bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis and properties of a magnetically hard Sm5(Fe,Co,Ti)17 phase in sintered magnets
Authors: Huang M. Q. , Simizu S. , Miller K. , Wallace W. E.
Published: July 2002

Magnetically hard Sm5(Fe,Co,Ti)17 phases have been synthesized by the conventional powder metallurgical method. A representative magnet of composition Sm5(Fe0.94Ti0.06)17 shows a room-temperature coercivity, iHc, ∼21.5 kOe, remanence, Br, ∼3.5 kG, and Curie temperature, Tc∼300 °C. The main phase of the magnets has the hexagonal Nd5Fe17 structure with a=20.146 Å, c=12.310 Å. The magnets contained minor amounts of Sm2(Fe,Ti)17 and Sm(Fe,Ti)3, as detected by thermomagnetic analyses (TMA). When Fe is partially substituted by Co, Tc increases, by ∼40 °C for the magnet of composition Sm5(Fe0.74Co0.20Ti0.06)17. This magnet exhibits iHc∼4.0 kOe and Br∼3.4 kG at room temperature. It consists of 2:17, 5:17, and 1:2 phases. The effects of heat treatment procedure on the phase formation and magnetic properties of the magnets have been studied. The best sintering temperature for forming the 5:17 phase, as well as for developing the highest iHc, is ∼700–850 °C. Above 850–1000 °C the 2:17 and 1:2 or 1:3 phases become dominant, the 5:17 hard phase almost disappears, and the coercivity of the magnets decreases substantially.
Growth and characterization of a new diluted magnetic semiconductor, Zn1−xEuxSe
Authors: Jonker B. T. , Peterson L. D. , Krebs J. J.
Published: July 2002

The growth and initial characterization of a new II-VI-based diluted magnetic semiconductor compound that incorporates a rare-earth element, Zn1−xEuxSe, is reported. The samples were grown by molecular-beam epitaxy on GaAs(001) substrates using elemental source ovens. Reflection high-energy electron diffraction confirms single-crystal growth with an in-plane symmetry like that of the GaAs substrate, although the crystalline quality deteriorates with increasing epilayer thickness. Growth on a ZnSe buffer layer substantially improves the crystalline quality, suggesting that the lattice constant of the Zn1−xEuxSe is larger than that of GaAs. This is confirmed by x-ray θ−2θ scans, which further show that the epilayers grow in a (001) orientation and retain the zinc-blende structure of the host lattice. Eu is expected to substitute for Zn as Eu2+ (4f7, 8S7/2 free ion) with an 8A1 ground state, an orbital singlet with a spin S=7/2. Superconducting quantum interference device (SQUID) magnetometry data (2–150 K, 0–5 T) exhibit the temperature and field dependence characteristic of Brillouin paramagnetic behavior, consistent with substitutional incorporation.
Interface formation of Ca with poly(p-phenylene vinylene)
Authors: Gao Y. , Park Ken T. , Hsieh Bing R.
Published: July 2002

We have investigated the interface formation of Ca with poly(p-phenylene vinylene) (PPV) using x-ray photoemission spectroscopy. The most significant part of our findings was the very late occurrence of band bending at the Ca/PPV interface and the lack of strong reaction between the Ca overlayer with the PPV substrate, indicating that the Schottky barrier formation in Ca/PPV was a slow process. The late barrier formation at the Ca/PPV interface may be due to the shielding by surface oxygen impurities, part of which underwent calcium oxide formation at the interface.
Determination of the valence band offset of Si/Si0.7Ge0.3/Si quantum wells using deep level transient spectroscopy
Authors: Vescan L. , Apetz R. , Lüth H.
Published: July 2002

Deep level transient spectroscopy (DLTS) was performed on p-isotype Si/SiGe/Si Schottky barrier diodes in order to obtain the valence band offset between Si and SiGe. A single strained Si0.7Ge0.3 layer was placed in such a depth in Si so as to be able to fill and empty the quantized SiGe well during the transient capacitance procedure. Broad capacitance transient peaks were obtained and interpreted as being due to the capture of holes by the quantum well. The broadness of the peaks was explained by thickness variations of the SiGe layer. From the dependence of the high temperature side of the DLTS peak on the rate window a valence band offset of 220±20 meV was evaluated.
Electron density and collision frequency of microwave-resonant-cavity-produced discharges
Authors: McColl W. , Brooks C. , Brake M. L.
Published: July 2002

A review of perturbation diagnostics applied to microwave resonant cavity discharges is presented. The classical microwave perturbation technique examines the shift in the resonant frequency and cavity quality factor of the resonant cavity caused by low-electron density discharges. However, the modifications presented allow the analysis to be applied to discharges with electron densities beyond the limit predicted by perturbation theory. An ‘‘exact’’ perturbation analysis is presented which models the discharge as a separate dielectric, thereby removing the restrictions on electron density imposed by the classical technique. The ‘‘exact’’ method also uses measurements of the shifts in the resonant conditions of the cavity. Third, an electromagnetic analysis is presented which uses a characteristic equation, based upon Maxwell’s laws, and predicts the discharge conductivity based upon measurements of a complex axial wave number. By allowing the axial wave number of the electromagnetic fields to be complex, the fields are experimentally and theoretically shown to be spatially attenuated. The diagnostics are applied to continuous-wave microwave (2.45 GHz) discharges produced in an Asmussen resonant cavity. Double Langmuir probes, placed directly in the discharge at the point where the radial electric field is zero, act as a comparison with the analytic diagnostics. Microwave powers ranging from 30 to 100 W produce helium and nitrogen discharges with pressures ranging from 0.5 to 6 Torr. Analysis of the data predicts electron temperatures from 5 to 20 eV, electron densities from 1011 to 3×1012 cm−3, and collision frequencies from 109 to 1011 s−1.
Magnetic properties of sintered Sm2Fe17Ny magnets
Authors: Hu Bo-Ping , Rao Xiao-Lei , Xu Jian-Min , Liu Gui-Chuan , Wang Yi-Zhong , Dong Xiao-Lin , Zhang Deng-Xia , Cai Ming
Published: July 2002

