bluuebunny/crossref_metadata_2025 huggingface dataset:
Multidimensional energy barrier distributions of interacting magnetic particles evaluated at different magnetization states
Authors: Chubykalo-Fesenko O. , Chantrell R. W.
Published: May 2005

We evaluate the energy barrier distributions of coupled Co particles as a function of their concentration, which changes the strength of magnetostatic interactions, in a multidimensional space to take into account possible collective reversal. The distributions are evaluated at the remanence, demagnetized state and coercivity for Co particles with 2D random anisotropy easy axes orientations and with concentrations up to 56%. We show that the overall influence of interactions in this system is always to broad the energy barrier distribution. Consequently, in all the cases the magnetization decay would start earlier when the strength of interactions increases. At the remanence, strong interactions displace the average energy barrier to larger values. However, at the coercive force it is displaced to smaller values. At the same time, at the demagnetized state, the peak of the distribution practically remains at the same position.
On Jacobian matrices for flows
Authors: Doyon B. , Dubé L. J.
Published: May 2005

We present a general method for constructing numerical Jacobian matrices for flows discretized on a Poincaré surface of section. Special attention is given to Hamiltonian flows where the additional constraint of energy conservation is explicitly taken into account. We demonstrate the approach for a conservative dynamical flow and apply the technique for the general detection of periodic orbits.
Enhancement of polar Kerr effect by forming Au nanoparticles on Ni surface
Authors: Jen S. U. , Chen K. C.
Published: May 2005

Au ( x ) ∕ Ni ( y ) ∕ glass ( sub ) bilayered films, with x ranging from 0to310Å and y=85Å, were made by the vapor evaporation method in vacuum. The surface morphology of each bilayer was examined by an atomic force microscope. The results show that when x=xm=25 and 100Å, respectively, there were, in particular, a large number of Au nanoparticles (or nanoislands) forming on top of the Ni surface. As a result, we observed considerable enhancement in either the polar Kerr rotation θK or the extraordinary Hall coefficient RS at these two thicknesses. As is well known, the penetration depth δP of the electron transverse flow, crossing the Au∕Ni interface from the Ni to the Au layer, is of the order of the electron mean free path ℓAu, i.e., δP=(3∕8)ℓAu≈116Å, in the Au layer. Hence, the situation xm<δP is always satisfied. Then, the strong enhancement of θK is due to the surface plasma resonance effect on the Au nanoparticles (e.g., by reducing the real part of the diagonal dielectric tensor εxx so that Re[εxx(ω)]≈1, where ℏω=1.96eV). Moreover, the formation of Au nanoparticles roughens the bilayer’s surface. That means the enhancement of RS is due to the increase of the surface resistivity ρS. Finally, since the side-jump mechanism is effective, it could affect θK too (e.g., by enhancing the imaginary part of the off-diagonal conductivity Im[σxy]).
[
Co
Fe
∕
Pt
]
×
n
multilayer films with a small perpendicular magnetic anisotropy
Authors: Ding Yunfei , Judy Jack H. , Wang Jian-Ping
Published: May 2005

The effects of CoFe thickness, Pt thickness, and number of CoFe∕Pt bilayers on the anisotropy and coercivity of [CoFe∕Pt]×n multilayer films have been studied. These parameters are varied in an attempt to deposite [CoFe∕Pt]×n multilayer films with well-defined small perpendicular magnetic anisotropies. Best results were obtained in a [CoFe3Å∕Pt10Å]×5 film with coercivity Hc=42Oe, perpendicular anisotropy Hk=2200Oe, and easy-axis remanence Mr∕Ms=1. Large Pt thickness tends to cause well-defined interfaces thus larger surface anisotropy. Large CoFe thickness and more number of bilayers tend to cause bow-tie shaped easy-axis loops and multiple domain structures.
Ferromagnetic shape memory alloys: Theory of interactions
Authors: Paul D. I. , O’Handley R. C. , Peterson Bradley
Published: May 2005

Twin boundary motion in a viscous discrete lattice is examined in the presence of a dislocation defect and a magnetic driving force supplemented by an acoustical signal. The results show, amongst others, a resistance to twin boundary motion due to lattice discreteness and a large but temporary reduction in twin boundary velocity during the interaction between the localized pulse and the dislocation.
Domain engineered states over various length scales in (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3 crystals: Electrical history dependence of hierarchal domains
Authors: Bai Feiming , Li Jiefang , Viehland D.
Published: May 2005

The dependence of the domain structure on poling for (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3 crystals with x=20, 30, 35, and 40 has been investigated by scanning force microscopy in the piezoresponse mode and by polarized optical microscopy. The results demonstrate a domain hierarchy on various length scales, ranging from nanometers to millimeters, which is notably dependent upon the electrical history of the specimen. The results provide important insights into the nature of the domain engineered state in these crystals.
Electromagnetic modeling of terahertz quantum cascade laser waveguides and resonators
Authors: Kohen Stephen , Williams Benjamin S. , Hu Qing
Published: May 2005

