bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of gas-flow patterns in a symmetric vertical vapor-phase-epitaxy reactor allowing asymmetric solutions
Authors: Weber C. , van Opdorp C. , de Keijser M.
Published: July 2002

Recently it was demonstrated for the first time that in axisymmetric cold-wall vapor-phase-epitaxy reactors with top inlet, for symmetric boundary conditions and at atmospheric pressure, the stable carrier-gas flow pattern was generally strongly asymmetric. As a result the epilayer thickness turned out to vary by as much as a factor of three over a nonrotating susceptor. The asymmetry of the flow pattern was shown both by visualization experiments and by numerical simulations. Only a few results of the simulations were presented at that time. The present paper sets forth in somewhat more detail the procedure used to solve the relevant differential equations, and presents a systematic series of flow and temperature patterns for different carrier-gas fluxes. At low enough as well as at high enough fluxes there is a single stable flow pattern, which is symmetric in both cases. They are of the natural and forced flow type, respectively. At intermediate fluxes, which include the fluxes used in practice, there are alternative solutions: In addition to the stable asymmetric pattern mentioned earlier, unstable symmetric patterns are found. Only the latter patterns were reported by other investigators. A synopsis of the various flow types is presented here in the form of a perspicuous bifurcation diagram. Apart from vapor-phase-epitaxy applications this appears to be an interesting case of bifurcation phenomena for their own sake.
Effect of the loop capacitance on resonant modes in superconducting two-junction interferometers
Authors: Camerlingo C. , Ruggiero B. , Russo M. , Sarnelli E.
Published: July 2002

Self-resonant current steps in superconducting interferometers are investigated. The case of interferometers with non-negligible loop capacitance is considered. A theoretical analysis based on an equivalent electrical circuit is performed to get the resonance frequencies in the current-controlled case. Careful measurements on a slightly asymmetrical (C1≠C2) interferometer are reported. The magnetic field dependence of the maximum amplitude of the resonant steps and their voltage position have been studied. A good agreement between theory and experiments is obtained.
Influence of the minority carriers on the transverse acoustoelectric voltage
Authors: Lippens P. E. , Lannoo M. , Pouliquen J. F.
Published: July 2002

A new expression of the transverse acoustoelectric voltage (TAV) is obtained for semiconductors including both majority and minority carriers. Numerical results are given and compared for LiNbO3/Si and LiNbO3/GaAs structures. It is shown that the influence of the minority carriers is restricted to quasi-intrinsic semiconductors. The change in sign of the TAV for low conductivities and low frequencies, previously predicted for extrinsic semiconductors, is confirmed.
Moving species during ion mixing in GexSi1−x/metal systems
Authors: Xia W. , Hewett C. A. , Fernandes M. , Lau S. S. , Poker D. B.
Published: July 2002

The origin of the motion of semiconductors during ion mixing was investigated by studying both the temperature and the atomic mass dependence of moving species in the GexSi1−x/Ni and the GexSi1−x/Pd systems. Ion mixing was performed with 280-keV Ar ions at temperatures between 30 K and room temperature. The atomic mass of the GexSi1−x alloy was adjusted by changing the concentration of Ge in the alloy. In thermally induced reactions, no preferential motion of Si or Ge was observed. During ion mixing, the atomic flux of Si was observed to be enhanced compared to that of Ge. The atomic flux of the sum of Si and Ge to metal decreases with increasing substrate temperature during mixing and with increasing Ge concentration in the GexSi1−x alloy. From the strong atomic mass dependence of the moving species during ion mixing it is concluded that the origin of the motion of semiconductors under ion mixing conditions is due to the effects of secondary recoils.
Interaction of amorphous MoNix alloys with silicon
Authors: Rastogi R. S. , Vankar V. D. , Chopra K. L.
Published: July 2002

The interaction of amorphous MoNix alloys with silicon has been investigated by glancing-angle x-ray diffraction, Auger-electron spectroscopy, and sheet-resistance measurements in the temperature range 500–700 °C. Mo-rich alloys (Mo80Ni20 and Mo65Ni35) were found to react with silicon at ∼550 °C while nickel-rich alloys (Mo32Ni68) react at ∼500 °C to form silicides. In Mo-rich alloys, the initial reaction occurs due to out-diffusion of nickel from the alloy and in-diffusion of silicon into the alloy, while in nickel-rich alloys the initial reaction occurs due to out-diffusion of nickel only. In the initial phase of the reaction, a mixture of several crystalline phases of molybdenum and molybdenum and nickel silicides is observed. At higher temperature, however, a two-layer structure MoSi2 +NiSi(NiSi2)/ NiSi(NiSi2)/ Si(100) is formed in both cases.
Recommended Bethe–Bloch stopping power parameters for Kapton and Havar
Authors: Porter L. E.
Published: July 2002

Previous measurements of the stopping power of Kapton and Havar for 1.1–4.4-MeV protons have been analyzed with modified Bethe–Bloch theory in order to extract some of the parameters required in the formalism. Results suggest the possibility of systematic errors, in that the Havar data appear to have been slightly high and the Kapton data quite low when compared with other reported measurements. Currently recommended values of the mean excitation energy and Barkas-effect parameter are, respectively, 79 eV and 1.34 (Kapton) and 296 eV and 1.36 (Havar). The need for further accurate measurements for Kapton in particular is emphasized.
Cavity eigenmodes for a free-electron laser
Authors: Riyopoulos S. , Tang C. M.
Published: July 2002

