bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and magnetic properties of arc-melted Sm2(Fe1−xCox)14Ga3C2 compounds
Authors: Shen Bao-gen , Kong Lin-shu , Wang Fang-wei , Cao Lei , Liang Bing , Cheng Zhao-hua , Gong Hua-yang , Guo Hui-qun , Zhan Wen-shan
Published: July 2002

The effects of the substitution of Co for Fe on the formation, structure, and magnetic properties of Sm2Fe14Ga3C2 compounds were studied. Alloys with composition Sm2(Fe1−xCox)14Ga3C2 (x=0, 0.1, 0.2, 0.3, 0.4, and 0.5) were prepared by arc melting. X-ray diffraction shows that these alloys are single phase compounds of the rhombohedral Th2Zn17-type structure. The lattice constants a and c, and the unit cell volumes v of Sm2(Fe1−xCox)14Ga3C2 compounds decrease monotonically with increasing cobalt concentration. It is found that the Curie temperature increases from 615 K for x=0 to 666 K for x=0.5. Room-temperature saturation magnetization is 90.3 emu/g for x=0, and it decreases to 79.6 emu/g at x=0.5. All compounds of Sm2(Fe1−xCox)14Ga3C2 studied in this work exhibit an easy c-axis anisotropy at room temperature. The anisotropy field is higher than 90 kOe for x≤0.2, and it decreases slightly with x≥0.3.
Neutron diffraction and Mössbauer effect study of the structure of DySixFe11−xCoN alloys
Authors: Pan Shu-ming , Chen Hong , Liu Den-ke , Xu Zu-xiong , Ma Ru-zhang , Yang Ji-lian , Zhang Bai-sheng , Xue De-yan , Ni Qiang
Published: July 2002

DySixFe11−xCo (x=0.5, 1.0, and 1.5) alloys and their nitrides are studied by x-ray diffraction, Mössbauer effect and neutron diffraction experiments. The results show that both Si and Co atoms can occupy the 8f and 8j sites, and more preferentially occupy the 8f sites. Nitrogen atoms as interstitial atoms enter into the 2b site. The nitrides have high Curie temperatures. The results of Mössbauer spectroscopy indicate that nitriding increases the hyperfine fields of all Fe sites.
Magnetic properties of amorphous nanocolumns created by heavy ion irradiation of paramagnetic YCo2 thin films (invited)
Authors: Givord D. , Nozières J. P. , Ghidini M. , Gervais B. , Otani Y.
Published: July 2002

Recent studies have demonstrated the possibility of inducing amorphous latent tracks in metallic materials by GeV heavy ion irradiation. In the present work, 0.880 GeV 238U beams have been used to induce ferromagnetic amorphous nanocolumns in nonmagnetic crystalline YCo2 films. The loss of crystallinity deduced by x-ray analysis is in good agreement with the one determined by magnetic measurements. The Co magnetic moment in the columns is approximately equal to 1 μB, as in bulk amorphous YCo2, but the ordering temperature (150 K) is strongly reduced with respect to the bulk. Magnetization measurements reveal the nanocolumns to be single domain. A perpendicular anisotropy is observed in samples irradiated at the smallest fluencies, which is interpreted to be due to shape anisotropy. A progressive decrease of the anisotropy with increasing fluence is observed and qualitatively described in terms of dipolar interactions between columns. At low temperature, the coercive field reaches 650 Oe. A simple nonuniform magnetization reversal process is suggested.
Giant magnetoresistance at low fields in [(NixFe1−x)yAg1−y]/Ag multilayers prepared by molecular beam epitaxy (abstract)
Authors: Farrow R. F. C. , Marks R. F. , Cebollada A. , Toney M. F. , Dobbertin D. , Beyers R. , Parkin S. S. P. , Rabedeau T. A.
Published: July 2002

