bluuebunny/crossref_metadata_2025 huggingface dataset:
Cooperative spin-crossover transition: Effects of the antiferro- and ferri-ordered phase (abstract)
Authors: Vekhter Benjamin G.
Published: July 2002

The considerable increase of magnetic moment M with increasing temperature T [the spin-crossover (SC) effect] in some transition metal complexes is caused by a predominant occupation of the excited high-spin (HS) state which has a larger statistical weight than the low-spin (LS) ground one. Rather unusual experimentally observed M(T) behavior that provides for molecular electronics applications with such compounds, gives evidence of the importance of cooperative intermolecular interactions. The microscopic theory of cooperative SC transitions is developed. This is based on the phonon-mediated ion-ion interaction and accounts for the possibility of a phase with antiferro-ordered local distortions around the magnetic ions, in addition to the homogeneous ferro-phase that was the only one considered previously. If the parameter ‖J‖ of the antiferro-coupling (J<0) is greater than the energy gap Δ between the LS and HS states, then at T=0 one-half of the ions are in the ‘‘excited’’ HS state while the other half reside in the LS state. A temperature increase causes a M(HS)/2→M(HS) transition to the phase where all the ions are in the HS states. The larger is the value of ‖J‖, the higher is the transition temperature. If ‖J‖<Δ, then at T=0 all the ions are in the LS state because the intermolecular correlations are too small to lead to antiferro-ordering. However, if ‖J‖ is close enough to Δ, then the intermediate ferri-ordered phase appears, resulting in two [M(LS)→M(HS)/2→M(HS)] consecutive transitions (two-step SC change). The closer ‖J‖ is to Δ, the lower the temperature of the first transition is and the wider the temperature interval of the ferri-phase existence is. Hysteresis phenomena and effects of magnetic dilution and external pressure, examined within the framework of the developed microscopic approach, are in a good agreement with experiment.
Temperature dependence of switching field distribution
Authors: Lin Jiang-Ching , Klik I. , Chen C. J. , Chang Ching-Ray
Published: July 2002

The peak value of switching field distribution χmax, coercivity Hc, and saturation magnetization Ms are measured as functions of temperature T in Co-γ-Fe2O3 and γ-Fe2O3 powders. It is found that χmax has a maximum at T=Tp and evidence is presented also for the existence of a theoretically predicted minimum at T=Tm<Tp. Coercivity is decreasing with T. For Ba-ferrite platelets we find that Hc has a maximum at T=T̃p and a minimum at T=T̃m≊Tp, where χmax has a local maximum. The possible origin of this unusual behavior is discussed.
Dynamics of fine particles observed in zero-field neutron scattering
Authors: Hennion M. , Bellouard C. , Mirebeau I. , Dormann J. L. , Ober R.
Published: July 2002

We present here a dynamical study of Fe particles (○/∼50 Å) dispersed in an alumina matrix, performed by inelastic neutron scattering, in zero applied field. The line shape of the energy spectrum, quasielastic or inelastic, allows us to define three temperature ranges, attributed to three distinct regimes for the transverse fluctuations. At high temperature, the line shape is quasielastic with a nearly temperature independent energy linewidth. At intermediate temperature, one observes the expected precession mode with an inelastic line shape. At low temperature, a ‘‘reentrant’’ behavior occurs, which locates the disappearance of the precession mode, likely due to the effect of interparticle coupling.
Polyphase eddy current testing
Authors: Marinova I. , Hayano S. , Ishida N. , Saito Y. , Hirama Y. , Yamamoto M.
Published: July 2002

Polyphase eddy current testing is proposed. The operating principle of this system is as follows. The polyphase eddy current testing system may be regarded as one of the polyphase transformers. Primary balanced polyphase coils are star connected and secondary polyphase circuits are composed of the target. When balanced polyphase currents are fed into the primary coils, zero phase voltage is detected if the target has defects. No zero phase voltage is detected if the target has no defects. It is found that this method has a higher sensitivity compared with that of conventional eddy current testing.
Electrical characteristics of spiral coil planar inductors using amorphous alloy ribbons as magnetic layers
Authors: Tanigawa K. , Hirano H. , Sato T. , Tanaka N.
Published: July 2002

Various types of planar inductors have been proposed for reducing the volume of switching converters. Planar inductors using amorphous alloy ribbons as magnetic layers are also studied. The purpose of this study is to clarify the relation between the electrical performance of the inductor, and the properties and composition of amorphous ribbon. The experimental results show that superimposed dc current, I80, increases linearly with the product of Bm and ribbon thickness t, where I80 is defined as the dc current corresponding to an inductance which decreases to 80% of an initial value. In the case of stacking amorphous alloy ribbons, I80 increases with gradual approach to saturation, as the stack thickness increases regardless of ribbon thickness. However, the saturation value of I80 depends on ribbon thickness. The saturation value increases with increasing ribbon thickness. The above results suggest that from a practical viewpoint the use of thick ribbons is preferable. With respect to alloy composition, the Fe-based alloy is more favorable compared with the Co-based alloy because of its higher stored magnetic energy.
Validity and the applicablity of magnetic-circular-dichroism sum rules for transition metals (invited)
Authors: Wu Ruqian , Wang Dingsheng , Freeman A. J.
Published: July 2002