The cavity transfer matrix for the round trip of a light pulse in the optical resonator of the National Institute of Standards and Technology/Naval Research Laboratory free-electron laser oscillator is derived. The cavity eigenmodes and the corresponding eigenvalues are obtained, using an expansion in Gaussian–Laguerre vacuum modes, by numerical diagonalization. The fractional power loss per eigenmode, caused by the finite sizes of the cavity mirrors and apertures, is determined. It is found that the losses are very small over the entire wavelength regime of operation.
Increase of blue-green dye laser energy by an energy converter dye
Authors: Oh Chul H. , Hwang In H.
Published: July 2002

In order to increase the laser energy through energy conversion of the pumping light, the converter dye BBQ was mixed in the laser dye LD490 solutions. The laser was pumped by an Ar flashlamp with a relatively long pulse width. Various concentrations of dye mixture LD490+BBQ in methanol were used. The experimental results show that the laser output energy increases by 180% with a dye mixture made of 4.5×10−6 mol/ℓ  BBQ over the maximum output obtainable with the laser dye alone, while the laser power increases by 92% with the same conditions. Comparison with previous work [K. S. Han et al., J. Appl. Phys. 60, 3414 (1986)] is discussed.
Ion-induced electrical breakdown in metal-oxide-silicon capacitors
Authors: Milgram Alvin A.
Published: July 2002

Metal-oxide-silicon (MOS) capacitors used in the breakdown mode were constructed to detect 252Cf fission fragments. The physical effects related to radiation-induced breakdown were studied and found to have a relationship to intrinsic dielectric breakdown. Physical studies were made of the effect of oxide annealing temperature between 100 and 400 K, oxide thickness, differing metal electrodes, and the materials interaction due to the spark discharge breakdown. By means of both silicon-ion and cesium-ion implants near the metal-SiO2 interface of the MOS capacitor, the current emission into the SiO2 was found to be significantly enhanced; however, the enhanced emission was found to have no effect on increasing the sensitivity to lower specific ionization radiation as seen by fission-fragment detection. The results indicate that the radiation-induced breakdown mechanism occurs within the bulk, or body, of the SiO2 film and is not influenced by conditions at the dielectric interfaces. It was found that thin films of the refractory metals HfO2 or ZrO2 between the oxide film and a top aluminum electrode prevented Al-SiO2 interaction during discharge and allowed operation of the MOS capacitor at higher electric fields. Obtaining higher operating fields significantly improved the minimum specific ionization detectability to 14 MeV cm2/mg from a prior value of 22 MeV cm2/ mg.
Electrical characteristics of GaAs grown from the melt in a reduced-gravity environment
Authors: Wang Z. G. , Li C. J. , Cao F. N. , Shi Z. W. , Zhou B. J. , Zhong X. R. , Wan S. K. , Xu S. D. , Lin L. Y.
Published: July 2002

The electrical properties and structural defects of Te-doped GaAs grown in space have been investigated by using various techniques. The experimental results confirm that the microgravity conditions offer some advantages for the melt growth of III-V compound semiconductor materials; improvements of homogeneity and perfection as well as purity of the space GaAs single crystal are expected.
Effect of flexoelectric coupling on helix distortions in cholesteric liquid crystals
Authors: Lee Sin-Doo , Patel J. S. , Meyer Robert B.
Published: July 2002

The static and dynamic properties of the flexoelectric effect in cholesteric liquid crystals are presented. The application of an external electric field, below the critical strength for helix unwinding, perpendicular to the cholesteric helix, results in a linear electro-optic effect. The experimental results are in good agreement with theoretical predictions made under the assumption of the uniform distortion of the helix due to the flexoelectric coupling with the field. The temperature dependence of the dynamic response to the field is found to be the Arrhenius type. It is suggested that the surface boundary conditions play a role in the properties of the flexoelectric effect.
Influence of surface impurity on impact response of lattices
Authors: Tasi James , Whitten Jerry L.
Published: July 2002

An analytical and computational study has been made of the influence of surface impurity on the normal impact response of two identical crystal lattices. Identical monolayers of adsorbed atoms are considered to cover each host lattice. The incoming lattice is assumed to move with uniform initial speed, impacting an initially motionless lattice. A nonlinear differential-integral equation governing the impact response of the surface impurity is derived and then solved analytically by asymptotic expansion for compressive interaction. For numerical calculations, oxygen surface impurity and a copper host lattice are considered, with surface interaction force determined from quantum-mechanical results. Because the surface interaction force exhibits a gradual long-range change in force with distance, surface oscillations do not occur during impact. The time required for the surface of an impacted lattice to accelerate to the classical continuum shock velocity is shown to depend on impact velocity and two parameters from the quantum-mechanical calculations. The subsequent distance at which breakdown of weak shock-wave motion far into the interior of the lattice occurs is determined by a simple approximation. Shock responses in the lattice beyond the breakdown distance are determined by numerical integration of both the lattice equations of motion, with quadratic nonlinear force interaction between atoms, and the Korteweg–de Vries (KdV) equation. The asymptotic solution for velocity response of the surface impurity provides the initial condition for numerical integration of the KdV equation. The calculations for both sets of equations show a good correspondence in the structure of the shock wave far into the lattice.
Structural and superconducting properties of artificially superstructured MoN-TiN films
Authors: Kawaguchi K. , Shin S.
Published: July 2002

