bluuebunny/crossref_metadata_2025 huggingface dataset:
Delayed expansions and ionization instabilities observed in the current decay phase of a SF6 spark discharge
Authors: Akitsu Tetsuya , Ohtsu Takayoshi , Matsuzawa Hidenori , Suganomata Shinji
Published: July 2002

In SF6 gas at 5–30 Torr, self-breakdown spark discharges starting in a constricted phase expanded spatially when the discharge current decayed to a critical value. The strength of the electric field increased from 400 to 2×103 Td in E/N, where 1 Td=10−21 V m2, where E is the electric field and N is the neutral gas number density. At the same time, the optical emission from the spark increased in strength and ionization instabilities started to grow. In the decay phase of SF6 spark discharge, negative ions strongly enhanced electron diffusion in the multipolar diffusion that resulted in the expansion and the excitation of ionization instabilities.
Excitation of optical transitions in the InxGa1−xAs-GaAs quantum well system by the free exciton in the barrier
Authors: Reynolds D. C. , Jogai B. , Yu P. W. , Stutz C. E.
Published: July 2002

In the InxGa1−xAs-GaAs coupled double quantum well system the free exciton in the GaAs barrier is directly coupled to the symmetric heavy-hole free exciton in the quantum well. In contrast the free exciton in the barrier does not couple to the symmetric light-hole free exciton in the quantum well, or to Landau levels associated with the lowest energy conduction and valence subband levels. These results are explained by momentum conservation rules.
Anisotropy, wall energy density, and exchange stiffness of amorphous (Gd,Dy)-(Fe,Co) films
Authors: Raasch D. , Reck J.
Published: July 2002

The uniaxial anisotropy Ku and the magnetic domain wall energy density σw are important quantities for the thermomagnetically information storage process and its modeling. Ku was measured on (Gd,Dy)-(Fe,Co) films in dependence on temperature and Dy content. It increased linearly with increasing Dy content. Ku and the coercive field Hc are related by the equation Hc=αKun/μ0Ms with the fitting parameters α and n. At room temperature σw was determined from the difference of the switching fields for expanding and collapsing of thermomagnetically written circular domains on magneto-optical disks. The disks were prepared by electron beam evaporation with Dy contents between 5 and 35 at. %; the composition was adjusted to yield samples with a Curie temperature TC close to 500 K. The wall energy density increased with the Dy content. The exchange stiffness A was derived from measured σw and Ku data using the simple Bloch wall equation A=σw2/16Ku. The exchange stiffness was found independent on the Dy content to be 3.1×10−12±0.8×10−12 J/m. The resulting A was in agreement to literature data for amorphous Gd33Fe67 alloys.
Dielectric polarization relaxation measurement in α-SiO2 by means of a scanning electron microscope technique
Authors: Oh K. H. , Le Gressus C. , Gong H. , Ong C. K. , Tan B. T. G. , Ding X. Z.
Published: July 2002

A scanning electron microscope is used as a tool to study dielectric relaxation processes in α-SiO2 by measuring the leakage current in the sample surrounded by a metallic aperture. A transient time (tt) of the order of a few seconds appears before the steady-state current is established. The time dependence of the trapping rate is found to follow a power law and to be related to relaxation processes of a dielectric under electrical and thermal stress.
The temperature characteristics of InGaAsP/InP buried heterostructure lasers
Authors: Gault M. , Mawby P. , Adams A. R. , Towers M.
Published: July 2002

The temperature characteristics of 1.3 μm InGaAsP/InP buried heterojunction lasers are investigated using a fully self-consistent two dimensional numerical model. Devices operating at 1.3 μm are very temperature sensitive and therefore the model includes for the first time coupled solutions to the thermal as well as the electrical and optical equation sets. Good agreement is found with experimental results, including the temperature dependence of the threshold current and the prediction of a break-point temperature.
Properties of the deep donor states of AlxGa1−xAs:Se
Authors: Oh E. G. , Hanna M. C. , Lu Z. H. , Szmyd D. M. , Majerfeld A.
Published: July 2002

A study of the electrical and optical properties of the Se deep donor states in AlxGa1−xAs:Se grown by metalorganic vapor phase epitaxy (MOVPE) process is reported. A novel experimental technique is presented to determine the absolute energy and the true density of the deep donor. The characteristics of the Se deep donor states are obtained from deep level transient spectroscopy, photoluminescence, photocapacitance, Hall effect measurements, isothermal capacitance transient measurements, and a quasistatic capacitance voltage technique. It is found that the Se dopant gives rise to at least two energy levels in the band gap. One is the generally observed deep donor level, commonly called the DX level and the other is a new shallower donor state which also exhibits DX-like properties. The concentration of the shallower state is less than 5% of the deep donor density. The densities of both donors increase with the mole fraction of H2Se used during MOVPE growth. Thermal emission activation energies of 0.29±0.01 and 0.24±0.01 eV were found for the deep and shallower donor states, respectively, for 0.23≤x≤0.41. The Se donor ionization energies relative to the Γ minimum are determined for samples with different AlAs mole fractions, and also, the true densities of Se donors are obtained. We propose a macroscopic model for the emission and capture mechanisms of Se donors in AlxGa1−xAs, which allows a consistent interpretation of the results obtained by different measurement techniques and provides a natural explanation for the low temperature Hall density saturation phenomenon and the persistent photoconductivity effect.
Micromagnetic structure of domains in Co/Pt multilayers. I. Investigations of wall structure
Authors: Ploessl R. , Chapman J. N. , Scheinfein M. R. , Blue J. L. , Mansuripur M. , Hoffmann H.
Published: July 2002

