bluuebunny/crossref_metadata_2025 huggingface dataset:
A magnetic, neutron diffraction, and Mössbauer spectral study of the Nd2Fe17−xAlx solid solutions
Authors: Long Gary J. , Marasinghe G. K. , Mishra S. , Pringle O. A. , Hu Z. , Yelon W. B. , Middleton D. P. , Buschow K. H. J. , Grandjean F.
Published: July 2002

The magnetic properties of a series of Nd2Fe17−xAlx solid solutions, with x equal to 2.04, 4.01, 5.97, 7.94, and 9.06, have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. Magnetization studies indicate that the Curie temperature increases from 330 K in Nd2Fe17 to a maximum of ∼470 K at an x of 3.5. The compounds crystallize in the Th2Zn17 structure with lattice parameters and unit cell volumes which increase linearly with increasing aluminum content. The neutron diffraction results indicate that aluminum atoms are excluded from the 9d site, prefer the 18h site at low aluminum content, and prefer the 6c and 18f sites at high aluminum content. At 10 K the magnetic moments of the iron and neodymium atoms are collinear and take up a basal orientation at all aluminum contents. The moments decrease with increasing aluminum content and the magnetic moments per unit cell at 10 K are in excellent agreement with the 4.2 K saturation magnetization values. At 295 K the Nd2Fe17−xAlx solid solutions for x equal to 7.94 and 9.06 are paramagnetic. The magnetic Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments. The weighted average isomer shift increases, as expected, with increasing aluminum content as a result of interatomic charge transfer and intraatomic iron 4s–3d charge redistribution. The weighted average maximum hyperfine field at 295 K shows a maximum of 221 kOe at x equal to 2.04 but at 85 K it decreases uniformly with increasing aluminum content. The weighted average decremental field, ΔH, the change in the hyperfine field per aluminum near-neighbor, decreases with increasing aluminum content. It is proposed that, as a consequence of the increase in the average distance between an iron atom and its next near-neighbor shell with increasing aluminum content, the wavelength of the Friedel oscillation increases and the ratio of this wavelength and the shell distance becomes more favorable for ferromagnetic exchange.
Raman and x-ray studies of Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb
Authors: McBride J. R. , Hass K. C. , Poindexter B. D. , Weber W. H.
Published: July 2002

Powdered samples of the type Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb, are synthesized over the range 0≤x≤0.5 starting from nitrate solutions of the rare earths. X-ray diffraction and Raman scattering are used to analyze the samples. These compounds, at least in the low doping regime and for strictly trivalent dopants, form solid solutions that maintain the fluorite structure of CeO2 with a change in lattice constant that is approximately proportional to the dopant ionic radius. The single allowed Raman mode, which occurs at 465 cm−1 in pure CeO2, is observed to shift to lower frequency with increasing doping level for all the rare earths. However, after correcting for the Grüneisen shift from the lattice expansion, the frequency shift is actually positive for all the strictly trivalent ions. In addition, the Raman line broadens and becomes asymmetric with a low frequency tail, and a new broad feature appears in the spectrum at ∼570 cm−1. These changes in the Raman spectrum are attributed to O vacancies, which are introduced into the lattice whenever a trivalent RE is substituted for Ce4+. This conclusion is supported by a simple model calculation of the effects of O vacancies on the Raman spectrum. The model uses a Green’s function technique with the vacancies treated as point defects with zero mass.
Dynamic response of molybdenum shock compressed at 1400 °C
Authors: Duffy Thomas S. , Ahrens Thomas J.
Published: July 2002

Wave profile measurements are reported for pure molybdenum initially heated to 1400 °C and shock compressed to stresses between 12 and 81 GPa. The Hugoniot states are consistent with previous results and all data can be described by the parameters: c0=4.78(2) km/s and s=1.42(2), where the numbers in parentheses are one standard deviation uncertainties in the last digits. The amplitude of the Hugoniot elastic limit is 1.5–1.7 GPa at 1400 °C compared with 25 °C values of 2.3–2.8 GPa. Unloading wave velocities range from 6.30(22) km/s at 12.0 GPa to 7.91(24) km/s at 80.7 GPa and are 4%–8% below extrapolated ultrasonic values and Hugoniot measurements from a room temperature initial state. These differences can be explained by the effect of temperature on the compressional elastic wave velocity. No temperature dependence of the dynamic tensile strength can be resolved from the present data.
Magnetic properties and molecular field theory analysis of RFe10Mo2 alloys
Authors: Xu Xie , Shaheen S. A.
Published: July 2002

The RFe10Mo2 compounds (R=Y, Nd, Gd, Dy) with ThMn12 type structure have been synthesized, and their magnetizations have been investigated in the temperature range from 2 to 800 K using a SQUID magnetometer. The Curie temperature, saturation moments, and measurements, as well as molecular field analysis of the temperature dependence of the magnetization are reported. Our work demonstrates that the molecular field analysis based on a two-sublattice model is capable of quite accurately describing the temperature dependence of the magnetization for the RFe10Mo2 compounds. The results also suggest that the moments of the R and Fe sublattices are nearly collinear in a large temperature range. According to the calculated molecular field coefficients, we find that the magnetic interactions are dominated by exchange between iron 3d electrons, and, on the other hand, the strength of the R–R interaction has even the same order as that of the R–Fe interaction.
Magnetic and structural properties of Fe-FeO bilayers
Authors: Lin X. , Murthy A. S. , Hadjipanayis G. C. , Swann C. , Shah S. I.
Published: July 2002