Finite-element numerical modeling and analysis of electromagnetic waveguides and resonators used in terahertz quantum cascade lasers are presented. Both metal-metal and semi-insulating (SI) surface-plasmon ridge waveguide geometries were investigated. Simulations and analysis of two types were performed: two-dimensional waveguides (eigenmode calculation), and two- and three-dimensional resonators (facet reflectivity calculation for infinite width and finite width waveguides, respectively). Waveguide simulations extend previous transverse one-dimensional analyses to two dimensions (for the lateral and transverse dimensions), and quantify the breakdown of the one-dimensional approximation as the ridge width is reduced. Resonator simulations in two and three dimensions are presented and are used to obtain facet reflectivities and output radiation patterns. For the metal-metal waveguide structures, these resonator simulations quantitatively show strong deviations for terahertz facet reflectivities from those predicted by the effective index method. However, the effective index method for calculating reflectivities is shown to still be valid for SI surface-plasmon waveguides.
Electrical and microstructural characterization of molybdenum tungsten electrodes using a combinatorial thin film sputtering technique
Authors: Jun Seung-Ik , Rack Philip D. , McKnight Timothy E. , Melechko Anatoli V. , Simpson Michael L.
Published: May 2005

A combinatorial rf magnetron sputter deposition technique was employed to investigate the electrical characteristics and microstructural properties of molybdenum tungsten (MoW) high temperature electrodes as a function of the binary composition. In addition to the composition, the effect of substrate bias and temperature was investigated. The electrical resistivity of MoW samples deposited at room temperature with zero bias followed the typical Nordheim’s rule as a function of composition. The resistivity increases with tungsten fraction and is a maximum around 0.5 atomic fraction of tungsten. A metastable β-W phase was identified and the relative amount of the β-W phase scales with the resistivity. Samples deposited at higher temperature (250°C) also followed Nordheim’s rule as a function of composition, however, it did not contain the metastable β-W phase and consequently had a lower resistivity. The resistivity of samples deposited with substrate bias is uniformly lower and obeyed the rule of mixtures as a function of composition. The molybdenum-rich compositions had a lower resistivity, contrary to expectations based on bulk resistivity values, and is attributed to high electron-dislocation scattering cross sections in tungsten versus molybdenum. The metastable β-W phase was not observed in the biased films even when deposited at room temperature. High resolution scanning electron microscopy revealed a more dense structure for the biased films, which is correlated to the significantly lower film resistivity.
Lagrange–Fedosov nonholonomic manifolds
Authors: Etayo Fernando , Santamaría Rafael , Vacaru Sergiu I.
Published: May 2005

We outline a unified approach to geometrization of Lagrange mechanics, Finsler geometry and geometric methods of constructing exact solutions with generic off-diagonal terms and nonholonomic variables in gravity theories. Such geometries with induced almost symplectic structure are modeled on nonholonomic manifolds provided with nonintegrable distributions defining nonlinear connections. We introduce the concept of Lagrange–Fedosov spaces and Fedosov nonholonomic manifolds provided with almost symplectic connection adapted to the nonlinear connection structure. We investigate the main properties of generalized Fedosov nonholonomic manifolds and analyze exact solutions defining almost symplectic Einstein spaces.
Magnetic properties of dilute FePt:C nanocluster films
Authors: Xu Yingfan , Yan M. L. , Zhou J. , Sellmyer D. J.
Published: May 2005

Nanocluster-assembled dilute (FePt)xC100−x films with 30⩾x⩾5 (x denotes volume fraction) were produced using a gas-aggregation technique. FePt clusters with an average size of about 4.0nm (with standard deviation σ∕d=0.09) were embedded in high volume fraction of carbon matrix, which is used to isolate the FePt clusters. Postdeposition annealing was used to realize the high-anisotropy L10 phase. Single-crystal features of well-isolated clusters were observed by transmission electron microscope (TEM) in annealed dilute films. The coercivity of the films annealed at 700°C for 5min was a few hundred Oersteds at room temperature, while a coercivity about 4kOe was observed at low temperature 10K for x=5, indicating partial ordering of the clusters. The coercivity strongly depends on annealing temperature and annealing time, and increases with decreasing FePt volume fraction. For a dilute FePt cluster film with x=5 annealed at 700°C for 60min, a room-temperature coercivity of about 30kOe and low-temperature (10K) coercivity of about 40kOe were obtained, which imply an anisotropy field of 83kOe, assuming the clusters are noninteractive. The results are discussed by comparing with simulations for Stoner–Wohlfarth particles.
Magnetic properties of Fe∕AlOx∕GaAs(001) structures
Authors: Vaz C. A. F. , Ionescu A. , Trypiniotis T. , Bland J. A. C. , Dalgliesh R. M. , Langridge S.
Published: May 2005

We report results on the effect of the AlOx barrier layer on the magnetic moment of Fe using polarized neutron reflection. The sample has nominal composition 25nmAu∕5nmFe∕1.3nmAlOx∕GaAs(001). The metal deposition was done in ultrahigh vacuum and the Al oxidation was performed in two steps. From the fit to the neutron data we obtain for the magnetic moment of Fe the value (2.164±0.026)μB, very close to the bulk value (2.185μB at room temperature). We conclude that there is no dead layer at the AlOx interface.
Magnetic field dependence of galfenol elastic properties
Authors: Petculescu G. , Hathaway K. B. , Lograsso T. A. , Wun-Fogle M. , Clark A. E.
Published: May 2005