An analysis of the micromagnetic structure of domains and domain walls in Co/Pt multilayer films is reported. Magneto-optically written domains have been imaged in a scanning transmission electron microscope by using the modified differential phase contrast mode of Lorentz electron microscopy. These have been compared with computer-simulated images based on a two-dimensional model of a circular, perpendicular magnetized domain with a Bloch-like wall structure. Agreement is found for the domain and stray field contrast, but the absence of wall contrast in the experimental images indicates a more complex wall structure in the multilayer than was assumed by the model. In a further series of calculations the magnetic microstructure of a Co/Pt multilayer was modeled by solving the Landau–Lifshitz–Gilbert equations. These suggest that the wall structure varies throughout the thickness of the multilayer, allowing significant saving of magnetostatic energy through the establishment of flux closure paths close to the walls, and are consistent with experimental observations.
Process-property correlations of excimer laser ablated bismuth titanate films on silicon
Authors: Maffei N. , Krupanidhi S. B.
Published: July 2002

Bismuth titanate thin films were deposited by the excimer laser ablation technique directly onto bare silicon substrates, SiO2 and Si3N4 coated silicon. The impact of process parameters such as gas pressure, laser fluence, processing temperature, and the presence of an oxygen plasma were studied with regards to the ferroelectric-semiconductor interface. The density of interfacial surface state (Nss) at the flatband voltage was found to be on the order of 1012–1014 eV−1 cm−2. Hysteretic capacitance-voltage data indicated charge injection from the substrate was the dominant mechanism, masking any polarization mode. Films deposited on SiO2 coated silicon did, however, exhibit polarization type switching.
Distribution of times for barrier traversal caused by energy fluctuations
Authors: Hagmann Mark J.
Published: July 2002

Quantum tunneling is attributed to energy fluctuations permitted by the uncertainty principle. The distribution of barrier traversal times is shown to be bimodal. The expression for the expectation value is similar to the semiclassical result, and the width of the distribution varies inversely with the size of the barrier.
Many-body effects in x-ray photoemission and high-T
c superconductivity of cuprate superconductors
Authors: Kohiki Shigemi , Sakai Masahiro , Mizuno Koichi , Hayashi Shigenori , Enokihara Akira , Setsune Kentaro , Fukushima Sei , Gohshi Yohichi
Published: July 2002

Many-body effects in Cu 2p x-ray photoemission of Bi2Sr2CaxCu1+xO6+2x (x=0,1,2), La2−xSrxCuO4 (x=0,0.13), and ErBa2Cu3O7−x,y (x<y) were observed and correlations between the many-body effects and high-Tc superconductivity of cuprate superconductors were reviewed. For the Bi2Sr2CaxCu1+xO6+2x (x=0,1,2) and La2−xSrxCuO4 (x=0,0.13) systems, proper high-Tc superconductivity was realized by increasing the transfer integral and the interaction strength of the Cu-O bond of the crystal determined the superconducting properties. However, for the ErBa2Cu3O7−x and YBa2Cu3O7−x systems, 90 K superconductivity was realized by increasing the charge transfer energy. The extended valence state including the Cu, Ba, and O valence states in the crystal determined the superconducting properties of the ErBa2Cu3O7−x and YBa2Cu3O7−x systems.
Diamond growth on turbostratic carbon by hot filament chemical vapor deposition
Authors: Yu Z.-M. , Rogelet T. , Flodström S. A.
Published: July 2002

Diamond films were grown on turbostratic carbon (TC) pregrown on Cu substrates by hot filament chemical vapor deposition. Auger electron spectroscopy, Raman spectroscopy, x-ray diffraction, infrared absorption spectroscopy, and scanning electron microscopy were used to characterize both the TC and the diamond films. It was shown that the TC films could form on Cu at low temperatures (≊650 °C) with the catalysis of Mo and that diamond crystallites rapidly formed on TC in less than 10 min at higher temperature (≊1000 °C). The sp3 bonds were found to coexist with sp2 bonds in TC. The sp3 bonds in TC provide nucleation sites for diamond crystals and improve the nucleation rate at the early stage of deposition of diamond on TC.
Synchrotron x-ray topography study of Er-doped LaP5O14 crystal
Authors: Hu Z. W. , Huang P. Q. , Jiang S. S. , Wang J. Y. , Jiang J. H. , Zhao J. Y.
Published: July 2002

An Er-doped LaP5O14 ferroelastic crystal has been studied by synchrotron radiation white beam x-ray topography. A set of normal ferroelastic domains is revealed in topographs by the intense contrast at the domain walls and the temperature dependence of the wall contrast is shown to be in good agreement with that expected from the structural consideration. In addition to growth bands and growth sector boundaries, a type of abnormal boundary is observed, and the contrast varies with the occurrence of phase transitions. A detailed discussion on experimental results is given in terms of structural and symmetry analyses.
Potential role of atomic carbon in diamond deposition
Authors: Zhang Y. F. , Dunn-Rankin D. , Taborek P.
Published: July 2002

This paper presents a numerical simulation of the gas-phase chemistry in diamond deposition processes. The simulation shows that the concentration of the two potential growth species CH3 and C in the boundary layer near the diamond film substrate are sensitive to these species’ concentration in the bulk gas. The concentrations in the bulk gas depend, in turn, on the physical arrangement of the reactor, and in particular on the time provided for the gas mixture to reach chemical equilibrium. With sufficient equilibration time, simulations of both a hot-filament reactor and a plasma torch reactors show that the concentration of atomic carbon at the substrate surface can be much higher than the concentration of CH3.
Monte Carlo simulation for dissociation of hydrogen during electron assisted chemical vapor deposition of diamond
Authors: Saitoh Hidetoshi , Mima Hiroyuki , Ishiguro Takashi , Ichinose Yukio
Published: July 2002

