bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface-oxidation-induced phase separation in FeMn thin films
Authors: Lefakis H. , Huang T. C. , Alexopoulos P.
Published: July 2002

Films of Fe50Mn50, 700 Å thick, were deposited on glass substrates by diode rf sputtering. Auger electron spectroscopy, and x-ray fluorescence analysis verified the film composition and x-ray diffraction determined the crystalline phase to be α-Mn(Fe), a complex bcc-like structure and a considerable deviation from that dictated by the equilibrium binary phase diagram. The films were annealed for 1 h at 260 °C and 10−7, 10−5, and 10−2 Torr. Auger depth profile analysis showed that (a) while no further oxidation (than that induced by ambient exposure) occurred during annealing at the lowest pressure, severe film oxidation took place at the highest pressure and (b) preferential surface oxidation of Mn resulted in a layered structure consisting of a surface Mn- and oxygen-rich layer followed by a region (deeper into the film) depleted in Mn and Fe enriched. The crystalline phases present were determined by x-ray diffraction analysis to be MnO (fcc) and α-Fe(Mn) (bcc). Magnetic measurements of the oxidized films revealed that they displayed in-plane isotropic ferromagnetic properties, with 4πMs∼15–18 kG for the most extensively oxidized films.
Metallurgical study of Ni/GaAs contacts. II. Interfacial reactions of Ni thin films on (111) and (001) GaAs
Authors: Guivarc’h A. , Guérin R. , Caulet J. , Poudoulec A. , Fontenille J.
Published: July 2002

The solid-state interactions at the Ni/(111) and (001) GaAs interfaces were investigated in the temperature range 25–600 °C by Rutherford backscattering spectrometry and channeling experiments, x-ray diffractometry, and four-point probe. The samples were prepared by depositing nickel films, 70 nm thick, onto clean (111) and (001) GaAs single-crystal substrates under 5×10−10 Torr vacuum. Then they were annealed for 1 h at increasing temperatures under a flow of forming gas (90% N2-10% H2). The sequence of phase formation was the following: first, a ternary phase at 200 °C, then a mixture of two ternary phases at 250 and 350 °C, and finally from 400 to 580 °C a mixture of a ternary phase and NiAs. This last structure was stable up to 600 °C on (111) GaAs, but at this temperature a further consumption of GaAs led to a mixture of (NiGa+NiAs) onto (001) GaAs. Small differences were observed versus the orientation of the GaAs samples but all the ternary and binary phases exhibited epitaxial arrangements with both the (111) and (001) GaAs substrates. The ternary-phase diagram for the bulk Ni-Ga-As system provides the basis for understanding the sequence of the above results and much of the information in the literature about Ni/GaAs interfacial reactions.
Use of the microindentation technique for determining interfacial fracture energy
Authors: Rosenfeld L. G. , Ritter J. E. , Lardner T. J. , Lin M. R.
Published: July 2002

The microindentation technique was used to determine the interfacial fracture energy of epoxy coatings on soda-lime glass substrates. An analytical model was developed for calculating fracture energy based on indenter load versus debond crack size measurements. Finite-element analysis was used to determine the relative amounts of opening and shear loadings at the debond crack tip. The calculated fracture energies are compared to values determined by the double-cantilever-beam technique and the four-point flexure-beam technique.
Particle size statistics in dynamic fragmentation
Authors: Grady Dennis E.
Published: July 2002

Condensed matter, when subjected to intense disrupting forces through impact or radiation deposition, will break up into a randomly distributed array of fragments. An earlier analysis of random fragmentation is extended to account for fragmentation in bodies which are finite in extent and for bodies within which the minimum fragment size is bounded. The statistical fragment size relations are compared with molecular dynamic simulations of dynamic fragmentation, with fragmentation caused by the high-energy collision of nuclear particles, and with the distribution of galaxies in the universe which are assumed to be fragment debris from the primordial Big Bang.
Relationship between the intrinsic coercivity and the phases of Nd-(Dy)-Fe-B sintered magnets
Authors: Kim K. Y. , Lee D. G. , Jeung W. Y. , Kang I. K.
Published: July 2002

The magnetic properties of sintered Nd-(Dy)-Fe-B magnets prepared with different annealing conditions have been investigated. Thermomagnetic analysis was carried out on the sintered and annealed Nd-(Dy)-Fe-B magnets in the temperature range of room temperature to 320 °C. The coercivity of gas-quenched Nd14.1Fe78.3B7.6 magnets after annealing for 1 h at temperatures between 600 and 900 °C is higher by 3 kOe than the furnace-cooled one after annealing. Thermomagnetic analysis found that the sintered Nd14.1Fe78.3B7.6 magnet consists of the Nd2Fe14B phase as well as an unknown magnetic phase (A1) with Tc around 150 °C. The annealed magnets consist of two unknown magnetic phases (A1,A2) and the Nd2Fe14B phase with a Tc of 150, 240, and 305 °C, respectively. Thermomagnetic analysis also showed that the Mr value of the A2 phase in the gas-quenched Nd-Fe-B magnet was higher than that of the furnace-cooled one. These results seem to indicate that increase of coercivity is related to the formation of the A2 phase.
Photothermal detection and characterization of a horizontal buried slab by the mirage technique
Authors: Salazar A. , Sánchez-Lavega A. , Fernández J.
Published: July 2002