Elastic shear moduli measurements on Fe100−xGax (x=12–33) single crystals (via resonant ultrasound spectroscopy) with and without a magnetic field and within 4–300 K are reported. The pronounced softening of the tetragonal shear modulus c′ is concluded to be, based on magnetoelastic coupling, the cause of the second peak in the tetragonal magnetostriction constant λ100 near x=28. Exceedingly high ΔE effects (∼25%), combined with the extreme softness in c′ (c′<10GPa), suggest structural changes take place, yet, gradual in nature, as the moduli show a smooth dependence on Ga concentration, temperature, and magnetic field. Shear anisotropy (c44∕c′) as high as 14.7 was observed for Fe71.2Ga28.8.
Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography
Authors: Teh W. H. , Dürig U. , Drechsler U. , Smith C. G. , Güntherodt H.-J.
Published: May 2005

We report the quantitative characterization and analysis on the solidification of SU-8, a chemically amplified near-ultraviolet ultrathick resist, based on two-photon-absorbed (TPA) near-infrared photopolymerization. The resolution of TPA photopolymerized SU-8 voxels and lines is studied as a function of laser-pulse energy, single-shot exposure time, and scanning speed. Two-photon microstereolithography using SU-8 as the matrix material was verified by the fabrication of SU-8 photoplastic structures with subdiffraction-limit resolution. We show that the nonlinear velocity dependence of TPA photopolymerization can be used as the shutter mechanism for disruptive three-dimensional (3D) lithography. This mechanism, when combined with low numerical-aperture optics is exploited for the rapid 3D microfabrication of ultrahigh-aspect-ratio (up to 50:1) photoplastic pillars, planes, and cage structures.
Nonlinear shunt paths in thin-film CdTe solar cells
Authors: McMahon T. J. , Berniard T. J. , Albin D. S.
Published: May 2005

Here we report on a nonlinear, sometimes metastable, shunt-path loss mechanism located at thin-film (TF) solar cell corners, and presumably to a lesser degree, edges that causes much of the degradation in performance during stress testing. We use dark I–V curves and current transients to study the voltage switching behavior for normal, undegraded cells, and degraded cells with these shunt paths that are devastating to performance. By physically removing the microshunt-path defect, located with infrared imaging, and knowing the light and dark I–V performance before and after its removal, we can model the electrical properties of that defect and demonstrate how it so severely compromises device performance. Using a one-dimensional model to describe such cell degradation is inappropriate and misleading. We propose a defect mechanism and discuss the implications for scribe lines and cell ends in full size modules.
Isothermal equations of state of beta octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine at high temperatures
Authors: Gump Jared C. , Peiris Suhithi M.
Published: May 2005

Isothermal pressure-volume equations of state of beta HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) at temperatures of 30, 100, and 140°C under both hydrostatic and nonhydrostatic compressions have been obtained using synchrotron angle-dispersive x-ray diffraction experiments. The samples were heated to the isotherm temperature and compressed up to 5.8GPa. At all temperatures HMX remained in the beta phase up to 5.8GPa. However, at 140°C upon decompression to ambient from nonhydrostatic pressures above 4GPa, HMX underwent a phase transition to the delta phase. The same transition was seen upon decompression to ambient from hydrostatic compression; however, parts of the sample remain in the β phase, resulting in a mixed-phase sample. The diffraction data were analyzed to yield unit-cell dimensions at each pressure, and further analyzed to yield thermal expansion, bulk modulus, and the pressure derivative of the bulk modulus.
Precipitation and high temperature magnetic properties of FeCo-based alloys
Authors: Yu R. H. , Zhu J.
Published: May 2005

The microstructure and high temperature magnetic properties of Fe–Co–2V and Fe–Co–2V–0.3Nb alloys have been investigated. Aging at 600 °C, γ2 phases precipitate whose size and volume fraction increase with increasing aging time. A roughly linear relationship between magnetic coercivity Hc and volume fraction of the γ2 precipitates was observed. Addition of Nb facilitates nucleation and growth of the γ2 precipitates, resulting in a dramatic deterioration of soft magnetic properties of the alloys. A pinning effect of the γ2 precipitates on the magnetic domain wall has been observed by Lorenz electron microscopy.
Diffracted magneto-optical Kerr effects of permalloy ring arrays
Authors: Zhu Xiaobin , Liu Zhigang , Freeman Mark R. , Metlushko Vitali
Published: May 2005

Diffracted magneto-optical Kerr effect (MOKE) technique was used to study the quasistatic magnetization switching of thin-film permalloy ring structures. Two different switching mechanisms were observed, dependent on the width of the annular portion of the ring. For small widths, a two-step switching is found, from an initial onion state to a state of circulation magnetization around the ring to a final onion state. In the wider rings, the switching from the circulation state to the onion state takes place via nucleation of a vortex within the annulus. The presence of the vortex is determined by the higher spatial frequency information revealed by diffracted MOKE, and through comparison with micromagnetic simulation.
Large energy absorption in Ni–Mn–Ga/polymer composites
Authors: Feuchtwanger Jorge , Richard Marc L. , Tang Yun J. , Berkowitz Ami E. , O’Handley Robert C. , Allen Samuel M.
Published: May 2005

Ferromagnetic shape memory alloys can respond to a magnetic field or applied stress by the motion of twin boundaries and hence they show large hysteresis or energy loss. Ni–Mn–Ga particles made by spark erosion have been dispersed and oriented in a polymer matrix to form pseudo 3:1 composites which are studied under applied stress. Loss ratios have been determined from the stress-strain data. The loss ratios of the composites range from 63% to 67% compared to only about 17% for the pure, unfilled polymer samples.
Ion kinetic effects in radio-frequency sheaths
Authors: Xiang Nong , Waelbroeck F. L.
Published: May 2005