An H2 dissociation model in the diamond deposition process is described and discussed using the results of a Monte Carlo computer simulation. A hot filament assisted chemical vapor deposition technique with substrate bias voltage was assumed and electron trajectories were computed. In this model, molecular hydrogen is dissociated by the impact of electrons accelerated towards the substrate. The number of dissociative collisions occurring while one electron flies from the hot filament to the substrate was calculated varying both the bias voltage and the gas pressure. The results we obtained here suggest that (1) the number of dissociations increases toward the biased substrate; (2) the production of H atoms generated in the gas phase is a function of the ratio of electric field to gas pressure, E/p; and (3) there is an optimum condition of E/p required to obtain efficient dissociation.
Optoelectronic modulation spectroscopy applied to the characterization of field effect transistors
Authors: Wang Q. H. , Swanson J. G.
Published: July 2002

Optoelectronic modulation spectroscopy has been applied to field effect transistors. Spectra include responses from bulk levels as well as from the continuum of states at the insulator-semiconductor interface. States that are energetically deep in a continuum can be probed but would not be accessible to the Fermi level in an electrical measurement.
X-ray refinement and magnetic properties of nitrogenated melt-spun Sm2Fe17 compound
Authors: Yang Choong-Jin , Lee Woo-Young , Shin Hyung-Sup
Published: July 2002

Sm2Fe17N2.9 compound powders were prepared from parent alloys of Sm2Fe17 which were made by both the arc-melting and rapid solidification processes. The cast Sm2Fe17 contained mixed phases of SmFe3 and α-Fe, and correspondingly formed less Sm2Fe17Nx with a rather large amount of residual α-Fe after nitrogenation. The melt-spun Sm2Fe17 compound, however, was single phase and exhibited a negligible amount of residual α-Fe after nitrogenation. The residual amount of free iron was found to increase as a function of milling time and impede the development of promising permanent magnetic properties. The melt-spun Sm2Fe17Nx compound powders exhibited a coercivity value iHc of 5 kOe, which is double that of the as-cast Sm2Fe17Nx powders and a high remanence, Br=60 emu/g. The average magnetic moment of Fe atoms in the crystal is estimated to be 2.29 μB and the Fe atom in the 6c site shows the highest magnetic moment of 2.65 μB. The expansion in the c-axial direction of the nitrogenated crystal was found to be mainly due to extension of Sm(6c)-Fe(6c) and Fe(18f)-Fe(18f) distances.
Numerical fitting of transient decays in the high defect density limit
Authors: Yang S. , Lamp C. D.
Published: July 2002

A numerical fitting method based on the deep level transient spectroscopy (DLTS) technique is presented. This method deals with a situation where the standard rate window DLTS is no longer sufficient, i.e., the assumption that the defect density NT is much less than the donor doping density ND is no longer valid. Digitized capacitance transients are numerically fit to extract the electron emission rate, defect density, and energy level. The defect center under study is EL2 in n-type liquid-encapsulated Czochralski gallium arsenide. The fitting method gives an EL2 thermal activation energy of 0.76 eV, different from the 0.82 eV obtained by standard DLTS, which only examines the maximum emission conditions. The advantages, as well as the limitations, of this fitting method are discussed.
Effects of ambient gas on photo-acoustic displacement measurement by laser interferometric probe
Authors: Sumie Shingo , Takamatsu Hiroyuki , Nishimoto Yoshiro , Kawata Yutaka , Horiuchi Takefumi , Nakayama Hiroshi , Kanata Takashi , Nishino Taneo
Published: July 2002

The effect of gas adjacent to the sample surface on the photo-acoustic displacement (PAD) measurement was studied by using an extremely sensitive laser interferometric probe with a sensitivity of 0.1 picometers. For silicon, the PAD signal measured at atmospheric pressure increased about 18% as compared to the signal obtained in vacuum, and varied by less than 0.7% for a change in pressure of 5% around 1 atm. It is shown, by a simple theoretical model, that the variation of the PAD is caused by a change in refractive index of the gas and the real PAD can be accurately obtained by correcting this effect.
The physics of ion impact cathode heating
Authors: Morgan W. L. , Pitchford L. C. , Boisseau S.
Published: July 2002

The heating of a metal surface by ion impact is described by a coupled electron and phonon model that follows from recent research on femtosecond laser heating of metals. The conduction electrons of the metal are directly heated to very high temperature by the impinging ion and then on a longer time scale transfer their energy to the lattice. This results in very efficient heating of the lattice. These processes are described by coupled partial differential equations for the electron and lattice temperatures. The results from three-dimensional numerical solutions of these equations are presented.
Trap filling measurements to determine the crossover point of Fermi level and midgap level in GaAs by constant capacitance technique
Authors: Tomokage Hajime , Furuta Hirosuke , Miyamoto Tokuo
Published: July 2002

Trap filling measurements on the midgap level in GaAs are carried out under constant capacitance conditions in the emission process. The transient voltage after application of a filling pulse is measured with varying capture time. A method to obtain the distance λ1 from the depletion edge to the crossover point of the midgap level and the quasi-Fermi level is proposed for the case of high trap concentration. Fitting the calculated λ1 to the experimental results enables estimation of the capture cross section of electron in the midgap level.
Relief of residual strains in gallium phosphide (100) wafers by cracking
Authors: Yamada Masayoshi
Published: July 2002

