bluuebunny/crossref_metadata_2025 huggingface dataset:
MoS
2
nanoparticle formation in a low pressure environment
Authors: Stoffels E. , Stoffels W. W. , Ceccone G. , Hasnaoui R. , Keune H. , Wahl G. , Rossi F.
Published: July 2002

Formation of MoS2 nanoparticles at pressures between 0.5 and 10 Torr has been studied. Two different chemistries for the particle nucleation are compared: one based on MoCl5 and H2S, and the other based on MoCl5 and S. In both cases particle formation has been studied in a thermal oven and in a radio-frequency discharge. Typically, the reaction rates at low pressures are too low for an efficient thermal particle production. At pressures below 10 Torr no particle production in the oven is achieved in H2S chemistry. In the more reactive chemistry based on sulfur, the optimal conditions for thermal particle growth are found at 10 Torr and low gas flows, using excess of hydrogen. In the radio-frequency discharge, nanoparticles are readily formed in both chemistries at 0.5 Torr and can be detected in situ by laser light scattering. In the H2S chemistry particles smaller than 100 nm diameter have been synthesized, the sulfur chemistry yields somewhat larger grains. Both in thermal and plasma-enhanced particle syntheses, using excess of hydrogen is beneficial for the stability and purity of the particles.
Double-pass high-gain laser amplifiers
Authors: Casperson Janet M. , Moore Frederick G. , Casperson Lee W.
Published: July 2002

Double-pass laser amplifiers have advantages of compactness and efficiency in the amplification of optical signals, and such amplifiers have been employed in a wide range of optical systems. Work in this area is reviewed briefly, and analytical solutions are obtained for the intensity of the electromagnetic waves in double-pass homogeneously-broadened high-gain laser amplifiers. Expressions are derived relating the output power to the input, including the effects of arbitrary mirror reflectivity and frequency detuning from line center. In the limits of weak saturation and of high reflectivity the results are consistent with earlier studies.
Charge carrier mobility in electroluminescent alternating block copolymers
Authors: Dongge Ma , Hümmelgen I. A. , Hu Bin , Karasz F. E.
Published: July 2002

We have investigated charge transport in two electroluminescent block copolymers, poly(1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene-3, 5-dimethoxy-1,4-phenylene) and poly(1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene1,4-phenylene-1,2-ethenylene-3,5-dimethoxy-1,4-phenylene), using current versus voltage measurements performed on metal/polymer/metal structures. The effective mobility of holes has been determined for both polymers. The characteristic charge carrier trap distribution energies of the polymers for the case of Al electrodes have also been determined.
Fermi level control and deep levels in semi-insulating 4H–SiC
Authors: Mitchel W. C. , Perrin R. , Goldstein J. , Saxler A. , Roth M. , Smith S. R. , Solomon J. S. , Evwaraye A. O.
Published: July 2002

Temperature dependent Hall effect, optical admittance spectroscopy, and optical absorption measurements of semi-insulating bulk 4H–SiC are reported. Both intentionally vanadium doped material and commercial grade semi-insulating material were investigated. The carrier concentration versus inverse temperature results from Hall effect measurements up to 1000 K indicated the samples were dominated by one of two deep levels near midgap. In addition to the deep donor level of substitutional vanadium, Ec−1.6 eV, we observed another level at Ec−1.1 eV in some samples, indicating that levels other than the vanadium donor can pin the Fermi level in semi-insulating SiC. Optical admittance measurements on the semi-insulating material indicate the presence of levels at Ec−1.73 and 1.18 eV that were previously observed in conducting samples with this technique and we attribute these levels to the same defects producing the 1.1 and 1.6 eV levels seen by Hall effect. Secondary ion mass spectroscopy measurements of dopant and impurity concentrations are reported. Even though vanadium is present in all of these samples, along with other impurities we are at present unable to definitively identify the 1.1 eV level.
InGaAs quantum wires and wells on V-grooved InP substrates
Authors: Schrimpf T. , Bönsch P. , Wüllner D. , Wehmann H.-H. , Schlachetzki A. , Bertram F. , Riemann T. , Christen J.
Published: July 2002

We report on the fabrication and characterization of In1−xGaxAs quantum structures on V-grooved InP substrates grown by metalorganic vapor phase epitaxy. The geometry of the quantum wells and wires was determined by scanning-electron microscopy and atomic-force microscopy. We optimized the InP buffer-layer thickness in order to obtain narrow quantum wires. The optical properties were studied by photoluminescence (PL) spectroscopy. The PL peaks of the different quantum structures can be identified by a self-aligned masking process. The interpretation of the PL measurements was verified by means of cathodoluminescence measurements with high spatial resolution. Transition energies were evaluated from the geometry of the quantum wells and quantum wires. The composition of the InGaAs was used as a parameter for the calculations. By comparison of the measured transition energies with the evaluated ones we determined the Ga content of the quantum structures. We found the quantum wires nearly lattice matched to the InP substrate and the quantum wells on {111} planes rich in In (1−x=71%).
Structure, magnetic properties, and magnetostriction of (Tb1−xNdx)(Fe0.4Co0.6)1.9
Authors: Guo Z. J. , Busbridge S. C. , Wang B. W. , Zhang Z. D. , Zhao X. G. , Geng D. Y.
Published: July 2002

