine) crystal with periodic laminar ferroelectric domains cannot be reversed with an applied electric field. By using the nematic liquid-crystal method, the direct observation of the dynamics of the domain walls in the switching processes has been carried out. It is reasonable to assume that above experimental phenomena are due to the existence of a series of inclined domain walls in the specimen. The experimental methods and some observed preliminary results are reported.
Photoluminescence measurements of tensile-strained GaAs/In0.07Al0.93As quantum wells
Authors: Yeh C. N. , McNeil L. E. , Daniels-Race T. , Blue L. J.
Published: July 2002

GaAs/In0.07Al0.93As tensile-strained quantum wells were grown on [001] GaAs substrates using molecular-beam epitaxy. The incorporation of tensile strain is made possible by preparing a 1-μm-thick In0.07Al0.93As relaxed buffer which is followed by the growth of quantum wells. The strain of the GaAs was measured using Raman spectroscopy and photoluminescence. The photoluminescence measurements from wells ranging in thickness from 25 to 100 Å reveal that the observed optical transition originates from the electron–light hole recombination for a 100 Å well and from the electron–heavy hole recombination if the well thickness is less than 40 Å. Therefore, a thick Al-rich InxAl1−xAs relaxed buffer on the GaAs substrate can be used to engineer the relative energy position of the light and heavy holes for GaAs-based quantum wells.
α″-Fe16N2 phase epitaxially grown by sputter beam method
Authors: Okamoto Satoshi , Kitakami Osamu , Shimada Yutaka
Published: July 2002

Epitaxial Fe-N films have been grown on Fe or Ag (001) single-crystal underlayers by the sputter beam method. The Fe-N films on both underlayers have a bct structure (α′-martensite), whose lattice is elongated along the [001] direction by nitrogen atoms located in octahedral interstices of α-Fe. These films possess well-defined crystal orientation and exhibit a higher saturation magnetization (4πMs) than that of α-Fe for a certain range of various nitrogen content. Postannealing promotes ordering of nitrogen atoms in the bct lattice, resulting in precipitation of a metastable α″-Fe16N2 phase in Fe-N films. The 4πMs of the annealed samples tends to increase with the increase of the α″ volume fraction, and the Fe-N film with 4πMs of about 25 kG contains 26–33 vol% α″ phase. While both Fe and Ag underlayers can promote epitaxial growth of the Fe-N films, very fast ordering of nitrogen atoms and easily decomposed α″-Fe16N2 have been observed in the sample on a Ag underlayer. This fact leads to the fact that the ordering rate and the stability of α″-Fe16N2 are very sensitive to its crystal quality.
Magnetic phase transitions in RENi2B2C (RE=Ho,Dy,Tb) studied by 57Fe Mössbauer spectroscopy (abstract)
Authors: Sánchez D. , Micklitz H. , Fontes M. B. , Bud’ko S. L. , Baggio-Saitovitch E.
Published: July 2002

The rare earth (RE) nickel borocarbides are intermetallic layered compounds which offer the possibility to study the interplay between superconductivity and magnetism. We have doped RENi2B2C (RE=Ho,Dy,Tb) with ∼1 at. % 57Fe in order to study the magnetic phase transitions occurring in these systems by a local probe, i.e., by 57Fe Mössbauer spectroscopy. Sample characterization by x-ray diffraction as well as ac susceptibility measurements showed mainly single phase. The room temperature and 4.2 K 57Fe ME spectra essentially show one quadrupole doublet (Δ∼0.20 mm/s) for all compounds, except for RE=Tb. A magnetic hyperfine (hf) field appears in the case of RE=Tb and Ho at TN∼15 and 8 K, respectively. The magnetic hf field for Tb at 4.2 K is Bhf=0.86 T, while it is quite smaller for Ho [Bhf(5 K)∼0.30 T]. Since 57Fe at the Ni sites in RENi2B2C does not have its own magnetic moment, all magnetic hf fields are resulting from the neighboring RE moments. The absence of a hf field in DyNi2B2C at 4.2 K confirms the results from neutrons diffraction experiments: the magnetically ordered state in these two compounds is a simple collinear antiferromagnet. For Tb and Ho, however, the spin structure in the magnetically ordered state is not collinear, resulting in a transferred magnetic hf field at the 57Fe nucleus.
Magnetic properties and grain growth stability of nanocomposite Fe–ZrO2 granular solids prepared by mechanical milling
Authors: Axtell S. C. , Schalek R.
Published: July 2002