Relief of residual strains in GaP(100) wafers by cracking is first presented by means of quantitative photoelastic measurement. It is found that the residual strains are reduced in proportion to the size of cracked piece. Also, the causes of cracking are searched and discussed.
Microstructural and electrical properties of epitaxial PtSi/p-Si(100) co-deposited under ultrahigh vacuum
Authors: Wang L. P. , Yang J. R. , Hwang J.
Published: July 2002

Epitaxial PtSi/p-Si(100) contacts were fabricated by co-depositing Pt and Si on p-Si(100) at 450 °C in a dual electron gun chamber at a base pressure of 10−10 Torr. The PtSi/p-Si(100) film possessed six types of epitaxial modes, as indicated in the electron diffraction patterns. The average grain size of the PtSi grains was measured to be 100–300 Å through the Moiré fringe image. The Schottky barrier height of the epitaxial PtSi/p-Si(100) diode was determined to be 0.249 eV at 100 K, with an ideality factor of 1.04. In contrast to this, the Schottky barrier height of the polycrystalline PtSi/p-Si(100) diode was 0.229 eV at 100 K, with an ideality factor of 1.05. The difference in Schottky barrier height was attributed to the difference in the atomic arrangement at interface.
Modification of GaAs/AlGaAs growth-interrupted interfaces through changes in ambient conditions during growth
Authors: Kopf R. F. , Schubert E. F. , Harris T. D. , Becker R. S. , Gilmer G. H.
Published: July 2002

The effect of the ambient conditions in the growth chamber of the molecular beam epitaxy machine during the growth of GaAs/Al0.35Ga0.65As structures was investigated. Both growth-interrupted (120 s at each heterointerface) and uninterrupted surfaces and interfaces were evaluated using a growth temperature of 580 °C. Two ambient conditions were studied: (a) ∼1×10−10 Torr O2; and (b) ultrahigh vacuum (UHV, ∼5×10−11 Torr, with no intentional introduction of contaminants). A striking difference was observed in both the 1.7 K photoluminescence (PL) spectra of single quantum well (SQW) structures and UHV scanning tunneling microscopy (STM) of surfaces, which were grown under ambient condition (a) as opposed to (b). When consecutive growth-interrupted SQW samples were grown with different well widths (25 and 28 Å) under condition (a), the emission energy splitting into several peaks was observed, indicating discrete thicknesses of the well. However, the peak energies shifted as the laser spot was scanned across each sample. Additionally, the peak energy shifted from sample to sample for the same nominal well width.

On the other hand, when SQW samples were grown under condition (b), no variation in the emission energy was observed as the laser was scanned across the sample, or from sample to sample for a given well width. Furthermore, the PL observations are supported by UHV-STM results. UHV-STM images indicated a very rough surface with large islands containing small terraces on top (a bimodal distribution) for condition (a). Conversely, when samples were grown under condition (b), only large islands were observed. For growth interrupted GaAs surfaces, 400 Å×600 Å islands were observed, and for Al0.35Ga0.65As, they were 150 Å×400 Å, with a one-monolayer step in between islands. These data are consistent with abrupt interfaces with only a single-mode distribution for growth-interrupted surfaces. On the other hand, UHV-STM images of uninterrupted GaAs surfaces grown under condition (b) showed islands that were 40–60 Å across. Photoluminesce spectra of a similarly grown SQW sample showed only a single broad emission line, consistent with an interface configuration of many steps which are smaller than the exciton diameter. The results show that interface roughness is sensitive to background O2.
Deep level transient spectroscopy on focused ion beam written in-plane capacitances
Authors: Bever T. , Pfeiffer G. , Prescha T. , Bohne D. I. , Weber J. , Wieck A. D. , Ploog K.
Published: July 2002

We discuss defects created by focused Ga ion beam implantation in GaAs or AlGaAs/GaAs heterostructures using deep level transient spectroscopy (DLTS). A novel contact configuration which is sensitive to defects located at the boundary between implanted and unperturbed regions at a well-defined depth is presented. The DLTS spectra for these samples are dominated by a peak with an activation energy of Ea=0.38 eV. The results show that this peak is associated with implantation-induced damage independent of the ion species. The defect is also found in a sample with Schottky contacts on top of a Ga-implanted GaAs layer.
Carrier transport in InAs/AlSb/GaSb interband tunneling structures
Authors: Liu M. H. , Wang Y. H. , Houng M. P.
Published: July 2002

A three-band model that considers the coupling effects among the conduction band, light-hole band, and spin-orbit split-off–hole band is used to investigate carrier transport in InAs/AlSb/GaSb interband tunneling structures. The E-k relations and the boundary conditions suitable for the three-band model are derived from the Hamiltonian. Good agreement in the peak current density and peak voltage between experiments and model has been achieved. It is also found that the three-band model shows better agreement in the peak current densities than those of two-band model. It indicates the importance of the coupling effects of the spin-orbit split-off–hole band to the InAs/AlSb/GaSb interband tunneling structures. The valley current components, the key ingredient of the peak-to-valley current ratios, such as the thermionic currents and hole tunneling current, are studied to fit the experimental peak-to-valley current ratios. It is found that the thermionic currents can be neglected due to the large band offset (barrier height). The hole tunneling current, the major part of valley current, decreases with the AlSb barrier thickness. However, deviations from the experiments still exist. Furthermore, the effect of the band bending at the interfaces influences the I-V characteristics.
Feasibility of a high-temperature bolometer based on Bi2Sr2CaCu2O8+x superconducting films
Authors: Bottiglione A. , Epifani M. , Balestrino G. , Milani E. , Montuori M. , Maritato L. , Ruosi A. , Scafuro R.
Published: July 2002

