bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the coaxial double z-pinch for the Al x
i - Mg i
x laser at 228 Å
Authors: Qi N. , Hammer D. A. , Apruzese J. P.
Published: July 2002

A magnetically embedded coaxial z-pinch configuration is proposed for the resonant photoexcitation laser at 228 Å in Mg ix pumped by the 48.338 Å line of Al xi. This laser configuration consists of an annular Al z-pinch plasma imploded (final electron density of about 1020 cm−3) onto an inner, Mg plasma (final electron density of about 1018 cm−3) on the axis. The Mg plasma is confined and stabilized by an axial magnetic Bz field, which is compressed to a final magnitude of about 130 T from an initially applied field of about 1 T by the Al implosion, thus serving to stabilize and physically separate the Al and Mg plasmas. Separate numerical simulations of the Al and Mg plasmas designed to guide exploratory experiments are reported. A 0D dynamic z-pinch model captures the Al implosion, including trapped Bz field and power losses due to line radiation. Initial conditions required to produce the intense Al xi pump line at 48.338 Å are derived assuming an 800 kA, 100 ns implosion current pulse. Assuming a pinch length of 2 cm, a pump power of 3.5 GW/eV is predicted for the 48.338 Å Al xi line. With the pump so defined, a separate, detailed atomic physics model is used to study the photoexcitation kinetics in the Mg ix lasant plasma. It is found that a gain of 1.0–1.5 cm−1 is obtainable on the 2s3d 1D2 - 2s4f 1F3 transition at 228 Å in Mg ix.
Observation by electroabsorption of strain-enhanced interface roughening in GaxIn1−xAs/Ga0.22In0.78As0.48P0.52 quantum wells prepared by gas-source molecular beam epitaxy
Authors: Simes R. J. , Starck C. , Weihofen R. , Weiser G.
Published: July 2002

This article discusses a low-temperature photoluminescence (PL) and electroabsorption (EA) study of GaxIn1−xAs/Ga0.22In0.78As0.48P0.52 single quantum well (QW) samples prepared by gas-source molecular beam epitaxy. The linewidth of PL emitted from these single QW samples increases monotonically with increasing QW strain. EA measurements on the same samples reveal a multipeaked response on the high energy side of the PL spectrum. The energy separation of the EA features corresponds to that expected for differences in QW thickness of one monolayer. The observed PL broadening results from PL emanating from different regions of the same well, differing in thickness, while the Stokes shift results from migration of excitons to wider well regions. Spectral features are lost at large strain which is attributed to strain-enhanced roughening of the QW surface during the crystal deposition.
The use of elastic recoil detection for stoichiometry determination of reactively evaporated TiN layers
Authors: Gagnon G. , Currie J. F. , Brebner J. L. , Gujrathi S. C. , Ouellet L.
Published: July 2002

Thin TiN layers have been successfully produced by reactive evaporation combined with rapid thermal annealing. Their stoichiometry as a function of depth has been measured by elastic recoil detection combined with time-of-flight. It is shown that this technique is the most appropriate for stoichiometry determination of TiN layers.
Interactions of wide band gap single crystals with 248 nm excimer laser radiation. III. The role of cleavage-induced defects in MgO
Authors: Dickinson J. T. , Jensen L. C. , Webb R. L. , Dawes M. L. , Langford S. C.
Published: July 2002

When single-crystal, arc-fused MgO is cleaved and exposed to single pulses of 248 nm, 3–10 J/cm2 excimer laser light (in vacuum), micron-sized holes are produced in the irradiated area. Cleavage produces micron-sized sites which exhibit highly localized absorption, resulting in decomposition, melting, and vaporization of the crystal at these sites. At fluences between 1 and 2 J/cm2, single laser pulses can produce localized, superficial melting and decomposition, leaving clusters of Mg-rich droplets on the surface. The mass-resolved charged and neutral particle emission produced by irradiation of the cleaved MgO surface with single laser pulses are examined. There are considerable variations in these emissions when single laser pulses strike different regions of the cleavage surface; these variations are strongly correlated with the production of conical holes in the irradiated region. A phenomenological model for the creation of these sites involving dislocation driven processes during cleavage and the resulting interactions of these sites with the laser beam are presented.
Antiferro-ferromagnetic transition and microstructural properties in a sputter deposited FeRh thin film system
Authors: Ohtani Y. , Hatakeyama I.
Published: July 2002

Magnetic and microstructural properties are studied in sputter deposited Fe100−xRhx alloy (28.0<x<57.0) thin films. As-deposited films show a preferred (111) oriented fcc phase at x≳35.0. Annealing at 600 °C results in the CsCl type structure which exhibits a preferred (110) orientation with the same lattice constant as that of bulk. However, an incomplete antiferro-ferromagnetic transition with large thermal hysteresis is observed in a wider composition range (x≳45.4) than bulk. Furthermore, magnetization at room temperature decreases continuously from x=35.0 to x=57.0 in contrast to the sharp drop at x=50 in bulk. By using scanning and transmission electron microscopy and Mössbauer spectroscopy, it is found that the retained fcc phase and compositional fluctuation strongly affect the magnetic transition. The retained fcc phase surprisingly reduces the grain growth of the CsCl type ordered phase from 4000 to 300 Å as the Rh content increases from 45.4% to 54.0%.
Thermal behavior of Al and Al-3 at. % Ge thin films on Si wafers
Authors: Lim B. S. , Pritchet W. C. , Rodbell K. P. , Tu K. N.
Published: July 2002

