bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties of defects in low-dimensional transition-metal complexes
Authors: Ramakrishna B. L.
Published: July 2002

The occurrence of chemical or structural defects in low-dimensional materials is quite a general phenomenon and can have drastic consequences on magnetic and transport properties. In a 1D magnetic chain of S= 1/2 spins which are coupled by antiferromagnetic interaction, a single-vacancy, misoriented chain member or impurity substitution cuts the chain into two segments by decoupling the spins on either side of the ‘‘defect.’’ The finite chains with odd-length segments behave essentially like Curie-type magnets and lead to electron paramagnetic resonance (EPR) spectra and divergencies in magnetic susceptibility at low temperatures. In the series of compounds we have chosen to study, viz., R+ Ni(mnt)−2, the d7 S= 1/2 platelike anions stack like poker chips along a chain and have strong antiferromagnetic interactions which can be varied easily by choosing different R+ cations. These are very good examples of 1D systems and have the great advantage of possessing a tunable interaction. We present here the static and dynamic magnetic properties investigated by dc magnetic susceptibility, EPR, and spin lattice relaxation time measurements. We have established that the so-called ‘‘impurity’’ is a structural defect. From a detailed study of the defect we have been able to learn about the various cooperative magnetic interactions such as the strong interaction in the chains and the weak interaction between the chain and the defect.
Stoner excitations in bcc Co (invited)
Authors: Idzerda Y. U. , Lind D. M. , Papaconstantopoulos D. A. , Prinz G. A. , Jonker B. T. , Krebs J. J.
Published: July 2002

Multiple structures in the spin-polarized electron energy loss spectra for thin films of bcc Co epitaxially grown on GaAs(110) are observed. Analyzing the specular beam (near-zero momentum transfer) for an incident polarized electron beam, three sharp loss features are observed at 1.6, 2.9, and 4.4 eV loss energy. These loss features are shown to be due only to incident minority-spin electrons. This is the first observation of multiple structures in the Stoner (spin-flip) loss spectrum. A theoretical Stoner density of states (DOS) and spin-nonflip transition DOS derived from band-structure calculations for bcc Co show excellent agreement with the measured energy positions and peak widths of the loss features. Further comparison between the two calculated transition DOS and the experimentally measured spectra allows for identification of the loss mechanisms associated with the three loss structures as spin-flip (Stoner) or spin-nonflip.
Soft heating for hyperthermia using temperature-sensitive magnetic powders and fine metal rings (abstract)
Authors: Matsuki H. , Ando H. , Yamaguchi M. , Watanabe T. , Murakami K.
Published: July 2002

In local hyperthermia it is important to leave the temperature of surrounding normal tissues essentially unchanged while increasing the temperature of tumors to a desired level. Soft heating1 with temperature-sensitive magnetic materials is advantageous in heating deep-seated tumors because of the self-limiting ability. In order for the method to become available for large-scale tumors we have proposed to use temperature-sensitive magnetic powders, in which fine metal rings are dispersed. As the magnetic flux distribution varies with the temperature of the powders, the fine rings immersed in the powders with low temperatures generate heat more than rings located in the powders with high temperatures. Furthermore, the transition from the ferromagnetic to the nonferromagnetic state becomes sharp effectively. As a result it is expected to make the temperatures of powders in a broad region uniform. Figure 1 shows the temperature dependence of the magnetic powders used in experiments. Figure 2 shows the relation between the attained temperature of the powders and the ring resistances, comparing them in the case of iron powder. The figure reveals that it exists appropriate combination between the ring resistance, and the magnetic permeability of the powders.
Magnetic properties of the spinel phase for FexCu1−xRh2Se4
Authors: Kim Chul Sung , Childress J. , Chien C. L.
Published: July 2002

FexCu1−xRh2Se4 is found to crystallize with a pure spinel structure in the composition range 0≤x≤0.3. Mössbauer spectra of FexCu1−xRh2Se4 have been taken at various temperatures ranging from 4.2 K to room temperature. Analysis of Mössbauer spectra at 4.2 K indicate that the effective hyperfine field increases with increasing iron concentration x and that the quadrupole splitting is negligible. The absence of quadrupole splitting above the magnetic ordering temperature TN indicates that iron ions occupy only the tetrahedral sites. The isomer shifts indicate that the charge states of the Fe ions have a ferric character. Magnetic susceptibility measurements by dc SQUID magnetometry show that long-range superexchange interactions on the tetrahedral sites are antiferromagnetic.
Off-site Cooper pairing in strong electron-phonon interacting systems
Authors: Lin D. L. , Zheng Hang
Published: July 2002

We have developed a new variational treatment to deal with the superconductivity properties of systems with strong electron-phonon interactions by introducing a ‘‘squeeze’’ transformation for phonon operators. It is shown that as long as the renormalized intersite electron correlation is attractive, a new type of Cooper pairing occurs whether the renormalized on-site electron correlation is repulsive or atttractive. The transformed electron-phonon interaction gives rise to the squeezed polarons which are, in general, different from the ordinary polarons. We have found from this theory that: (1) the squeezed polaronic state corresponds to a stable minimum of the total energy; (2) the effective mass of the squeezed polaron is much smaller than that of ordinary polarons; (3) both the gap equation and the expression for the critical temperature Tc in squeezed polaronic states are different from those in ordinary polaronic states as well as from those in BCS theory; and (4) high Tc is obtained for simple cubic and body centered cubic lattice structures with the electron concentration n=1.
The effect of particle interactions on Curie–Weiss behavior in ferrofluids
Authors: Popplewell J. , Abu Aisheh B. , Ayoub N. Y.
Published: July 2002