The design of a high-temperature bolometer with large area that uses the resistive transition of a Bi2Sr2CaCu2O8+x superconducting thin film as a sensitive thermometer is discussed. Load curve analysis and discussion of nonthermal behavior are presented. Measurements of the electrical noise voltage spectrum and the analysis of the optical response performed with a chopped He-Ne laser at λ=633 nm gave a zero-frequency responsivity between 1.3 and 2.4 V/W and a noise equivalent power of 1×10−8 W/Hz1/2 in the frequency range 2–10 Hz, with an effective time response of 1.5 s. Measurements of the excess noise voltage for different values of the bias current and temperature are used in order to predict the performances of a suitable bolometric composite architecture.
YBa2Cu3O7−y/Y2BaCuO5 composites: Growth and characterization
Authors: Broussard P. R. , Allen L. H. , Cestone V. C. , Wolf S. A.
Published: July 2002

We have prepared thin film composites of Y1Ba2Cu3O7−y (‘‘123’’) and Y2Ba1Cu1O5 (‘‘211’’) by off-axis sputtering from separate targets of the 123 and 211 material. X-ray diffraction on the films shows the presence of c-axis oriented 123, and (00L) ordered Y2O3, but no indication of 211 lines. The c-axis lattice constant does not show any change compared to our pure 123 films. As the volume percentage of 211 material increases, we see a reduction in Tc and Jc, and an increase in the resistivity. The temperature and field dependence of the critical current are different for our composites compared to our pure 123 films.
Response to ‘‘Comment on ‘Evaluation of the minority carrier diffusion length and edge surface-recombination velocity in GaAs p/n solar cells’ ’’ [J. Appl. Phys. 74, 3620 (1993)]
Authors: Hakimzadeh Roshanak , Möller Hans J. , Bailey Sheila G.
Published: July 2002

This reply to Dr. Luke’s comment on our findings [J. Appl. Phys. 72, 2919 (1992)] explains the intent of our work and answers the questions raised.
Morphological analysis of discontinuous thin films on various substrates
Authors: Blacher S. , Brouers F. , Gadenne P. , Lafait J.
Published: July 2002

We study and characterize the morphology of granular gold films deposited on amorphous and polycrystalline substrates using some of the more efficient tools of the mathematical morphology. This analysis shows the occurrence of characteristic lengths in the morphology which are strongly dependent on the nature of the substrate. This implies that a completely random model such as the theory of percolation can only be considered as a first approximation to interpret the dielectric and optical properties of granular thin films.
Various bonding forms of OH groups in hydrogen-treated silica
Authors: Kitagawa Manabu , Nishikawa Hiroyuki , Ohki Yoshimichi , Hama Yoshimasa
Published: July 2002

The bonding state of OH groups in silica glasses is studied by examining infrared absorption spectra and thermal annealing characteristics of γ-induced paramagnetic detect centers. When an oxygen-rich silica is treated by hydrogen, OH groups exist in the form of those hydrogen-bonded with nonbridging oxygen (≡Si–O−--- –H–O–Si≡) besides the forms of H2O and ≡Si–O–H.
Influence of Ca on the irreversibility line of YSr2Cu2.67Ti0.33O6+z
Authors: Suryanarayanan R. , Leelaprute S.
Published: July 2002

Preparation, x-ray diffraction, and ac susceptibility measurements of Y1−xCaxSr2Cu2.67Ti0.33O6+z are presented. Tc increases from 18 K for x=0 to 22 K for x=0.1 and then decreases to 18.5 K for x=0.15. The irreversibility line derived from the susceptibility data indicates an enhancement of x=0.05.
A thin-film second-order gradiometer with integrated dc-SQUID
Authors: Carelli P. , Castellano M. G. , Chiaventi L. , Leoni R. , Cirillo M. , Modena I.
Published: July 2002

We have fabricated and characterized a superconducting planar gradiometer sensitive to the second-order spatial derivative of the magnetic field. Our device has been patterned on a 12.8×12.8 mm silicon chip on which the sensitive area of the gradiometer and the SQUID inductance are generated by the same four 0.5-mm-square holes. We measured the sensitivity both in flux-locked-loop (FLL) and in open-loop mode. With the second method we used another dc-SQUID as amplifier and in this case we obtained the best noise performance. The gradiometer sensitivity was 18 fT/cm2 √Hz. The magnetic isoflux line distribution generated by a dipolar source was measured by the gradiometer in FLL mode.
Low hydrogen content silicon nitride films from electron cyclotron resonance plasmas
Authors: Flemish J. R. , Pfeffer R. L.
Published: July 2002

The material and electrical properties of SiNx:H films deposited using a 2% SiH4/N2 mixture with additional N2 in an electron cyclotron resonance reactor have been evaluated. Deposition rate, refractive index, and stoichiometry have been determined using ellipsometry, Rutherford backscattering spectrometry, and infrared spectroscopy. Current-voltage and dielectric breakdown characteristics have been measured on metal-insulator-silicon structures. Stoichiometric material with a hydrogen content of 1.5 at. % is created using a ratio of SiH4/N2=0.003, P=2 mTorr, T=250 °C, and power=650 W. Hydrogen levels are reduced by using lower ratios of SiH4/N2, lower total pressure, or higher microwave power. Higher total pressure results in significantly enhanced deposition rates, but with greatly increased H and O content. The low-field resistivity of these films is largely independent of the process parameters over the range investigated. The dielectric breakdown strength is significantly greater in films deposited at higher temperature, but is somewhat degraded in films deposited at lower ratios of SiH4/N2, despite the higher density and lower hydrogen content of these films.
Ultrafast relaxation of hot minority carriers in p-GaAs
Authors: Alencar A. M. , Sampaio A. J. C. , Freire V. N. , da Costa J. Alzamir P.
Published: July 2002