The mechanical stresses of a pure Al film and a low-thermal-expansion Al-3 at. % Ge thin film on (100) Si wafers are measured and compared in the temperature range of room temperature to 400 °C by the vibrating membrane method. The results are discussed including the comparison with those obtained by the popular wafer bending method. It was found that the chemical reaction between the Al and the Si substrate affects the mechanical stress. The relatively low stress in the Al(Ge) film is due to the interference of Ge precipitation. The relatively slow relaxation in the Al film during annealing may be caused by the dissolution of Si into Al.
Large, low-voltage absorption changes and absorption bistability in GaAs/AlGaAs/InGaAs asymmetric quantum wells
Authors: Trezza J. A. , Larson M. C. , Lord S. M. , Harris J. S.
Published: July 2002

Three-step asymmetric coupled quantum wells have unique excitonic properties, particularly under bias. We demonstrate these properties through the absorption changes in quantum well optical modulators. The samples consisted of p-i-n diodes with an active region of 20 coupled wells, each coupled well containing a 50 Å GaAs well and a 20 Å In0.2Ga0.8As well separated by a 10 Å Al0.33Ga0.67As barrier. Analysis of the structure shows that field-induced enhancement and suppression of electron and hole envelope wave function overlap can be observed through a corresponding increase or decrease in exciton absorption peaks. Our devices showed suppressed absorption with bias for the electron-heavy hole 1 exciton and enhanced absorption with bias for the electron-heavy hole 2 exciton. Stress-related effects on the electron-light hole 1 exciton are also observed. Absorption change per applied bias is five times lower than at the zero-field exciton wavelength in quantum well devices utilizing the conventional quantum-confined Stark effect (QCSE). At higher bias, the QCSE becomes dominant, producing absorptive bistability. Our devices exhibit lower chirp and lower-voltage operation than single-well devices and the flexibility of design allows for further optimization of absorption changes.
Measurement of thermal stress and stress relaxation in confined metal lines. I. Stresses during thermal cycling
Authors: Moske M. A. , Ho P. S. , Mikalsen D. J. , Cuomo J. J. , Rosenberg R.
Published: July 2002

A method based on the bending beam technique has been developed to measure the thermal stresses of fine lines confined by a dielectric layer. This method has been employed to determine the thermal stress of Al (2 at. % Cu) lines passivated by a SiO2 overlayer between room temperature and 400 °C. The effect of quartz confinement was analyzed by matching the thermal displacement at the metal/passivation interfaces and by imposing a mechanical equilibrium condition on the structure. The analysis enables us to deduce the triaxial stress components of metal and passivation from measurements of the substrate bending parallel and perpendicular to the length direction of the lines. Results of the measurements show a substantial stress enhancement as a result of the confinement, with the stress level significantly higher than that of a passivated blanket film. Parameters that influence the magnitude of the stress components are line geometry, layer deposition conditions, and the extent of plastic deformation during thermal cycling. Results of the measurements are consistent with those determined using x-ray techniques.
Characterization of stain etched porous Si with photoluminescence, electron paramagnetic resonance, and infrared absorption spectroscopy
Authors: Jia Y. Q. , Zhang L. Z. , Fu J. S. , Zhang B. R. , Mao J. C. , Qin G. G.
Published: July 2002

Porous Si (PS) layers are prepared by stain etching in a HF/NaNO2 solution on both p- and n-type crystal Si substrates, and are characterized by photoluminescence (PL), electron paramagnetic resonance (EPR), and infrared absorption (IR) spectroscopy. The PL spectra under 488 nm laser excitation exhibit a strong peak at 680–720 nm for various samples of different substrate parameters and remain stable upon aging in air or γ irradiation; as-etched (∼20 min in air before measurement) and aged (for up to six months) samples show no detectable EPR signal but the γ-irradiated samples show an isotropic g=2.006 signal of peak-to-peak linewidth of 1.1 mT supporting an amorphous Si structure; the IR spectra show both hydrogen and oxygen related IR modes in the as-etched samples and the former decreases with aging time in air while the latter increases. Comparing our results with those of anodically etched PS samples we conclude that: (1) the PL peak position of the stain PS seems to be unique and stable as compared with that of the anodic PS varying in 620–830 nm; (2) the isotropic EPR signal of the stain PS reflects no crystallinity, in contrast with the anisotropic signal of the anodic PS; and (3) obvious oxidation in the as-etched stain PS is also in contrast with the nonobservation of oxygen-related IR modes in the as-etched anodic PS. We discuss the results in terms of structural properties and PL mechanism of PS.
Characterization of 28Si+ and 40Ar+ ion-implanted epitaxial ReSi2 films on an n-Si(100) substrate
Authors: Kim Kun Ho , Kim Do Hee , Nam Sang Tack , Lee Joung Ju , Kim In Ho , Kim Sung Chul , Lee Jeong Yong , Nicolet Marc-A. , Bai Gang
Published: July 2002