The initial susceptibility of a number of Fe3 O4 particle ferrofluids with volume concentrations, ε=0.08, 0.07, 0.05, 0.03, 0.02, and 0.01 (prepared by diluting a stock fluid of Ms =40 kA m−1) has been measured in the temperature range 220–450 K. The mean particle sizes Dm (magnetic) and Dp (physical) are 74 and 81 Å, respectively. The ferrofluids exhibit Curie–Weiss-type behavior with negative (Néel) temperature temperature intercepts To . The form of the Curie–Weiss behavior is interpreted as arising from particle interactions which are strong enough to reduce the initial susceptibility below the noninteracting value in the temperature range of measurements. It is suggested that interactions lead to the formation of closed particle structures of zero moment in zero field and, as would be the case for an antiferromagnetic material, leads to Curie–Weiss behavior with a negative ‘‘Néel’’ temperature. Negative values of To are thus characteristic of an interacting particle system.
Preparation and properties obtained by borohydride reduction process magnetic powders (abstract)
Authors: Dragieva I. D. , Slavcheva M. S. , Buchkov D. T. , Mechandjiev D. R.
Published: July 2002

An overview on the variation of the magnetic properties, the grain size, the structure, and the heat of crystallization in the amorphous-crystalline state transition of single-domain metal particles depending on the element metals, boron, and the hydrogen contents is presented. The existence of the two types of chains of spheres, constituted from similar single-domain particles, prepared with and without dc magnetic field, is discussed. The differences between these two kinds of chains are explained by measurements of the sizes and shapes of the voids, the magnetic properties, the heat of crystallization, the rate of a gaseous nitrogenation, the press densities, and Mössbauer spectra of the samples. The essentially different states of the one single-domain particle and the complex of single-domain particles in the chains are discussed according to the application of the powders.
Magnetic relaxation and critical current in an YBaCuO crystal
Authors: Yeshurun Y. , Malozemoff A. P. , Holtzberg F.
Published: July 2002

We have measured the temperature dependence of magnetic relaxation and critical current in an YBaCuO single crystal. Unusually strong time-logarithmic magnetic relaxation is observed. The relaxation rate increases initially with temperature, peaks at ≂25 K and slows down at higher temperatures. The results, interpreted with a thermally activated flux model, yield a low-temperature pinning energy U0≂0.02 eV. This low pinning energy results in an unusually rapid decrease of the critical current as temperature is increased.
Magnetic properties and ferromagnetic resonance of Li-Mg-Ti microwave ferrite systems (abstract)
Authors: Wang Huizong , Li Guodong
Published: July 2002

The magnetic properties and ferromagnetic resonance of the Li-Mg-Ti microwave ferrite system were investigated in this paper.1,2 The chemical compositions of the samples studied were as follows: Li0.44Me0.12(MgTi)xFe2.44−2xO4 where Me represent Ni, Mn, Zn, etc., ions. It was verified by x-ray diffraction spectra that all specimens were of the spinel structure. The lattice constants were linearly increased with the substitution amount of Mg and Ti ions. The saturation magnetizations were measured as a function of composition and temperature. The results of experiments indicate that the saturation magnetizations and Curie temperatures were decreased with the increase of the substituted amount of Mg and Ti ions. The magnetic properties of this ferrite system were as follows: 4πMS≊300–3000 G, Hc≊0.8–1.5 Oe, Br/Bm≥0.8, and ΔH≊66–350 Oe. In the range of 3.6–16.5 GHz, except the composition of x=0, the room-temperature ferromagnetic resonance linewidths ΔH were linearly decreased with lowering frequency, up to 6 GHz. It can be explained by the phenomenological theory of ferromagnetic resonance. Below 4 GHz the resonance linewidths rose steeply. It was caused by the unsaturation effect of the spherical specimen. The resonance linewidths (x≥0.20) and the intensities of the internal magnetic fields (x≥0) were decreased with the increasing Mg and Ti ions, while the g factors were slightly increased. On the basis of the theory of ferrimagnetism and ferromagnetic resonance, the experimental results were analyzed and discussed.
Magnetic properties and synthesis of high i
H
c Sm-Ti-Fe
Authors: Kamprath N. , Liu N. C. , Hegde H. , Cadieu F. J.
Published: July 2002

The room-temperature intrinsic coercive force, iHc, of the hard Sm-Ti-Fe phase observed in film samples has been measured as a function of composition. The maximum iHc observed at room temperature was 38.5 kOe for a composition Sm19Fe73Ti8. Such samples were first made amorphous and then subsequently crystallized with an in-plane magnetic field, Hs, applied. Tentative tetragonal lattice parameters have been assigned to this phase based on a fitting of 30 x-ray lines between 30° and 80° 2θ for Cu radiation. The fitted tetragonal parameters are a=8.403 Å and c=12.401 Å. This high iHc Sm-Ti-Fe phase has not been observed in films that were directly crystallized onto heated substrates. The ferromagnetic Fe-rich phases in Sm-Ti-Fe samples have been synthesized by rf sputtering films onto heated as well as room-temperature substrates. The samples synthesized onto heated substrates were crystallized directly upon deposition, while the samples deposited onto room-temperature substrates were amorphous and were then subsequently crystallized. An in-the-film-plane magnetic field, Hs, was applied during the deposition as well as during the subsequent crystallization of amorphous film.
Temperature dependence of coercivity and permeability of pearlitic rail steels (abstract)
Authors: Bussière J. F. , Lord M. , Fiset M.
Published: July 2002