We present a theoretical and experimental analysis of the mirage detection and measurement of a semi-infinite horizontal buried slab of air within opaque solids. A detailed study on the effects introduced by the mirage parameters (pump and probe beam sizes, probe beam height) as well as by the surface features is discussed. We use pump and probe beams orthogonally superimposed, and detection is made of the amplitude and phase of the normal component of the mirage deflection. Air slabs with a thickness (e2) as low as 0.1 μm and located down to a depth (e1) of 0.3√ks (ks is the thermal diffusivity of the material) can be easily observed. Detection improves as the pump radius (a) and probe height (z0) increase, although a compromise is necessary because of signal attenuation. Determination of e1 and e2 through the model fitting, requires an independent control of a and z0. The use of low frequencies avoids the effects of surface features on the slab detection. However, for surface planar horizontal steps, their height must be incorporated into the model as a variation of z0 in order to characterize the slab.
On the reaction of Pt with SiO2 substrates: Observation of the Pt3Si phase with the Cu3Au superstructure
Authors: Lamber Ryszard , Jaeger Nils I.
Published: July 2002

The reaction of platinum with SiO2 substrates during thermal treatment in hydrogen has been studied by electron microscopy, electron diffraction and microdiffraction. Following prolonged heating at 840 K in H2 a new Pt-Si phase with the cubic Cu3Au structure (a=0.388 nm, Pm3̄m space group) could be identified. This phase can be transformed into the monoclinic Pt3Si alloy after heating at higher temperatures. The formation of the cubic structure is interpreted in terms of a platinum rich Pt3Si phase which is intermediate to the monoclinic Pt3Si.
Salt-finger instability in an anisotropic and inhomogeneous porous substrate underlying a fluid layer
Authors: Chen Falin
Published: July 2002

In the directional solidification of a binary liquid, salt-finger and plume convection are the two major types of fluid motion that occur when the lighter species is rejected from the solid. Fingering convection is usually observed prior to the initiation of plume formation. When the nominally planar liquid-solid interface is morphologically unstable, solidification is dendritic. Consequently, the melt is separated from the solid by a mushy zone consisting of solid dendrites and interdendritic liquid. This can be considered to be a porous layer with anisotropic and inhomogeneous permeability and, possibly, thermal and solutal diffusivities. A linear stability analysis is empoyed to study salt-finger instability in a fluid layer bounded above by a rigid wall and below by an anisotropic and inhomogeneous porous medium saturated by the same fluid. The effects of anisotropy and inhomogeneity on the stability characteristics are discussed for a variety of porous media. Results are also presented for a porous medium for which the anisotropy and inhomogeneity of the permeability and thermal and solutal diffusivities have been obtained experimentally during the directional solidification of aqueous ammonium chloride solutions. The nature of the instability modes relevant to the present problem are also discussed. Based on the present results, a relationship is proposed between salt-finger and plume convection during directional solidification. Methods for inhibiting salt-finger convection are also suggested, and the feasibility of their implementation is discussed.
Electron trap generation in thermally grown SiO2 under Fowler–Nordheim stress
Authors: Zhang J. F. , Taylor S. , Eccleston W.
Published: July 2002

Electron trap generation in thermally grown silicon dioxide (SiO2) during Fowler–Nordheim (FN) stress is investigated by using an aluminium-gated capacitor structure. The generated electron traps are characterized by the avalanche electron injection technique. The experimental results support the model that electron trapping in oxide follows the first-order kinetics and may have multiple-capture cross sections. It is found that both donorlike (positive charge related) and acceptorlike (neutral before capturing electron) traps are generated and they behave differently. The donorlike trap is not stable at or above room temperature and its effective density saturates as the stressing time increases, while the opposite is true for the acceptorlike trap. The electron-capture cross section of donorlike trap spreads from 10−18 to over 10−14 cm2, but the capture cross section of the generated acceptorlike trap is limited in the range of (4.5–9)× 10−17 cm2. The acceptorlike trap is generated by the interaction between free holes and SiO2 and hole trapping leads to donorlike traps. The relation between the generated trap and the as-grown trap will be discussed. Comparison of the electron traps generated by FN stress with those by irradiation and hot hole injection indicates that the electron trap generation under these different stressing conditions is controlled by the same mechanism. The necessary condition for electron trap generation is the presence of holes in the oxide, rather than a high electrical field.
Love-type surface-acoustic waves propagating in amorphous iron-boron films with multilayer structure
Authors: Yokokawa Narutoshi , Tanaka Shin’ichi , Fujii Toshitaka , Inoue Mitsuteru
Published: July 2002

The propagation of Love-type surface-acoustic waves (SAWs) in a multilayer structure was investigated. The structures consisted of rf-sputtered iron-boron (FeB) amorphous magnetic films stacked alternatively to SiOx insulating layers i.e., FeB-(SiOx−FeB)×n (where n denotes the stacking number) onto a soft glass substrate. The phase of waves was controlled over a wide range by a magnetic field applied in the film plane, through change in the strength of magnetoelastic coupling between the magnetic moment of the FeB film and SAW propagating in the substrate. The transducers for the Love-type SAW excitation were constructed by combining the interdigital electrodes and ZnO films, prepared by sputtering in an oblique electrode arrangement. The controllable phase shift (ΔΦ) under a magnetic field increases linearly with n, when the total FeB film thickness DM is kept constant (DM=1.5 μm). The ΔΦ of a device with n=8 is about 2300°/cm within the magnetic field smaller than about 120 Oe.
Physics of the ferroelectric nonvolatile memory field effect transistor
Authors: Miller S. L. , McWhorter P. J.
Published: July 2002

