bluuebunny/crossref_metadata_2025 huggingface dataset:
Field effect on thermal emission from the 0.40 eV electron level in InGaP
Authors: Zhu Q. S. , Hiramatsu K. , Sawaki N. , Akasaki I. , Liu X. N.
Published: July 2002

Results are reported of electric-field dependence on thermal emission of electrons from the 0.40 eV level at various temperatures in InGaP by means of deep-level transient spectroscopy. The data are analyzed according to the Poole–Frankel emission from the potentials which are assumed to be Coulombic, square well, and Gaussian, respectively. The emission rate from this level is strongly field dependent. It is found that the Gaussian potential model is more reasonable to describe the phosphorus-vacancy-induced potential in InGaP than the Coulombic and square-well ones.
Frequency control of semiconductor lasers
Authors: Ohtsu M. , Nakagawa K. , Kourogi M. , Wang W.
Published: July 2002

This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency.

For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.
Measurements of electron density, decay time, and floating potential in methane discharges
Authors: Snyder H. R. , Fleddermann C. B.
Published: July 2002

Measurements of electron density, electron density decay times, and floating potential in mixtures of methane, hydrogen, oxygen, and argon excited by a radio-frequency helical resonator discharge are reported. These studies were undertaken in a range of gas mixtures that correspond to those used for plasma deposition of diamond thin films. Measurements were performed using microwave cavity perturbation to measure electron density and decay times, and a Langmuir probe to measure electron density and floating potential. It is shown that there is little variation in electron density over the range of gas mixtures used, and that negative ion formation is not a significant process in these types of discharges.
Evaluation of crystal diffractor parameters for curved diffractors
Authors: Wittry D. B. , Chang W. Z.
Published: July 2002

The general equation for the effective region of doubly curved diffractors for radiation from a point source that was derived by Wittry and Sun [J. Appl. Phys. 71, 1 (1992)] has been extended to include the effect of source misalignment. Intensity profiles across an x-ray topograph recorded on a photographic film located between the diffractor and the focus have been calculated for the Johann case including misalignment effects. Topographs and intensity profiles obtained using TiKα radiation generated by a focused electron beam have confirmed the theoretical calculations and have provided values of the peak reflectivity and rocking-curve width for graphite, mica, and silicon diffractors of Johann geometry with a radius of curvature of ∼100 mm.
The lattice locations of silicon atoms in delta-doped layers in GaAs
Authors: Ashwin M. J. , Fahy M. , Harris J. J. , Newman R. C. , Sansom D. A. , Addinall R. , McPhail D. S. , Sharma V. K. M.
Published: July 2002

We have used secondary ion mass spectrometry, local vibrational mode infrared absorption, and electrical characterization to study the incorporation of Si delta-doped planes in GaAs grown by molecular beam epitaxy at 400 °C, in the concentration range 0.01–0.5 monolayers. A correspondence is observed between the density of SiGa donors, the free electron concentration and the total Si coverage, up to a coverage of ∼1013 cm−2; however, above this value, the electron density falls, while [SiGa] remains constant up to a coverage of ∼1014 cm−2, and then falls below the detection limit at 0.5 monolayer coverage. These effects have been interpreted in terms of a model which takes account of Si migration and aggregation on the delta-doped plane during deposition.
Interpretation of the activation energy derived from a stretched-exponential description of defect density kinetics in hydrogenated amorphous silicon
Authors: Benatar Lisa E. , Redfield David , Bube Richard H.
Published: July 2002

At least three quantities have been referred to as ‘‘activation energies’’ in association with fits to metastable defect kinetics in hydrogenated amorphous silicon. Most commonly cited is Eτ, the activation energy determined from stretched-exponential fits to kinetics data measured over a range of temperatures. The stretched exponential can also be written in terms of a rate constant, K, which has been reported as being thermally activated. In addition, a stretched-exponential model describing defect kinetics includes an energy, E2, in its rate equation. In this article, we clarify the interpretations of Eτ, EK, and E2, and discuss the possible physical significance of Eτ.
Γ-X phonon-assisted thermionic currents in the GaAs/AlxGa1−xAs interface system
Authors: Tammaro David , Hess Karl , Capasso Federico
Published: July 2002

The decrease of the Richardson constant by more than 3 orders of magnitude in the indirect energy-gap range of composition of the GaAs/AlxGa1−xAs interface has been interpreted in the past in several ways. We present here a phenomenological model based on envelope wave functions that can describe the transmission coefficient involving two mechanisms: zero-phonon transitions due to Γ-X mixing and phonon assisted transitions. The model shows reasonable agreement with a wide range of experimental data. We conclude that a general relation for the thermionic current across heterojunctions must include phonon effects when the Al mole fraction exceeds 0.45. We also find that calculated transmission coefficients are very different from the step function used in the classical theory.
Growth kinetics of micron-size nickel lines produced by laser-assisted decomposition of nickel tetracarbonyl
Authors: Boughaba S. , Auvert G.
Published: July 2002