The structural and magnetic properties of sputtered Fe/Fe-O films were studied by x-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy, transmission electron microscopy, and superconducting quantum interference device. XPS studies showed the presence of FeO and Fe2O3 on the surface of as-made samples. Microstructure studies showed a uniform nanostructure with the grain size in the range of 50–150 Å with smaller grains corresponding to thinner films. The coercivity at 10 K was found to increase substantially with decreasing film thickness below 60 Å. A high Hc (2.7 kOe) was observed in samples with a thickness about 20 Å. Magnetization curves showed a planar anisotropy with a shifted hysteresis loop characteristic of an exchange anisotropy between the Fe and Fe-O coating. The coercivity was found to drop steeply with increasing temperature. This may be attributed to the superparamagnetic behavior of the Fe-O surface layer.
An analytical moderate inversion drain current model for polycrystalline silicon thin-film transistors considering deep and tail states in the grain boundary
Authors: Chen S. S. , Kuo J. B.
Published: July 2002

This paper presents an analytical moderate inversion drain current model for polycrystalline silicon thin-film transistors based on localized deep and tail states in the grain boundary regions. As verified by the published data, using the analytical model, that as compared to the subthreshold region in the bulk iliccon metal-oxide-silicon (MOS) devices, the less steep slope of the moderate inversion region has been explained as due to the lowering in the potential barrier height. In addition, the analytical model provides an accurate prediction that with a smaller average trap state density from the grain boundary regions, the polysilicon thin-film transistor shows a sharper moderate inversion behavior.
Diffusion controlled growth of metallic nanoclusters at selected surface sites
Authors: Francis G. M. , Kuipers L. , Cleaver J. R. A. , Palmer R. E.
Published: July 2002

We have investigated the growth of three-dimensional Ag particles at atomic steps on the surface of highly oriented pyrolytic graphite using a scanning electron microscope. By controlling the growth parameters the cluster growth was confined to the steps avoiding terrace nucleation. In this way quasi-one-dimensional chains of Ag nanoclusters of approximately 10 nm diam were produced. The results suggest the viability of an important new route to the creation of controlled nanoscale structures. A comprehensive surface study indicates that cluster mobility and coalescence play an important role in the growth mechanism on the steps. Evidence was also found that the graphite surface has several different types of surface steps. A quantitative analysis of the cluster distribution on the steps was performed, to investigate the nucleation and growth processes at temperatures from 50 to 205 °C.
Comparative Raman studies of cubic and hexagonal GaN epitaxial layers
Authors: Tabata A. , Enderlein R. , Leite J. R. , da Silva S. W. , Galzerani J. C. , Schikora D. , Kloidt M. , Lischka K.
Published: July 2002

Hexagonal and cubic GaN layers are grown on (001) GaAs substrates by means of molecular beam epitaxy. First order Raman spectra are taken from these layers at various incident laser wavelengths and temperatures. The T2 transverse-optical (TO) and longitudinal-optical (LO) frequencies of cubic GaN are determined, as well as the A1 TO and LO, E1 TO, and E2 frequencies of hexagonal GaN. The T2 TO frequency of cubic GaN lies between the A1 and E1 TO frequencies of hexagonal GaN as one expects comparing the lattice dynamics of zincblende and wurtzite type crystals. The T2 TO frequency is close to the calculated value but disagrees with a recently reported experimental value. For the hexagonal layer, all frequencies are close to those previously measured. A broad Raman structure below the A1 LO peak is interpreted in terms of a disturbed long range order of the hexagonal layer.
Structural and elastic behavior of Fe50Al50 nanocrystalline alloys
Authors: Bonetti E. , Scipione G. , Frattini R. , Enzo S. , Schiffini L.
Published: July 2002

Pure iron and aluminum powders were mixed in the equiatomic ratio and mechanically alloyed in a high-energy ball mill for different times. Structure refinement of x-ray powder diffraction data was performed to study the structural transformations induced by mechanical and subsequent thermal annealing treatments. The mechanical alloying (MA) process induces a progressive dissolution of aluminum phase into the bcc iron phase. After 32 h of MA a single-phase Fe(Al) bcc extended solid solution, with lattice parameter a0=2.891 Å, average coherent domain size 〈D〉≊50 Å, and lattice strain 0.5%, was observed. The annealing of the specimens after MA up to 8 h favored the aluminum dissolution in α-iron and the precipitation of the Al5Fe2 phase, whereas a nanostructured B2 FeAl intermetallic compound was observed in the annealed samples which were previously milled for 8, 16, and 32 h. In the same specimens a minority cubic phase Fe3AlCX, anti-isomorphous with perovskite, derived from contamination of ethanol and introduced in the steel vial as a lubricant agent, was also observed. Anelasticity measurements have shown the occurrence of two main transient effects during the first thermal run. The first one occurring at 500 K in all mechanically alloyed specimens was attributed to thermally activated structural transformations, whereas the second at about 700 K was attributed to a magnetic order–disorder transition. During the second run of anelasticity measurements a relaxation peak P1 in the nanostructured B2 FeAl intermetallic compound, attributed to grain-boundary sliding mechanisms and with an activation energy of 1.8±0.2 eV was observed. In specimens milled for 8–32 h a second small peak P2 at the low-temperature tail of the P1 peak was observed and tentatively attributed to a Zener-type relaxation.
Study of InxGa1−xAs/InAsyP1−y structures lattice mismatched to InP substrates
Authors: Kim Dong-Su , Forrest Stephen R. , Lange Michael J. , Cohen Marshall J. , Olsen Gregory H. , Menna Ray J. , Paff Richard J.
Published: July 2002