The operation of the ferroelectric nonvolatile memory field effect transistor is theoretically examined extensively for the first time. The ferroelectric transistor device properties are derived by combining the silicon charge-sheet model of metal-oxide-semiconductor field-effect transistor device operation with Maxwell’s first equation which describes the properties of the ferroelectric film. The model we present describes ferroelectric transistor I-V and C-V behavior when time-dependent voltages are applied which result in hysteresis due to ferroelectric switching. The theoretical results provide unique insight into the effects of geometrical and material parameters on the electrical properties of the transistor. These parameters include the ferroelectric spontaneous and remanent polarization, the coercive field, and dielectric layer thicknesses. We have found that the conventional concept of threshold voltage is no longer useful, and that increasing the spontaneous polarization has only a minor impact on memory operation due to reverse dipole switching of the ferroelectric layer. The application of the model to optimize design and fabrication parameters is illustrated with a virtual prototyping example. The model is also used to develop a practical testing methodology for this unique device.
Magnetic reversal in Sm2Fe17Ny permanent magnets
Authors: Skomski R. , Müller K.-H. , Wendhausen P. A. P. , Coey J. M. D.
Published: July 2002

Coercivity and nucleation fields for three-dimensional inhomogenities are calculated and discussed. For soft spherical inclusions in a hard magnetic matrix a (δ/R)2 dependence of the nucleation field on inclusion radius R and domain wall width δ has been obtained. To calculate propagation fields for spherical inhomogenities, a nonlinear model has been used. The corresponding coercive field can be much higher than the nucleation field, particularly if the magnitude of the anisotropy changes very steeply. Soft inclusions with sharp interfaces are ineffective at producing complete magnetic reversal. For partly nitrided particles coercivity and nucleation field are predicted to be identical, which is confirmed by experiments on high-performance Sm2Fe17N3−δ magnets.
Effects of oxide traps, interface traps, and ‘‘border traps’’ on metal-oxide-semiconductor devices
Authors: Fleetwood D. M. , Winokur P. S. , Reber R. A. , Meisenheimer T. L. , Schwank J. R. , Shaneyfelt M. R. , Riewe L. C.
Published: July 2002

We have identified several features of the 1/f noise and radiation response of metal-oxide-semiconductor (MOS) devices that are difficult to explain with standard defect models. To address this issue, and in response to ambiguities in the literature, we have developed a revised nomenclature for defects in MOS devices that clearly distinguishes the language used to describe the physical location of defects from that used to describe their electrical response. In this nomenclature, ‘‘oxide traps’’ are simply defects in the SiO2 layer of the MOS structure, and ‘‘interface traps’’ are defects at the Si/SiO2 interface. Nothing is presumed about how either type of defect communicates with the underlying Si. Electrically, ‘‘fixed states’’ are defined as trap levels that do not communicate with the Si on the time scale of the measurements, but ‘‘switching states’’ can exchange charge with the Si. Fixed states presumably are oxide traps in most types of measurements, but switching states can either be interface traps or near-interfacial oxide traps that can communicate with the Si, i.e., ‘‘border traps’’ [D. M. Fleetwood, IEEE Trans. Nucl. Sci. NS-39, 269 (1992)]. The effective density of border traps depends on the time scale and bias conditions of the measurements. We show the revised nomenclature can provide focus to discussions of the buildup and annealing of radiation-induced charge in non-radiation-hardened MOS transistors, and to changes in the 1/f noise of MOS devices through irradiation and elevated-temperature annealing.

Border-trap densities of ∼1010–1011 cm−2 are inferred from changes in switching-state density during postirradiation annealing, and from a simple trapping model of the 1/f noise in MOS devices. We also present a detailed study of charge buildup and annealing in MOS capacitors with radiation-hardened oxides through steady-state and switched-bias postirradiation annealing. Trapped-hole, trapped-electron, and switching-state densities are inferred via thermally stimulated current and capacitance-voltage measurements. A lower bound of ∼3×1011 cm−2 is estimated for the effective density of border traps that contribute to the electrical response of the irradiated devices. This is roughly 20% of the observed switching-state density for these devices and irradiation conditions. To our knowledge, this represents the first quantitative separation of measured switching-state densities into border-trap and interface-trap components. Possible physical models of border traps are discussed. E′ centers in SiO2 (trivalent Si centers associated with oxygen vacancies) may serve as border traps in many irradiated MOS devices.
A shock-tracking algorithm for surface evolution under reactive-ion etching
Authors: Hamaguchi S. , Dalvie M. , Farouki R. T. , Sethuraman S.
Published: July 2002

A new algorithm that determines the evolution of a surface eroding under reactive-ion etching is presented. The surface motion is governed by both the Hamilton–Jacobi equation and the entropy condition for a given etch rate. The trajectories of ‘‘shocks’’ and ‘‘rarefaction waves’’ are then directly tracked, and thus this method may be regarded as a generalization of the method of characteristics. This allows slope discontinuities to be accurately calculated without artificial diffusion. The algorithm is compared with ‘‘geometric’’ surface evolution methods, such as the line-segment method.
Kinetics of the amorphous-to-nanocrystalline transformation in Fe73.5Cu1Nb3Si13.5B9
Authors: Allia P. , Baricco M. , Tiberto P. , Vinai F.
Published: July 2002