Polycrystalline nickel lines as small as 1 μm in width were deposited by pyrolysis of nickel tetracarbonyl [Ni(CO)4] on polysilicon/silicon dioxide/monosilicon substrates. As heat source, a cw argon-ion laser operating at several wavelengths around 0.5 μm was used. The growth kinetics, morphology, and electrical resistivity of the nickel microstructures were investigated at various scanning speeds of the laser spot, laser beam powers, and reactant gas pressures. From the deposition kinetics, the vertical deposition rate of nickel was calculated. For Ni(CO)4 pressures below 0.3 mbar, deposition of flat-topped nickel lines occurs. The deposition rate was proportional to the Ni(CO)4 pressure and independent of the laser output power. For Ni(CO)4 pressures above 0.3 mbar, lines with a Gaussian profile are obtained. The deposition rate was found to be independent of the Ni(CO)4 pressure and exhibits an activation energy of 11.5 kcal mol−1. The measured electrical resistivity of the flat-topped lines was about ten times higher than that of the bulk (7 μΩ cm) while for the Gaussian lines this ratio falls below 1.5. Kinetic measurements with carbon monoxide (CO) as buffer gas were also performed. The nickel deposition rate was found to decrease as the CO partial pressure increases for all the Ni(CO)4 partial pressures considered (0.2–20 mbar). The morphology and roughness of the deposited lines were investigated using both a scanning electron microscope and an atomic force microscope. The effect of the laser beam on the quality of the interfaces underlying the deposited lines was investigated by means of a focused ion beam system. On the basis of the experimental results, a decomposition mechanism of the Ni(CO)4 molecules was elaborated.
Radiation detection from Fiske steps in Josephson junctions above 200 GHz
Authors: Cirillo M. , Modena I. , Santucci F. , Carelli P. , Castellano M. G. , Leoni R.
Published: July 2002

We have measured the Josephson radiation emitted at millimeter-wave frequencies by a long Josephson junction dc-current biased on Fiske steps of the current-voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero-field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in-line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon-assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30-Ω normal-state resistance tunnel junction.
Wide-band frequency response measurements of photodetectors using low-level photocurrent noise detection
Authors: Xie Fu Zeng , Kuhl D. , Böttcher E. H. , Ren S. Y. , Bimberg D.
Published: July 2002

We report a novel measurement technique for the wide-band determination of the frequency response of photodetectors. It is based upon the accurate measurement of the photocurrent noise spectra under illumination with a light-emitting diode. The high sensitivity of −200 dBm/Hz within a frequency regime from 10 MHz to 1.6 GHz renders it particularly attractive for investigating the response behavior at low optical input levels and for characterizing frequency-dependent gain phenomena. The practical potential of the method is illustrated by applying it to various types of InGaAs-based photodetectors (p-i-n and avalanche photodiodes and metal-semiconductor-metal photodetectors).
Magneto-optics of excitons in a center Si δ-doped GaAs/AlGaAs quantum well
Authors: Rimmer J. S. , Evans J. H. , Innes A. , Hamilton B. , Missous M.
Published: July 2002

Observation of magneto-optical transitions in a Si center δ-doped quantum well is reported. The excitons are stabilized by the application of the field. With photoluminescence excitation spectroscopy (PLE), transitions below the Fermi level are observed. At magnetic fields ≳8 T broadened Landau levels can be seen. The features in the PLE spectra sharpen up significantly when the Larmor diameter becomes less than the average interimpurity spacing. The effect of the ionized donors in the well on the optical spectra is discussed.
Onset of helimagnetism in weakly strained epitaxial FCC antiferromagnet Cd1−xMnxSe (abstract)
Authors: Giebultowicz T. M. , Nunez Valerie , Samarth N. , Luo Hong , Furdyna J. K.
Published: July 2002

The FCC Heisenberg spin lattice with AF NN interactions is a known model of an inherently frustrated system. Due to the continuous ground state degeneracy, the spin order in this lattice is particularly sensitive to weak symmetry-breaking perturbations, such as Dzyaloshinski–Moriya (D-M) interactions or anisotropic strains. These effects may create a rich variety of colinear and noncolinear structures. The limited availability of naturally existing prototypes has resulted in little experimental work in this field. However, the emergence of novel epitaxial structures containing zinc-blende type MnSe and MnTe, which are very close approximations of the NN FCC Heisenberg model, opens new opportunities for such studies. In many of these systems the MnSe and MnTe layers are strongly strained (c/a∼0.95–1.05) due to lattice mismatch effects. The resulting anisotropy in the Mn-Mn exchange may lead to a dramatic change in the spin structure—e.g., studies of MnSe/ZnTe superlattice systems have revealed a transition from a type III AF order to an entirely new incommensurate helical state not yet seen in any FCC aniferromagnet.1 In this paper we report neutron diffraction data from ‘‘thick’’ (∼1 μm) Co0.25Mn0.75Se films. Detailed mapping of the magnetic diffraction intensity reveals a shift from perfect type III lattice points, showing that detectable incommensurate effects may be produced even by weak residual stresses. Our results point out that noncolinear effects arising from strains may be indistinguishable from canting produced by D-M exchange,2 which may additionally complicate the detection of this weak interaction in Mn chalcogenides.
Oscillatory interlayer magnetic coupling of sputtered Fe/Nb superlattices
Authors: Mattson J. E. , Fullerton Eric E. , Sowers C. H. , Huang Y. Y. , Felcher G. P. , Bader S. D.
Published: July 2002