X-ray magnetic-circular-dichroism (MCD) spectra are calculated using the thin film full potential linearized augmented plane wave energy band method for transition metal surfaces. Together with a model analysis, a clear physical insight is provided for this new and exciting phenomenon. The MCD sum rule for the orbital moments originally derived from a single ion model are found to be valid to within 5–10% even in the band approach which treats the strongly hybridized multiband structure. However, the spin sum rule may result in a larger error due to the effects of s,p−d hybridization. In addition, the magnetic dipole term is very important in the spin sum rule, especially for atoms with lower local symmetry.
X-ray magnetic circular dichroism studies of multilayered thin films of 3d transition metals (invited) (abstract)
Authors: Weller D. , Samant M. G. , Stöhr J. , Wu Y. , Hermsmeier B. D. , Held G. , Chappert C.
Published: July 2002

X-ray magnetic circular dichroism (XMCD) is used to probe both the spin and orbital d moments of Co in Co/Pd, Co/Pt, and Co/Ni multilayers, and in a Au/Co-wedge/Pd structure. While the spin moment per Co atom is found to be constant within the experimental error, the orbital moment shows a strong dependence both on the chemical environment and on the thickness of the Co layer. In particular, we find an orbital moment enhancement of up to a factor of three for a Co layer thickness of four atomic layers in the present Au/Co/Pd sandwich structure. This enhancement decays to the bulk Co value, roughly following a 1/dCo dependence, thus confirming its interfacial origin. The relevance of these observations with respect to the perpendicular magnetic anisotropy in these samples will be discussed.
Magnetic properties in the Ising mixed spin-1/2–spin-1 superlattice
Authors: Sarmento E. F. , Cressoni J. C. , dos Santos R. J. V.
Published: July 2002

An effective-field theory that accounts for the self-spin correlation function is used to derive the phase diagrams and magnetization curves of a magnetic superlattice, consisting of a mixed spin- 1/2–spin-1 (with a single-ion anisotropy D) alternate monolayers of ferromagnetic material. It is found that the phase diagrams exhibit a number of interesting phenomena resulting from the dependence of the crystal-field interaction. Both limit cases for very large positive and very large negative D reproduce respectively the T(3D)=5.07 and the T(2D)=3.08 Ising critical temperatures which are in agreement with earlier results. On the other hand, in particular, no tricritical point could be found. The magnetization curves are presented for a range of interaction strengths and D, respectively.
A hysteresis model with interactions
Authors: Planès J.
Published: July 2002

The magnetic properties of an assembly of monodomain ferromagnetic particles are computed, considering the whole set of dipolar interactions in competition with magnetocrystalline anisotropy. Coercive field, remanent magnetization, initial susceptibility, are studied as a function of the ratio p of competing energies, and also of geometrical parameters such as density. p≂1 corresponds to a crossover region, separating an anisotropy-dominated regime from a coupling-dominated regime. An example of a possible experimental realization is presented, for which the magnetization curve and hysteresis loop present strong similarities with calculated ones; a numerical discrepancy, however, remains.
3-D micromagnetic modeling of domain configurations in soft magnetic materials
Authors: Guo Zhenzhou , Della Torre Edward
Published: July 2002

This paper describes a micromagnetic model that computes the vector magnetization in a three-dimensional geometry. Since the magnitude of the magnetization is constant, the energy minimization is performed with respect to polar and azimuthal angles. This generates a pair of coupled nonlinear partial differential equations solved numerically in spherical coordinates. The demagnetizing field, on the other hand, is not constrained in magnitude and therefore requires three variables at each point to fully describe it. Applying this to a Permalloy film whose thickness is equal to one mesh unit permits direct comparison to our 2-D study that only permitted pure Neél walls to form. The extra degree of freedom allows the magnetic spins to have an out-of-plane component, which is especially noticeable in the region where a vortex formed in the 2-D model. This numerical model is being used to study and optimize the flux conduction in a pipe whose cross section permits the elimination of closure domains, thus improving both the magnetic susceptibility and the magnetic stability of the recording head.
Mathematical modeling of lubrication for the head-disk interface using incompressible fluids
Authors: Peck Paul R. , Jhon Myung S. , Simmons Ralph F. , Janstrom Thomas J.
Published: July 2002

In order to keep pace with the demand for higher storage densities in magnetic recording disk drives, new technology must be developed to reduce the tribological problems associated with ultralow flying. Non-Newtonian liquid bearings may become a feasible alternative to slider miniaturization due to the decrease in slider-disk contacts. Issues pertaining to modeling the head-disk interface are presented, and non-Newtonian effects are described. Generalized Newtonian fluid models describe the dependence of the apparent viscosity on the shear rate for bulk fluids, but the extremely high shear rate and the confined geometry at ultralow flying heights complicate conventional non-Newtonian rheological models. Equations are developed for generalized Newtonian fluids which can determine the flying characteristics of recording sliders in viscoelastic drives. Order-of-magnitude analyses are performed in order to determine the effect of drive parameters and fluid properties on the load bearing capacity of the slider and power consumption due to frictional forces. The results obtained are new and may become important in providing design criteria for VISqUS drives or to related design engineers.
Wear and stiction regimes of thin-film magnetic disks
Authors: Bowen Arlen
Published: July 2002

