bluuebunny/crossref_metadata_2025 huggingface dataset:
Co–Pt micromagnets by electrodeposition
Authors: Zana Iulica , Zangari Giovanni
Published: July 2002

Arrays of cylindrical Co–Pt micromagnets with 2 μm diameter have been successfully fabricated by optical lithography and electrodeposition. The influence of applied current density and the aspect ratio on the magnetic properties of the micromagnets have been investigated. Intrinsic coercivities (MHc) up to 3.7 kOe, retentivities (Br) up to 4.9 kG, and large energy products (BH)max up to 34 kJ/m3 (4.31 MGOe) have been achieved. The cylinder aspect ratio strongly influences the magnetic properties of the magnet array. Specifically, coercivity HcM, squareness (Mr/Ms), and energy product (BH)max, all increase with increasing aspect ratio. Current density only slightly influences the magnetic properties. The large energy product attained and the compatibility with complementary metal–oxide–semiconductor fabrication, render these structures interesting candidates for hard magnetic components in microelectromechanical systems.
Step surface induced unidirectional exchange anisotropy in PtMn/Ni80Fe20 on Mo(001)
Authors: Yu C. C. , Huang J. C. A. , Yao Y. D.
Published: July 2002

Exchange anisotropy for a PtMn/Ni80Fe20 bilayer grown on a Mo(001) step surface is studied. The PtMn/Ni80Fe20 bilayers were mainly grown as a (110) quad-crystal structure mixed with a minor (001) component. The PtMn/Ni80Fe20 bilayers show a unidirectional exchange anisotropy with exchange field up to about 190 Oe with easy axis parallel to the underlying Mo step-edge direction. The azimuthal distribution of the exchange field is explained by interplay of a step-induced unidirectional anisotropy and crystalline-induced anisotropy.
Rotational level dependence of ground state recovery rates for OH X 2Π(v″=0) in atmospheric pressure flames using the picosecond saturating-pump degenerate four-wave mixing probe technique
Authors: Tobai J. , Dreier T. , Daily J. W.
Published: July 2002

We report the first direct measurement of the rotational level dependency of the rate of recovery of initially depleted levels in the electronic ground state X 2Π(v″=0) of OH produced in different flame environments at atmospheric pressure. The initial depopulation of a specific rotational level is accomplished by an intense picosecond pump pulse at 308 nm to partially saturate the electronic A 2Σ–X 2Π(0,0) transition. The recovery of the depleted ground state population then is monitored by probing the same level via the (1,0) band at 283 nm using picosecond degenerate four-wave mixing (DFWM). Both laser wavelengths were derived from the pulse-amplified and frequency doubled output of two independently tunable distributed feedback dye lasers operated with Rh101 and Rh6G in ethanol, respectively, and pumped with the second harmonic of a frequency doubled ps-Nd:YAG laser. It is shown that the rate of repopulation of the depleted ground state levels decreases by 54% and 50% with increasing rotational quantum number, N″, ranging from 2–16 and 2–13 for stoichiometric CH4/air and H2/O2/He flames, respectively. Within experimental error their absolute values in both flames are equal and are not noticeably sensitive to an unequal depletion of the Zeeman sublevels, as created for different polarization configurations of the saturating pump beam and the DFWM probe beams. The rate of (1.8±0.4)×109 s−1 averaged over all rotational transitions investigated is smaller by a factor of 3 than the corresponding average rate of the temporal DFWM signal intensity decay determined by us previously. The rate also is smaller than total depopulation rates obtained in the excited A 2Σ+ state of OH for similar flame conditions.
Variable-range hopping conductivity and absence of a true metal–insulator transition in La0.7−δ Ca0.3Mn1−yFeyO3
Authors: Laiho R. , Lähderanta E. , Salminen J. , Lisunov K. G. , Zakhvalinskii V. S. , Safontchik M. O. , Shakhov M. A. , Shubnikov M. L.
Published: July 2002

Resistivity of La0.7−δ Ca0.3Mn1−yFeyO3 (LCMFO) with y=0–0.05, δ=0 and 0.017 is investigated between 4.2–350 K, showing that La vacancies increase the effect of Fe doping by influencing electronic states through additional microscope disorder. The Shklovskii–Efros variable-range hopping conductivity, governed by generation of a soft parabolic gap in the spectrum of localized states due to Coulomb interaction between charge carriers, is observed both above and well below the ferromagnetic-to-paramagnetic transition temperature. Our results give evidence for absence of a true metal–insulator transition both in undoped material and in LCMFO.
Mechanisms of visible photoluminescence from nanoscale silicon cones
Authors: Wellner A. , Palmer R. E. , Zheng J. G. , Kiely C. J. , Kolasinski K. W.
Published: July 2002

We explore the origin of visible photoluminescence in nanoscale silicon cones fabricated by reactive ion etching in silicon-on-insulator substrates utilizing rough silver films as masks. Photoluminescence (PL) visible to the naked eye was observed after oxidation and annealing. Samples oxidized at 900 °C exhibit intense yellow/green photoluminescence centered at about 530 nm. Samples oxidized at 1000 °C luminesce in the red-to-infrared region with peak positions between 650 and 730 nm. Transmission electron microscopy characterization is employed to show that PL at 530 nm can be understood in terms of defect states, while the PL at 650–730 nm can be explained by a combination of defect state and quantum confinement effects.
Photoelectron spectroscopy via the 1 1Δu state of diacetylene
Authors: Ramos C. , Winter P. R. , Zwier T. S. , Pratt S. T.
Published: July 2002

