bluuebunny/crossref_metadata_2025 huggingface dataset:
Antiferromagnetic behavior in S=2 layered RbMnF4
Authors: Morón M. C. , Palacio F. , Rodríguez-Carvajal J.
Published: July 2002

The magnetic properties of layered-perovskite RbMnF4 have been investigated by neutron powder diffraction. This compound exhibits a magnetic ordering below Tc=3.7±0.2 K. The resolution of the magnetic structure at 1.5 K indicates that RbMnF4 orders as a collinear antiferromagnet. The propagation vector is k=0 and the magnetic moment of each sublattice has a value of 2.97(3) μB. The strong reduction of the observed magnetic moment should be attributed to a particularly high two-dimensional character of this compound.
A new guideline for the improvement of soft magnetic properties of crystalline films deposited on tilted substrates
Authors: Iwasaki Y. , Hayashi K. , Itabashi M.
Published: July 2002

The coercive forces and anisotropy fields of Fe-Ru-Ga-Si films deposited on 30° tilted substrates were measured for varying chemical compositions and crystal orientations. The minimum coercivity and minimum in-plane magnetic anisotropy were observed at an iron-rich composition corresponding to a substantial magnitude of anisotropy constant: K1=6×103 (J/m3). From an orientation analysis of the crystal aggregate, this optimization with the large K1 was understood to be a result of the cancellation of the shape anisotropy by the crystalline anisotropy.
Magnetic aftereffect in nanocrystalline Fe88Zr7B4Cu1 alloy
Authors: Kim K. Y. , Noh T. H. , Lee Y. H. , Kang I. K. , Kang T.
Published: July 2002

The magnetic relaxation phenomenon was investigated by using the pulse method for an Fe88Zr7B4Cu1 alloy with nanoscale grain structure, which was prepared by melt quenching and a subsequent annealing. In order to know the relationship between the microstructure and the magnitude of magnetic aftereffect in the nanocrystalline state, annealing temperature dependence of volume fractions of crystalline bcc-Fe and residual amorphous phase was also measured. It was found that disaccommodation occurred in this nanocrystalline alloy, and the intensity of disaccommodation decreased as the fraction of the crystalline bcc-Fe phase increased. Moreover, the jumping field, defined by the magnetic field, at which initial magnetization curve rises abruptly decreased as the fraction of residual amorphous phase was reduced. From the observed results, the magnetic aftereffect was considered to result from the residual amorphous phase.
Hydrogen decrepitated anisotropic Nd-Fe-B powder: Hydrogen desorption and magnetic properties
Authors: Meisner G. P. , Panchanathan V.
Published: July 2002

The hydrogen decrepitation (HD) process easily pulverizes rapidly solidified, hot-deformed Nd-Fe-B (MQ3) magnets but degrades the magnetic properties. Recovery of high-performance magnetism in HD MQ3 powder has always required hydrogen desorption at temperatures above 600 °C. It is reported that desorption at the relatively low temperature of 240 °C restores magnetic coercivity Hci to above 10 kOe despite residual hydrogen in the material. Desorption between 240 and 580 °C significantly degrades Hci but not the intrinsic magnetic anisotropy. Hydrogen is reabsorbed when the desorbed powder is cooled from above 600 °C to room temperature in the presence of small amounts of residual hydrogen gas. This hydrogen reabsorption strongly degrades the coercivity but does not affect the intrinsic magnetic anisotropy.
Curie paramagnetism of chromium ultrafine particles
Authors: Furubayashi Takao , Nakatani Isao
Published: July 2002

Magnetic properties of ultrafine particles of chromium, 2 nm in diameter, were investigated by magnetization measurements from 4.2 K to room temperature. Observation of anomalous Curie paramagnetism with μeff=1.8 μB is reported and its possible origin is discussed.
Magnetic neutron scattering in single crystal Bi2CuO4
Authors: Roessli B. , Fischer P. , Furrer A. , Petrakovskii G. , Sablina K. , Valkov V. , Fedoseev B.
Published: July 2002

The spin-wave excitations and the critical behavior of the sublattice magnetization in Bi2CuO4 have been investigated by means of neutron scattering. The three-dimensional character of the antiferromagnetic ordering with propagation vector K=(1,0,0) below TN=45.83(8) K is reflected by the value of the critical exponent β=0.352(3). The spin-wave excitations have been measured along the symmetry directions [100] and [001]. The spin-wave dispersion is found to split into two branches with energy gaps Δ1=2.1 meV and Δ2=3.4 meV at the zone center. The data analysis is based on an S=1/2 spin Hamiltonian of tetragonal symmetry with anisotropic bilinear exchange including a four-site exchange interaction term.
Magnetization reversal in Co/Cr multilayer films
Authors: Wang D. , Kirby R. D. , Robertson B. W. , Sellmyer D. J.
Published: July 2002

Magnetization reversal of Co/Cr multilayer films is studied by utilizing initial magnetization curves, minor loops, isothermal and dc demagnetization remanence curves, and temperature dependence of the loops. Electron microscope observations reveal that the films consist of columnar grains of size from 400 to 800 Å. It is suggested that the magnetization reversal is dominated by rotation of single-domain grains. Numerical calculations are performed based on the Stoner–Wohlfarth coherent rotation model for noninteracting, single-domain particles of uniaxial anisotropy. The calculated results agree qualitatively with the experimental results. To investigate the quantitative differences, magnetic interactions among the grains are measured in terms of deviations from the linear Hankel plot predicted by Wohlfarth.
Magnetic properties of BaFe12−2xCoxSnxO19 prepared by coprecipitation annealing
Authors: Kuo P. C. , Yao Y. D. , Tzang W. I.
Published: July 2002