The saturation field of sputtered Fe/Nb superlattices oscillates as a function of the Nb thickness with a periodicity of ∼9 Å. In contrast to the case of Fe/Cr superlattices, the concurrent magnetoresistance oscillations were found to be very weak. Yet polarized neutron reflection measurements confirm that the Fe/Nb superlattices with high saturation field possess a magnetic ground state of the +−+− type. Neutron and x-ray measurements indicate that, while the crystalline and antiferromagnetic order is well developed along the thickness of the film, the average lateral size of the crystallites (as well as of the magnetic domains) is quite small. This effect (thought to be related to the gross mismatch of the iron and the niobium crystal lattices) may be the cause of the high overall resistance of the material, and its weak dependence on the magnetization.
Mathematical modeling of the head-disk interface (abstract)
Authors: Crone Robert M. , Jhon Myung S.
Published: July 2002

State-of-the-art theoretical and numerical techniques required to simulate the head-disk interface (HDI) of future magnetic storage devices is presented. The severity of operating conditions (i.e., attempts to achieve flying heights as low as 40 nm) pose several challenges. Large transient pressure gradients can be established within air bearing leading to numerical oscillations as well as to increased program execution times. Enhanced gaseous rarefaction effects must also be incorporated into the analysis. In the present study, accurate nonoscillatory air bearing pressure distributions were obtained using a high resolution finite element algorithm to solve the generalized Reynolds equation. Higher order gaseous rarefaction effects are incorporated into generalized Reynolds equations using the total mass flow rate coefficient predicted from the linearized Boltzmann equation. The form of the generalized Reynolds equation that is presented in this paper is an improved version of the continued fraction approximation previously proposed by Crone et al.1 A simple scaling analysis, which is based upon the results of the linearized Boltzmann equation, will also be presented to study the effect of slider miniaturization, as well as to obtain a novel interpretation of accelerated wear and accelerated flyability test results.
Anomalous anisotropy in the RCo4B compounds
Authors: Hong N. M. , Thuy N. P. , Hien T. D. , Hilscher G. , Zhao T. S. , Grössinger R.
Published: July 2002

The magnetic anisotropy of YCo4B determined by the singular point detection (SPD) method and magnetization measurements over a large range of temperature is reported. The anisotropy energy of CeCo4B estimated by magnetization measurements is one order of magnitude larger than those of compounds with Y, La, or Gd. The temperature dependence of anisotropy energy in YCo4B is discussed in the light of the temperature variation of the c/a ratio and the giant anisotropy of CeCo4B is presumed to be due to the itinerant character of the 4f states in the compound.
Film Sm–Co permanent magnets for the biasing of thin permalloy strips
Authors: Hegde H. , Jen S. U. , Chen K. , Cadieu F. J.
Published: July 2002

Highly textured Sm–Co-based permanent magnet films which exhibit in-plane anisotropy, with thicknesses ranging from 500 Å to several microns, have been synthesized on polycrystalline Al2O3 substrates. The textured Sm–Co-based films of the disordered TbCu7-type structure were directly crystallized by sputter deposition onto heated substrates. Remanent inductions BR were ≊8 kG, and the intrinsic coercivities, iHc, ranged from 4–10 kOe. The crystallite grain size as a function of film thickness was determined. The use of these films in small scale geometry devices was tested in a sandwich configuration of strips of areal size 7×1 mm2. The Sm–Co films were used to bias Ni81.5Fe18.5 permalloy films. The sandwich permanent magnet was magnetized across its width. The permalloy films were sputter synthesized under conditions very similar to those of the permanent magnet films. Substrate biasing was not used for either the permanent magnet or the permalloy strips. The permalloy films exhibited a well-defined easy axis along the length of the strip. Unbiased, the permalloy showed a maximum magnetoresistance MRmax of 3.0%. On biasing with a 25-μm-thick permanent magnet film with a spacer thickness of 25 μm, the longitudinal biasing field value at MR/MRmax=0.5 was more than a 100 Oe, indicating an average biasing field of the same value.
Emerging technologies in magnetic storage (invited) (abstract)
Authors: Thompson David A.
Published: July 2002

This tutorial paper deals with the continuing evolution of magnetic data storage, and opportunities for the application of new magnetic materials and phenomena to this field. It is intended for people who are knowledgeable about magnetism, but not specialists in storage technology. Hot topics include giant magnetoresistance and spin valves, direct overwrite optical media, advanced head and media materials, and the materials requirements of high data rate contact recording.
Measurements of the ac susceptibility and the spontaneous resistive anisotropy in dilute P_d_Fe
Authors: Wang Z. , Kunkel H. P. , Williams Gwyn
Published: July 2002