Isothermal measurements of the electrical resistance R were performed in samples of the alloy Fe73.5Cu1Nb3Si13.5B9 at temperatures where the nanocrystalline phase develops. At each temperature, nanocrystallization is associated to a very slow decrease of R(t) compared to conventional crystallization processes. X-ray-diffraction data were obtained on similar samples annealed at the same temperatures for selected times. The evolving nanocrystalline fraction turns out to be essentially proportional to the variation in the electrical resistance. The form of the kinetic law describing the R(t) decays is obtained by making use of a novel procedure, developed to analyze time-dependent properties characterized by a complex behavior. Nanocrystallization in the considered alloy appears as initially described by a Johnson–Mehl–Avrami kinetics with exponent n≂4, suggesting that the early stages of the process involve homogeneous nucleation and three-dimensional grain growth. For longer times, corresponding to the hearth of the nanocrystallization process, and at all temperatures, the functional form of the kinetic law drastically changes to a much slower power law of the type R∼At−α. The exponent α turns out to be essentially independent of temperature. Such a change presumably reflects a reduction in the grain growth velocity and/or in the nucleation rate.
Amorphous twisted nematic–liquid-crystal displays fabricated by nonrubbing showing wide and uniform viewing-angle characteristics accompanying excellent voltage holding ratios
Authors: Toko Yasuo , Sugiyama Takashi , Katoh Kazuhisa , Iimura Yasufumi , Kobayashi Shunsuke
Published: July 2002

A new twisted-nematic (TN)–liquid-crystal display (LCD) device has been prepared without rubbing, which is called the no-rubbing (NR) technique. The device is shown to exhibit wide and uniform viewing-angle characteristics without showing a contrast inversion in the vertical direction of the cell. This characteristic is achieved in particular by filling chiral-molecular-doped nematic LCs (NLCs) in the isotropic phase into a cell. The substrate surfaces of the cells are either bare indium-tin-oxide films or those coated with polymer materials such as polyimide. These films are optically and structurally isotropic in a quasi-macroscopic scale (say, 100×100 μm2). In particular, the polymer films are capable of stabilizing and homogenizing the distribution of the NLC directors at the surfaces. The TN-LCDs fabricated by the NR technique, called amorphous TN-LCDs, are also featured by the excellent voltage holding ratio which is necessary to achieve good active matrix driving, and, furthermore, the device exhibits a very small viewing angular dependence in the electro-optic (EO) characteristics, which provides a significant benefit for achieving a good gray-scale operation. This rubbing-free technology may contribute not only to the significant improvement of the EO performance of TN-LCDs but also to the simplification of the production process and in turn to the reduction of the production cost.
Interaction of aluminum with hydrogenated amorphous silicon at low temperatures
Authors: Shahidul Haque M. , Naseem H. A. , Brown W. D.
Published: July 2002

Annealing effects on aluminum/hydrogenated amorphous silicon (a-Si:H) contacts in the temperature range from 100 to 300 °C were studied. Al was evaporated on device-quality, phosphorus-doped (n+) a-Si:H films deposited in a UHV plasma-enhanced chemical- vapor-deposition system. Both electrical measurements and surface morphological analyses were performed to characterize the interaction. The transmission line model technique was used to measure sheet resistance and contact resistivity. For samples where Al covered the entire a-Si:H surface during annealing, sheet resistance and contact resistivity were found to decrease monotonically with annealing temperature; whereas, samples annealed after patterning of the Al pads exhibited a minimum in sheet resistance and contact resistivity at temperatures between 150 and 200 °C. Optical and scanning electron microscopy, surface profilometry, and Raman spectroscopy were used to study the surface morphology. Interaction of Al with a-Si:H was observed to initiate at a temperature of 150 °C. Crystallization of the interacted Al/a-Si:H film starts at a temperature as low as 180 °C. A model, involving Al counterdoping of the n+ a-Si:H film at the surface, is proposed to explain the electrical behavior of the contacts and films following different annealing cycles.
Crystallization in fluorinated and hydrogenated amorphous silicon thin films
Authors: Edelman F. , Cytermann C. , Brener R. , Eizenberg M. , Khait Yu. L. , Weil R. , Beyer W.
Published: July 2002

The amorphous-to-crystalline (AC) transition of amorphous Si thin films containing fluorine or hydrogen is studied by transmission electron microscopy. The AC transition can be described quantitatively by the incubation time prior to the onset of crystallization t0. This parameter is found to decrease exponentially with temperature with an activation energy of 1.7 eV for a-Si:F and 3.1 eV for a-Si:H:D. It is found that during the crystallization process in a-Si:F the crystallites organize as dendrite single crystals oriented along the 〈110〉 axis perpendicularly to the film surface. a-Si samples that had been covered by Pd or Al crystallize at appreciably lower temperatures. In the case of Al lower activation energies of 0.7 eV for hydrogenated and 0.4 eV for fluorinated a-Si are measured. In the case of Pd/a-Si:H,F for both kinds of a-Si an activation energy of 1.7 eV is found.
Measurement of piezoelectric coefficients of ferroelectric thin films
Authors: Lefki K. , Dormans G. J. M.
Published: July 2002