Photoelectron spectra are reported for one-photon resonant, two-photon ionization of jet-cooled diacetylene via a number of vibronic levels of the 1 1Δu state. An improved value for the adiabatic ionization threshold is found to be 82 064±30 cm−1 (10.175±0.004 eV), in good agreement with the earlier result. The photoelectron spectra of different vibronic bands of the 1 1Δu state nearly all show long progressions in what appear to be low frequency bending vibrations. At energies just above the ionization threshold, the observed progressions can be understood in terms of excitation of a single Renner-Teller active mode in the ion, with Renner-Teller parameters similar to those of the ν4+ trans-bending mode in the ground state acetylene cation.
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and Na23 nuclear magnetic resonance study of V6 magnetic molecular clusters
Authors: Jung J. K. , Procissi D. , Jang Z. H. , Suh B. J. , Borsa F. , Luban M. , Kögerler P. , Müller A.
Published: July 2002

We report H1 and Na23 nuclear magnetic resonance (NMR) and relaxation measurements in two variants of the V6 cluster with formula [CH3H6]4Na2[H4{V3L}2P4O4]⋅14H2O and Na6[H4{V3L}2P4O4]⋅18H2O, respectively. Both the NMR linewidth and the spin-lattice relaxation (NSLR) data can be explained semiquantitatively by assuming a dipolar coupling between the proton (sodium) nuclei and the three V(IV) magnetic moments forming an almost equilateral triangle with nearest neighbor antiferromagnetic exchange interactions. From the temperature dependence of the Na23 NSLR we obtain the value of the two gaps separating the lower energy total spin states.
Enhanced Fe moment at Pd/Fe interfaces studied by low-temperature conversion electron Mössbauer spectroscopy
Authors: Cheng L. , Altounian Z. , Ryan D. H. , Ström-Olsen J. O.
Published: July 2002

Conversion electron Mössbauer spectroscopy (CEMS) has been employed to study the magnetic moment of Fe in proximity to Pd at the interfaces of [Pd 53 Å/Fe t Å]25 multilayers grown by dc magnetron sputtering (t was varied from 5 Å to 40 Å). At room temperature, all of the multilayers were ferromagnetically ordered. Analysis gives an interface Fe layer thickness of ∼2 monolayers. The ground state hyperfine field (Bhf), extracted from the temperature dependence of Bhf, was found to be as high as 38.9 T, corresponding to a ground state Fe moment of 2.8 μB.
Effect of soft-phase anisotropy on reversal behavior in two-phase exchange-coupled bilayer ferromagnetic thin films
Authors: Crew D. C.
Published: July 2002

A two-dimensional micromagnetic simulation has been used to study a thin-film bilayer consisting of two ferromagnetic phases with different magnitudes of magnetocrystalline anisotropy. The aim was to investigate the effect the magnitude of magnetocrystalline anisotropy of the soft-phase has upon the resulting hysteresis properties. For low soft-phase anisotropy, reversal of the soft phase occurs in low fields while the hard phase remains unreversed until higher fields, whereupon the domain wall becomes unpinned from the interface. For higher values of the soft-phase anisotropy, the soft phase is more resistant to change in magnetization, but once reversal begins the domain wall propagates more easily into the hard phase. The result is that the coercivity of the hard phase decreases with increasing soft-phase anisotropy. The ultimate point is reached when the nucleation field of the soft phase is larger than the pinning field of the interface anisotropy mismatch and an almost single-phase reversal process results. This is explained in terms of previous analytic theories of domain-wall pinning developed for two-section nanowires and planar defects.
Coupled particle-in-cell and Monte Carlo transport modeling of intense radiographic sources
Authors: Rose D. V. , Welch D. R. , Oliver B. V. , Clark R. E. , Johnson D. L. , Maenchen J. E. , Menge P. R. , Olson C. L. , Rovang D. C.
Published: July 2002

Dose-rate calculations for intense electron-beam diodes using particle-in-cell (PIC) simulations along with Monte Carlo electron/photon transport calculations are presented. The electromagnetic PIC simulations are used to model the dynamic operation of the rod-pinch and immersed-B diodes. These simulations include algorithms for tracking electron scattering and energy loss in dense materials. The positions and momenta of photons created in these materials are recorded and separate Monte Carlo calculations are used to transport the photons to determine the dose in far-field detectors. These combined calculations are used to determine radiographer equations (dose scaling as a function of diode current and voltage) that are compared directly with measured dose rates obtained on the SABRE generator at Sandia National Laboratories.
H
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nuclear magnetic resonance and spin dynamics in the tetranuclear iron(III) cluster {Fe4}
Authors: Procissi D. , Suh B. J. , Lascialfari A. , Borsa F. , Caneschi A. , Cornia A.
Published: July 2002

The spin dynamics of a cluster of four iron(III) ions characterized by a total spin ground state ST=5 and Ising anisotropy have been investigated by the H1 nuclear magnetic resonance (NMR) linewidth and H1 NMR spin-lattice relaxation rate 1/T1 as a function of the temperature (0.5–295 K) and external magnetic field (0.3–7.2 T). At very low T (0.5 K) the spectrum becomes very broad indicating freezing of the Fe3+ moments in a superparamagnetic state. The temperature dependence of T1−1 and T2−1 shows strong field dependent enhancement below 50 K which must be related to the spin dynamics of the Fe3+ moment freezing process, although no quantitative theory of this is currently available.
Magnetic and electronic transport properties of YbxCa1−xMnO3 compounds
Authors: Guo Yongquan , Roy Sujoy , Ali Naushad , Sardela Mauro R.
Published: July 2002