Sintered magnets of Sm2Fe17Ny nitrides, with a density of 6.0–7.4 g/cm3, have been prepared by using an explosion technique. Both crystalline structure and the magnetic properties of Sm2Fe17Ny nitrides were retained in the process. The sintered magnet had a remanence Br=0.83 T, an intrinsic coercivity μ0iHc=0.57 T and an energy product (BH)max=88 kJ/m3. The temperature dependence of coercivity and remanence were also measured. The temperature coefficients α of remanence and β of coercivity are −0.076%/°C and −0.51%/°C, respectively.
Pyroelectric properties of ferroelectric-polymer composite
Authors: Wang Yuguo , Zhong Weilie , Zhang Peilin
Published: July 2002

General formulae of the pyroelectric coefficient and the figure of merit (p/ε) of a ferroelectric-polymer composite are theoretically derived in terms of the volume fraction q, the depolarization coefficient nx, the dielectric constant of the ferroelectric particles and the dielectric constant of the composite. The value of p/ε can be expected to be larger than that of pure ferroelectric ceramics. Good agreement is obtained between theoretical and experimental results.
Microstructural origin of giant magnetoresistance in a new sensor structure based on NiFe/Ag discontinuous multilayer thin films
Authors: Parker M. A. , Hylton T. L. , Coffey K. R. , Howard J. K.
Published: July 2002

The origin of giant magnetoresistance (GMR) in a new sensor structure incorporating discontinuous multilayers (DML) of NiFe thin films separated by Ag interlayers is elucidated by means of x-ray diffraction, x-ray reflectivity, and cross-section transmission electron microscopy. It is shown that the observed magnetoresistance, ∼4%–6% in fields on the order of 5–10 Oe at 25 °C, is associated with the breakup upon annealing of the initially dense, columnar superlattice structure into discontinuous multilayers of NiFe due to highly mobile Ag forming Ag bridges between the Ag layers. This observation supports a micromagnetic model for these structures that is based on the concept of initially large ferromagnetic domains in the as-deposited NiFe structure, exhibiting negligible GMR, breaking up into discontinuous layers, exhibiting an appreciable GMR, with antiferromagnetic ordering across the Ag interlayers.
Temperature-dependent interface magnetism and magnetization reversal in Co/Pt multilayers
Authors: Shan Z. S. , Shen J. X. , Kirby R. D. , Sellmyer D. J. , Wang Y. J.
Published: July 2002

We report on the temperature dependence of the magnetic properties and interface magnetism of Co/Pt multilayers. The magnetic properties including magnetization and anisotropy change substantially as the temperature varies from 300 to 10 K for samples with Co layer thickness in the range from 3 to 7 Å. The interface anisotropy of about 0.38 erg/cm2 is nearly independent of temperature. The magnetization reversal is dominated by domain wall motion for the thinner Co layers and dominated by nucleation for the thicker Co layers.
Reflection high energy electron diffraction observation of anion exchange reactions on InAs surfaces
Authors: Collins D. A. , Wang M. W. , Grant R. W. , McGill T. C.
Published: July 2002

We have used time-resolved reflection high energy electron diffraction (RHEED) measurements to study anion exchange reactions in molecular beam epitaxy (MBE) grown III-V semiconductors. In the experiment, InAs surfaces are exposed to Sbx fluxes and subsequent changes in the crystals RHEED patterns are examined. We find that when an InAs surface is initially exposed to an Sb flux the specular spot intensity first decreases, then recovers back toward its initial value. The shape of the intensity versus time curves is extremely reproducible if the absolute Sb flux and the Sb species are kept constant. The length of time required for the RHEED pattern to stabilize is much shorter for cracked Sb than for uncracked Sb. The RHEED dynamics are also faster if the total Sb flux increases. The behavior of the RHEED dynamics as a function of Sb flux and Sb species is consistent with the changes in the RHEED pattern being due to an Sb/As exchange reaction on the crystal’s surface. The RHEED data are compared to previously published x-ray photoelectron spectroscopy (XPS) data which studied exchange reactions on InAs surfaces exposed to Sb fluxes. The XPS study confirmed that the incident Sb did indeed exchange with As in the epilayer and estimated the exposure time needed to complete the Sb/As exchange reaction. The time scales for exchange associated with the RHEED and XPS data are in good agreement. This further indicates that RHEED could be used to indirectly probe anion exchange reactions, potentially opening up several avenues of research ranging from basic materials science to MBE process control in manufacturing.
Crystallization of sol-gel derived lead-zirconate-titanate thin films in argon and oxygen atmospheres
Authors: Bursill L. A. , Brooks Keith G.
Published: July 2002

