bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ characterization of titanium silicide formation: The effect of Mo interlayer, temperature ramp-rate, and annealing atmosphere
Authors: Zhang S.-L. , Lavoie C. , Cabral C. , Harper J. M. E. , d’Heurle F. M. , Jordan-Sweet J.
Published: July 2002

The formation of titanium silicides has been studied using simultaneous in situ x-ray diffraction with millisecond time resolution and sheet resistance measurements. The effect of a Mo interposed layer between Ti films and Si substrate was investigated by varying the thickness of the Mo interlayer from 0 (Ti/Si) to 1.8 nm (Ti/Mo/Si). The thickness of Ti was kept to 55 nm for all samples. Both isothermal annealing and ramp annealing in helium were performed in order to study the mechanism of silicide formation. While C49 TiSi2 was the only disilicide found after annealing Ti/Si at 650 °C for 20 min, C54 TiSi2 was readily formed in the presence of Mo. The formation of C49 TiSi2 was not observed with a 1.8 nm thick Mo interlayer. Instead, there was indication that C40 (Mo,Ti)Si2 was formed. In addition, broad diffraction peaks, weak in intensity, could be all assigned to Ti5Si4. However, the presence of Ti5Si4 alone did not induce the formation of C54 TiSi2. Even with a 20 °C increase in isothermal annealing temperature to 670 °C for 20 min, the presence of Mo was found to be a necessity for the formation of C54 TiSi2. When annealed in nitrogen, instead of helium, the formation of surface titanium nitride competes with the formation of silicides. The formation of C54 TiSi2 was even suppressed in the sample with a 1.8 nm Mo interlayer when annealed at 650 °C in nitrogen. The formation of Ti5Si4 and the role it plays in the formation of other silicides (C40, C49, and C54) are discussed on the basis of simple crystallographic considerations. The effect of a Mo interlayer and/or ramp-rate on the formation of C40 (Mo,Ti)Si2 and C49 TiSi2 is discussed in conjunction with variations of the preferential orientation of C54 TiSi2 films.
Contribution of interface roughness to the infrared spectra of thermally grown silicon dioxide films
Authors: Ishikawa Kenji , Ogawa Hiroki , Fujimura Shuzo
Published: July 2002

Interfacial oxide structures were studied by Infrared reflection absorption spectroscopic analysis. The longitudinal optical mode of the interfacial oxide films arose with lower frequencies and wider widths. We assumed and later verified that these changes resulted from the contribution of interfacial roughness. We therefore examined the contribution of interfacial roughness to the infrared spectra using the effective medium approximation model and via infrared spectra simulation. The experimental results explained by a model of that interface had an atomically roughness of within 1 nm and chemical structure changes effectively little fluctuation.
Photobleaching and photodegradation of photoluminescence in thin films of poly(methylphenylsilane)
Authors: Nakayama Yoshikazu , Inagi Hiroyuki , Zhang Mei
Published: July 2002

The photobleaching process has been studied systematically in poly(methylphenylsilane) thin films with thickness comparable to or less than the penetration depth of incident ultraviolet light. It is found out that the photoscission of σ bonds proceeds from longer segments of Si chains to shorter segments, which has been overlooked in an analysis of the photobleaching process so far. Taking into account this phenomenon and its temperature dependence, a thermally assisted photoscission model is proposed. In the model the activation energy ε has a distribution combining two exponential functions of exp(ε/ε1) in a low energy region and exp(−ε/ε2) in a high energy region with the transition energy εT separating the two. The shape of the energy distribution is independent of the length of segments and the photoscission cross section Cr is larger for longer segments. The analysis based on this model is fairly consistent with the experimental results. The parameters of ε1, ε2, εT and Cr are confirmed to be 38 meV, 31 meV, 73 meV, and 5.0×10−17–1.3×10−16 cm2, respectively. The photodegradation of the photoluminescence from the thin films is also well explained on the basis of the proposed model.
Optical properties of the bulk amorphous semiconductor ZnIn2Te4
Authors: Matsumoto Yasuyuki , Ozaki Shunji , Adachi Sadao
Published: July 2002

The bulk amorphous-like (a-) semiconductor, a-ZnIn2Te4, is prepared by the melt-quench technique. The pseudodielectric-function spectra, 〈ε(E)〉=〈ε1(E)〉+i〈ε2(E)〉, of a-ZnIn2Te4 are measured by spectroscopic ellipsometry in the 1.2–5.3 eV photon-energy range at room temperature. The measured 〈ε(E)〉 spectra strongly resemble those of amorphous tetrahedrally bonded semiconductors. These spectra are analyzed using a simplified model of the interband optical transitions. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of a-ZnIn2Te4 are also been presented.
Application of a dc glow discharge source with controlled plasma potential in plasma immersion ion implantation
Authors: Ueda M. , Berni L. A. , Gomes G. F. , Beloto A. F. , Abramof E. , Reuther H.
Published: July 2002