The structure and magnetostriction of (Tb1−xNdx)(Fe0.4Co0.6)1.9 (0<x<1) alloys have been investigated at room temperature and the magnetic properties determined over the temperature range 1.5–300 K. It was found that the matrix of (Tb1−xNdx)(Fe0.4Co0.6)1.9 is essentially that of a (Tb,Nd) (Fe,Co)2 phase with a MgCu2-type cubic Laves structure for all values of x. The lattice parameter of the Laves phase increases from 0.7297 to 0.7363 nm as x increases from 0 to 1 and obeys Vegard’s law. We have measured the Curie temperature to decrease with increasing x from 667 K for Tb(Fe0.4Co0.6)Fe1.9 to 454 K for Nd(Fe0.4Co0.6)Fe1.9. The resultant macroscopic magnetization becomes zero when x=0.45 at a temperature of 1.5 K. At 300 K, a slightly smaller value of x is found to produce no resultant magnetization, and the spontaneous magnetostriction constant λ111 decreases slowly with increasing Nd content and vanishes abruptly from a value of 1630×10−6 when x>0.65. The polycrystalline isofield magnetostriction curves exhibit a minimum at x=0.45 and a peak at x=0.65. The observed behavior can be explained on the basis of opposing rare-earth Tb and Nd moments and an anisotropy minimum close to x=0.65. As the temperature is reduced below 300 K, the spin reorientation occurs at greater Nd concentrations.
Quantification of the surface- and bulk-order parameters of a homogeneously aligned nematic liquid crystal using fully leaky guided modes
Authors: Hallam B. T. , Brown C. V. , Sambles J. R.
Published: July 2002

An optical waveguide technique, involving the excitation of fully leaky guided modes, has been used to quantify the director distortions induced by the application of weak in-plane electric fields to a homogeneously aligned nematic liquid crystal, E7. The twist profile of the nematic is determined by fitting angle-dependent reflectivity and transmission data to multilayer optical theory. By considering the Frank–Oseen free energy in the bulk, with Rapini–Papoular-type anchoring at the surfaces, we have determined accurate values for both the azimuthal anchoring strength Wa and the twist elastic constant k22 as a function of temperature. Values for the temperature-dependent surface and bulk average-order parameters have also been deduced by allowing for a spatially varying nematic-order parameter.
Microwave-mixing scanning capacitance microscopy of pn junctions
Authors: Schmidt J. , Rapoport D. H. , Behme G. , Fröhlich H.-J.
Published: July 2002

We describe an approach to scanning capacitance microscopy. A mixing technique is employed for imaging local capacitance variations simultaneously with the sample topography using an atomic force microscope (AFM) with a conductive tip. A SiO2/Si sample with lateral pn junctions formed by ion implantation has been investigated. Microwave signals incident on the metal–oxide–semiconductor (MOS) structure formed by the AFM tip and the sample give rise to mixing signals due to the nonlinear voltage dependence of the space charge capacitance in the Si. In our experiments two microwave input signals with frequencies f1 and f2 and a variable dc bias voltage were applied to the tip-sample MOS structure. The dependence of the generated sum frequency and third harmonic signals on the dc sample voltage shows that the f1+f2 and 3 f signals are proportional to dC/dV and d2C/dV2, respectively. Images of the sum frequency and third harmonic signals delineating the pn junctions on our model sample are presented and the dc bias voltage dependence of the images is discussed.
Local atomic volume dependence of Tb and Co magnetic moments
Authors: Caspi El’ad N. , Pinto Haim , Melamud Mordechai
Published: July 2002

The individual sizes and orientations of the magnetic moments of Tb and Co in different sites were studied by neutron diffraction of Tbn+1Co3n+5B2n (n=1, 2, 3, and ∞). Total magnetic moments obtained by our neutron diffraction measurements are in agreement with the literature and our superconducting quantum interference device measurements. The local atomic volumes dependence of Tb and Co magnetic moments are discussed using the Wigner–Seitz cell construction method.
Dynamic study of the surfaces of (001) gallium arsenide in metal-organic vapor-phase epitaxy during arsenic desorption
Authors: Pristovsek M. , Trepk T. , Klein M. , Zettler J.-T. , Richter W.
Published: July 2002

We have investigated by reflectance anisotropy spectroscopy the arsenic desorption from GaAs (001) at various temperatures in metal–organic vapor-phase epitaxy to obtain reaction orders and activation energies. The highest arsenic coverage, found at low temperatures with arsine stabilization, corresponds to a (4×3) reconstruction. Without arsine, arsenic starts to desorb and less arsenic-rich reconstructions are observed, depending on temperature: c(4×4) (below 800 K), β2(2×4) (below 920 K), α(2×4), and only with hydrogen carrier gas finally (4×2) (above 950 K). Above 920 K the reaction order differs in hydrogen and nitrogen atmosphere, probably due to an etching effect of hydrogen radicals. The five different desorption processes show either a first- or zero-order time dependence. First order is related to the desorption from the terraces and zero order to desorption from the steps (or kinks) on the surfaces. The activation energies for all processes are around 2.5 eV. This energy is, therefore, assumed to be the activation energy for the removal of an arsenic dimer from the surface.
Electron paramagnetic resonance study of the Eu2+ ion in a PbWO4 single crystal
Authors: Yeom T. H. , Kim I. G. , Lee S. H. , Choh S. H. , Kim T. H. , Ro J. H.
Published: July 2002