The mechanical integrity of a thin-film disk drive is determined by contact start-stop (CSS) testing. Poor results in CSS testing are called either a wear or a stiction failure. Ambiguities are identified between wear and stiction observations and properties of the head/disk interface that caused the failure. The CSS test results are categorized into five wear and stiction regimes that are related to the physical properties of the head/disk interface.
Magnetostrictive hysteresis in nickel polycrystalline thick films
Authors: Brard Isabelle , Georges Roland , Le Blevennec Gilles
Published: July 2002

Simultaneous magnetization and linear magnetostriction measurements have been performed on a 0.1-mm-thick polycrystalline nickel film. First magnetization and hysteresis loop type experiments are reported. The magnetostriction versus magnetization behavior appears to follow an approximate M4 law, and to exhibit a weak but clear-cut hysteresis. Moreover, the film shape is changed by the applied magnetic field as the saturation magnetization is approached. Magnetostriction has also been measured as a function of the applied field direction for both saturation and intermediate magnetization states. Hysteresis effects appear in the second case. These results are interpreted in terms of domain wall motion and magnetization reorientation mechanisms within each domain.
High pulsed field magnetostriction in RMn2 (R=Gd, Tb, Nd, and Y)
Authors: Ibarra M. R. , Marquina C. , Garcia-Orza L. , Arnold Z. , del Moral A.
Published: July 2002

We have performed thermal expansion and magnetostriction experiments on several compounds of the series RMn2 and R(Mn1−xNix)2 (R=Nd, Gd, Tb, and Y) using high pulsed magnetic field up to 15 T in the temperature range 4–300 K. A giant volume magnetostriction of ω=−17×10−3 was observed in TbMn2 at 15 T and 4 K. A negligible forced volume magnetostriction was observed in YMn2. In GdMn2 paramagnetic magnetostrictive behavior was found below TN. Magnetostriction experiments were confirmed as a powerful tool in order to determine the stability of the magnetic moment on Mn in this series of compounds.
Model calculations of rotational hysteresis for ferromagnetic particles with competing anisotropies
Authors: Yoshida Y. , Templeton T. L. , Arrott A. S.
Published: July 2002

The net work Wr(360°) on rotating a particle through 360° about an axis perpendicular to an applied field is called the rotational hysteresis. Its dependence on field is calculated for model particles with large surface anisotropies that compete with the bulk anisotropies. Both anisotropies are uniaxial and twofold but acting along different axes. If the axes are separated by any angle except 0° or 90°, Wr(360°) is quite different for clockwise and counterclockwise rotations, for the range of fields where it is not zero. Because the magnetization is nonuniform, the results depend upon the exchange interaction and the particle size. The results are given for a range of particle sizes from small to large compared to a domain wall width. The results for particles with uniform anisotropy and magnetization are given for comparison.
Electrochemical evaluation of the effect of binder additives on iron corrosion
Authors: Arroyo J. Carlos , Saffarian Hassan M. , Warren Garry W.
Published: July 2002

Recent research efforts have emphasized the archivability of metal particle media which utilizes ‘‘passivated’’ iron particles as the magnetic material. Two fundamental processes limit the archival lifetime of such media (1) corrosion of metal particles, and (2) degradation of polymer binder. This paper addresses the first process by quantifying the effect of pH and various binder additives (lubricant, dispersant, polymer degradation products, etc.) on the corrosion of pure iron in aqueous 0.05 M K2SO4. This study is unique in its use of electrochemical techniques to adequately quantify the corrosion process. Both ac [electrochemical impedance spectroscopy, (EIS)] and dc polarization techniques have been used. Experimental data showed that the addition of stearic acid (a lubricant) and lecithin (a dispersant) resulted in a decrease in the corrosion rate of iron in K2SO4. Five different carboxylic acids of various chain lengths were investigated since such compounds can be viewed as simple models for binder additives, or as polymer degradation products. Except for stearic acid, all of the carboxylic acids significantly accelerated iron dissolution at the corrosion potential. The corrosion rate increased as chain length or pH decreased and as acid concentration increased. The charge involved in passive film formation increased as the solution becomes more acidic, indicating that the passive film was less stable or more difficult to form. In the passive region the corrosion rate was lower than that of the base electrolyte for high concentrations of carboxylic acids.
Rheo-optical measurements on suspensions of magnetic recording particles (abstract)
Authors: Choi Hyoung J. , Park Yoon D. , Frattini Paul L. , Jhon Myung S.
Published: July 2002

New rheo-optic measurements of the orientation of magnetic particles (used in particulate recording media) in suspension under imposed external fields (hydrodynamic and/or magnetic) are presented. The apparatus consists of a rheo-optical device1 employing polarization modulation technology, and the external magnetic and flow field generating equipment (plane Poiseuille flow cell and quadrupole electromagnet). The polarization modulation dichroism measurement technique, which has the advantage of improved sensitivity and precision over conventional optical measurements, is able to characterize uniaxial single domain magnetic particle suspensions under external magnetic and hydrodynamic fields. Linear dichroism measurements, which reflect the degree of particle alignment about the field axis, are shown to be sensitive indicators of particle magnetic interactions, of effects of particle intrinsic magnetic properties, and of effects of particle shape. In this paper we focus on the comparison between effects of imposed magnetic and hydrodynamic fields on particle orientation for rodlike γ-Fe2O3, CrO2, and plate-like Ba ferrites. Relationships between the particle orientation due to the two external fields are established through dimensional analysis, and the ‘‘equivalent magnetic field,’’ which yields the same magnitude of dichroism obtained under a given flow field, is measured. The flow field and the equivalent magnetic field show a nearly linear relationship. Once the equivalent magnetic field plot is obtained experimentally, one can conveniently transform hydrodynamic field to applied magnetic field and then determine the particle orientation. Preliminary results indicate that this device represents the best technique for systematic investigation of magnetic particle orientation during simultaneous imposition of magnetic and hydrodynamic fields.
Forward volume wave microwave envelope solitons in yttrium iron garnet thin films: Peak profiles and multisoliton signatures (abstract)
Authors: Tsankov M. A. , Chen M. , Patton C. E.
Published: July 2002