A dc glow discharge source with controlled plasma potential was developed for application in plasma immersion ion implantation processing of materials surfaces. This type of ion implantation system allows cost effective surface modification of workpieces with complex shapes. The effects of the nitrogen plasma etching during the plasma immersion ion implantation process was studied using Si wafers as monitors, as we varied the externally controlled plasma potential between 0 and 350 V. When the plasma potential is controlled below 70 V, the ion implantation is dominant, otherwise the etching overtakes. The nitrogen implanted silicon wafers were analyzed by high resolution x-ray diffraction and Auger electron spectroscopy which revealed successful implantation of ions with accumulated nitrogen dose of 1.5×1017 cm−2, for the low potential case.
X-ray photoelectron spectroscopy analysis of GaN/(0001)AlN and AlN/(0001)GaN growth mechanisms
Authors: King S. W. , Carlson E. P. , Therrien R. J. , Christman J. A. , Nemanich R. J. , Davis R. F.
Published: July 2002

The mechanisms of growth of GaN on AlN and AlN on GaN via gas source-molecular beam epitaxy with NH3 as the nitrogen source have been investigated using x-ray photoelectron spectroscopy, low energy electron diffraction, and Auger electron spectroscopy. The growth of GaN on AlN at low temperatures (650–750 °C) occurs via a Stranski–Krastanov 2D→3D type mechanism with the transition to 3D growth occurring at ≈10–15 Å. The mechanism changes to Frank van der Merwe (FM)/layer-by-layer growth above 800 °C. The growth of AlN on GaN occurred via a FM layer-by-layer mechanism within the 750–900 °C temperature range investigated. We propose a model based on the interaction of ammonia and atomic hydrogen with the GaN/AlN surfaces which indicates that the surface kinetics of hydrogen desorption and ammonia decomposition are the factors that determine the GaN growth mechanism.
Surface roughness at the Si–SiO2 interfaces in fully depleted silicon-on-insulator inversion layers
Authors: Gámiz F. , Roldán J. B. , López-Villanueva J. A. , Cartujo-Cassinello P. , Carceller J. E.
Published: July 2002

The effect of surface roughness scattering on electron transport properties in extremely thin silicon-on-insulator inversion layers is carefully analyzed. It is shown that if the silicon layer is thin enough (thinner than 10 nm) the presence of the buried interface plays a very important role, both by modifying the surface roughness scattering rate due to the gate interface, and by itself providing a non-negligible scattering rate. The usual surface roughness scattering model in bulk silicon inversion layers is shown to overestimate the effect of the surface-roughness scattering due to the gate interface as a consequence of the minimal thickness of the silicon layer. In order to account for this effect, an improved model is provided. The proposed model allows the evaluation of the surface roughness scattering rate due to both the gate interface and the buried interface. Once the scattering rates are evaluated, electron mobility is calculated by the Monte Carlo method. The effect of the buried interface roughness on electron mobility is carefully analyzed by changing the height of the roughness. The effect of the silicon layer thickness on this scattering mechanism is also considered.
Suppression of bulk wave radiation from leaky surface acoustic waves by loading with thin dielectric films
Authors: Kakio Shoji , Hishinuma Kenji , Nakagawa Yasuhiko
Published: July 2002

The bulk wave, which is radiated into the substrate from the leaky surface acoustic wave (LSAW), can be suppressed by loading the substrate with a thin film of dielectric such as tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2). Rotated Y-cut X-propagating lithium niobate (LiNbO3) is used for the substrate. For a certain range of the rotation angle, the attenuation of the LSAW can be suppressed by choosing the appropriate material and thickness for the film. For the free surface of 64° Y−X LiNbO3 with Ta2O5 film, and the metallized surface of 41° Y−X LiNbO3 with SiO2 film, the measured propagation losses decreased approximately to one-fourth to half of that of the sample without the thin film. It is shown that, even if the propagation path is a free surface, the particle displacement distribution in the LSAW is concentrated near the substrate surface by the loading with the thin film, as for the metallized surface. This result suggests that the bulk wave radiation in the LSAW excitation can be suppressed. The bulk wave radiation loss for the 41° Y−X LiNbO3 with the Ta2O5 film decreased approximately to one-sixth of that of the sample without the thin film.
Field-induced antiferroelectric–ferroelectric phase transitions in the Pb0.98La0.02(Zr0.70Hf0.30)1−xTixO3 system
Authors: Heremans Catherine , Tuller Harry L.
Published: July 2002

Field-induced phase transitions of antiferroelectric–ferroelectric nature have been investigated in the Pb0.98La0.02(Zr0.70Hf0.30)1−xTixO3 system. Samples of compositions 0.065⩽x⩽0.10, located in the vicinity of an antiferroelectric–ferroelectric phase boundary, were analyzed by x-ray diffraction, by dielectric, and by electro-mechanical measurements. Samples with x⩽0.095 exhibited a metrically tetragonal structure, whereas the sample with x=0.1 exhibited a rhombohedral structure. From the electromechanical measurements, the compositions deepest in the antiferroelectric phase field (x⩽0.075), were initially located in the antiferroelectric phase field (AFEI). The samples showed a pure electrostrictive behavior at low electric fields but underwent a phase transition into another antiferroelectric phase AFEII when submitted to higher electric fields. For intermediate compositions with 0.0775⩽x⩽0.095, the electric field enforced a transition between the antiferroelectric AFEII phase and the ferroelectric phase. In the latter compositional range, remanent ferroelectric behavior was detected at zero field and increased with Ti content until the ferroelectric phase remained metastable, in analogy with the better studied PbNbZrSnTiO3 and PbLaZrSnTiO3 systems.
Textured MoS2 thin films obtained on tungsten: Electrical properties of the W/MoS2 contact
Authors: Gourmelon E. , Bernède J. C. , Pouzet J. , Marsillac S.
Published: July 2002