The chemical coprecipitation annealing method is shown to be suitable for preparing BaFe12−2xCoxSnxO19 powders with a hexagonal fine platelet structure and a narrow particle size distribution. The proper annealing temperatures are between 800 and 900 °C. The saturation magnetization decreases slowly with increasing x; however, the coercivity decreases rapidly with increasing x. When x=0.8, the coercivity is reduced to below 1000 Oe, while the value of the saturation magnetization is near 52 emu/g. The temperature coefficient of coercivity decreases roughly from 2.6 to −1.0 Oe/°C, when x increases from 0.0 to 1.2.
Photo-magneto-acoustic effect in hematite (abstract)
Authors: Ozhogin V. , Kiselev A. , Moshkin V. , Preobrazhenskii V. , Shumilov V.
Published: July 2002

Single crystals of antiferromagnets α-Fe2O3 and FeBO3 having the ‘‘easy plane’’-type anisotropy and the high Neel temperature are convenient model objects for studying various magneto-acousto-optical phenomena, see Review.1 A light intensity dependent change ΔF of the eigenfrequency of magnetoelastic ‘‘contour shift’’ mode has been observed for a hematite (α-Fe2O3) lamellar resonator whose surface was irradiated with a He–Ne laser beam with the wavelength 0.63 μm and the power 7 mW. The pulse length varied within 10–100 ms. The light was focused on the resonator surface with a spot diameter 0.3 mm. The resonator had a shape of a disk (6.0 mm diameter and 0.35 mm thickness) cut in the basis plane of a crystal. Magnetoelastic oscillations of the contour shift at frequency F=0.5 MHz were excited and detected using the induction technique. The photoinduced shift of the resonance frequency is of a magnetic nature which is suggested by a strong dependence of ΔF on the in-plane magnetizing field; ΔF=300 Hz at H=150 Oe. Time τ≊15 ms of the sample response to the light impulse is close to the relaxation time of magnetoelastic oscillations (the Q factor of the resonator Q≊10−4 for H=0.3 kOe). The presented experimental data show that this photo-magneto-acoustic effect in hematite which is not transparent for the visible light seems to result from the action of light on the magnetic subsystem of a crystal in the undersurface layer of a sample. The mechanism of such action may be ascribed to local changes of the in-plane anisotropy under lighting, these changes being similar to those discussed in Ref. 2 as applied to transparent FeBO3.
Improved cross-talk performance from multilayer longitudinal media using a Hall head
Authors: Mallary M. , Batra S. , Liu S. , Rauch G. C. , Ng P. K. , Neely G.
Published: July 2002

A new concept for improving the cross-talk performance of flux sense heads is proposed and is experimentally verified. The concept makes use of a new longitudinal media and a shielded flux sense Hall head. The new longitudinal media uses a soft magnetic layer separated by a nonmagnetic layer underneath the hard magnetic layer. The Hall head consists of a three pole structure. The outer two magnetic poles are the write poles which also act as a shield for the center pole during read. For a carefully selected thickness of the soft magnetic layer and the nonmagnetic layer, a significant reduction in fringing at low frequencies is observed. Using a 3.9-μm-wide Hall head on new longitudinal media, the fringe width at 0.5F[4F of 30 K flux reversal per in. (KFRI)] is reduced by 51% compared to the fringe width on a conventional media. This concept entails a loss of on-track amplitude. The loss of amplitude is significantly less at high frequencies compared to low frequencies, thereby improving the head resolution.
Micromagnetic study of dynamic domain instability in magnetoresistive sensors (abstract)
Authors: Zhu Jian-Gang , Guo Yi-min
Published: July 2002

Dynamic domain instability in magnetoresistive sensors produces noise in the output voltage pulses and forces complex head designs to suppress the irregular domain activities. In this paper, the micromagnetic modeling study of dynamic domain instability is presented. A Permalloy magnetoresistive sensor 10 μm in width, 2.5 μm in height, and 200 Å in thickness is modeled by a two dimensional array containing 512×128 cubic elements with identical intrinsic magnetic properties of Permalloy material. The Landau–Lifshitz equation is utilized for calculating the dynamic magnetic domain configurations as well as structures within domain walls. The study focuses on the transient processes that generate complex multidomain structures and the irregular dynamic domain activities. Figure 1 shows a simulated static ‘‘single’’ domain configuration, obtained by initially aligning the magnetization along the long axis of the Permalloy stripe. Circular edge domains are formed with patterns well agree with experimental observations. A high frequency (200 MHz) sinusoidal external field (amplitude 40 Oe) is then applied vertically with 10° deviation. Figure 2 shows the transient domain configuration at the time t=0.6T (T=period). The center portion of the stripe have become multidomain with irregular domain configuration and 180° Neel walls containing irregularly distributed Bloch lines and crossties. Note the irregularity of the domain configuration even though the material is homogeneous. The magnetization within the Neel walls alternates its direction along the walls. Detailed analysis of the onset of the multidomain state will be presented. Investigation of dynamic domain behavior under various bias schemes will be reported.
The Curie–Weiss behavior of birefringence in ferrofluids
Authors: Yusuf N. A. , Abu-Safia H. , Abu-Aljarayesh I.
Published: July 2002