Electron diffraction and high-resolution electron microscopic techniques are applied to reveal the mechanisms of crystallization of free standing 75-nm-thick thin films of ferroelectric lead-zirconate-titanate (PZT). Sol-gel methods, followed by pyrolysis at 350 °C, were used to provide a common starting point after which a variety of rapid-thermal annealing (RTA) experiments in the temperature range 400–700 °C were made in argon, oxygen, and nitrogen/hydrogen atmospheres. The results are interpreted in terms of a crystal chemical analysis, which points out that partial pressure of oxygen and heating rate are important experimental parameters which must be controlled if ferroelectric perovskite-type Pb2(Zr,Ti)2O6, rather than pyrochlore-type Pb2(Zr,Ti)2O6+X, where 0<X<1 or −1<X<0, is to be obtained after the RTA step. Thus significant improvements in the crystallization of perovskite-type PZT were clearly demonstrated by using argon atmospheres for the RTA step. The results may have significance for the production of high-quality ferroelectric thin films, with improved switching and fatigue characteristics, since even small amounts of the pyrochlore phase may prove detrimental for these properties.
The dependence of room-temperature oxidation of silicon catalyzed by Cu3Si on the silicide grain size
Authors: Liu C. S. , Chen L. J.
Published: July 2002

The dependence of room-temperature oxidation of silicon catalyzed by Cu3Si on the silicide grain size has been investigated by transmission electron microscopy and x-ray diffractometry. The thickness of the SiO2 layer was found to decrease with the average grain size of the starting Cu3Si layer. High-resolution transmission electron microscopy revealed that oxidation is initiated at the grain boundaries. Oxide film as thick as 4.5 μm, compared to a previous record of about 2 μm, was grown at room temperature over a period of two weeks in (001) samples. The growth of thick oxide films was achieved by minimizing the grain size of Cu3Si through a reaction between Cu and an intermediate amorphous silicon layer at 200 °C.
The viscosity of amorphous Pd-Si and Pd-Si/Pd-Si-Fe multilayers determined from stress relaxation in thin films on a substrate
Authors: Witvrouw Ann , Spaepen Frans
Published: July 2002

Substrate curvature measurements were used to measure viscous flow in ion-beam sputtered amorphous Pd-Si and amorphous Pd-Si/Pd-Si-Fe multilayered thin films. Viscosities were found to increase linearly with time, characteristic of a bimolecular flow defect annihilation process. The results for Q=2.4±0.8 eV, the isoconfigurational enthalpy, and η̇, the change in viscosity with time, of the Pd78Si22 thin films are in agreement with the values found in tensile creep experiments on thin films and melt-quenched ribbons. No dependence on repeat length was found for the viscosity of the multilayered films. Although constant η̇ values were found for the multilayers studied here, an analysis shows that this is not necessarily the case for multilayers in which sublayers have a constant η̇.
An investigation on the magneto-optic and magnetic properties of Tb:YIG
Authors: Yang Jie Hui , Xu You , Zhang Guo Ying
Published: July 2002

The calculations of the spontaneous electric Faraday rotation (FR) and the magnetization contributed by Tb3+ ions in Tb substituted iron garnets based on the quantum theory are presented. The crystal field (CF) and the superexchange interaction are two crucial factors in determining FR and the magnetization. Every CF splitting level is nondegenerate. But the energy differences between the lowest seven CF split levels of the ground configuration are not very large comparing with the superexchange interaction upon the Tb3+ ions, so the strong admixing of these levels by the high-order perturbation of the exchange interaction makes FR and the magnetization not small. Except FR below 150 K, the calculated results are in good agreement with experiments.
In situ magnetic and structural analysis of epitaxial Ni80Fe20 thin films for spin-valve heterostructures
Authors: Hashim I. , Atwater H. A.
Published: July 2002

We have investigated structural and magnetic properties of epitaxial (100) Ni80Fe20 films grown on relaxed Cu/Si(100) seed layers. The crystallographic texture and orientation of these films was analyzed in situ by reflection high energy electron diffraction (RHEED), and ex situ by x-ray diffraction and cross-sectional transmission electron microscopy (XTEM). In particular, RHEED intensities were recorded during epitaxial growth, and intensity profiles across Bragg rods were used to calculate the surface lattice constant, and hence the film strain. XTEM analysis indicated that the epitaxial films had atomically abrupt interfaces. The magnetic properties of these epitaxial films were measured in situ using magneto-optic Kerr effect magnetometry. Large Hc (10–20 Oe) was observed for epitaxial Ni80Fe20 (100) films less than 10.0 nm thick whereas for larger thicknesses, Hc decreased to a few Oe with the appearance of a uniaxial anisotropy. Correlations were made between magnetic properties of these epitaxial films and the strain in the film.
Quench in superconducting magnets. I. Model and numerical implementation
Authors: Shajii A. , Freidberg J. P.
Published: July 2002

A new simple, self-consistent theoretical model is presented that describes the phenomena of quench propagation in Cable-in-Conduit superconducting magnets. The model circumvents many of the difficulties associated with obtaining numerical solutions in more general existing models. Specifically, a factor of 30–50 is gained in CPU time over the general, explicit time-dependent codes used to study typical quench events. The corresponding numerical implementation of the new model is described and the numerical results are shown to agree very well with those of the more general models, as well as with experimental data.
Magnetic states of magnetic multilayers at different fields
Authors: Schroeder P. A. , Lee S.-F. , Holody P. , Loloee R. , Yang Q. , Pratt W. P. , Bass J.
Published: July 2002