The electron paramagnetic resonance of the Eu2+ ion in a PbWO4 single crystal, grown by the Czochralski method, has been investigated using an X-band spectrometer. The rotation patterns in the crystallographic planes together with spin-Hamiltonian parameters of Eu2+ show that the local site symmetry of the Eu2+ ion is tetragonal. The spectroscopic splitting tensor g, zero-field splitting parameters Bkq, and hyperfine tensor A are determined with the effective spin Hamiltonian. Energy levels of the ground state for Eu2+ embedded in the PbWO4 crystal are calculated. It turns out that the Eu2+ ion substitutes for the Pb2+ ion without any charge compensation.
Optical constants of Ga1−xInxAsySb1−y lattice matched to GaSb (001): Experiment and modeling
Authors: Muñoz M. , Wei K. , Pollak Fred H. , Freeouf J. L. , Wang C. A. , Charache G. W.
Published: July 2002

The optical constants ε(E)[=ε1(E)+iε2(E)] of two epitaxial layers of GaInAsSb/GaSb have been measured at 300 K using spectral ellipsometry in the range of 0.35–5.3 eV. The ε(E) spectra displayed distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E0+Δ0 component, spin-orbit split (E1,E1+Δ1) and (E0′,E0′+Δ0′) doublets, as well as E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Holden et al., Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb-enhancement effects at E0, E0+Δ0, and the E1, E1+Δ1 doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R1) of the two-dimensional exciton related to the E1, E1+Δ1 CPs was obtained. The value of R1 was in good agreement with effective mass/k⋅p theory. The ability to evaluate R1 has important ramifications for recent first-principles band-structure calculations which include exciton effects at E0, E1, and E2 [M. Rohlfing and S. G. Louie, Phys. Rev. Lett. 81, 2312 (1998); S. Albrecht et al., Phys. Rev. Lett. 80, 4510 (1998)]. The experimental absorption coefficients in the region of E0 were in good agreement with values obtained from a linear interpolation of the end-point materials. Our experimental results were compared to a recent evaluation and fitting (Holden model) of the optical constants of GaSb.
Near-ultraviolet electroluminescent performance of polysilane-based light-emitting diodes with a double-layer structure
Authors: Hoshino Satoshi , Ebata Keisuke , Furukawa Kazuaki
Published: July 2002

We fabricated double-layer light-emitting diodes (LEDs) by utilizing poly[bis(p-nbutylphenyl)silane] (PBPS) and oxadiazole derivatives, and investigated their basic LED characteristics. The near-ultraviolet electroluminescence (EL) performance, such as the EL threshold electric field and the current density, depended on the oxadiazole derivatives used as the electron transport materials as well as the components of the EL emission. We observed better EL performance where the EL external quantum efficiency in a double-layer LED with a 2-(4′-tert-butylphenyl)-5-(4″-biphenyl)-1,3,4-oxadiazole based electron transport layer was twice that of a PBPS single-layer LED. By contrast, we observed a worse EL threshold electric field and current density when we used 2,5-bis(1-naphthyl)-1,3,4-oxadiazole as an electron transport material. The reason for the difference in the EL performance was revealed by investigating the charge carrier injection and transport dynamics of the two LEDs.
Magnetic frustration in Y-doped manganites: Electron spin resonance and magnetization
Authors: Alejandro G. , Causa M. T. , Tovar M. , Fontcuberta J. , Obradors X.
Published: July 2002

Magnetic susceptibility (χdc) and electron spin resonance (ESR) measurements on La0.67−xYxCa0.33MnO3 in the paramagnetic (PM) regime Tc⩽T⩽1000 K are presented, for 0⩽x⩽0.20. For T>750 K,χdc follows a Curie–Weiss (CW) law with C=2.95(15) emu K/mol, and a CW temperature, Θ, showing a monotonic variation as a function of x. The ESR linewidth follows a T dependence given by ΔHpp(T)=[C/Tχdc(T)]ΔH∞. The parameter ΔH∞ increases from 2600 G for x=0 to 3300 G for x=0.20. The correlation with expected variation of the anisotropic spin–spin interactions is discussed. The results appear to be insensitive to the changes of the double-exchange isotropic interaction. A comparison of our experimental results in the PM regime with reported data on the ordered phase indicates a complex magnetic behavior associated with Y substitution. The CW temperature, a measure of the average strength of the magnetic interactions, decreases slowly with x (17% for x=0.20) reflecting the softening of the double exchange due to the bending of the Mn–O–Mn bond. At low temperatures the materials order ferromagnetically (FM) in all cases. The ordering temperature, defined as the point of the maximum slope for M vs T, decreases rapidly with x and the transition becomes increasingly broad. For x=0, we found Tc/Θ≈0.68, value close to 0.78 which is expected within a constant coupling model. This ratio, usually a measure of magnetic frustration, decreases rapidly with x reaching Tc/Θ≈0.26 for x=0.20. However, for this concentration the FM transition is already extremely broad and the relation of this behavior with structural disorder and possible phase separation is discussed.
Magnetic and optical properties of ordered and disordered B2-phase Co–Al alloys
Authors: Rhee J. Y. , Kudryavtsev Y. V. , Kim K. W. , Lee Y. P.
Published: July 2002