Measurements of birefringence in an Fe3O4 magnetic fluid with isopar-M as a liquid carrier were undertaken as a function of temperature for 100<T<320 K and in magnetic fields up to 2 kOe. The results show that the birefringence increases with temperature exhibiting a maximum at a certain temperature Tm. The position of Tm decreases with increasing field intensity. For T≳Tm the birefringence follows a Curie–Weiss-type behavior. The ordering temperature changes not only in magnitude but also in sign with increasing field intensity, being negative for H≳500 Oe.
Optimum geometrical structure of highly magnetostrictive multilayer films for horizontally polarized high frequency magneto-surface-acoustic-wave propagation
Authors: Inoue Mitsuteru , Fujita Naoyuki , Fujii Toshitaka
Published: July 2002

Optimum geometrical structure of highly magnetostrictive multilayer films for the propagation of horizontally polarized magneto-surface-acoustic-wave (Love-type MSAW) was determined theoretically. The multilayered medium is composed of highly magnetostrictive alloy films and insulating films which are stacked alternately on a glass substrate. The analysis revealed that outstanding magnetoacoustic quantities comparable to those of an ideal nonconductive medium can be realized with the conductive multilayer films whose geometrical parameters meet the following conditions: DM/DL≥1, DM/DI≥10, and N∼20, where N is the number of insulating layers having the whole thickness of DI, and DM is the whole thickness of magnetic layers. DL is a limiting thickness of the magnetic film with a single-layer structure, above which the magnetoacoustic quantities are considerably deteriorated due to the micro-eddy current losses.
Comments on the thermoelectric properties of pressure-sintered Si0.8Ge0.2 thermoelectric alloys
Authors: Rowe D. M. , Fu L. W. , Williams S. G. K.
Published: July 2002

The recent results of C. B. Vining, W. Laskow, J. O. Hanson, R. R. Van der Beck, and P. D. Gorsuch [J. Appl. Phys. 69, 4333 (1991)] regarding the effect of grain size on the thermoelectric figure of merit of heavily doped p-type silicon germanium alloys are compared to earlier results on similar materials. The data confirm that the room-temperature figure-of-merit is significantly increased in materials with a small grain size.
Photothermal luminescence spectroscopy of GaAs/AlxGa1−xAs quantum wells
Authors: Chen Y. F. , Shen J. L. , Lin L. Y. , Huang Y. S.
Published: July 2002

A novel luminescence technique named photothermal luminescence has been developed. For the photothermal luminescence spectroscopy, the emission signal is caused by electronic transitions via the absorption of photons, followed by thermal excitation via electron-phonon interactions, and is monitored as a function of the excitation photon energy, in which the excitation photon energy is less than that of the emission signal. This new technique has been applied to the study of electronic transitions in GaAs/AlxGa1−xAs quantum wells. In addition to the observation of the n=1 electron-heavy-hole 1s and 2s exciton recombinations, a previously unreported fine structure in the n=1 electron-heavy-hole 1s exciton spectrum has also been observed. By measuring the temperature dependence of the spectra on different quantum wells, we suggest that the fine structure is due to the formation of the standing waves of acoustic vibrations in GaAs/AlxGa1−xAs quantum wells. We emphasize that due to the underlying mechanism of the technique, the photothermal luminescence provides a powerful tool to investigate the processes of electron-phonon interactions.
Description of the exponent in the Bodner–Partom model based on deformation kinetics theory
Authors: Krausz A. S. , Krausz K.
Published: July 2002

The Bodner–Partom model defines the constitutive law of plastic deformation for a large range of materials with a system that is economical for computer processing. Because the model was designed to combine physically defined concepts with empirical descriptions it is of interest to extend its validity by expressing the empirical exponent, which represents temperature dependence, in terms of macroscopic and material characteristic quantities derived rigorously from basic physical principles in the context of deformation kinetics theory. The expression derived provides guidance for the development of improved material properties, testing methods, manufacturing process and product design, and maintenance procedures.
p-type CdTe epilayers grown by hot-wall-beam epitaxy
Authors: Pauli H. , Hingerl K. , Abramof E. , Sitter H. , Zajicek H. , Lischka K.
Published: July 2002

Highly conducting p-type CdTe films were grown by photoassisted hot-wall-beam epitaxy using Li3N as a dopant source. Doping levels can be controlled from p=4×1016 to 2×1018 cm−3, as determined by Van der Pauw measurements. The hall mobility ranges from 30 to 100 cm2/V s, depending on the hole concentration. Photoassisted growth enhances the incorporation of dopants by one order of magnitude and decreases the growth rate.
Submillimeter optical properties of hexagonal boron nitride
Authors: Gatesman A. J. , Giles R. H. , Waldman J.
Published: July 2002

The submillimeter optical properties of hot-pressed polycrystalline boron nitride with a predominantly hexagonal crystal structure were studied at room temperature from ∼20 to 120 cm−1 (λ=500–84 μm) with a Fourier-transform spectrometer. Several grades were studied and probed both parallel and perpendicular to the material’s optic axis. The material was found to behave as a negatively uniaxial birefringent crystal. The birefringence, defined as the difference between the real part of the extraordinary and ordinary indices of refraction (ne–no), was quite large in one case with a value of −0.152. The material’s absorption properties were also studied. For certain grades, a modest dichroism was observed. The low absorption (α<1 cm−1) for grade A at frequencies below 38 cm−1 suggests the possibility for millimeter/submillimeter wave applications. Results are compared with data obtained by other researchers on related materials.
Effects of reducing and oxidizing thermal treatments on the transport critical current density in epitaxial Bi2Sr3−xCaxCu2O8+y films
Authors: Balestrino G. , Marinelli M. , Milani E. , Paoletti A. , Paroli P.
Published: July 2002