The polycrystalline YbxCa1−xMnO3 (x=0.1,0.2) were studied by x-ray powder diffraction, magnetic, and electrical resistivity measurements. The YbxCa1−xMnO3 crystallizes in an orthogonally distorted perovskite structure, and shows the ferromagnetic ordering with TC more than 110 K. However, the field dependence of magnetization and electrical resistivity exhibits very complicated behavior. A magnetic field induced insulator–metal transition has been found in the Yb0.2Ca0.8MnO3 compound. In addition, the large asymmetry in magnetization and resistivity hysteresis loops has been observed in this compound at 10 K, which might be due to the charge ordering and magnetocrystalline anisotropy.
Spontaneous Hall-effect studies on ion-beam-deposited GdCoZr amorphous alloy thin-film sensors
Authors: Raja M. Manivel , Lin K. W. , Gambino R. J.
Published: July 2002

Magnetic properties and Hall resistivity of amorphous films depend on the energy of bombarding ions during film growth. The saturation magnetization was found to oscillate with ion energy. The ion bombardment induces in-plane anisotropy and results in a harder perpendicular axis. An exchange field of as high as 95 Oe was observed due to the compositional gradient. The amorphous films show a large spontaneous Hall effect. A maximum Hall angle of 1.7% was observed, which is considered to be an important parameter for a Hall device. The large spontaneous Hall resistivity and the low coercivity of the amorphous films offer potential applications in weak magnetic-field sensors.
4 °C preparation of ferrite nanoparticles having protein molecules immobilized on their surfaces
Authors: Nishimura K. , Hasegawa M. , Ogura Y. , Nishi T. , Kataoka K. , Handa H. , Abe M.
Published: July 2002

Trypsin, a proteolytic enzyme or a protein, was immobilized onto the surfaces of ferrite (a Fe3O4–γFe2O3 mixed solution) fine particles, ∼8 nm in size, during the process in which the particles were synthesized from an aqueous solution. The process was performed in the open air at a temperature as low as 4 °C and on near-neutral condition of pH⩽9, which is compatible with most of the bioactive molecules as well as trypsin. Therefore this technique is advantageous for preparing magnetite particles having biomolecules immobilized on their surfaces, which will be used for biomedical applications utilizing magnetic separation technique.
Detection and quantification of Barkhausen noise in magnetoresistive read heads
Authors: Arnold C. S. , McKinstry Kevin D.
Published: July 2002

We recently designed and implemented a new method for detecting and quantifying Barkhausen noise in magnetoresistive (MR) read heads. The new method employs a time-domain digital filter that detects discontinuities in the signal output, measures the magnitude of the associated jump, and determines the parts of the read output cycle where the Barkhausen jumps occur. The filter and an efficient algorithm suitable for real-time processing on a laboratory PC are described. Its transfer characteristics are also presented. Finally, we demonstrate the action of the filter on MR elements.
Spin polarized electronic reflections at metal–oxide interfaces: A technique for characterizing tunneling barriers in magnetic random access memory devices
Authors: Ounadjela K. , Da Costa V. , Iovan A. , Dimopoulos T. , Dahmani F. , Mahenthiran D. , Dieny B. , Allen W. , Gregg J. F.
Published: July 2002

One of the most critical steps in the realization of high quality tunneling devices is the fabrication of ultrathin oxide layers. This article describes a powerful technique for controlling the oxidation of ultrathin metallic layers. The technique consists of depositing the metallic layer to be oxidized on top of the soft magnetic layer of a bottom spin valve. The oxidation kinetics are then monitored by measuring the current in plane sheet resistance and magnetoresistance. The technique relies on the extreme sensitivity of the magnetoresistance on the degree of specular reflection at the soft magnetic layer/oxide interface.
A direct two-dimensional comparison of magnetic circular dichroism and magnetic linear dichroism in ultraviolet photoemission spectroscopy
Authors: Gao Xingyu , Koveshnikov Alexey N. , Madjoe Reginaldt H. , Subramanian Krishnan , Mankey Gary J. , Stockbauer Roger L. , Kurtz Richard L.
Published: July 2002

The angular dependence of magnetic dichroism in photoemission (MDAD) from valence bands is more complicated than that from core levels. It gives access to spin-resolved information about the bands involved in the photoexcitation process. By using a unique ellipsoidal-mirror analyzer, we are able to observe MDAD from valence bands as a two-dimensional image, which gives a direct view of their angular dependence over a large slice through the Brillouin zone. In this work we have investigated Co films grown on a Cu(001) substrate. By using either circularly polarized light or linearly polarized light at a photon energy of 21 eV, magnetic circular dichroism and magnetic linear dichroism (MLD) from the Fermi edge shows different angular dependencies. This is due to the different excitation processes involved with the different light polarizations and it reflects the symmetry of the 3d bands of the Co. Furthermore, we also find that the sign of the MLD can change if differing initial states are involved. First principles calculations of Co bands and their photoelectron angular distributions with the different light polarizations will be presented. This shows that dichroism in valence band photoemission can provide valuable additional information on the electronic and magnetic properties of ultrathin films.
Magnetic properties of a metal-organic network Fe(N3)2(4,4′-bpy)
Authors: Yuen Tan , Lin C. L. , Fu Aihua , Li Jing
Published: July 2002