The kinetic effects of ions on the radio-frequency collisionless sheath dynamics are explored by solving the ion Vlasov equation with an ion source term. The ion velocity distribution function is obtained numerically. It is found to depend strongly on the source term. The macroscopic variables such as the ion density, velocity, electric field, and sheath voltage predicted by the kinetic model are in good agreement with the fluid model. The predicted period-averaged ion energy distributions at the cathodes, however, are different. In the high frequency regime, multiple peaks can be formed with some process parameters such as bias voltage and rf frequency. The mechanism for this multiple-peak structure can be attributed to the rf modulation of the sheath potential in combination with the creation of cold ions in the sheath as shown by Wild and Koidl for charge-exchange collisional plasma [J. Appl. Phys. 69, 2909 (1991)]. It is also found that the ion source temperature plays an important role in determining the ion velocity distribution function.
Er-doped ZnO thin films grown by pulsed-laser deposition
Authors: Pérez-Casero Rafael , Gutiérrez-Llorente Araceli , Pons-Y-Moll Olivier , Seiler Wilfrid , Defourneau Reine Marie , Defourneau Daniel , Millon Eric , Perrière Jacques , Goldner Philippe , Viana Bruno
Published: May 2005

Crystalline erbium(Er)-doped zinc oxide thin films have been grown by pulsed-laser deposition and were analyzed by the complementary use of Rutherford backscattering spectroscopy, x-ray diffraction analysis, atomic force microscopy, and photoluminescence. The composition, structure, and surface morphology of films were studied, as a function of the growth conditions (temperature from 300 °C to 750 °C and oxygen pressure from 10−6 to 0.5 mbar) and Er-doping rate, and were correlated to the emission spectroscopy of Er in the infrared domain. While these studies lead to the determination of optimal conditions for the growth of high crystalline quality films, results of photoluminescence experiments show that the insertion of Er ions in the ZnO matrix does not follow a simple pattern. The Er ions are incorporated from two pathways, one population is found inside the crystallites and another one at the grain boundaries, as a consequence of the differences in valence and ionic radius of Zn and Er.
Theoretical analysis of numerical aperture increasing lens microscopy
Authors: Ippolito S. B. , Goldberg B. B. , Ünlü M. S.
Published: May 2005

We present a detailed theoretical analysis and experimental results on a subsurface microscopy technique that significantly improves the light-gathering, resolving, and magnifying power of a conventional optical microscope. The numerical aperture increasing lens (NAIL) is a plano-convex lens placed on the planar surface of an object to enhance the amount of light coupled from subsurface structures within the object. In particular, a NAIL allows for the collection of otherwise inaccessible light at angles beyond the critical angle of the planar surface of the object. Therefore, the limit on numerical aperture increases from unity for conventional subsurface microscopy to the refractive index of the object for NAIL microscopy. Spherical aberration associated with conventional subsurface microscopy is also eliminated by the NAIL. Consequently, both the amount of light collected and diffraction-limited spatial resolution are improved beyond the limits of conventional subsurface microscopy. A theoretical optical model for imaging structures below the planar surface of an object, both with and without a NAIL, is presented. Experimental results demonstrating the predicted improvements in resolution of subsurface imaging are also presented.
Stress evolution during and after sputter deposition of Cu thin films onto Si (100) substrates under various sputtering pressures
Authors: Pletea M. , Brückner W. , Wendrock H. , Kaltofen R.
Published: May 2005

The stress evolution during and after dc magnetron sputter deposition of Cu thin films with thicknesses of 20 and 300 nm and deposited with a constant rate of 0.1nm∕s onto Si (100) substrates is studied for various sputtering pressures (0.05–6 Pa). The stress was determined by means of in situ wafer curvature measurements using an optical two-beam deflection method. To correlate the stress evolution with the microstructure development, microstructure investigations were performed by scanning electron microscopy, atomic force microscopy, and electron backscatter diffraction. The results show the transition from tensile to compressive stress with decreasing sputtering pressure at different stages of the deposition. The features of the stress evolution during the early stage of deposition can be ascribed to the Volmer–Weber mechanism. For thicker films, three regions of the sputtering pressure can be distinguished concerning their effect on the stress evolution. The transition from compressive to tensile stress was correlated with the evolution from a dense to an open microstructure and with increasing surface roughness by increasing sputtering pressure. The results of the stress and microstructure evolution are interpreted in the context of the mechanisms being discussed in the literature.
Diagnostic of the self-healing of metallized polypropylene film by modeling of the broadening emission lines of aluminum emitted by plasma discharge
Authors: Tortai J.-H. , Bonifaci N. , Denat A. , Trassy C.
Published: May 2005

Metallized-film capacitors have the property, even under high continuous voltage, to self-heal i.e., to clear a defect in the dielectric. The self-healing process is a consequence of a transient arc discharge. It has been previously shown that during the discharge, due to Joule effect, the metal is vaporized until the arc extinguishes. The discharge duration has been found to be inversely proportional to the mechanical pressure applied on the layers of metallized films making up a capacitor. The aim of this study is to understand the physical processes involved in this spontaneous extinction of the arc discharge. Emission spectroscopy has been used to provide information about the physical properties (temperatures, electronic and neutral particles densities, etc.) of the plasma induces by a self-healing. An analysis, based on the broadenings and shifts of Al atomic lines, of the experimental light spectra obtained has shown that the self-healing process leads to the generation, from the vaporized metal, of a high-density and relatively weakly ionized aluminum plasma. The plasma density increases with the pressure applied on the film layers and, consequently, the density power needed to extend the plasma zone increases as well and the arc discharge goes out faster as experimentally observed.
Energy absorption of superparamagnetic iron oxide nanoparticles by microwave irradiation
Authors: Kim Do Kyung , Amin M. Shahrooz , Elborai Shihab , Lee Se-Hee , Koseoglu Yüksel , Zahn Markus , Muhammed Mamoun
Published: May 2005