The resistance of an as-sputtered magnetic multilayer in zero applied magnetic field can be considerably different from the maximum resistance measured after the multilayer has been cycled to above its saturation field. We examine the relation between these two resistances in some Ag/Co, Cu/Co, and Cu/NiFe multilayers, and its implications for interpreting giant magnetoresistance.
Structure of the defect perovskite [Pb0.85La0.10]TiO3 between 10 and 1023 K
Authors: Rossetti George A. , Rodriguez Mark A. , Navrotsky Alexandra , Cross L. Eric , Newnham Robert E.
Published: July 2002

The structure of the defect perovskite [Pb0.85⧠0.05La0.10]TiO3 was investigated by powder neutron and x-ray diffraction and by specific heat measurements made at various temperatures. Refinement of the structure by the Rietveld technique at 10, 200, and 300 K from time-of-flight powder neutron diffraction data revealed no low temperature transitions from the ferroelectric tetragonal (4mm) phase. On average, the oxygen octahedral framework of the parent compound was not distorted by the incorporation of lanthanum and vacancies, and the ratio of cation displacements along the polar axis was comparable to that observed for pure PbTiO3. However, the material showed a large nonuniform strain that increased with decreasing temperature. As a result, the effective coupling between the uniform tetragonal deformation and cation displacement was reduced. X-ray diffraction and specific heat measurements at 298–1023 K indicated that the ferroelectric m3m→4mm transition was nearly tricritical and took place at 565 K. The diffuse nature of the transition was reflected in a temperature-dependent x-ray profile broadening and anomalous excess specific heat that persisted to ≊775 K, close to the phase transition temperature of pure lead titanate.
Optical absorption study of ion beam synthesized polycrystalline semiconducting FeSi2
Authors: Yang Z. , Homewood K. P. , Finney M. S. , Harry M. A. , Reeson K. J.
Published: July 2002

Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three-parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.
Preparation and electrical properties of Bi2S3 whiskers
Authors: Mizoguchi H. , Hosono H. , Ueda N. , Kawazoe H.
Published: July 2002

Bi2S3 whiskers ∼10 mm long were synthesized from vapor phase. The electrical conductivities of the resulting whiskers (along the longitudinal direction) were ∼101 S cm−1 at around room temperature and their temperature coefficient was negative. The growing direction was in the direction of the crystallographic c axis, which is parallel to the direction of running chains composed of (Bi4S6)n. The measurements of electrical conductivities in sulfur-free and sulfur-loading atmosphere strongly suggest that carrier electrons are generated through the formation of sulfur deficiencies.
Transient photocurrent in hydrogenated amorphous silicon and implications for photodetector devices
Authors: Shen D. S. , Wagner S.
Published: July 2002

The transient photocurrent of amorphous silicon photodetectors is analyzed, with emphasis on its device implications. The transient photoresponse in two types of photodetector device, photoconductor and photodiode, are compared. The difference between transient photocurrent and steady-state photocurrent is discussed. Numerical modeling is used to analyze the transient photocurrent decay. The results show that response speeds in the GHz range can be achieved in amorphous silicon photodetectors with proper device structures and proper detection schemes. Potential applications are discussed.
Modified surface charge spectroscopy for the characterization of insulator/semiconductor structures
Authors: Chan R. W. M. , Kwok R. W. M. , Lau W. M.
Published: July 2002

In the present study, a modified charge neutralizer was used to generate both the negative and positive surface potentials at the dielectric surface for the surface charge spectroscopy (SCS). A stable surface potential and a uniform potential area can be easily achieved. The SCS results of plasma enhanced chemical vapor deposition SiO2/Si samples without annealing and thermal SiO2/Si samples with and without annealing in forming gas (4%H2 in N2) are used to demonstrate the applicability of the SCS analysis.
Kinetics of residual stress evolution in evaporated silicon dioxide films exposed to room air
Authors: Leplan H. , Robic J. Y. , Pauleau Y.
Published: July 2002

Deposition of silicon dioxide thin films on silicon, germanium, and glass substrates has been accomplished by electron gun evaporation under various experimental conditions. The level of compressive residual stresses in evaporated SiO2 films determined just after deposition of films on Si or Ge substrates and the mass density of silica were found to be dependent on the substrate temperature and oxygen partial pressure in the evaporation chamber. Moreover, the intensity of compressive residual stresses decreased linearly as the logarithm of the exposure time of films to room air (aging time) was increased. After a compressive–tensile stress transition observed at a given aging time for films with relatively low densities, the intensity of tensile stresses continued to increase progressively with increasing aging time. The relative variation of residual stresses in SiO2 films was dependent on the mass density of the deposited material. Since the intensity of the absorption band of Si–OH radicals in the infrared spectra of SiO2 films exposed to room air increased with increasing aging time, the evolution of residual stresses was attributed to the effect of the hydration of silica by water vapor contained in room air. A reaction mechanism involving the physical adsorption of water vapor molecules on the SiO2 surface and reaction between adspecies and Si–O–Si species is proposed and discussed to interpret the logarithmic kinetics of residual stress evolution in evaporated SiO2 films exposed to room air.
How predictable is the current perpendicular to plane magnetoresistance? (invited)
Authors: Pratt W. P. , Yang Q. , Henry L. L. , Holody P. , Chiang W.-C. , Schroeder P. A. , Bass J.
Published: July 2002