This article presents measurements of piezoelectric coefficients of lead zirconate titanate (PZT) thin films. The normal load method is used to measure the coefficients for PZT films with various compositions prepared by the sol-gel technique or by organometallic chemical vapor deposition (OMCVD). The as-deposited OMCVD films have a piezoelectric coefficient of 20–40×10−12 m/V, whereas the unpoled sol-gel films are not piezoelectric. After poling the thin films having a composition near the morphotropic phase boundary; these values increase to 200×10−12 m/V for OMCVD films and 400×10−12 m/V for sol-gel films. The difference may arise from an incomplete poling of the OMCVD films.
Experimental and theoretical analysis of shadow-masked growth using organometallic vapor-phase epitaxy: The reason for the absence of facetting
Authors: Armour E. A. , Sun S. Z. , Zheng K. , Hersee S. D.
Published: July 2002

Shadow-masked growth using organometallic vapor-phase epitaxy allows the creation of nonplanar regions having smoothly varying profiles devoid of macroscopic facets. The shape of these profiles is independent of gas flow direction and stripe orientation, suggesting that they are determined by lateral gas phase gradients. It is proposed that the absence of facets is due to the unique growth regime underneath the shadow mask, where organic radicals combine with group-III surface species, temporarily returning them to the vapor phase. This process has been simulated using a two-dimensional finite-element analysis that accurately describes the experimental behavior. Furthermore, this hypothesis predicts other experimentally observed phenomena.
Structure and optical properties of tungsten thin films deposited by pyrolysis of W(CO)6 at various temperatures
Authors: Davazoglou D. , Pallis G. , Psycharis V. , Gioti M. , Logothetidis S.
Published: July 2002

Tungsten thin films, 200 nm thick, have been chemically vapor deposited by pyrolysis of W(CO)6 on oxidized silicon substrates at atmospheric pressure and temperatures varying from 350 to 500 °C. The structure of the films has been studied with x-ray-diffraction measurements and it was found that at deposition temperature of 350 °C a face-cubic-centered phase is formed with a unit-cell dimension of 0.4111 nm. At higher temperatures, up to 450 °C, the deposited films crystallize in a body-cubic-centered phase of tungsten with a unit cell of 0.317 nm. The complex dielectric constant (ε=ε1+iε2) of the films has been measured with spectroscopic ellipsometry within the energy range 1.5–6.5 eV. The energy variation of ε1 changes with deposition temperature; instead, that of ε2 does not change significantly with it. The energy variation of ε1 for the films deposited at temperatures below 400 °C corresponds to that of insulators while that of films deposited at higher temperatures, up to 450 °C, reveals their metallic character.
Frequency dispersion and temperature variation of complex permeability of Ni-Zn ferrite composite materials
Authors: Tsutaoka Takanori , Ueshima Masahiro , Tokunaga Toshihiko , Nakamura Tatsuya , Hatakeyama Kenichi
Published: July 2002

Permeability spectra in Ni-Zn ferrite composite materials were studied at the volume loading of ferrite above 30% and at temperatures from 100 to 400 K. The permeability decreased with decreases in the volume loading of ferrite. This decrease was much larger than that expected from the empirical mixing law. This was attributed to the demagnetizing field, generated by the magnetic poles on the surface of the ferrite particles. Simultaneously, the demagnetizing field increased spin resonance frequency. For the sintered ferrite, the primary peak of the permeability was just below the Curie temperature. The peak becomes obscure and disappeared as the volume loading decreased. The temperature dependence of the spin resonance frequency was lower in the ferrite composite material than that in the sintered ferrite. These features were also discussed from the view point of the demagnetizing field.
Pyroelectric, dielectric, and piezoelectric properties of LiB3O5
Authors: Guo Ruyan , Markgraf Steven A. , Furukawa Yasunori , Sato Masayoshi , Bhalla Amar S.
Published: July 2002

Selected tensor property coefficients of the nonlinear optical crystal LiB3O5 are reported. The full set of pyroelectric, dielectric, and thermal expansion coefficients were measured as a function of temperature. The five independent piezoelectric coefficients, the full set of electromechanical coupling coefficients, along with the compliance constants, were measured at room temperature. The material was found to be quite anisotropic in most property coefficients. The piezoelectric and electromechanical coupling coefficients were reasonably low, and comparisons are made between LiB3O5 and other borate crystals. The material showed strong ionic conductivity along the 〈001〉 direction, with a calculated activation energy for Li transport of 0.43 eV.
Nuclear magnetic resonance relaxivity and surface-to-volume ratio in porous media with a wide distribution of pore sizes
Authors: Borgia G. C. , Brown R. J. S. , Fantazzini P.
Published: July 2002

The simplest model for the contribution of pore surfaces to nuclear magnetic resonance (NMR) relaxation of a pore fluid gives R, the average relaxation rate minus the bulk rate, equal to a constant ρ, the velocity at which nuclear magnetization flows out of the pore fluid at the surfaces, times the pore-space surface-to-volume ratio S/V. Although ρ can vary widely, a great variety of porous media exhibit ρ values of the order of a few μm/s for longitudinal relaxation when S/V is measured by gas adsorption by the Brunauer, Emmett, and Teller (BET) method or high pressure mercury injection. For samples with wide distributions of relaxation rates it is of interest to find what functions of the relaxation data correlate best with S/V measurements and how different relaxation parameters relate to each other. Longitudinal relaxation data were taken for 77 sandstone samples of different origin, which had been cleaned and saturated with brine. After the NMR measurements the samples were dried and surface areas measured by BET. The samples have S/V from 1.5 to 150 (μm)−1, porosity from 3% to 28%, and permeability from less than 0.1 mD to more than 1 D. Longitudinal relaxation data were taken from 400 μs to 6 s and analyzed in many different ways, including stretched-exponential fits and multiexponential fits up to five components. S/V and ln(S/V) were correlated with various relaxation rates derived from these computed parameters.