Microwave envelope solitons have been observed in yttrium iron garnet (YIG) films for different field configurations and surface pinning conditions. Earlier work on magnetostatic forward volume wave (MSFVW) solitons in YIG films has focused on the break from the low power linear dependence of the output pulse peak power Pout on input pulse power Pin,1 or on the evolution of the shape of the microwave pulse envelope with increasing power.2 In this work, both (1) the nonlinear dependence of Pout on Pin and (2) the shape of the output pulses for MSFVW solitons have been examined over a wide range of power levels and pulse widths. The experiments used 7.2-μm-thick single crystal YIG films with unpinned surface spins. A field of 3744 Oe was applied perpendicular to the plane of a 15×2 mm film sample. Microwave pulses at 5.4–6 GHz, with a width τ of 5–50 ns and peak powers up to 3 W, were launched by a microstrip antenna about 5 mm from one end and picked up by a second, parallel antenna about 5 mm from the other end. Data were processed with a microwave transition analyzer operated in a time domain mode. For a given pulse width, the initial break and nonlinear increase in Pout relative to Pin noted in Ref. 1 is followed by a gradual leveling off in Pout at high power. For a given pulse power above soliton threshold, the Pout versus τ profiles show an initial linear increase which is followed by a decrease and an oscillatory response as multiple solitons are formed. These profiles are directly correlated with the formation of a single soliton pulse which evolves into multiple soliton wave packets with increasing either power or pulse length.
The influence of heat treatments on order–disorder phenomena in a naturally occurring manganese-ferrite
Authors: Bluncson Cherie R. , Evans B. J.
Published: July 2002

Naturally occurring samples of iwakiite, Mn1.1Fe1.9O4, have been annealed for 17 and 77 h at 1000 K in an investigation of order–disorder phenomena in Mn–Fe oxide spinels. Two distinct magnetic hyperfine patterns, which correspond to Fe- and Mn-rich phases, were observed in the 57Fe Mössbauer spectra for the two annealed samples. While the spectrum of the Fe-rich phase corresponds to a magnetically and crystallographically highly ordered α-Fe2O3-like phase, the Mn-rich phase exhibits considerable disorder, as indicated by very broad linewidths and a low value for Heff. Annealing for longer periods of times leads to a considerable increase in the ordering of the Mn-rich spinel phase, as evidenced by the considerable narrowing of the lines, from Γ=2.40 mm s−1 to Γ=1.17 mm s−1, and an increase in Heff from 318 to 417 kOe at 298 K. The Fe-rich, α-Fe2O3-like phase is not affected by further annealing, as expected. At 85 K, the 57Fe Mössbauer spectrum of the Mn-rich, spinel phase continues to exhibit rather broad lines, indicating the presence of significant cation disorder.
Infinite stage Ettingshausen cooling in Bi-Sb alloys
Authors: Scholz K. , Jandl P. , Birkholz U. , Dashevskii Z. M.
Published: July 2002

An infinite staged Ettingshausen cooler of exponential shape has been constructed, using an oriented single crystal of Bi97Sb3. The magnetic field of B=0.75 T was provided by a permanent magnet system. A maximal temperature difference ΔT=42 K was achieved maintaining the heat sink at T=160 K. Referring to the temperature range and magnetic field strength used, this result represents a large cooling effect for an Ettingshausen cooling device. A simulation program that takes into account the temperature dependence of the transport coefficients was developed. Not only the predicted maximal temperature difference but also the electrical current necessary to obtain this maximum are in reasonable agreement with experimental results.
Chlorophyll b-modified nanocrystalline SnO2 semiconductor thin film as a photosensitive electrode
Authors: Bedja Idriss , Hotchandani Surat , Carpentier Robert , Fessenden Richard W. , Kamat Prashant V.
Published: July 2002

Nanocrystalline thin SnO2 semiconductor films (thickness≤1 μm) have been modified with chlorophyll b (Chl-b) by electrodeposition and adsorption methods for use as novel photosensitive electrodes in photoelectrochemical cells. Excitation of Chl-b with monochromatic light produced photocurrents with an incident photon-to-photocurrent efficiency of around 8.5%. The charge injection from excited Chl-b into the conduction band of the semiconductor SnO2 crystallites has been probed by time-resolved microwave absorption.
Surface structure of laser ablated C60 pellet
Authors: Yoda Osamu , Miyashita Atsumi
Published: July 2002