We have studied the effects of oxidizing and reducing thermal treatments on the critical current density in epitaxial Bi2Sr3−xCaxCu2O8+y films. Reducing treatments, while increasing the transition temperature up to 94 K, reduces the critical current density relative to the as-grown value in the whole range of reduced temperatures. On the contrary, oxidizing treatments, while decreasing the transition temperature, increases the critical current density. Such effects are shown to be fully reversible under a final air annealing. Such effects are attributed to a decrease of the Bi2Sr3−xCaxCu2O8+y anisotropy which occurs in connection with the increase of the oxygen content in the compound.
Conduction mechanisms in erbium silicide Schottky diodes
Authors: Unewisse M. H. , Storey J. W. V.
Published: July 2002

Conduction mechanisms in erbium silicide Schottky diodes on n-type silicon have been studied over a temperature range of 25 to 160 K. Thermionic emission is the dominant carrier transport mechanism above 70 K. Below this temperature, deviations are apparent in the zero-bias barrier height and ideality factor. However, the flat-band barrier height is shown to remain constant over the entire temperature range. The Fermi level is demonstrated to be pinned to the conduction band. A new quantity, the flat-band saturation current (Isf) is defined. Plots of n ln(Isf/T2) vs 1/T are found to give an excellent fit to the data over 28 orders of magnitude. From these plots the flat-band barrier height and the modified Richardson constant are obtained directly. This technique provides a completely self-consistent and more reliable way of obtaining these parameters than do previous methods. For low temperatures and low forward bias, recombination via tunneling through surface states becomes the dominant conduction mechanism.
Monoenergetic positron beam study of Si-doped GaAs epilayers grown by low-pressure metalorganic chemical vapor deposition using tertiarybutylarsine
Authors: Chichibu S. , Iwai A. , Nakahara Y. , Matsumoto S. , Higuchi H. , Wei L. , Tanigawa S.
Published: July 2002

Si-doped GaAs epilayers grown by low-pressure metalorganic chemical vapor deposition (MOCVD) using tertiarybutylarsine (tBAs) were investigated using a slow positron beam. The concentration of Ga vacancies, VGa, generated in GaAs epilayers was increased drastically by heavy Si doping of more than 1019 cm−3, where the deactivation of Si occurred. This result suggests that the deactivation of Si in GaAs is mainly caused by a VGa-related defect, such as a VGa-SiGa complex. The VGa concentration in the samples grown using tBAs was found to be almost the same as that grown using arsine (AsH3). On the other hand, the VGa concentration in MOCVD-grown Si-doped GaAs is lower than that in molecular-beam-epitaxy-grown material for the same Si concentrations. The generation mechanisms of VGa were found to be greatly dependent on the growth and/or doping methods, in addition to the Si doping concentration.
Interparticle force measurements on ferromagnetic steel balls
Authors: Tan Chak , Jones T. B.
Published: July 2002

Interparticle force measurements have been performed upon various configurations of linear chains and planar layers of chrome steel balls immersed in a steady uniform applied magnetic field. These force measurements, made for particles in contact and at various separations, reveal that the attractive forces are enhanced in the case of particle chains parallel to the applied magnetic field and slightly decreased in the case of perpendicular layers. The magnetic saturation exhibited by the steel balls has only a modest effect on the force. Experimental data obtained for two-particle chains correlate reasonably well to force calculations based upon a linear multipolar expansion using a relative particle permeability value of μr=100. It is also found that data obtained with longer linear chains as well as with planar particle arrays do not deviate drastically from the predictions of the two-sphere problem.
Temperature sensitivity of strained-layer InGaAs/Ga(In)As(P)/GaInP separate-confinement-heterostructure quantum well lasers (λ∼980 nm)
Authors: Zhang G. , Ovtchinnikov A.
Published: July 2002

We have studied the temperature sensitivity of strained-layer InGaAs/Ga(In)As(P)/GaInP separate-confinement-heterostructure quantum well lasers emitting at λ∼980 nm. We observed that the lasers with GaAs confinement layers exhibited higher temperature sensitivity than those with GaInAsP confinement layers. It was also found that using the GaAs confinement layers, the lasing characteristics dramatically degraded at elevated temperatures, in particular, as the quantum well width was reduced. These phenomena could be attributed to a poor carrier confinement present in the case of InGaAs/GaAs quantum wells.
Enhanced drift mobility under in-plane conduction in doping-modulated amorphous silicon multilayers
Authors: Oheda Hidetoshi
Published: July 2002

The photoconductivity under in-plane conduction in nipi multilayers takes an enhanced value at low temperatures as compared with an n-type single layer. By evaluating temperature dependences of the drift mobility and the lifetime separately, it becomes clear that the enhanced photoconductivity has a close correlation with unusual behavior of the drift mobility in the nipi multilayers. The drift mobility is limited by hopping conduction at low temperatures and its magnitude depends on the undoped sublayer thickness; the maximum value at 100 K is higher than that for the n-type single layer by about three orders of magnitude. Such an enhanced drift mobility in the nipi multilayers is caused by an unusual excess-carrier population at tail states in the n-type sublayers, which is different from that in the n-type single layer.
The effect of aluminum mole fraction on the surface microroughness and morphology of AlxGa1−xAs grown at low temperature by molecular beam epitaxy
Authors: Fatt Yoon Soon
Published: July 2002