The main complexity in hyperthermia is generating and controlling the temperature distribution within tumor cells without damaging the normal tissue. Superparamagnetic iron oxide nanoparticles (SPIONs) with a diameter of 11nm were prepared by controlled coprecipitation and coated with oleic acid to prevent agglomeration and flocculation in the solvent. In situ monitoring of the temperature increment was performed to interpret the microwave absorption rate of the SPION dispersed in appropriate host media (polar or nonpolar solvents) during microwave irradiation. This approach allowed for the prediction of heating mechanisms as a result of the excitation of unpaired electrons of iron, effects of coating agents, particle size, and volume fraction (ϕ). The conversion efficiency from microwave irradiation to thermal energy was predicted by applying the conservation of energy to a differential volume. The rates of heat loss and energy absorption were obtained by nonlinear fitting of the experimental data.
The magnetic and magnetocaloric properties of Gd5Ge2Si2 compound under hydrostatic pressure
Authors: Carvalho A. Magnus G. , Alves Cleber S. , de Campos Ariana , Coelho Adelino A. , Gama Sergio , Gandra Flavio C. G. , von Ranke Pedro J. , Oliveira Nilson A.
Published: May 2005

The Gd5Ge2Si2 compound presents a giant magnetocaloric effect with transition temperature at around 276 K and is a very good candidate for application as an active regenerator material in room temperature magnetic refrigerators. Recently it has been shown that pressure induces a colossal magnetocaloric effect in MnAs, a material that presents a giant magnetocaloric effect and a strong magnetoelastic coupling, as also happens with the Gd5Ge2Si2 compound. This motivated a search of the colossal effect in the Gd5Ge2Si2 compound. This work reports our measurements on the magnetic properties and the magnetocaloric effect of Gd5Ge2Si2 under hydrostatic pressures up to 9.2 kbar and as a function of temperature. Contrary to what happens with MnAs, pressure increases the Curie temperature of the compound, does not affect the saturation magnetization and decreases markedly its magnetocaloric effect.
Activation volume and magnetization reversal of nanoparticle films composed of magnetically isolated Fe–Pt nanoparticles
Authors: Chen Mu-Pei , Nishio Hiroaki , Kitamoto Yoshitaka , Yamamoto Hiroshi
Published: May 2005

We report on activation volume and magnetization reversal of Fe–Pt nanoparticle films. In order to diminish coalescence and sintering during high-temperature annealing above 700K, carbon overcoats are deposited on Fe–Pt nanoparticle-films. TEM observations reveal that grains in the Fe–Pt nanoparticle-films are kept to be 5–6nm in size even after annealing at 873K; the particle size is close to the crystalline size estimated by Scherrer’s formula as 5nm from x-ray diffraction. In-plane coercivity at room temperature is 2.5kOe; the activation volume (1.16×10−19cm3) at around the coercivity is almost equal to the physical volume of the particles observed by TEM, suggesting that the nanoparticles are magnetically isolated and the magnetization reversal is based on rotation mode in the carbon-coated nanoparticle films.
Pulse electrodeposition and electrochemical quartz crystal microbalance techniques for high perpendicular magnetic anisotropy cobalt nanowire arrays
Authors: Ursache Andrei , Goldbach James T. , Russell Thomas P. , Tuominen Mark T.
Published: May 2005

This research is focused on the development of pulse electrodeposition techniques to fabricate a high-density array of vertically oriented, high-magnetic anisotropy cobalt nanowires using a porous polymer film template. This type of array is a competitive candidate for future perpendicular magnetic media capable of storage densities exceeding 1Terabit∕in.2 The polymer template, derived from a self-assembling P(S-b-MMA) diblock copolymer film, provides precise control over the nanowire diameter (15nm) and interwire spacing (24nm), whereas nanowire length (typically 50to1000nm) is controlled accurately with the aid of real-time electrochemical quartz crystal monitoring. Pulse and pulse-reversed electrodeposition techniques, as compared to dc, are shown to significantly enhance the perpendicular magnetic anisotropy of the magnetic nanowire array and ultimately result in coercivity as large as 2.7kOe at 300K. Magnetic and structural characterizations suggest that these properties arise from an improved degree of magnetocrystalline anisotropy (due to c-axis oriented crystal growth and improvements in crystal quality) that strongly supplements the basic shape anisotropy of the nanowires. Low temperature magnetometry is used to investigate exchange bias effects due to the incorporation of CoO antiferromagnetic impurities during the electrodeposition process and subsequent Co oxidation in air.
Kerr measurements on single-domain SrRuO3 thin films
Authors: Herranz G. , Dix N. , Sánchez F. , Martínez B. , Fontcuberta J. , García-Cuenca M. V. , Ferrater C. , Varela M. , Hrabovsky D. , Fert A. R.
Published: May 2005