Prior measurements of the current perpendicular to the layer planes (CPP) resistances, taken on Co/Cu/Py/Cu (Py=Ni84Fe16) multilayers with a single pair of Co and Py thicknesses, are extended to three additional pairs of thicknesses. The same parameters, obtained from independent measurements on Co/Cu and Py/Cu multilayers that fit the original pair reasonably well, fit the three new pairs almost as well, from which we conclude that there is substantial predictability in the CPP magnetoresistance (MR). Because the predictability is not perfect, we examine the extent to which we can improve the fits to the Co/Cu/Py/Cu data by varying the Py/Cu and Co/Cu parameters within their uncertainties, without substantially weakening the fits to the original Co/Cu and Py/Cu data. We conclude by presenting the first CPP-MR measurements on Co/Ag/Py/Ag multilayers. The data are similar to those for Co/Cu/Py/Cu multilayers, but the Co/Ag/Py/Ag MRs are noticeably larger.
Perpendicular transport and magnetic properties in patterned multilayer magnetic microstructures (invited)
Authors: Krebs J. J. , Vavra W. , Prinz G. A. , Cheng S. F. , Fink Anita
Published: July 2002

Using micron-level photolithography, we have prepared a set of patterned multilayer samples which have the form [Co/Cu/NiFeCo/Cu]×10 and which permit perpendicular current transport through the structure. Based on vibrating sample magnetometry, the multilayer stacks show a loss of ≊2 Å of magnetic material at each interface. The magnetoresistance was measured as a function of temperature, magnetic layer thickness, and element size and the ΔR/R values range up to 8% at 300 K and 18% at 10 K. The thickness dependent data are modeled using an extension of existing methods and can be fit using comparable resistivity, interface resistance, and spin asymmetry parameters. The magnetic fringe fields of the elements have an important effect on the observed R vs H curves.
Measurements of local magnetization by Kerr effect on Si-Fe nonoriented sheets
Authors: Defoug S. , Kaczmarek R. , Rave W.
Published: July 2002

Magnetic domains have been visualized by Kerr effect on nonoriented silicon-iron sheets. A simple methodology on how to determine the surface magnetization is proposed. For complex patterns, this method provides interesting results which could not be obtained by any reasoning based on physical principles.
New methods to measure the current perpendicular to the plane magnetoresistance of multilayers
Authors: Dauguet P. , Gandit P. , Chaussy J.
Published: July 2002

The experimental devices presented here enable us to measure variations with magnetic field of electrical resistances in the range (0.1 nΩ, 1 μΩ), for applied magnetic fields up to 3 T, and for temperatures less than 9 K. The setups work with three measurement methods. Two provide direct access to R(H), one in constant current, the other in alternating current. Their absolute sensitivity is about 1 pΩ, and their relative resolution ΔR/R is limited to 0.3%. The third method is a sinusoidal field modulation technique giving the field derivative of the resistance: dR/dH. It is sensitive to variations of the resistance as small as 1 pΩ independent of the absolute resistance of the sample, thus the resulting resolution ΔR/R is, for example, 10−6 for a 1 μΩ sample. With these devices, the magnetoresistance of multilayers can be studied with the current perpendicular to the plane.
Study of the frequency dependence of the ferromagnetic resonance linewidths of nickel ferrites from 8–60 GHz
Authors: Torres L. , Zazo M. , Flores A. G. , Raposo V. , Iñiguez J.
Published: July 2002

In this communication we present in a systematic way experimental data on ferromagnetic resonance (FMR) linewidths of polycrystalline nickel ferrites (NixFe3−xO4 with 0.8<×<1.5) from 8 to 12.4 GHz and from 26.5 to 60 GHz. Data from 26.5 to 60 GHz were taken at room temperature while measurements at X band have been carried out from 77 to 400 K. Neither in X band nor from 26.5 to 60 GHz was a clear frequency dependence found. Classical contributions to the FMR linewidth according to Sparks’ outline are analyzed in the whole range of frequencies from 8 to 60 GHz. Contributions of the porosity, anisotropy, slowly and rapidly relaxing impurities, valence exchange and eddy current mechanisms are considered.
Grain boundary diffusion through thin films. Application to permeable surfaces
Authors: Preis W. , Sitte W.
Published: July 2002

A detailed mathematical analysis of isolated as well as multiple grain boundary diffusion through thin films of finite thickness has been performed. The thin film is bounded by two permeable and parallel surface planes, and a constant flux of diffusing particles through both surfaces is maintained during the entire diffusion process. The flux is kept small in order to guarantee the applicability of Fick’s second law of diffusion with constant and position independent diffusion coefficients. The boundary conditions considered can be found in symmetric electrochemical cells applied to the determination of chemical diffusion coefficients of mixed conductors. Both Fourier–Laplace transforms and Fourier analysis techniques are used to solve Fick’s second law of diffusion. Various concentration profiles for different parameter sets (grain boundary/volume diffusion coefficient ratio and grain boundary distance) have been calculated. The diffusion process in polycrystalline thin films is considerably enhanced by an increase of the grain boundary/volume diffusion coefficient ratio as well as a decrease of the grain boundary distance. The results for multiple grain boundaries are compared with those obtained from the isolated grain boundary model. A satisfactory coincidence between the concentration profiles of these two models is achieved when the grain boundary distance is sufficiently high or the diffusion time fairly small.
Determination of the spatial distribution of a physical parameter from the distribution of another physical variable— a differential inverse problem
Authors: Sumi Chikayoshi , Suzuki Akifumi , Nakayama Kiyoshi
Published: July 2002

