bluuebunny/crossref_metadata_2025 huggingface dataset:
Filamentation instability of an electromagnetic wave in an expanding plasma
Authors: Ghanshyam  , Tripathi V. K.
Published: July 2002

A laser beam propagating at an angle to the density gradient in an expanding plasma is susceptible to filamentation instability. For a linear density profile and self consistent temperature and expansion velocity profiles, the amplitude of the perturbation grows as an Airy function with the distance of propagation. The characteristic growth length decreases with the size of perturbation and the ratio of expansion velocity to sound velocity. It increases the angle laser k vector makes with the density gradient. Moreover, the growth length does not always increase with θ0: it depends on vb(0). The spatial growth of perturbation of larger size starts occurring deeper into the plasma.
Crystallization of small and separated magnetic particles of Nd2Fe14B alloy
Authors: Ram S. , Joubert J. C.
Published: July 2002

Well-separated magnetic particles (∼1 μm size) of Nd2Fe14B alloy are successfully crystallized using an oxygen-reduction-diffusion process. In this process suitable mixtures of Nd2O3, Fe, and B precursor powders were sintered (at 900–1000 °C for 1–3 h) with finely dispersed calcium granules (reducing agent) and anhydrous CaCl2 under a pure argon atmosphere. The crystallization of promisingly small alloy particles in this reaction is analyzed by studying the microstructure and by x-ray diffractometry of reaction products before and after washing away the oxidized calcium and other by-product impurities using fresh water. It is shown that during the crystallization process, the alloy crystals grow in a matrix of oxidized calcium with CaCl2 and possibly unoxidized calcium left in the reaction (if the reaction takes place with an excess calcium). Since oxidized calcium is a thermally highly rigid material (mp∼2580 °C), it controls large-grain growth of the alloy by slowing down diffusion of the reaction species on sintering at such a low temperature Ts≤950 °C for a few hours. When washed with water, oxidized calcium is dissolved with other by-product impurities and results in a loose powder of separated crystallized Nd2Fe14B alloy particles.
Electronic structure and magnetism of YFe11Ti
Authors: Sakuma Akimasa
Published: July 2002

Self-cosistent band calculations for YFe11Ti have been performed with LMTO-ASA method in the frame of local spin density functional formalism. The electronic total energy calculations predict that the system is most stable when the Ti atoms are located at the 8f sites, while experimental results suggest the 8i sites for the Ti atoms. On the other hand, the calculated equilibrium volume of the unit cell exhibits fair agreement with the measured value. It is also shown that the unit cell volume increases with the Ti concentration, X in YFe12−XTiX, with a rate ΔV/ΔX=10 (Å3/unit cell), and the magnetic moment decreases with X with a rate ΔM/ΔX=−4μB. The total magnetic moment of YFe11Ti (X=1) is given as about 21.7μB. With a inclusion of the spin–orbit interaction in the LMTO Hamiltonian, the orbital magnetism and the magnetocrystalline anisotropy were further examined. The orbital magnetic moment per Fe atom is about 0.06μB and the anisotropy energy is obtained as 97μRy/f.u. The value leads to about 12.1×106 erg/cc, which is about the half of the measured value, 23×106 erg/cc at 1.5 K.
Modeling the influence of intergranular phases on the hysteretic behavior of hard magnetic polycrystals
Authors: González J. M. , Cebollada F. , Hernando A.
Published: July 2002

We have modeled, in the framework of a micromagnetic approximation, the field evolution of the configuration of magnetic moments induced by the occurrence of interfacial exchange coupling at grain boundary regions. The models describe hard ferromagnetic polycrystals where secondary intergranular phases are present. Depending on the paramagnetic or soft ferromagnetic nature of the secondary phases, on the exchange to anisotropy ratio of the main phase and on the intensity of the exchange coupling at the interfaces the reversal, from a remanence state, of the magnetization of a linear chain of magnetic moments perpendicular to the interface takes place for demagnetizing fields which vary from the anisotropy field of the hard phase (grains decoupled by a thick paramagnetic layer) down to tenths of that field (coupling through a soft layer). The dependence of these phenomena on the thickness of the intergranular phases has been also examined.
Magnetic properties and crystal texture of Co alloy thin films prepared on double bias Cr
Authors: Deng Y. , Lambeth D. N. , Lee L.-L. , Laughlin D. E.
Published: July 2002

A double layer Cr film structure has been prepared by sputter depositing Cr on single crystal Si substrates first without substrate bias and then with various substrate bias voltages. Without substrate bias, Cr{200} texture grows on Si at room temperature; thus the first Cr layer acts like a seed Cr layer with the {200} texture, and the second Cr layer, prepared with substrate bias, tends to replicate the {200} texture epitaxially. CoCrTa and CoNiCr films prepared on these double Cr underlayers, therefore, tend to have a {112̄0} texture with their c-axes oriented in the plane of the film. At the same time, the bias sputtering of the second Cr layer increases the coercivity of the subsequently deposited magnetic films significantly. Comparison studies of δM curves show that the use of the double Cr underlayers reduces the intergranular exchange interactions. The films prepared on the Si substrates have been compared with the films prepared on canasite and glass substrates. It has also been found that the magnetic properties are similar for films on canasite and on glass.
DyAlO3: A more promising refrigerant than Dy3Al5O12?
Authors: Kuz’min Michael D. , Tishin Alexander M.
Published: July 2002