The results of magnetic susceptibility, isothermal magnetization, and heat capacity measurements on a newly discovered iron complex with mixed ligands Fe(N3)2(4,4′-bpy) (4,4′-bpy=4,4′-bipyridine) are reported. The study of high-temperature magnetic susceptibility data indicates antiferromagnetic correlation of the Fe2+ moments. However, the results of low-temperature measurements reveals a ferromagnetic transition for this compound, and the Curie temperature is determined from the Arrott plot with TC=20 K. The large jump in the heat capacity data confirms the magnetic phase transition and the transition temperature.
Integration of a generalized Hénon–Heiles Hamiltonian
Authors: Verhoeven C. , Musette M. , Conte R.
Published: July 2002

The generalized Hénon–Heiles Hamiltonian H=1/2(PX2+PY2+c1X2+c2Y2)+aXY2−bX3/3 with an additional nonpolynomial term μY−2 is known to be Liouville integrable for three sets of values of (b/a,c1,c2). It has been previously integrated by genus two theta functions only in one of these cases. Defining the separating variables of the Hamilton–Jacobi equations, we succeed here, in the two other cases, to integrate the equations of motion with hyperelliptic functions.
Three-color polymer light-emitting diodes by stamped dye diffusion
Authors: Nakamura A. , Tada T. , Mizukami M. , Hirose S. , Yagyu S.
Published: July 2002

A method to pattern the integrated emissive colors of polymer light-emitting diodes (PLEDs) using stamped dye diffusion is introduced. In this method, the emitting colors are tuned selectively by the energy transfer processes from the host polymer to the diffused dye. The diffusion of the dye into the emitting polymer layer was successfully achieved by transferring the prefabricated dye pattern onto a silicon substrate at an elevated temperature. Coumarin6, 4-(dicyanomethylene)2-methyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran and Nile Red dyes were diffused selectively into a poly (9-vinylcarbazole)-based emitting layer using this method. We have investigated the electroluminescent properties of the dye diffused PLEDs and also have demonstrated the integration of three-color PLEDs with dimensions of the order of 10 μm.
Influence of crystalline structures on the domain configurations in controlled mesoscopic ferromagnetic wire junctions
Authors: Hirohata A. , Xu Y. B. , Bland J. A. C. , Holmes S. N. , Cambril E. , Chen Y. , Rousseaux F.
Published: July 2002

A role of crystalline structures on domain wall formation is investigated by magnetic force microscopy (MFM) using mesoscopic wire junctions of both epitaxial bcc Fe and polycrystalline permalloy. Three sets of structures were fabricated with ferromagnetic wires (30 nm thick, 0.2⩽w⩽50 μm wide and 200 μm long); straight wires, two wires connected at the ends (L shapes), and crosses. Fe samples show complicated domain configurations due to magnetocrystalline anisotropy, while permalloy samples show simple domain with shape anisotropy. These results show that spin configurations at the junction can be precisely tuned by varying not only the shape and the size but also the crystalline structures of the sample.
On the magnetic structure of γ-FeMn alloys
Authors: Stocks G. Malcolm , Shelton W. A. , Schulthess Thomas C. , Újfalussy Balazs , Butler W. H. , Canning A.
Published: July 2002

The alloy γ-FeMn is a rare example of a fcc antiferromagnet. It has become a prototype for pinning layer studies in magnetoelectronic devices. Here we report the results of first principles calculations of the magnetic structure of γ-FeMn based on large cell models of the disordered alloy. The calculations are based on the constrained local moment model and use of first principles spin dynamics to obtain the ground state orientational configuration. In agreement with previous layer KKR-CPA studies, we find the 3Q-state to be lowest of the three prototype structures studied (1Q,2Q,3Q). However, the constraining fields introduced into the theory to maintain a specific orientational configuration are not zero indicating that even the 3Q-structure is not the ground state. Subsequent optimization of the magnetic configuration using first principles spin dynamics yields a state that is lower in energy by 2.5 meV/atom.
Shock loadings on thin film manganin gauges packaged by vapor-deposited alumina
Authors: Du X. S. , Yang B. C. , Zhou H. R.
Published: July 2002

To extend the measurement range to 100 GPa, a modified manganin gauge was prepared and examined. The modifications were focused on the package materials and technique. The manganin sensing elements were first deposited by magnetron sputtering on Al2O3 substrates, and then covered by a layer of Al2O3 films several μm thick by electron beam evaporation. After being placed on slabs of Al2O3 ceramic, the gauges were shocked to 40–80 GPa with a light gas gun. The stress-time profiles showed a good plateau of about 1 μs in duration. The initial calibration resulted in a linear curve with piezoresistance coefficient of 0.0198±0.0002 GPa−1.
Surface transverse waves in polymer-coated grating configurations
Authors: Yantchev V. M. , Strashilov V. L.
Published: July 2002

The propagation characteristics of surface transverse waves (STW) in polymer-coated surface gratings on rotated Y-cut quartz substrates are studied theoretically. A two-layer Love wave theory is combined with Floquet analysis to derive the wave dispersion and attenuation. The thickness of the polymer layer is found to affect the position and width of the frequency stop band, while the polymer viscosity introduces out-of-band attenuation. Both parameters effectively act in the vicinity of the lower-stop-band edge where STW resonators are mostly operated. The presented algorithm is applicable to the analysis of STW gas sensors and to the improvement of the resonator performance.
Equilibrium polymerization of cyclic carbonate oligomers. II. Role of multiple active sites
Authors: Ballone P. , Jones R. O.
Published: July 2002