Textured films of molybdenum disulfide have been obtained by solid state reaction between the constituents in thin films form when a (200) oriented tungsten sheet is used as substrate. The crystallites have their c axis perpendicular to the plane of the substrate. The annealing conditions are T=1073 K and t=30 min. The films are stoichoimetric and p type. Such highly textured films are achieved without foreign atom addition (Ni, Co…). It appears, as shown by x-ray photoelectron spectroscopy, that a thin WS2 layer is present at the interface W/MoS2. The crystallization process is discussed by a van der Waals texturation (pseudoepitaxy) onto dangling bond sulfur terminated surfaces, these surfaces being ordered. After characterization of the W/MoS2 structure by x-ray diffraction and x-ray photoelectron spectroscopy, an upper electrode of tungsten was deposited by sputtering. The electrical properties of these W/MoS2/W structures have been investigated by analyzing the behavior of the current–voltage characteristics as a function of the measuring temperature. It is shown that an ohmic contact is obtained with a contact resistance smaller than the resistance of the MoS2 film.
Appearance of toroidal structure in dissipating laser-generated sparks
Authors: Nassif D. , Hüwel L.
Published: July 2002

We have investigated the temporal and spatial evolution of laser-induced plasmas in pure nitrogen. A 1064 nm, 20 ns pulse from a neodymium-doped yttrium–aluminum–garnet (Nd:YAG) laser with pulse energies ranging from 175 to 500 mJ is tightly focused to produce a spark at various, near-atmospheric pressures. Spatially resolved Rayleigh scattered light from a time-delayed, 355 nm Nd:YAG laser pulse traversing the spark at right angles is collected with an image intensifier gated, linear diode array. At a delay time of 30 μs, the laser plasma remnant appears as a nearly spherically symmetric region with a center temperature of about 4500 K. After around 100 μs, the hot gas starts to change into a toroidal shape expanding radially at an average speed of a few meters per second. The final torus size increases with decreasing pressure and increasing laser power. This general behavior of the plasma afterglow can be reconciled with a numerical model, where in the aftermath of the spark shock wave a pair of vortices is produced which in turn moves the residual hot gas into the observed toroidal geometry.
Cavities and dislocations induced in silicon by MeV He implantation
Authors: Godey S. , Sauvage T. , Ntsoenzok E. , Erramli H. , Beaufort M. F. , Barbot J. F. , Leroy B.
Published: July 2002

We implanted n-type silicon with 1.6 MeV helium at fluences ranging from 1×1016 to 1×1017 He/cm2 while keeping a constant dose rate. These samples were then subjected to 800 °C annealing for 30 min. The results obtained by means of cross-sectional transmission electron microscopy indicate that the density of cavities is fluence dependent with homogeneous distribution of cavity sizes when fluences of 5×1016 and 1017 He/cm2 are used. The threshold fluence required to form cavities is found to be between 1 and 2×1016 He/cm2. For the 2×1016 He/cm2 dose, we observed loop punching induced by a concerted action of overpressurized bubbles, whereas He implants at doses of 5×1016 and 1×1017/cm2 lead to the formation of {311} defects. At the same time, non Rutherford elastic backscattering (NREBS) experiments using 2.5 MeV H+ provide the fraction of helium remaining in cavities after different annealing times at 800 °C. The NREBS data show a fast He release process for short annealing times (<2000 s). Then, the He amount decreases slowly and after 30 000 s about 40% of the helium still remain in cavities. Finally, an additional implantation with 50 keV He at 5×1016 He/cm2 shows the difference in cavity size distribution between MeV and keV implantation.
Magnetic and thermal properties of IrMn/FeTaN films
Authors: Byeon S. C. , Rantschler J. , Alexander C.
Published: July 2002

The antiferromagnetic–ferromagnetic exchange coupling between IrMn and FeTaN films has been investigated to study the effects of the exchange field on the high frequency response and the thermal stability of the anisotropy field of FeTaN. Ferromagnetic resonance, vibrating sample magnetometer, and torque measurements were used to determine the values of the unidirectional exchange bias field Hp, the uniaxial anisotropy field Hk and the interfacial energy J. Permeameter measurements to 3 GHz were used to study the effect of the exchange field on the low-frequency permeability and the high-frequency cutoff. Measurements on varying thicknesses of IrMn and FeTaN films gave Hp values from 4 to 115 Oe, Hk values from 10 to 20 Oe, Hc values from 10 to 20 Oe, and an interfacial energy J=0.075 ergs/cm2. The exchange coupling was found to have significant effects on the permeability and the thermal stability of the anisotropy.
Magnetic properties of polycrystalline LnNi0.3Co0.7O3 (Ln=La, Pr) compounds
Authors: Escote M. T. , Westphal C. H. , Jardim R. F.
Published: July 2002