The results of detailed measurements of the field and temperature dependent ac susceptibility (χac) and the spontaneous resistive anisotropy (SRA) of P_d_Fe samples with between 0.35 and 2.4 at. % Fe are summarized. The χac data enable estimates for the critical exponents γ, β, and δ to be made, and in the 1.4 at. % sample these are close to Heisenberg model predictions. In the other samples evidence is found for exchange bond disorder. The SRA results are consistent with this transport coefficient exhibiting critical behavior governed by percolationlike behavior.
Magnetic behavior of (Fe0.90Cr0.03Ni0.07)2P and (Fe0.90Cr0.05Ni0.05)2P
Authors: Ghose Vipasha , Sivayogan N. , Krishnamurthy Anjali , Srivastava Bipin K. , Dolia S. N.
Published: July 2002

(Fe0.90Cr0.03Ni0.07)2P and (Fe0.90Cr0.05Ni0.05)2P are found to crystallize in Fe2P-like hexagonal symmetry (hereafter referred to as FCN37 and FCN55, respectively). dc magnetization measurements have been made in external fields upto 8 kOe and in the temperature range 80–800 K. ac susceptibility measurements have been performed in the temperature range 80–300 K. dc measurements show that the two alloys are ferromagnetic with Tc∼250 K for FCN37 and ∼180 K for FCN55. Magnetic moments of the polycrystalline samples at 7 kOe and ∼90 K are 67 and 47 emu/g for FCN37 and FCN55, respectively. Both of them also exhibit small but finite remanence and coercivity. The temperature dependent magnetization measurements made on FCN37 and FCN55 in zero field cooling and field cooling modes branch off at ∼230 and ∼125 K, respectively. Remanent magnetization, measured in the remanent field of the magnet (∼20 Oe), preceded by isothermal cooling of the samples down to 85 K and applications of different fields for fixed interval, relaxes over a period of time following log t. Peak in the ac susceptibility-temperature curve, measured at three frequencies in the interval 27–983 Hz, shifts with frequency obeying Vogel–Fulcher law. Results suggest existence of magnetic clusters in the two ferromagnetically ordered alloys, or spin glass state at low temperatures, or both.
Metal-metal versus anion-anion bonding in B31 structures
Authors: Lewis L. Henderson , Goodenough J. B.
Published: July 2002

The evolution with x in V1−xTixS of the lattice parameters and magnetic susceptibility are contrasted with the situation for CoAs. The V1−xTixS system exhibits metal-metal bonding with a transition from the B81 structure of TiS to the B31 structure of VS as the metal-metal bonding in the basal planes of the B81 structure weakens with increasing vanadium concentration. In the transition region, the loss of up to 7 mol % sulfur stabilizes the B81 versus the B31 phase. The V1−xTixS system is Pauli paramagnetic. In CoAs, on the other hand, a distortion to B31 symmetry is shown to be due to metalloid-metalloid bonding, and the compound exhibits antiferromagnetic long-range order below TN=54 K, but with a weak ferromagnetic component if cooled in a magnetic field.
Quasi-two-dimensional antiferromagnetism in Fe[S2CNC4H8]2I
Authors: Lukin J. A. , Friedberg S. A. , DeFotis G. C. , Chamberlain R. V.
Published: July 2002

Recent studies of the magnetic susceptibility of powdered Fe[S2CNC4H8]2I confirm that Fe(III) in this salt is in the unusual spin state S=3/2 and that the system behaves as a quasi-2D antiferromagnet. Analysis of the data indicates that the ‖±1/2≳ ground doublet is separated by Δ/k=18.7 K from an excited ‖±3/2≳ doublet. We have now measured the heat capacity of polycrystalline Fe[S2CNC4H8]2I between 0.5 and 80 K in a liquid 3He-cooled calorimeter. CP exhibits a λ-type peak at TN=2.18±0.01 K. The critical entropy, S(TN)=0.25R ln2, indicates that most of the spin order is of a short-range, low-dimensional nature. The contribution of lattice vibrations to CP was estimated using the method of Sorai and Seki [J. Phys. Soc. Jpn. 32, 382 (1972)], which assumes a Schottky contribution from two doublets whose separation Δ was determined from the magnetic data. At the same time, the cooperative peak was reasonably represented by the results for the 2D S=1/2 XY model with Jxy/k=−2.18 K. In order to obtain a self-consistent fit of the lattice heat capacity, however, it was necessary to use a larger splitting (Δ/k=32 K) than that found from susceptibility data. This discrepancy between the doublet-doublet separations indicated by χ and CP measurements may suggest a possible temperature dependence of Δ.
Spiraling algorithm in Monte Carlo studies of incommensurate spin systems (abstract)
Authors: Saslow W. M. , Gabay M. , Zhang W.-M.
Published: July 2002