We report on the magneto-optical measurements of an epitaxial SrRuO3 film grown on SrTiO3 (0 0 1), which previously was determined to be single domain orientated by x-ray diffraction and Raman spectroscopy techniques. Our experiments reveal a large Kerr rotation, which reaches a maximum value of about 0.5° at low temperature. By measuring magnetic hysteresis loops at different temperatures, we determined the temperature dependence of the Kerr rotation in the polar configuration. Values of the anisotropic magnetoresistance ∼20% have been measured. These values are remarkably higher than those of other metallic oxides such as manganites. This striking difference can be attributed to the strong spin-orbit interaction of the Ru 4d ion in the SrRuO3 compound.
Measurement of ultrafast dynamics in the interaction of intense laser pulses with gases, clusters, and plasma waveguides
Authors: Kim K. Y. , Alexeev I. , Milchberg H. M.
Published: May 2005

Femtosecond time-resolved dynamics in the interaction of high intensity, ultrashort laser pulses with various targets—gases, nanometer-size clusters, and plasma waveguides—was studied using a new ultrafast optical diagnostic: Single-shot supercontinuum spectral interferometry (SSSI). The diagnostic measures ultrarapid transients induced by an intense laser pulse in the complex refractive index of the evolving target medium, providing a direct view of how the laser-produced disturbances, such as plasma densities, evolve in time and space. Using the SSSI diagnostic (i) the laser-induced double step ionization of helium, (ii) time-resolved explosion dynamics of intense-laser-heated clusters, and (iii) the coupling and guiding of intense laser pulses injected into a plasma waveguide were examined.
Phase behavior of short-range square-well model
Authors: Pagan D. L. , Gunton J. D.
Published: May 2005

Various Monte Carlo techniques are used to determine the complete phase diagrams of the square-well model for the attractive ranges λ=1.15 and λ=1.25. The results for the latter case are in agreement with earlier Monte Carlo simulations for the fluid-fluid coexistence curve and yield new results for the liquidus-solidus lines. Our results for λ=1.15 are new. We find that the fluid-fluid critical point is metastable for both cases, with the case λ=1.25 being just below the threshold value for metastability. We compare our results with prior studies and with experimental results for the γII-crystallin.
Laser-driven fast-electron transport in preheated foil targets
Authors: Honrubia J. J. , Kaluza M. , Schreiber J. , Tsakiris G. D. , Meyer-ter-Vehn J.
Published: May 2005

Laser-driven relativistic electron transport through aluminum foils preheated and expanded by amplified spontaneous emission (ASE) prepulses has been studied by means of two- and three-dimensional hybrid particle-in-cell simulations. This study is motivated by recent proton acceleration experiments [M. Kaluza, J. Schreiber, M. I. K. Santala, G. D. Tsakiris, K. Eidmann, J. Meyer-ter-Vehn, and K. J. Witte, Phys. Rev. Lett. 93, 045003 (2004)] showing a significant effect of the ASE prepulse on the proton spectra. Here, it is found that electron-beam collimation due to magnetic fields is reduced and resistive heating by return currents is significantly enhanced, when considering ASE-expanded rather than unperturbed solid target foils. It is shown that this allows for a consistent picture of the new proton spectra and the parameters of the driving electron pulse (angular spread at injection, laser-to-electron conversion, and energy spectrum).
Computer simulations of polymer chain structure and dynamics on a hypersphere in four-space
Authors: Råsmark Per Johan , Ekholm Tobias , Elvingson Christer
Published: May 2005

There is a rapidly growing interest in performing computer simulations in a closed space, avoiding periodic boundary conditions. To extend the range of potential systems to include also macromolecules, we describe an algorithm for computer simulations of polymer chain molecules on S3, a hypersphere in four dimensions. In particular, we show how to generate initial conformations with a bond angle distribution given by the persistence length of the chain and how to calculate the bending forces for a molecule moving on S3. Furthermore, we discuss how to describe the shape of a macromolecule on S3, by deriving the radius of gyration tensor in this non-Euclidean space. The results from both Monte Carlo and Brownian dynamics simulations in the infinite dilution limit show that the results on S3 and in R3 coincide, both with respect to the size and shape as well as for the diffusion coefficient. All data on S3 can also be described by master curves by suitable scaling by the corresponding values in R3. We thus show how to extend the use of spherical boundary conditions, which are most effective for calculating electrostatic forces, to polymer chain molecules, making it possible to perform simulations on S3 also for polyelectrolyte systems.
Spin-up of a liquid metal flow driven by a rotating magnetic field in a finite cylinder: A numerical and an analytical study
Authors: Nikrityuk Petr A. , Ungarish Marius , Eckert Kerstin , Grundmann Roger
Published: May 2005