The structural and electrical characteristics of 300 keV Si+ or 380 keV Ar+ ion-implanted epitaxial ReSi2 films grown on an n-Si(100) substrate have been studied by using 2 MeV 4He+ ion backscattering spectrometry, x-ray diffraction, high-resolution transmission electron microscopy, and electrical measurement. Ion implantation causes static disorder in the film, which overlap and grow to become an amorphous layer. The threshold dose for amorphizing the ReSi2 film is ∼5×1014/cm2 for 28Si+ and ∼1×1014/cm2 for 40Ar+. Although the resistivity of the implanted ReSi2 film decreases when the degree of disorder (or the implantation dose) is increased, the resistivity reaches a minimum value at a dose of ∼1×1015/cm2 for Si+ or ∼5×1014/cm2 for Ar+. The 28Si+-implanted amorphous ReSi2 films recovered original epitaxy after thermal annealing at 700 °C for 30 min in vacuum, as do the partly amorphized ReSi2 films by 40Ar+ implantation. On the other hand, those films fully amorphized by 40Ar+ implantation (dose≥1×1014/cm2) did not recover after thermal annealing, even when exposed to a temperature as high as 1000 °C for 30 min.
Hot carrier induced interface trap annealing in silicon field effect transistors
Authors: Das N. C. , Nathan V.
Published: July 2002

Interface traps are created in metal-oxide-semiconductor field-effect transistors when hot carrier stressing is done with the maximum substrate current biasing condition (Vd=6.0 V and Vg=2.9 V). Unlike trapped oxide charge, the interface traps are not annealed by keeping the device at room temperature for 24 h. However, by applying reverse bias with high positive drain voltage Vd and negative gate voltage Vg, the hot carrier induced interface traps can be completely annealed out. This is confirmed by both transconductance and charge pumping measurements. There is a direct relationship between the substrate current and annealing of interface states by reverse stressing. The possible mechanism of interface state annealing is discussed.
Capacitance studies of charge redistribution between Γ and X states in a GaAs/AlAs double barrier structure at high pressure
Authors: Austing D. G. , Klipstein P. C. , Roberts J. S. , Hill G.
Published: July 2002

Capacitance measurements as a function of bias provide evidence for the redistribution of electrons above the pressure induced type I to type II transition in a 43, 72, and 40 Å AlAs/GaAs double barrier structure (DBS). Measurements at 77 K for pressures up to 16 kbar allow the identification of low and high pressure regimes. In the low pressure regime the DBS exhibits a capacitance whose characteristic length includes both the barriers and the well, and any surrounding depletion regions. In the high pressure regime, the behavior is like that of a ‘‘quantum capacitor,’’ with plates only ∼70 Å apart. This causes a marked increase in the zero bias capacitance, which is explained by electron transfer to the X wells in the AlAs regions. The subsequent variation of the capacitance with bias is consistent with a band bending model introduced previously, in which the Fermi level is pinned to the lowest X level in each well at low bias, but where the collector well is eventually depleted at larger bias.
Low frequency and low magnetic field effects on the alternating current volume susceptibility of cuprate superconductors
Authors: Bensebaa Farid , Hankiewicz Janusz , Kevan Larry
Published: July 2002

The real and imaginary parts of the alternating current volume susceptibility are simultaneously measured on a cylindrical sample of ceramic superconducting YBa2Cu3O7−δ in the frequency range from 0.5 to 1×103 Hz. A relatively small frequency dependence of the shielding diamagnetism is observed below 2 Hz and above 250 Hz. A logarithmic dependence of the peak loss temperature is observed which is characteristic of an activated process with an activation energy of 2.5 eV. A weak decrease of the shielding signal takes place when the amplitude of the alternating current magnetic field is increased from 0.002 to 0.02 G. Above 0.02 G this shielding signal is field independent. The data are interpreted in terms of flux creep and viscous drag effects.
High-quality focused-ion-beam-made mirrors for InGaP/InGaAlP visible-laser diodes
Authors: Overwijk M. H. F. , de Poorter J. A.
Published: July 2002

We employed a focused ion beam (FIB) to sputter the end mirrors on InGaP/InGaAlP laser diodes. This particular diode operates at a wavelength of about 670 nm. For this wavelength the quality of the sputtered mirrors is far more critical than for longer wavelength devices. The present lasers do, in fact, show a relatively large increase in their threshold current after FIB micromachining. In this article, we investigate the origins of this increase and how to prevent it. After optimization, the lasers with a FIB-made end mirror have a threshold current comparable to that of cleaved-facet devices. We have seen that the polycrystalline structure of the device metallization on top of the laser diode causes roughening of the mirror during sputtering, which results in severe scatter losses of the laser light. A method to decrease the surface roughness is given. A further reason for the increase in threshold current is optical absorption at the mirror, presumably by a Ga-rich layer. This can be circumvented by chemical etching after FIB treatment to remove the absorbing layer. Finally, to demonstrate the quality and the applicability of the FIB-made mirrors, the influence of the tilt angle of the end mirror on the threshold current of the laser has been investigated. This dependency can be excellently described by theory.
Improved optical model for resonant tunneling diode
Authors: Zohta Yasuhito , Tanamoto Tetsufumi
Published: July 2002

The optical model to incorporate the effect of scattering is improved by using the energy dependent optical potential that is determined by experimental data on hot electrons. Using this model, the I−V characteristic of a resonant tunneling diode is calculated, and the calculated result explains experiments well. From this analysis it turns out that the valley current of the resonant tunneling diode is intimately related to the scattering rate of hot electrons in the well. As this model is easy to use and simple to interpret, it is quite suitable for simulation of device characteristics.
Asymmetry of the screw dislocation core structure in cadmium sulfide crystals
Authors: Negrii V. D.
Published: July 2002