Correlations between the grain size [or the diffracting particle size (DPS)] and the magnetic properties of mechanically milled Fe–ZrO2 nanocomposite powders are reported. Nanocomposite powders with iron volume fractions below, near, and above the percolation value were annealed at temperatures between 573 and 1083 K and investigated. The composite as-milled powders exhibited enhanced coercivities (≳300 Oe) compared to similar mechanically milled iron powders. The iron diffracting particle size (grain size) as a function of annealing temperature showed two regimes. The first regime, corresponding to temperatures at and below 773 K, consisted of enhanced coercivities and remanence ratios with a relatively stable iron diffracting particle size of ∼25 nm. In the second temperature regime, above 773 K, the DPS increased, but remained well below the value for pure iron. Concomitant with this increase, large reductions in the coercivities were observed in the second regime.
The role of interface crystalline and magnetic structure in exchange anisotropy (abstract)
Authors: Nogués J. , Lederman D. , Schuller Ivan K. , Rao K. V.
Published: July 2002

Large exchange bias effects were measured in antiferromagnetic FeF2-ferromagnetic Fe bilayers. The interface spin structure and roughness was controlled by using different substrates and growth temperatures. The samples were characterized ex situ using low and high angle x-ray diffraction, in-plane x-ray diffraction, and atomic force microscopy. We discovered that the exchange bias is largest in the (110) FeF2 spin-compensated surface films, in contradiction with simple models that predict a low exchange anisotropy for spin-compensated surfaces. For these samples, the exchange anisotropy decreases as the interface roughness increases.
Pressure effect on the resistivity in GMR La0.60Y0.07Ca0.33MnO3 compound (abstract)
Authors: Arnold Z. , Kamenev K. , Ibarra M. R. , Algarabel P. A. , Marquina C. , Blasco J. , García J.
Published: July 2002

The substitution of La+3 by smaller Y+3 ions in mixed valence Mn+3–Mn+4 perovskites La0.67−xYxCa0.33MnO3 gives rise to a decrease of the order temperature (Tc=160±5 K for x=0.07 and Tc=240 K for x=0). One of the most relevant features found in this compound was the extremely large magnetoresistance observed in bulk polycrystals (≊−10 000%) near Tc. This huge effect was associated with the reduction of the lattice parameter. In order to have an insight into the influence of volume change on transport properties in this compound, we have performed measurement of the thermal dependence of the resistivity under hydrostatic pressure up to 8 kbar. At ambient pressure a peaklike anomaly in the resistivity is observed at Tc. Under the former assumptions, one would expect an increase of the resistivity value at the maximum as well as a decrease of Tc. However, we have found a shift toward higher temperatures and a drastic decreasing of the maximum in the resistivity for increasing pressures. At a pressure of 7.5 kbar the anomaly in the resistivity almost disappears and consequently the GMR effect. As a consequence, the effect of pressure seems to be identical to the effect of the applied magnetic field. Ibarra et al. recently observed a large spontaneous magnetovolume effect at Tc with an extra contribution to the lattice thermal expansion. This extra contribution was found to be suppressed by an applied magnetic field. From these preliminary results it seems that the interplay between magnetostrictive deformations and the shrinkage of the lattice by external pressure could be in the origin of the observed behavior.
Alternating current-excited magnetoresistive sensor
Authors: Ripka P.
Published: July 2002

Thin-film anisotropic magnetoresistors are superior to semiconductor magnetic field sensors. ac excitation of the sensor bridge reduced the noise level to 10 nT p-p. The offset reduction and sensor characteristic stabilization was achieved by periodical flipping by means of 100 mA/100 μs current pulses into a 400 turns coil wound across the sensor perpendicular to the sensitive axis. The use of the flipping mechanism can significantly improve the performance of the permalloy anisotropic magnetoresistors: long-term offset stability of 10 nT, hysteresis below 100 ppm FS and 5 nT p-p noise can be achieved with low-power sensor electronic circuits. The array of eight magnetoresistors was used in a contactless dc current meter. A sensitivity of 0.1 mA and high geometrical selectivity was obtained.
Magnetic field distribution caused by a notebook computer and its source searching
Authors: Doi T. , Hayano S. , Saito Y.
Published: July 2002

Previously, we have proposed a method of solution for the inverse problems, and successfully applied it to the biomagnetic fields. In the present article, we apply our inverse approach to the leakage magnetic field source searching for the notebook computers. As a result, it is found that our inverse approach is quite effective in searching for the leakage magnetic field source. The validity of our solutions is carefully examined by comparing the measured and calculated magnetic field distributions.
Self-stabilized magnetic colloids: Ultrafine Co particles in polymers
Authors: Leslie-Pelecky Diandra L. , Zhang X. Q. , Rieke Reuben D.
Published: July 2002