In principle, the relaxation parameter to use with a ρ value is the average rate, which is initial slope divided by initial amplitude, namely, R(0), where R(t)=(d/dt)ln S(t) at t=0 and S(t) is the relaxing signal. One can extrapolate an n component fit to t=0 to get Rn(0), but very good signal quality is required even to get small short components reliably for t well within the times covered by the data. Over half of the points have ρ’s within a factor of 2 of the minimum value 0.9 μm/s when the average rate of a five-component fit to the data is used. There are numerous points with ρ up to 7 μm/s, but none of the high-ρ points are for samples with high S/V. All samples with high S/V have wide distributions of relaxation rates, but not vice versa. The best simple correlation with ln(S/V) was ln(S/V)≊1.81 ln(R33)−5.73, where R33 is the highest rate of a three-component fit without regard to the corresponding amplitude, and where S/V is in (μm)−1 and rate in s−1. This result was unexpected. This fit does not represent proportionality to a velocity ρ and does not correspond to any obvious physical model, but it can be of practical interest to estimate in a very simple and noninvasive manner S/V at the BET scale in sandstones.
Fundamental properties in the formation of Co, Ni, and Pt metal thin films using pulsed laser deposition
Authors: Hiroshima Y. , Ishiguro T. , Urata I. , Makita H. , Ohta H. , Tohogi M. , Ichinose Y.
Published: July 2002

Co, Ni, and Pt metal thin films have been formed using the pulsed laser deposition method. Several fundamental properties of this method, e.g., deposition rate for the KrF excimer laser with various irradiation energy densities, distribution of film thickness, change of the deposition rate as a function of the number of shots, change of the target morphology, and corresponding film structure, are examined. It is found that the deposition rate and the film structure (amorphous and/or polycrystalline) depend on the state of the morphology of the target. An attempt to control the target morphology was performed, i.e., the rotation of the Co target restrained the target surface from the formation of laser cones and as a result, the amorphous Co films could be obtained.
Manganese-activated luminescence in ZnGa2O4
Authors: Yu Chang Feng , Lin Pang
Published: July 2002

Manganese-activated ZnGa2O4, prepared at a sintering temperature of 1375 °C with 0.1% MnO dopant, emitted red and green light with peaks at 666 and 508 nm when excited with a laser source (325 nm). The luminescence centers of red emission were ascribed to Mn4+ ions in the octahedral (B) sites of spinel-structure zinc gallate. Monitored at these two peaks, the sample exhibited absorption bands at 342 and 464 nm (for red) and 245 and 290 nm (for green) when excited with radiation from a xenon discharge lamp, indicating an excitation process occurring in Mn4+ ions distinct from that of Mn2+ ions (in A sites) which were known to be responsible for the green emission. Cathodoluminescence spectra showed that the integrated intensity of an UV band (peak at 349 nm) increased steadily with beam current while the green light intensity saturated at high current ranges. Several possible saturation mechanisms were discussed. The variation of the intensities versus current was interpreted and formulated based on an energy process between Ga–O groups and Mn2+ ions. Good fittings between the calculated curves and measured intensity data were attained with appropriate parameter values in the calculation. Assuming a resonant transfer process with electric dipole–dipole interaction prevailing between the Mn2+ and Ga3+ ions, a critical interaction distance of ∼9.9 Å for the ions was derived from an associated relation and the parameters used.
Search for high moment soft magnetic materials: FeZrN
Authors: Chakraborty A. , Mountfield K. R. , Bellesis G. H. , Lambeth D. N. , Kryder M. H.
Published: July 2002

FeN materials exhibiting high moment, low coercivity and small magnetostriction have previously been reported. Zr has been known to reduce the magnetostriction in other Fe alloys. The criteria for an ideal recording head pole material as well as shields for magnetoresistive sensors include high moment, low coercivity, high permeability, and zero magnetostriction. We present here the properties of 0.3 μm thick rf sputtered FeZrN films measured as a function of the N2 partial pressure. The films were deposited at a pressure of 3 mT using a Perkin Elmer sputtering system. The target was composed of Fe with Zr chips covering approximately 2% of the surface area. The easy axis and hard axis coercivities show minima of 1.8 and 0.6 Oe, respectively, at 7–10 % N2 partial pressure with a saturation magnetization of about 18 kG. The magnetic anisotropy field is approximately 5 Oe yielding a dc permeability of approximately 4000 along the hard axis. X-ray data reveal a systematic change in the ratio of α–Fe and γ–Fe4 N; the amount of the γ–Fe4N phase increasing with increasing N2 partial pressure. The magnetostriction increases with increasing N2 content crossing zero at approximately 6%. The grain size, as probed by atomic force microscopy, is an increasing function of the N2 partial pressure; from a few nm for N2 partial pressures of 5% to being as large as 50 nm for a partial pressure of 15%. The surface roughness is about 1 nm.
Ferromagnetic resonance and eddy currents in high-permeable thin films
Authors: van de Riet E. , Roozeboom F.
Published: July 2002