The magnetic properties and the optical conductivity spectra of the ordered and disordered Co1−xAlx (x=0.5, 0.46, and 0.38) alloy films were measured. The ordered off-stoichiometric alloy films exhibit a ferromagnetic behavior and the magnetic property is enhanced when the disorderedness is introduced. The electronic band structures, magnetic moments, and optical conductivity spectra of the ordered Co1−xAlx (x=0.5, 0.4375, and 0.375) alloys were calculated using the linearized-muffin-tin-orbital method. The calculated magnetic moments were in a reasonable agreement with the experiments. Both the antistructure Co atoms and their neighboring eight Co atoms contribute to the magnetic moments. The charge transfer between Co and Al atoms is closely related to the magnetic behavior. The minority-spin bands of the antistructure Co atoms form a very intense and sharp density of states peak at the Fermi level, resulting in the magnetic moments. The evolution of the low-energy structures below 2 eV in the measured optical conductivity spectra upon decreasing the Al concentration is explained by an increased number of the antistructure Co atoms.
Quantitative imaging of magnetization reversal in FeGd multilayers by magnetic transmission x-ray microscopy
Authors: Eimüller Th. , Kalchgruber R. , Fischer P. , Schütz G. , Guttmann P. , Schmahl G. , Köhler M. , Prügl K. , Scholz M. , Bammes F. , Bayreuther G.
Published: July 2002

A multilayered FeGd system sputtered on a Si3N4-membrane was used to demonstrate that magnetic transmission x-ray microscopy enables the imaging of the domain structure with high resolution in a quantitative way and in varying external magnetic fields. The field-dependent evolution of sub-100 nm out-of-plane magnetized domains, i.e., the transition from stripe into bubble domains, could be observed in detail. It has been shown that critical fields like the bubble collapse field can be described by a wall energy model. Contrary to the assumption of the theory, the magnetization does not remain perpendicular to the film plane in all cases. This is only true inside the bubbles. In the stripe domains the magnetic moments rotate. Their angle could be measured as a function of the external applied field. Internal stray fields can cause a rotation of moments leading to a contrast at the edges of stripe domains. The results indicate that the magnetization reversal in 3d/4f multilayers is not only determined by wall motions and is therefore more complex than previously assumed.
Frequency- and time-resolved measurements of FeTaN films with longitudinal bias fields
Authors: Alexander C. , Rantschler J. , Silva T. J. , Kabos P.
Published: July 2002

The high frequency characteristics of 100 nm FeTaN films have been studied by using both time-resolved inductive techniques and frequency-resolved permeability measurements. Experiments performed as a function of longitudinal bias fields from 120 to 2400 A/m (1.5–30 Oe) showed precessional frequencies from 1.3 to 2.5 GHz, initial permeabilities from 1600 to 500, and damping constants α=0.013 to 0.0045. It is illustrated that the magnetization precessional data obtained from the time-resolved inductive technique can be Fourier transformed to the frequency domain to give the real and imaginary components of a permeability spectrum; this spectrum compares well with data obtained from frequency-resolved permeability measurements. It is also demonstrated that accurate values of the damping constant α can only be determined from permeameters whose bandwidth is two to three times the ferromagnetic resonance frequency of the material to be measured.
Magnetic pinning strength for the new Sm-TM magnetic materials for use up to 550 °C
Authors: Chen Christina H. , Walmer Marlin S. , Walmer Michael H. , Liu Jinfang , Liu Sam , Kuh G. E.
Published: July 2002

A new series of Sm(CowFevCuxZry)z alloys has been developed to produce magnets with high intrinsic coercivity Hci at high temperatures for use up to 550 °C. The maximum use temperature is defined as TM, which is the maximum temperature at which the extrinsic demagnetization curve is a straight line. An important feature of the alloys with high TM is a lower temperature coefficient of Hci, β. These magnets have a higher resistance to demagnetization from increasing thermal agitation which occurs with increasing temperature. A study on magnetic pinning field Hp, which will be explained in the text, vs TM was conducted by measuring the initial magnetization curves of the magnets at 25 and 300 °C. The study shows that as TM increases, Hp increase at high temperatures. At 25 °C, all magnets with TM from 250 to 550 °C have Hp higher than 24 kOe, which is too high to be determined using a hysteresigraph. At 300 °C, the magnet with TM=250 °C has Hp=7.0 kOe, and the magnet with TM=550 °C has Hp=15.1 kOe. Higher pinning field at high temperatures (⩾300 °C) provides a greater resistance to thermal demagnetization, which leads to better performance at high temperatures.
Magnetic properties of epitaxial (110) multilayer films of DyFe2 and YFe2
Authors: Sawicki M. , Bowden G. J. , de Groot P. A. J. , Rainford B. D. , Ward R. C. C. , Wells M. R.
Published: July 2002

Laves phase DyFe2/YFe2 multilayers have been grown epitaxially on a YFe2 seed layer, with a (110) growth direction. Magnetic measurements taken in applied fields of up to 12 T, and from 5 K to room temperature, show that short period multilayers (∼100 Å) behave, collectively, as a single magnetic entity. As a result, it is possible to engineer magnetic compensation points, in a digital manner, by adjusting the thicknesses of the alternate DyFe2 and YFe2 layers. Nevertheless, the magnetic response of the DyFe2/YFe2 structure and that of the YFe2 seed layer are not completely independent of one another. Because of a mismatch in the Fe–Fe magnetic exchange at the multilayer/seed interface, a 180° magnetic soliton-like domain (“magnetic twister”) is set up in the top of the YFe2 seed layer. A semiquantitative model describing the properties of the magnetic twister is presented and discussed.
Tuneable microstrip device controlled by a weak magnetic field using ferromagnetic laminations
Authors: Adenot A. L. , Acher O. , Taffary T. , Quéffélec P. , Tanné G.
Published: July 2002