A simplified mean-field calculation of the magnetic entropy change for the Ising-like rare-earth magnets DyAlO3 and Dy3Al5O12 is performed, showing DyAlO3 to be a more promising working material for the 4.2–20 K range Carnot-type magnetic refrigerators than the conventional refrigerant Dy3Al5O12.
A crystallographic methodology for modeling dislocation dynamics in GaAs crystals grown from melt
Authors: Tsai C. T. , Gulluoglu A. N. , Hartley C. S.
Published: July 2002

The objective is to develop a quantitative model for simulating dislocation motion and multiplication in gallium arsenide (GaAs) crystals during growth from the melt by taking the crystallography of slip into account. A constitutive model that couples microscopic dislocation motion and multiplication to macroscopic plastic deformation is employed for developing the model. The density of dislocations in the crystal is predicted by assuming the crystal to be grown under quasi-steady-state conditions. The thermoelastic stresses are calculated from a two-dimensional finite-element analysis, and then transformed into the resolved shear stresses in each slip system for the simulation of dislocation motion and multiplication. A numerical example is presented to verify the validity of the model. The calculated distribution of dislocation density on (001) GaAs wafer shows fourfold symmetry which is consistent with etch pit observations made by Jordan, Caruso, and Von Neida [Bell Syst. Tech. J. 59, 593 (1980)]. Although the emphasis is placed on GaAs, the model can be applied to other electronic and photonic materials.
Structure and magnetic properties of Nd6Fe13Cu
Authors: Knoch K. G. , Le Calvez A. , Qi Qinian , Leithe-Jasper A. , Coey J. M. D.
Published: July 2002

A new phase δ in the ternary system Nd-Fe-Cu is identified as Nd6Fe13Cu. X-ray powder diffraction patterns of a sample annealed at 540 °C for 1000 h are indexed on a tetragonal unit cell with a=8.09 Å and c=22.26 Å. The Curie temperature Tc is 190±5 °C and the small net magnetization at 4.2 K of 18 mT (2 J T−1 kg−1) after correcting for the presence of 1.5% Nd2Fe17 suggests a ferrimagnetic spin structure. There is a spin rearrangement in a field of 1.5 T at 4.2 K and a metamagnetic transition is also observed on hydrogenation. Mössbauer spectra at 15 K are fitted with four subspectra with hyperfine fields of 36.9, 33.2, 30.6, and 23.0 T. X-ray measurements on oriented powder indicate that the easy direction of the net magnetization lies along the c axis, but Mössbauer spectra show that none of iron sublattices have their magnetization aligned along c.
Effect of the metal film thickness on surface-plasmon-enhanced Raman scattering in an attenuated total reflection experiment
Authors: Primeau N. , Coutaz J. L. , Abello L.
Published: July 2002

We present the measured dependence of the Raman intensity on the thickness of the metal film in a Kretschmann geometry when surface plasmons are excited. To perform our work, we deposited a Ag film of varying thickness onto a prism, and then we coated it with a thin layer of copper phthalocyanine. We show that the maximum of the Raman signal is achieved for an overall optimization of the coupling at both pump and Stokes frequencies, demonstrating the simultaneous excitation of surface plasmons at these two frequencies.
The energetics of dislocation array stability in strained epitaxial layers
Authors: Gosling T. J. , Willis J. R. , Bullough R. , Jain S. C.
Published: July 2002

Two aspects of the energetics of dislocation array stability in lattice-mismatched strained layers are addressed. The first concerns criteria for determining equilibrium dislocation distributions in strained layers; the second concerns the difference between the energies of arrays of dislocations in which the Burgers vectors of all dislocations are identical, and those in which the screw components of the Burgers vectors alternate. The conclusions reached are at variance with those of recent work by Feng and Hirth on periodic arrays of dipoles in an infinite body [X. Feng and J. P. Hirth, J. Appl. Phys. 72, 1386 (1992); J. P. Hirth and X. Feng, J. Appl. Phys. 67, 3343 (1990)]. In particular, it is emphasized that if layers remain in equilibrium then there is always a residual mean strain; in other words, the mismatch strain is never completely relaxed. Also it is shown, via a direct calculation, that although alternating the screw components of the Burgers vectors of dislocations within a single array is energetically favorable, it is preferable to have all screw components of the same sign within an array if two orthogonal arrays are considered. Although for comparison with the work of Feng and Hirth arrays of dipoles in an infinite body are considered in more detail, the stated conclusions are also shown to hold for arrays of unpaired dislocations near a free surface.
Expansion and electron emission characteristics of a hollow-cathode plasma contactor
Authors: Parks Donald E. , Katz Ira , Buchholtz Brett , Wilbur Paul
Published: July 2002

A theory is presented to explain the observed electron emission characteristics of a hollow-cathode-based plasma source. The theory is compared with measurements made in a laboratory vacuum facility and is used to predict emission characteristics when the source plasma expands into an ambient space plasma. Crucial to understanding the observed emission current-voltage characteristic of hollow-cathode-type devices is the recognition of the role of emission current, not just the current in the main discharge circuit, in ionization of the neutral gas flowing through the device. This ionization can lead to breakdown of the sort that is familiar in many gas discharge devices. Equally crucial to understanding the low impedance capability of the device in coupling spacecraft to an ambient space plasma is recognition of the role of escaping as well as trapped source electrons in the formation of the space potential profile. The presence of these electrons, according to the theory, results in space electron emission current-voltage characteristics which are well approximated by those observed in laboratory chambers.
Single crystalline GaSe/WSe2 heterointerfaces grown by van der Waals epitaxy. II. Junction characterization
Authors: Lang O. , Tomm Y. , Schlaf R. , Pettenkofer C. , Jaegermann W.
Published: July 2002