The effect of eddy currents and ferromagnetic resonance (FMR) on the frequency dependence of the permeability of FeTaN thin films is studied. An equation is given that describes the combination of these two effects. The agreement between the theoretical calculations and the experimental data is excellent. Already at 1 MHz the imaginary part of the permeability is significantly changed by FMR. Above 10 MHz the real part of the permeability is decreased by FMR. The combined effect is best observed in extremely high permeable, 1 μm thick films. A value for the ferromagnetic damping constant is obtained.
Large ion yields in hydrogen scattering from a graphite surface
Authors: Tsumori K. , Koppers W. R. , Heeren R. M. A. , Kadodwala M. F. , Beijersbergen J. H. M. , Kleyn A. W.
Published: July 2002

In this article we report on surface scattering experiments at graphite as to model the interactions at divertor surfaces in tokamaks and conversion electrodes in negative ion sources. The formation of negative and positive hydrogen ions on a graphite surface during positive hydrogenous ion scattering at a projectile energy of 400 eV is observed. A remarkably high H− and H+ ion fraction, both varying between 10% and 30% of all scattered particles, is found during scattering of positive hydrogenous ions.
Anomalous magnetoresistance in antiferromagnetic polycrystalline materials R2Ni3Si5 (R=rare earth)
Authors: Mazumdar Chandan , Nigam A. K. , Nagarajan R. , Gupta L. C. , Chandra G. , Padalia B. D. , Godart C. , Vijayaraghaven R.
Published: July 2002

Magnetoresistance (MR) studies on polycrystalline R2Ni3Si5, (R=Y, rare earth) which order antiferromagnetically at low temperatures, are reported here. MR of the Nd, Sm, and Tb members of the series exhibit positive giant magnetoresistance, largest among polycrystalline materials (85%, 75%, and 58% for Tb2Ni3Si5, Sm2Ni3Si5, and Nd2Ni3Si5, respectively, at 4.4 K in a field of 45 kG). These materials have, to the best of our knowledge, the largest positive GMR reported ever for any bulk polycrystalline compounds. The magnitude of MR does not correlate with the rare earth magnetic moments. We believe that the structure of these materials, which can be considered as a naturally occurring multilayer of wavy planes of rare earth atoms separated by Ni–Si network, plays a role. The isothermal MR of other members of this series (R=Pr,Dy,Ho) exhibits a maximum and a minimum, below their respective TN’s. We interpret these in terms of a metamagnetic transition and short-range ferromagnetic correlations. The short-range ferromagnetic correlations seem to be dominant in the temperature region just above TN.
Current status and future outlook for bonded neodymium permanent magnets (invited)
Authors: Croat J. J.
Published: July 2002

Bonded neodymium magnets can provide significant size and weight reduction and/or performance enhancement over sintered and, particularly, bonded ferrite permanent magnets and, moreover, provide these benefits at reasonable cost. Primarily for these reasons, these bonded magnets are now used in a wide and growing range of computer peripheral, office automation, and consumer electronic applications and now constitute the fastest growing segment of the permanent magnet market. The current status of these materials will be reviewed. Included is a brief overview of the manufacture of these magnetically isotropic magnets and a discussion of their unique properties and features from the perspective of both bonded magnet producer and user. Major applications are discussed as are some of the factors that will drive the market for these materials in the future. New technical developments, including the status and outlook for anisotropic bonded materials, high remanance isotropic materials and high temperature bonded magnets will also be discussed.
Highly resistive grain boundaries in doped MnZn ferrites for high frequency power supplies
Authors: Drofenik M. , Žnidaršič A. , Zajc I.
Published: July 2002

The power loss of MnZn ferrites and its relation to average grain size and grain boundary properties was investigated. The power loss was found to be dependent on the average grain size and on the highly electrically resistive grain boundaries which are formed by introduction of aliovalent ions in the outer grain region of a MnZn ferrite. Analysis of ac impedance data, assisted by Auger electron spectroscopy, elucidated the chemical and electrically diversity of doped and undoped samples and its influence on eddy current loss.
Low-temperature growth and critical epitaxial thicknesses of fully strained metastable Ge1−xSnx (x≲0.26) alloys on Ge(001)2×1
Authors: Gurdal O. , Desjardins P. , Carlsson J. R. A. , Taylor N. , Radamson H. H. , Sundgren J.-E. , Greene J. E.
Published: July 2002