Self-stabilized magnetic colloids consist of magnetic particles dispersed in an appropriate matrix. Fixing the particles in a stabilizing matrix has the advantage of preventing particle agglomeration, increasing resistance to oxidation, and allowing control over the interparticle spacing and particle size. We describe the chemical synthesis of cobalt nanoparticles in a polystyrene/triphenylphosphine polymer matrix. Depending on the synthesis parameters, magnetic properties of the as-synthesized nanocomposites range from superparamagnetic to ferromagnetic with coercivities on the order of 130 Oe. Solvent choice and polymer crosslinking significantly affect the magnetic properties. Annealing in vacuum produces coercivities of up to 600 Oe and remanence ratios of up to 0.4. Measurement of the isothermal remanence magnetization and dc demagnetization indicate the presence of both magnetizing and demagnetizing interactions, in contrast to particles synthesized without the polymer, which show no evidence for magnetizing interactions. The zero-field-cooled temperature-dependent magnetization displays a cusp, while the field-cooled magnetization increases monotonically below the cusp temperature. Glassy behavior is observed for temperatures below the cusp, although the mechanism producing this behavior is not yet understood.
Complementary imaging of granular Co-Ag films with magneto-optical indicator film technique and magnetic force microscopy
Authors: Donahue M. J. , Bennett L. H. , McMichael R. D. , Swartzendruber L. J. , Shapiro A. J. , Nikitenko V. I. , Gornakov V. S. , Dedukh L. M. , Khapikov A. F. , Matveev V. N. , Levashov V. I.
Published: July 2002

A magneto-optical indicator film (MOIF) technique and magnetic force microscopy were used for visualization and direct real-time experimental study of the magnetization processes of magnetic Co90Ag10 granular films. It is shown that the magnetization reversal of the as-deposited films follows a specific two-step course. The first stage is characterized by gradual spin rotation to large angles up to 90° without domain formation. Further magnetization reversal proceeds by the nucleation and motion of zigzag-shaped domain walls. The dendritic structure of the domain walls was observed using both techniques. Also tracks of magnetic inhomogeneities remaining behind moving zigzag-shaped domain walls was revealed by MOIF.
Electro-optic tuning of the phase mismatch in quasi-phase-matched frequency doubling waveguides
Authors: Watts D. J. M. , Davis G. M. , May P. G. J. , Brown R. G. W.
Published: July 2002

The effect of applying an electric field across a quasi-phase-matched frequency doubling lithium tantalate waveguide has been investigated. The waveguide was fabricated by a two-stage ion exchange process in pyrophosphoric acid. An electric field of 2 kV/mm was found to shift the phase matching wavelength by 0.05 nm. It is estimated that more optimized waveguides could produce wavelength shifts of ±4 nm for an applied electric field of ±20 kV/mm and could compensate for temperature variations of ±67 °C.
A time dependent propagator method for long mean free path transport of neutral particles in plasma processing reactors
Authors: Tan Wen-yi , Hoekstra Robert J. , Kushner Mark J.
Published: July 2002

Plasma etching reactors for microelectronics fabrication are moving towards operating at lower gas pressures (<10 mTorr). These pressures are sufficiently low that simulations using continuum modeling techniques may not be strictly applicable. A time dependent kinetic method based on the use of a transition matrix (propagator) has been developed and applied to the calculation of long mean free path transport of neutral species in an inductively coupled plasma (ICP) etching reactor. The propagator P(r,r′) provides the probability that particles originating at location r′ will have their next collision at location r. The species densities obtained from this model are compared with results from fluid and Monte Carlo simulations for various mean free paths. We find that the propagator model is valid when the mean free path of the particles is larger than the numerical cell dimension and that fluid methods for long mean free path transport can be corrected to obtain the Monte Carlo or propagator results by employing an effective diffusion coefficient. Time dependent results are generating by employing a retarded time in which flights of particles beginning at past times from remote locations are used to determine the present value of the local collision frequency. Self-consistent neutral densities in ICP discharges for various pressures are obtained by employing the propagator model in a hybrid ICP model.
X-ray polarization-contrast imaging of crystals
Authors: Hirano K.
Published: July 2002