Superconducting artificially superstructured films (ASFs) composed of MoN and TiN have been prepared by an alternate reactive deposition method. Alternate layers of MoN and TiN were epitaxially grown on MgO(100) single-crystal substrates. Those ASFs with the periodicities of [MoN(0.4 nm)/TiN(0.4nm)], [MoN(0.2 nm)/TiN(3.4 nm)], and [MoN(2.4 nm)/TiN(0.2 nm)] were successfully synthesized. It was confirmed that the precise thickness of ASFs could be controlled on the atomic level. Average lattice constants of ASFs were calculated from the positions of satellite peaks observed in x-ray diffraction. Structural analyses indicate the formation of a single B1-MoN phase when conditions are optimized. ASFs containing B1-MoN layers showed lower superconducting transition temperatures (Tc ’s) than those of ASFs with γ-Mo2N. A few probable reasons for the lower Tc are discussed.
The effect of heat treatment on the superconducting properties of Cu-Nb composites
Authors: Klein J. D. , Rose R. M.
Published: July 2002

Microfilamentary composites of continuous Nb fibers in Cu matrices were tested for superconducting transport properties after heat treatments at 200–900 °C. The critical current density at applied magnetic field was reduced dramatically as the heat treatment temperature was increased. The scaling relations describing the critical current as a function of applied field shifted from conventional (1−h)2 scaling to higher-order relations as the heat treatment history was varied. Critical temperature transitions were abnormally broad in the fine-filament composites examined. The upper critical fields and transition temperatures approached bulk values after severe heat treatment.
Heteroepitaxy of GexSi1−x on porous Si substrates
Authors: Xie Y. H. , Bean J. C.
Published: July 2002

Molecular-beam epitaxial growth of GexSi1−x on 〈100〉 porous Si substrate was studied using Rutherford backscattering spectrometry, transmission electron microscopy, and defect etching. This study was stimulated by the theoretically predicted possibility of stress field reduction in a lattice-mismatched film grown on a patterned substrate. The experimental results show predominantly 60° dislocations with long misfit segments, and no reduction of dislocation density is observed by the use of porous Si substrates. The nature of interconnected growth area of the porous Si substrate is believed to be the reason for the ineffectiveness of the stress reduction.
Capping and decapping of InP and InGaAs surfaces
Authors: Lau W. M. , Sodhi R. N. S. , Jin S. , Ingrey S. , Puetz N. , SpringThorpe A.
Published: July 2002

An investigation of techniques for the preparation and preservation of stoichiometric InGaAs and InP surfaces, and techniques for regenerating these surfaces in vacuum have been carried out using x-ray photoelectron spectroscopy for surface characterization. It was found that InGaAs stoichiometric surfaces grown by molecular-beam epitaxy (MBE) can be preserved from oxidation and contamination by condensation of a thin layer of arsenic in the MBE chamber. The stoichiometric surfaces can be regenerated by thermal desorption of the arsenic cap in vacuum. Although stoichiometric InP surfaces can be prepared and arsenic capped in the MBE chamber, interfacial alloying between the arsenic cap and the substrate occurred during the thermal desorption process. Stoichiometric InGaAs and InP surfaces can also be preserved by a UV/ozone-formed oxide overlayer. For the removal of ozone-formed native oxides, thermal desorption at 458 °C regenerated clean and stoichiometric InP surfaces with a band bending of less than 0.3 eV on both n- and p-type substrates. However, complete oxide desorption on InGaAs required a temperature, above 565 °C, at which the surface decomposed. It was found that the ozone-formed surface oxides on InGaAs and InP could be removed at a substrate temperature below about 250 °C using a remote hydrogen plasma. The resultant n-InGaAs substrate surface was close to flat band, whereas the surface Fermi levels of the n- and p-type InP substrates were at 0.4 and 0.7 eV from the conduction-band minimum, respectively.
Field theory of a common sidewall, single-hole directional coupler
Authors: Li Guifang , Seshadri S. R.
Published: July 2002

A rigorous electromagnetic theory of the common sidewall, single-hole directional coupler is presented. The characteristics of the directional coupler are obtained retaining the propagating modes as well as the evanescent modes, and the effect of the evanescent modes is determined. The standing-wave ratio at the input port, the directivity, the transmission, and the coupling factors, together with their spectral responses, are investigated. Maximization of the power coupled to the output port with respect to the aperture width and the optimization with respect to the variation in the width of the waveguide are also discussed.
Microanalysis on the (200) diffraction intensity to determine the Al concentrations for AlGaAs-GaAs multiple-quantum-well structures
Authors: Ou H.-J. , Cowley J. M. , Chyi J. I. , Salvador A. , Morkoç H.
Published: July 2002

By using a nanoprobe electron beam 4 Å in diameter, the [001] nanodiffraction patterns of AlGaAs-GaAs multiple-quantum-well structures have been formed and the epitaxial relationship between AlGaAs and GaAs is confirmed. The intensities of the (200) diffraction disk, monitored by a spot detector, are displayed in two ways: (1) the (200) dark-field scanning transmission electron microscopy (STEM), which shows the layers of AlGaAs and GaAs in contrast, and (2) the (200) line-scan profile, which reveals the (200) intensity distribution of a specimen region of uniform thickness. The thickness and the absolute Al concentration of AlGaAs layers are, respectively, determined from the contrast of, and the (200) thickness contour position in, the (200) dark-field STEM images. The microanalysis on the (200) line-scan profile is used to find the local Al concentrations in AlGaAs layers and to study the interface boundary between the layers of AlGaAs and GaAs. Diffusion of the Al atoms from the AlGaAs layer into the GaAs layer is also reported.
Experimental and theoretical evidence for a new universality class in FeF3: A 3D lattice with frustrated Heisenberg spins (abstract)
Authors: Reimers J. N. , Greedan J. E. , Bjorgvinsson M.
Published: July 2002