In an effort to relate microstructure, mechanical properties, and magnetic properties as a tool for nondestructive testing, the coercivity and permeability of fully pearlitic rail steels having different compositions, microstructures, and hardnesses were measured between −200 and 600 °C. A pronounced peak in coercivity, Hc, is generally observed at the Curie temperature of the cementite phase, Tc(Fe3C), ≊200 °C for plain carbon rail steels. This peak shifts rapidly to lower temperatures with small additions of chromium and with heat treatment (annealing). This is interpreted as being due to a lowering of Tc(Fe3C) which results even for small Cr concentrations (≊1%) because of preferential partitioning of carbide-forming elements such as Mn and Cr to the carbide phase. As a result, coercivity at room temperature is found to be strongly dependent on the presence of chromium additions and on thermal history and relates poorly to microstructure and hardness. Above Tc(Fe3C), however, Hc is found to vary more consistently with pearlite morphology and with mechanical properties such as hardness.
Effect of sputtering parameters on the magnetic properties of Mo-Permalloy
Authors: Zhang Z. S. , Yang L. , Yang C. S. , Cai B. C.
Published: July 2002

Sputtered Mo-Permalloy thin film is expected to possess great potentiality in soft magnetic properties which can be realized chiefly by controlling sputtering conditions. The effects of sputtering parameters, such as argon pressure, bias voltage, and preheating temperature, on the magnetic properties, of FeNiMo sputtered films have been investigated. The magnetic anisotropy field Hk was found to decrease with the raising preheating temperature while coercivity Hc presented the contrary trend. High effective permeability can be acquired at low argon pressure, adequate preheating temperature, and bias voltage. Close relationships among effective permeability, anisotropy field, magnetostriction coefficient, and half-height width Δθ50 were found. Annealing is available for increasing effective permeability.
Is there a microscopic foundation for the theory of superconductivity in heavy fermion systems? (abstract)
Authors: Grewe N.
Published: July 2002

An attempt is made of collecting reliable information from microscopic models, which may be relevant to the problem of heavy fermion superconductivity. For this purpose direct perturbation theory with respect to hybridization between band states and local states is used to cope with the influence of strong local correlations in lattice periodic systems.1,2 Results taken into account concern the one-particle excitation spectra and the collective behavior, resulting from quasiparticle interactions, of such lattice systems. We discuss the prospects of electronic and phononic mechanisms for superconductivity and the possible resulting symmetry of the order parameter. The particular difficulties found in a microscopic treatment of superconductivity in heavy Fermion systems are pointed out and compared to standard BCS theory.
Reentrant ferromagnetism in very dilute P
dFe
Authors: Kornik K. , Kunkel H. , Roshko R. M.
Published: July 2002

The static magnetization of a very dilute PdFe alloy containing 700-ppm Fe has been measured as a function of temperature, between 0.05 and 1.0 K, and in applied magnetic fields between 0 and 10 Oe. Comparison of the experimental data with numerical calculations based on a mean-field effective-field model for bond-disordered systems with arbitrary spin suggests that the alloy is in the reentrant regime of the magnetic phase diagram. Measurement of the time dependence of the thermoremanent magnetization is consistent with a logarithmic, and possibly a stretched exponential, decay.
Properties of amorphous FeCoB alloy particles (abstract)
Authors: Charles S. W. , Wells S. , Meagher A. , Mo/rup S. , van Wonterghem J.
Published: July 2002

Amorphous and crystalline alloy particles (0.05–0.5 nm) of FexCoyBz in which the ratio x:y ranges from 0 to 1 have been prepared by the borohydride reduction of iron and cobalt salts in aqueous solution. The structure of the particles has been studied using Mössbauer spectroscopy and x-ray diffraction. Magnetic measurements of the saturation magnetization, coercivity, and remanence of the particles have been measured. The transition from the amorphous-to-crystalline state has been studied using differential scanning calorimetry (DSC) and thermomagnetometry up to a temperature of 450 °C (see Fig. 1). It has been shown that the fraction of boron in the alloys (10–35 at. %) is dependent upon the rate of addition of salts to borohydride and the concentration of cobalt present; this in turn influences the crystallinity and magnetic properties .
Dynamical susceptibility of Ho2Fe14B single crystal: Spin rotation and domain wall motions
Authors: Rillo C. , Chaboy J. , Navarro R. , Bartolomé J. , Fruchart D. , Chenevier B. , Yaouanc A. , Sagawa M. , Hirosawa S.
Published: July 2002

The complex ac magnetic initial susceptibility (χ=χ′−jχ″) has been measured along the [100], [110], and [001] directions from 4.2 to 300 K. Along the two basal directions the spin-reorientation phase transition (SRT) is detected at TS =57.8 K as an abrupt steplike anomalous decrease in χ′ for increasing temperature. The value of the experimental ratio χ′[110]/ χ[100] ≂2 holds for T≥TS. The reversible magnetic moment rotation set on by the exciting ac magnetic field explains these results, allowing a fit from which the stress constant σ=42±1 kg/mm2 is derived. Along the [001] direction no anomaly in χ′(T) is detected at TS while a strong continuous increase is manifest above 150 K. It is explained by the onset of narrow wall motions between 180° domains induced by thermal excitations.
Composition and temperature dependence of coercivity of Nd-Fe-B alloys crystallized from the amorphous state (abstract)
Authors: Tu Guo-hua , Chen Wan-rong , Muir W. B. , Altounian Z.
Published: July 2002