Material properties of several lattice-mismatched InxGa1−xAs/InAsyP1−y (0.66<x<0.84, 0.29<y<0.66) alloys grown on InP substrates are investigated. The lattice constants and compositions were measured using x-ray diffraction and electron probe microanalysis. Photoluminescence, white light transmission, and detector cutoff wavelengths were used to determine the band gap of InxGa1−xAs as a function of In concentration x. The three methods agree to within 3%. The quality of the grown layers was also investigated using these techniques, in addition to cross-section transmission electron microscopy and Nomarsky optical microscopy. The dependence of the experimental measurements of band gap and lattice constant on material composition, measured by electron probe microanalysis, was compared against theoretical values.
Composite materials for Ericsson-like magnetic refrigeration cycle
Authors: Smaı̈li A. , Chahine R.
Published: July 2002

The ideal magnetic Ericsson refrigeration cycle should have a constant induced magnetic entropy change as a function of temperature over the whole refrigeration range. To realize this condition using composite materials, a numerical method has been developed to determine the optimum proportions of the component materials in such refrigerants. This paper investigates the effects of increasing the number (n) of the components on the constancy of the magnetic entropy change of the composite (ΔScom), and suggests some new composite refrigerants. For this purpose, the Gd1−x–Dyx (with x=0, 0.12, 0.28, 0.49, and 0.70) alloys, have been used. The values of ΔS have been calculated both from mean-field theory as well as from experimental magnetization curves of these alloys, in the 0.1–7 T magnetic field range and the 200–300 K temperature range. Two sets of composite materials have then been proposed as refrigerants, operating, respectively, over the temperature range 240–290 K and 210–290 K. The ΔS data of the individual Gd–Dy alloys were then used to calculate the optimum mass ratio of the composites. The resultant ΔScom is practically constant in the required temperature range and amounts to 8.0 and 7.3 J/kg K for the two respective sets. The results show that a subsequent increase of n can improve the constancy of the value of ΔScom, and, hence, the corresponding refrigerant should operate more efficiently. Thus, it is found that the appropriate values of n are 3 and 4, respectively, for the first and the second set.
Combined texture and structure analysis of deformed limestone from time-of-flight neutron diffraction spectra
Authors: Lutterotti L. , Matthies S. , Wenk H.-R. , Schultz A. S. , Richardson J. W.
Published: July 2002

The orientation distribution of a textured polycrystalline material has been traditionally determined from a few individual pole figures of lattice planes hkl, measured by x-ray or neutron diffraction. A new method is demonstrated that uses the whole diffraction spectrum, rather than extracted peak intensities, by combining the orientation distribution calculation with the crystallographic Rietveld method. The feasibility of the method is illustrated with time-of-flight neutron diffraction data of experimentally deformed polycrystalline calcite. It is possible to obtain quantitative information on texture, crystal structure, microstructure, and residual stress from highly incomplete pole figures and from regions of the diffraction spectrum containing many overlapping peaks. The approach provides a key for quantitative texture analysis of low symmetry compounds and of composites with complicated diffraction spectra.
Delayed failure in shocked silicon carbide
Authors: Bourne Neil , Millett Jeremy , Pickup Ian
Published: July 2002

Plate impact and split Hopkinson pressure bar (SHPB) experiments have been conducted on three grades of silicon carbide produced by different routes. Data are presented which indicate that the failure of the materials was delayed for some time after the maximum stress had been achieved. In particular, the measured lateral component of the stress in plate impact was found to increase across a front which traveled behind the shock. This phenomenon is akin to the failure wave which has been observed to occur in glasses but has not previously been reported in polycrystalline materials. Hopkinson bar experiments have revealed significant differences in the behaviors between the three materials. These may be related to the effects observed in the plate impact experiments. These results explain the anomalous ballistic phenomena that have been reported for the penetration behavior of SiC. Additionally the Hugoniot elastic limit (HEL) and shear strength were found to vary with the production route used.
Stabilization of the photoluminescence from porous silicon: The competition between photoluminescence and dissolution
Authors: Dudel Frank P. , Gole James L.
Published: July 2002

Two experiments are reported which demonstrate the means to stabilize the photoluminescence from an electrochemically etched (100) Si surface (porous silicon). The strong stabilizing influence of small quantities of water on a nonaqueous [2 mol/ℓ HF in methylcyanide (MeCN)] etching process and the stabilizing effect of hydrochloric acid on a porous silicon surface photoluminescing in solution are described. Mechanisms consistent with these observations are considered as they are commensurate with a silicon oxyhydride emitting species.
Microwave absorption in single crystals of lanthanum aluminate
Authors: Zuccaro Claudio , Winter Michael , Klein Norbert , Urban Knut
Published: July 2002