The junction properties of GaSe/WSe2 heterointerfaces grown by van der Waals epitaxy have been characterized by photoelectron spectroscopy and surface photovoltage measurements (SPV). The surfaces of p-WSe2 substrates doped with Se excess convert to n doping during annealing at T≥720 K, leading to a SPV of 330 mV. Deposited p GaSe forms an n-p heterodiode opposing the p-n homodiode within the substrate. Promising results are obtained for n-WSe2/p-GaSe heterointerfaces with SPV of at least 0.3 eV. The valence band and conduction band offsets are 0.6 and 0.2±0.1 eV, respectively, in accordance with the Anderson model of heterojunction formation.
Spin-valve structures exchange biased with a-Tb0.23Co0.77 layers
Authors: Freitas P. P. , Leal J. L. , Plaskett T. S. , Melo L. V. , Soares J. C.
Published: July 2002

Spin-valve structures, glass/NiFeI/Cu/NiFeII/a-TbCo/Cu, were prepared where the pinned Permalloy layer is exchanged biased by a 200 to 400 Å thick a-Tb0.23Co0.77 layer. Exchange fields between 50 and 250 Oe were achieved with TbCo thicknesses below 400 Å, for a pinned Permalloy layer 150 Å thick. The exchange fields are strongly dependent on substrate bias. The magnetoresistance of these structures reaches 4.5% when thin Co layers are added at the NiFe/Cu interfaces.
Analytical transport cross section of medium energy positrons elastically scattered by complex atoms (Z=1–92)
Authors: Dapor Maurizio
Published: July 2002

Transport positron-atom elastic scattering cross sections have been computed for kinetic energies in the range 500–4000 eV. The phase shifts have been calculated by numerically solving the Dirac equation for a central electrostatic field up to a large radius in which the atomic potentials are negligible. Atomic potentials were that of Hartree–Fock for Z≤18 and of Dirac–Hartree–Fock–Slater for Z≳18. An analytical expression depending on three parameters, obtained by fitting the numerical results, is proposed for the energetic range examined to calculate transport cross sections for positron-atom elastic scattering in the atomic number range 1–92. The accuracy of the given expression for energies lower than 500 eV and greater than 4000 eV has been discussed by comparison with other tabulations.
Room-temperature photoreflectance and photoluminescence of heavily Si-doped GaAs
Authors: Lee Chul , Lee Nam-Young , Lee Kyu-Jang , Kim Jae-Eun , Park Hae Yong , Kwak Dong-Hwa , Lee Hee Chul , Lim H.
Published: July 2002

Room-temperature photoreflectance (PR) and photoluminescence (PL) spectra in heavily n-doped GaAs were compared. It was found that for highly degenerate semiconductors the critical energy measured by the PR equals to the peak energy of the PL spectrum. When Fermi level lies below the conduction-band minimum, the PR spectra revealed the band-gap energy as well as the energy Emax at which the electron concentration per unit energy in the donor band becomes maximum, and this maximum was observed to merge in the conduction-band at about 3×1017 cm−3 electron concentration.
Improvement of microstructure of amorphous silicon–germanium alloys by hydrogen dilution
Authors: Middya A. R. , Ray Swati , Jones S. J. , Williamson D. L.
Published: July 2002

The microstructures of two sets of hydrogenated amorphous silicon–germanium (a-Si1−xGex:H) alloys prepared by the plasma-enhanced, chemical-vapor-deposition technique with and without hydrogen dilution of the source gases (silane and germane) have been analyzed by small-angle x-ray scattering (SAXS), infrared vibrational spectroscopy, and flotation density measurements. Optoelectronic properties of codeposited films have also been characterized. Hydrogen dilution suppresses dihydride/polyhydride formation, reduces bonded H content, and reduces the SAXS-detected microstructure for x≳0. Studies of anisotropy in the SAXS intensity indicate an increased amount of oriented microstructure as Ge is added, consistent with a trend toward columnarlike growth in both undiluted and hydrogen-diluted films, but the diluted films have a significantly reduced degree of such oriented microstructure. The improvement in the microstructure of a-Si1−xGex:H films by H2 dilution correlates with concomitant improvement of optoelectronic properties. The modification of microstructure due to H2 dilution of the source gases is discussed in terms of growth mechanisms of alloy films.
Phase relations in the system of (Na1/2Bi1/2)TiO3–PbTiO3. II. Dielectric property
Authors: Hong Kug Sun , Park Seung-Eek
Published: July 2002