As a first step to study the role of second phases in achieving excellent hard magnetic properties in Nd-Fe-B permanent magnets we have prepared Nd-Fe-B alloys, near the Nd2Fe14B composition, with a maximum variation of 10 at. % Nd and 5 at. % B. The alloys were first quenched into an amorphous state by melt spinning, as previous studies on crystallization kinetics suggest better control on the microstructure and hence better hard magnetic properties of the recrystallized alloys. For all compositions with Nd concentrations 1 at. % greater than that of Nd2Fe14B the intrinsic coercivity Hci is above 1.2 T at 100 °C. For a constant Nd/B ration, Hci increases monotonously with Nd constant while for compositions where even small amounts of α. Fe form as a crystallization product Hci shows a sharp drop. On the other hand, it appears that B content is much less significant in determining Hci. The temperature dependence of Hci is almost linear for all compositions between 20 and 200 °C and is much better than that for sintered materials. Hci at room temperature of our samples is about 20% higher than that of commercial rapidly quenched materials with similar composition.
Exchange anisotropy in amorphous Gd-Co films
Authors: Toxen A. M. , Geballe T. H. , White R. M.
Published: July 2002

It was previously reported that multilayer samples of Co/GdCo2 annealed at 500 °C exhibited exchange anisotropy. One observes a B-H hysteresis loop centered at a finite field, the ‘‘exchange’’ field. To identify the ‘‘hard’’ magnetic phase(s) responsible for the shifted loops we have studied thin films of GdCox as well as multilayers of Co/GdCox, where x varies from 2 to 8.5. We saw shifted loops in two distinct groups of samples, neither of which is multilayered. The first group was GdCox in the composition range between GdCo3 and GdCo5. The loop displacement at room temperature is about 5 Oe. The coercivities of these samples are about 10 Oe. The second group of samples had a composition close to GdCo2. For these samples most of the B-H loop is centered at zero field with a coercivity of 1–2 Oe. However, at ±80 Oe there are small loops which account for about 20% of the total magnetization. X-ray measurements indicate that both groups of samples are microcrystalline or amorphous. The second set of samples discussed above exhibit a maximum in diffracted intensity at 2θ=36°, where GdCo2 has a strong (220) peak. The first group of samples have a broad maximum at 38°, where neither GdCo3 nor GdCo5 has peaks. Gd2Co7 has a very weak peak at this angle and Gd2Co17 has strong peaks but would have to be metastable.
Theory of electronic density of states of a two-dimensional disordered system in the presence of a strong magnetic field (invited)
Authors: Das Sarma S. , Xie X. C.
Published: July 2002

We calculate the density of states (DOS) of a two-dimensional electron gas in the presence of an external magnetic field. We take into account the self-consistent screening of the impurity scattering and the Landau level coupling effects. Using realistic parameters, we find qualitative agreement with the existing experiments. Our calculated DOS is broad and fairly featureless with considerable overlap between the Landau levels and is quite different from the well-separated Landau levels derived from the short-range interaction model. This implies that the calculated level broadenings are much larger than that obtained from the short-range model.
Ballistic propagation of large wave vector magnons in magnetically ordered materials (invited)
Authors: Yen W. M. , Rotter L. D. , Grill W.
Published: July 2002

We present evidence of ballistic transport of energy by high-energy, large wave vector magnons in the antiferromagnet MnF2. Nonequilibrium magnons are created by heat pulses or by selective laser excitation throughout the Brillouin zone, using the magnon sideband or far-infrared (FIR) two-magnon absorptions. The ballistic magnon signal is extracted from the total observed signal and used to calculate the magnon velocity as a function of magnon energy. The results compare favorably with theory. Implications of our results to various types of magnetically ordered lattices are discussed.
Magnetoelastic waves during thermal cycles from room to Curie temperature in Metglas (abstract)
Authors: Lanotte L. , Luponio C. , Porreca F.
Published: July 2002

The study of the magnetoelastic waves as a function of temperature appears as a useful method to be able to investigate both the magnetization critical point (particularly in amorphous metals) and structural relaxation effects. Our experimental apparatus1,2 used to produce and pick up standing magnetoelastic waves in ferromagnetic samples was modified by building an anti-inductive furnace and a water cooling system. This allows us to perform measurements ‘‘on line’’ on the wave amplitude A versus temperature in the range ∼30–400 °C. By slowly increasing the temperature, one observes that the amplitude A falls to zero when the Curie point Tc is reached. Only if the samples were properly preannealed, during subsequent heating or cooling, the same A values were measured at any fixed temperature. When the annealing is unaccomplished, the thermal cycles revealed in the ‘‘as-produced’’ samples can be correlated with the structural relaxation. In 2826 Metglas, magnetic anisotropy abatement, large increase of the wave amplitude, and sensitive increment to Tc agree with each other and represent, in this study, the most important consequence of the structural relaxation.
(110)Fe/(111)Ag superlattices by MBE (abstract)
Authors: Gutierrez C. J. , Mayer S. H. , Walker J. C.
Published: July 2002