Polycrystalline samples of LnNi0.3Co0.7O3 (Ln=La, Pr) were prepared through sol-gel precursors. The samples were subjected to several heat treatments at temperatures ranging from 650 to 1000 °C and under oxygen pressure as high as 70 bar. Characterizations of x-ray powder diffraction XRD magnetization M(T), and ac magnetic susceptibility χac(T) were performed on these compounds. The results of XRD showed that samples of LaNi0.3Co0.7O3 crystallize in a rhombohedral distorted perovskite structure (R3̄c) and that PrNi0.3Co0.7O3 compounds displayed an orthorhombically distorted structure (Pbnm). The temperature dependence of M(T) revealed the presence of a sharp cusp occurring at temperatures TP∼60 and ∼ 30 K for Ln=La and Pr, respectively. Below TP, the M(T) curves taken after different thermal cycles, i.e., field cooled and zero-field cooled processes, are histories dependent on the magnitude of M(T) decreasing with increasing applied magnetic field. We have also observed that TP decreases with increasing applied magnetic field. Measurements of χac(T) at zero applied magnetic field in all samples studied revealed one peak anomaly occurring at temperatures Tf=58 and 30 K for La and Pr, respectively. In fact, increasing frequency over four orders of magnitude results in a shift of the peak anomaly to higher temperatures, as expected for a spin-glass system. Also, a broadening of the peak anomaly is observed with the application of an external magnetic field and it evolves in two peaks at H as high as ∼ 500 Oe. Thus the χac(T) curves reveal two well-defined peaks: one, which is essentially frequency independent and another that is shifted to higher temperatures with increasing frequency. The origin of these magnetic behaviors is discussed.
Hysteresis-loop overskewing in the light of a novel nucleation mode
Authors: Skomski R. , Liu J. P. , Sellmyer D. J.
Published: July 2002

It is demonstrated that the magnetostatic demagnetizing factor is not the only sample-shape dependent contribution to the hysteresis loop. There are also exchange contributions, which lead to interesting effects, particularly in nanostructures. An example is the recently discovered bulging mode in two-phase nanomagnets. The bulging mode exhibits the angular symmetry of the coherent mode, but it is incoherent due to its radial variation and gives rise to negative demagnetizing factors.
Ultrafast microscopy and numerical simulation study of magnetization reversal dynamics in permalloy
Authors: Ballentine G. E. , Hiebert W. K. , Stankiewicz A. , Freeman M. R.
Published: July 2002

We have measured and simulated dynamics of magnetization reversal in a micrometer scale Ni80Fe20 polycrystalline thin film element in the time domain. The sample is probed experimentally using time-resolved scanning Kerr microscopy. This allows for spatially resolved images of the dynamical magnetization states. We have simulated these magnetic reversals micromagnetically for the case of an ideal rectangular element. The reversal rates agree semiqualitatively, and gross spatial features such as a stripe instability in the directions perpendicular to the applied field are reproduced.
Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass
Authors: Berger René , Kliava Janis , Bissey Jean-Claude , Baïetto Vanessa
Published: July 2002

In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in the particle diameters, while at higher Ta these diameters reach a limit value. When the measurement temperature is increased, the resonance spectra show a reversible narrowing and an increase in intensity. The temperature dependence of the individual linewidths is attributed to thermal fluctuations of the orientations of the magnetic moments with respect to the magnetic anisotropy axes.
High-resolution x-ray diffraction, x-ray standing-wave, and transmission electron microscopy study of Sb-based single-quantum-well structures
Authors: Mukhamedzhanov E. Kh. , Bocchi C. , Franchi S. , Baraldi A. , Magnanini R. , Nasi L.
Published: July 2002

Ga 0.6 Al 0.4 Sb/GaSb single-quantum-well structures grown by molecular-beam epitaxy on GaSb substrates with different well thicknesses, have been studied by high-resolution x-ray diffraction and x-ray standing-wave methods. By fitting the diffraction curves, thickness, composition, and the static Debye–Waller factor were obtained for each layer of the structures. The analysis of the angular dependence of the yield of photoelectrons emitted by the x-ray standing-wave field in the range of the dynamical x-ray diffraction was used for selecting the most appropriate set of layer parameters among those which gave virtually identical fittings of the diffraction curves. Relatively broadened GaAlSb/GaSb interfaces were found in all of the samples. This result was confirmed by high-resolution transmission electron microscopy investigation of one of the samples. The effect of the surface degradation due to the chemical reaction with the atmosphere of the free surface of the upper Ga0.6Al0.4Sb layer was considered.
Analyzing mutual inductance measurements to determine the London penetration depth
Authors: Coffey Mark W.
Published: July 2002

The mutual inductance of a superconducting slab of infinite extent, in the Meissner state, with both point dipole and circular-current-loop applied magnetic fields, is examined. The effect of a distinct, constant permeability of the slab is considered and the full mutual inductance and various approximations are obtained. Closed form representations are also found for an inductance per length factor which has proven useful in inductance data inversion in order to obtain an absolute measurement of the London penetration depth.
Ferromagnetic resonance spectra of thick yttrium–iron–garnet films
Authors: Pardavi-Horvath Martha , Keszei Bela , Vandlik Janos , McMichael R. D.
Published: July 2002