The region of the phase diagram in the system (Na1/2Bi1/2)1−xPbxTiO3 from x=0 to 0.2 was established using dielectric measurements along with the phase diagram established in part I of this work. The diffuse phase transition behavior of solid solutions with low PbTiO3 content was also investigated. When x is lower than xrc (composition at which rhombohedral↔cubic transition exists) the rhombohedral↔tetragonal transition accompanied an anomaly in the ε(T) curve which shows an abrupt jump, followed by diffuse behavior of the ε(T) curve during heating. When xrc<x<xMPB (rhombohedral–tetragonal morphotropic phase boundary composition at room temperature) the rhombohedral↔tetragonal transition was a normal ferroelectric-to-ferroelectric transition but the tetragonal↔cubic transition showed diffuse behavior following the abrupt jump in the ε(T) curve. The temperature of the structural change coincided with the abrupt jump in ε(T) curve. Normal ferroelectric phase transition from ferroelectric tetragonal to paraelectric cubic was found when x≳xMPB. It was found that the temperature range of the reported antiferroelectric phase would follow that of the diffuse transition range regardless of the crystal symmetry. It can be said that the antiferroelectric phase of (Na1/2Bi1/2)1−xPbxTiO3 is strongly connected with the diffuse transition behavior during ferroelectric phase transition.
Effect of Knudsen-layer formation on the initial expansion and angular distribution of a laser-produced copper plasma at reduced pressure of air
Authors: Pietsch W.
Published: July 2002

The initial expansion of a copper plasma generated at reduced pressure (20 mTorr air) by XeCl excimer laser irradiation of 308 nm has been studied by fast intensified charge-coupled device photography of the overall visible plume emission with a time resolution of 3 ns. The spatial distribution and its temporal development along the target normal as well as in a plane parallel to the sample surface were measured in a single laser shot (28 ns). We report the first in situ measurement and observation of a one-dimensional plasma expansion with very small divergence during the initial stage (δt<150 ns) at reduced pressure for low laser fluence (5.4 J cm−2). The related angular distribution of ablated particles has a cosn Θ form where n≳36 for δt≤150 ns and Θ<20°. The observed effect corresponds to a highly collisional plasma, where strongest forward peaking beyond the Knudsen-layer edge takes place. For times greater than 150 ns, the plasma switches into a three-dimensional expansion corresponding to an effusion model with recondensation. A nearly constant center-of-mass velocity in the range of 6×103 m s−1 has been observed. A particle velocity at the surface of uk=2×103 m s−1, in good agreement with the theory of unsteady adiabatic expansion, has been confirmed through a measured excitation temperature of T≊104 K revealing an image of the kinetic energy.
Carrier spillover at 300, 195, and 77 K in InGaAs and GaAs single quantum wells
Authors: Ongstad Andrew P. , Tilton Michael L. , Bochove Erik J. , Dente Gregory C.
Published: July 2002

The carrier recombination rates in GaAs and strained InGaAs–GaAs single-quantum-well lasers of varying well width and potential depth, respectively, have been measured at 300, 195, and 77 K. For the InGaAs quantum wells (QWs), the carrier lifetime saturates at high inversion, with both the Shockley–Read (SR) lifetime and the saturation lifetime showing substantial reductions with decreasing temperature. The large reduction in the SR lifetime may be attributed to the increased effectiveness of acceptor ions as trap sites, due to the reduced carrier momentum at lower temperature. In a similar vein, the saturation lifetime is also reduced, due to the enhanced carrier confinement in the QW, brought about by the decrease in the carrier thermalization. For the GaAs QWs at 300 K, the saturation lifetime decreases as the well width is increased. The recombination rate law of bulk material is inadequate to predict the recombination rates in these QWs. Consequently, a local recombination model has been developed which accurately predicts the observed lifetime saturation behavior as a function of well width, potential depth, and temperature. Further, T0 of 95 and 162 K are calculated for the shallow and deep QW lasers, respectively. This calculation suggests that it is the temperature dependence of the differential gain that is the dominant factor in setting the temperature sensitivity of the InGaAs QW lasers.
Band lineup of lattice mismatched InSe/GaSe quantum well structures prepared by van der Waals epitaxy: Absence of interfacial dipoles
Authors: Lang O. , Klein A. , Pettenkofer C. , Jaegermann W. , Chevy A.
Published: July 2002

Epitaxial growth of the strongly lattice mismatched (6.5%) layered chalcogenides InSe and GaSe on each other is obtained with the concept of van der Waals epitaxy as proven by low-energy electron diffraction and scanning tunnel microscope. InSe/GaSe/InSe and GaSe/InSe/GaSe quantum well structures were prepared by molecular beam epitaxy and their interface properties were characterized by soft x-ray photoelectron spectroscopy. Valence and conduction band offsets are determined to be 0.1 and 0.9 eV, respectively, and do not depend on deposition sequence (commutativity). As determined from the measured work functions the interface dipole is 0.05 eV; the band lineup between the two materials is correctly predicted by the Anderson model (electron affinity rule).
Giant magnetoresistance and phase decomposition of Fe–XCr–Co (WFe/WCo=1) thin film granular alloys
Authors: Chang H. C. , Chang Y. H. , Yao S. Y.
Published: July 2002

In this study, the giant magnetoresistance (MR) effect was observed in Fe–XCr–Co (WFe/WCo=1, X=27, 40, 50, 60, 70, and 80 wt %) ternary heterogeneous alloy films. After aging, the separated phases and lattice images of the α1 (Fe–Co rich phase) and α2 (Cr rich phase) were further determined by high resolution transmission electron microscope. Experimental results indicate that the MR variations in Fe–Cr–Co thin films alloys were associated with the heterogeneous microstructure. By isothermal aging at 550 °C for 80 min, the largest MR of −43% appears for the film with the composition of 40(Fe/Co)–60Cr at 77 K under a field of 14 kOe. At the same treatment condition, the MR ratios of 50(Fe/Co)–50Cr and 30(Fe/Co)–70Cr are −13.2% and −21.3%, respectively. Cr content, therefore, significantly affected the MR ratio.
Domain populations in epitaxial ferroelectric thin films: Theoretical calculations and comparison with experiment
Authors: Pertsev N. A. , Zembilgotov A. G.
Published: July 2002