The application to incommensurate spin systems of the Monte Carlo method has been an ongoing problem for many years. We resolve it by employing a spiraling algorithm that enables the system to choose its own boundary conditions (i.e., its pitch Δ). The model considered is a modification of the 2D fully frustrated triangular lattice of XY spins: all horizontal bonds are multiplied by a factor η.1,2 This system has two ordered phases: spiral and antiferromagnet. In the spiral phase, even for small sizes of the system, the algorithm gives the bulk value for the pitch; convergence as a function of size is far better than can be obtained with free or periodic boundary conditions. Moreover, the algorithm yields the temperature dependence of the incommensurate pitch in the spiral phase. The spiral-to-antiferromagnetic phase transition is continuous and a Lifshitz point occurs at finite temperature. The spiral-to-antiferromagnetic phase transition does not appear in the specific heat Cv or the spin stiffness ρyy along y, but it does appear in the helicity ψhel (a measure of the sense of twist of the spiral), the helicity susceptibility χhel, and in the spin stiffness ρxx along x.3 This latter quantity cannot be computed by the usual procedures when the boundary conditions are permitted to vary, but it is inversely proportional to χΔ, the fluctuation-determined susceptibility of the pitch.3
Magnetization correlations in thin film media in the presence of a recorded transition
Authors: Bertram H. Neal , Beardsley I. A. , Che Xiaodong (Carl)
Published: July 2002

Magnetization correlations in thin film media in the presence of a written transition have been studied utilizing numerical micromagnetic simulation. Polycrystalline films are modeled by a hexagonal array of identical grains each with random anisotropy axis orientation. A transition is written in the medium and the positional variance, the stationary cross track correlation, and the nonstationary down track correlation functions are determined by suitable averaging over statistical ensembles.
Temperature dependence of the magnetic properties of amorphous rare earth-iron boron alloys
Authors: Clegg S. J. , Greenough R. D. , Hagston W. E.
Published: July 2002

The temperature dependences of the magnetization and magnetostriction have been measured in Tbx(Fe82B18)100−x, Smx(Fe82B18)100−x, and TbxLa5(Fe82B18)95−x, where 0≤x≤50. Below a spin freezing temperature Tf, which depends on the rare earth (RE) content, a random anisotropy phase is established when the anisotropy to exchange ratio is large. Between Tf and the Curie temperature Tc, the temperature dependences of the magnetostrictive contributions from the rare earth ions are compared with the theoretical single ion and two ions predictions. For low Tb concentrations (∼10 at. %) or larger Tb content (∼30 at. %) with small additions of La, the data follow the Î9/2(x) power law whereas the Î5/2(x) is otherwise more appropriate. For Sm, when x∼50, single ion Î5/2(x) behavior is seen but with x∼10, a two ion (M/M0)2 dependence is observed. These different temperature dependences are a reflection of the symmetry of the fields surrounding the RE ions and provide useful information for the modeling of RE ions in an amorphous matrix.
Granular magnetic composites employing cobalt based amorphous alloys in a polymeric host
Authors: Göktürk Halit S. , Fiske Thomas J. , Kalyon Dilhan M.
Published: July 2002

Cobalt based soft ferromagnetic amorphous alloys have unique properties which make these materials good candidates to be employed in magnetic composites prepared by dispersing granules of the alloy in a polymeric host. These alloys have excellent soft magnetic properties which are minimally affected by external stresses induced during the processing operations involved in the preparation of such a composite. Continuous ribbons of a cobalt based amorphous alloy, Metglas 2705M, were cut into large aspect ratio flakes and blended into a polyethylene host in volume fractions ranging from 0.001 to 0.15. The relative magnetic permeability values of the composites increased with increasing volume fraction of the amorphous alloy, reaching a value of 26 at the volume fraction 0.15. The magnetic properties of the composites exhibited two distinguishing features: The permeability values of these composite samples were found to vary only slightly, about 10%, as a function of frequency in the range 10 Hz–100 kHz; the samples remained unsaturated up to magnetic fields of 5800 A/m. These features are believed to be a result of the magnetic properties of the composite being heavily influenced by the polymeric gaps between the magnetic filler particles.
Fast thermoremanent magnetization in the insulating spin glass CsNiFeF6
Authors: Enkler D. , Roters A. , Steiner M.
Published: July 2002

With a newly built superconducting quantum interference device magnetometer, the time range of measurements of the thermoremanent magnetization (TRM) was extended to less than 10 ms. This new method allows a direct investigation of aging and relaxation in the spin glass region for short times. As a first example for fast TRM, the magnetization in a single crystal of the insulating spin glass CsNiFeF6 was measured in the time range −2<log(t/s)<3 at magnetic fields between 2 and 20 G at temperatures between 4.7 and 1.7 K. Additionally, the zero field and field cooled magnetization of this sample to temperatures up to 9 K was studied.
A unique feature of some simple many body quantum spin systems
Authors: Sen Surajit , Long Ming , Florencio João , Cai Z.-X.
Published: July 2002