This paper presents a combined numerical and analytical study of the impulsive axisymmetric spin-up from rest of an isothermal liquid metal in a closed cylinder. The motion of the liquid is caused by the action of a low-frequency, low-induction rotating magnetic field, whose magnetic Taylor number is in the range (0.01–0.9) Tacr3D with Tacr3D given by Grants and Gerbeth [“Linear three-dimensional instability of a magnetically driven rotating flow,” J. Fluid Mech. 463, 229 (2002)]. The computations were performed for cylindrical containers of aspect ratios (diameter/height) R equal to 0.5, 1, and 2. The numerical simulations are compared with the predictions of an analytical model, valid for small Ekman numbers E extending a former work by Ungarish [“The spin-up of liquid metal driven by a rotating magnetic field,” J. Fluid Mech. 347, 105 (1997)]. The first phase of the motion from rest is an initial adjustment: the inviscid fluid begins to rotate due to the externally forced azimuthal acceleration, and concomitantly a meridional velocity field is induced by the unbalanced centrifugal effects; this occurs during, approximately, the first revolution about the axis. Subsequently, the spin-up flow is dominated by an axially independent swirl in the core (which can be regarded as a geostrophic mode) and Bödewadt (Ekman) layers on the top-bottom boundaries. This is accompanied by inertial oscillations. Both the development of the swirl motion and the decay of the inertial mode occur on the spin-up time scale predicted by the asymptotic model. The investigation also points out the important role of the aspect-ratio parameter in the analysis of the magnetohydrodynamics-driven spin-up. The efficiency of the driving decays strongly for R>0.6 and becomes close to zero at R≈3.3. The numerical results confirm the analytical inference that the thickness of the sidewall layer for the angular velocity adjustment is E1∕4∕R, and when this parameter is larger than 0.3 the sidewall influence gains dominance over the Ekman-layer effects. The frequency of the inertial oscillations increases with R.
Role of inclined threading dislocations in stress relaxation in mismatched layers
Authors: Cantu P. , Wu F. , Waltereit P. , Keller S. , Romanov A. E. , DenBaars S. P. , Speck J. S.
Published: May 2005

(0001)-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained III-nitride layers may gradually relax. We report on the stress relaxation of Al0.49Ga0.51N layers grown on nominally relaxed Al0.62Ga0.38N buffer layers on sapphire. The reduction in elastic strain of the Al0.49Ga0.51N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al0.49Ga0.51N layer was determined by on-axis ω-2θ scans of (000l) peaks and reciprocal space maps of inclined (off-axis) peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al0.49Ga0.51N layer inclined from the [0001] direction towards ⟨11¯00⟩ directions by ∼15–25°, perpendicular to their Burgers vector (13⟨112¯0⟩). These inclined threading dislocations have a misfit dislocation component and thus provide stress relief. The contribution of the dislocation inclination to the degree of relaxation has been formulated and the energy release has been determined for dislocation inclination in mismatched stressed layers.
Dynamic Kerr effect responses in the terahertz range
Authors: Häberle Uli , Diezemann Gregor
Published: May 2005

Dynamic Kerr effect measurements provide a simple realization of a nonlinear experiment. We propose a field-off experiment where an electric field of one or several sinusoidal cycles with frequency Ω is applied to a sample in thermal equilibrium. Afterwards, the evolution of the polarizability is measured. If such an experiment is performed in the terahertz range it might provide valuable information about the low-frequency dynamics in disordered systems. We treat these dynamics in terms of a Brownian oscillator model and calculate the Kerr effect response. It is shown that frequency-selective behavior can be expected. In the interesting case of underdamped vibrational motion we find that the frequency dependence of the phonon damping can be determined from the experiment. Also the behavior of overdamped relaxational modes is discussed. For typical glassy materials we estimate the magnitude of all relevant quantities, which we believe will be helpful in experimental realizations.
Probing interactions between colloidal particles with oscillating optical tweezers
Authors: Mellor Christopher D. , Sharp Melissa A. , Bain Colin D. , Ward Andrew D.
Published: May 2005

The potential of optical traps for the study of weak interactions in colloidal systems is well recognized. In this paper, a theoretical model is developed for two oscillating interacting hard spheres in a fluid in the low Reynolds number limit. The amplitude of oscillation of the particle and the phase lag between the motion of the particles and the optical traps are a function of both the hydrodynamic coupling between the particles and the interaction potential. The effect of the particle-particle interaction is most pronounced for particles oscillated asynchronously along their line of centers. The experimental realization of this scheme is demonstrated for the model system of polystyrene latices in water and the theoretical predictions are compared with the experimental results. The extension of this idea to oil-in-water emulsions is discussed.
Structure and magneto-optic Kerr measurements of epitaxial MnSi films on Si(111)
Authors: Schwinge K. , Müller C. , Mogilatenko A. , Paggel J. J. , Fumagalli P.
Published: May 2005

MnSi films are grown by evaporation of Mn onto Si(111) substrates under ultrahigh-vacuum conditions. Films are characterized with real- and reciprocal-space surface-science techniques such as scanning tunneling microscopy and low-energy electron diffraction. The bulk structure is determined ex situ by transmission electron microscopy (TEM). Thin (<60Å Mn-deposited) MnSi films show a regularly modulated surface due to a 3% lattice mismatch of the interfaces. TEM confirms epitaxial growth and demonstrates smooth, atomically flat interfaces. Temperature-dependent ex situ measurements of the magneto-optic Kerr effect show ferromagnetism with an in-plane easy axis magnetic anisotropy for 60- and 100-Å-thick Mn films.
Structural and dielectric properties of Nd3+-doped Ba0.77Ca0.23TiO3 ceramics
Authors: Mazon T. , Hernandes A. C. , Souza Filho A. G. , Moraes A. P. A. , Ayala A. P. , Freire P. T. C. , Mendes Filho J.
Published: May 2005