An equilibrium domain theory is used to calculate the thickness and temperature dependencies of relative domain populations in tetragonal ferroelectric thin films epitaxially grown on cubic substrates. The relative coherency strain between the film and substrate that governs domain configurations is evaluated with the account of strain accomodation caused by the generation of misfit dislocations at the growth temperature. The threshold coherency strain, which provides the energetic equivalency of the c and a single-domain films and plays an important role in predicting multidomain patterns, is computed as a function of temperature with the aid of the Landau–Ginsburg–Devonshire-type thermodynamic theory. Theoretical predictions are compared with the experimental measurements for PbTiO3 films grown on (001)-oriented MgO and KTaO3 substrates and found to be in good agreement in both the thickness and temperature dependence of the c-domain volume fraction in these epitaxial heterostructures.
Thickness dependence of the switching voltage in all-oxide ferroelectric thin-film capacitors prepared by pulsed laser deposition
Authors: Cillessen J. F. M. , Prins M. W. J. , Wolf R. M.
Published: July 2002

Thin-film ferroelectric capacitors consisting of PbZr0.53Ti0.47O3 sandwiched between La0.5Sr0.5CoO3 electrodes have been deposited using pulsed laser deposition. The combination of oxidic perovskite-type materials results in capacitors with a coercive field (Ec) which is comparable with values for bulk ceramics. Textured thin-film capacitors with a columnar microstructure show lower switching voltages than epitaxial films. No thickness dependence of Ec and a good endurance up to 1011 cycles have been observed for epitaxial as well as textured capacitors with oxidic electrodes. In contrast, capacitors with a metallic top electrode show an increase of Ec with decreasing thickness of the ferroelectric layer. We show that charge injection can explain the experimentally observed increase of Ec with decreasing ferroelectric layer thickness. An overview is given of the growth conditions needed for PbZr0.53Ti0.47O3 films, because the precise stoichiometry is of the utmost importance for the capacitor quality.
Electromigration-induced transgranular failure mechanisms in single-crystal aluminum interconnects
Authors: Joo Young-Chang , Thompson Carl V.
Published: July 2002

Passivated and unpassivated Al single-crystal lines with (110), (133), and (111) planes parallel to the substrate have been fabricated and electromigration tests have been performed to study transgranular failure mechanisms. Both erosion voids and slitlike voids with {111} facets were observed in single-crystal lines. The slitlike voids lie along the in-plane direction, which leads to minimum-surface-area voids among the crystallographically possible directions for the {111}-faceted voids. Voids that nucleate in the lines appear to be mobile and can move toward the cathode end and sometimes accumulate in the cathode pad. (110)-textured lines fail due to erosion voids, slitlike voids, and pad voids, with roughly equal probability. However, (111)- and (133)-textured lines failed predominantly due to pad voids. In both passivated and unpassivated single-crystal lines, the median time to failure, t50, is texture dependent, with t50(111)>t50(133)>t50(110), and with t50(111)⩾10×t50(110). The activation energy for failure for (110) single-crystal lines is about 1 eV, suggesting that Al/oxide interface diffusion may be the dominant mechanism in these lines, and by inference, in bamboo grains in lines with near-bamboo or fully bamboo microstructures. The current density exponent for failure times of single-crystal lines is about 2.5, which is close to that of polycrystalline lines. Based on the results of this study, coupled with results from an earlier model for calculation of lifetimes due to polygranular failure mechanisms, an improved scaling methodology for projection of lifetimes from test conditions to service conditions for near-bamboo interconnects is proposed.
Antiferromagnetic domains and the spin–flop transition in MnF2 (abstract)
Authors: Felcher G. P. , Kleb R. , Jaccarino V.
Published: July 2002

Polarized neutron diffraction measurements were carried out on a single crystal of anti- ferromagnetic MnF2 at different magnetic fields up to the spin–flop transition. This compound orders antiferromagnetically below 67 K, and the magnetic moments of the two manganese atoms in the rutile structure are aligned antiferromagnetically along the tetragonal axis of the crystalline cell. By applying a magnetic field of H=9.27 T (at 4.2 K) along the axis the antiferromagnetic moments flop in the basal plane. From the polarization dependence of the (210) reflection it was found that just below the spin–flop field the entire crystal becomes aligned in a single antiferromagnetic domain, and remains in such state even after the field is removed. The symmetry relations of the polarization for neutron diffraction peaks in different quadrants of the [001] zone were analyzed in the paramagnetic, antiferromagnetic, and spin–flop regions (here polarization analysis sorted out the axial and basal plane components of the sublattice magnetization). In the spin–flop region the magnetization induced along the tetragonal axis was found to be different for the two manganese atoms, an effect quantitatively explained by the misalignment of a fraction of degree of the applied field with the crystal axis. What is remarkable here is that a minute breakdown of symmetry provides a way to control antiferromagnetic domains.
The analysis of background gas heating in direct current sputtering discharges via particle simulation
Authors: Serikov Vladimir V. , Nanbu Kenichi
Published: July 2002