We have successfully produced a variety of high-quality (110)Fe/(111)Ag superlattices using a Perkin-Elmer Model 430B molecular-beam epitaxy system (MBE) with Fe structures as thin as 2 atomic layers and Ag structures as small as 7 layers. The growth of the superlattices was continually monitored by reflection high-energy electron diffraction (RHEED). The magnetic properties determined by Mössbauer spectroscopy show a strong dependence on the respective thicknesses of the Fe and Ag repeat structures which makes up the superlattice.
Correlation of aluminum content, inclusion structure, and core loss of nonoriented electrical steels
Authors: Bóc I. , Csébi J. , Gróuf T. , Cziráki A. , Nádor B.
Published: July 2002

The addition of 0.3%–0.6% aluminum is widely used to reduce the core loss of nonoriented silicon steels. The core loss reduction due to Al was found much more intensive than expected. It was suggested that the effect of Al can be mainly connected to the change of inclusion structure. In this work silicon steel samples containing different amounts of Al (0.008–0.49 wt. %) were investigated by scanning and transmission electron microscopy, and the inclusion structures were analyzed.
Microstructure and magnetostriction in rare-earth-iron alloys
Authors: Westwood P. , Abell J. S. , Clarke I. H. , Pitman K. C.
Published: July 2002

The microstructure of the magnetostrictive alloy terfenol, Tb0.27Dy0.73Fex, has been studied in the range 1.55≤x≤2.30. Image analysis has confirmed and quantified observations of the reabsorption of the rare-earth phase into the matrix after annealing, implying the existence of a solid solubility range. Similar measurements have demonstrated the growth of the primary RFe3 phase for all alloys with x>2.00. Eutectic and peritectic temperatures of 892 and 1239 °C, respectively, have been determined by differential thermal analysis (DTA) for this composition range. The homogeneous, elongated grain structure induced by vertical float zoning increases k33 and allows the development of a uniform static strain throughout the rod.
Raman spectroscopy of amorphous Ge/SiOx superlattices
Authors: Williams G. V. M. , Bittar A. , Trodahl H. J.
Published: July 2002

Raman spectroscopic studies have been performed on a series of vacuum evaporated amorphous Ge/SiOx superlattice films with germanium layer thicknesses between 8 and 200 Å and silicon oxide layer thicknesses between 12 and 300 Å. The results show the presence of a Gey (SiOx )1−y interface alloy of thickness (0.7±0.2) nm and average composition of y∼0.35. This alloy region is identified as the residence of the observed germanium bond angle disorder, leaving the pure germanium layers in the superlattices essentially undistorted from the bulk a-Ge structure.
Oxidation process of indium-alloyed TbFe amorphous films
Authors: Iijima Tetsuo
Published: July 2002

Oxygen diffusion behavior in indium-alloyed TbFe films is studied under several aging conditions. The oxygen-suppression mechanism is discussed. The oxygen-diffusion coefficients of TbFeIn films after aging are 1.5×10−22 m2/s at 80 °C and 2×10−24 m2/s at room temperature, both at 85% relative humidity. These coefficients are one-tenth those previously reported for aged GdTbFe and TbFeCo films. The activation energy of TbFeIn films is 1.5 times larger than that of GdTbFe films. A diffusion model based on In characteristics and involving vacancy sites in the films is presented.
Epitaxial growth of silicon at low temperature by ultrahigh vacuum electron cyclotron resonance plasma chemical vapor deposition
Authors: Nagai I. , Takahagi T. , Ishitani A. , Kuroda H. , Yoshikawa M.
Published: July 2002

Epitaxial growth of a silicon thin film has been accomplished at a temperature lower than 650 °C by electron cyclotron resonance plasma chemical vapor deposition using an ultrahigh vacuum chamber. Ar plasma from the same electron cyclotron plasma ion source is also used for surface cleaning of the substrate. The relationship between the cleaning condition of a substrate and the quality of an epitaxially grown silicon film is examined in detail. The energy of Ar plasma in the cleaning process should be kept lower than 100 eV in order to obtain films of good crystal quality. Otherwise, damage by Ar ion bombardment cannot be restored by annealing at lower than 700 °C. The epitaxial growth also should be carried out without the accelerating bias voltage to avoid residual compressive stress in the film. An epitaxial film with perfect crystal structure (indicating only a Kikuchi pattern of reflection of high-energy electron diffraction) is obtained at the substrate temperature of 710 °C.
Loop transducer matching for low insertion loss magnetostatic surface wave resonator
Authors: De D. K.
Published: July 2002

In this paper we describe the matching of a loop transducer to significantly reduce the insertion loss and to improve the off-resonance rejection of a magnetostatic surface wave resonator.
A real time Monte Carlo simulation of thin film nucleation in localized-laser chemical vapor deposition
Authors: Kotecki David E. , Herman Irving P.
Published: July 2002