The structural and dielectric properties of both pure and Nd3+-doped Ba0.77Ca0.23TiO3 (BCT23) ceramics obtained from solid-state reaction were investigated. The BCT23 ceramics sintered at 1300 °C for 3 h showed a dense microstructure without the presence of secondary phases. The use of Nd3+ as a dopant affected the microstructure of these ceramics. Samples prepared with Nd3+ exhibit a more homogeneous microstructure without a liquid phase on the grain boundaries. The dielectric constant and Raman scattering measurements indicate that the Curie temperature of the ferroelectric phase transition depends on Nd3+ content. It shifts toward lower temperatures with Nd3+ addition.
Adhesion effects on contact opening dynamics in micromachined switches
Authors: Jensen Brian D. , Huang Kuangwei , Chow Linda L.-W. , Kurabayashi Katsuo
Published: May 2005

We propose a technique to measure the opening time for micromachined switches and present substantial experimental data for switches with gold–gold contacts. The data demonstrate that contact opening time increases dramatically as apparent contact area increases or as pull-apart force or contact resistance decreases. A model of opening time is also presented with model parameters that fit the experimental data. Moreover, we show that transient mechanical vibrations can play an important role in reducing switch opening time.
Electron tunneling of photochemical reactions on metal surfaces: Nonequilibrium Green’s function–density functional theory approach to photon energy dependence of reaction probability
Authors: Nakamura Hisao , Yamashita Koichi
Published: May 2005

We have developed a theoretical model of photoinduced reactions on metal surfaces initiated by the substrate/indirect excitation mechanism using the nonequilibrium Green’s function approach. We focus on electron transfer, which consists of (1) electron-hole pair creation, (2) transport of created hot electrons, and (3) tunneling of hot electrons to form an anion resonance. We assume that steps (1), (2), and (3) are separable. By this assumption, the electron dynamics might be restated as a tunneling problem of an open system. Combining the Keldysh time-independent formalism with the simple transport theory introduced by Berglund and Spicer, we present a practical scheme for first-principle calculation of the reaction probability as a function of incident photon energy. The method is illustrated by application to the photoinduced desorption/dissociation of O2 on a Ag(110) surface by adopting density functional theory.
Enhanced annealing of damage in ion-implanted 4H-SiC by MeV ion-beam irradiation
Authors: Kinomura A. , Chayahara A. , Mokuno Y. , Tsubouchi N. , Horino Y.
Published: May 2005

The effect of ion-beam annealing for implantation-induced damage in single-crystalline 4H silicon carbide has been studied. Four sets of samples, implanted with two types of ions (C or Si) and two different damage levels (complete or incomplete amorphization), were prepared to investigate the influence of damaging conditions. The damaged samples were irradiated with a 3-MeV Ge ion beam at 600 °C in the range of 1×1015–2×1016cm−2 to induce the ion-beam annealing. Some of the damaged samples were thermally annealed without the Ge irradiation to evaluate pure thermal effects. Rutherford backscattering/channeling for these samples revealed substantial enhancements of damage annealing under the MeV ion-beam irradiation. The enhanced annealing effect was stronger for the incompletely amorphized samples than for the completely amorphized samples. For both cases, the annealing effects almost saturated with increasing ion fluence. The results suggest the competition between the annealing and damaging effects induced by the annealing beam, at least, for the incompletely amorphized samples.
Scattering kernel for polyatomic molecules
Authors: Méolans J. Gilbert , Dadzie S. Kokou
Published: May 2005

A polyatomic scattering kernel phenomenologically presented in a previous paper is derived from an integral operator formulation. The five parameters involved in the scattering kernel expression are shown to be equal to the accommodation coefficients of various fluxes at the wall, namely, the fluxes of the three components of the momentum and the fluxes of the rotational and vibrational energies of molecules. Under its present form the model is especially convenient for the diatomic molecules.
On the infimum of quantum effects
Authors: Gheondea Aurelian , Gudder Stanley , Jonas Peter
Published: May 2005

The quantum effects for a physical system can be described by the set E(H) of positive operators on a complex Hilbert space H that are bounded above by the identity operator. While a general effect may be unsharp, the collection of sharp effects is described by the set of orthogonal projections P(H)⊆E(H). Under the natural order, E(H) becomes a partially ordered set that is not a lattice if dimH⩾2. A physically significant and useful characterization of the pairs A,B∊E(H) such that the infimum A∧B exists is called the infimum problem. We show that A∧P exists for all A∊E(H), P∊P(H) and give an explicit expression for A∧P. We also give a characterization of when A∧(I−A) exists in terms of the location of the spectrum of A. We present a counterexample which shows that a recent conjecture concerning the infimum problem is false. Finally, we compare our results with the work of Ando on the infimum problem.
Dopant and defect interactions in polycrystalline silicon thin-film transistors
Authors: Valletta A. , Mariucci L. , Bonfiglietti A. , Fortunato G. , Brotherton S. D.
Published: May 2005

The influence of channel doping on the threshold voltage of polycrystalline silicon thin-film transistors (TFTs) has been investigated using both boron and phosphorus implants. For each dopant type, asymmetric TFT behavior was seen between enhancement and depletion mode TFTs, with the enhancement mode TFTs always showing a greater shift in threshold voltage for a given dose. From a detailed analysis of the boron-doped samples, using a two-dimensional device simulator, it was demonstrated that the results could be best explained in terms of an asymmetric density of states, in which the deep trapping state density in the lower half of the band gap increased with increasing boron concentration, and the effect of the dopant on threshold voltage was itself determined by this local defect concentration.