The properties of lamellar insulator ferromagnetic on the edge (LIFE) material in a microwave device under a static field are studied. The LIFE composite is manufactured by gluing sheets of a ferromagnetic thin film deposited by physical vapor deposition on a polymeric substrate. The 0.154-μm-thick ferromagnetic material is an amorphous CoNiZr alloy deposited on a 12.7-μm-thick kapton film. According to gyromagnetic model, when an external static field is applied along the easy axis, the gyromagnetic resonance frequency is shifted towards high frequencies. Then, a piece of LIFE composite was integrated in a microstrip line, using a substrate of low permittivity. The displacement of the minimum of the transmission coefficient in the microstrip line due to a static field applied is presented. The operation of a microwave filter with a stopband tuneable in the 2–6 GHz range by a small static magnetic field (<500 Oe) is demonstrated, as well as a magnetic switch.
Enhanced giant magnetoresistance sandwich film
Authors: Everitt B. A.
Published: July 2002

Devices have been patterned from enhanced sandwich material as well as more conventional giant magnetoresistance (GMR) sandwich material. The enhanced GMR sandwich consists of essentially adding a Cu spacer and a synthetic antiferromagnet layer to either side of a conventional sandwich stack. While devices patterned from simple sandwich material are always high resistance in the zero field bias state and decrease in resistance in an applied field, similar devices fabricated from enhanced sandwich material exhibit more complex characteristics. Depending upon the bias current density, the zero field biased state may be either low or high resistance for this structure. This effect could be useful for applications such as current or magnetic field sensing.
Effect of Fe-exchange-field penetration on the residual amorphous phase in nanocrystalline Fe92Zr8
Authors: Suzuki K. , Cadogan J. M.
Published: July 2002

The temperature dependence of hyperfine interactions has been studied by means of Mössbauer spectroscopy for rapidly quenched amorphous Fe92Zr8 in as-quenched and partially crystallized states. An analysis of the temperature dependence of the mean hyperfine field based on the molecular-field approximation showed that the Curie temperature of the residual intergranular amorphous phase (TCam) is 330±5 K, well above the maximum Curie temperature in amorphous Fe–Zr binary alloys (TC*≈280 K). The difference of TCam−TC* (ΔTCam≈50 K) cannot be attributed to the possible magnetovolume effect in the sample. The difference of ΔTCam could be explained by an enhancement of the molecular field by ∼50 T at 0 K, induced by the Fe exchange field penetrating from the bcc-Fe nanocrystallites.
Effect of neutron irradiation on soft magnetic properties in amorphous FeCuNbSiB alloy
Authors: Kim H. C. , Yu S. C. , Kim C. G. , Han H. S. , Cho W. K. , Kim D. H.
Published: July 2002

The changes in soft magnetic properties on amorphous Fe73.5Cu1Nb3Si13.5B9 alloy after neutron irradiation are studied by complex permeability spectra, magnetoimpedance ions and M–H curve measurements. The total integration fluence of fast neutrons (nf,En>1 MeV) is varied from 1.92×1014nf cm−2 to 4.85×1016nf cm−2. Two different types of relaxation processes in permeability spectra, which were observed in low and high frequency regions, are related to the domain wall motion and the rotational magnetization, respectively. As neutron irradiation fluence increases, the permeability due to domain wall motion decreases but the permeability due to rotational magnetization increases. The anisotropy field estimated from magnetoimpedance profile decreases with neutron irradiation fluence. The measurement of M–H curves shows the improvement of softness after neutron irradiation of 4.85×1016nf cm−2, due to the enhancement of rotational magnetization.
Ferrimagnetic order in Ca2FeMoO6
Authors: Pinsard-Gaudart L. , Surynarayanan R. , Revcolevschi A. , Rodriguez-Carvajal J. , Greneche J-M. , Smith P. A. I. , Thomas R. M. , Borges R. P. , Coey J. M. D.
Published: July 2002

The double perovskite Ca2FeMoO6 is a poor metal which orders magnetically at 345 K. The Fe57 Mössbauer hyperfine field and isomer shift at 4.2 K (48 T and 0.70 mm/s relative to α-Fe) indicate an Fe3+ configuration 3d5↑ which is strongly hybridized with the Mo 4d↓t2g orbitals. The saturation magnetization of 3.6μB/formula is consistent with neutron diffraction measurements which indicate iron and molybdenum moments of 4.0(1) and −0.2(3)μB per iron or molybdenum, respectively. The sample exhibits approximately 6% cation disorder relative to the ideal NaCl-type order of Fe and Mo in the double perovskite structure.
Current noise spectra in CdTe semiconductor diodes
Authors: Sampietro M. , Ferrari G. , Bertuccio G.
Published: July 2002

By using a correlation spectrum analyzer, we succeeded in performing direct measurements of the current noise spectra of cadmium telluride (CdTe) diodes, commonly used in γ-ray spectrometers. The current noise spectra have been measured over a wide range of frequencies, from below 1 Hz up to 100 kHz, and for diode operating points from 0 up to 150 V. The device showed linear I–V characteristics in all the bias range with a dynamic resistance of about 2 GΩ. Around the equilibrium condition (0–0.5 V), the white component of the noise spectrum is in agreement with the Johnson noise associated to the device resistance. As the bias is increased up to 150 V, the white noise level is shown to slowly approach the shot noise behavior. The white noise shows a cut-off frequency consistent with the carriers transit time across the device. In all the nonequilibrium conditions, the noise spectra also show a significant 1/f component whose power density increases with the square of the device current.
Transmission electron microscopy study of thermal effects on the free-layer reversal of a crossed-anisotropy spin valve
Authors: Murdoch S. J. T. , Chapman J. N. , Pokhil T. G. , Mao S. , Murdock E. S.
Published: July 2002