Ring opening polymerization of bisphenol A polycarbonate is studied by Monte Carlo simulations of a model comprising a fixed number of Lennard-Jones particles and harmonic bonds [J. Chem. Phys. 115, 3895 (2001)]. Bond interchanges produced by a low concentration (0.10%⩽ca⩽0.36%) of chemically active particles lead to equilibrium polymerization. There is a continuous transition in both 2D and 3D from unpolymerized cyclic oligomers at low density to a system of linear chains at high density, and the polymeric phase is much more stable in three dimensions than in two. The steepness of the polymerization transition increases rapidly as ca decreases, suggesting that it is discontinuous in the limit ca→0. The transition is entropy driven, since the average potential energy increases systematically upon polymerization, and there is a steady decline in the degree of polymerization as the temperature is lowered. The mass distribution functions for open chains and for rings are unimodal, with exponentially decaying tails that can be fitted by Zimm–Schulz functions and simpler exponential forms.
Theoretical study of excited state proton transfer in 3,6-bis(benzoxazolyl)pyrocatechol (BBPC)
Authors: Zgierski Marek Z. , Fernández-Ramos Antonio , Grabowska Anna
Published: July 2002

Ab initio density functional theory and approximate instanton methods are used to study proton transfer processes in the first excited electronic state of 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Geometries of di-enol, keto-enol, and di-ketone tautomers as well as transition states for single and double proton transfer processes and the corresponding force fields are obtained with the CIS/6-31G* method and verified with CISD/6-31G* single point calculations. It is shown that keto-enol tautomer is the most stable in the S1 state while the least stable is di-ketone. The single proton transfer in the 2A1 state of di-enol leads to a somewhat more stable keto-enol tautomer. This result nicely reproduces the experimental assignment stating that BBPC, a symmetric molecule with two equivalent proton transfer reaction sites, undergoes a single proton transfer in the S1 state. The excited system has to overcome the barrier of about 9 kcal/mol and proton transfer is therefore dominated by tunneling. Dynamics calculations with the instanton method yield the rate of transfer of 9.8×1010 s−1, again in a very good agreement with the experimental value of kPT=(5.1±0.4)×1010 s−1 [Chem. Phys. Lett. 169, 450 (1990)]. Theory predicts a large kinetic isotope effect on this process. It is also shown that the reverse proton transfer leading back to di-enol has the rate strongly dependent on the stabilization energy of keto-enol. It effectively competes with the radiative decay of the latter, resulting in the observed weak di-enol fluorescence of BBPC. Finally, the calculations demonstrate why BBPC is not a photochrome unlike many typical Schiff bases.
Evidence of swelling of SiO2 upon thermal annealing
Authors: Banerjee S. , Chakraborty S. , Lai P. T.
Published: July 2002

Ultrathin SiO2 film was thermally grown on Si(001) substrate by dry oxidation and wet oxidation processes. The films were then subjected to thermal annealing (TA) at 1000 °C for 30 min. The structural characterization of the as-grown and the TA samples was carried out using the grazing incidence x-ray reflectivity technique. The analysis of the x-ray reflectivity data was carried out by using a model independent formalism based on the distorted wave Born approximation for obtaining the electron density profile (EDP) of the film as a function of depth. The EDP of both films show a decrease in the electron density as well as an increase in their thickness when the films are subjected to TA. It has also been observed that the total number of electrons is conserved in the oxide film after TA. Our analysis of the x-ray reflectivity data indicates that the SiO2 film swells and its interface with the substrate modifies upon TA.
Atomic force microscope detector drift compensation by correlation of similar traces acquired at different setpoints
Authors: Kindt Johannes H. , Thompson James B. , Viani Mario B. , Hansma Paul K.
Published: July 2002

The atomic force microscope measures surface topography by maintaining a certain cantilever deflection or vibration amplitude as the cantilever is scanned over a sample surface. The desired cantilever deflection or amplitude is referred to as the setpoint, and is maintained by moving the sample toward or away from the cantilever. The signal from the cantilever deflection detector has a real component, due to cantilever deflection, and a drift component due to various sources of drift. We present a method of eliminating the drift component by sensing and correcting it in real time. Our method involves automatically changing the setpoint so as to maintain a certain set difference in the relative feature richness of two traces taken with slightly offset setpoints. We show how the system maintains a setpoint only 70 mV above minimum, perturb it with a gentle blow of air that causes 200 mV of detector drift, and observe its recovery within 13+−6 s.
Magnetic properties of Nd60−xYxFe30Al10 (x=0,10,30) melt-spun ribbons containing two amorphous magnetic phases
Authors: Sun Z. G. , Löser W. , Eckert J. , Müller K.-H. , Schultz L.
Published: July 2002

The magnetic properties of Nd60−xYxFe30Al10 (x=0, 10, 30) melt-spun ribbons were studied. The ribbons contain two amorphous magnetic phases characterized by their Curie temperature TC1 and TC2. The Y atoms may be unevenly distributed in the two magnetic phases. The Y atoms prefer to substitute Nd in the low TC1 phase. Thermo-history effects are found in the thermomagnetic curves for field-cooled and zero-field-cooled samples. The bifurcation temperature Tb decreases with Y addition, suggesting a decrease of the anisotropy energy and the coercivity. The room-temperature coercivity and remanence ratio also strongly decrease with Y addition. The apparent coercivity μ0HC for x=0 first increases with decreasing temperature, reaching a maximum at about 60 K, near the Curie temperature of the TC1 phase, then strongly decreases in the two-magnetic phase region. The coercivity mechanism is assumed as domain-wall pinning effect.
The Green’s function for multistate Schrödinger-type equation. Magnetic-field effects in radical pair recombination in the presence of interaction potential
Authors: Shushin A. I.
Published: July 2002