A very sensitive dielectric resonator technique is employed to measure loss tangent tan δ and relative permittivity εr of lanthanum aluminate (LaAlO3) single crystals at 4–300 K and 4–12 GHz. A variety of single crystals grown by different techniques and purchased from different suppliers are considered. For T>150 K the loss tangent tan δ is almost sample independent with linear frequency dependence and monotonous temperature variation from 8×10−6 at 300 K to 2.5×10−6 at 150 K and 4.1 GHz. In this temperature range the experimental data are explained by a model based on lifetime broadened two-phonon difference processes. The loss tangent below 150 K is characterized by a peak in tan δ(T) at about 70 K. The height of this peak is frequency and strongly sample dependent. This leads to a variation of the loss tangent from 10−6 to 1.5×10−5 at 77 K and 8.6 GHz, the lowest values are generally achieved with Verneuil grown crystals and approach the intrinsic lower limit predicted by the phonon model. The peak is explained by defect dipole relaxation (local motions of ions). The activation energy of the relaxation process is determined from the measured data to be 31 meV. This low value indicates that the defect dipoles are associated with interstitials, possibly impurities in interstitial positions. Considering absorption due to phonons and due to defect dipole relaxation the loss tangent is calculated for a wide frequency range.
Distinction between bulk and interface states in CuInSe2/CdS/ZnO by space charge spectroscopy
Authors: Herberholz R. , Igalson M. , Schock H. W.
Published: July 2002

We present a detailed study of admittance spectroscopy and deep level transient spectroscopy on CuInSe2/CdS/ZnO thin film solar cells. The admittance spectra reveal an emission from a distribution of hole traps centered at an activation energy of 280 meV and a shallower level with a sharp activation energy of ∼ 120 meV. After repetitive annealing of the device in air at 200 °C, the activation energy of the latter level increases continuously from 120 to 240 meV, while the 280 meV hole traps remain unaffected. Deep level transient spectroscopy with optical excitation reveals an emission of minority carriers with time constants comparable to those observed for the shallow level in admittance spectroscopy. The shift of the activation energy after annealing also occurs in deep level transient spectroscopy and ascertains that the emissions observed in both techniques have the same origin. The magnitude and continuous shift of the activation energy of the minority carrier emission indicates a distribution of levels in the vicinity of the CdS/CuInSe2 heterointerface. In the case of interface states, the activation energy deduced from admittance spectroscopy corresponds to the position of the electron quasi-Fermi level at the interface, pointing to an inversion of the carrier type at the absorber surface. Measurements with an applied dc bias indicate that the electron Fermi level is pinned at the interface.
Effect of surface roughness on magnetic properties of Co films on plasma-etched Si(100) substrates
Authors: Li M. , Wang G.-C. , Min H.-G.
Published: July 2002

Magnetic and morphological properties of ∼970-Å-thick Co films, deposited simultaneously on ten plasma-etched Si(100) substrates, were measured through the magneto-optic Kerr effect (MOKE) technique, ferromagnetic resonance (FMR), magnetic force microscopy (MFM), and atomic force microscopy. As the etch time t increased from 0 to 100 min, the vertical interface width w of Co films increased from ∼5 to ∼1400 Å; the lateral correlation length ξ, from ∼300 to ∼10 500 Å. The MOKE and FMR measurements gave the in-plane azimuthal angular dependence of the hysteresis loops and the ferromagnetic resonance absorption spectra, respectively. From MOKE and FMR, the smoother films showed uniaxial magnetic anisotropy (t⩽40 min). The uniaxial anisotropy decreased with the increase of the surface roughness and disappeared for the roughest films (t=60 and 100 min). The MOKE hysteresis loop measurements suggested that, with the increasing surface roughness, the magnetization reversal changed gradually from magnetization rotation dominated for the smoothest films to domain-wall motion dominated for the roughest films. The MFM images of the films showed anisotropic magnetic domain contrasts for the smooth film, and the subsequent fragmentation of these domains as the roughness parameters increased. This supports the results of the MOKE and FMR measurements.
Chemistry of naturally layered manganites (invited)
Authors: Battle P. D. , Kasmir N. , Millburn J. E. , Rosseinsky M. J. , Patel R. T. , Spring L. E. , Vente J. F. , Blundell S. J. , Hayes W. , Klehe A. K. , Mihut A. , Singleton J.
Published: July 2002

Experiments on three double-layer (n=2) Ruddlesden–Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (Tc=126 K) with Nd to form Sr1.8La1.2−xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x=0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x⩾0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0<P/GPa⩽1.8) is shown to modify the magnetotransport properties of Sr2NdMn2O7 to resemble those of Sr1.9Nd1.1Mn2O7. The changes can be explained by considering the relative strength of ferromagnetic and antiferromagnetic interactions within the material. Finally, the need for careful phase analysis of n=2 RP materials is demonstrated by the misleading magnetization data recorded for a sample of Sr1.8Sm1.2Mn2O7 containing ∼2.8% of an n=∞ perovskite phase.
Nonlinear acoustic response from precipitate-matrix misfit in a dislocation network
Authors: Cantrell John H. , Zhang X.-G.
Published: July 2002