A particle-in-cell/Monte Carlo numerical model has been developed to simulate a direct current discharge self-consistently with the motion and thermalization of both energetic charge-exchange neutrals and sputtered cathode atoms. In the model the charged particle motions are considered in a self-consistent electric field. A one-dimensional glow discharge in Ar has been simulated for the cases of Al and Cu cathode. The background argon gas heating has been predicted, with the temperature rise being larger for the case of Cu cathode which is characterized by a higher sputtering yield than Al. The balance of power input into the gas due to the energetic neutrals, sputtered atoms, and ions is analyzed. The dominant contribution is from energetic neutrals. Comparison of the calculated fluxes of these three species at the cathode surface shows a great contribution of the energetic neutrals into sputtering of the cathode material. The effect of applied voltage has been also investigated. Lastly, the influence of the gas heating on discharge characteristics is discussed.
Optical spectra of the layered Cd2P2S6 and Cd2P2Se6 compounds in the region from 1.6 to 5.5 eV
Authors: Calareso C. , Grasso V. , Silipigni L.
Published: July 2002

Room temperature absorption and reflectivity data are presented for Cd2P2S6 and Cd2P2Se6 crystals in the range of photon energies between 1.6 and 5.5 eV. For both compounds the characteristic feature of the optical absorption is that their absorption edge appears to correspond to an indirect allowed transition with an optical energy gap of 3.06 eV for Cd2P2S6 and 2.29 eV for Cd2P2Se6. An associated phonon energy of 74 meV for Cd2P2S6 and 50 meV for Cd2P2Se6 is also deduced. The Cd2P2X6 (with X=S or Se) reflectivity spectra are compared with the early reflectivity spectrum of Zn2P2S6. As observed in previous reflectivity spectra of the Hg2P2X6 compounds, near the fundamental absorption edges the Cd2P2X6 reflectivity spectra show a pronounced rise in reflectivity, a feature common to most layered compounds. For photon energies greater than their absorption edges, the reflectivity spectra of both materials look similar to one another and to that of Zn2P2S6. On the basis of these similarities we have adopted the so-called transition metal weakly interacting model and interpreted the spectral structures as direct allowed interband transitions. For both compounds, these electronic transitions are described by a simplified energy-band scheme.
Anomalies of ferromagnetic resonance in garnets and their relation to the magnetization process
Authors: Fukui Kouichi , Uno Katsuhiro , Ohya-Nishiguchi Hiroaki , Kamada Hitoshi , Tanno Naohiro
Published: July 2002

X - and L-band ferromagnetic resonance (FMR) measurements were made for a [111]-film sample of a bismuth-substituted garnet (YbTbBi)3Fe5O12. From the angular variation of the FMR signal, magnetic parameters have been determined as uniaxial anisotropy constant Ku=4.17×103 J/m3, cubic anisotropy constant K1=−6.7×102 J/m3, and gyromagnetic ratio γ/2π=23.9 GHz/T. Besides the usual FMR signal, anomalous-shaped features have been observed in the range 100–150 mT. The occurring fields of the features do not depend on the microwave frequency employed. The features always appear at the characteristic fields of the magnetization process such as the saturation-complete field under increasing field and the saturation-breakdown field under decreasing field. Faraday rotation measurements were also made to confirm the coincidence between the characteristic fields of the magnetization curve and the anomalous FMR features. These results establish that the anomalous features reflect the magnetization process of the sample. Such features so far have not been known. The origin of the anomalous features is also considered.
Tracer-based representation of the mobility in a diffuse antiphase boundary
Authors: Kirkaldy J. S.
Published: July 2002

A tracer representation of the rate coefficient involved in the relaxation of diffuse spherical phase–antiphase boundaries in symmetric B2 binary ordering alloys is presented. Adopting broadly accepted Allen–Cahn [Acta Metall. 27, 1085 (1979)] diffusion-reaction phenomenology, but incorporating vacancy kinetics together with a conventional Landau driving force based upon mean curvature K and the surface free energy σ following Lifshitz, we obtain a non-Arrhenius rate coefficient D=2Mκ appearing within the Allen–Cahn [Acta Metall. 27, 1085 (1979)] velocity expression which pertains to the symmetric β-brass type coarsening kinetics: v/K=2Mκ≃2D*(Tc−T)/(Tc)1/2, where M is a non-Einstein mobility coefficient, κ is the gradient energy coefficient, D* is a near-Arrhenius empirical mean tracer diffusion coefficient, and Tc is the critical temperature. This temperature dependency, together with Arrhenius D*, offers satisfactory agreement with the experiments of Allen, Cahn, and Krzanowski. On the basis of an Allen–Cahn [Acta Metall. 27, 1085 (1979)] σ for a diffuse interface in the mean field approximation this translates to a velocity curvature relation of the form v∼D*σ1/3K. This represents an element of conflict since Allen and Cahn [Acta Metall. 27, 1085 (1979)] have favored a velocity or mobility which is independent of σ.
Plastic deformation modeling of die-upset process for magnequench NdFeB magnets
Authors: Guruswamy S. , Wang Y. R. , Panchanathan V.
Published: July 2002