Compressed C60 powder, a C60 pellet, has been irradiated by second harmonics of Nd:YAG/glass pulsed laser light. From the x-ray photoelectron spectroscopic measurement, it was suggested that some reorganization occurred on the surface of the ablated C60 pellet. A periodic structure was discovered on the laser ablated C60 pellet and the spacing of that periodic structure was in good agreement with the prediction based on the surface scattering model.
Comment on ‘‘Structural properties of CuInxGa1−xSe2 thin films prepared by rf sputtering’’ [J. Appl. Phys. 72, 5657 (1992)]
Authors: Rincón C. , Quintero M. , Tinoco T.
Published: July 2002

An attempt to clarify the controversy related to the crystal structure of the CuInxGa1−xSe2 alloy system is given. For In-rich samples the crystal structure can be identified by the appearance of lines due to the order of cations in the chalcopyrite structure, while for Ga-rich samples the structure is identified by line splitting of the structure lines.
The Preisach model with stochastic input as a model for aftereffect
Authors: Mayergoyz Isaak D. , Korman Can E.
Published: July 2002

In previous papers, we have suggested to use the Preisach model driven by stochastic inputs as a model for aftereffect. However, in these papers the stochastic inputs have been modeled by discrete time I.I.D. (independent identically distributed) random processes. Here, we further extend the aforementioned approach by modeling the stochastic inputs by continuous time diffusion processes. It is shown that the mathematical machinery of the ‘‘exit problem’’ is instrumental for calculations of time evolutions of the expected value of the output of the Preisach model.
Experiments on quasiparticle-waves interference using Si/Nb boundary
Authors: Hatano Mutsuko , Nishino Toshikazu , Saito Kazuo , Takagi Kazumasa
Published: July 2002

Experiments on interference in solids analogous to the Michelson interferometer can be achieved using a specific mirror of Si/Nb boundary. Current is changed periodically by interference between quasiparticle waves with path length difference. Oscillation in differential conductance by interference due to changes in quasiparticle wavelength is observed. These measured results are found to be caused by the phase difference in quasiparticle waves.
Platinum enhanced textured growth of grain-aligned YBa2Cu3O7−x thick films
Authors: Wegmann M. R. , Lewis J. A. , Platt C. E.
Published: July 2002

The effects of magnetic alignment, heat treatment, and substrate interactions on the microstructural development and properties of YBa2Cu3O7−x (Y123) thick films were studied. Aligned films were formed by vacuum filtrating a particulate suspension in a 7 T applied field. These films and nonaligned control films were fired on either platinum (Pt) foil or magnesium oxide (MgO) substrates to various maximum temperatures between 930 and 1040 °C. Optical microscopy revealed large differences in microstructural development between the various films. Aligned Y123 films fired on Pt foil exhibited the best microstructural properties. Via plasma emission spectroscopy and secondary ion mass spectroscopy, approximately 0.1 wt % Pt was found distributed throughout the films fired on platinum, while negligible amounts of Mg were detected in the films fired on MgO substrates. Differential thermal analysis revealed that, in the presence of Pt, the peritectic temperature (1030 °C for pure Y123 in O2) is reduced 70 °C, thereby opening a substantial thermal processing window for partial melt assisted growth of textured Y123. SQUID measurements of magnetic hysteresis and Tc provided quantitative evidence that, relative to the films fired on MgO, those fired on Pt exhibited enhanced texture development [ΔM(Happ∥c axis)/ΔM(Happ⊥c axis)=2.6 at 5 K, 1 T] and properties (Bean model Jc,m=5×104 A/cm2 at 5 K, 4 T) without degradation of the Tc characteristics.
Atomic position and electronic field gradient of Fe dopant in Y-124 superconductor
Authors: Li Yang , Zhao Zhongxian , Wang Yunbo , Cao Guohui , Ma Ruzhang
Published: July 2002

We attempt to provide a model calculation of the electric field gradient. With reference to the observed 57Fe Mössbauer hyperfine interaction parameters, the effective valence and electric field gradient on different Fe lattice sites have been computed for the Fe-doped Y-124 superconductor by means of the point-charge-effective-valence model. According to the present calculations, the sign of electric-field gradient for the pyramid Fe(B′) site and the quasioctahedral Fe(B) is estimated to be positive and negative, respectively, and furthermore Fe is found to be displaced relative to the CuO2 plane in Y-124 superconductors.
Photoresponse of semiconductor on semi-insulator structure
Authors: Maimon S. , Schacham S. E.
Published: July 2002

A new type of photoconductivity is demonstrated in a p-type GaAs epitaxial layer grown on a semi-insulating substrate. Rather than increasing the hole concentration in the epilayer, the photons absorbed in the substrate reduce the junction depletion region, thus increasing the thickness of the conducting path in the epilayer. Small photon fluxes increase drastically the conductance of the structure. A flux of 3×1013 cm−2 s−1 is sufficient to double the conductance at room temperature. Voltage responsivities of the order of 5×106 V/W were obtained at a wavelength of 0.84 μm at room temperature. Hall measurements indicate that the total hole concentration increases slowly at low fluxes (below 1015 cm−2 s−1) and steeply at higher ones. These two regions correspond to two processes contributing to the increased population. Photons absorbed in the substrate, originally n type, turn it into p type in the vicinity of the junction, since the lifetime of holes, dominated by recombination with EL2 traps, is four orders of magnitude larger than that of electrons. Thus, the configuration changes from a p/n junction to p/p−. Hence, the depletion region is reduced significantly and the conductance increases accordingly. At higher intensities the hole concentration in the substrate becomes the dominant component in the integral hole concentration thus increasing linearly with photon flux. This process saturates as the injection starts affecting trap occupancy.
Island-to-percolation transition during growth of metal films
Authors: Jeffers G. , Dubson M. A. , Duxbury P. M.
Published: July 2002