The local symmetry of dislocation centers in CdS crystals was studied by piezospectroscopy of space resolved photoluminescence of individual screw dislocations. The limited set of dislocation Cs centers (2 from a total of 6) was found to be characteristic for each screw dislocation and that set determines the asymmetry of the core structure. The space- and time-nonstationarity of the dislocation domains observed in the polarized emission of dislocations are explained by the reorientation of one-dimensionally ordered Cs centers in the core of the screw dislocation gliding by the Peierls mechanism.
Growth of YbSb2/GaSb(001) and GaSb/YbSb2/GaSb(001) heterostructures by molecular beam epitaxy
Authors: Guivarc’h A. , Ballini Y. , Auvray P. , Caulet J. , Minier M. , Dupas G. , Ropars G.
Published: July 2002

Successful growth by molecular beam epitaxy of YbSb2 (010) single crystal films on GaSb(001) has been demonstrated. The YbSb2 films have a room temperature resistivity equal to 40 μΩ cm. YbSb2 has a basal plane rectangular (but almost square) and nearly lattice matched to GaSb. The memory of the GaSb(001) surface orientation is transferred to the YbSb2 surface films, avoiding the formation of inversion domains in the GaSb overlayers. The effects of the lattice mismatches in the basal plane and in the growth direction are discussed.
Laser ablation deposition of YBa2Cu3O7−x thin films using a microwave plasma disk reactor oxygen source
Authors: Pawlowski C. , Aslam M. , Rimai Lajos
Published: July 2002

Thin films of YBa2Cu3O7−x were prepared on polished (100) yttria stabilized zirconia single crystal substrates by pulsed laser deposition in a low-pressure oxygen background (<10 mTorr) by using a microwave plasma disk reactor (MPDR) oxygen source. The MPDR oxygen source provides high ion densities (≳1010/cm3) at low pressures (10−5–10−2 Torr), making it possible to form the superconducting YBCO phase in a low background pressure of oxygen. The best film was prepared at 4 mTorr and had a TC of 79.5 K as-deposited and the measured JC was 2.9×105 A/cm2 at 77 K. Films prepared under the same conditions using molecular oxygen were nonsuperconducting as-deposited.
Interband tunneling through a heavy hole state
Authors: Nogaret A. , Maldonado M. A. , Carnahan R. E. , Martin K. P. , Higgins R. J. , Maude D. K. , Portal J. C. , Cury L. A. , Chen J. F. , Cho A. Y.
Published: July 2002

We investigated interband tunneling transport through a hole well in five GaSb/AlSb/GaSb/AlSb/InAs type II tunnel devices in which the effect of the variation of barrier and well widths is systematically studied. Low temperature measurements were performed using high magnetic fields applied perpendicular to the current and hydrostatic pressures as external perturbations. A resonant current through the ground heavy hole subband in the GaSb well could be identified for the first time. This examination points out (i) the role of in-plane momentum conservation in determining the resonance onset voltage, and (ii) the occurrence of a shoulder in the current when a maximum of states conducts through a resonant subband.
A self-consistent approach to spectral hole burning in quantum wire lasers
Authors: Vurgaftman Igor , Singh Jasprit
Published: July 2002

In a semiconductor laser above threshold, carriers are extracted at the lasing energy at a high rate due to stimulated emission and are injected at higher energies. This creates a ‘‘hole burning’’ phenomenon resulting in gain compression. This effect is studied in a quantum wire laser by solving the Boltzmann equation with sink and source terms by a novel Monte Carlo technique. The results for various values of the characteristic injection times are given. A formalism is also proposed for the fully self-consistent determination of the laser operating parameters from the rate equations with the inclusion of nonlinear gain effects by substituting the correct form of the distribution function in presence of hole burning into the standard expressions for the laser material gain. The nonlinear gain effect is then described completely starting from the wire band structure and scattering rates. The generality of the proposed technique and its possible extensions and applications to the problem of nonlinear gain in quantum-well lasers are discussed.
A simple sequence-band CO2 waveguide laser
Authors: Ho Tsan-Kuang , Shaw Sen-Yen , Shy Jow-Tsong
Published: July 2002

A CO2 waveguide laser with an intracavity hot cell, which is capable of being operated on the 00°2 sequence transitions, is described. More than 15 lines in the 10.4 μm sequence band having maximum output power up to 1.2 W have been observed from a simple grating-tuned cavity.
Deposition of metallic films by laser ablation of oxides in hydrogen atmosphere
Authors: Ogale S. B. , Joshi Sushama , Bilurkar P. G. , Mate Nitant
Published: July 2002

Pulsed laser ablation from oxide surfaces in hydrogen atmosphere is shown to be a powerful new method for deposition of high quality metal films. Specifically, the cases of ablation from CuO and α-Fe2O3 are studied. By examining the effects of hydrogen pressure, laser energy density, and the substrate temperature on the nature of the deposited films, optimum parameters have been identified for realization of high quality films of metallic Cu and Fe.
Growth and characterization of ZnSe epitaxial layers grown by the solution growth method
Authors: Sano Michihiro , Yamashita Yoji , Okuno Yasuo
Published: July 2002

Experiments which led to the growth of a ZnSe layer suitable for a blue-light-emitting diode are presented. The ZnSe epitaxial layer was obtained by the solution growth method and, in the process, the electrical and optical properties of the ZnSe layers doped with various impurities were investigated. Through research it was concluded that the group-VII elements (Cl,Br,I) were much more suitable than the group-III elements (Al,Ga,In). It has also been proven that the Hall mobility and the intensity ratio of the deep to edge emission in the photoluminescence spectrum depended on the Se pressure applied during the annealing of the ZnSe source crystal. The deep emission in the photoluminescence spectrum was suppressed by using a ZnSe source material annealed prior to growth at a Se pressure of 8 atm. The maximum Hall mobility attained was 380 cm2/V s at a carrier density of 1.6×1017 cm−3.
Real-time monitoring of the reaction of H2S on GaAs
Authors: Jönsson Jan , Deppert Knut , Samuelson Lars
Published: July 2002