An analytical model is presented for the effects on the acoustic nonlinearity of the interaction of matrix dislocations with precipitate coherency strains in metallic alloys. The acoustic nonlinearity parameters are shown to depend linearly on the dislocation density, volume fraction of precipitates and precipitate-matrix misfit parameters, and on the fourth power of the dislocation loop length. The model predicts changes in the acoustic nonlinearity parameter associated with the growth of precipitates during the artificial aging of aluminum alloy 2024 that are in good agreement with experimental measurements reported in the literature.
Characterization and optimization of the detection sensitivity of an atomic force microscope for small cantilevers
Authors: Schäffer Tilman E. , Hansma Paul K.
Published: July 2002

The detection sensitivity of an atomic force microscope with optical beam deflection for small cantilevers is characterized experimentally and theoretically. An adjustable aperture is used to optimize the detection sensitivity for cantilevers of different length. With the aperture, the signal-to-noise ratio of cantilever deflection measurements is increased by a factor of 1.5 to nearly 3. A theoretical model is set up that generally describes the optical beam deflection detection in an atomic force microscope. This model is based on diffraction theory and includes the particular functional shape of the cantilever.
Breakdown and destruction of heterogeneous solid dielectrics by high voltage pulses
Authors: Lisitsyn I. V. , Inoue H. , Nishizawa I. , Katsuki S. , Akiyama H.
Published: July 2002

A mechanism explaining the breakdown of dielectrics with high intrinsic inhomogeneity like natural rocks and concrete is proposed and proved experimentally. This work has a very promising industrial application in the drilling and demolition of natural and artificial solid materials by electric pulses. The mechanism includes the breakdown of gas cavities inside the dielectric and on its surface. At a very high applied voltage, the high electric field causes the breakdown in the cavities. The displacement and conduction currents flowing through a number of such cavities result in the heating of the plasma and high pressure pulse generation. The pulsed pressure results in crack formation and finally, in the destruction of a solid material.
Carrier recombination at silicon–silicon nitride interfaces fabricated by plasma-enhanced chemical vapor deposition
Authors: Schmidt Jan , Aberle Armin G.
Published: July 2002

Using the light-biased microwave-detected photoconductance decay method, injection level dependent measurements of the effective surface recombination velocity Seff at silicon surfaces passivated by plasma-enhanced chemical vapor deposited (PECVD) silicon nitride (SiNx) films are performed on monocrystalline silicon wafers of different resistivities and doping types. In order to theoretically simulate the measured dependences of Seff on the bulk injection level Δn, the extended Shockley-Read-Hall formalism is used. Simulation input parameters are the energy dependent interface state densities and capture cross sections of the involved interface defects as well as the positive insulator charge density Qf. The energy dependent properties of the interface defects are experimentally determined by means of small-pulse deep-level transient spectroscopy. These measurements reveal the existence of three “deep” silicon dangling bond defects at the Si-SiNx interface with similar interface state densities but very different capture cross sections and hence recombination rates. Another defect is found very close (⩽0.1 eV) to the edge of the silicon conduction band. This defect is identified with the K+ center which is responsible for the large positive Qf values (∼1012 cm−2) at Si-SiNx interfaces obtained from standard dark capacitance-voltage measurements. In order to get a good agreement between measured and calculated Seff(Δn) dependences, a reduction of Qf by one order of magnitude is found to be necessary. The explanation for this reduction is the capture of electrons from the silicon conduction band into the K+ centers. The comparison of Si-SiNx interfaces fabricated by different PECVD techniques shows that the dominant interface defect is produced by the ion bombardment during the SiNx deposition. Thus, avoidance of the ion bombardment leads to a strongly reduced interface recombination and hence a better surface passivation quality.
Magnetic properties of HITPERM (Fe, Co)88Zr7B4Cu1 magnets
Authors: Willard M. A. , Huang M.-Q. , Laughlin D. E. , McHenry M. E. , Cross J. O. , Harris V. G. , Franchetti C.
Published: July 2002

A new class of nanocrystalline alloys with composition Fe44Co44Zr7B4Cu1 has been developed. This and similar alloys of general composition (Fe, Co)–M–B–Cu (where M=Zr, Hf, Nb, etc.) have been named HITPERM. They offer large magnetic inductions and excellent soft magnetic properties at elevated temperatures. Thermomagnetic properties, permeability, and frequency dependent losses are described in this report. These alloys exhibit high magnetization that persists to the α→γ phase transformation at 980 °C. Alternating current permeability experiments reveal a high permeability at 2 kHz with a loss value of 1 W/g at Bs=10 kG and f=10 kHz.
Electronic structure, exchange interactions, and Curie temperature of FeCo
Authors: MacLaren J. M. , Schulthess T. C. , Butler W. H. , Sutton Roberta , McHenry Michael
Published: July 2002

Fe–Co alloys in the α phase are soft magnetic materials which have high saturation inductions over a wide range of compositions. However, above about 1250 K, an α to γ phase transition occurs. The fcc-based, γ, high-temperature phase is paramagnetic at this temperature. In this work the low-temperature ordered B2, or α′, phase, as well as the disordered bcc phase of FeCo alloys, have been studied with first-principles electronic-structure calculations using the layer Korringa–Kohn–Rostoker method. The variation of moment with composition (Slater–Pauling curve) is discussed. For equiatomic FeCo, interatomic exchange couplings are derived from first principles. These exchange interactions are compared to those obtained for pure Fe and Co, and are used within a mean-field theory to estimate the hypothetical Curie temperature of the α phase.
Characterizing interactions in fine magnetic particle systems using first order reversal curves
Authors: Pike Christopher R. , Roberts Andrew P. , Verosub Kenneth L.
Published: July 2002