Due to the finite aspect ratio of the ferrite, the distribution of the internal field in small planar microwave ferrite devices is strongly inhomogeneous. This is a source of additional losses through magnetostatic modes. To study the magnetostatic modes, the thickness dependence of the ferromagnetic resonance spectra was measured for a series of 2 mm square single crystal yttrium–iron–garnet films in the 35<h<220 μm thickness range. The in-plane resonance field Hr increases with thickness by 150 G, while Hr normal to film plane decreases. The uniform mode linewidth, dHr is in the 2–20 G range. A large number of magnetostatic wave modes modes is excited, covering the “full spectral width,” DH, increasing with thickness from 400 to 960 G. In a ferrite device DH has to be taken into account, giving rise to large losses in spite of the small intrinsic linewidth dHr of the material.
Uniform rotation of magnetization measured in single nanometer-sized particles
Authors: Bonet Orozco E. , Wernsdorfer W. , Barbara B. , Benoı̂t A. , Mailly D. , Thiaville A.
Published: July 2002

We show in this article the first full three dimensional measurements of switching fields of a nanometer-scale magnetic particle. The field direction dependence of the switching fields can be successfully described by the uniform rotation model. Nevertheless, we could see that the Stoner–Wohlfarth form for the anisotropy function, which is often assumed together with the uniform rotation model, is an oversimplification. In order to reproduce the measured fields, one has to take into account higher order anisotropy terms. The complete form of the anisotropy function seems to be accessible only by magnetic measurements on each individual nanoparticle.
Fluctuation induced precriticality in heavy fermions
Authors: Schlottmann P.
Published: July 2002

The nesting of the Fermi surfaces of an electron pocket and a hole pocket separated by a wave vector Q interacting via a repulsive potential gives rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as TN→0. The renormalization group flow leads to a strong coupling fixed point. We study the linear response to spin- and charge-density waves, to singlet and triplet superconducting fluctuations, the low-T specific heat, and the magnetic susceptibility. All quantities increase on a logarithmic scale when the temperature is lowered. The results are discussed in the context of non-Fermi-liquid behavior observed in some heavy fermion compounds.
Magnetic properties of Sm2Co17−xGax (x=0–7) compounds
Authors: Liang B. , Kleinschroth I. , Shen B. G. , Kronmüller H.
Published: July 2002

The study of the effect of Ga substitution on the magnetic properties of arc-melted Sm2Co17−xGax (x=0–7) compounds has been carried out by means of x-ray diffraction (XRD) and magnetic measurements. XRD patterns show that all samples of Sm2Co17−xGax (x=0–7) crystallize in the rhombohedral Th2Zn17-type structure. The Ga substitution for Co leads to an approximately linear decrease in the Curie temperature and a rapid decrease in the saturation magnetic moment which is faster than that in the case of magnetic dilution. Spin-reorientation transition is observed for Sm2Co11Ga6 compound at 32 K. In order to determine the room-temperature easy magnetization direction (EMD), XRD measurements are performed on magnetically aligned samples with x⩽5. The result shows that the EMD of these compounds corresponds to the c axis. The anisotropy constants K1 and K2 of Sm2Co17−xGax compounds were derived by fitting the hard-direction magnetization recoil curves measured on magnetically aligned powder samples as well as by fitting the magnetization recoil curves measured on polycrystalline bulk samples. The fitting results indicate that the substitution of Ga for Co in Sm2Co17 compound decreases the anisotropy constants, thus indicating the weakening of the easy-axis anisotropy.
Low temperature magnetic hysteresis in Mn12-acetate single crystals
Authors: Kent Andrew D. , Zhong Yicheng , Bokacheva Louisa , Ruiz Daniel , Hendrickson D. N. , Sarachik M. P.
Published: July 2002

We report precise magnetic hysteresis measurements down to 0.4 K of spin-10 magnetic clusters in small single crystals of Mn12-acetate. At temperatures above 1.6 K, steplike changes in magnetization are observed at regularly spaced magnetic field intervals, as found in earlier measurements. The steps gradually shift to higher magnetic fields as the temperature is decreased to 1.2 K, consistent with the presence of a second-order uniaxial magnetic anisotropy. An abrupt shift in step position is observed at still lower temperature which may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunneling suggested by recent theory.
Spin diffusion dynamics in double exchange manganites
Authors: Zou Liang-Jian , Campbell D. K. , Lin H. Q.
Published: July 2002

We present theoretical studies on the spin diffusion dynamics of the double exchange model including Jahn–Teller distortion for manganites. It is shown that due to the trapping of composite polarons in the magnetic transition regime, the spin diffusion dynamics becomes important. The spin diffusion coefficient obtained by our theory agrees well with experimental data.
Investigation of magnetization behavior in nanoelements using the finite element method
Authors: Ridley P. H. W. , Roberts G. W. , Chantrell R. W.
Published: July 2002

A model of thin film permalloy using an efficient finite element variational approach to the magnetostatic field calculation is described. The material is discretized into a nanoelement structure at the micron level which enables us to investigate material properties due to patterning. Predicted domain structures agree well with experimental data. Interactions between elements are significant and it will be shown that the domain structure in the central element differs from that of its neighbors. As expected, the addition of pointed ends stabilizes the single-domain state. Elements with two pointed ends exhibit pseudo-single-domain behavior. We have studied the single-domain/pseudo-single-domain transition for permalloy platelets, results for noninteracting platelets are given as a function of the elongation. Interactions are shown to increase or decrease the critical size depending on the geometry.
The reversal mechanism and coercivity of permanent magnetic materials
Authors: Ong C. K. , Feng Y. P. , Zhao G. P. , Lim H. S. , Dan Wei
Published: July 2002