Dynamical correlations in simple quantum spin models can be conveniently studied via the continued fraction formalism in which the Laplace transformed dynamical two-point correlation can be written as C(z) = 1/(z + Δ1/(z + Δ2/(z + ..to ∞))), where Δn’s are functions of static correlations. Very often, for systems without any natural dominant characteristic frequencies, as n increases, Δn ≊ κnα, α=0 or 1, κ being some constant, for large n for simple quantum spin systems. It is suggested that this property of Δn’s could be related to some underlying ‘‘nearly noninteracting fermionic nature’’ of these quantum spin systems.
Dynamical spin pair correlations in s=1/2 Heisenberg chain
Authors: Sen Surajit , Long Ming
Published: July 2002

Calculations are presented for the on-site dynamical spin pair correlation of the s=1/2 Heisenberg chain at T=∞. The result for the isotropic chain is consistent with short time analysis. The long time behavior may be interpreted as compatible with exponential decay as well as with power law decay interpretations. The chain with small Ising-like anisotropy also exhibits exponential-like long time decay as found in recent Mössbauer effect spectroscopy work. Preliminary calculations are also presented for the mathematically challenging case of extreme Ising-like anisotropy.
Critical behavior of ferromagnetic Ising films in a transverse field (abstract)
Authors: Tucker J. W. , Sarmento E. F.
Published: July 2002

The critical behavior of a spin-1/2 Ising thin film in the presence of transverse fields, both at the surface, and within the bulk of the film, is investigated. The method adopted makes use of an effective field theory that includes the single-site spin correlations. Surface exchange enhancement within the surface and between the surface and the nearest-neighboring layer is accounted for. An expression for the critical ratio of the exchange couplings that lead to a Curie temperature higher than that for the infinite bulk material is obtained. This value is independent of the number of layers, L, in the film. It is shown that the surface and bulk transverse fields act in opposite ways on this critical ratio of the exchange couplings, with the result that in the presence of a uniform field the critical ratio is practically insensitive to the field strength. The dependence of the critical temperature on the thickness of the film is examined, and the reason for the conflicting results that have recently appeared in the literature for the zero field case resolved. The magnetic field dependence of the critical temperature is studied and the phase diagram in the (hs, hb, Js/Jb) space presented. (hs, Js and hb, Jb are the transverse field and the exchange coupling for the surface layer and the bulk of the film, respectively.) In the limit that L→∞, contact with previously reported results obtained for a semi-infinite slab is made.
The absence of giant magnetoresistance on antiferromagnetically coupled NiFe/Cr multilayers
Authors: Watson M. L. , Doyle W. D. , Fujiwara H. , Hossain S.
Published: July 2002

To further investigate the relationship between the giant magnetoresistance (GMR), anisotropic magnetoresistance (AMR), and antiferromagnetic (AFM) coupling found in magnetic multilayers we have deposited NiFe/Cr multilayers onto rectangular Corning 7059 glass substrates at ambient temperatures. Uniaxial anisotropy was induced by applying a 90 Oe field along the length of the substrate strips during deposition and in all cases a 50–100-Å-thick Cr underlayer was used. For Cr interfilms 9–12-Å thick, the multilayers were found to have hard axis saturation fields of 90–200 Oe and easy axis hysteresis loops which exhibit a complex behavior dependent on whether the multilayer is composed of an even or an odd number of NiFe films. Both of these effects result from AFM coupling of the NiFe films. The magnetoresistive (MR) behavior was also found to be complex and dependent on the number of NiFe films in the multilayer. A theoretical model based on coherent rotation and including AFM coupling between the NiFe films has been developed and both the hysteresis loops and MR of the multilayers calculated. From these calculations it has been found that, despite the existence of AFM coupling, the experimentally observed magnetoresistive behavior can be described using only AMR. Thus the existence of AFM coupling does not result in GMR NiFe/Cr multilayers. This is discussed in terms of spin filtering at the multilayer interfaces.
Spin correlations in dense Kondo systems
Authors: Santini P. , Sólyom J. , Andreani L. C. , Beck H.
Published: July 2002

We performed numerical diagonalization studies of the periodic Anderson Model on finite chains up to 6 sites, in the integral valence limit for T=0. We find that in the weak coupling (Kondo) region the hybridization between f and conduction electrons induces pronounced correlations between the f spins. These correlations are ferromagnetic for low filling and become antiferromagnetic close to half-filling, with a smooth transition between the limiting cases. In no limit can the physics be completely described by a Ruderman–Kittel–Kasuya–Yosida effective spin Hamiltonian, as f and conduction band electrons are always correlated. To study the onset of long range order we considered also a simpler model for the half-filled case, called ‘‘Kondo necklace.’’ We studied the model at T=0 by numerical diagonalization and finite-size scaling, on chains up to 12 sites. We find that below a finite critical value of the Kondo coupling there exists a gapless phase with quasi long range antiferromagnetic correlations.
UPd2Al3 singlet-ground-state magnetism in a heavy-fermion superconductor
Authors: Grauel A. , Böhm A. , Geibel C. , Weber G. , Steglich F. , Sato N. , Komatsubara T. , Horn S.
Published: July 2002