We demonstrate a powerful and practical method of characterizing interactions in fine magnetic particle systems utilizing a class of hysteresis curves known as first order reversal curves. This method is tested on samples of highly dispersed magnetic particles, where it leads to a more detailed understanding of interactions than has previously been possible. In a quantitative comparison between this method and the δM method, which is based on the Wohlfarth relation, our method provides a more precise measure of the strength of the interactions. Our method also has the advantage that it can be used to decouple the effects of the mean interaction field from the effects of local interaction field variance.
Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices (invited)
Authors: Sun Shouheng , Murray C. B.
Published: July 2002

High temperature, solution phase reduction of cobalt chloride in the presence of stabilizing agents was employed to produce magnetic colloids (ferrofluids) of cobalt nanocrystals. We systematically synthesized and isolated nearly monodisperse nanocrystal samples ranging in size from 2 to 11 nm while maintaining better than a 7% std. dev. in diameter. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (ε-Co). Annealing the nanocrystals at 300 °C converts them quantitatively to the more common hexagonal-close-packed crystal form. Deposition of these uniform cobalt particles on solid substrates by evaporation of the carrier solvent results in the spontaneous assembly of two-dimensional and three-dimensional magnetic superlattices (colloidal crystals). A combination of x-ray powder diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry were used to characterize both the dispersed nanocrystals and the assembled superlattices.
Hydrogen role on the properties of amorphous silicon nitride
Authors: de Brito Mota F. , Justo J. F. , Fazzio A.
Published: July 2002

We have developed an interatomic potential to investigate structural properties of hydrogenated amorphous silicon nitride. The interatomic potential used the Tersoff functional form to describe the Si–Si, Si–N, Si–H, N–H, and H–H interactions. The fitting parameters for all these interactions were found with a set of ab initio and experimental results of the silicon nitride crystalline phase, and of molecules involving hydrogen. We investigated the structural properties of unhydrogenated and hydrogenated amorphous silicon nitride through Monte Carlo simulations. The results show that depending on the nitrogen content, hydrogen has a different chemical preference to bind to either nitrogen or silicon, which is corroborated by experimental findings. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material, and consequently the concentration of dangling bonds.
Role of oxygen vacancy defect states in the n-type conduction of β-Ga2O3
Authors: Hajnal Zoltán , Miró József , Kiss Gábor , Réti Ferenc , Deák Péter , Herndon Roy C. , Kuperberg J. Michael
Published: July 2002

Based on semiempirical quantum-chemical calculations, the electronic band structure of β-Ga2O3 is presented and the formation and properties of oxygen vacancies are analyzed. The equilibrium geometries and formation energies of neutral and doubly ionized vacancies were calculated. Using the calculated donor level positions of the vacancies, the high temperature n-type conduction is explained. The vacancy concentration is obtained by fitting to the experimental resistivity and electron mobility.
Dielectric loss anomaly of BaBiO3
Authors: Lee Seung-Hoon , Jung Woo-Hwan , Sohn Jeong-Ho , Lee Joon-Hyung , Cho Sang-Hee
Published: July 2002

Dielectric properties of oxygen deficient BaBiO3−δ have been analyzed, and dielectric loss anomalies at 175 and 225 K were found, which were dependent on oxygen deficiency. The activation energies corresponding to the relaxation processes of dielectric anomalies at 175 and 225 K were 0.26 and 0.50 eV, respectively. The dielectric anomaly at 175 K is believed to be caused by hole movement from an occupied Bi 6s band to hole polaron accommodating band state. Another anomaly at 225 K is believed to occur from the hopping of a hole between a Bi 6s band and the Fermi level.
Electrical, optical, and structural properties of indium–tin–oxide thin films for organic light-emitting devices
Authors: Kim H. , Gilmore C. M. , Piqué A. , Horwitz J. S. , Mattoussi H. , Murata H. , Kafafi Z. H. , Chrisey D. B.
Published: July 2002

High-quality indium–tin–oxide (ITO) thin films (200–850 nm) have been grown by pulsed laser deposition (PLD) on glass substrates without a postdeposition annealing treatment. The structural, electrical, and optical properties of these films have been investigated as a function of target composition, substrate deposition temperature, background gas pressure, and film thickness. Films were deposited from various target compositions ranging from 0 to 15 wt % of SnO2 content. The optimum target composition for high conductivity was 5 wt % SnO2+95 wt % In2O3. Films were deposited at substrate temperatures ranging from room temperature to 300 °C in O2 partial pressures ranging from 1 to 100 mTorr. Films were deposited using a KrF excimer laser (248 nm, 30 ns full width at half maximum) at a fluence of 2 J/cm2. For a 150-nm-thick ITO film grown at room temperature in an oxygen pressure of 10 mTorr, the resistivity was 4×10−4 Ω cm and the average transmission in the visible range (400–700 nm) was 85%. For a 170-nm-thick ITO film deposited at 300 °C in 10 mTorr of oxygen, the resistivity was 2×10−4 Ω cm and the average transmission in the visible range was 92%. The Hall mobility and carrier density for a 150-nm-thick film deposited at 300 °C were 27 cm2/V s and 1.4×1021 cm−3, respectively. A reduction in the refractive index for ITO films can be achieved by raising the electron density in the films, which can be obtained by increasing the concentration of Sn dopants in the targets and/or increasing deposition temperature. Atomic force microscopy measurements of these ITO films indicated that their root-mean-square surface roughness (∼5 Å) was superior to that of commercially available sputter deposited ITO films (∼40 Å). The PLD ITO films were used to fabricate organic light-emitting diodes. From these structures the electroluminescence was measured and an external quantum efficiency of 1.5% was calculated.
Deuterium diffusion through hexagonal boron nitride thin films
Authors: Checchetto Riccardo , Miotello Antonio
Published: July 2002