The dynamics of hot minority carriers in p-GaAs is calculated for doping concentrations in the range of 1.5×1017 cm−3 to 1.5×1018 cm−3. It is shown that the electron-hole interaction increases the rate of dissipation of the excess energy of the minority carriers in the early stages of the process. However, this channel for energy dissipation becomes weaker as the cooling of the minority carriers proceeds, an effect more noticeable in the case of high doping levels. When the electron-hole interaction is disregarded, the dissipation rate is always smaller for low doping concentrations.
Circular magnetic x-ray dichroism in rare-earth magnets
Authors: Wang Xindong , Antropov V. P. , Harmon B. N. , Lang J. C. , Goldman A. I.
Published: July 2002

With the availability of dedicated high-intensity synchrotron radiation sources, circular magnetic x-ray dichroism (CMXD) is developing as a technique for the investigation of magnetic properties. CMXD is defined as the difference in absorption between right and left circularly polarized beams at an absorption edge and provides information about the local (element and angular momentum specific) spin and orbital magnetic moments. Both theoretical and experimental work on the CMXD spectra of rare-earth magnets is presented. The nature of the feature below the absorption edge in the rare-earth CMXD spectra for the L2 and L3 edges is discussed.
Resonant photoemission and magnetic x-ray circular dichroism in the M shell of ultrathin films of Fe
Authors: Tobin J. G. , Waddill G. D.
Published: July 2002

Using magnetic ultrathin films (2–4 monolayers) of Fe on Cu(001) and bulklike Fe, the Fe 3p and Fe 3s core states have been investigated with resonant photoemission and core-level photoemission, including magnetic x-ray circular dichroism (MXCD) experiments. The resonant photoemission experiment has been done, to probe the parentage of various spectral structures. All the observations can be explained as simply due to the presence of Auger features: It is not necessary to invoke configuration interaction combined with strong photoemission resonance.
Structural and magnetic properties of Co/Cr(001) superlattices
Authors: Donner W. , Zeidler T. , Schreiber F. , Metoki N. , Zabel H.
Published: July 2002

Epitaxial Co/Cr(001) superlattices were prepared by Molecular-Beam-Epitaxy (MBE). The crystal structure was determined both with Reflection High Energy Electron Diffraction (RHEED) and x-ray diffraction experiments. The Co layers grow in an hcp phase with their (11.0) planes oriented parallel to the Cr(001) planes. The magnetic anisotropy was measured using the Magneto-optical Kerr Effect (MOKE) and Ferromagnetic Resonance (FMR) technique. It is found that the easy axis of the magnetization is perpendicular to the film plane below a Co thickness of about 14 Å. The value of the interface anisotropy constant Ks is determined to (0.85±0.1)erg/cm2 assuming the bulk magnetization of hcp Co.
Electronic structure and Curie temperature of YFe12−xMoxNy compounds
Authors: Fernando A. S. , Woods J. P. , Jaswal S. S. , Welipitiya D. , Patterson B. M. , Sellmyer D. J.
Published: July 2002

The electronic structures of YFe12−xMoxNy , where x=1.0, 2.0 and y=0, 0.7, have been studied with photoemission and spin-polarized calculations. The peak near the Fermi level in the energy distribution curves (EDC) becomes successively broader with larger Mo concentration. The features in the calculated density of state at 1.3 and 2.7 eV are not readily seen in the EDC, and this may be due to lifetime effects in these compounds. Finally, changes in Curie temperature (Tc) with the change of N or Mo concentration are compared with prediction of the theory of Mohn and Wohlfarth. Reasonable agreement is obtained in the N case but not in the Mo case, the latter most likely due to hybridization of Fe and Mo d bands.
Photoinduced disaccommodation of magnetic permeability in yttrium iron garnet (abstract)
Authors: Matsubara I. , Hisatake K. , Maeda K.
Published: July 2002

After illuminating a single crystal of yttrium iron garnet grown by a modified floating zone method at 77 K, the complex permeability was measured from 77 to 300 K under low alternating field. As well as the decreasing of permeability and the appearance of a peak in μ″-T curve, disaccommodation (DA) or the change of permeability with time was found to be induced around 140 and 210 K. As the result of the superposition of disaccommodation, the peak in the μ″-T curve is observed to be divided. These photoinduced magnetic effects cannot be attributed to a single origin, for only photoinduced DA disappears in the process of warming. The origins are discussed and a model is proposed, taking account of the mobility of oxygen vacancy depending on the charge state and the photoexcited electron.
Collective magnetic excitations of Ho3+ ions in grain-aligned HoBa2Cu3O7
Authors: Staub U. , Fauth F. , Guillaume M. , Mesot J. , Furrer A. , Dosanjh P. , Zhou H. , Vorderwisch P.
Published: July 2002

Inelastic neutron scattering has been employed to study the magnetic excitations of Ho3+ in a grain-aligned sample of HoBa2Cu3O7. The lowest lying branch exhibits dispersion for spin waves propagating in the (a,b) plane, whereas excitations along the c axis remain constant in energy. The data are analyzed in the random phase approximation in terms of the Heisenberg and an anisotropic model, resulting in an enormous spatial anisotropy of the magnetic coupling parameters, which are weakly ferromagnetic, antiferromagnetic, and almost zero for nearest-neighbor Ho3+ pairs along the a, b, and c axes, respectively. In addition, we have been able to separate the spin wave excitations in longitudinal and transverse parts.
Raman spectra of two-dimensional spin-1/2 Heisenberg antiferromagnets
Authors: Haas Stephan , Dagotto Elbio , Riera Jose , Merlin Roberto , Nori Franco
Published: July 2002