The reaction of H2S on (100)-oriented GaAs surfaces has been studied in real time by an optical technique. Reflectance difference was used to follow the surface anisotropy. Kinetic studies were made of the reaction of H2S on an As-terminated as well as on a Ga-terminated surface. We show that the sulfur incorporation process is different for these surfaces. The time constant for the reaction of H2S on an As-terminated surface was measured for different conditions. Based on the temperature dependence of the reaction rate, and its nonlinear dependence on the flow of H2S, the process of incorporation is discussed. We show also that the probability for formation of [110]-oriented Ga dimers during a TEG supply is significantly decreased if the surface was exposed to H2S for a prolonged period.
Construction of higher-moment terms in the hydrodynamic electron-transport model
Authors: Woolard D. L. , Tian H. , Littlejohn M. A. , Kim K. W. , Trew R. J. , Ieong M. K. , Tang T. W.
Published: July 2002

A critical step in the development of all hydrodynamic transport models (HTMs), derived from moments of the Boltzmann transport equation, is the introduction of accurate closure relations to terminate the resulting infinite set of macroscopic equations. In general, there are a number of resulting integral terms that are highly dependent on the form of the true electron distribution function. The so-called heat flux term is one very important higher-moment term that requires attention. Methods for the accurate construction of an improved heat-flux model are presented. In this construction, a higher-moments approach is combined with a unique definition of electron temperature (i.e., based upon an ansatz distribution) to investigate the effects of conduction-band nonparabolicity and distributional asymmetry. The Monte Carlo method has been used to evaluate the resulting model closures and to study microscopic electron dynamics. These investigations have identified an important relationship between a particular symmetric (i.e., thermal) component of the electron distribution function and the heat flow vector. This knowledge is important because all the parameters in the HTM must be closed (i.e., related to each other through a common set of system variables) before the technique can be accurately applied to the study of electron transport in semiconductor devices.
Intrinsic and extrinsic 1/f noise sources in proton-irradiated n-GaAs epitaxial layers
Authors: Ren L.
Published: July 2002

The low-frequency resistance noise of proton-irradiated n-GaAs epitaxial layers was studied at temperatures from 77 to 300 K. Two types of 1/f noise were identified from the temperature dependence of the 1/f noise parameter α. One type of 1/f noise that is dominating at high temperatures seems to be of intrinsic origin related to lattice phonon scattering. The other dominating one at lower temperatures is, then, of extrinsic origin induced by the irradiation. The extrinsic type of 1/f noise is consistent with the quantum ‘‘local-interference’’ effect and can reasonably be described by the Dutta–Dimon–Horn model [P. Dutta and P. M. Horn, Rev. Mod. Phys. 53, 497 (1981)].
Post-metallization annealing of metal-tunnel oxide-silicon diodes
Authors: Lundgren P. , Andersson M. O. , Farmer K. R.
Published: July 2002

We report a post-metallization annealing study of very thin oxide (2.4–3.2 nm), aluminum gate metal-tunnel oxide-(p) silicon devices. Voltage dependence measurements of both tunnel current and high-frequency capacitance as functions of anneal time and temperature reveal that annealing the thin oxide devices after metallization leads to a decrease in interface state density, with dynamics which are similar to, though slower than, what has been observed in thicker oxide aluminum gate systems.
Hydrogen absorption and disproportionation of melt-spun Nd–Fe–B ribbons
Authors: Meisner G. P. , Panchanathan V.
Published: July 2002

Hydrogen absorption and disproportionation of melt-spun Nd–Fe–B-based ribbons are significantly affected by quench state (wheel speed). The initial hydrogen absorption temperature corresponding to the formation of the Nd2Fe14BHx-type phase shifts from ∼120 °C for ingot to 250–300 °C for ribbons spun at 13, 15, 17, 20, 22, and 24 m/s wheel speeds. This shift is presumably related to the fact that ribbons are considerably more inert than ingot, which is typical of Nd–Fe–B materials. The hydrogen disproportionation temperature, however, monotonically decreases with increasing wheel speed from ≳700 °C for ingot to 530 °C for 24 m/s ribbons. Hydrogen absorption due to disproportionation exhibits a two-step feature for wheel speeds ≥20 m/s. The onset and size of the lower temperature step correlates with the presence and amount of an amorphouslike component in the x-ray diffraction spectra of as-spun ribbons. The amorphouslike component is detectable only for wheel speeds ≥20 m/s, and both the amount of amorphouslike component and the size of the lower temperature step in the hydrogen absorption increase with increasing wheel speed.
Omega head—an experimental 120-turn inductive head
Authors: Tang D. D. , Lee R. E. , Su J. L. , Chu F. , Lo J. , Santini H. , Lane L. , Robertson N. , Ponce M. , Cisneroz P. , Guthmiller G.
Published: July 2002