We evaluated the deuterium diffusion coefficient in hexagonal boron nitride (h-BN) thin films deposited by radio frequency magnetron sputtering on metallic substrate. The measurements were carried out by studying the transient of the deuterium permeation flux through substrates coated with h-BN thin films 400 nm thick, for temperatures ranging from 535 to 752 K. The deuterium diffusion coefficient was in the range between 1.4×10−13 and 5.3×10−12 cm2/s and was characterized by an activation energy of 0.52±0.04 eV and pre-exponential factor of the order of 10−8 cm2/s. In steady-state transport conditions the deuterium concentration in the h-BN layers was close to 3×1021 at./cm3. Starting from the earlier data we suggest a model in which the deuterium migration process is controlled by diffusion of D atoms in the volume fraction of the h-BN films relative to grain boundaries, just in connection with the structure of the deposited samples which consists of nanocrystals with 2 nm average diameter.
Microcontact impedance measurements of individual highly resistive grain boundaries: General aspects and application to acceptor-doped SrTiO3
Authors: Fleig J. , Rodewald S. , Maier J.
Published: July 2002

The electrical properties of grain boundaries in polycrystalline materials can vary from boundary to boundary and conventional impedance measurements yield only averaged information. Local measurements at individual grain boundaries using microelectrodes can partly overcome this restriction. Finite element calculations on the impedance of such microelectrode measurements are performed to reveal the interrelations between the measured grain boundary impedance and the grain boundary properties (grain boundary resistivity, permittivity and geometry). We will discuss how far adjacent grain boundaries influence the results. In particular, the calculations revealed that even in the case of identical grain boundaries a considerable part of the current between two microelectrodes may flow across neighboring grain boundaries. It is shown how these additional current paths can be taken into account with respect to a reliable data analysis. Experiments on SrTiO3 polycrystals demonstrate that spatially resolved impedance investigations on single grain boundaries are possible in this way. The results are interpreted in terms of a double Schottky barrier at the grain boundaries.
Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements
Authors: Friedrich I. , Weidenhof V. , Njoroge W. , Franz P. , Wuttig M.
Published: July 2002

Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputtered Ge2Sb2Te5 films. The pronounced changes of film resistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the Ge2Sb2Te5 films undergo a structural change from an amorphous to rock salt structure (Fm3m) around 140 °C and finally a hexagonal structure (p3̄m) around 310 °C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method [Anal. Chem. 29, 1702 (1957)] the activation energy for crystallization to the rock salt structure is determined to be 2.24±0.11 eV, and for the phase transformation to the hexagonal phase to be 3.64±0.19 eV, respectively. A thin capping layer of ZnS–SiO2 leads to an increase of the first transition temperature as well as of the corresponding activation energy (2.7±0.2 eV).
Thermal expansion coefficient of polycrystalline silicon and silicon dioxide thin films at high temperatures
Authors: Tada Haruna , Kumpel Amy E. , Lathrop Richard E. , Slanina John B. , Nieva Patricia , Zavracky Paul , Miaoulis Ioannis N. , Wong Peter Y.
Published: July 2002

The rapid growth of microelectromechanical systems (MEMS) industry has introduced a need for the characterization of thin film properties at all temperatures encountered during fabrication and application of the devices. A technique was developed to use MEMS test structures for the determination of the difference in thermal expansion coefficients (α) between poly-Si and SiO2 thin films at high temperatures. The test structure consists of multilayered cantilever beams, fabricated using standard photolithography techniques. An apparatus was developed to measure the thermally induced curvature of beams at high temperatures using imaging techniques. The curvatures measured were compared to the numerical model for multilayered beam curvature. The model accounts for the variation in thermomechanical properties with temperature. The beams were designed so that the values of Young’s moduli had negligible effect on beam curvature; therefore, values from literature were used for ESi and ESiO2 without introducing significant error in curvature analysis. Applying this approximation, the difference in thermal expansion coefficients between αSi and αSiO2 was found to increase from 2.9×10−6 to 5.8×10−6 °C−1 between room temperature and 900 °C. These results suggest that the α for poly-Si thin films may be significantly higher than values for bulk, crystalline Si.
Complex permittivity, permeability, and X-band microwave absorption of CaCoTi ferrite composites
Authors: Singh Praveen , Babbar V. K. , Razdan Archana , Puri R. K. , Goel T. C.
Published: July 2002