The Raman spectrum of two-dimensional spin-1/2 Heisenberg antiferromagnets is calculated by exactly diagonalizing clusters of up to 26 sites. The obtained spectra are compared to experimental results for various high-Tc precursors, such as La2CuO4 and YBa2Cu3O6.2. In spite of good agreement in the position of the main excitation in the B1g channel, i.e, the two-magnon peak around 0.4 eV, an additional mechanism has to be invoked to account for the broad and asymmetric shape of the overall spectrum. Here, we consider the phonon-magnon interaction which, in a quasistatic approximation, renormalizes the Heisenberg exchange integral. This mechanism is motivated in part by recent experimental observations that the Raman linewidth broadens with increasing temperature. Our results are in good agreement with Raman scattering experiments performed by various groups; in particular, the calculations reproduce the broad line shape of the two-magnon peak, the asymmetry about its maximum, the existence of spectral weight at high energies, and the observation of nominally forbidden A1g scattering.
Theory of two-dimensional antiferromagnets with a nearly critical ground state (abstract)
Authors: Chubukov Andrey V. , Sachdev Subir
Published: July 2002

We present the general theory of clean, two-dimensional, quantum Heisenberg antiferromagnets which are close to the zero-temperature quantum transition between ground states with and without long-range Néel order. For Néel-ordered states, ‘‘nearly-critical’’ means that the ground state spin stiffness, ρs, satisfies ρs≪J, where J is the nearest-neighbor exchange constant, while ‘‘nearly-critical’’ quantum-disordered ground states have an energy gap, Δ, towards excitations with spin-1, which satisfies Δ≪J. The allowed temperatures, T, are also smaller than J, but no restrictions are placed on the values of kBT/ρs or kBT/Δ. Under these circumstances, we show that the wave vector /frequency-dependent uniform and staggered spin susceptibilities, and the specific heat, are completely universal functions of just three thermodynamic parameters. On the ordered side, these three parameters are ρs, the T=0 spin-wave velocity c, and the ground state staggered moment N0. Explicit results for the universal scaling functions are obtained by a 1/N expansion on the O(N) quantum nonlinear sigma model. These calculations lead to a variety of testable predictions for neutron scattering, NMR, and magnetization measurements. Our results are in good agreement with a number of numerical simulations and experiments on undoped and lightly doped La2−δSrδCuO4.
Magnetic phase diagram of (Tb1−xGdx)Fe11Ti alloys
Authors: Nagamine L. C. C. M. , Rechenberg H. R. , Algarabel P. A. , Ibarra M. R.
Published: July 2002

The spin reorientation transition (SRT) in (Tb1−xGdx)Fe11Ti alloys has been studied by using susceptibility, angular-dependent magnetization, and Mössbauer measurements. Alloys with x<0.4 are shown to undergo an axial-to-planar SRT on cooling, while alloys with x≥0.4 remain axial down to 4.2 K. TbFe11Ti has an easy-axis magnetization at room temperature. A theoretical magnetic phase diagram for the (Tb1−xGdx)Fe11Ti system has been calculated, showing reasonable agreement with experiment.
Effect of Sm substitution on structure and magnetic properties of high-carbon Er2Fe17Cy compounds
Authors: Shen Bao-gen , Cao Lei , Kong Lin-shu , Ning Tai-shan , Hu Ming
Published: July 2002

The carbides (Er1−xSmx)2Fe17Cy (0≤x≤0.8) with a carbon concentration up to y=3.0 for the Er-rich region and up to y=1.5 for the Sm-rich region have been successfully prepared by melt spinning. They are found to crystallize in the hexagonal Th2Ni17-type or the rhombohedral Th2Zn17-type structure and to be stable at high temperature. The introduction of the interstitial carbon results in a large increase in the unit-cell volumes and Curie temperatures. The room-temperature saturation magnetization is found to increase with increasing carbon or samarium concentrations. X-ray diffraction studies of magnetically aligned powder show that the samples with x<0.2 or y≤0.5 exhibit easy plane anisotropy while the samples with x≥0.2 and y≥1.0 exhibit easy c-axis anisotropy at room temperature. For (Er0.2Sm0.8)2Fe17C1.5, the room-temperature anisotropy field is found to be 8 T, which is comparable to that of Nd2Fe14B.
Influence of electron damping and reservoir on the magnetic phase diagram of chromium alloys
Authors: Fishman R. S. , Liu S. H.
Published: July 2002

The magnetic phase diagram of chromium alloys sensitively depends on both electron damping and the presence of an electron reservoir. If the damping energy Γ vanishes and the power ρ of the reservoir is infinite, then lightly doped CrMn alloys would experience a first-order transition from an incommensurate (I) to a commensurate (C) spin-density wave (SDW) state with decreasing temperature. Either damping or a finite reservoir may flip the phase boundary from one side of the triple point to the other, allowing a commensurate-to-incommensurate transition with decreasing temperature as observed experimentally. Both damping and a finite reservoir suppress the first-order jumps in the SDW order parameter and wave vector. When ρ≤2, the C-I transition is second order for all temperatures. When ρ≳2, the transition is second order near the tricritical point but first order at lower temperatures. Unlike electron damping, an electron reservoir does not shift the paramagnetic phase boundary and triple point.
The durability of new rigid disks by molded plastic substrates (abstract)
Authors: Morita Osamu , Sano Takanobu , Imai Yasuyuki , Hisayama Naoko , Takino Hiroshi
Published: July 2002