A new technique of x-ray polarization-contrast imaging to characterize crystals is described. The polarization-contrast imaging visualizes birefringent properties of a crystal. The contrast formation mechanism is explained by the dynamical theory of x-ray diffraction. The novel x-ray optics using synchrotron radiation are proposed, and the first x-ray polarization-contrast images of a crystal are reported.
Conductive sphere in a radio frequency field: Theory and applications to positioners, heating, and noncontact measurements
Authors: Jackson H. W. , Watkins J. L. , Chung S. , Wagner P.
Published: July 2002

An electrically conductive spherical sample located in an electromagnetic field excited by rf (radio frequency) current in a system of coaxial coils is treated theoretically. Maxwell’s equations are solved exactly and all integrals in the formulas for the fields are evaluated analytically for the case where the sphere is on the axis and the coil system is modeled by a stack of filamentary circular loops. Formulas are also derived for electromagnetic force exerted on the sphere, excess impedance in the coil system due to the presence of the sphere, and power absorbed by the sphere. All integrals in those formulas have been evaluated analytically. Force measurements are presented and they are in excellent agreement with the new theory. A low-power electromagnetic levitator that is accurately described by the theory has been demonstrated and is discussed. Experimental measurements of excess impedance are presented and compared with theory, and those results are used to demonstrate an accurate noncontact method for determining electrical conductivity. Theoretical formulas for power absorption are evaluated numerically and their usefulness in both rf heating and in making noncontact measurements of a number of thermophysical properties of materials is discussed.
Resonant exciton effects in InAs monolayer insertions in a GaAs matrix
Authors: Wang P. D. , Ledentsov N. N. , Torres C. M. Sotomayor , Zhukov A. E. , Kop’ev P. S. , Ustinov V. M.
Published: July 2002

We report resonant luminescence studies in ultrathin InAs quantum-well structures. Under double resonant conditions, sharp lines induced by longitudinal optical phonons were observed in the emission spectrum, in addition to the broad band luminescence. These sharp lines exhibited moderate circular polarizations whereas the linear polarization was negligible, indicating a resonant luminescence process. The properties of these sharp lines observed in InAs monolayer quantum wells are quite different from those observed previously in GaAs-AlxGa1−xAs quantum wells under similar conditions. InAs phonons were found to participate in the exciton resonant scattering process at high magnetic fields where double resonant conditions are fulfilled. InAs-like phonon energy is determined to be close to 29.9 meV, in good agreement with the energy expected for a uniform ultrathin elastically strained InAs layer. We performed various circular and linear polarizations to investigate the resonant luminescence process in thin InAs layers inserted in GaAs.
A study of Franz–Keldysh oscillations in the photo reflectance spectrum of the δ-doped GaAs film
Authors: Wang D. P. , Chen C. T. , Hsu T. M.
Published: July 2002

Our photo reflectance (PR) spectroscopy measurements of the δ-doped GaAs film at 300 K reveal many Franz–Keldysh oscillations (FKOs) above the valence band edge, E0 and the spin-orbit split energy, E0+Δ0, which enables us to determine the electric field strength from periods of FKOs provided reduced masses of the electron and holes are known. The reduced masses can be determined unambiguously at E0+Δ0, but not at E0, at which the heavy- and light-hole transitions are degenerate. However, the ambiguity at E0 can be resolved by applying the fast Fourier transform to the PR spectrum to separate the contributions from the heavy and light holes.
Simulation of a three-dimensional electrorheological suspension
Authors: Jian Li , Jiapeng Shui
Published: July 2002

The results of a computer simulation of structure formation in a three-dimensional electrorheological suspension are described. A neutrally buoyant and nonconducting hard-sphere suspension was used. It was found that the ground state of the suspension is a body-centered tetragonal structure and the process of the structure formation is as follows: initial state→random chains→chains A and B→body-centered tetragonal structure.
Conversion of a whistler wave into a controllable helical wiggler magnetic field
Authors: Kalluri D. K.
Published: July 2002

Plasma in the presence of a static magnetic field supports a whistler wave. It is shown that when the static magnetic field is switched off, the energy of the whistler wave is converted into the energy of a helical wiggler magnetic field.
Nucleation of semicircular misfit dislocation loops from the epitaxial surface of strained-layer heterostructures
Authors: Zou J. , Cockayne D. J. H.
Published: July 2002

The nucleation of semicircular misfit dislocations from the epitaxial surface in strained-layer heterostructures is considered. It is shown that the model of J. W. Matthews, A. E. Blakeslee, and S. Mader [Thin Solid Films 33, 253 (1976)], which considered a semicircular dislocation loop expanding in a semi-infinite layer, needs modification when an epilayer/substrate interface is involved.
Barkhausen noise in FeCoB amorphous alloys (abstract)
Authors: Durin G. , Bertotti G.
Published: July 2002