Metal films grown on nonwetting substrates evolve from an early stage of isolated compact islands to a later stage of elongated islands and percolation. Results are presented of a scanning electron microscopy study of Pb on SiO2 showing that the critical island radius Rc at which this crossover occurs is strongly dependent on temperature and weakly dependent on deposition rate. The experimental results are semiquantitatively described by a kinetic freezing model, in which the rate of island coalescence due to surface diffusion competes with the rate of island growth due to deposition.
Quantum scattering states in open two-dimensional electronic systems
Authors: Harbury Henry K. , Porod Wolfgang
Published: July 2002

We present a method to solve the two-dimensional effective-mass Schrödinger equation for scattering states on open boundary domains. The boundary conditions are developed by partial wave expansion of the known far-field solution and by enforcing continuity of the complex wavefunction and its derivative across the domain periphery. The problem is formulated within the finite element method and the algorithm is used to solve for the scattering states of a two-dimensional electron gas with point scatterers. This technique may be used to study the local field effects within the vicinity of scattering defects in mesoscopic structures.
Optical spectra, energy levels, and emission intensity calculations of trivalent thulium ions in gadolinium scandium gallium garnet
Authors: Seltzer Michael D. , Gruber John B. , Rosenblatt Gregg H. , Morrison Clyde A. , Filer Elizabeth D.
Published: July 2002

Absorption spectra of trivalent thulium ions in gadolinium scandium gallium garnet are reported between 1.9 and 0.25 μm at 4 K. Laser-induced fluorescence was observed at 4 K from the 1D2, 1G4, and 3H4 (4 and 300 K) manifolds. Site-selective excitation experiments reveal large fractions of thulium ions in both regular D2 sites and alternate sites. A crystal-field splitting calculation was carried out in which a parameterized Hamiltonian (including Coulombic, spin-orbit, and crystal-field terms in D2 symmetry) was diagonalized for all multiplet manifolds of the Tm3+ (4f12) configuration. The rms deviation between 56 experimental and calculated Stark levels was 10 cm−1. Calculations were carried out to predict branching ratios for emission from the 3H4 manifold to the 3H5, 3F4, and 3H6 manifolds, as well as line-to-line transition probabilities for transitions between the 3H4 manifold and the 3H5 manifold.
Femtosecond time-resolved optical response of phthalocyanine Langmuir–Blodgett film
Authors: Yuan Ping , Xia Zongju , Zou Y. H. , Qiu Ling , Shen Jifeng , Shen Yuquan , Xu Huijun
Published: July 2002

The mechanism of 2,9,16,23-tetra-heptadecylamido-substituted vanadyl phthalocyanine Langmuir–Blodgett film was studied with femtosecond laser pulses. After the excitation into the first electronic excited band, an ultrafast decay due to phonon-exciton coupling greatly affected by temperature was observed. Following this process, exciton-exciton annihilation leads to the recovery of the ground state with a decay time of about 10 ps. The Frenkel exciton theory was employed to give a complete interpretation of the dynamics. An even slower relaxation was also observed.
Photocapacitance investigation of the ionized levels in n-GaAs crystals and its association with the ‘‘photoquenching phenomenon’’
Authors: Nishizawa Jun-ichi , Oyama Yutaka , Dezaki Kazushi
Published: July 2002

Results are reported of a photocapacitance (PHCAP) investigation to evaluate the ionized deep levels in n-GaAs crystals. The PHCAP measurements reveal two sorts of ionized deep levels at 0.50 and 0.74 eV above the valence band at 45 K only during the photoquenching phenomenon. The changes of ion densities are shown as a function of the course of the photoquenching phenomenon. From the change of ion density of each level, it is shown that the generation of the 0.50 eV+Ev level by photoexcitation plays a vital role in the occurrence of the photoquenching phenomenon.
The effect of contact barriers on space-charge-limited current injection in impurity-band-conduction photodetectors
Authors: Martin B. G.
Published: July 2002

A theoretical investigation was made on the effect of contact barriers on space-charge-limited (SCL) current injection in impurity-band-conduction (IBC) photodetectors. Parmenter and Ruppel [J. Appl. Phys. 30, 1548 (1959)] considered the effect of contact barriers on SCL current injection in insulators. They solved the steady-state, one-dimensional transport equations, obtaining the solution in terms of integrals, which they evaluated for certain limiting cases. We have modified their approach by using the current injection ratios at the contacts as boundary conditions. This is a very general formulation that can be used for any type of contact barrier. We then applied this technique to model contact barriers in IBC devices. The fact that the IBC hopping mobility is several orders of magnitude smaller than the electron mobility enables us to evaluate the integrals and obtain a closed form solution to the transport equations. Calculated results include current versus voltage behavior, electrostatic potential and electric field versus position for a given bias. It is concluded that the injected electron current is dominant for the store-mode applied bias.
Nondestructive determination of free-electron concentration and mobility in Hg1−xCdxTe, n-type InSb, and n-type GaAs
Authors: Clarke Frederick W.
Published: July 2002