Analysis of the leading nonlinear term of the susceptibility χ3(T) of the singlet-ground-state heavy-fermion superconductor UPd2Al3 shows evidence for a jump at TN and a ‘‘third-order’’ Curie–Weiss type behavior at higher temperatures, inferring quadrupolar or magnetoelastic interactions. These effects are closely related to quadrupolar correlations observed in measurements of the nonlinear susceptibility and calculated in a crystal-electric-field approach for the singlet-ground-state systems URu2Si2 and PrNi5 .
Enhancement of the electronic specific heat coefficient in (Co1−xNix)2ScSn
Authors: Fuller C. J. , Chen Z. W. , Anbalagan N. , Lin C. L. , Mihalisin T.
Published: July 2002

Both Co2ScSn and Ni2ScSn crystallize with the cubic L21 Heusler structure. The electrical resistivity, magnetization, and Mossbauer studies have shown that Co2ScSn undergoes a magnetic transition at Tc=238 K and Ni2ScSn is a paramagnet. When Ni is substituted for Co in (Co1−xNix)2ScSn, Tc decreases linearly with increasing Ni concentration x and goes to zero in the vicinity of x=0.40. The electronic specific heat coefficient γ is enhanced from 13 mJ/mol K2 for Co2ScSn (x=0.0) to 58 mJ/mol K2 for x=0.40. γ remains nearly constant from x=0.40 to x=0.50 as the sample moves away from the magnetic instability, and then decreases to about 5 mJ/mol K2 for Ni2ScSn (x=1.0). The γ enhancement will be discussed and compared with that observed in the other d-electron and f-electron systems.
The prediction of current distribution in an induction heating installation by a coupled finite-boundary-elements method (abstract)
Authors: Fouladgar J. , Develey G.
Published: July 2002

The finite elements method (F.I.M.) is a powerful means to resolve the diffusion equation and to predict the current distribution in the induction heating installations. However, it is confronted with two difficulties: the relative movements of different parts and definition of the infinity for the open boundary systems. The boundary element method (B.E.M.) can resolve these two problems but it is not applicable for the nonlinear systems.1 In this paper we propose a coupled finite-boundary-elements method to study a planar circular inductor parallel to a metallic load and fed by a voltage U with a variable frequency up to several kHz. We resolve the diffusion equation in the volume of the inductor and load by the F.E.M. and we use the B.E.M. to define the boundary conditions at their surfaces. The F.E.M. leads to a symmetrical band matrix but the coupled method changes it to a nonsymmetrical one and increases its width. To improve it, we use an iterative method that preserves the structure of the F.E.M. matrix. The calculations shows that the current is distributed nonuniformly around the coils. It is important near the load and very low near the neighboring coils. For the middle coils the current changes the direction in some areas. This nonuniform distribution of current increases the resistance of the inductor and can improve the generator design.
Numerical analysis of field perturbation due to a crack in a conductor (abstract)
Authors: Ismagilov E. M. , Komarov V. A.
Published: July 2002

A wide range of nondestructive testing (NDT) applications to detect cracks and defects in conducting materials are based on eddy current effects. A study of electromagnetic fields and eddy currents that induced around defects is important to design and optimize eddy current probes. In order to determine the current density in the region that includes conducting material and field perturbation in the air, the solution of Maxwell’s equations is required. For most practical problems the geometry of the region is too difficult for analytical solution and numerical method must be used. Numerical analysis techniques such as the finite element method have been successfully developed for modeling eddy current NDT phenomena.1 We used the three-dimensional finite element formulation as a basis for calculation of the induced current density around the crack in a conductor. Space components perturbation both for current and flux densities is investigated. The influence of different crack sizes and excitation coil dimensions is studied. The computational results can be used for optimizing eddy current probe dimensions. Much information can be obtained to identify and quantify defects. Our future work will be devoted to this.
Magnetic circular dichroism of FexCo1−x alloy single crystal thin films (abstract)
Authors: Idzerda Y. U. , Gutierrez C. J. , Tjeng L. H. , Lin L.-J. , Meigs G. , Chen C. T.
Published: July 2002

Soft x-ray magnetic circular dichroism (SXMCD) of the L2 and L3 edges is used to independently determine magnetic information for single crystal FexCo1−x alloy thin films (0≤x≤1) grown on ZnSe(001). Similar to surface magneto-optical Kerr effect (SMOKE) but with element and site specificity, SXMCD, which is the difference between the absorption of left and right circularly polarized soft x rays, can be used to determine a variety of magnetic parameters including spin-orbit and exchange interaction energies. Measurements made at the National Synchrotron Light Source (NSLS) U4B beamline on the Fe–Co alloy system exhibit unique changes in SXMCD signal as a function of alloy concentration. The SXMCD signal is found to rise linearly with reducing atomic concentration for both the Co and Fe. At the dilute limit of 10% FE (x=0.1), the Fe L3 SXMCD is found to increase by almost a factor of three to near 40%. In some Fe/Co structures, the largest reported SXMCD values in excess of 70% are observed! This example shows the utility of SXMCD in obtaining magnetic information in alloy systems in an element specific and even site specific manner.
Giant Barkhausen effects with second order instability (abstract)
Authors: Savage H. T. , Chen D-X , Polo Christina Gomez , Vazquez M. , Wun-Fogle M.
Published: July 2002