The die upset of hot pressed NdFeB magnets modifies the equiaxed grains to platelets, develops the c-axis texture parallel to press direction, and improves magnetic properties. The mechanism of c-axis alignment has been suggested to be a combination of grain-boundary sliding and anisotropic grain growth in a direction normal to the applied stress. To clearly understand the role of deformation process in grain-boundary sliding or anisotropic grain growth simulations of the die-upset process were performed using ANATARES, a three-dimensional finite element method based deformation modeling software. The stresses and strains in the different regions of a cylindrical Nd–Fe–B magnet at different stages of the die-upset process were determined. The average value of the maximum principal stress (parallel to the upset direction) and total effective strain increased as the upset increased from 50% to 70%. The maximum principal stress and total effective strain show a maximum at the center and decrease in both the thickness and the radial direction due to friction at the die-wall/magnet interface. The stress and effective strain uniformity improves with increase in upset. The data agree well with the variations of texture in the magnet observed using pole figure measurements in this study and texture studies using a synchrotron source.
Particle size dependence of rotational responses in Ni–Zn ferrite
Authors: Anil Kumar P. S. , Sainkar S. R. , Shrotri J. J. , Kulkarni S. D. , Deshpande C. E. , Date S. K.
Published: July 2002

Active powders of Ni–Zn ferrite were synthesized by soft chemical approaches and were processed with varying concentrations of Bi2O3. The sintered products were characterized extensively using a vibrating sample magnetometer, scanning electron microscopy, and resistivity techniques. The loss factor and the permeability have been evaluated. At an optimum concentration of Bi2O3 doping the loss factor has been reduced by 20%. The rotational responses are found to be strongly dependent on particle size.
Role of the antiferromagnet in exchange-biased Fe3O4/CoO superlattices (invited)
Authors: Ijiri Y. , Borchers J. A. , Erwin R. W. , Lee S.-H. , van der Zaag P. J. , Wolf R. M.
Published: July 2002

Using neutron diffraction techniques, we have directly characterized the antiferromagnetic spin structure of CoO in exchange-biased Fe3O4/CoO (001) superlattices. The CoO antiferromagnetic order is long-range and extends coherently across intervening ferrimagnetic Fe3O4 layers. The ordering is influenced significantly by not only epitaxial growth constraints but also by coupling to the Fe3O4 layers. The antiferromagnetic order in these Fe3O4/CoO (001) samples exhibits significant dependence on field cooling preparations. The CoO spins show a strong tendency to align perpendicular to the net ferrimagnetic moment with some changes in the average domain size associated with field cooling. The CoO spin structure in the exchange-biased state illustrates important aspects of recent biasing theories but with unusually large domain sizes.
Magnetization process and magnetoimpedance in (110)[001]FeSi3%
Authors: Carara M. , Gündel A. , Baibich M. N. , Sommer R. L.
Published: July 2002

The present article investigates the relation between magnetoimpedance (MI) and the evolution of the magnetic domain structure along the magnetization curves of commercial FeSi3% sheets cut at different angles with respect to the easy magnetization axis [001]. The role of the magnetocrystalline anisotropy on MI is studied and the correlation between the basic features of the MI vs H and magnetization (M) vs H curves is established. From data obtained at appropriate frequency ranges, the frequency dependence of the effective transverse differential permeability is obtained. The frequency and field spectra of the impedance allow the identification of the rotation of the magnetization as the main contribution of the magnetization processes to the MI effects in the studied samples.
Phase development and electrical property analysis of pulsed laser deposited Pb(Mg1/3Nb2/3)O3–PbTiO3 (70/30) epitaxial thin films
Authors: Maria J.-P. , Hackenberger W. , Trolier-McKinstry S.
Published: July 2002

(001)-oriented heterostructures consisting of LaAlO3 substrates, SrRuO3 bottom electrodes, and Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) (70/30) films were deposited by pulsed laser deposition from Pb-enriched ceramic targets. The epitaxial growth of all layers was confirmed by four-circle x-ray diffraction. Film growth was carried out over a wide range of processing space where substrate temperature, ambient pressure, and laser parameters were varied in order to determine suitable conditions for producing high quality crystals with good electrical properties. In general, strong similarities were observed between thin film and ceramic processing of the same compositions. The associated pyrochlore phase was very difficult to avoid and, in the absence of excess Pb-containing second phases, could only be done at very high temperatures and high oxygen/ozone pressures for films on LaAlO3 substrates. When deposited at temperatures below approximately 640 °C, PMN-PT films grown from lead-enriched targets exhibited depressed ferroelectric transition temperatures. For depositions at or above 660 °C, the electrical properties approached those of bulk single crystals. In particular, films showed room temperature dielectric constants greater than 1500, well developed Curie–Weiss behavior, and remanent polarization values between 13 and 20 μC/cm2.
Noncollinear magnetism in substitutionally disordered face-centered-cubic FeMn
Authors: Schulthess T. C. , Butler W. H. , Stocks G. M. , Maat S. , Mankey G. J.
Published: July 2002

We use first principles electronic structure techniques to study the magnetic structure of γ-FeMn using the Korringa–Kohn–Rostocker multiple-scattering approach in conjunction with an extension of the single site coherent potential approximation to noncollinear magnetic structures. Our results show that the noncollinear 3Q and 2Q structures are both stable solutions with the former being slightly lower in energy. The collinear solutions could only be converged in a traditional spin-polarized calculation and are unstable in a noncollinear treatment.
Transport, optical and electronic properties of the half metal CrO2
Authors: Mazin I. I. , Singh D. J. , Ambrosch-Draxl Claudia
Published: July 2002