AlGaAs layers of varying mole fraction x grown by molecular beam epitaxy are analyzed using scanning force microscopy. This relatively new technique allows high resolution imaging and analysis of surface morphology and roughness of conductors as well as nonconductors. The surface roughness and morphology of AlGaAs grown at low temperature (580 °C) are analyzed as its mole fraction x is changed from 0 to 0.8. Over an area of 400 μm2, the average roughness increases by as much as 26 Å as the AlGaAs surface morphology changes to be highly irregular with increasingly finer grains at higher mole fractions. The results show that high arsenic partial pressure and low substrate temperature inhibit the surface mobility of the aluminum atoms resulting in degradation in surface roughness.
Room temperature spontaneous emission in five-micron-long Fabry–Pérot vertical cavities
Authors: Lei C. , Pinzone C. J. , Huang Z. , Huffaker D. L. , Deppe D. G. , Dupuis R. D.
Published: July 2002

Five-micron long AlGaAs/GaAs Fabry–Pérot vertical cavities with thin GaAs active regions of two different thicknesses are analyzed both theoretically and experimentally in terms of their optical transmission characteristics, spontaneous radiation patterns, and spontaneous spectral emission characteristics. The effects on spontaneous emission of the precise placement of the GaAs active region and also the thickness of this region, as compared to the emitted wavelength, are demonstrated. The measured results are compared with theoretical calculations which are based on the first-order perturbation field theory presented in an earlier publication. Good agreement is found in the comparison between the theoretical predictions and the experimental results, and indicates the controllable spontaneous emission in these long vertical cavities.
Wave velocity and diffusion constant in strongly scattering media
Authors: Ferrari L. A. , Gangji Al-Karim
Published: July 2002

The transport mean free path in a strongly scattering medium consisting of polystyrene spheres (1.27 cm diameter, volume fraction f≊0.55) in air, is measured using a combination of scale-dependent transmission and reflection experiments at microwave frequencies (≊20 GHz). The diffusion coefficient is obtained from a pulse propagation experiment and is approximately 50% of the diffusion coefficient obtained by means of an analysis of the intensity-intensity correlation function. These data yield a transport velocity that agrees with the velocity calculated from a formula proposed by van Albada et al.
Thin gold wires as reference for thermoelectric power measurements of small samples from 1.3 K to 350 K
Authors: Wendling N. , Chaussy J. , Mazuer J.
Published: July 2002

We measured the absolute thermoelectric power (TEP) of a thin gold wire between 1.3 and 350 K. Low-temperature measurements of the electrical resistivity of the wire have been performed and used to determine the Fe concentration and to discuss the low-temperature behavior of the TEP. Comparison with previous measurements on pure gold is given. With this calibration, these thin gold wires are quite suitable as reference for TEP measurements on small samples on which they can be easily bonded with thermosonic or ultrasonic bonders. More specifically these results are very convenient for thermoelectric measurements on high critical temperature superconductors, by using the relation S(T)=5×10−3T+38T−1 (μV K−1) as a reference for the TEP of the gold leads between 80 and 350 K.
Origin of current instabilities in GaAs/AlxGa1−xAs heterostructures: Avalanche ionization in the AlxGa1−xAs layer
Authors: Zwaal E. A. E. , Hendriks P. , Vermeulen M. J. M. , van Helmond P. T. J. , Haverkort J. E. M. , Wolter J. H.
Published: July 2002

Pulsed current-voltage measurements on modulation-doped GaAs/AlxGa1−xAs heterostructures are presented at electric fields up to 2 kV/cm. At fields between 0.5 and 2.0 kV/cm we observe up to three well-defined avalanche type current jumps as a function of time. These current jumps show hysteresis effects as a function of the electric field. At even higher electric fields the current becomes irregular and we observe chaotic behavior. To explain the current instabilities we assume that at high electric fields electrons are injected into the AlxGa1−xAs layer parallel to the two-dimensional electron gas. The injected electrons subsequently cause avalanche ionization of occupied DX centers in the AlxGa1−xAs layer. Due to this process, a current filament is created with an exceptionally high mobility which is about 2×104 cm2/V s at 10 K.
Effect of Cr addition on magnetization of Cu-Co alloy
Authors: Kondo Shin-ichiro , Yamamoto Takeshi , Yamamoto Kazuhiro , Morimura Takao , Hasaka Masayuki
Published: July 2002

Magnetization of a Cu-2 at. % Co-0.5 at. % Cr alloy was measured in the temperature range from 77 to 296 K using vibrating sample magnetometer to investigate the effect of Cr addition on magnetization of the Cu-Co alloy. Aging temperature and the range of aging time were 723 K and 1–50 h. The Cu-Co-Cr alloy, as well as the Cu-Co alloy, is found to be superparamagnetic; this indicates the presence of magnetic clusters in the Cu-Co-Cr alloy. From the magnetization versus H/T (H: applied field, T: temperature) plots, magnetization of the Cu-Co alloy can be expressed as a function of H/T while that of the Cu-Co-Cr alloy cannot. Such analytical results suggest that the magnetic moment of the cluster existing in the Cu-Co-Cr alloy is temperature dependent. The temperature dependence of the magnetic moment is attributable to the presence of Cr atoms in the magnetic cluster. From the calculation of nonlinear least-squares fitting in the magnetization, the magnetic moments of the cluster in the Cu-Co-Cr alloy aged for 1 and 5 h were estimated to be about 590 and 4800μB at room temperature.
Comparison of glancing angle extended x-ray absorption fine structure obtained from fluorescence and reflectivity measurements
Authors: Chen Huaiyu , Heald Steve M.
Published: July 2002