A method for nondestructive determination of free-electron concentration and mobility using Faraday rotation and absorption at mid-infrared wavelengths is presented. Faraday rotation measurements were made at four CO2 laser wavelengths between 9 and 11 μm. The component due to free electrons was extracted using its wavelength dependence. Concentration N was determined for N≳1014 cm−3 from calibration plots exploiting Faraday rotation’s linear dependence on free electron concentration. Electron rotation was combined with absorption due to electrons to determine their mobility. These mobilities agreed reasonably well with Hall test mobilities in materials where electron absorption could be determined separately from hole absorption. In materials where hole and electron absorption cannot be determined separately, a reference mobility was calculated using an electron absorption cross section. Samples tested were Hg1−xCdxTe for 0.20≤x≤0.30, n-type and intrinsic InSb, and n-type GaAs at room temperature.
Study of hydrogenation on near-surface strained and unstrained quantum wells
Authors: Chang Ying-Lan , Tan I.-Hsing , Hu Evelyn , Merz James , Emiliani V. , Frova A.
Published: July 2002

We have studied the effects of hydrogenation on the luminescence efficiency of near-surface strained InGaAs/GaAs and unstrained GaAs/AlGaAs quantum wells (QWs). By using two different materials with an analogous structure, we have been able to clarify the effects of substrate temperature, ion dosage, strain profile in the material, and material quality on the local hydrogen concentration. This in turn modifies the behavior of hydrogen, the formation of hydrogen-related defects, and the variation of luminescence efficiency from the near-surface QW.
Static and dynamic behavior of a Si/Si0.8Ge0.2/Si heterojunction bipolar transistor using Monte Carlo simulation
Authors: Galdin Sylvie , Dollfus Philippe , Hesto Patrice
Published: July 2002

A theoretical study of a Si/Si1−xGex/Si heterojunction bipolar transistor using Monte Carlo simulations is reported. The geometry and composition of the emitter-base junction are optimized using one-dimensional simulations with a view to improving electron transport in the base. It is proposed to introduce a thin Si-P spacer layer, between the Si-N emitter and the SiGe-P base, which allows launching hot electrons into the base despite the lack of natural conduction-band discontinuity between Si and strain SiGe. The high-frequency behavior of the complete transistor is then studied using 2D modeling. A method of microwave analysis using small signal Monte Carlo simulations that consists of expanding the terminal currents in Fourier series is presented. A cutoff frequency fT of 68 GHz has been extracted. Finally, the occurrence of a parasitic electron barrier at the collector-base junction is responsible for the fT fall-off at high collector current density. This parasitic barrier is lowered through the influence of the collector potential.
Magnetic domain structures and dynamics of CoTaZr/Cr multilayered films
Authors: Yamamoto Kazuhiro , Matsuyama Hideo , Hamakawa Yoshihiro , Kitada Masahiro
Published: July 2002

The permeability, domain structure, and dynamic magnetization change of CoTaZr/Cr multilayered stripes are studied. The frequency dependence of permeability is improved by lamination with Cr layers, which eliminates the closure domain structure. An edge curling wall (ECW) is observed near the edge of the multilayered stripes. The width of the ECW agrees well with calculations and is independent of stripe width. The magnetization in the ECW is difficult to rotate by the external magnetic field; therefore, the permeability of a narrow stripe is less than that of wider stripes.
Anomalous photovoltage in Cd0.80Zn0.20Te thin films
Authors: Samanta B. , Chaudhuri A. K. , Sharma S. L. , Pal U.
Published: July 2002

The mechanism of anomalous photovoltage in Cd0.8Zn0.2Te films is explained on the basis of the formation of junctions at the grain boundaries and the surface band bending. The role of atmospheric oxygen on the surface space charge and, indirectly, on photovoltage is demonstrated. The change of polarity with ambient temperature is explained as is the combined effect of the two models.
Formation of stress reduced silicon oxide films by Ar/H2 sputtering method
Authors: Takahashi Hiroki , Nagata Hirotoshi , Kataoka Haruki , Takai Hiroshi
Published: July 2002

The compressive internal stresses of sputtered silicon oxide films are remarkably reduced by use of an Ar/H2 gas mixture instead of the conventional Ar/O2 one. The film deposition rates are also found to be larger than those using an Ar/O2 gas mixture. The effect is thought to be caused mainly by structural relaxation of Si-O-Si network via partial Si-H terminations. The oxygen deficiency in films leads to optical absorption loss at short wavelengths (<700 nm). However, these films are still suitable as mechanically stable layers of optical devices operating above 700 nm.
The effects of He plasma interactions with SiH4 in remote plasma enhanced chemical vapor deposition
Authors: Theil Jeremy A. , Powell Gary
Published: July 2002