Experimental 120-turn thin-film inductive heads have been built. The key features of this head are the 6-μm pitch helical coils and an omega-shaped, planar yoke structure having dual easy axes. Hardbaked photoresist insulator layers are used to encapsulate the yoke and to smooth out the wafer surface topography. Micro-Kerr studies show that the easy axis remains in the transverse direction in the yokes after multiple anneals. The P1/G/P2 is 3.8/0.3/3.8 μm, and the yoke length is close to 1 mm. The helical coils were built with a novel process that combines yoke/stud coplating and a photoresist planarization process. The coil resistance is 68Ω and the inductance is 5.5 μH. The yoke saturates at 5 mA. The heads were tested over disks having Mrt of 2.5 memu/cm2 and Hc of 1500 Oe. The write threshold current is 5.3 mA (peak-to-peak) and the overwrite is 30 dB. The isolated pulse amplitude Vpp is 10.3 μV/(TwV), where track width Tw is in μm and the linear velocity V in m/s.
The percolation limit for the disappearance of ferromagnetism in melt spun Co-B-C amorphous alloys
Authors: Nicolaides G. K. , Inoue A. , Rao K. V.
Published: July 2002

Low-temperature magnetic properties of the Co56−xB32+xC12, x=0, 2, 4, and 4.5 ternary amorphous alloys are investigated by using low-field ac magnetic susceptibility measurement techniques and also a superconducting quantum interference device magnetometer. The ac susceptibility data show that these alloys undergo a paramagnetic to ferromagnetic state transition at cryogenic temperatures. The Curie temperature Tc decreases linearly with decreasing Co content at a rate 31 K per Co at %, suggesting a percolation limit for the disappearence of ferromagnetism at around 50 at % Co. A scaling analysis performed on magnetic isotherms obtained near Tc; even for the sample almost close to the percolation limit, yields critical exponent values consistent with the 3D-Heisenberg model for an ideal ferromagnet. Thus in these Co-based amorphous alloys, long-range order persists to the concentrations almost at the percolation limit and the mean-field theory is found to describe well the paramagnetic to the ferromagnetic state phase transitions.
Unidirectional pinning in irradiated Bi2Sr2CaCu2O8 (invited)
Authors: Klein L. , Yacoby E. R. , Tsameret A. , Yeshurun Y. , Kishio K.
Published: July 2002

Bi2Sr2CaCu2O8 crystals were irradiated with heavy ions (Pb, Xe) to produce columnar defects along the c direction or at 45° with respect to it. We describe the dependence of the critical currents, irreversibility fields, pinning force, and magnetic relaxation rates on the dose and on the type of ions. The irreversible properties of the Pb-irradiated samples are more enhanced than those of the Xe-irradiated sample at low temperatures, and vice versa at high temperatures. At low temperatures the efficiency of the columnar defects is enhanced with the ion dose but, at high temperatures, the maximum dose (2.5×1011 ions/cm2) is less efficient than intermediate doses. The unidirectional nature of the columnar pinning centers serves to demonstrate that the vortices maintain linelike features. For most of the irradiated samples, the pinning force density data are reduced to a single curve below 45 K. The relaxation rates at low temperatures reflect thermal reduction of the effective pinning barriers.
Magnetic properties of Nd(Fe,Ti)12 and Nd(Fe,Ti)12Nx films of perpendicular texture
Authors: Wang D. , Sellmyer D. J. , Panagiotopoulos I. , Niarchos D.
Published: July 2002

Nd(Fe,Ti)12 and Nd(Fe,Ti)12Nx films textured with c axis perpendicular to the film plane were synthesized by sputtering. For the Nd(Fe,Ti)12 films a spin reorientation took place at about 170 K. For the Nd(Fe,Ti)12Nx films, the anisotropy field is about 85 kOe at room temperature and 130 kOe at 5 K, corresponding to a coercivity of 2.7 and 12 kOe, respectively. Initial magnetization curves show a behavior characteristic of nucleation type of magnetization reversal mechanism. Attempts were made to fit the experimental data with Kronmüller’s magnetization reversal model. The results suggest that nucleation with extended sites is responsible for the reversal.
A contribution to the knowledge of phase equilibria and the magnetic properties of the Nd–Fe–B–X systems (X=Al,Co,V)
Authors: Micski Anders , Uhrenius Björn
Published: July 2002

A technique based on diffusion couples for the determination of the compositions of different phases present in the Fe–Nd–B and Fe–Nd–B–Al systems was used. By this technique the composition of the liquid in equilibrium with the solid phases could be determined. Since all elements are encapsulated in an iron capsule, the study of oxidation-sensitive elements like neodymium was easily made. By using this technique, new information on the composition of the liquid phase in equilibrium with solid phases in the Fe–Nd–B system as well as the Fe–Nd–B–Al system was determined at two temperatures. This information was used to improve the representation of the liquidus surfaces of the systems. The effects of Al, V, and/or Co additions on the coercivity and energy product of sintered Fe–Nd–B magnets are also presented.
Temperature-dependent non-Heisenberg exchange coupling of ferromagnetic layers (invited)
Authors: Erickson R. P.
Published: July 2002