The pyrochlore form of FeF3 is a Heisenberg antiferromagnet [TN=15.2(1)K], in which the Fe3+ atoms form a high-symmetry lattice of corner-sharing tetrahedra. The low-temperature magnetic structure is noncollinear with four sublattices oriented 109° from each other. Neutron diffraction was used to determine the critical exponent, β=0.17(2), which does not correspond to any known universality class. Monte Carlo simulations with finite-size scaling analysis confirm the measured value of β and also provide the exponent ν=0.34(4). The existence of this new universality class supports Kawamura’s hypothesis that critical properties of a system depend on the symmetry properties (and not just the dimensionality) of the order parameter.
Chemical processing using magnetic fluids (abstract)
Authors: Chagnon Mark
Published: July 2002

Magnetic fluid colloids have been widely accepted for use as low vapor pressure seals in vacuum rotary feedthroughs, dust exclusion seals for computer disk drives, and as liquid coolants for loudspeakers for many years. Chemical separations based on colloidal size magnetic particles have been recently employed in magnetic radioimmuno assay techniques. Our research demonstrates the usefulness of the magnetic liquid colloid as a method of in situ separation of a reaction product from its reactants during chemical processing. Two chemical reactants are placed into a reaction vessel in contact with an immiscable magnetic liquid colloid. As the reaction proceeds, a product that is completely soluable in the magnetic colloid is formed. The product dissolves in the colloidal suspension and is rendered magnetic. Upon completion of the reaction, the product and magnetic fluid colloid is collected by application of an external magnetic fluid. The residual reactants are decanted. The colloid/product matrix is contacted with acetone to extract the desired product from the colloid matrix. The magnetic colloid is collected with a magnet, and recycled for further use. The product is purified by distillation of the acetone. Using this method we have generally found higher product yields and faster separation times to occur in the materials studied.
ac magnetic susceptibility of tetramethylammonium copper chloride (TMCuC): S=(1)/(2), Heisenberg one-dimensional ferromagnet
Authors: Ravindran K. , Haines Donald N. , Drumheller John E.
Published: July 2002

The ac susceptibility of powder sample TMCuC, a S= (1)/(2) , 1D Heisenberg ferromagnet, was measured in the temperature range 4.2 K≤T≤20 K and in the fields 1, 1.5, and 2 T. The data are compared with the theory of Johnson and Bonner [J. D. Johnson and J. C. Bonner, Phys. Rev. B 22, 251 (1980)], which includes the spin-wave and bound spin-wave excitations. Preliminary fittings yield parameters J/k=45 K, average g value=2.1, and γ=0.955 (anisotropy 4.5%), and show that the bound-state excitations have a significant effect on the magnetic behavior.
Domain-wall excitations in the spin-(1)/(2) X
X
Z linear chain: A new variational approach
Authors: Gómez-Santos G.
Published: July 2002

The linear chain XXZ spin- (1)/(2) Hamiltonian is mapped onto a model where the local degrees of freedom are domain walls between opposite antiferromagnetic domains. The domain-wall Hamiltonian is solved within the simplest Hartree–Fock approximation with results in very good agreement with the exact solution in the antiferromagnetic regime. The location of ordered and disordered (critical) phases is correct and low-lying excitations are well described and easily interpreted in terms of domain walls. Our domain-wall version of the original Hamiltonian is shown to provide the natural framework for a physical understanding of this system in the antiferromagnetic regime.
One-dimensional magnetic materials with dominant next-nearest-neighbor interactions
Authors: Benelli Cristiano , Caneschi Andrea , Gatteschi Dante , Pardi Luca
Published: July 2002

A series of one-dimensional magnetic compounds in which trivalent gadolinium ions and organic radicals alternate regularly in space is reported. The magnetic properties of these compounds are interpreted in terms of a two-spin-up, two-spin-down ground-state configuration. Such spin configurations may be stabilized by next-nearest-neighbor interactions. A quantitative fit of the experimental data is attempted using the Ising model.
Effect of local magnetic anisotropy on the spin-wave resonance in amorphous thin films
Authors: Suran G. , Naili M. , Rivoire M. , Levy J. C. S.
Published: July 2002

On the spin-wave-resonance spectra corresponding to Co100−xZrx thin films, all the resonance fields, including the one corresponding to the uniform mode, follow a quadratic dispersion law with excellent precision. The intensities of the spin-wave modes are very weak. The overall results are fairly well explained by a new model where it is shown that the characteristics of the spectra are determined by the random local anisotropy Kl. The value of the deduced Kl decreases with decreasing Ms as predicted by the single-ion anisotropy theory.
Frequency dependence of magnetic anisotropy in Ni/Mo multilayers (abstract)
Authors: Kingsbury David E. , Pechan Michael J.
Published: July 2002

Based upon earlier findings,1 an investigation of frequency-dependent effects in interface anisotropy has been undertaken, preliminary results of which are presented. Six equal-layer-thickness Ni/Mo multilayer samples ranging in Ni layer thickness d from 20 to 1000 Å were measured in the parallel geometry by 4, 9, 18, and 36 GHz ferromagnetic resonance at room temperature. The total anisotropy decreases with decreasing frequency for small d samples and increases with decreasing frequency for large d samples, thereby establishing the existence of frequency dependence. Measurements at low temperature, where the interface anisotropy (IA) is much more pronounced, are underway to investigate the IA frequency dependence.
Studying hyperfine fields with resonant nuclear diffraction of x rays (invited)
Authors: Arthur J. , Brown D. E. , Ruby S. L. , Brown G. S. , Shenoy G. K.
Published: July 2002