In recent years, the Barkhausen effect has been indicated as a promising tool to investigate and verify the ideas about the self-organization of physical complex systems displaying power law distributions and 1/f noise. When measured at low magnetization rates, the Barkhausen signal displays 1/fα-type spectra (with α=1.5÷2) and power law distributions of duration and size of the Barkhausen jumps. These experimental data are quite well described by the model of Alessandro et al. which is based on a stochastic description of the domain wall dynamics over a pinning field with brownian properties. Yet, this model always predicts a 1/f 2 spectrum, and, at the moment, it is not clear if it can take into account possible effects of self-organization of the magnetization process. In order to improve the power of the model and clarify this problem, we have performed a thorough investigation of the noise spectra and the amplitude distributions of a wide set of FeCoB amorphous alloys. The stationary amplitude distribution of the signal is very well fitted by the gamma distribution P(ν)=νc−1 exp(−ν)/Γ(c), where ν is proportional to the domain wall velocity, and c is a dimensionless parameter. As predicted in Ref. , this parameter is found to have a parabolic dependence on the magnetization rate. In particular, the linear coefficient is related to the amplitude of the fluctuations of the pinning field, a parameter which can be measured directly from the power spectra. In all measured cases, the power spectra show α exponents less than 2, and thus poorly fitted by the model.

Actually, the absolute value of the high frequency spectral density is not consistent with the c parameter determined from the amplitude distribution data. This discrepancy requires to introduce effects not taken into account in the model, as the propagation of the jumps along the domain wall. This highly enhances the fit of the data and indicates effects of propagation on the scale of a few millimeters. These results are analyzed in terms of new descriptions of the statistical properties of the pinning field based on fractional brownian processes.
Soft magnetic properties of Fe–Zr–B thin films (abstract)
Authors: Shevchenko N. B. , Christodoulides J. A. , Meng-Burany X. , Murthy A. S. , Hadjipanayis G. C.
Published: July 2002

The Fe–Zr–B system has been shown to exhibit superior soft magnetic properties in melt-spun ribbons. These ribbons with composition Fe87Zr7Cu3B3 had very low coercivity (0.02 Oe) and high magnetization (190 emu/g) after crystallization at 540 °C from the amorphous state. We have recently extended these studies to thin films with similar composition. These films were prepared by dc magnetron sputtering from a solid alloy target. Transmission electron microscopy and differential scanning calorimetry were used to study the evolution of the microstructure upon annealing. Magnetic measurements were taken with a hysteresigraph. Unlike the as-spun ribbon samples, the as-sputtered films showed 10 nm crystallities. Selected area diffraction patterns indicated a two phase microstructure, with the bcc phase which gave rise to the soft magnetic properties in the ribbons, coexisting with a second phase leading to degraded soft magnetic properties (Hc=20 Oe and Ms=150 emu/g). Subsequent annealing of the thin film samples enhanced the formation of the second phase and further degraded the soft magnetic properties. Work is currently underway to determine a means of suppressing the formation of the secondary phase while preserving the phase with superior soft properties.
Micro/nanoscale studies of boundary layers of liquid lubricants for magnetic disks (abstract)
Authors: Koinkar Vilas N. , Bhushan Bharat
Published: July 2002

The atomic force/friction force microscope is used to study the micro/nanotribiological properties of perfluoropolyether lubricants. Single-crystal silicon wafers were lubricated with nonpolar (Fomblin Z-15) and polar (Fomblin Z-DOL and Demnum S-100) lubricants. The nanowear tests show that the nonpolar (Z-15) lubricant depleted from the wear track within few cycles whereas polar (Z-DOL) lubricant exhibits excellent nanowear resistance with no degradation. The polar lubricant results in a lower value of microfriction as compared to the nonpolar lubricant and unlubricated silicon sample. The effect of thickness of polar lubricant is studied for the thermally bonded Z-DOL lubricant before and after wash. Unwashed polar lubricant film with unbonded fraction exhibited better resistance to wear than that of washed lubricant film. Thicker films are also more durable. Wear experiments with magnetic disks show that lubricant film on a supersmooth disk is more effective in reduction of friction and wear than a smooth disk. Coefficients of friction on micro- and macroscales are compared. Variation in lubricant film thickness results in variation in the coefficient of friction. Thus, friction force microscopy can be used to measure lubricant uniformity with spatial resolution on the order of tens of nanometers. Finally, adhesive experiments show that bonded film behaves as a soft polymeric solid.