The underlying mechanism of coercivity in permanent magnets has been a topic of intense interest for many years. It is motivated by the fact that the measured coercivity approaches only 20%–40% of the theoretical nucleation fields as derived from micromagnetic theory. We address this problem by proposing an analytical model, within the framework of the micromagnetic approximation, to examine the mechanism of magnetization reversal in hard magnetic materials. The exchange interaction between neighboring grains with different easy axes orientations can result in the formation of a domain wall-like magnetization structure (transition region) in the grain boundary. We propose that the transition region can propagate between neighboring grains provided that it is energetically favorable. The subsequent nucleation of a domain wall is shown to reduce the critical field considerably. Applying our model to a thin film, whose magnetic grains have a randomly oriented in-plane easy axis distribution, we have calculated the coercivity for the film to be 0.14HK, where HK is the anisotropy field. It is found that the coercivity decreases with increasing film thickness. For a material with a three-dimensional random easy axis distribution, we obtain the coercivity as 0.16HK. These results are substantially lower than that given by the Stoner-Wohlfarth model and are consistent with available experimental results.
Interwoven magnetic and flux line structures in single crystal (Tm,Er)Ni2B2C (invited)
Authors: Gammel P. L. , Lopez D. , Bishop D. J. , Eskildsen M. R. , Andersen N. H. , Mortensen K. , Fisher I. R. , Cheon K. O. , Canfield P. C.
Published: July 2002

We review studies of the interactions between magnetic order and the flux line lattice (FLL) in the (RE)Ni2B2C intermetallic borocarbides for (RE)=Tm and Er using small angle neutron scattering (SANS) and magneto-transport. For (RE)=Tm the magnetic order and the FLL assume a common symmetry, sharing a phase transition at ∼2 kOe, despite an order of magnitude difference in periodicity. For (RE)=Er, the penetration depth λ and the coherence length ξ, both of which are derived from the FLL form factor, are modified near TN=6 K by a theoretically predicted weakly divergent pairbreaking. Finally, below 2.3 K, (RE)=Er shows a coexistence of weak ferromagnetism and superconductivity. This state reveals a highly disordered FLL and a striking increase in the critical current, both arising from the strong ferromagnetic pairbreaking.
Magnetic remanence of Josephson junction arrays
Authors: Passos W. A. C. , Araujo-Moreira F. M. , Ortiz W. A.
Published: July 2002

In this work we study the magnetic remanence exhibited by Josephson junction arrays in response to an excitation with an ac magnetic field. The effect, predicted by numerical simulations to occur in a range of temperatures, is clearly seen in our tridimensional disordered arrays. We also discuss the influence of the critical current distribution on the temperature interval within which the array develops a magnetic remanence. This effect can be used to determine the critical current distribution of an array.
In- and out-diffusion of oxygen during the buried-oxide formation in oxygen-implanted silicon
Authors: Ono Haruhiko , Ogura Atsushi
Published: July 2002

Formation mechanism of buried oxide in silicon wafers during the annealing process after oxygen implantation has been investigated by using Fourier-transform infrared absorption spectroscopy and secondary ion mass spectrometry. The implanted Si wafers were annealed at a different temperature, for a different time, and in a different atmosphere. We found that the implanted oxygen density decreased by the out-diffusion after a very short time at an elevated temperature. We also found that the in-diffusion of oxygen took place after a long time at the same temperature. The in- and out-diffusion behaviors clearly depended on the annealing atmosphere. This can be explained by taking account that the equilibrium surface concentration of oxygen differs between the annealing atmospheres.
Rate constants for the reaction of H2 with defects at the SiO2/Si(111) interface
Authors: Gheorghita Ligia , Ogryzlo Elmer
Published: July 2002

A radio frequency probe has been used to monitor changes in charge-carrier recombination centers at a SiO2/Si interface by following the steady-state photogenerated carrier concentration in the silicon. A silicon surface covered with ∼200 Å of thermal oxide was exposed to gaseous molecular hydrogen at temperatures between 135 and 300 °C. The reaction was found to occur with a nonexponential rate law that could be described by a cluster of rate constants governed by a Gaussian distribution of activation energies. At high temperatures, the dominant reaction was found to be that of “Pb” centers with a rate law that is consistent with that previously obtained by electron paramagnetic resonance. At low temperatures the reaction is dominated by the passivation of a carrier recombination center with a much lower activation energy. This new species makes up about 25% of the total recombination center concentration at the Si/SiO2 interface. Almost 20% of the recombination centers could not be passivated by H2.
The effects of thermal annealing on ZnO thin films grown by pulsed laser deposition
Authors: Lu Y. F. , Ni H. Q. , Mai Z. H. , Ren Z. M.
Published: July 2002

ZnO thin films were grown on silicon (100) by pulsed laser deposition. Highly textured crystalline ZnO thin films can be grown at 600 °C. The films were then annealed at 600 °C in oxygen. The effects of annealing on chemical composition of the ZnO films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra indicate that water has been adsorbed and then dissociated into H and OH groups. The surface properties of ZnO were studied both by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A narrow potential well has been formed on the surface of the ZnO thin films due to high density of surface states and negatively biasing the ZnO thin films during STS measurement. The discrete energy levels can be measured by STS.
General susceptibility functions for relaxations in disordered systems
Authors: Bergman R.
Published: July 2002