At glancing angles the extended x-ray absorption fine structure (EXAFS) can be determined from either the fluorescence or reflectivity signals. This article presents a critical comparison of the two detection channels as they apply to thin film interface problems. Both signals are distorted by anomalous dispersion effects, and our previous method for correcting the distortions in the fluorescence signal is shown to work equally well for the reflectivity signal. Using results from a pure gold film and a Cu-Al bilayer as examples, it is demonstrated that both signals can be corrected to obtain accurate EXAFS information. Fluorescence detection generally offers greater sensitivity, improved signal-to-noise and less distortion of the EXAFS, but the reflectivity signal may be easier to obtain for in situ measurements. A complication for fluorescence is the possibility of an additional contribution to the signal caused by x rays scattered into the substrate layer. While generally small, this contribution is difficult to remove from the measured signal, and may make the reflectivity the preferred signal for some experiments.
Damage and deuterium trapping in highly-oriented pyrolytic graphite
Authors: Siegele R. , Roth J. , Scherzer B. M. U. , Pennycook S. J.
Published: July 2002

In this work the buildup of damage due to deuterium implantation in highly-oriented pyrolytic graphite (HOPG) is investigated. HOPG was implanted with 10–30 keV D3+ at different target temperatures between room temperature and 773 K with fluences from 1014 to 1018 D/cm2. Subsequently, the damage due to the implantation and the retained deuterium were measured by Rutherford backscattering (RBS) in a channeling direction (RBSc) and by the D(3He, p)α nuclear reaction analysis (NRA), respectively. The damage of selected samples was additionally observed with transmission electron microscopy (TEM). The initial trapping efficiency is unity in the whole temperature and energy range. The maximum retention of the deuterium, however, depends on the temperature and implantation energy. The damage in HOPG measured with RBSc starts to saturate at 5×1015 D/cm2 (295 K) and 1.3×1017 D/cm2 (773 K). Both fluences are well below the fluence at which amorphization is observed in TEM.
Properties of diffraction gratings used as output couplers in a quasi-optical gyrotron
Authors: Tran M. Q. , Cao H. , Hogge J. Ph. , Kasparek W. , Tran T. M. , Paris P. J.
Published: July 2002

The design of a diffration grating used as an output coupler for a Fabry–Pérot resonator is presented. We then consider the problem of determining the distortion and the cross-polarization of the incident Gaussian beam. Different types of gratings and planes with straight or curvilinear grooves and elliptical, have been designed and built. The output pattern from a resonator using such a grating has been measured experimentally. We found that the elliptical grating generates the least distortion and cross-polarization. This result is attributed to the geometry of the grooves and is in excellent agreement with numerical calculations.
Effect of electromagnetic radiation on alternating current losses in high-T
c superconductors
Authors: Kumar Binod , Das Kamal K.
Published: July 2002

This communication reports a relatively simple technique to measure the alternating current (ac) loss in superconductors and also determines the effect of electromagnetic radiation on the measurement of ac losses. The interaction of the electromagnetic radiation with the superconductor was found to increase the ac loss. The enhancement of ac loss has been explained on the basis of generation of additional charge carriers and their interaction with the lattice.
Generation mechanisms of paramagnetic centers by gamma-ray irradiation at and near the Si/SiO2 interface
Authors: Awazu Koichi , Watanabe Kikuo , Kawazoe Hiroshi
Published: July 2002

P b (⋅Si≡Si3) and E′ (⋅Si≡O3) centers in the Si/SiO2 structure under gamma-ray radiation are studied with the electron-spin-resonance technique. The Si/SiO2 structures of (111), (110), and (100) planes are obtained using three different oxidation methods to control the concentration of the hydrogen-related impurities. It is first observed that the concentration of Pb centers is decreased with increasing radiation dosage in samples with lower concentrations of impurities; the concentration dramatically increases in the samples with a higher concentration of impurities. The concentration of E′ centers also increases with accumulated dosage. When the Si/SiO2 structure is fabricated under different oxidation temperatures, the saturated concentrations of Pb centers after irradiation depend upon the oxidation temperature. The concentration of E′ centers increases with oxidation temperature after irradiation. When the Si/SiO2 structure is fabricated under different cooling rates, concentrations of the Pb centers are saturated after irradiation. The concentration of E′ centers increases with increasing cooling rate.
Characterization of hydrogenated GaAs/AlGaAs multiple quantum well structures
Authors: Zavada J. M. , Voillot F. , Lauret N. , Wilson R. G. , Theys B.
Published: July 2002

We report on the photoluminescence properties and the hydrogen depth distributions of plasma treated GaAs/AlGaAs multiple quantum well (MQW) structures. Specimens grown by molecular beam epitaxy were exposed to a deuterium plasma under different temperature-time conditions. Photoluminescence measurements were made at 4.2 K, using low and high excitation powers, on the hydrogenated samples and on untreated partners. A decrease in the linewidth of the free exciton and an increase in the peak intensity were observed in specimens following plasma treatment. In general, each of the hydrogenated MQW specimens displayed an increase in luminescence efficiency and a diminution of impurity-related peaks after hydrogenation. Secondary ion mass spectrometry measurements yielded depth distributions for 2H and Al atoms. In samples having the best luminescence, the 2H was nearly constant throughout the MQW region, at about 1018 cm−3.
Development of coercivity in Sm2Fe17(N,C)x magnets by mechanical alloying, solid–gas reaction, and pressure-assisted zinc bonding
Authors: Kuhrt C. , Schnitzke K. , Schultz L.
Published: July 2002