We propose a new type of inverse problem, in which the spatial distribution of the relative value of a physical parameter can be determined only from the distribution of another physical variable in the region of interest if there is no source in that region. The inverse problem proposed here has two features different from conventional remote probing type problems. One is that the physical variable data are given throughout the region of interest although the number of data variables is insufficient to determine directly the physical parameter of interest, while only remote data are given in a conventional problem. The other is the mathematical structure of the inverse problem: this new inverse problem yields a spatial differential equation on the target parameter, while the conventional problem becomes an integral equation on the target parameter. To show the nature of the problem, we formulated an illustrative inverse problem on the steady-state electric current field, in which the spatial distribution of the relative conductivity can be determined from the distribution of either the potential or the current density. We give examples of computational results obtained when using the simulated potential data and the current density data. This proposed formalism for a new type of inverse problem could provide a new approach and open up new measurement schemes in many fields of science and engineering.
Electrode effects in positive temperature coefficient and negative temperature coefficient devices measured by complex-plane impedance analysis
Authors: Cann D. P. , Randall C. A.
Published: July 2002

Impedance spectroscopy in conjunction with capacitance measurements were utilized to study the electrical contact behavior of commercial positive temperature coefficient and negative temperature coefficient ceramics. The impedance spectroscopy data, measured at frequencies ranging from 100 Hz to 40 MHz, exhibited a double semi-circle shape characteristic of systems which have both barrier and bulk contributions to the impedance of the device. From the impedance data, equivalent capacitive-resistive circuits were assigned to grain, grain boundary, and barrier elements. These barrier capacitances and resistances were measured for Au, Ag, Pt, and In-Ga electrodes. The trends in the magnitude of the barrier resistance are discussed with relevance to the thermodynamic work of adhesion and the effect of metal-oxygen and metal-metal interactions at the interface.
On Hall scattering factors for holes in GaAs
Authors: Look D. C. , Stutz C. E. , Sizelove J. R. , Evans K. R.
Published: July 2002

Hall scattering factors for electrons and holes in molecular beam epitaxial GaAs layers have been determined by comparing carrier concentrations measured by the Hall effect with those measured by the electrochemical capacitance–voltage technique. The conclusion is that both the electron and hole scattering factors are near unity for n ranging from 2×1016 to 7×1017 cm−3, and p ranging from 5×1016 to 4×1019 cm−3. This conclusion is consistent with the present theory for electrons, but not with that for holes.
Photovoltaic effects in porphyrin polymer films and heterojunctions
Authors: Marée C. H. M. , Roosendaal S. J. , Savenije T. J. , Schropp R. E. I. , Schaafsma T. J. , Habraken F. H. P. M.
Published: July 2002

Electropolymerized porphyrin films on indium–tin–oxide substrates have been characterized using Rutherford backscattering spectrometry, absorption spectroscopy, electrical characterization methods and with step profiling. With these methods the density of the films (ρ=1.35 g/cm3) and the absorption coefficients α(λ) have been determined. For film thicknesses exceeding 40 nm, silver electrical contacts without shunts are achieved by evaporation. The dark conductivity of the films amounts to 10−13–10−12 Ω−1 cm−1. When applying a band model for the conduction in the films, the dark space charge limited current and the exponent in the relation between photoconductivity and illumination intensity (σ∼Iγ, γ=0.65±0.05) indicate an exponential trap distribution in the band gap of the films. From the action spectra, filter effects of the photoconductance and low mobilities are inferred. Spin coating of acceptor layers on top of the polymer films results in the formation of heterojunctions showing photovoltaic behavior, with an open-circuit voltage 0.4–0.6 V. The short-circuit current is controlled by electron transfer at the donor/acceptor interface only and is limited by filter effects in the bulk and by the low conductivity of the materials. The optoelectrical properties of the layers are different if analyzed using a mercury contact (higher dark conductivity, no photoconductivity) which is attributed to the intro- duction of dopants from ambient air in this case.
Hole transport properties of bulk zinc-blende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure
Authors: Oğuzman İsmail H. , Kolník Ján , Brennan Kevin F. , Wang Rongping , Fang Tzu-Ning , Ruden P. Paul
Published: July 2002

In this paper, we present calculations of the hole transport properties of bulk zinc-blende and wurtzite phase GaN at field strengths at which impact ionization does not occur significantly. The calculations are made using an ensemble Monte Carlo simulator, including the full details of the band structure and a numerically determined phonon scattering rate based on an empirical pseudopotential method. Band intersection points—including band crossings and band mixings—are treated by carefully evaluating the overlap integral between the initial and possible final drift states. In this way, the hole trajectories in phase space can be accurately traced. It is found that the average hole energies are significantly lower than the corresponding electron energies for the field strengths examined. This result is most probably due to the drastic difference in curvature between the uppermost valence bands and the lowest conduction band. The relatively flat valence bands impede hole-heating, leading to low average hole energy.
Ordered phase and field-induced domains in a short-range Ising spin glass: Fe0.25Zn0.75F2
Authors: Raposo E. P. , Coutinho-Filho M. D.
Published: July 2002