The structural changes that accompany the development of GMR (giant magnetoresistance) at low (≲10 Oe) fields in annealed magnetic multilayers are of current interest because of potential applications of such structures in sensors. In this paper we report a study of the development of GMR in [111]-oriented multilayers comprising ferromagnetic films of a mixture of Ag and permalloy (NixFe1−x, x∼0.8) alternating with Ag spacer films. The multilayers were grown by molecular beam epitaxy (MBE) on Pt(111) seed films on sapphire (0001) substrates at temperatures in the range 20 to 200 °C. The structure of the multilayers was investigated using x-ray diffraction and electron microscopy. For a series of multilayers grown with nominally identical ferromagnetic and spacer layer thicknesses, the magnetoresistance is found to be strongly dependent on both growth temperature and subsequent annealing temperature. The multilayers exhibited a negative magnetoresistance in the as-grown state which more than doubled when the growth temperature was increased from 20 to 100 °C. However, the highest magnetoresistance (peak 5.6%; maximum slope 0.4% per Oe) was obtained by annealing (at 400 °C) multilayers grown at 100 °C. Transmission electron microscopy studies of such multilayers showed no evidence for discontinuities or penetration of the ferromagnetic films by Ag along grain boundaries. Thus, we conclude that discontinuous or granular multilayers with complete phase separation are not necessary for GMR with low saturation fields.
Structure and magnetic anisotropy of Sm2Fe17−xAlxC (x=2–8) compounds prepared by arc melting
Authors: Cheng Zhao-Hua , Shen Bao-Gen , Zhang Jun-Xian , Wang Fang-Wei , Gong Hua-Yang , Zhan Wen-Shan , Zhao Jian-Gao
Published: July 2002

In previous work it was discovered that the 2:17-type rare-earth–iron compounds with high carbon concentration could be formed by the substitution of Ga, Si, or Al, etc., for Fe in R2Fe17Cx. The effect of Al substitution for Fe on the structure and magnetic anisotropy of Sm2Fe17C has been investigated. Alloys with the composition of Sm2Fe17−xAlxC (x=2, 3, 4, 5, 6, 7, and 8) were prepared by arc melting. The carbides are single phase with rhombohedral Th2Zn17-type structure except for Sm2Fe17C which contains a small amount of α-Fe. The addition of Al results in an approximately linear increase in the lattice constants and the unit-cell volumes. The Curie temperature Tc is found to increase slightly when x≤3, then decrease rapidly with increasing Al concentration, while the room-temperature saturation magnetization decreases monotonically with the addition of aluminum. X-ray-diffraction and magnetization measurement studies of magnetic-field-oriented powders demonstrate that the samples with x≤6 exhibit an easy c-axis anisotropy at room temperature and the room-temperature anisotropy field increases from 5.3 T for x=0 to about 11 T for x=2. Further substitution decreases the anisotropy field. For the sample with x=2, the room-temperature anisotropy field is higher than that of Nd2Fe14B, and the saturation magnetization is about 110 emu/g. In this alloy, the substitution of a small amount of other elements, such as Co, Ni, etc., may yield a further improvement in its magnetic properties. Thus, it is possible that these carbides can be used as the starting materials for producing high-performance 2:17-type sintered permanent magnets.
Spin wave spectra in semi-infinite magnetic superlattices with nonuniaxial single-ion anisotropy
Authors: Albuquerque E. L.
Published: July 2002

A microscopic theory, based on the Heisenberg model, is employed to investigate the spin wave spectra in a semi-infinite superlattice made up of two ferromagnetic materials, where the constituents may present both the uniaxial and nonuniaxial components of the single-ion anisotropy. Our calculations are carried out for the exchange-dominated regime, and a transfer-matrix approach is used to simplify the algebra, which otherwise could be quite heavy. The calculated spectra show that, in addition to bulk spin wave modes, there may be surface modes associated with the truncation of the superlattice.
A comparison of the giant magnetoresistance and anisotropic magnetoresistance in Co/Cu sandwich films (abstract)
Authors: Miller B. H. , Chen E. Youjun , Tondra Mark , Dahlberg E. Dan
Published: July 2002