Highly coercive isotropic Sm2Fe17Nx and Sm2Fe17Cy powders were prepared by a mechanical alloying of elemental Fe and Sm, a heat treatment in vacuum at 700–800 °C to form microcrystalline Sm2Fe17, and, finally, a solid–gas reaction in N2 and C2H2 atmosphere, respectively, at 450–500 °C to introduce nitrogen and carbon, respectively, on interstitial sites. The solid–gas reaction parameters, such as temperature and time as well as, in the case of carbides, the C2H2 partial pressure, are very critical with respect to dynamical decomposition of the Sm2Fe17 phase during reaction progress. Optimum parameters result in a coercivity of up to 25.6 kA/cm (32.2 kOe) and 18.5 kA/cm (23.2 kOe) for the nitride and carbide, respectively. Zinc bonding of such powders assisted by a uniaxial pressure of 200–300 MPa at a temperature of 425–450 °C leads to further substantial coercivity enhancement. The room-temperature coercivity of Zn-bonded Sm2Fe17Nx magnets reached 34.7 kA/cm (43.6 kOe), which represents an increase by about 50% referred to the starting powder value. The assistance of Zn bonding by pressure is required for the coercivity enhancement in such microcrystalline material.
Crystallographic and magnetic properties of UPdSn
Authors: Johnson S. W. , Robinson R. A. , Nakotte H. , Brück E. , de Boer F. R. , Larson Allen C.
Published: July 2002

A single crystal of the intermetallic compound UPdSn has been studied by means of neutron diffraction using the white-beam Laue diffractometer (single-crystal diffractometer) at the Los Alamos spallation neutron source. Data were taken in the paramagnetic phase at 55 K and just below the Néel point at 37 K. The crystallographic data refine well within the space group P63mc and confirm the results obtained previously on powders, namely that the Pd and Sn atoms are ordered. At low temperature, UPdSn undergoes two phase transitions at 40 and 25 K to complicated noncollinear antiferromagnetic structures. All three pairs of magnetic domains are observed, in roughly equal populations, in the intermediate-temperature orthorhombic magnetic structure (phase I). A uranium moment of 0.863μB and canting angles φ=56.51° and θ=21.42° were obtained, in reasonable agreement with previous powder neutron data for this temperature. The observed form factor yields an orbital-to-spin moment ratio −μL/μS=2.6 which is characteristic of the trivalent f3 configuration.
A new type of spin ordering in a random magnet with competing anisotropies
Authors: Katsumata K. , Shapiro S. M. , Matsuda M. , Shirane G. , Tuchendler J.
Published: July 2002

The result of an elastic neutron-scattering study of the phase transition in the random magnet with competing anisotropies Fe0.75Co0.25Br2 is reported. It was found that the ordering of the spin component parallel to the crystalline c axis at the transition from the paramagnetic to antiferromagnetic phase induces an ordering of the spin component perpendicular to it. Thus, there is no boundary separating the higher-temperature antiferromagnetic and lower-temperature mixed-ordering phase. This type of spin ordering is interpreted as arising from a coupling between the orthogonal spin components.
Spin polarization in paramagnetic layers on magnetic substrates
Authors: Shi Zhu-Pei , Levy Peter M. , Fry John L.
Published: July 2002

The spin polarization of conduction electrons in a paramagnetic layer due to an adjacent magnetic layer has been derived based on the mixing interaction between the local and conduction electrons. Specifically, based on the band structure of paramagnetic chromium, we have calculated the spin polarization in a Cr layer on Fe(100) substrates. The results are in reasonably good agreement with recent experimental data.
Thin-film head materials for quasi-contact head/disk interface
Authors: Tsujimoto Shinji , Wada Toshiaki , Nakano Osamu , Sasaki Toyoshige , Furukawa Mitsuhiko
Published: July 2002

The mechanical, physical, and tribological properties of ZrO2-Y2O3-Ta2O5 have been investigated in this article. Under noncontact conditions, the frictional coefficient of the composite was found to be less than 50% of the Al2O3-TiC material. And, under contact conditions, the damage against the media was less than 0.2 dB, comparable to the ferrite material. The slicing resistance of this material rapidly changed from 1 to 3 mol % Ta2O5 because of its fracture mode. It was found that addition of Ta2O5 stabilizes the tetragonal phase which was found to be unchanged by aging at 200 °C.
Magnetic properties of highly stable carbides (Er0.9Y0.2)2Fe17Cx (x=0–2.8)
Authors: Cao Lei , Kong Lin-shu , Shen Bao-gen
Published: July 2002

Carbides with nominal compositions of (Er0.8Y0.2)2Fe17Cx (x=0, 0.5, 1.0, 1.5, 2.0, 2.5, 2.8) were prepared by arc-melting and melt-spinning method. The arc-melted and heat-treated samples with x=0, 0.5, 1.0 are single-phase, hexagonal Th2Ni17 structure. When x=1.5, the Th2Ni17 structure transforms to the Th2Zn17 structure, while for x≥1.5, only optimal melt-spun samples crystallize in the Th2Zn17 structure. The carbides with x≥1.5 are stable and maintain their rhombohedral structure after being heat treated at 1000 °C. The carbides with x=2.8 have a 6.5% volume expansion and a 345-K Curie temperature enhancement over those of the carbon-free compound. The magnetic moment per iron atom μFe remains 1.96 μB throughout the entire series. The spin reorientation temperature TSR increases, when carbon enters interstitially, from 71 K when x=1.0 to about 122 K when x=2.8.
Structural and magnetic properties of rare-earth iron nitride Ce2(Fe1−xCox)17Ny series
Authors: Xu Xie , Shaheen S. A.
Published: July 2002