Media that have discrete data tracks and servo pits for rigid disk systems have been developed.1 Discrete data tracks and servo pits are embossed on plastic substrates by a stamping technique similar to that used in optical disk technology. The outer diameter of the substrates is 65 mm, and the thickness is 1.2 mm. To enhance the durability of contact-start-stop (CSS) environment and reduce glide height, we have developed a method of texturing the surface with a fine dispersion of SiO2 particles, whose diameters are between 10 and 40 nm, by dipping. Thin film layers are then sputtered on the substrates, and the shapes of SiO2 particles are reflected in the surface profile of the thin film layers. The initial friction coefficient (μi) is 0.46, and the friction coefficient after 50 000 CSS events is 0.56 for CSS tests on molded plastic substrates at a radius of 28 mm. In addition, glide height depends on surface roughness of the disks. Surface roughness of a glass template used on stampers is carried over to the molded plastic substrates. Therefore the arithmetic average (Ra) of the molded plastic substrates with SiO2 particles is 1.5 nm. The peak-to-valley height (PV) is 20 nm, reducing the glide height to less than 65 nm. Simultaneously, with these media, a slider flying height of 0.1 μm has been achieved. Therefore, the media satisfy parameters of CSS environment, glide height, and flying height for rigid disks.
Magnons in ferromagnetic terbium under high pressure
Authors: Kawano S. , Fernandez-Baca J. A. , Nicklow R. M.
Published: July 2002

We have performed an inelastic neutron scattering experiment in order to investigate the pressure dependence of the magnon dispersion of terbium in its ferromagnetic phase. Our measurements were performed along the crystal c axis at 90 K (well below the Curie temperature Tc=220 K) at ambient pressure, and at 4.3 and 15.2 kbar of applied hydrostatic pressure. The difference between the magnon dispersion curves at ambient pressure and at 4.3 kbar is small, while at 15.2 kbar the dispersion curve shifts appreciably towards higher energies. The measured magnon dispersion curves have been analyzed using a Hamiltonian that includes Heisenberg exchange as well as single-ion anisotropy terms. For each dispersion curve we have calculated five exchange constants and two anisotropy terms. The energy gap at q=0 due to the anisotropies is enhanced with pressure and exchange interactions acting between c planes appear to decrease with the application of high pressure.
Phase transitions in bubble lattice under temperature lowering (abstract)
Authors: Gerasimchuk V. S. , Gorobets Yu. I. , De Ville K.
Published: July 2002

It is usually considered that bubbles forming a close-packed hexagonal bubble lattice have equal diameters. It turned out that bubbles of different diameters can exist in a certain temperature interval in zero bias field in bubble lattices. With temperature-lowering one of them collapses, and others exist as a lattice-like graphite-type lattice. In the present paper theoretical grounds of the experimental results are given. Energy of a lattice containing bubble of different diameters is calculated. Variations of full energy in bubble diameters give the system of equations. Joint solution of the system with appropriate conditions of stability characterize stability of a bubble lattice in respect to collapse. The conditions of loosing of stability determine critical points of (1) collapse of a lattice with bubbles of equal diameters;1 (2) structural phase transitions under which hexagonal bubble lattice transforms into lattice with bubbles of different diameters. Diameters of bubbles situated in the center of a hexagonal cell are shown to decrease with lowering temperature, while bubble diameters situated in junctions of a cell increase. On achieving a certain critical size the former collapses and the latter increases its size by a sudden leap. Further bubble lattices such as graphite-type lattices exist. Conditions of existence and collapse of the latter lattice are reviewed.
An analysis of the high-temperature relaxation in polycrystalline magnetite
Authors: Castro J. , Martinez D. , Rivas J. , Blythe H. J.
Published: July 2002

Magnetic relaxation measurements were performed in the temperature range 200<T(K)<800 on vacancy-doped polycrystalline magnetite. Important relaxation peaks have been observed in the temperature ranges 250<T(K)<350 (peak III) and 400<T(K)<600 (peak I). Particular attention has been focused on the behavior of peak I where isothermal measurements have extended over times up to 105 s. It is argued that the main features of peak I are compatible with a dislocation relaxation.
Hydrogenation decomposition desorption recombination magnets based on Sm2+δ Fe17M0.4 carbonitrides (M=IVB/VB/VIB group elements)
Authors: Chen X. , Altounian Z.
Published: July 2002

The hydrogenation decomposition desorption recombination (HDDR) process is carried out to produce permanent magnets based on Sm2+δFe17M0.4 carbonitrides, where M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W; and δ≤0.6. It is found that minor additions of M significantly reduce the amounts of α-Fe and SmFe3 in the starting materials. The best isotropic HDDR magnet obtained is for M=Ti, which has an intrinsic coercivity of 18 kOe and an energy product of 9.8 MGOe at room temperature.
An improved method for magnetic flux density visualization using three-dimensional edge finite element method
Authors: Čingoski Vlatko , Yamashita Hideo
Published: July 2002

Visualization of a magnetic flux density distribution in three-dimensional (3-D) finite element analysis (FEA) is very important in order to grasp the real behavior of the magnetic field, especially in the design process. In this paper, we present an improved method for visualizing magnetic flux density distributions calculated by 3-D first-order edge FEA. Compared with traditional methods, this method provides more accurate, smoother magnetic flux density distribution inside the analyzed region and satisfies the proper boundary conditions across intermaterial boundaries. The usefulness of our algorithm is demonstrated with several examples.