The half metal ferromagnet CrO2 has interesting and unusual electronic properties. As in many other cases when kinetic properties are not easily described by the conventional Bloch–Boltzmann theory, strong correlation effects of the Hubbard type were called for explanation of the electronic properties of CrO2. Here we report detailed analysis of the electronic structure and related properties of CrO2, based on state-of-the-art density functional calculations. We conclude that (1) the most current experimental data do not indicate an anomalously large mass renormalization, (2) temperature dependence of the resistivity is due mainly to the scattering by spin fluctuations, (3) coupling between charge (electrons) and spin (magnons) degrees of freedom is anomalously strong, (4) recent optical measurements can be reasonably interpreted within the quasi-one-electron density functional calculations, and (5) their deviation from the experiment is opposite to that expected in the Hubbard model, and agrees with a physical picture of electrons dressed by spin fluctuations.
Increasing coercivity of CoCrTaPt/Cr thin film media by interdiffusion of Mn and Cu overlayers by low temperature annealing
Authors: Peng Wenbin , Qian Zhenghong , Yang Cheng , Sivertson John M. , Judy Jack H.
Published: July 2002

Mn and Cu overlayers were studied with CoCrTaPt/Cr films on glass substrates. Low temperature postannealing reveals an increase in the in-plane coercivity of 700 Oe for Mn and 400 Oe for Cu. X-ray diffraction patterns indicate that the change in magnetic properties is unlikely due to the improvement of crystalline structure. It is suggested that the interdiffusion between magnetic layer and overlayer plays a key role in the coercivity increase. ΔM curves show reduced exchange coupling between grains. It is believed that during low temperature annealing, the diffusion of the overlayer atoms most likely occurs at the grain boundaries, which results in better segregation between grains that may lead to higher coercivity and lower media noise.
High-transmission defect modes in two-dimensional metallic photonic crystals
Authors: Gadot F. , de Lustrac A. , Lourtioz J.-M. , Brillat T. , Ammouche A. , Akmansoy E.
Published: July 2002

An experimental and numerical study of point defect modes in a two-dimensional photonic crystal of metallic rods is presented. A method is developed to optimize the characteristics of transmission resonances in the second (and true) photonic gap of these structures. Transmission maxima close to −2 dB have been obtained by using a combination of a small number of point defects. Such a value is among the highest ones reported to date for two-dimensional photonic crystals with point defects. Besides, the width of the transmission window is shown to be adjustable in a wide range by changing the rod diameter. These results could be applicable to the fabrication of low-cost and compact filters in both the microwave and near-terahertz domains.
Residual stress distribution in the direction of the film normal in thin diamond films
Authors: Kamiya Shoji , Sato Masaki , Saka Masumi , Abé Hiroyuki
Published: July 2002

The residual stress distribution in the direction of the film normal in thin diamond films deposited on Si substrate has been evaluated together with the distribution of Young’s modulus. The films were deposited on the substrate by the microwave chemical vapor deposition method. It has been observed that the curvature of the diamond films delaminated from the Si substrate is functionally dependent on the film thickness. Young’s modulus, which has been estimated by the film bending test in conjunction with a finite element method of analysis, appears to be gradually decreasing towards the adhesion interface. On the basis of detailed measurement of curvature and with the aid of Raman spectroscopy, the residual strain distribution in the film has been evaluated. Although the average intrinsic stress was tensile as reported earlier, we have found that a huge compression concentrates in the very small region near the adhesion interface. This finding shows evidence that something happens on the interface, which is absolutely different from the subsequent process of film growth.
A universal relation for the stress dependence of activation energy for slip in body-centered cubic crystals
Authors: Ngan A. H. W. , Zhang H. F.
Published: July 2002

By analyzing experimental data in the literature, the activation energy H for slip in body-centered cubic metals is found to approximately obey the simple relation H≈0.1 μb3(1−t)2, where t is the applied stress normalized by the zero-temperature Peierls stress, μ is the 〈111〉 shear modulus, and b the 1/2〈111〉 Burgers vector. Such universal relation is explained by analyzing the activation processes of kink-pair generation and expansion along threefold screw dislocations using the generalized Peierls–Nabarro model [A. H. W. Ngan, J. Mech. Phys. Solids 45, 903 (1997)]. The model also predicts qualitatively the general form of the orientation dependence of Peierls stress at zero temperature.
The nonlinear surface impedance of YBa2Cu3O7−δ thin films in zero and large applied fields
Authors: Powell J. R. , Porch A. , Kharel A. P. , Lancaster M. J. , Humphreys R. G. , Wellhöfer F. , Gough C. E.
Published: July 2002

We present coplanar resonator measurements of the nonlinear microwave surface impedance of laser ablated and electron-beam coevaporated YBa2Cu3O7−δ thin films at 8 GHz in zero and applied magnetic fields, including the effects of irradiation with heavy ions. Correlations between the changes in surface resistance and reactance as a function of microwave current suggest that there are different contributions to the nonlinear behavior at low and high microwave currents in both zero and applied fields. In zero field, we suggest that microwave-induced flux lines are responsible for the observed nonlinear behavior, resulting in thermal runaway at large, sample dependent, microwave currents. The observed nonlinearity completely masks any possible contribution from the intrinsic pair breaking expected for a d-wave superconductor. In a magnetic field at relatively low currents, the nonlinearity can be described by the nonlinear dynamics of flux lines pinned by defects with a continuous range of pinning strengths. At higher currents, microwave field nucleation of flux lines is again considered important. On irradiation with 3.6 MeV/nucleon Xe-129 ions, the microwave losses and nonlinearity are significantly decreased in both zero and applied fields, which could have important implications for microwave device applications.