A general equation for the susceptibility of disordered systems is proposed. It is based on the experimental observation of power laws at frequencies far from the peak frequency of the imaginary part of the frequency dependent relaxation function, the susceptibility. The obtained general expression contains the equations of other proposed relaxation functions as special cases and, thus, it might be considered as a generalization of these. From this general equation we derive an equation specially adapted for the α relaxation in glass-forming materials. This equation contains only three fit parameters and it is thus very suitable for fitting real experimental data. It is shown that this equation is a good frequency domain representation of the time domain Kohlrausch–Williams–Watts stretched exponential. From the general equation we also derive a four-parameter “universal” equation that describes most types of responses and even inverted response data, i.e., response peaks more stretched on the low frequency side than on the (as is normal) high frequency side. The physical significance of the different parameters is qualitatively discussed and the proposed functions are shown to satisfactorily describe typical experimental data.
Interface properties of organic/indium–tin oxide and organic/GeS(001) studied using photoemission spectroscopy
Authors: Peisert H. , Schwieger T. , Knupfer M. , Golden M. S. , Fink J.
Published: July 2002

We present an investigation of interface properties of the hole transport material N,N′-diphenyl-N,N′-bis (3-methylphenyl)–(1,1′)–biphenyl-4,4′diamine (TPD) on indium–tin oxide (ITO) and on GeS(001) using x-ray and ultraviolet photoelectron spectroscopy. On both the TPD/ITO interface and the TPD/GeS(001) heterojunction we found shifts of the vacuum level indicating a formation of an interfacial dipole barrier. We obtained a barrier for the hole injection for TPD on Ar ion sputter-cleaned ITO of 1.5 eV which is significantly higher than the commonly cited value of 0.5 eV for ozone-oxidized ITO, whereas the interface dipole barrier of 0.3 eV is in good agreement with reported data. At low coverages of TPD on ITO additional features in the C 1s core level spectra were found, which can be ascribed to a partially oxidized TPD species at the interface.
Theory of the second virial coefficients in polymeric solutions below the theta temperature
Authors: Tanaka Fumihiko
Published: July 2002

Recent experimental measurements on the temperature variation of the second virial coefficients A2 in polymer solutions below the Θ point are analyzed theoretically. In contrast to the existent theories we incorporate the ternary collision integral between monomers into our theory in order to recover the stability of the system beyond the Θ region. A scaling law A2=f(τ(M)1/2)/(M)1/2 is derived for the virial coefficient, where τ ≡ T/Θ−1 is the temperature deviation and M is the molecular weight of the polymer. Based on this scaling law, observed molecular weight independence is explained. It is also pointed that the tendency of A2 to increase through a minimum is expected in connection with the coil–globule transition.
Relative kinetic isotope effects of heavy atoms
Authors: Paneth Piotr
Published: July 2002

The dependence of relative kinetic isotope effects of heavy atoms on a reaction mechanism was studied. For complex reactions it was found that while the r value is in the normal range for primary isotope effects, it may deviate from that range for the secondary effects.
Vibrational motion effects on molecular polarizabilities. Shifts in vibrational transition frequencies and transition moments of lithium hydride from applied electrical fields
Authors: Malik David J. , Dykstra Clifford E.
Published: July 2002

Lithium hydride has been used to provide representative information on the effects of vibrational motion on molecular polarizabilities. Dipole and quadrupole polarizability and hyperpolarizability tensors were calculated using derivative Hartree–Fock (DHF) theory. Many of the calculated tensor elements were found to be changing with internuclear separation even in the vicinity of the equilibrium. Vibrational wave functions obtained from a numerical vibrational analysis were used to vibrationally average the electrical properties (tensor elements) of the lowest four vibrational states, and this showed that the averaged polarizabilities of the states have important differences. A manifestation of these differences is that there are shifts in the vibrational transition frequencies along with changes in the transition moments of LiH from applying electrical fields and field gradients. Field gradients can augment the shifts arising from uniform axial fields. This was determined by variationally finding vibrational state energies and transition frequencies as a function of the applied electrical potential.
A comparison of the one-dimensional band structures of Ni tetrabenzoporphyrin and phthalocyanine conducting polymers
Authors: Kutzler Frank W. , Ellis D. E.
Published: July 2002

The one-dimensional band structures of (tetrabenzoporphyrinato) Ni (II) and (phthalocyaninato) Ni (II) were calculated. The reported band structures include the effects of the stacking perturbation and of the overlap between nearest neighbors. The basis functions for the intermolecular integrals were obtained from a discrete variational X-α calculation of the monomeric species. Electron spin resonance (ESR) experiments on conductive columnar stacks of these metallomacrocycles, partially oxidized with iodine, indicate that carrier electrons are primarily localized onto the ligands in the phthalocyanine species, but also involve the metal in the tetrabenzoporphyrin complex. The present calculations suggest that the ESR data results from a band crossing or near degeneracy between the highest occupied (ligand centered) porphyrin molecular orbital, and the lowest occupied (metal centered) molecular orbital. The analogous levels are more separated in energy in the phthalocyanine derivative.
Orientational correlations in fluids of convex bodies. II. Explicit results for two- and three-dimensional systems
Authors: She Richard S. C. , James Corinne , Evans Glenn T.
Published: July 2002