A real time Monte Carlo simulation is used to model the nucleation and initial stages of thin film growth during localized-laser chemical vapor deposition (LLCVD). This model includes the effects of laser–substrate heating, heterogeneous pyrolytic decomposition of parent molecules on the laser-heated region of the surface, and adatom migration and desorption dynamics. The amount of material deposited as a function of time is obtained over a surface area of 150×150 Å for various values of the substrate temperature, parent gas pressure, and adsorbate–substrate binding energy. Additional information is obtained about the cluster density, and the role of surface defects and two-atom cluster dynamics on the initial growth rate. The deposition of silicon by heterogeneous pyrolytic decomposition of silane (SiH4) is used as a base case for the simulation. Predictions of the initial thin film morphology and its temporal evolution during static laser heating of micron-dimensional regions of the surface are presented. Simulation results indicate that for a given silane pressure and adsorbate–substrate binding energy, there is a critical temperature Tc such that for laser-induced peak temperatures TpTc, nucleation occurs initially in an annulus region centered with respect to the incident laser irradiation. The inclusion of two-atom cluster dynamics in the simulation is shown to increase the value of Tc and alter the initial morphology for low adsorbate–substrate binding energies. The simulation results are extended to scanning LLCVD to predict the maximum scan speed at which nucleation will occur.
Temperature dependence of atomic transport in ion mixing
Authors: Hung L. S. , Xia W. , Poker D. B. , Fernandes M. , Tao K. , Lau S. S. , Mayer J. W.
Published: July 2002

The moving species during ion beam mixing of Si/Ni and Si/Pt bilayers were investigated at temperatures between liquid-nitrogen temperature (LN2) and 180 °C using imbedded markers and Rutherford backscattering. For Si/Ni samples irradiated with Ar ions, the flux ratio of Si to Ni decreased from 1.6 to 0.2 as the substrate temperature increased from LN2 to 180 °C. Over this range of substrate temperatures, the individual amount of Si atoms transported was found to remain unchanged; whereas the transport flux of Ni atoms was observed to increase. Similar temperature dependence of the flux ratio was found for the Si/Pt system. The experimental results indicate that the substantial Si motion is due to the temperature-independent part of ion mixing which is associated with collision cascades. The Ni motion is characteristic of radiation-enhanced diffusion which is substrate temperature dependent.
Polarization profiles of polyvinylidene fluoride films polarized by a focused electron beam
Authors: Schilling D. , Dransfeld K. , Bihler E. , Holdik K. , Eisenmenger W.
Published: July 2002

The depth profiles of the polarization in films of polyvinylidene fluoride (PVDF) as well as in vinylidene-fluoride–trifluoroethylene (VDF-TrFE) copolymer films polarized by a focused electron beam were investigated using the piezoelectrically generated pressure step method. The dominant polarization exhibits a broad maximum inside the film. The position of this maximum depends not only on the energy of the incident electrons but also on the material parameters of the sample. Close to the surface exposed to the electron beam we have in addition observed a small secondary maximum of opposite polarization (amounting to about 1 mC/m2). A qualitative model is presented for the poling of films of PVDF and its copolymers with TrFE by focused electron beam accounting for most of the observed features. The application of electron beams for the poling of ferroelectric films allows the production of piezoelectric bimorphs. By using a well-focused electron beam also ferroelectric domains of very small lateral dimensions can be created which could become important for ferroelectric data storage.
Measurement of nonlinear refractive index coefficients of NH3 gas for transversely excited atmospheric CO2 laser lines
Authors: Oh Cha Hwan , Lee Sang Soo
Published: July 2002

The positive and negative nonlinear refractive index coefficients of NH3 gas are determined for the nine transversely excited atmospheric CO2 laser lines by observing the far-field spatial distortions of the output Gaussian beam passed through the nonlinear cell. The expression of the distorted field, which is a series expansion of Gaussian beams and derived by D. Weaire et al. [Opt. Lett. 4, 331 (1979)], is used. By taking the first two terms of the series, the experimental results are computer analyzed to obtain the nonlinear refractive index coefficient. Among the nine lines observed, one line 9R(16) has shown self-defocusing and eight lines the self-focusing effect.
Stimulated emission in indirect gap AlxGa1−xAs
Authors: Kalt H. , Smirl Arthur L. , Boggess Thomas F.
Published: July 2002

Stimulated emissions from both direct and indirect valleys are observed simultaneously in the picosecond time-resolved photoluminescence spectra of the indirect gap semiconductor AlxGa1−xAs, for samples with Al concentrations near (but slightly above) the direct-to-indirect band-gap crossover value. Indirect stimulated emission is possible in this material because the long-wave-vector scattering is enhanced by the near degeneracy of the direct and indirect conduction-band valleys and by alloy scattering. Two distinct bands are observed in the luminescence spectra. One of these we associate with the stimulated indirect recombination of electrons from states in the X valley that are below the Γ band gap and are, therefore, not resonantly coupled to states in the direct valley. The other we associate with the recombination of electrons from the central Γ valley or of electrons from the indirect X valley that are energetically resonant with states in the direct valley. The dramatically different temporal dynamics of these two stimulated processes are determined by the generation, relaxation, accumulation, and recombination of the electron-hole plasma.
Coevaporation and ion implantation of Pd5Ti5 and Pt5Ti5 on AISI stainless steels for reducing wear and friction
Authors: Zheng L. R. , Hung L. S. , Mayer J. W.
Published: July 2002

Pd50Ti50 and Pt50Ti50 alloy films were deposited on stainless-steel substrates and irradiated with ion beams. The samples were tested on pin-on-disk wear equipment and analyzed by Rutherford backscattering spectrometry and scanning and transmission electron microscopy. Adding Pd50Ti50 and Pt50Ti50 as a surface layer to stainless steel resulted in reduced wear and friction. Xe implantation slightly improved tribomechanical properties, while N implantation led to a dramatic reduction in wear and friction. The improvement was thought to be associated with nitride precipitation hardening and/or forming lubricious oxides during sliding. The as-deposited films exhibited amorphous structures and transformed to equilibrium compounds upon annealing. Sliding tests on amorphous and crystalline films did not show a significant difference in friction and wear behavior.
Methods for reducing stress birefringence in cadmium telluride electro-optic modulators
Authors: Herrit Gary L. , Reedy Herman E.
Published: July 2002