Transmission electron microscopy has been used to study the reversal of the free layer of a NiMn-pinned crossed-anisotropy spin valve as a function of applied field orientation and specimen temperature. By choosing the orientation of the applied field correctly it was possible to avoid the formation of domains in the reversal process. As the temperature was raised above room temperature, the mechanism remained qualitatively unchanged until temperatures of ≈200 °C were reached, beyond which irreversible behavioral change began to take place. From analysis of image sequences magnetoresistance characteristics have been constructed.
Monodomain configurations due to bias effect in NiO/NiFe microstructures
Authors: Wu J. C. , Huang H. W. , Lai Chih-Huang , Wu Te-ho
Published: July 2002

Magnetic domain configurations of microstructured permalloy exchange coupled by an antiferromagnetic (NiO) thin film is presented. NiO/NiFe bilayer micrometer array elements were fabricated using electron beam lithography through a lift-off technique. The magnetic force microscopy images of the elliptical and rectangular elements with various aspect ratios showed dipole-like magnetic domain structures. The bright/dark arc contrast associated with the magnetic pole strength was dependent on the anisotropic exchange field. Furthermore, the shape anisotropy can overwhelm anisotropic exchange in patterned elements with a high aspect ratio and thicker permalloy film.
Proper oxidation for spin-dependent tunnel junctions
Authors: Song Dian , Nowak Janusz , Covington Mark
Published: July 2002

From wafers where the AlOx barrier varies from junction to junction, we determine the proper oxidation conditions for spin-dependent tunnel junctions. We obtain large variation in the resistance×area product (R×A) of the junctions within a wafer by either using nonuniform plasma oxidation on uniform Al films, or using uniform oxidation on Al films with a wedge thickness profile. When plotted against R×A, the tunneling magneto-resistance (TMR) for all junctions on the wafer falls on one curve that exhibits a broad maximum in the TMR. We propose that this maximum is where most metal Al has been oxidized while the oxidation of the bottom electrode is minimal. With annealing, we achieved our highest TMR, 38%, in highly resistive Co–AlOx–Co junctions. The most conductive junctions we made have about 18% TMR and R×A of 140 Ω μm2. They are made by natural oxidation on about 5 Å of Al. For barriers thinner than 13 Å Al, we start to lose TMR for junctions larger than 2 μm2. This is possibly caused by pinholes in thin barrier and indicates the importance of studying small size junctions.
Voltage controlled spintronic devices for logic applications
Authors: You Chun-Yeol , Bader S. D.
Published: July 2002

We consider logic device concepts based on our previously proposed spintronics device element whose magnetization orientation is controlled by application of a bias voltage instead of a magnetic field. The basic building block is the voltage-controlled rotation (VCR) element that consists of a four-layer structure—two ferromagnetic layers separated by both nanometer-thick insulator and metallic spacer layers. The interlayer exchange coupling between the two ferromagnetic layers oscillates as a function of applied voltage. We illustrate transistorlike concepts and reprogrammable logic gates based on VCR elements.
Plasma nitrogenation of Pr2Fe17 and Sm2Fe17
Authors: Ardisson Jose D. , Macedo Waldemar A. A. , Araujo Roberto C. , Gama Sergio
Published: July 2002

The effect of plasma nitrogenation on the structural and magnetic properties of Pr2Fe17 and Sm2Fe17 compounds was investigated by Mössbauer spectroscopy, thermomagnetic measurements, optical metallography, and electronic microprobe analysis. Samples were prepared starting from Pr2Fe17 and Sm2Fe17 commercial powders with 20 μm granulometry, submitted to nitrogenation treatment at 400 °C in 10 Torr atmosphere of 80%H2–20%N2 during periods up to 8 h. Our results show the formation of R2Fe17N3 and metallic Fe and indicate the existence of two other phases. The Mössbauer parameters of the latter are characterized by magnetic hyperfine fields (peak values) of 21 (24) and 3 (4) T and isomer shifts of 0.17 and 0.24 (0.24 and 0.29) mm/s for Pr (Sm). Their magnetic-phase transitions were determined to be close to 250 and 90 °C. Our results suggest that these 2:17 phases present stoichiometries close to R2Fe17N8 and R2Fe17N11, respectively.
Magnetic diffuse scattering in disordered systems studied by neutron polarization analysis (invited)
Authors: Stewart J. R. , Andersen K. H. , Cywinski R. , Murani A. P.
Published: July 2002

Neutron polarization analysis studies of both nuclear and magnetic correlations in disordered alloys and compounds provide a unique insight into the character and extent of magnetic disorder in these systems and its dependence on local atomic environment. We present here a review of polarization analysis studies of magnetic and structural defects in a variety of materials, performed using the diffuse scattering spectrometer D7 at the Institut Laue–Langevin. The diverse range of magnetic materials studied using D7 include spin-glasses, frustrated itinerant electron systems, pyrochlores, and quasicrystals.
Zeeman and orbital effects of an in-plane magnetic field in cuprate superconductors
Authors: Yang Kun , Sondhi S. L.
Published: July 2002

We discuss the effects of a magnetic field applied parallel to the Cu–O (ab) plane of the high Tc cuprate superconductors. After briefly reviewing the Zeeman effect of the field, we study the orbital effects, using the Lawrence–Doniach model for layered superconductors as a guide to the physics. We argue that the orbital effect is qualitatively different for in-plane and interlayer mechanisms for superconductivity. In the case of in-plane mechanisms, interlayer couplings may be modeled as a weak interlayer Josephson coupling, whose effects disappear as H→∞; in this case Zeeman dominates the effect of the field. In contrast, in the interlayer mechanism the Josephson coupling is the driving force of superconductivity, and we argue that the in-plane field suppresses superconductivity and provides an upper bound for Hc2 which we estimate very crudely.
Pr-doping effect on the depinning of vortices in Bi2Sr2Ca1−xPrxCu2Oy single crystals
Authors: Li X.-G. , Sun X. F. , Fan X. J. , Zhao X. , Wu W. B.
Published: July 2002