Using a local mean-field numerical approach to describe disordered antiferromagnets, with application to FexZn1−xF2, it is shown that the characteristics of the spin glass phase found for x=0.25 is much in agreement with the scenario predicted by the scaling theory of the droplet model. In particular, the spin-reversal domains induced by a magnetic field give rise to a metastable irreversibility line.
Effects of lattice distortion on the physical properties and surface morphology of magnetoresistive perovskite epitaxial films
Authors: Yeh N.-C. , Fu C.-C. , Wei J. Y. T. , Vasquez R. P. , Huynh J. , Maurer S. M. , Beach G. , Beam D. A.
Published: July 2002

The effects of lattice distortion on the physical properties of La0.7Ca0.3MnO3 epitaxial films are investigated. Our results suggest that larger substrate-induced lattice distortion gives rise to larger zero-field resistivity and larger negative magnetoresistance. Similar effects are also observed in samples of different thicknesses and on the same substrate material, with larger resistivity and magnetoresistance associated with thinner samples. In addition to x-ray diffraction spectroscopy, the degrees of lattice distortion in different samples are further verified by the surface topography taken with a low-temperature scanning tunneling microscope. Quantitative analyses of the transport properties suggest that the high-temperature (T→TC) colossal magnetoresistance (CMR) in the manganites is consistent with the conduction of lattice polarons induced by the Jahn–Teller coupling, and that the low-temperature (T≪TC) magnetoresistance may be attributed to the magnetic domain wall scattering. In contrast, the absence of the Jahn–Teller coupling and the large conductivity in La0.5Ca0.5CoO3 epitaxial films yield much smaller negative magnetoresistance, which may be attributed to disorder-spin scattering.
Pressure induced giant volume magnetostriction in the (Fe1−xNix)49Rh51 alloys
Authors: Kamenev K. , Arnold Z. , Kamarád J. , Baranov N. V.
Published: July 2002

We present the results on magnetovolume anomalies in (Fe1−xNix)49Rh51 alloys. At ambient pressure, they exhibit phase transition from the ferro- to the antiferromagnetic state at temperatures decreasing from about 300 to 150 K for x increasing from 0 to 0.036. The thermal expansion and compressibility of the alloys with x=0.025 and 0.045 were measured under high hydrostatic pressure up to 14 kbar. The phase transition to the antiferromagnetic state was induced in the alloy with x=0.045 by critical pressure of about 2.4 kbar. In the result of the phase transition to the antiferromagnetic state the volume decrease in the both alloys reaches the value of about 0.8%. The critical temperature Tt increases under pressure with the ratio of 9 and 13 K/kbar for x=0.025 and 0.045, respectively.
Synthesis of Z-type hexagonal ferrite films by rf magnetron sputtering techniques (abstract)
Authors: Ramakrishnan E. S. , Cornett Kenneth D. , Srinivasan G.
Published: July 2002

The synthesis of textured stoichiometric Z-type hexagonal ferrite films, Ba3Me2Fe24O41 (Me2-Z), where Me is a divalent ion, by sputtering techniques is reported. The compound Co2-Z shows a large planar anisotropy with a high permeability at room temperature, and thus, are promising candidates for microwave device applications. The objective here was to develop thin films of these materials for devices on integrable substrates. The films were deposited on unheated Si substrates by rf magnetron sputtering using oxide targets in a mixed argon/oxygen atmosphere. Studies on the crystal structure, composition, and magnetic properties were performed on 1.6 μm thick films. The films were characterized by x-ray diffraction and scanning electron microscopy, and the composition was analyzed by energy dispersive x-ray analysis and Auger electron spectroscopy/secondary-ion-mass-spectroscopy techniques. As-deposited films were amorphous and paramagnetic. The films were subjected, either to a rapid thermal anneal at 900 or 1000 °C for 2 min, or a conventional furnace anneal at 800 °C for 60 min. The rapid annealed films were found to contain 10%–40% of M- and Y-phases, and the saturation magnetization M was 50% smaller than the expected values. The furnace annealed films were textured with the c axis perpendicular to the film plane and showed a small amount of impurity phases. The measured M of 28.2 μB at 4.2 K is in excellent agreement with the theoretical value of 29.2 μB. Room-temperature data on the field dependence of M show a squareness ratio of 0.8 for in-plane fields and an in-plane anisotropy field on the order of 6 kOe.
Comparison of hysteresis loops from giant magnetoresistance and magnetometry for perpendicular-current exchange-biased spin valves
Authors: Steenwyk S. D. , Hsu S. Y. , Loloee R. , Bass J. , Pratt W. P.
Published: July 2002

We compare for permalloy (Py)/Cu/Py or Py/Ag/Py exchange-biased spin valves, hysteresis loops of: (1) giant magnetoresistance (GMR) with current perpendicular to the layer planes (CPP-MR), and (2) magnetization M measured for the same portion of the sample used to measure the CPP-MR. As expected, the two kinds of loops are mostly very similar. There are, however, some slight differences, the source of which is not yet clear.
Variation of multilayer magnetoresistance with ferromagnetic layer sequence: Spin-memory loss effects
Authors: Chiang W.-C. , Yang Q. , Pratt W. P. , Loloee R. , Bass J.
Published: July 2002

We find the magnetoresistance (MR) with current flow perpendicular to the layer planes (CPP-MR) of multilayers made of the ferromagnetic metals Co and Py (permalloy=Ni84Fe16) and the nonmagnetic metal Ag to vary with the Co and Py layer sequencing. That is, [Co/Ag/Py/Ag]N (type I) multilayers have larger MRs than [Co/Ag]N[Py/Ag]N (type II) ones, and the difference grows with the number of repeats N. We ascribe the differences to an apparent short spin-diffusion length in Py. Interestingly, similar differences appear also in the current in plane MR, probably due to finite mean-free paths in all three metals, plus interface scattering.
Mechanisms influencing “hot-wire” deposition of hydrogenated amorphous silicon
Authors: Molenbroek Edith C. , Mahan A. H. , Gallagher Alan
Published: July 2002