The ternary nitrides Ce2(Fe1−xCox)17Ny have been studied to determine the effect of nitrogenation on their crystal structure and magnetic properties. X-ray diffraction results show that the nitrides retain the rhombohedral Th2Zn17 structure of their parent compounds, but with an expansion in the unit cell volume. For x≥0.6, no Th2Zn17-type nitride compounds are formed. The room temperature saturation magnetization of nitrides is greater than that of the parent compounds for low Co concentrations. The 3d sublattice anisotropy is modified upon nitrogenation; the transition from easy plane to easy axis shifts from x≊0.5 in Ce2(Fe1−xCox)17 to x≊0.15 in Ce2(Fe1−xCox)17Ny.
Magnetic properties of YCo4B-based compounds
Authors: Zhao Z. G. , Collocott S. J. , Dunlop J. B. , de Boer F. R. , Sun X. K. , Chuang Y. C.
Published: July 2002

The magnetic properties of YCo4B-based compounds have been investigated. The Curie temperature TC of YCo4B does not change upon substitution of 20% of the Y ions by Nd or 20% of the B ions by C. TC increases significantly upon substitution of Fe for Co. The Co moment in YCo4B0.8C0.2 is nearly equal to that in YCo4B.
Dynamic bubble array method of imaging magnetic inclusions in substrates (abstract)
Authors: Argyle Bernell , Fuselier Charles
Published: July 2002

Nonmagnetic ceramic substrates supporting magnetic devices occasionally contain magnetic inclusions or particles, often submicron in size. This report describes a convenient method for locating and characterizing such particles, especially those not visible in direct optical inspection because they lie just beneath the surface or are suboptical in size. We make use of their stray fields which can extend beyond the optical resolution limit (particles this small do not usually demagnetize). A recently reported scheme places a bubble-type magnetic garnet film in contact with the substrate.1 The film is saturated in-plane and its Kerr contrast imaged while the field is reduced. The particle’s stray field causes a visible deviation in an otherwise normal contrast pattern of the critical magnetic phase transformation. Large fields (H0≳Hk) and critical alignment of the field are required. We describe and demonstrate another method not requiring large fields at critical alignment. It relies on rf or pulsed fields to excite fast motions in arrays of bubble domains. The time average of these motions produces concentric rings of Kerr contrast centered on each particle whose stray fields influence the motion. This ‘‘bull’s-eye’’ pattern is useful as it is viewable with modest magnifications, thereby allowing larger fields of optical view. A bull’s-eye Faraday pattern was discovered and applied earlier2 to defect detection in garnet films. The fundamental origin of this pattern and the practical application to substrates for magnetic devices will be discussed.
Application of a vector Preisach model in a magnetic circuit
Authors: Bergqvist A. , Tiberg H. , Engdahl G.
Published: July 2002

Magnetic hysteresis effects have been included in a finite-element description of a magnetic circuit by using the classical vector Preisach model for the constitutive relation between H and B. The influence of an external electric circuit is taken into account by adding equations derived from Faraday’s law. Computational results are presented for a magnetic circuit used in a magnetostrictive device.
Evaluation of a magnetostrictive drive element model
Authors: Tiberg H. , Bergqvist A. , Engdahl G.
Published: July 2002

A voltage-fed magnetostrictive drive element is modeled. The model involves the Terfenol rod, a magnetizing coil, permanent magnets, a magnetic circuit, and predicts displacements and stresses under different load conditions. In this model the performance of the drive element is calculated using a finite difference method for the active material, Faraday’s and Ampere’s laws for the electric circuit, and a finite-element method for the magnetic circuit comprising permanent magnets. The resulting system of equations is solved using a dynamic simulation program. A comparison between the results of the model and experimental data from a commercial drive element is presented.
Magnetic and atomic order in dilute Pd and Pt based alloys
Authors: Parra R. E. , Gonzalez A. C.
Published: July 2002

Unpolarized magnetic neutron scattering data, from dilute PdFe, PtCo, PtFe, and PdMn alloys, have been analyzed using a magnetic environmental model and Monte Carlo simulation methods. The neutron scattering shows large forward peaks at low K values due to the magnetization clouds and to the presence of critical scattering. The effects of atomic short range order, when present, were also included in the analysis. The analysis allows the calculation of the neutron diffuse scattering and the determination of the critical temperatures of the ferromagnetism in the alloys.
Compaction of amorphous iron–boron powder (abstract)
Authors: Hendriksen P. V. , Mo/rup S. , Koch C. B. , Linderoth S. , Bo/dker F.
Published: July 2002

Large scale practical use of bulk amorphous alloys requires the capability of molding the material to a desired design, for instance by compaction of an amorphous powder. This is a difficult task because the sintering temperature is limited by the crystallization temperature of the alloy.1 Here we report on attempts to compact amorphous iron–boron particles prepared by chemical reduction of Fe(II) ions in aqueous solution by NaBH4 (Ref. 2). The particles prepared in this way are pyrophoric, but can be passivated. The small particle size (10–100 nm), characteristic of this preparation technique, should facilitate a compaction. The passivation layer, however, impedes a compaction. Isostatic pressing at 540 K at a pressure of 200 MPa clearly illustrated this; pellets pressed from passivated powder were much more brittle than pellets pressed from unpassivated powder. The density of the pellets was very low (≊25% of the density of bulk FeB). We have designed a die for uniaxial pressing in which the compaction can be performed without exposing the powder to air and have obtained densities larger than 60% of that of bulk FeB. We have reported studies of the dependence of density and structure on compaction pressure and compaction temperature.