Epitaxial metastable Ge1−xSnx alloys with x up to 0.26 (the equilibrium solid solubility of Sn in Ge is <0.01) were grown on Ge(001)2×1 by low-temperature molecular beam epitaxy. Film growth temperatures Ts in these experiments were limited to a relatively narrow range around 100 °C by the combination of increased kinetic surface roughening at low temperatures and Sn surface segregation at high temperatures. All Ge1−xSnx films consisted of three distinct sublayers: the first is a highly perfect epitaxial region followed by a sublayer, with an increasingly rough surface, containing 111 stacking faults and microtwins, while the terminal sublayer is amorphous. Based upon reflection high energy electron diffraction and cross-sectional transmission electron microscopy (XTEM) analyses, critical epitaxial thicknesses tepi, defined as the onset of amorphous growth, were found to decrease from 1080 Å for pure Ge to ≃35 Å for alloys with x=0.26. TEM and XTEM analyses revealed no indication of misfit dislocations (except in Ge0.74Sn0.26 samples) and high-resolution x-ray reciprocal lattice mapping showed that epitaxial Ge1−xSnx layers were essentially fully strained. From an analysis of tepi(x) results, surface morphological evolution leading to epitaxial breakdown is controlled by kinetic roughening for alloys with x≲0.09 and by strain-induced roughening at higher Sn concentrations. We propose that the thermal activation required for the cross-over, reported here for the first time, from kinetic to strain-induced roughening is partially overcome by the fact that kinetic roughening provides local surface chemical potential gradients over lateral length scales which are sufficiently small to initiate strain-induced roughening even at these low temperatures.
A phenomenological model for systematization and prediction of doping limits in II–VI and I–III–VI2 compounds
Authors: Zhang S. B. , Wei Su-Huai , Zunger Alex
Published: July 2002

Semiconductors differ widely in their ability to be doped. As their band gap increases, it is usually possible to dope them either n or p type, but not both. This asymmetry is documented here, and explained phenomenologically in terms of the “doping pinning rule.”
Crystal grain nucleation in amorphous silicon
Authors: Spinella Corrado , Lombardo Salvatore , Priolo Francesco
Published: July 2002

The solid phase crystallization of chemical vapor deposited amorphous silicon films onto oxidized silicon wafers, induced either by thermal annealing or by ion beam irradiation at high substrate temperatures, has been extensively developed and it is reviewed here. We report and discuss a large variety of processing conditions. The structural and thermodynamical properties of the starting phase are emphasized. The morphological evolution of the amorphous towards the polycrystalline phase is investigated by transmission electron microscopy and it is interpreted in terms of a physical model containing few free parameters related to the thermodynamical properties of amorphous silicon and to the kinetical mechanisms of crystal grain growth. A direct extension of this model explains also the data concerning the ion-assisted crystal grain nucleation.
Laser generation of acoustic waves in the ablative regime
Authors: Murray Todd W. , Wagner James W.
Published: July 2002

A practical model of acoustic wave generation by a pulsed laser source in the ablative regime is presented. The pressure exerted on the surface during Q-switched laser heating is calculated through a finite difference solution of the vaporization problem. The epicentral displacement is found through summation of the displacement field induced by the vaporization process with that caused by thermoelastic expansion. The model is restricted to the weakly ablative regime in the absence of a backing gas. The results are compared to the epicentral displacements generated in aluminum samples under rough vacuum conditions at generating wavelengths of 532 and 1064 nm. The waveforms compare well over a limited irradiance range. The effects of rough vacuum conditions on the generated acoustic signals are also examined and compared to signals generated in the presence of a backing gas. The divergence in the shape and amplitude of these signals observed under highly ablative conditions is discussed.
Foucault imaging of superconducting fluxons
Authors: Yoshida T. , Endo J. , Harada K. , Kasai H. , Matsuda T. , Kamimura O. , Tonomura A. , Beleggia M. , Patti R. , Pozzi G.
Published: July 2002

The Foucault method, one of the classical Lorentz microscopy techniques, is here investigated in order to demonstrate its applicability to in-focus observations of superconducting fluxons. As the deflections involved are of the order of 10−5−10−6 rad, low-angle techniques, high brightness, coherent illumination, and a low aberration magnetic stage are needed. The first experimental results are presented and discussed in relation with a theoretical model for the fluxon which allows the interpretation of the main features of the observed patterns.
The rate of radiative recombination in the nitride semiconductors and alloys
Authors: Dmitriev Alexey , Oruzheinikov Alexander
Published: July 2002

The radiative recombination rates of free carriers and lifetimes of free excitons have been calculated in the wide band gap semiconductors GaN, InN, and AlN of the hexagonal wurtzite structure, and in their solid solutions GaxAl1−xN, InxAl1−xN and GaxIn1−xN on the base of existing data on the energy band structure and optical absorption in these materials. We determined the interband matrix elements for the direct optical transitions between the conduction and valence bands, using the experimental photon energy dependence of absorption coefficient near the band edge. In our calculations we assumed that the material parameters of the solid solutions (the interband matrix element, carrier effective masses, and so on) could be obtained by a linear interpolation between their values in the alloy components. The temperature dependence of the energy gap was taken in the form proposed by Varshni [Physica 34, 149 (1967)]. The calculations of the radiative recombination rates were performed in a wide range of temperature and alloy compositions.
Particle–particle interactions in electrorheological fluids based on surface conducting particles
Authors: Gonon P. , Foulc J.-N. , Atten P. , Boissy C.
Published: July 2002

We develop a simple, analytical conduction model for the case of electrorheological fluids based on surface conducting particles. By modeling two contacting spheres in a dielectric liquid by a distributed impedances network we derive analytical expressions for the potential and current at the spheres surface, and for the electric field and the current in the liquid phase. The knowledge of the electric field in the dielectric liquid allows us to calculate the interparticle interaction force as a function of the applied voltage. The theoretical interaction force is compared with experimental results obtained on insulating spheres coated with a thin conducting polyaniline film. We find a good agreement between the theory and experiment. The materials properties which govern the response of the system are outlined. In this regard, the product of the liquid conductivity by the sheet resistance of the surface coating appears as a key parameter. Some applications of this model for the practical design of electrorheological fluids are given.