Intrinsic hydrogenated amorphous silicon (a-Si:H) has been deposited using a hot tungsten filament in pure silane to drive the deposition chemistry—the “hot-wire” deposition method. The electronic and infrared properties of the film have been measured as a function of deposition parameters, leading to three principal conclusions. First, to obtain a high quality material, the Si atoms evaporated from the filament (distance L from the substrate) must react with silane (density ns) before reaching the substrate; this requires nsL greater than a critical value. Second, radical-radical reactions cause deterioration of film properties at high values of G(nsL),3 where G is the film growth rate; this requires G(nsL)3 less than a critical value. Finally, the film quality is a function of G, and as G is increased the substrate temperature must be correspondingly increased to obtain high film quality. By optimizing these parameters, we have produced films with excellent electronic properties (e.g., ambipolar diffusion length >200 nm) at >5 nm/s deposition rate. Based on these insights, formulas are also given for optimizing film properties in multiple-filament geometries and in diluted silane.
Bulk metallic multilayers produced by repeated press-rolling and their perpendicular magnetoresistance
Authors: Yasuna K. , Terauchi M. , Otsuki A. , Ishihara K. N. , Shingu P. H.
Published: July 2002

Bulk Fe/Ag multilayers with layer thickness of about 10 nm have been successfully fabricated by repeated press-rolling directly from a macroscopic stack of metal sheets. The press-rolled multilayers exhibited giant magnetoresistance of 13% in the current perpendicular to the plane geometry and 4% in the current in the plane geometry at 5 K.
Enhanced magneto-optical Kerr effect in oxidized Co thin films
Authors: Rellinghaus B. , Fernandez de Avila S. , Weller D. , Armelles G. , Beyers R. , Kellock A.
Published: July 2002

We have studied the structural and magneto-optical properties of postdeposition oxidized Co thin films. The oxidization process leads to the formation of a double-layered structure of fcc Co3O4 on top of metallic Co. The magneto-optical Kerr effect (MOKE), measured in the range 0.8 eV⩽EPh⩽5.5 eV reveals characteristic dependencies of the MOKE spectra on annealing temperature and time. In particular, we observe resonance-type enhancements of the Kerr effects by up to a factor of 10 compared with unannealed metallic Co. The experimental data are quantitatively reproduced by bilayer optical stack calculations.
Electrical properties of Ga and ZnS doped ZnO prepared by mechanical alloying
Authors: Cook B. A. , Harringa J. L. , Vining C. B.
Published: July 2002

A series of n-type ZnO alloys doped with Ga and ZnS were prepared by mechanical alloying. Densities of 95% to 98% of theoretical density were achieved by hot pressing the milled powders at 1000 and 1200 °C, respectively. The electrical resistivity and Seebeck coefficient of alloys containing 0.25–3.0 at. % Ga were characterized between 22 and 1000 °C. The magnitude of the resistivity and Seebeck coefficient at 22 °C ranged from 0.2 mΩ cm and −25 μV/°C for the most heavily doped specimen to 1.1 mΩ cm and −70 μV/°C for the lightly doped material. The alloys exhibit a positive temperature coefficient of resistivity and Seebeck coefficient with a nearly constant slope over the temperature range studied. Thermal diffusivity measurements on a specimen containing 1.0 at. % Ga were performed over the same temperature range. The thermal conductivity appears to follow a T−1 dependence, decreasing from 180 mW/cm °C at 22 °C to 82 mW/cm °C at 1000 °C. An estimate of the maximum dimensionless thermoelectric figure of merit, ZT, in this system at 1000 °C gives a value of 0.26, a factor of three to four less than current state-of-the-art materials such as Si–Ge. A significant reduction in thermal conductivity would be required to make these alloys competitive with existing thermoelectric power generation materials.
Random-exchange and random-field Ising model-like behaviors in Fe0.48Zn0.52F2
Authors: Raposo E. P. , Coutinho-Filho M. D.
Published: July 2002

By using a local mean-field microscopical numerical approach, we investigate the role of frustration, random vacancies, and magnetic field cycles on the random-exchange and random-field Ising model-like behaviors of the diluted antiferromagnet Fe0.48Zn0.52F2. The analysis includes studies on microscopic configurations, distribution of local effective fields, and the crossover exponent φ, which were found to be in good agreement with the experimental results.
The determination of dominant diffusing species in the growth of amorphous interlayer between Gd and Si thin films by a Mo cluster marker experiment
Authors: Chen J. C. , Shen G. H. , Chen L. J.
Published: July 2002

The dominant diffusing species in the formation of amorphous interlayer between Gd and Si thin films have been determined by a Mo cluster marker experiment. Multilayered metal thin films were deposited on (001) Si in an ultrahigh vacuum electron beam evaporator. The positions of the Mo cluster markers relative to the Si substrate, before and after heat treatment, were determined by transmission electron microscopy and energy dispersive x-ray as well as autocorrelation function analysis. The displacement of the Mo cluster markers in the amorphous interlayer during the Gd–Si interdiffusion indicates that Si atoms constitute the dominant diffusing species during the growth of the amorphous interlayer.