We consider with temperature the exchange coupling of two identical ferromagnetic films separated by a paramagnetic metal of thickness d. The minority-spin band of the ferromagnets is matched to the band of the paramagnet. Majority-spin electrons experience a repulsive potential of maximum height equal to the exchange gap of the ferromagnets. A spin-flip current, which flows as a result of a virtual rotation of the ferromagnetic moments, is calculated at temperature T, assuming single-particle excitations only. This current, equated to the torque exerted by one ferromagnet on the other, defines the exchange energy as an infinite sum of terms with coefficients A12(d,T),B12(d,T),..., extending beyond the Heisenberg-like A12(d,T) term. For certain d, the biquadratic B12(d,T) term is of the proper magnitude and sign to cause intrinsic 90° coupling between the moments of the two ferromagnets. Additionally, we calculate the T dependence of A12(d,T) and B12(d,T) for different Fermi energy to exchange-gap ratios, showing that, in general, a nonmonotonic dependence of both A12(d,T) and B12(d,T) vs T is expected. Monotonic T dependence occurs in the asymptotic regime of large d.
The effect of Co doping on spin cluster resonance in the one-dimensional Ising ferromagnet FeTAC
Authors: Rubins R. S. , Black T. D. , Ravindran K. , Drumheller John E.
Published: July 2002

Electron paramagnetic resonance measurements at 36 GHz and in the range 12–18 GHz were made on a single crystal of the 1D Ising ferromagnet [(CH3)3 NH] FeCl3⋅2H2O, known as FeTAC, containing a nominal 10% of the isomorphous compound CoTAC. While the 4.2 K spectra obtained with the external field parallel to the chain axis were similar to the spin cluster resonance spectra previously identified in pure FeTAC, differences in the low-field satellite structure were observed, which are tentatively attributed to differences in the dipolar interaction and to shorter ferrous chains. An appreciable decrease in the strength of the demagnetizing field is associated with the tendency of the Co2+ moments to align perpendicularly to the chain axis.
An adaptive computational method for domain wall dynamics (abstract)
Authors: Osipov Sergey G.
Published: July 2002

The dynamics of magnetic domain walls is a subject of intensive computer simulation of last years. Now it is well understood, that strongly twisted structures (like horizontal Bloch lines) of domain walls can be created. In some cases this HBL can have a very small localization region. Appearance of such inconvenient objects, like HBL, states a new scale for numerical model and it required a significant increase of node number for the uniform grid model. The main idea of the new adaptive computational method for LLG equation for two-dimensional (2D) domain wall dynamics is to inject and delete additional nodes only to the HBL localization region, using a special target function, which is based on the twisting angle of the domain wall. Node injection occurs when HBL is created, and node deleting occurs when HBL is breakthrough. The conjugated gradient method was used for fast demagnetization field computation by two Dirichlet problems for Poisson equations. We show an effective application of this method for 2D computations of domain wall dynamics for bubble film with parameters: Q=4, D=3(l), α=0.2, h=(0,8,−3), (Ms) [Here Q is the quality factor of the film, D the film thickness in characterization units, α the damping parameter, h the external magnetic field in Ms units, 8(Ms) the value of in-plane magnetic field via chirality, −3(Ms) is the driven magnetic field.] We found that HBL has very strong localization in this case. To carry out this computation we need nx×nz=65×162 for uniform grid sytem, and only nx×nz=33×60 for adaptive grid system. A color computer movie was created for this case. The new mechanics of HBL generation and breakthrough is discussed. Here we can see significant asymmetry in HBL generation: two HBL created at one moment at the lower surface of the film, but upper HBL goes to the upper surface of the film and the lower goes to the opposite direction. The next pair of HBL is created at the lower surface of the film at the same place.
Improvement of anisotropy of perpendicular magnetic recording tape by Ta addition and Kr sputtering gas (abstract)
Authors: Kuga Kiyoshi , Yoshimoto Hideaki , Yoneda Yoshiro , Numazawa Junji , Naoe Masahiko
Published: July 2002

A perpendicular magnetic recording tape requires a high magnetic anisotropy field Hk and a small distribution of the perpendicular magnetic anisotropy field σhk/Hk. It can be expected that by the extension of the interplane distance of (002) plane d(002) in Co-Cr, a perpendicular magnetic anisotropy energy will be increased and Hk becomes higher. It can be also expected that the σhk/Hk becomes smaller by the improvement of the crystallinity of Co-Cr film. Therefore, in order to achieve the above, the following experiments were attempted. At first, a small amount of Ta was added to Co-Cr. This, as a result, extended the d(002) of the Co-Cr from 2.033 to 2.046 Å. Consequently, Hk was increased from 4.4 to 5.1 kOe and σhk/Hk was reduced from 0.14 to 0.11. Second, as a sputtering gas, Kr gas was used instead of Ar gas. The use of Kr resulted in greater crystallinity in the film, increasing the peak intensity I(002) of x-ray diffraction of the Co-Cr-Ta from 2090 to 2500 cps. Consequently, σhk/Hk was reduced up to 0.07. Last, a recording and reproducing experiment with the Co-Cr-Ta tape was attempted with the use of a hilical-scan VTR machine. As a result, a reproduced output of 56 nVO-P (μm T m/s) was obtained at the recording wavelength of 0.33 μm. This figure is equivalent to the output at the recording wavelength of 0.66 μm of metal-powder tape.
Optimum design of voice coil motor with constant torque coefficients using evolution strategy
Authors: Koh Chang Seop , Mohammed Osama A. , Kim Jun-o , Hahn Song-yop
Published: July 2002

An effective optimum design method is presented. The method combines the three-dimensional boundary element method with the (1+1) evolution strategy. The reduced scalar potential formulation with the magnetic surface charge as the unknown variable is used under the assumption that the yokes are not saturated. It is found, through the numerical example, that the global optimum shape of the magnet can be easily found within a reasonable number of generations. More elaborate design can be achieved by increasing the number of design variables.
Comparative analysis of two methods for time-harmonic solution of the steady state in induction motors (abstract)
Authors: DeWeerdt Robrecht , Brandisky Kostadin , Pahner Uwe , Belmans Ronnie
Published: July 2002