By a systematic variation of structures, recent measurements conclude the scattering associated with the high resistance giant magnetoresistance (GMR) state occurs within 0.25 nm of the magnetic interface. We have accomplished a similar measurement that does not require such stringent control of the sample structure. Instead, the present work uses measurements of the anisotropic magnetoresistance (AMR) in both the high resistance and low resistance GMR states. The samples are sandwiches of Co/Cu/Co/CoO with Co thicknesses ranging from 1 to 10 nm and Cu thickness of approximately 2.5 nm. The AMR is measured with the Co magnetizations aligned parallel to one another (the low resistance GMR state) and with the Co magnetizations aligned antiparallel to one another (the high resistance GMR state). The data show that the AMR in the antiparallel configuration is less than that in the parallel configuration. An analysis that relates the reduced AMR to the magnetic interfacial region giving rise to the high resistance GMR state indicates the scattering occurs within approximately 0.5 nm of the interface.
A Monte Carlo study of the temperature dependence of magnetic order on ferromagnetic and antiferromagnetic surfaces: Implications for spin-polarized photoelectron diffraction (abstract)
Authors: Zhang F. , Thevuthasan S. , Scalettar R. T. , Singh R. R. P. , Fadley C. S.
Published: July 2002

We have used Ising-model Monte Carlo calculations to study the magnetic order near cubic ferromagnetic and antiferromagnetic surfaces. The antiferromagnets were studied with and without frustrated next-nearest-neighbor interactions. Intralayer and interlayer spin-spin correlation functions have been calculated as a function of the relative coupling strength in the surface layer. If this coupling strength is more than a few times the bulk value, a distinct surface phase transition is observed at temperatures TC,surf or TN,surf that can be significantly above the corresponding bulk values, TC,bulk or TN,bulk. These calculations suggest that previous spin-polarized photoelectron diffraction measurements on antiferromagnetic KMnF3(110) and MnO(001) could in fact have been observing such surface transitions at TN,surf values of 2.7 and 4.5 times TN,bulk, respectively.
Circular dichroism in core-level photoemission from nonmagnetic and magnetic systems: A photoelectron diffraction viewpoint (abstract)
Authors: Kaduwela A. P. , Xiao H. , Thevuthasan S. , Westphal C. , Van Hove M. A. , Fadley C. S.
Published: July 2002

Magnetic circular dichroism (MCD) in x-ray absorption represents an exciting new technique for studying and imaging magnetic systems. However, there are to date relatively few studies of dichroism in the inherent process involved: photoelectron emission. We will here illustrate that photoelectron diffraction (PD) theory provides a fruitful way of analyzing dichroism data for both nonmagnetic and magnetic systems. Circular dichroism (CD) has been observed in core-level photoemission from nonmagnetic systems: C 1s from CO/Pd(111) and Si 2p from Si(100). For CO/PD(111), chirality in the experimental geometry is readily discernible, but for Si(100), it is more difficult to define the chirality with simple vector relationships. PD effects implicitly contain all information on such core-level dichroism, and we will present multiple-scattering simulations of the observations to date. We will also discuss the role of such CD effects in core-level MCD measurements, using Fe 2p emission from magnetically aligned Fe(110) as an example. The analysis of such MCD data has so far been qualitative. We will present a more quantitative analysis including final-state effects such as the interference of l±1 photoelectron channels and spin-dependent scattering and diffraction.
Magnetization-related transport anomalies in metal/ferromagnetic insulator heterostructures
Authors: Roesler G. M. , Filipkowski M. E. , Broussard P. R. , Osofsky M. S. , Idzerda Y. U.
Published: July 2002

Magnetoresistance measurements have been performed on epitaxial metal/ferromagnetic insulator bilayers. They are more sensitive to magnetic behavior at the interface of such structures than magnetization measurements. It is clear from the magnetoresistance data that previously reported slope discontinuities in the resistance versus temperature of such heterostructures are magnetic in origin. These studies demonstrate that the conduction electrons of the metal are coupled to the spins in the magnetic insulator, and act as probes of the magnetic state at the interface between the two materials. An example of the usefulness of this probe is shown by magnetoresistance measurements on a Ag/EuO bilayer.
Magnetic anisotropy of small clusters and very thin transition-metal films
Authors: Dreyssé H. , Dorantes-Dávila J. , Pick Š. , Pastor G. M.
Published: July 2002