The irreversibility lines of Pr-doped Br2Sr2Ca1−xPrxCu2Oy single crystals have been studied systematically. It was found that for the Pr-doped samples as compared with the undoped Bi2Sr2CaCu2Oy the effective superconducting area shrinks, the irreversibility line Birr decreases, and the anisotropic parameter γ increases strongly due to Pr doping. It is evident that the substitution of Pr for Ca in the system effectively reduces the hole concentration on CuO2 layers to the underdoped state and destroys the Josephson coupling of the two superconducting regions of adjacent CuO2 layers. The vortex depinning is responsible for the irreversibility and the displacement value for vortex depinning is similar to that for vortex lattice melting, but with different physical meaning.
Strain distribution and carrier confinement in Si/SiO2 quantum point contacts and wires
Authors: Boxberg F. , Virkkala R. , Tulkki J.
Published: July 2002

The strain distribution and strain-induced confinement of carriers in Si/SiO2 quantum wires (QWRs) and quantum point contacts (QPCs) have been analyzed by elastic continuum and envelope wave function models. Recently, a compressive strain up to 1% has been predicted to exist in the thermally oxidized SiO2 surrounding the Si waveguide. We show that 1% radial strain in the thermal oxide leads to lowering of the band edge inside the Si wire and to confinement of electrons in a quantum-dot-like potential having a depth of ∼40 meV. The binding energy of the lowest electron level is −34 meV in a 240 nm long and 60 nm high QWR. The lowest energy level rises above the band edge in the contact pads when the QWR is made narrower than 12 nm. For the QPC, no bound states exist according to our calculations.
Carbon particle phase transformation kinetics in detonation waves
Authors: Viecelli J. A. , Ree F. H.
Published: July 2002

A much simplified model of nonequilibrium soot phase mixture kinetics is derived for use in hydrodynamic computation. This model is based on a review of existing shock and detonation soot recovery experiments as well as the available data on the carbon phase diagram. It yields a time and pressure–temperature path dependent value for the nonequilibrium diamond fraction of the soot mixture. Approximate methods for computing the detonation products pressure for the kinetics derived mixture of diamond and graphite are discussed. The proposed model increase in hydrodynamic computation cost is negligible.
Transmission electron microscopy characterization of secondary defects created by MeV Si, Ge, and Sn implantation in silicon
Authors: Wong-Leung J. , Fatima S. , Jagadish C. , Fitz Gerald J. D. , Chou C. T. , Zou J. , Cockayne D. J. H.
Published: July 2002

Extended defects created in Si by ion implantation to doses below the amorphization threshold have been studied after annealing at 800 °C for 15 min. The implant species were the group IV elements Si, Ge, and Sn, and structural defects created by similar damage distribution were compared. The mass of the implanted ion influences the type of defect observed. For all three implant species, rod-like {311} planar defects were observed. Additionally, in Ge and Sn implanted samples, small {111} interstitial faulted dislocation loops were observed.
Photoluminescence and Raman scattering of silicon nanocrystals prepared by silicon ion implantion into SiO2 films
Authors: Li G. H. , Ding K. , Chen Y. , Han H. X. , Wang Z. P.
Published: July 2002

Photoluminescence (PL) and Raman spectra of silicon nanocrystals prepared by Si ion implantion into SiO2 layers on Si substrate have been measured at room temperature. Their dependence on annealing temperature was investigated in detail. The PL peaks observed in the as-implanted sample originate from the defects in SiO2 layers caused by ion implantation. They actually disappear after thermal annealing at 800 °C. The PL peak from silicon nanocrystals was observed when thermal annealing temperatures are higher than 900 °C. The PL peak is redshifted to 1.7 eV and the intensity reaches maximum at the thermal annealing temperature of 1100 °C. The characterized Raman scattering peak of silicon nanocrystals was observed by using a right angle scattering configuration. The Raman signal related to the silicon nanocrystals appears only in the samples annealed at temperature above 900 °C. It further proves the formation of silicon nanocrystals in these samples.
Matrix isolation spectroscopy in methane. Isotropic ESR spectrum of HC17O
Authors: Adrian F. J. , Kim B. F. , Bohandy J.
Published: July 2002

The isotropic electron spin resonance (ESR) spectrum of HC17O has been observed in methane at 20 K (prepared by photolysis of HI in CH4 containing 1% of 17O enriched CO), and the isotropic oxygen hyperfine splitting determined [a0(HCO)=−15.1 G]. Isotropic spectra of HO2 (prepared similarly with O2 replacing CO) and NO2 in methane were also observed. Superhyperfine interactions in the ESR spectrum of H atoms in CH4 indicate H is trapped in the octahedral interstitial site of the matrix. ‘‘Forbidden’’ proton spin-flip transitions in the ESR spectrum of CH3 in methane indicate it is trapped at a substitutional site in the matrix.
Vibrational relaxation of carbon dioxide (101) and carbon monoxide (v=2) during gas–surface collisions
Authors: Misewich J. , Houston P. L. , Merrill R. P.
Published: July 2002