The recently proposed expression for the Green’s function (GF) of the multistate Schrödinger-type equation is applied to obtaining the GF for the stochastic Liouville equation which describes the chemically induced dynamic electron polarization (CIDEP) generation in diffusion assisted radical pair recombination in liquids in the presence of interrradical interaction (for example, the Coulomb interaction). The analytical formulas for CIDEP amplitude are derived with the use of the sudden perturbation approximation, properly treating the effect of the spin exchange interaction, and two approaches, the cage and interpolation ones, which allow for the accurate description of diffusive motion of radicals in the potential. The accuracy of these two approaches and specific features of CIDEP generation in the presence of the interaction potential are discussed in detail.
A correlation of exchange energy-exchange potential at the nucleus in atoms
Authors: Sen K. D.
Published: July 2002

We report the results of numerical tests on atom He–Lu, which suggest that the exchange energy calculated within the spin-polarized exchange-only optimized potential model for atoms can be expressed as a power-law in terms of the local effective exchange potential evaluated at the nucleus. The total effective exchange potential increases linearly with the atomic number. These results are useful in improving the energy density functionals at the small r region where most of the popularly used energy functionals provide rather poor functional derivatives. The significance of the present results in obtaining detailed information on the correlation-kinetic energy of the Kohn–Sham model is discussed.
Efficient methods to calculate dynamic hyperpolarizability tensors by time-dependent density-functional theory
Authors: Hait Heinze Habbo , Della Sala Fabio , Görling Andreas
Published: July 2002

A derivation of density-functional response theory is presented which is based directly on the Runge–Gross theorem and does not invoke the action formalism and thus does not violate causality requirements in time. Working equations to treat linear and nonlinear response properties of electronic systems by time-dependent density-functional theory in the frequency domain are given. Efficient noniterative methods to calculate dynamic hyperpolarizability tensors of molecules for arbitrary frequency combinations are presented. With the introduced methods, hyperpolarizabilities of benzene and stilbene derivatives are investigated. The results show that the influence of substituents on the frequency-dependent hyperpolarizability is well described by time-dependent density-functional theory at the adiabatic local density level. Inspection of the linear and nonlinear response of the electron density shows the importance of screening effects for the response properties of larger molecules on electric fields and electromagnetic radiation.
Unimolecular decomposition of nitrooxyalkyl radicals from NO3–isoprene reaction
Authors: Zhang Dan , Zhang Renyi
Published: July 2002

The unimolecular decomposition of the nitrooxyalkyl radicals arising from the NO3-initiated reaction of isoprene is investigated. Density functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energies of the transition states of decomposition of the nitrooxylalkyl radicals and the corresponding oxirane products. Geometry optimizations were performed using density functional theory at the B3LYP/6-31G(d,p) level and the single-point energies were computed using second-order Møller–Plesset perturbation theory and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G(d) level of theory, the decomposition barriers range from 13 to 21 kcal mol−1, and the decomposition products (i.e., the separated oxirane and NO2) are 9 to 25 kcal mol−1 more stable than the nitrooxylalkyl radicals. The rate constants of decomposition of the nitrooxylalkyl radicals and the pressure-dependent oxirane yields have been calculated using the transition state theory and master equation formalism.
Calculation of third-rank molecular hypermagnetizabilities by continuous transformation of the origin of the current density
Authors: Caputo M. C. , Lazzeretti P.
Published: July 2002

The third-rank hypermagnetizabilities of three small molecules have been evaluated at the Hartree–Fock level of accuracy within the conventional common-origin approach as well as alternative procedures formally based on continuous transformation of the origin of the electronic current density induced by a perturbing electromagnetic field. Gaugeless basis sets of increasing size and flexibility have been employed in a numerical test (i) to assess the practicality of distributed origin methods whereby the diamagnetic contribution to the current density is formally annihilated, and (ii) to estimate the degree of convergence of diamagnetic and paramagnetic contributions to tensor components. It is shown that two nice features characterize the computational scheme adopted in this study: (i) the results depend only linearly on a shift of origin; (ii) a permutational symmetry condition of tensor indices yields a natural criterion for the near-Hartree–Fock quality of computed values.
Bifurcations and traveling waves in a delayed partial differential equation
Authors: Rey Alejandro D. , Mackey Michael C.
Published: July 2002

Here cell population dynamics in which there is simultaneous proliferation and maturation is considered. The resulting mathematical model is a nonlinear first-order partial differential equation for the cell density u(t,x) in which there is retardation in both temporal (t) and maturation variables (x), and contains three parameters. The solution behavior depends on the initial function φ(x) and a three component parameter vector P=(δ,λ,r). For strictly positive initial functions, φ(0)≳0, there are three homogeneous solutions of biological (i.e., non-negative) importance: a trivial solution ut≡0, a positive stationary solution ust, and a time periodic solution up(t). For φ(0)=0 there are a number of different solution types depending on P: the trivial solution ut, a spatially inhomogeneous stationary solution unh(x), a spatially homogeneous singular solution us, a traveling wave solution utw(t,x), slow traveling waves ustw(t,x), and slow traveling chaotic waves uscw(t,x). The regions of parameter space in which these solutions exist and are locally stable are delineated and studied.
Wave collapse in plasmas and fluids
Authors: Kuznetsov E. A.
Published: July 2002