It is shown that the hyperfine field in a crystal can be characterized in a short measurement using resonant nuclear diffraction at a high-brightness synchrotron x-ray beamline. In addition to the strength of the field, this technique is sensitive to the field direction and to the crystal-structure factor of the nuclear sites. It is possible to separately measure the field at different crystallographic subgroups of nuclei. These principles are illustrated with an experiment using a crystal of 57Fe-enriched yttrium iron garnet, and future applications are discussed.
Soft magnetic properties of multilayer FeY/SiN films
Authors: Okayama H. , Sugenoya S. , Kohmoto O. , Narumiya Y.
Published: July 2002

Magnetic properties of sputtered multilayer Fe100−xYx/Si3N4 films (x=0–3.5 at. %) were studied. We developed heat resistant and corrosion resistant soft magnetic films having high saturation induction (Bs). The Fe97.7Y2.3/Si3N4 film has a Bs value of 17.3 kG, a coercive force (Hc) of 0.9 Oe, permeability at 5 MHz (μ) of 2800 and magnetostriction (λs) of ∼−1×10−6 after being annealed at 600 °C.
Anisotropy of RKKY coupling strength in Dy/Y superlattices (abstract)
Authors: Tsui F. , Flynn C. P. , Salamon M. B. , Erwin R. W. , Rhyne J. J.
Published: July 2002

We have investigated the dependence of magnetic coupling strength on growth direction in superlattices made from alternating layers of Dy and Y metals. Using new molecular-beam epitaxy techniques, we have succeeded in growing high-quality single-crystal materials with structural coherence lengths >500 Å and with the growth direction oriented along a, b, or c crystallographic axes by choice. In earlier work, we showed that, in c-axis samples, Dy layers order coherently via RKKY-like interactions propagated through Y layers up to about 150 Å thick. Here we report neutron scattering data which show that Dy layers in b-axis samples fail to couple coherently even when the Y spacer layers are as thin as 26 Å. This observation can be explained in a semiquantitative way in terms of the anisotropy of the q-dependent susceptibility of Y, which exhibits a peak that is sharp in the c* direction but broad along a* and b*. As a result, the conduction-electron mediated exchange coupling through the Y layer is long ranged along c but short ranged along a and b. An interesting anisotropy of the magnetic coherence in b-axis samples is also observed.
Superconducting fluctuations in the Hubbard model with an attractive on-site interaction in one dimension
Authors: Schlottmann P. , Lee Kong-Ju-Bock
Published: July 2002

The low-temperature thermodynamics of the one-dimensional Hubbard model with attractive on-site interaction of the electrons is discussed within the framework of the Bethe ansatz for the wave function. At low T the dominant states consist of spin-paired electrons (Cooper-pair-like) and excitations of such pairs (without breaking up the singlet bound states). A critical field Hc (energy required to depair a Cooper pair) is obtained at T=0 (note that the superconducting Tc =0), which disappears for T≠0. There is no magnetic response at T=0 for H<Hc . The one-electron correlation function falls off exponentially with distance, while the singlet pair–singlet pair correlation decreases with a power law. The specific heat is linear in temperature, which is possibly a consequence of the dimension. The elementary excitations of the system are discussed. The excitation spectrum shows some analogies to that of resonant valence bonds in two dimensions.
Zero-temperature statics of a random exchange model for the magnetic properties of La2−xSrxCuO4
Authors: Huber D. L. , Ching W. Y.
Published: July 2002

Results are reported for the static properties of a random exchange model proposed for the magnetic propeties of La2−xSrxCuO4. Numerical calculations of the zero-temperature staggeredmagnetization and static structure factor are presented for 0.05≤c≤0.25, where c is the fraction of ferromagnetic exchange integrals in an otherwise perfect antiferromagnetic lattice with nearest-neighbor Heisenberg interactions between classical spins. The relevance of the findings to the magnetic properties of La-Sr-Cu-O in the insulating phase is assessed.
Magnetic critical behavior and exponents of the quasi-two-dimensional ferromagnet [NH3(CH2)9NH3]CuBr4
Authors: Zhou P. , Drumheller John E.
Published: July 2002

The dc initial susceptibility, spontaneous magnetization, and critical isothermal magnetization measurements of powder [NH3(CH2)9NH3]CuBr4 (9DACuBr) are reported with emphasis on the anomalous behavior of the critical exponents. Because of the weak exchange interaction between the layers, it behaves as a nearly ideal two-dimensional Heisenberg ferromagnet at finite temperature. The transition temperature Tc=11.35 K was determined by the measurements of spontaneous magnetization and initial susceptibility. The critical exponents β=0.28, γ=1.38 (T>Tc), and γ′=0.45 (T<Tc), as well as an empirical relation δ′=δ+cHd for the effective critical exponent with δ=0.65, c=0.185, and d=0.175, were found to fit the data.
Susceptibilities of the two-dimensional Heisenberg antiferromagnet [Cu(C5H5NO)6](BF4)2
Authors: Burriel R. , Lambrecht A. , Carlin R. L.
Published: July 2002

The zero-field magnetic susceptibilities of polycrystalline samples as well as oriented single crystals of [Cu(C5H5NO)6](BF4)2 are reported. Broad maxima are observed in all the data, but the existence of domains with threefold symmetry prevents the establishment of an easy axis. The data have been compared to the high-temperature series expansions and the Schwinger boson mean-field theory for the square lattice, S= (1)/(2) , Heisenberg antiferromagnet.
Structural and magnetic properties of Mn(SCN)2(i-C3H7OH)2 (abstract)
Authors: DeFotis G. C. , Wiese R. S. , Remy E. D. , Scherrer C. W.
Published: July 2002