The magnetic anisotropy of 3d-TM clusters is studied in the framework of a d-electron tight-binding Hamiltonian including hopping, Coulomb, and spin-orbit interactions on the same electronic level. Results for the magnetic anisotropy energy and spin-polarized DOS are given as a function of the bond length for the Fe6 cluster. In particular it is shown that the MAE may be qualitatively related to the projections of the orbital moment along the magnetization directions, and that the ‘‘in-plane’’ anisotropy can be of the same order of magnitude as the ‘‘perpendicular’’ anisotropy. Using the same Hamiltonian, the problem of the convergence of the magnetic anisotropy energy of very thin films is revisited. By the choice of a basis which mixes spherical harmonics, it is shown that the irregular oscillations of the MAE versus the band filling disappear. For Co(111) excellent stability of the in-plane anisotropy as observed experimentally was found. Finally, the crystal-field effects are also discussed.
Study of magnetohistory effects in YFe12−xMox (x=1.5–3.0)
Authors: Wang Yi-Zhong , Hu Bo-Ping , Liu Gui-Chuan , Song Lin , Wang Kai-Ying , Hu Ji-Fan , Lai Wu-Yan
Published: July 2002

Magnetohistory effects in YFe12−xMox (x=1.5–3.0) have been investigated. The freezing temperature Tf increases with increasing Mo concentration. Irreversible behavior still appears in an applied field up to 6 T for YFe9.5Mo2.5. These magnetohistory behaviors may be understood by domain-wall pinning.
Interlayer correlations and helical spin ordering in MnTe/CdTe multilayers (abstract)
Authors: Nunez V. , Giebultowicz T. M. , Faschinger W. , Bauer G. , Sitter H. , Furdyna J. K.
Published: July 2002

Mn-VI/II-VI superlattices belong to the family of artificial multilayered structures composed from zinc-blende II-VI semiconducting compounds and Mn chalcogenides using MBE and ALE. While all naturally existing crystals of MnTe have the NiAs (hexagonal) structure, the zinc-blende form is a fcc Heisenberg antiferromagnet with dominant nearest neighbor interactions. The magnetic properties of such frustrated systems are strongly influenced by lattice mismatch strain. For example, tensile strain produces an incommensurate helical antiferromagnetic phase with an in-plane axis. The strain and therefore helical period increases with thickness of CdTe, but in the presently studied samples is weak enough to allow investigation of the onset of incommensurate helical effects. In previously studied MBE-grown Mn-VI/Zn-VI systems, the nonmagnetic spacers were too thick to allow interlayer coupling. We report neutron diffraction studies of new MnTe/CdTe superlattices prepared by ALE with extremely thin CdTe spacers (from 2 to 6 monolayers). The results indicate the formation of spin helices, consistent with the tensile nature of strain in the MnTe layers. In addition, the widths of the AF diffraction peaks and the presence of satellite peaks clearly indicate that the magnetic interactions propagate through the CdTe spacers, introducing coherence between the helices in different MnTe layers. The nature of the interaction responsible for this transfer is not yet clearly understood. Some possible mechanisms are discussed.
Enhancement of the localized behavior in CeNi0.8Pt0.2 Kondo compound replacing Ce by magnetic ions (Pr,Nd)
Authors: Gómez Sal J. C. , Blanco J. A. , Espeso J. I. , Rodríguez Fernández J. , Gignoux D.
Published: July 2002

The substitution of Ce by magnetic ions such as Pr or Nd in the CeNi0.8Pt0.2 Kondo ferromagnetic compound not only favors the magnetic interactions increasing the Curie temperatures, but also disturbs the coherence of the Kondo lattice state leading to a decrease of the Kondo temperatures and an increase of the Ce intrinsic magnetic moment. These effects have been studied by resistivity, magnetization, and neutron diffraction.
Quantum tunneling in magnetic particles (invited) (abstract)
Authors: DiVincenzo D. P.
Published: July 2002

Recent advances both in materials preparation techniques for ultrafine magnetic particles, and in low-temperature magnetometry, have made possible the observation of a new kind of collective quantum-mechanical phenomenon. In our studies of horse-spleen ferritin particles (naturally occurring 75 Å iron-oxide crystallites), a resonance in the magnetic susceptibility is observed at low temperature (<200 mK) and exceedingly low ambient magnetic fields (of order 10−3 Gauss and below). I will discuss the accumulation of evidence which indicates that this resonance arises from the quantum tunneling of the magnetization between the ferritin particles’s two easy-axis states. This involves a unique collective effect: the orientation of the thousands of spins in the particle flip over as a unit, passing through an energy barrier. I will discuss various new theoretical results which have been stimulated by these recent experiments. For example, quantum mechanics predicts that the tunneling behavior is radically different for particles with even and odd number of iron atoms,—in particular, tunneling is predicted to be forbidden in the odd case. Finally, I will discuss the possibilities for quantum effects in a new class of nanoscopic Fe particles, produced by a novel STM deposition technique by A. Kent. It is clear that new materials preparation techniques will continue to provide novel testing grounds for quantum theory.
The effects of microcrystalline structure on the photoluminescence of porous silicon-on-sapphire
Authors: Russell S. D. , Dubbelday W. B. , Georgief P. , Shimabukuro R. L. , de la Houssaye P. R.
Published: July 2002