Idealized giant Barkhausen (BG) jumps in amorphous wires result in a simple rectangular hysteresis loop. There is a range of domain wall depinning fields so that if the depinning field for magnetization in the central section of the wire is high enough, magnetization reversal takes place by domains becoming depinned at the end of the wire. This leads to the rectangular hysteresis loop characteristic of GB. We have observed a different kind of Barkhausen effect in Fe-B-Si amorphous wires under a variety of conditions. The magnetization starts linearly with field, then displays a roughly quadratic field dependence, saturating at lower fields than that given by the low field linear extrapolation. In the quadratic region the plot of log(Hs−H) vs log(Ms−M), where Hs is the apparent saturation magnetization taking place at field Hs, is linear with a slope of 1/2. This is typical of simple second order instability processes. Thus, instead of a first order giant Barkhausen jump we have observed a second order Barkhausen process that is continuous. The only jump that occurs is in demagnetizing. This is caused by hysteresis. We believe this is the first time that this second order process has been described. From the details of the measurement we believe that the magnetization takes place by a longitudinal wall moving radially rather than a radial wall moving longitudinally as in the GB. The circumstances under which this takes place are in well-annealed wires magnetized with an imposed twist and longitudinal stress. In as-cast wires we imposed a curvature by bending along the longitudinal axis. The solenoid of the exciting field is bent the same way. The magnetization in wires with the imposed curvature displays a GB behavior for high and low curvatures. In an intermediate regime of curvature a behavior midway between GB and our second order effect is displayed. The low field magnetization looks like the second order effect but has a small jump to apparent saturation in the quadratic region described above. In summary we have found a range of depinning fields that lead to different magnetization processes when the depinning field values are less than that for central magnetization.
Self-consistent model of a double-barrier resonant tunneling diode: Dependence of intrinsic bistability on structural parameters
Authors: Hu Yuming , Stapleton Shawn
Published: July 2002

A self-consistent model of the double-barrier resonant tunneling diode (RTD) that is based on the damped Fabry–Perot model and the hot-electron distribution approximation, is presented. Dependence of intrinsic bistability on structural parameters in the RTD has been investigated both analytically and numerically. It was found that intrinsic bistability increases while the extrinsic bistability that is due to the load line effect decreases with the barrier width. This difference may be used as a criterion to distinguish intrinsic bistability from the extrinsic bistability caused by the load line effect. It was found that intrinsic bistability is likely to be observed in a RTD, at low temperatures, with a thick barrier and low doping at the collector.
Momentum gaps and laser stability
Authors: Brown T. G. , Olofsson L.
Published: July 2002

The impact of a momentum gap on the stability of a semiconductor laser is examined. We identify a condition which, when satisfied, suppresses the first order perturbation on both the intensity and the phase of such a laser. The same condition also provides a uniform intensity envelope in the laser cavity.
Efficient ablation of an organic polymer by a laser driven shock wave
Authors: Fletcher T. Rick
Published: July 2002

A new method for the efficient removal of an organic polymer is described. Fragments on the order of 1 mm diam are produced when a 0.02 J, 5 ns pulse of 532 nm light is focused into a red organic polymer that is coated on the front side of a transparent window. Quantities as large as 200 μg and craters as deep as 200 μm have been ablated in a single laser pulse. The large mass particles resulting from the ablation generate high transient pressures on the order of 100 kbar.
Photothermal rate-window spectrometry for noncontact bulk lifetime measurements in semiconductors
Authors: Chen Z. H. , Bleiss R. , Mandelis A. , Buczkowski A. , Shimura F.
Published: July 2002

A new noncontact technique for the determination of excess carrier lifetimes in semiconductors is presented. The technique employs a square laser pulse (hν≥Eg) and measures the infrared photothermal radiometric response of the sample. By applying the photothermal rate-window concept, the excess photoexcited carrier bulk lifetime was measured with optimal signal-to-noise (S/N) ratio and simple, unambiguous interpretation from the maximum position of the rate-window signal. The technique has been applied to Au-, Fe-, and Cr-doped Czochralski silicon crystals. The experimental results from boxcar and lock-in rate-window methods were found to agree very well. The results are further mostly in agreement with those from the noncontact laser/microwave detection method.
Photoconductivity transients and photosensitization phenomena in semi-insulating GaAs
Authors: Šantić B. , Desnica U. V. , Radić N. , Desnica D. , Pavlović M.
Published: July 2002

A model is proposed for the photoconductivity transients and the photosensitization in semi-insulating GaAs induced by low-intensity light at low temperatures. During photoconductivity transients, the lifetime of free carriers is shown to be a time-dependent quantity, determined mainly by the trapping processes, recombination being negligible. Photosensitization and thermal restoration are explained by the filling and emptying of deep traps. Contrary to some other explanations, in this model it is not necessary to utilize the metastability of EL2 or other defects. The model is verified experimentally by an original use of the thermally stimulated current method. It is also possible to determine values of capture cross sections for dominant traps.