Among systems of the type Mn(SCN)2(ROH)2, where R=CH3, C2H5, i-C3H7, or n-C3H7, only the title compound with R=i-C3H7 fails to exhibit a magnetic ordering transition in the vicinity of 10 K. As with the other members of this series, superexchange interactions between manganese ions are mediated by bridging thiocyanate ions, but for the isopropanol compound a distinctly different type of bridging network is revealed by a crystal structure determination.1 The structure, presented and described in detail, is monoclinic, P21/n, Z=2 with a=11.584(2) Å, b=5.781(1) Å, c=11.163(2) Å, and β=111.86(1)°. Only along the b axis do efficient thiocyanate superexchange bridges occur, so that magnetically one-dimensional behavior can be anticipated. This is evident in susceptibility measurements, which reveal a broad maximum near 16 K and which can be fit reasonably well assuming one-dimensional antiferromagnetic Heisenberg model behavior. A magnetic anisotropy of a few percent is also evident in the single-crystal susceptibility. No magnetic ordering appears to occur down to a temperature of about 1.65 K.
Properties of rapidly quenched Ti(Ni,Fe)Si alloys
Authors: Christie I. A. , Dunlap R. A. , McHenry M. E.
Published: July 2002

Alloys of the series Ti56Ni28−xFexSi16 (x=0,...,20) can be prepared either in the amorphous phase or as single phase quasicrystals by rapid solidification. Here electrical resistivity and magnetic susceptibility measurements are reported. All amorphous alloys and quasicrystalline samples with x≥10 reveal Curie-like magnetic susceptibility and electrical resistivity dominated by structural scattering. The quasicrystalline samples with x<10 demonstrate nonCurie susceptibility and a minimum in ρ(T) where Tmin varies inversely with Fe content.
Magnetic force microscopy of recording media (abstract)
Authors: Mamin H. J. , Rugar D. , Lambert S. E. , Franco L. P. , Sanders I. L. , Yogi T. , Beaulieu T.
Published: July 2002

Magnetic force microscopy (MFM) is an imaging technique which is particularly well suited to studying magnetization patterns and other issues in magnetic recording physics. The technique provides high magnetic contrast, submicrometer resolution, requires a minimum of sample preparation, and can be used with both soft and hard magnetic materials. We have developed a MFM designed around a novel optical-fiber interferometer incorporating a laser diode, and have used it to study a variety of thin films of recording media. We have imaged transitions written with a recording head in both CoPtCr, a longitudinal medium in which the magnetization lies in plane, and CoCr, a perpendicular medium. The MFM revealed fine structure such as intrinsic media noise on virgin media, detail within magnetic transitions, and side-writing effects. We have also used the MFM to study the erase band created during overwrite under various conditions. In films of SmCo, a novel thin-film medium, the MFM revealed that there exists a minimum spacing between transitions below which the magnetization pattern cannot be sustained. In order to improve lateral imaging resolution, we have fabricated tips by sputter coating nonmagnetic tungsten tips with 50 nm of CoPtCr. Using these tips we have achieved resolution well under 100 nm.
Phase diagram for a model exchange-disordered magnetic alloy: Physical principles
Authors: Parker G. N. , Saslow W. M.
Published: July 2002

The mean-field phase diagram for an XY model on a square lattice is studied. At low temperatures, noncollinearity can be produced by individual ‘‘strong’’ impurity bonds or a finite concentration of ‘‘weak’’ impurity bonds. Thermal fluctuations tend to destroy this noncollinearity, leading to a collinear state at low enough impurity concentrations. For both ‘‘weak’’ and ‘‘strong’’ impurities the phase diagram has a multicritical point like that found by Gabay and Toulouse. However, ‘‘very strong’’ impurity bonds lead to noncollinearity at all temperatures where there is magnetic order, and yield a phase diagram with a multicritical point unlike that of Gabay and Toulouse.
1D antiferromagnetism in spin-alternating bimetallic chains
Authors: Coronado E. , Sapiña F. , Drillon M. , de Jongh L. J.
Published: July 2002

The magnetic and thermal properties of the ordered bimetallic chain CoNi(EDTA)⋅6H2O in the very low-temperature range are reported. The magnetic behavior does not exhibit the characteristic features of 1D ferrimagnets, but a continuous decrease of χmT towards zero at absolute zero. This 1D antiferromagnetic behavior results from an accidental compensation between the moments located at the two sublattices. This behavior, as well as the specific-heat results, are modeled on the basis of an Ising-exchange model that considers both alternating spins and Landé factors, and a zero-field splitting on the Ni site.
1D ferrimagnetism in homometallic chains
Authors: Coronado E. , Gómez-García C. J. , Borrás-Almenar J. J.
Published: July 2002

The magnetic properties of the cobalt zigzag chain Co(bpy)(NCS)2 (bpy=2,2′-bipyridine) are discussed on the basis of an Ising-chain model that takes into account alternating Landé factors. It is emphasized, for the first time, that a homometallic chain containing only one type of site can give rise to a 1D ferrimagneticlike behavior.
Magnetostructural correlations in some one-dimensional exotic systems (invited) (abstract)
Authors: Drillon M.
Published: July 2002