Silicon-on-sapphire was fabricated in as-deposited, improved, and bonded wafer forms. Each of the fabrication techniques imparts characteristic microcrystalline defects in the silicon layer. Microstructural defects such as microtwins and threading dislocations in the starting material have no observed effect on the light emitting properties of porous silicon-on-sapphire. Vacancies imparted by ion implantation damage, however, can amorphize the material resulting in a quenching of the photoluminescence (PL). An apparent increase in the density of light emitting structures leading to enhanced PL can also be obtained by limited ion damage of the material prior to the fabrication of the porous layer.
Precipitation of 3d transition-metal silicides in Czochralski silicon crystals
Authors: Wijaranakula W. , Kim S. S.
Published: July 2002

Precipitation of metal silicides was observed in as-grown Czochralski silicon crystals contaminated with 3d transition metals. The result indicates that a copper silicide precipitation occurs favorably in a crystal grown under slow cooling conditions. In the case of titanium disilicide formation, which involves silicon interstitial absorption, supersaturation of silicon interstitials produced by oxygen precipitation could lead to a coprecipitation of titanium and silicon interstitials. Based upon the present analysis, it is suggested that 3d transition-metal atoms incorporated into the crystal may remain in the silicon lattice as isolated interstitial impurities or precipitate via silicide formation, depending upon the crystal cooling conditions. The precipitation of metal silicides could take place at the crystal lattice defects which were created during crystal solidification.
Effect of nitrogen on the properties of hard magnets
Authors: Melamud M. , Bennett L. H. , Watson R. E.
Published: July 2002

The enhancement of magnetic properties of RE-TM hard magnet materials, such as RE2Fe17, upon nitriding is studied using the Wigner–Seitz (Voronoi) construct. In analogy with other RE-TM nitrides, it is concluded that nitrogen has a strong preference to occupy the octahedral 9e site in the 2:17 compounds, to the exclusion of the other proposed sites. Additional materials are suggested as candidates for nitriding on the basis of the availability of such an octahedral site. The site preference and its effect on the magnetism in RE2Fe17 is discussed in relation to the Wigner–Seitz cell of the atoms. The magnetic moments on the iron are shown to be correlated with the WS cell volumes, and this dependence differs from that associated with αFe.
The effects of hydrogen disproportionation, desorption, and recombination on the structure and magnetic properties of Sm2Fe17Nx and NdFe10Mo2Nx compounds
Authors: Yang Jun , Dong Shengzhi , Mao Weihua , Xuan Ping , Yang Yingchang
Published: July 2002

Hydrogen disproportionation, desorption, and recombination (HDDR) has been used as a pretreatment to prepare high performance Sm2Fe17Nx and NdFe10Mo2Nx compounds. Isotropic Sm2Fe17Nx and NdFe10Mo2Nx compounds with intrinsic coercivity larger than 12 and 4 kOe have been obtained by nitriding the HDDR treated powders, respectively. It is found that the magnetic properties are sensitively dependent on the time and temperature of the HDDR process, which determine the grain size, nitrogen content, and the amount of α-Fe in the nitrides.
Nonstationary theory of mode competition in gyrotrons with allowance for small inhomogeneity of the guiding magnetic field
Authors: Dumbrajs O.
Published: July 2002