This lecture is a review of recent results (obtained mainly by the author and his co-authors) in the wave collapse theory with applications to plasma physics, fluid dynamics and nonlinear optics as well. The main attention in the review is paid to the qualitative reasons of the wave collapse and to the exact methods based on the integral estimations. Both approaches are applied to both the nonlinear Schrodinger equation and the two-dimensional generalized Benjamin–Ono equation which describes self-focusing of low-frequency oscillations in the boundary layer.
Directly sputter synthesized high-energy product Sm-Co based ferromagnetic films
Authors: Aly S. H. , Cheung T. D. , Wickramasekara L. , Cadieu F. J.
Published: July 2002

Different target configurations have been used to directly synthesize Sm-Co based films by selectively thermalized rf sputtering onto heated substrates in an in-plane magnetic field. Samples have been sputtered so that in certain cases a systematic gradient in Sm-to-Co concentration was created along the length of the substrates and in other cases so that films of a fixed uniform composition were deposited. Small subregions of a single substrate then serve as samples of a specific composition. For the Sm-to-Co composition range from 70 to 95 at. % Co gradient synthesized samples exhibit high magnetization and moderate intrinsic coercive forces. The maximum energy product of 20 MG Oe was obtained for a sample with 90.6 at. % Co containing mostly the Sm2Co17 phase. In addition, samples were synthesized from uniform composition targets at certain fixed compositions. Films sputtered from uniform composition Sm2(Co, Fe, Zr)17 targets exhibited square in-plane hysteresis loops and static energy products up to 21.2 MG Oe. It was observed that uniform composition SmCo5 films could be grown which exhibited either the (200) or the (110) texture as a function of the sputtering rate. Relatively higher sputtering rates of approximately 5 Å/sec resulted in the (200) texture growth pattern while lower rates resulted in (110) textured films. These differently textured SmCo5 films differ in both their microstructure and magnetic properties. The (110) textured film are finer in microstructure and magnetically harder than the (200) films. The uniform composition SmCo5 films of either texture in general were of smaller grain size and higher coercive forces than the SmCo5 subregions from films sputtered with a composition gradient spanning the 1–5 phase. A uniform composition SmCo5 film sputtered at 1.5 Å/sec which was (110) textured exhibited an intrinsic coercive force of 23 kOe and an energy product of 18 MG Oe at −63 °C. The high-energy products reported here are due to both the high induction and to the squareness of the in-plane hysteresis loops when measured in the direction parallel to the field applied during the sputter synthesis.
Magnetic susceptibility and magnetic resonance in the ordered state of single-crystal [NH3(CH2)2HN3]CuBr4
Authors: Rubenacker G. V. , Waplak S. , Hutton S. L. , Haines D. N. , Drumheller J. E.
Published: July 2002

We report here the single-crystal magnetic susceptibility data and the magnetic resonance behavior in the ordered state of (NH3(CH2)2NH3)CuBr4, which is a strongly coupled, pseudo-1D, Heisenberg antiferromagnet with strong in-plane ferromagnetic coupling. The Neel temperature is 58 K and appears to be sample-dependent. The observed resonances occur near g=2 with small anisotropies and suggest hidden spin canting with a second magnetic phase transition into a different ordered state at 20 K.
Hot-electron-induced defects at the Si-SiO2 interface at high fields at 295 and 77 K
Authors: Fischetti M. V. , Riccó B.
Published: July 2002

Fowler–Nordheim tunneling electron injection is performed at 295 and 77 K in metal-oxide-semiconductor capacitors. In both cases the positive charge generated at the Si-SiO2 interface is found to be the ‘‘anomalous’’ positive charge related to the slow states. At low temperature this charge is created at a faster rate than at 295 K for both positive and negative polarity. Its saturated density, on the contrary, strongly depends on the polarity of the applied bias voltage at low temperature, being much smaller during injection at positive bias. Fast and slow states appear only after the sample is heated to room temperature. While interband impact ionization is ruled out as a possible generation mechanism, the results are consistent with the idea that the hot injected electrons lose their energy at the anode-SiO2 interface by emitting an unidentified species which is responsible for the generation of the interfacial damage. A field- and temperature-activated migration of the positive defects from the sites of formation to the interface must occur before slow and fast states are observed. The correlation between slow states and hole traps is discussed.
X-ray diffuse scattering by composition waves in GaAlAs
Authors: Bocchi C. , Franzosi P. , Ghezzi C.
Published: July 2002

The possibility of using x-ray diffuse scattering experiments to investigate the atomic arrangement within the substituted sublattice in III-V solid solutions has been demonstrated. The Icw scattering by static composition waves has been separated from Compton and phonon scattering along the 100 direction in reciprocal space for a thick Ga1−xAlxAs (x=0.32) layer grown by liquid-phase epitaxy. The dependence of Icw on the scattering angle has given a detailed experimental proof that the Ga and Al atoms are distributed in a random way within the cation sublattice. An exception to this behavior, which is related to a slight (13%) increase of Icw at the 100 point, is discussed in the light of the pseudochalcopyrite model for the local bonding in III-V alloys.
Preparation of γ-LaSy (1.33<y<1.50) alloys by the pressure-assisted reaction sintering method and their thermoelectric properties
Authors: Takeshita T. , Gschneidner K. A. , Beaudry B. J.
Published: July 2002