The abstract presents two methods of solving induction motor problems using a time-harmonic approach, taking into account the saturation of the iron material. The first method uses the following algorithm. Initially, two static nonlinear problems are solved: one problem using the real part of the stator currents, and the other using the imaginary part. From both solutions, a reluctivity vector is generated. This reluctivity vector is then used in solving a time-harmonic problem to calculate the induced rotor currents. These currents are used to solve two new static problems. From the solution, a more accurate reluctivity vector can be generated. Convergence of this method occurs after 4 or 5 steps. The second method is an iterative method of solving nonlinear time-dependent problems by harmonic representation. It is assumed that H(t) is a sinusoidal function of time. A new sinusoidal Beq is introduced based on energy equivalence with the real nonsinusoidal B. This new Beq is used to calculate the new B-H curve for the iron materials involved and after that an equivalent reluctivity. The nonlinear algorithm represents under-relaxation of the equivalent reluctivity, based on the formula: RELUCTnew=RELUCTold+ALPHA*(RELUCTcrnt−RELUCTold), where ALPHA is a relaxation factor usually chosen between 0 and 1. The algorithm shows a good convergence rate (from 10 to 20 steps) if the initial starting vector for reluctivities and the relaxation factor are chosen appropriately. Rules for this choice are given. Both methods are compared. The difference between the induced currents in both methods is about 1%, with a linear solution it is about 300%. Also stored energy, losses, reluctivities, and other quantities are compared.
Dispersivity measurement of magnetic powders using the magnetic noise method (abstract)
Authors: Vasilyev V. M.
Published: July 2002

In spite of the different nature of high coercivity in hexaferrites (Ba, Sr, Pb) and intermetallic compounds (Sm CO5, Fe-Nd-B) the grain size distribution function in the powders is of a great importance for both types of sintered magnets. The conventional techniques in many cases could not provide a reliable estimate of the powders’ dispersion. The fine particles (particularly the single-domain ones) are magnetized almost to saturation and therefore are stretched together into the flocculas. These flocculas are taken by the various methods as a large particle, i.e., the methods fail. On the contrary, the jumplike domain wall motion parameters, obtained in BN spectrum study, let us analyze the distribution function of fine Ba-Sr hexaferrites powder particles by size. Later it was found that such dependence of the BN spectrum G(f) on the mean grain size dm is true for a wide variety of the powders whose dispersivity is far from a single-domain one. The stable dependencies of BN parameters on a powder dispersity was observed for a wider range in Ba-Sr ferrite 0.5<dm<4 mkm (since the single-domain size is about 1.3 mkm) and in Fe-Nd-B powders with 1.5<dm<20 mkm, and in pulverized iron powders with 30<dm<400 mkm and in fine electrolytic dendrite iron particles with a mean long axis size of 0.1<lm<0.2 mkm. The applied field and G(f) values are quite different in the above cases, but the main features remain the same. So the method may be widely used in various branches of magnetic powder and related technology for easy, fast, and reliable measurement and analysis of the distribution function of the powders’ particle numbers and specific volumes by size. We have developed a complete range of devices for the SPDA (system for powders dispersion analysis) equipped with a PC and proper software for analyzing and displaying the information obtained.
Spin glasses with cubic anisotropy
Authors: Domański Z. , Kopeć T. K. , Pázmándi F. , Erdös P.
Published: July 2002

The infinite-range quantum spin glasses with cubic anisotropy (K) are studied using a combination of the imaginary-time representation with the n-replica approach and the thermofield dynamic method. Mean-field theory phase diagrams in the temperature-anisotropy plane (T,K) are presented for quantum spin numbers S ranging from 2 to 9/2. At T=0, the ground state is nonmagnetic and the spin-glass order is absent for integer spin and large cubic anisotropy, the sign of which depends on S. For half-integer S and sufficiently low temperatures the spin-glass phase persists for arbitrary K.
The frequency dependence of the ferro-to-spinglass transition of amorphous Fe-rich Fe-Zr (abstract)
Authors: Nogués J. , Rao K. V.
Published: July 2002

It is well known that competing magnetic interactions in amorphous Fe-rich Fe-Zr alloys result in a ferromagnetic-like transition below room temperature (for example, Tc≊235 K for a Fe90Zr10 alloy). The field dependence of the ac response of such a system with competing interactions is thus complex, with the possible coexistence of ferromagnetic domains and spin freezing processes. One consequence of this is the observed1 Hopkinson peak in ac susceptibility and its strong field dependence when measured at ordinary fields (≳80 A/m). The existence of such a complex Hopkinson peak, which arises from the contribution of domain wall motion and hysteretic effects, makes it very difficult to identify, unambiguously, a spin freezing temperature. The purpose of this paper is to clarify this point. Complex ac susceptibility [χ′(T), χ″(T)] measurements have been carried out over an extended range of field amplitude (0.08–8000 A/m) and three decades of frequency (1–1000 Hz) along the axis of a single sample of melt spun Fe90Zr10 ribbon. We find that, below the threshold field, there is a unique spin freezing temperature with almost no frequency or field dependence. At higher fields (≳80 A/m) the viscous susceptibility, χ″(T), peak at low temperatures shows a strong temperature, frequency, and field dependence, indicating a superposition of the effects of ‘‘induced’’ long range order with the spinglass state. The ferro-to-spin glass transition temperature using this approach has been found to be Tsf=25 K.