High-power millimeter wave generators can induce electron cyclotron waves in a plasma which can fulfill many functions (plasma breakdown, heating, current drive) in fusion plasma reactors including both tokamak and stellarator concepts. All these applications require tens of megawatts of power in the frequency range 100–300 GHz and power 0.5–1.5 MW per unit. Among various microwave sources gyrotrons are the most developed source. Because of the heat losses in resonator walls at such high frequencies and powers oversized cavities must be used. In such cavities the mode competition becomes a severe problem. The most advanced theory of mode competition in gyrotrons is the nonstationary nonlinear theory. In the present article we extend this theory by allowing for inhomogeneity of the guiding magnetic field. The importance of this generalized theory is illustrated in the case of a 3 MW 140 GHz gyrotron with a coaxial cavity working in the TE21,13 mode.
Three-beam phase conjugation and mode competition in BaTiO3
Authors: Zhang Chun-ping , Song Q. Wang , Corderro Dave
Published: July 2002

A new configuration of phase conjugation (PC) with three beams in BaTiO3 crystal is demonstrated. When two beams are incident, they can generate their own self-pumped PC or mutually pumped PC depending on their intensity ratio. Upon the application of a third beam, one of the two mutually pumped beams is changed into a self-pumped PC. The other forms a double PC with the new beam. This configuration has more flexibility and functionality than that in the two-beam case. The analysis of mode competitions between self-pumped, mutually pumped, and double PC is given.
Large-band-gap SiC, III-V nitride, and II-VI ZnSe-based semiconductor device technologies
Authors: Morkoç H. , Strite S. , Gao G. B. , Lin M. E. , Sverdlov B. , Burns M.
Published: July 2002

In the past several years, research in each of the wide-band-gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high-temperature electronics and short-wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high-power operation. The present advanced stage of development of SiC substrates and metal-oxide-semiconductor technology makes SiC the leading contender for high-temperature and high-power applications if ohmic contacts and interface-state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high-temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra.

Blue SiC light-emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN-based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short-wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN-based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe-based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide-band-gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.
Magnetic anisotropy of metal/Co/metal and metal/Co/insulator sandwiches
Authors: Wiedmann Michael H. , Engel Brad N. , Falco Charles M.
Published: July 2002

In situ polar Kerr-effect measurements have been used to study the magnetic anisotropy of MBE-grown Au(111)/Co/X and Pd(111)/Co/X sandwiches, where X is the nonmagnetic metal Ag, Au, Cu, and Pd or the insulator MgO. For the metals it was recently found that the magnitude of the Co/X perpendicular interface anisotropy is strongly peaked at ∼1 atomic layer (1.5–2.5 Å) coverage. To investigate structural influences on the anisotropy, reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) have been used to measure changes resulting from overlayer coverage. Analysis of digitized RHEED images captured every ∼1 Å during metal overlayer coverage shows no abrupt change of the in-plane lattice constant. The out-of-plane lattice spacing has also been investigated as a function of nonmagnetic metal coverage by measuring LEED I-V curves along the (0,0) rod. In the case of Cu, where the LEED behavior is nearly kinematic, no evidence was seen of any abrupt structural changes at ∼1 atomic layer coverage. These results suggest the observed peak in magnetic anisotropy is not structural in origin. To further study this phenomenon, the influence of an insulating overlayer, MgO, on the perpendicular magnetic properties has been measured.
Threshold energies of high-field-induced hole currents and positive charges in SiO2 layers of metal-oxide-semiconductor structures
Authors: Gao Xiaoping , Yee Sinclair S.
Published: July 2002

The threshold energies of high-field-induced oxide hole currents and positive oxide charges were examined by using metal-oxide-semiconductor field-effect transistors (MOSFETs) with oxide layers of 6 nm. Effects of three anode materials, polysilicon, aluminum, and gold, were studied. The same threshold energy was observed for generating both hole current and positive charges on MOSFETs with the same anode material. This result supports the proposal that both high-field-induced oxide hole current and positive oxide charges have the same origin. Different threshold energies were observed for different anode materials. The larger the anode electron barrier height, the smaller the observed threshold energy. Analyzing the experimental results indicates that holes which cause hole current and positive charges are generated at anode mainly due to electron direct excitation. The anode holes with energies larger than anode hole barrier height have dominant contribution to both high-field-induced oxide hole current and positive oxide charges. Contribution from the anode holes with energies smaller than anode hole barrier height can be ignored.
Reliability test of popular fractal techniques applied to small two-dimensional self-affine data sets
Authors: Talibuddin S. , Runt J. P.
Published: July 2002