The pressure-assisted reaction sintering (PARS) method was used to prepare thermoelectric elements of the γ-phase lanthanum sesquisulfide solid solution alloys which exist over a wide homogeneity range: LaSy (1.33<y<1.5). A mixture of powders of stoichiometric lanthanum sesquisulfide (La2S3) and lanthanum trihydride (LaH3) was reaction sintered in a hot-press to make the desired composition of γ-LaSy. This method produced black dense single-phase γ-LaSy pellets with a theoretical density of 97% or better. The thermoelectric properties (electrical resistivity and Seebeck coefficient) of the samples prepared by the PARS technique were measured. The room temperature electrical resistivity and Seebeck coefficient of γ-LaSy varied in a systematic manner with the electron concentration indicating that γ-LaSy alloys behave like a simple metal. Although the current carriers appear to be quite heavy (drift mobility ∼3 cm2/V s), the temperature dependence (up to 1000 °C) of these properties of LaS1.42 were found to be linear functions, which is in accord with the above observations. The thermoelectric figure of merit based on the measured resistivity and Seebeck coefficient and an estimated thermal conductivity for LaS1.42 is 0.75×10−3 °C−1 at 1000 °C, which is comparable to those of the state-of-the-art materials such as silicon–germanium alloys at 1000 °C.
Evidence for graphitic-type bonding in glow discharge hydrogenated amorphous silicon carbon alloys
Authors: Mahan A. H. , von Roedern B. , Williamson D. L. , Madan A.
Published: July 2002

Amorphous silicon carbon (a-SiC:H) films have been deposited by the glow discharge technique using SiH4 and CH4 gas mixtures. At high discharge powers and low deposition chamber pressures, evidence for graphitic-type bonding in C-deficient a-SiC:H is found and correlations are made between the appearance of this bonding with significant changes in the electronic and structural properties. This graphitic-type bonding can be minimized by significant H attachment to C via CHn (n=2, 3) bonding. This results in a-SiC:H films with low gap state densities and sharp Urbach tails.
Growth-induced uniaxial anisotropy of bismuth-substituted iron-garnet films
Authors: Hansen P. , Witter K.
Published: July 2002

The temperature dependence of the growth-induced uniaxial anisotropy constant Kgu of epitaxially grown bismuth-substituted garnet films of composition R3−xBixFe5O12 with R=Y, Lu has been measured in the temperature range 4.2 K≤T≤TC. Films with (100), (110), (112), and (111) orientation have ben investigated and exhibit a very different temperature variation of Kgu. The experimental data are compared with theoretical results based on a recently reported single-ion theory applying to bismuth-induced uniaxial anisotropies. Kgu is calculated in terms of the actual sublattice magnetizations inferred from the fit of the molecular field theory to the measured saturation magnetization.
Effective thermal conductivity of a misoriented short fiber composite
Authors: Hatta Hiroshi , Taya Minoru
Published: July 2002

This paper examines the effective thermal conductivity of a misoriented short fiber composite. The analysis is based on the equivalent inclusion method for steady-state heat conduction in composite which we have recently proposed. The present approach is unique in that it takes into account the interaction among fibers at different orientations. Closed form solutions are given for the thermal conductivity of a misoriented short fiber composite. Then, numerical results are presented to demonstrate the effects of volume fraction, fiber aspect ratio, and distribution function of fiber orientation on the thermal conductivity.
Thermoelectric properties of lanthanum sulfide
Authors: Wood C. , Lockwood A. , Parker J. , Zoltan A. , Zoltan D. , Danielson L. R. , Raag V.
Published: July 2002

The Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall effect have been studied in γ-phase La3−xS4 (LaSy) in the composition range 0.04≤x≤0.3 (1.35≤y≤1.48) in order to ascertain its suitability for high-temperature (300 to 1400 K) thermoelectric energy conversion. In this temperature and composition range the material behaves as an extrinsic semiconductor whose degenerate carrier concentration is controlled by the stoichiometric ratio of La to S. A maximum figure-of-merit (Z) of ∼5×10−4 K−1 at a composition x=0.3, y=1.48 (LaS1.48) was obtained.
A model for the low-temperature growth of epitaxial Ge and Si films from GeH2 and SiH2 radicals produced by UV photolysis of GeH4 and SiH4
Authors: Motooka T. , Greene J. E.
Published: July 2002

Recent experimental results have demonstrated low-temperature growth of polycrystalline and single-crystal Ge films by UV laser-induced chemical vapor deposition (UV-LCVD) from GeH4. It is also known from experiment that the initial photofragments produced during the UV photolysis of GeH4 and SiH4 are GeH2 and SiH2 radicals, respectively. In this paper, we present an idealized model for the low-temperature epitaxial growth of Ge and Si films from the initial GeH2 and SiH2 radicals. This work is meant to serve as a first step in the development of a more detailed model which can also account for the role of other radicals, such as GeH3 and SiH3, which are formed in subsequent gas-phase reactions. The rate-limiting step for film deposition in the present case was found to be the formation and desorption of H2 molecules through bond switching from M-H (M=Ge or Si) to H-H. A cluster description of GeH2-adsorbed Ge(100) surfaces was combined with molecular orbital calculations to estimate the activation energy for H2 desorption as 40 meV, a value which is in reasonable agreement with the measured activation barrier, 85±20 meV, for UV-LCVD growth of polycrystalline Ge from GeH4.