Plasma generation and the effects of the plasma on SiH4 are examined for the remote plasma enhanced chemical vapor deposition (remote PECVD) process. The electrical characteristics of two plasma generation techniques using He were compared, one at 13.56 MHz and the other at 2.45 GHz. It has been shown that the plasma generation technique has an effect upon the power coupling to the discharge in that the 13.56 MHz technique yields a capacitively coupled discharge, while the 2.45 GHz technique yields an inductively coupled discharge. There are two distinguishable methods for generating charged species in the main portion of the reactor for 13.56 MHz discharges, electron impact ionization, and Penning ionization. What has been demonstrated is that SiH4 forms polysilanes as a direct consequence of interactions with charge created with the capacitively coupled discharge (13.56 MHz) by forming polysilanes, however Penning ionizations cannot sustain appreciable gas-phase SiH4 excitation. These polysilanes account for a significant fraction of the SiH4 in the reactor. In fact, due to the chemical similarity of these polysilanes and their concentration relative to SiH4, they should be considered in models of silicon thin-film growth in remote PECVD. Hydrogen has been shown to reduce the presence of polysilane species in the plasma for conditions that are conducive to microcrystalline Si growth, and cause silane species to exhibit a higher degree of hydrogenation.
Chemistry and electronic structure of the H2 plasma passivated surface of CdTe
Authors: Nelson Art J. , Frigo S. P. , Rosenberg R. A.
Published: July 2002

The effects of low energy H2 plasma exposure on the surface defect chemistry and the electronic structure of CdTe were studied by synchrotron radiation soft x-ray photoemission spectroscopy and optical emission spectroscopy as a function of substrate temperature. The low energy H2 plasma was generated with a commercial electron cyclotron resonance plasma source using pure H2 with the plasma exposure being performed at ambient temperature, 100 °C, and 200 °C. Plasma species were identified with optical emission spectroscopy. In situ photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence band electronic structure as well as changes in the Cd 4d and Te 4d core lines. The results were correlated in order to relate the plasma species and characteristics to changes in surface defect chemistry and electronic structure. These measurements indicate that the H2 plasma exposure type converts the CdTe(100) surface from p- to n-type and passivates defect states.
Deep level defects in proton irradiated Zn and Cd doped InP
Authors: Rybicki George C. , Williams Wendell S.
Published: July 2002

InP is a relatively radiation-resistant semiconductor and an attractive material for solar cells exposed to radiation in Earth orbit. To better understand this useful property of InP, proton irradiation induced defects in Zn and Cd doped InP have been studied by deep level transient spectroscopy. After 2 MeV proton irradiation the defects H3, H4, and H5 were observed in lightly Zn doped InP, while the defects H3 and H5 were observed in more heavily Zn and Cd doped InP. The concentrations of the irradiation induced defects were measured and the introduction rates were calculated and compared. The activation energies of the defects were measured and corrected for the effect of electric field on carrier emission. The capture cross sections of the defects were measured directly by the pulse width variation technique. The energy levels and capture cross sections of the defects were not affected by the substitution of Cd for Zn, but the introduction rate of the defect H5 was substantially lower in Cd doped InP, suggesting that this material would make an improved solar cell in a high radiation environment.
Surface analysis of floatglass by means of x-ray absorption, reflection, and fluorescence analysis
Authors: Hüppauff Martin , Lengeler Bruno
Published: July 2002

The surface structure of the top and bottom sides of floatglass are investigated by x-ray absorption, reflection, and fluorescence at grazing incidence. The data analysis includes the determination of the thickness, density, and interface roughness of surface layers and the depth profiling of different atomic species. Expressions for the reflectivity and the fluorescence from a substrate and from layers on a substrate are given and discussed in detail, with special emphasis on the influence of interface roughness on these quantities. It turns out that iron and tin are enriched on the bottom side of the floatglass. In addition, iron changes its valence with depth. Leaching the glass for 48 days in NaOH has a strong influence on the iron concentration, on the surface density, and on the surface roughness of the glass.
Influence of longitudinal bias field on magnetization distribution in magnetoresistive head with shield films
Authors: Ishikawa Chiaki , Suzuki Kaori , Yoshida Kazuetsu , Sugita Yutaka , Shinagawa Kiminari , Nakatani Yoshinobu , Hayashi Nobuo
Published: July 2002

The magnetization distribution in the magnetoresistive (MR) film has been calculated by self-consistently solving the three-dimensional field of the MR head. The magnetization distribution was calculated based on the Landau–Lifshitz–Gilbert equation and the head field was obtained by the Maxwell equation. The longitudinal bias field for the domain control was generated by exchange-coupled antiferromagnetic or permanent magnetic films which were formed outside the sensing region of the MR film. The resistance change of the MR film was calculated from the magnetization distribution with shield films and without shield films. It was found that the resistance change with the antiferromagnetic film without the shields was about two times larger than that with the permanent magnetic film with the remanence Br of 0.7 T. The difference between them was reduced when the shields were formed because the stray field from the permanent magnetic film which is applied to the MR film was decreased by the shields. Further, the effect of the longitudinal bias field on the magnetization distribution was analyzed for the MR film without the shields. The influence of the antiferromagnetic film reached the region of 2–3 μm from the end of the film, while that of the permanent magnetic film reached more than 10 μm.
Enhanced field control of second-harmonic generation in compositionally asymmetrical quantum wells
Authors: Davé Digant P.
Published: July 2002

The results presented here show that field control of second-harmonic generation (SHG) can be substantially enhanced in compositionally asymmetrical wells. In an optimized step well the magnitude of second order susceptibility for moderate field (±80 kV/cm) is 2 orders of magnitude larger than that in the square well with the same applied field. Numerical results are presented for SHG at a CO2 wavelength for a square, graded, and step well.