The fractal dimensions of five fractional Brownian motion (fBm) surfaces of 257×257 pixel size, with Hurst exponent H ranging from 0.1 to 0.9, were computed by profile, contour, and surface area analyses. A technique was deemed reliable if it demonstrated accuracy, consistency and sensitivity. Of all the techniques examined, surface area analysis methods, namely, two-dimensional pyramid and Peleg methods, were found to be most reliable and efficient for the data size studied. Hence, these were employed in a preliminary fractal analysis of poly(methyl methacrylate) and poly(styrene) fracture surfaces. The surfaces were imaged at scan sizes ranging from 1–6.5 μm by atomic force microscopy (AFM). The images indicated the existence of fractal structure and a high degree of roughness at microstructural scales for both the surfaces. These observations were supported by the results of the two surface area analysis techniques. A more conclusive study was prevented by severe scoring of the surfaces by the AFM tip at smaller scan sizes and the availability of only a narrow range of scan sizes.
Theoretical calculation of longitudinal-optical-phonon lifetime in GaAs
Authors: Bhatt A. R. , Kim K. W. , Stroscio M. A.
Published: July 2002

The anharmonic decay of longitudinal-optical (LO) phonons in zinc-blende semiconductors has been studied. Based on an approach in which the anharmonic crystal potential is estimated using the theory of elasticity, the lifetime of LO phonons via emission of two acoustic phonons is calculated as a function of lattice temperature and phonon wave vector. Application of this model to bulk GaAs shows an excellent agreement with available experimental data. Since the parameters employed in the model can be obtained experimentally, the approach provides a useful tool to investigate LO-phonon lifetimes in semiconductors.
Control of reflectivity and bistability in displays using cholesteric liquid crystals
Authors: Yang D.-K. , West J. L. , Chien L.-C. , Doane J. W.
Published: July 2002

It is demonstrated that the reflective properties and bistability of cholesteric liquid crystals can be controlled by proper surface treatment and dispersed polymers. Dispersing a polymer in the liquid crystal or using a cell with an inhomogeneous surface anchoring creates permanent defects which result in long-term bistability, high contrast at large viewing angles, and gray scale. The wide-angle, reflective feature makes cholesteric materials suitable for displays without backlights and bistability provides flicker-free operation.
Lightweight, distortion-free access to interiors of strong magnetic field sources
Authors: Leupold H. A. , Potenziani E. , Tilak A. S.
Published: July 2002

A permanent magnet structure has been designed to provide field distortion-free access to large magnetic fields in cylindrical and spherical cavities without the unacceptably large mass increases usually incurred to provide it. For example, a spherical shell that produces a 1.2-T field in a spherical cavity of 2-cm radius has a mass of about 2.3 kg if material of 12.2-kOe coercivity and 12.2-kG remanence is used. If the sphere is compensated for distortion-free equatorial access, its mass must rise to 23 kg to maintain the same field in the cavity. If the geometry is altered by the new technique to maintain field integrity under access, the mass is only 1.7 kg. Some applications are discussed that afford, up to this time, unviable devices.
Luminescence spectra and structure of Er complexes implanted in annealed semiconductors
Authors: Phillips J. C.
Published: July 2002

The titled spectra exhibit a wealth of fine structure between 795 and 810 meV, including two strong, narrow peaks at 805 meV (λ=1.54 μm) and 808 meV. The microstructural model explains the origin of this fine structure and chemical trends in the host-dependent width of the strongest peak at 805 meV. It also explains the host dependence of the quenching temperature T0[A(N)B(8−N)] in Si, InP, GaAs, AlGaAs, GaP, InAlGaAs, and ZnTe. From the data it is inferred that the annealed luminescence spectra originate in (111) c-Er2O3 nanoplatelets with dimensions 1.0×2.5 nm2. The two strong peaks originate from recombination at the two inequivalent Er sites of the 16 formula unit cell of Er2O3, while the remaining fine structure is associated with nanoplatelet surface sites. The temperature dependence of the luminescence intensity scales with the host energy gap EG[A(N)B(8−N)], which is explained by thermal activation over saddle-point energy barriers located near the platelet-host interfaces.
Carrier transport properties of iodine-doped (ZnS)3(ZnSe)42 ordered alloys grown by atomic layer epitaxy
Authors: Fujiwara Hiroyuki , Kiryu Hideaki , Shimizu Isamu
Published: July 2002