The probability for deactivation of CO(v=2) and CO2(101) on collision with polycrystalline silver surfaces has been measured. The deactivation probability for CO(v=2) was found to decrease from 0.33 at 300 K to 0.20 at 440 K, while the deactivation probability for CO2 (101) was found to decrease from 0.72 at 300 K to 0.37 at 440 K. Since no population was observed in the CO(v=1) and CO2 (001) intermediate levels, it appears that each deactivation proceeds completely to produce the vibrational ground level. The magnitudes for the deactivation probabilities and the temperature dependencies indicate that a dominant mechanism for relaxation involves trapping and subsequent deactivation by one or more of several mechanisms, including electron–hole pair formation, vibration-to-rotation energy transfer, or perhaps even transfer of energy to the surface phonons. The experiments were performed in a UHV chamber by using a tunable infrared laser source to excite gas-phase molecules vibrationally before their collision with the surface and by measuring the population of vibrationally excited molecules through their time-resolved infrared fluorescence.
Determination of the longitudinal compliance of poly(vinyl acetate) by using the dynamic light scattering technique
Authors: Wang C. H. , Fytas G. , Fischer E. W.
Published: July 2002

Using the Kramers–Kronig relation, we have provided a correlation between the longitudinal compliance and the time correlation function of density fluctuations that can be directly measured in a light scattering experiment. Photon correlation spectroscopic experiments have been carried out to test the theory. Good agreement between the dynamic light scattering data and the mechanical dynamic compressibility data has been obtained.
The lower Rydberg states of trans-hexatriene
Authors: Sabljić A. , McDiarmid Ruth
Published: July 2002

The one photon (optical), and two and three photon resonant multiphoton spectra of trans-hexatriene have been recorded and analyzed. Transitions to the 3s, 3px(Ag), 3pȳ(Bg), 3dȳz̄(Au), and 4s(3d) orbitals (states) have been observed. Vibrational modes ag: C=C stretch, CH2 rock, and CH and CCC deformations, au: CH and skeletal torsions, bu: C=C stretch have been observed in many of the studied Rydberg states. The assignments of the electronic states and vibrational modes are discussed.
Stochastic sensitivity analysis applied to gas–surface scattering
Authors: Lucchese Robert R.
Published: July 2002

The results of the application of sensitivity analysis to the simulation of gas–surface scattering are presented. The simulation technique was the three dimensional generalized Langevin method of Tully, and the sensitivity analysis methods were the coupled solution of system and sensitivity equations approach and a new stochastic sensitivity analysis method. We found that the coupled solution approach only converged when trajectories which involved multiple collisions of the gas with the surface were excluded from the Monte Carlo averaging of this simulation approach. The stochastic sensitivity method was found to give convergent results for both single collision trajectories and multiple collision trajectories. In the stochastic sensitivity method the sensitivity coefficients are computed by selecting the parameters of the system randomly from a given distribution and then performing a linear least-squares analysis of the variations of the output variables of the simulation model with respect to variations of the input parameters. The stochastic sensitivity analysis method should also be applicable to completely deterministic trajectory simulations which however exhibit ergodic behavior in individual trajectories. In addition to first-order coefficients, we also used the stochastic sensitivity analysis method to compute higher order coefficients (selected second and third order) and derived sensitivity coefficients. We have applied both the coupled solution and stochastic sensitivity analysis method to the Ar–Pt (111) scattering system, where we found that the surface corrugation and the steepness of the respulsive two-body interactions were essential features of the interaction potential in determining the quantity of energy transfered.
Dynamics of superionic conduction: A generalized Langevin equation study of mobile cation channeling and activated barrier crossing in α-AgI
Authors: Olson Mark A. , Adelman Steven A.
Published: July 2002

MTGLE and Langevin equation stochastic classical trajectory studies of the mobile silver cation channeling and activated barrier crossing processes which underly superionic conduction in α-AgI are presented. To interpret non-Kramers effects on the channeling kinetics, a new description of the condensed phase effect on activated barrier crossing is developed. This picture emphasizes the competition between ‘‘whipback’’ of the reaction coordinate, arising from instantaneous environmental restoring forces, and frequency-dependent dissipation of reaction coordinate energy due to environmental relaxation. The simulations show that while the roughest features of the channeling trajectories are determined by the cation friction coefficient the finer details are governed by short-time scale cation-cage correlations. Analogously, for α-AgI, the gross features of the kinetics, e.g., the rough extent of the breakdown of the transition state model, are described by a Kramers-like picture. The finer details of the kinetics, however, are governed by the competition between whipback, which makes realistic barrier crossing less efficient than Kramers model barrier crossing, and dissipation, which works in the opposite direction.
On the use of the Breit–Pauli approximation for evaluating line strengths for spin-forbidden transitions: Application to NF
Authors: Havriliak Stephen J. , Yarkony David R.
Published: July 2002

An ab initio treatment of the Breit–Pauli spin-orbit interaction is presented for use with large (state-averaged) MCSCF/CI wave functions and Cartesian Gaussian atomic orbitals. The first order perturbation theory equation which results from treating the Breit–Pauli interaction (Hso) as a perturbation to the nonrelativistic Born–Oppenheimer Hamiltonian (H0) is formulated in the configuration state function basis thereby permitting the coupling of all roots of H0 into the perturbed wave function. The basic atomic orbital integrals of Hso over Cartesian Gaussian functions are evaluated using a Rys quadrature approach. The method presented here is used to consider the spin forbidden transitions b 1Σ+→X 3Σ− and a 1Δ→X 3Σ− in NF. A comparison with a frequently used approach based on the few lowest eigenfunctions of H0 is presented.