By studying the static birefringence pattern of cadmium telluride electro-optic modulator crystals after fabrication, it has been determined that standard fabrication processes induce excessive stress birefringence. This stress birefringence is produced during the grinding stage of fabrication. Two methods for removing this birefringence have been explored: (1) mechanical polishing of the sides of the crystal to an optical finish, and (2) chemical etching of the sides of the crystal with a 5% bromine solution. Both methods reduced the static birefringence of the crystal. The stress birefringence pattern can be recreated by grinding the sides of the crystal with 9 μm polishing grit.
Phase formation and identification of an epitaxial Fe-Ni alloy silicide
Authors: Chang Y. S. , Hsieh I. J. , Chen Haydn
Published: July 2002

Epitaxial iron-nickel alloy silicide has been successfully grown on (111) Si for the first time using a new method of chemically electroless plating of Fe-Ni thin films on Si with subsequent annealing schemes. The deposited film was found to be amorphous by analytical scanning transmission electron microscopy. The alloy silicide phase formed during two-step vacuum annealing and was found to have an epitaxial relationship with the substrate Si. The composition of the eptaxial alloy silicide was identified by an energy dispersive analysis for x rays in the electron microscope. A small amount of nickel was found in the alloy silicide. The crystal structure of the silicide phase was analyzed to be that of an orthorhombic phase of FeSi2. The orientation relationships of the silicide phase with respect to the substrate were identified to be (033̄) (Fe,Ni)Si2∥(022̄) Si, (055) (Fe,Ni)Si2∥(4̄22) Si, and [100] (Fe,Ni)Si2∥[111] Si. Typical arrays of regular interfacial dislocations of edge type, 1000 Å in spacing, were found throughout the epitaxial regions. The formation and some potential applications of the epitaxial Fe-Ni alloy silicide are discussed.
Electrical conduction mechanism and breakdown property in sputter-deposited silicon dioxide films on polycrystalline silicon
Authors: Suyama S. , Okamoto A. , Serikawa T.
Published: July 2002

Silicon dioxide films are sputter deposited on polycrystalline silicon (polysilicon) in an oxygen-argon atmosphere at 200 °C. Electrical conduction and breakdown are measured at various gate fields and temperatures. In addition to the Fowler–Nordheim tunneling current reported for thermal oxide, the sputter-deposited films have another, newly identified high-activation-energy current as electrical conduction mechanisms. The high-activation-energy current contribution to the total current is considerably reduced by increasing the oxygen mixing ratio and/or by decreasing the sputtering gas pressure, resulting in a considerable total current reduction. High-activation-energy current contributes little to the triggering of breakdown, unlike Fowler–Nordheim tunneling injections. In contrast to polysilicon thermal oxide, sputter-deposited films on polysilicon have no observable field enhancement in Fowler–Nordheim tunneling current, which indicates smooth surfaces. Thus, electrical properties in oxygen-argon sputter-deposited films on polysilicon are explained by high-activation-energy current and Fowler–Nordheim tunneling current.
Cooperative oscillations of electron-copper-oxygen interaction: Possible relevance for superconduction theory
Authors: Tributsch Helmut
Published: July 2002

Interfaces of copper containing crystallized materials (Cu5FeS4, CuFeS2, Cu2S, CuS, CuInS2, and CuInSe2), exposed to oxidation by H2O2, undergo sustained electrochemical oscillations. They involve electronic (Cu+), (Cu2+), and (Cu3+) interactions with chemically bound oxygen similar as in superconducting YBa2Cu3O7 or (La,Sr)2CuO4 and may be a phenomenon of irreversible thermodynamics which is related to the kind of cooperative electron-copper-oxygen interactions which also give rise to high-temperature superconduction. The relevance of the model system is supported by the formation of the H2O2 species on the surface of high-temperature superconducting materials in contact with ambiental humidity. It is shown how, by a simple modification of the interaction mechanism, an oscillatory phenomenon such as that observed with oxidized copper sulfide interfaces, which follows a limiting cycle, can be converted into a triple-state phenomenon. Such a cooperative mechanism that involves a lowest steady state which is unable to turn over energy may be the background for the pairing of electrons or a related cooperative interaction process in superconduction. As a possible support, no interfacial oscillations could yet be induced with Ba2Cu3Y7.
Developmental experiments on large-area silicon solar cells
Authors: Silard Andrei P. , Nani Gabriel
Published: July 2002

A comprehensive design/material/technological study was conducted with the aim of obtaining high-efficiency back-surface-field- (BSF-) type bifacial solar cells with reproducible parameters on large-area n- and p-silicon wafers. Practical ways of attenuating the severe limitations imposed by different kinds of areal inhomogeneities on the electro-optical performance of large-area BSF single-crystal silicon solar cells are described and tested experimentally. Various procedures leading to a substantial increase of both the emitter and the base contributions to the generated photocurrent are implemented and discussed in detail. The p+-n-n+ and n+-p-p+ cells were processed as bifacial devices and tested under both front and backside AM1 illumination. The trial devices were fabricated on 2-in and 3-in. commercially available lower-grade silicon and the minimization of the cell cost was one of the sought-after objectives. The combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, high-efficiency (17%–18%) large-area silicon solar cells with good overall electro-optical performance as bifacial devices. The results of this work show clearly that simple, but adequately designed/processed cells fabricated on an industrial scale on large-area silicon wafers could possess parameters similar to those of sophisticated laboratory samples elaborated on small-area silicon. With the design/technological approaches outlined in this work, the use of even cheaper, i.e., lower-grade single-crystal silicon, could still yield 2- and 3-in. cells with fairly acceptable conversion efficiencies.
Total dose effect on soft error rate for dynamic metal-oxide-semiconductor memory cells
Authors: Benumof Reuben
Published: July 2002