The thermodynamics of one-dimensional (1D) magnetic systems is now well established for uniform ferro-or antiferromagnetic chains. In the last few years, several exotic arrays of metal ions exhibiting unusual magnetic behaviors have been isolated both in molecular and solid-state chemistry. Among these, the engineering of structurally ordered bimetallic chains and the breakthrough in the understanding of magnetostructural correlations have allowed the development of new classes of magnetic materials. This paper will focus on several types of low-dimensional materials characterized by exotic stackings of the metal ions. (i) The bimetallic complexes MnM(edta) 6H2O (M=Ni, Cu). The low-temperature magnetic behavior will be discussed in terms of alternating Heisenberg ferrimagnetic chains. (ii) The orbitally degenerate chain CoCu(pbaOH)(H2O)3⋅4H2O. The similarity with a branch chain, due to the spin-orbit-coupling effect for Co (II) ions, will be used for the magnetic analysis. (iii) The Cu(II) phosphates A3Cu3(PO4)4, where A=Ca, Sr. In this case, 1D ferrimagnetism will be related for the first time to a pure topology effect. The bahavior of Cu3(PO4)2 in which Cu(II) trimers interact through outer ions will be examined. (iv) The vanadyl phosphate VO(HPO4)⋅4H2O exhibiting frustrated chains. For all these low-dimensional systems, the behavior will be closely related to the stacking of magnetic ions and their connections through bridging anions. Specific models of exchange coupling will be developed. On the other hand, the conditions for the occurrence of long-range ferrimagnetic ordering will be investigated.
Simulation of x-ray diffraction intensity variation of W/Si multilayer after annealing
Authors: He Xian-chang , Shen He-sheng , Wu Zi-qin
Published: July 2002

A computer simulation of small-angle diffraction intensities is performed for a W/Si amorphous multilayer structure. The different models which are related to different interface structures after annealing are set up. Calculated results show that the existence of silicide interface layers which induces a change of both direction and magnitude of the diffraction amplitude is not always accompanied by a decrease of the diffraction intensity. In some cases, proper thickness and composition of interface layers may increase the diffraction intensity. However, the existence of systematic and random deviations of layer thicknesses always induces a decrease of x-ray diffraction intensity.
Low-noise and high-power operation in high reflectivity coated nonabsorbing mirror GaAlAs lasers
Authors: Naito H. , Nakanishi H. , Nagai H. , Yuri M. , Yoshikawa N. , Kume M. , Hamada K. , Shimizu H. , Kazumura M. , Teramoto I.
Published: July 2002

We have successfully attained low-noise and high-power operation in GaAlAs lasers by applying high reflectivity facet coating approach to nonabsorbing mirror lasers. High reflectivity coating is used to reduce feedback induced noise coming from hopping between the diode cavity modes and between the external cavity modes. A nonabsorbing mirror structure is used to obtain high-power operation by suppressing mirror degradation and catastrophic optical damage effectively even with the use of high reflectivity coating. Nonabsorbing mirror lasers with the front facet reflectivity 50% showed the relative intensity noise level less than −130 dB/Hz at 3 mW under 0%–10% optical feedback and showed the cw output power as high as 50 mW in fundamental spatial mode operation. Under 30-mW cw operation at 50 °C, on the other hand, obvious degradation has not been observed over 4000 h in the nonabsorbing mirror lasers with 50% front facet reflectivity. Thus the present laser is useful as a low-noise source at a low-power level and also as a high-power source with high reliability.
Transport critical current densities of partially aligned bulk samples of YBa2Cu3O7−x superconductors
Authors: Hascicek Y. S. , Testardi L. R. , Leventouri Th. , Liarokapis E. , Martinez L.
Published: July 2002

Temperature and magnetic field dependence of critical current densities (Jc) of partially oriented bulk YBa2Cu3O7−x samples are presented. A series of partially oriented samples has been fabricated by repeating the steps of the solid-state reaction method. X ray and Raman studies detect the orientation of the grains on the surface and in the bulk of the samples. Jc increases with the degree of alignment of the microcrystals, up to a factor of three. The effect of partial bulk-preferred orientation on the critical current anisotropy is also studied.
Investigation of diethylarsine as a replacement for arsine in organometallic vapor-phase epitaxy of GaAs
Authors: Kachi Tetsu , Ito Hiroshi , Terada Shigeo
Published: July 2002

Epitaxial growth of GaAs has been performed with diethylarsine (DEAs) to investigate its potential as a replacement of arsine. Triethylgallium (TEG) and trimethylgallium (TMG) were used as group-III sources and carbon incorporation into the epilayers was compared. The growths were carried out under low-pressure conditions (130 Pa–1.3 kPa) to avoid a gas-phase reaction between the precursors. All epilayers exhibited p-type conductivity, and the main acceptor impurity was carbon. The lowest hole concentrations were ∼1016 and ∼1017 cm−3 for TEG and TMG, respectively. The dependence of hole concentration on V/III ratio suggests that the carbon incorporation comes mainly from TMG and DEAs for the TMG/DEAs mixture, and from DEAs for the TEG/DEAs mixture. These results indicated that the contribution of the H atom from the As—H bond in DEAs on reduction of carbon incorporation was not enough to grow high-purity GaAs.
Characterization of defects produced during self-annealing implantation of As in silicon
Authors: Lulli G. , Merli P. G. , Migliori A. , Matteucci G. , Stanghellini M.
Published: July 2002