The effect of Co2+Ti4+ substitution on complex permeability, permittivity, and microwave absorption has been studied for [Ca(CoTi)xFe12−2xO19]96.0[La2O3]4.0 ferrite-epoxy composites, wherein x varies from 0 to 1.0 in steps of 0.2, in the frequency range from 8.0 to 12.4 GHz. The ferrites with x>0 exhibit significant dispersion in complex permittivity (ε′−jε″) with the maximum value of ε″ observed for x equal to 0.2. The dispersion in complex permeability (μ′−jμ″) is not significant. The variations of reflection loss and percentage absorption have been studied as a function of frequency, Co2+Ti4+ content, and thickness of the absorber. A maximum reflection loss of 31.0 dB is obtained for composites with x equal to 0.2 and absorber thickness of 2.5 mm. The experimental values of the matching frequency and the matching thickness agree well with the theoretical values obtained from an impedance-matching solution map.
Interaction of Pb defects at the (111)Si/SiO2 interface with molecular hydrogen: Simultaneous action of passivation and dissociation
Authors: Stesmans A.
Published: July 2002

The simultaneous action of passivation and dissociation during thermochemical interaction of trivalent interfacial Si traps (Pb’s;Si3≡Si⋅) with molecular hydrogen has been analyzed. A unified description is attained through solution of the simultaneous set of the first-order rate equations describing passivation and dissociation, under the restriction that the H2 concentrations at the interface and in the ambient are continuously equal. The analysis is given allowance by the recently attained physically consistent pictures for each of the separate steps of passivation in H2 and dissociation in vacuum, incorporating the existence of distinct spreads σEf and σEd in the respective activation energies. The assessment of heat treatment in H2 shows that, as compared to the fictitious case σEf=σEd=0, the effect of the existence of the spreads, for usual anneal times of 10–60 min in 1 atm H2, is to reduce the passivation efficiency by two orders of magnitude, while enhancing the optimum anneal temperature Tan from ∼330–360 to the range 400–430 °C—the latter being commonly used. The optimum anneal time–Tan curve is established. The analysis, and as experimentally verified, shows that the Pb passivation level is not decreased (Pb regenerated) by successive annealings at successively lower Tan, in contrast with previous reports on annealing of electrically detected interface traps in atomic H. The results are discussed within technological context. A general inference is that Pb may be readily optimally passivated (in 1 atm H2) to sub-1-ppm levels, rendering negligible their role in the typically attained residual interface trap densities of (2–10)×109 cm−2; at these levels, the interface traps left must be of different type, as concluded previously. During passivation in H2, the Pb system appears as an efficient atomic H mill.
Exchange biasing by Ir19Mn81: Dependence on temperature, microstructure and antiferromagnetic layer thickness
Authors: van Driel J. , de Boer F. R. , Lenssen K.-M. H. , Coehoorn R.
Published: July 2002

We have investigated the thermal stability of the exchange biasing interaction in antiferromagnetic/ferromagnetic bilayers of Ir19Mn81 and Ni80Fe20 or Co90Fe10. The exchange-biasing field and the coercive field were found to depend strongly on the thickness of the Ir19Mn81 layer and on the crystallographic texture of the bilayer. The exchange-biasing field at room temperature has a maximum at 4 nm Ir19Mn81 layer thickness and then decreases for increasing Ir19Mn81 layer thickness. This coincides quite well with the thickness dependence of the (111) texture measured in Ir19Mn81 layers. Removing the Ta seed layer resulted in the disappearance of the (111) texture and at the same time the exchange-biasing field decreased for all Ir19Mn81 layer thicknesses. For films with a Ta seed layer and 8 nm Ir19Mn81 or more, we have found a blocking temperature of 560 K, decreasing with decreasing Ir19Mn81 layer thickness to around room temperature for 2 nm Ir19Mn81. Removing the Ta seed layer resulted in a decrease of the blocking temperature for all layer thicknesses investigated. We have concluded that the (111) texture is an important factor governing the size of the exchange-biasing field and the thermal stability.
Microscopic mechanisms of laser ablation of organic solids in the thermal and stress confinement irradiation regimes
Authors: Zhigilei Leonid V. , Garrison Barbara J.
Published: July 2002

The results of large-scale molecular dynamics simulations demonstrate that the mechanisms responsible for material ejection as well as most of the parameters of the ejection process have a strong dependence on the rate of the laser energy deposition. For longer laser pulses, in the regime of thermal confinement, a phase explosion of the overheated material is responsible for the collective material ejection at laser fluences above the ablation threshold. This phase explosion leads to a homogeneous decomposition of the expanding plume into a mixture of liquid droplets and gas phase molecules. The decomposition proceeds through the formation of a transient structure of interconnected liquid clusters and individual molecules and leads to the fast cooling of the ejected plume. For shorter laser pulses, in the regime of stress confinement, a lower threshold fluence for the onset of ablation is observed and attributed to photomechanical effects driven by the relaxation of the laser-induced pressure. Larger and more numerous clusters with higher ejection velocities are produced in the regime of stress confinement as compared to the regime of thermal confinement. For monomer molecules, the ejection in the stress confinement regime results in broader velocity distributions in the direction normal to the irradiated surface, higher maximum velocities, and stronger forward peaking of the angular distributions. The acoustic waves propagating from the absorption region are much stronger in the regime of stress confinement and the wave profiles can be related to the ejection mechanisms.