A simple model for the soft error rate for dynamic metal-oxide-semiconductor random access memories due to normal galactic radiation was devised and then used to calculate the rate of decrease of the single-event-upset rate with total radiation dose. The computation shows that the decrease in the soft error rate is less than 10% per day if the shielding is 0.5 g/cm2 and the spacecraft is in a geosynchronous orbit. The decrease is considerably less in a polar orbiting device.
Magnetic fields of direct currents in horizontally stratified conductors
Authors: Inayat-Hussain A. A.
Published: July 2002

A new proof is given of the result that, for direct current flow (from a constant current source) in a horizontally stratified conductor, the magnetic field outside the conductor is independent of its electrical conductivity. The original proof is based on symmetry considerations and the application of Ampére’s law to a contrived current circuit linking the horizontally stratified conductor. The new proof, however, does not rely on the use of any contrived current circuits, but is based on a rigorous analysis of the boundary value problem for the direct current flow in the horizontally stratified conductor. The mathematical techniques employed in this proof would be useful for future calculations in electromagnetism.
Anomalous electron-beam-induced-current collection in denuded-zone-treated low-doped silicon
Authors: Burk D. E. , Chung B.-C.
Published: July 2002

The electron-beam-induced current (EBIC) for denuded-zone-treated π-silicon has been attributed to the collection of the excited carriers by a top-surface inversion region. A model for this collection by a parasitic metal-oxide-semiconductor field-effect transistor (MOSFET) is proposed. The MOSFET channel, drain, and source are the top surface inversion region, the adjacent metal-insulator-semiconductor (MIS) junction, which is reverse biased for the normal EBIC measurement, and the electron-hole pair excitation volume, respectively. This model can explain the increase in EBIC with reverse bias, especially at large distances from the MIS junction and the EBIC’s linear dependence (for long diffusion lengths) on the distance near the junction. It is shown how the diffusion length can be obtained in the presence of this channel.
Pressure effects in GaAs/AlGaAs heterostructures
Authors: Ko David Y. K. , Inkson J. C.
Published: July 2002

The trends in the pressure dependence of the electronic transmission in single- and multiple-barrier GaAs/AlGaAs heterostructures are calculated using a microscopic pseudopotential method. Electric effects are also included in our model. We find that in single-barrier systems the pressure induced direct to indirect band-gap transition alone does not affect the electron tunneling properties. It is only when the barrier’s X minimum is lowered below the electron energy that significant changes are found in the electron transmission. In a multiple-barrier system the lowering of the barrier X minimum due to the applied pressure and bias effectively pushes up the Γ resonant levels in the GaAs such that a clear negative differential resistance signal is lost.
Thermally stable ohmic contacts to n-type GaAs. V. Metal-semiconductor field-effect transistors with NiInW ohmic contacts
Authors: Murakami Masanori , Price W. H. , Greiner J. H. , Feder J. D. , Parks C. C.
Published: July 2002

Thermal stability of self-aligned refractory metal-semiconductor field-effect transistors (MESFETs) with the universally used AuNiGe ohmic contact metallurgy or the recently developed NiInW ohmic contact metallurgy has been studied. In these devices WSi0.1 films with length down to 1 μm were used as the gate material, and AlCu was used as the overlayer material on the ohmic contacts, where a very thin Ti layer was deposited prior to the AlCu deposition. The performance of the as-fabricated devices with the NiInW ohmic contacts was as good as those with the AuNiGe ohmic contacts. During subsequent annealing at 400 °C, deterioration of the device performance (defined by the decrease in the FET square-law coefficient) was observed after annealing at 400 °C for 2 h in the devices with the AuNiGe ohmic contacts. However, excellent stability was observed in the devices with the NiInW ohmic contacts; no deterioration was observed at 400 °C for 180 h, 450 °C for 18 h, and 500 °C for 2 h. The device deterioration with NiInW contacts, which was observed after annealing for longer times, is believed to be due to an increase in the contact resistances caused by In movement away from the metal/GaAs interfaces. Based on this assumption, an activation energy for In diffusion in GaAs was determined from onset times of the device deterioration to be ∼2.0 eV, which is close to the reported value of 1.9 eV.
Scaling of critical current density in field-aligned grains of (Tl0.75Bi0.25)1.33Sr1.33Ca1.33Cu2 O6.67+δ, the ‘‘1212’’ phase
Authors: Chen K. , Maheswaran B. , Haldar P. , Markiewicz R. S. , Giessen B. C.
Published: July 2002

We have measured intragrain critical currents in field-oriented samples of a rare-earth-free high-Tc superconductor (Tl0.75Bi0.25)1.33Sr1.33Ca1.33Cu2 O6.67+δ, the ‘‘1212’’ phase. The critical current Jc is anisotropic. For fields oriented along the conducting planes, a simple scaling of Jc with field is found. The critical current scales to zero at a field HBc2 (T) which is in good agreement with the form predicted by the flux creep theory.