Monte Carlo (MC) simulations and scaled particle theory (SPT) are used to determine a few anisotropic equilibrium properties of an infinitely dilute solution of an ellipse in a bath of circular disks. MC and SPT calculations of orientational polynomials for ellipse-disk pairs in contact are in qualitative agreement for the systems studied over a range of densities. Quantitative agreement degrades with increasing ellipse eccentricity and at high density. The ellipse-disk radial distribution function (rdf) has a contact value different than that of a neat fluid of disks at the same density. However, apart from the local surface effect, the full ellipse-disk rdf is strikingly similar to that of a neat fluid of disks, with the same extrema and a nearly superimposable radial dependence. An SPT-derived rdf for a three-dimensional fluid comprised of a single ellipsoid in a bath of spheres is used to calculate the density dependence of the mean-square angular momentum fluctuation in atom–(ellipsoidal)diatom collisions. The mean-square angular momentum fluctuation has a density dependence due primarily to the isotropic collision frequency, with small corrections owing to fluid orientational correlations. The solvent induced shift in the torsional free energy of butane is also determined using SPT.
Experimental study of thermal fluctuation in spinodal decomposition of a binary polymer mixture
Authors: Okada Mamoru , Han Charles C.
Published: July 2002

The kinetics of spinodal decomposition of a binary polymer system of polystyrene/poly(vinylmethylether) near its critical composition has been studied by the temperature jump light scattering technique. The results indicate that the linearized theory of the Cahn–Hilliard–Cook type which includes thermal fluctuation term can describe the polymer system very well in the eary-time and shallow-quench region. The virtual structure factor Sx(q), which is a consequence of including the thermal fluctuation, has been obtained for the first time. For experiments of deeper quench, the virtual structure factor becomes small in our experimental q range, and the time dependent static structure factor, S(q,t), reduces back to the original Cahn–Hilliard type.
The Ã 2Π–X̃ 2Σ+ transition of HC2 isolated in solid argon
Authors: Jacox Marilyn E. , Olson W. Bruce
Published: July 2002

Fourier transform absorption spectra have been obtained between 700 and 7900 cm−1 at a resolution of 0.2 cm−1 for Ar:C2H2 samples codeposited at 12 K with a beam of argon atoms that had been excited in a microwave discharge. Detailed isotopic substitution studies have confirmed that the predominant product species is HC2, which contributes not only the absorptions previously assigned to its two stretching fundamentals but also several weaker absorptions in the 2000–3600 cm−1 spectral region and a prominent, complicated pattern of absorptions between 3600 and 7800 cm−1. The previous assignment of the 3611 cm−1 HC2 absorption as the CH-stretching fundamental is reviewed, and the assignment of an absorption at 2104 cm−1 as ν2+ν3 of ground-state HC2 is discussed. The near infrared absorption band system has been assigned to the Ã 2Π–X̃ 2Σ+ transition of HC2, extensively perturbed by interaction with high vibrational levels of the ground state. The position of the transition origin could not be definitively established. The previous assignment of gas-phase absorptions of HC2 at 3786, 4012, and 4108 cm−1 to ground-state combination bands has been confirmed, and carbon-isotopic data have been obtained for these bands. Because all of the HC2 absorptions observed in an argon matrix must originate in the X̃(000) state and because the observed matrix shifts are relatively small, these studies may provide a map to facilitate more detailed gas-phase studies.
Monte Carlo studies on the long time dynamic properties of dense cubic lattice multichain systems. I. The homopolymeric melt
Authors: Kolinski Andrzej , Skolnick Jeffrey , Yaris Robert
Published: July 2002

Dynamic Monte Carlo simulations of long chains confined to a cubic lattice system at a polymer volume fraction of φ=0.5 were employed to investigate the dynamics of polymer melts. It is shown that in the range of chain lengths n, from n=64 to n=800 there is a crossover from a weaker dependence of the diffusion coefficient on chain length to a much stronger one, consistent with D∼n−2. Since the n−2 scaling relation signals the onset of highly constrained dynamics, an analysis of the character of the chain contour motion was performed. We found no evidence for the well-defined tube required by the reptation model of polymer melt dynamics. The lateral motions of the chain contour are still large even in the case when n=800, and the motion of the chain is essentially isotropic in the local coordinates. Hence, the crossover to the D∼n−2 regime with increasing chain length of this monodisperse model melt is not accompanied by the onset of reptation dynamics.
Linearized microscopic theories of nonequilibrium solvation
Authors: Wolynes Peter G.
Published: July 2002

Linearized microscopic theories of equilibrium solvation can be straightforwardly extended to treat the dynamic situation. A particular example, the solvation of a newly formed ion, is developed within the mean spherical approximation. The MSA exhibits significant deviations from the Born–Marcus formula for nonequilibrium solvation energies. The solvation dynamics proceeds on several time scales. The solvation structure first forms distant from the ion, in harmony with a remark of Onsager and with previous numerical calculations of Calef and Wolynes [J. Chem. Phys. 78, 4145 (1983)].