Several electron microscopy techniques were used to investigate in detail the structure of the defects produced by implantation of Si with high current density As+ ion beam under self-annealing conditions (i.e., with simultaneous damage recovery activated by the transient beam heating). The defects, which display a basic octahedral shape, appear if the irradiation is prolonged after the time for which the transient beam heating has produced the complete crystallization of the previously amorphized surface layer. With increasing irradiation time and temperature they grow up to a maximum diameter of about 20 nm, and concentrate in a band located around the position of the maximum of nuclear energy loss of As+ ions, where a segregation peak of electrically inactive As is developed simultaneously. The results of different microstructural and microanalytical techniques used to investigate the defects (conventional and high resolution electron microscopy, electron holography, and energy dispersion x-ray microanalysis) give information which leads us to identify them as large vacancy aggregates, or voids, with As atoms segregated in proximity of the inner surfaces. The role of As atoms is likely to reduce the strain energy which a vacancy aggregate is expected to produce in the surrounding lattice.
Thermally stable ohmic contacts to n-type GaAs. VIII. Sputter-deposited InAs contacts
Authors: Kim H.-J. , Murakami Masanori , Wright S. L. , Norcott M. , Price W. H. , La Tulipe D.
Published: July 2002

The electrical properties and microstructure of InAs ohmic contacts to n-type GaAs, prepared by sputter-depositing a single target, were studied by measuring the contact resistance (Rc) by the transmission line method and analyzing the interfacial structure by x-ray diffraction and cross-sectional transmission electron microscopy. Current-voltage measurement of an as-deposited InAs/W contact showed Schottky behavior, where the W layer was used as a cap layer. The InAs layer had an amorphous structure and a uniform oxide layer was observed at the InAs/GaAs interface. Even after annealing at 800 °C, ohmic behavior was not obtained in this contact because the intervening oxide layer prevented the InAs and GaAs interaction. By adding Ni to the InAs/W contacts (where Ni was deposited by an evaporation method), the interaction between the InAs and the GaAs was enhanced. Nickel interacted with As in the InAs layer and formed NiAs phases after annealing at temperature above 600 °C. The excess In in the InAs layer reacted with the GaAs substrate, forming InxGa1−xAs phases which covered about 80% of the GaAs interface. The Rc values of ∼0.4 Ω mm were obtained for InAs/Ni/W and Ni/InAs/Ni/W contacts at annealing temperatures in the range of 750–850 °C. These contacts contained only high melting point compounds and the contacts were stable during annealing at 400 °C for more than 100 h after ohmic contact formation.
Surface-contamination-related thermal instability in GaAs
Authors: Wang Faa-Ching , Rau Mann-Fu , Liao De-Dui , Carter Ronald
Published: July 2002

The instability of the electrical properties of semi-insulating GaAs materials has been investigated. The presence of chemical contaminants on the surface of the finished wafers is responsible. Simulated thermal conversion using intentional carbon-ion implantation indicates larger concentrations for the possible contaminants on the surface than that in the bulk. The contribution to instability due to preferential gettering of imperfections from bulk by the implant and annealing process was eliminated using intentional Ar-ion implantation. For the samples which exhibited a change in electrical conduction type, a linear relationship between the thickness of the type-converted surface layer and the square root of heat treatment time was observed. It is proposed that the causes for the instability are governed by conventional diffusion mechanisms with multiple electrically active species involved. A positive correlation was also observed between thermally induced electrical instability in ion-implanted material and a reduction in activation, peak carrier density, and Hall mobility.
Ion-induced electrical breakdown in metal-insulator-silicon capacitors
Authors: Milgram Alvin A. , Franco Edward D.
Published: July 2002

Metal-insulator-silicon self-healing capacitors were constructed with thin insulators of either thermal oxide, plasma-enhanced chemical vapor deposition (PECVD) oxide, low-pressure chemical vapor deposition (LPCVD) nitride, PECVD nitride, bilayers of oxide-nitride, or PECVD oxynitrides. Studies were made of the dependence of the ion-induced electrical field breakdown (RIB) on substrate temperature, ion mass, ion energy, and film thickness. For all of the dielectrics, the ion-induced threshold electric field, ET, at breakdown is dependent only upon the coulombic energy loss due to the incident ion for films greater than 70-nm thickness. For decreasing film thickness, both the intrinsic breakdown field and the RIB ET show increasing electric field values. The RIB ET is inversely proportional to the square root of the charge density created by the incident ion. A phenomenological model which agrees with experimental results is developed to explain the observations. The model indicates that the ion-induced breakdown occurs within the bulk of the thin film and is not sensitive to the interface or to the type of conduction mechanism preceding the ion-induced breakdown. With the use of nitride films or oxynitride films, the specific ionization limit is lowered to less than 6 MeV cm2/mg, compared to the previous limit of 14 MeV cm2/mg for oxide films.
The properties of a-C:H films deposited by bias sputtering of carbon
Authors: Zou J. W. , Schmidt K. , Reichelt K. , Dischler B.
Published: July 2002

Carbon films were deposited by rf bias sputtering of a carbon target in argon. Bias voltage and argon pressure were varied systematically. Hardness, internal stress, density, hydrogen content, and infrared absorption were measured as functions of the preparation parameters. From the IR measurements the bond ratio of sp3 to sp2 was calculated and the total amount of hydrogen in the films was determined by elastic recoil detection. The hydrogen content was found to be much lower than for films prepared by plasma decomposition of hydrocarbons. Because the film thickness was in the range of 500 nm, the hardness was measured with an ultralow-load diamond indenter. For these measurements the indentation depth was about 60 nm. The internal stress was determined by the bending beam method. The coordination number, number density, and the filling factor were calculated from the measured values. The results have been compared to those for films deposited by plasma decomposition of hydrocarbons.