Iodine-doped ordered alloys, given in a form of (ZnS)3(ZnSe)42, were grown on a GaAs(100) substrate by hydrogen radical-enhanced chemical vapor deposition using atomic layer epitaxy. Iso-buthyliodide was used for the first time as the doping source. A very high Hall mobility of 470 cm2/V s was obtained at room temperature in the slightly I-doped sample (3×1016 cm−3). This high mobility is due to the elimination of structural fluctuations by forming a two-dimensional ordered structure, which was confirmed by the satellite peaks in x-ray diffraction spectra. Blue band-edge emissions were found to be dominant at 35 K and room temperature in photoluminescence spectra. These results indicate that the formation of defects was minimized in these crystals by the layer-by-layer structure and the low-temperature growth at 200 °C.
Evidence for bias dependent barrier heights in gold-epitaxial CdTe Schottky diodes
Authors: Sands D. , Scott C. G.
Published: July 2002

The forward bias and reverse bias current-voltage characteristics of some gold-epitaxial CdTe-on-InSb Schottky diodes have been measured at room temperature. Series resistance is evident in undoped material (base electron concentration ∼1014 cm−3) which has been measured and corrected for and identified as arising from the reverse biased CdTe-InSb junction. Two straight lines of differing slopes are present in the semilog plots for all the diodes, which we interpret as a bias-dependent barrier height. The two barrier heights, 0.91±0.04 eV and 0.74±0.02 eV, are well known in the Au-CdTe system, from which we conclude that two discrete states are present at the CdTe-Au interface. The occupancy of the states is determined by the applied bias, and hence the barrier height changes.
Hall–Petch relationship in the nanocrystalline selenium prepared by crystallization from the amorphous state
Authors: Zhang H. Y. , Hu Z. Q. , Lu K.
Published: July 2002

Bulk nanocrystalline selenium (nc-Se) specimens with various grain sizes were prepared by isothermally crystallizing as-quenched amorphous selenium. Microhardness measurements indicated that with a decrease in grain size, from 25 to 9 nm, hardnesses of nc-Se samples increase dramatically from 0.34 to 0.98 GPa, i.e., a normal Hall–Petch relationship is obtainable over the grain size range studied for nc-Se samples. Structural characterization of nc-Se using x-ray diffraction revealed an evident lattice distortion in the nanocrystalline state, namely the lattice parameter a is increased, whereas c is decreased simultaneously with a reduction of grain size. The variation tendency of microhardness with grain size can be well correlated with the change of the unit cell volume in nc-Se, which implies that the lattice structure of the nanocrystalline state is a factor affecting the mechanical properties.
Thin lens simulations of inertial confinement fusion focusing systems for light ions
Authors: Olson J. C. , Kusse B. R.
Published: July 2002

Systems for focusing a light ion beam onto an inertial confinement fusion (ICF) target are studied using a thin lens model to compute ion trajectories. Three focusing systems are analyzed. The systems are (1) an uncorrected single focusing lens; (2) a focusing lens with chromatic correction due to the self magnetic field of the beam; and (3) the combination of a focusing lens, a defocusing lens for divergence correction, and chromatic correction from the beam’s self field. Systems (2) and (3) both produce energy efficiencies of greater than 50% for transporting an ion beam with 6 mrad divergence over 4 m and focusing it onto a 1 cm radius target. The predictions of the thin lens model are compared to predictions made by codes using numerical integration of particle trajectories. The implications for ICF applications are discussed.
Pulse propagation in sea water
Authors: Margetis Dionisios
Published: July 2002

The propagation in sea water of a low-frequency electromagnetic pulse generated by an electric dipole is investigated analytically. The dipole is excited by a rectangular current pulse with a finite, nonzero rise and decay time. In order to obtain an explicit formula for the field in the equatorial plane of the dipole source that is uniformly valid in distance and time, Fourier-transform methods are applied. Certain limiting forms of the current pulse are studied separately. Simple analytic expressions of the field are obtained, compared to previous results, and examined thoroughly. The effect of the finite rise and decay time is discussed. It is noted that the present analysis may be used for studying pulse propagation in any highly conducting medium besides sea water.
Distribution of light-induced defect states in undoped amorphous silicon
Authors: Hattori Kiminori , Anzai Makoto , Okamoto Hiroaki , Hamakawa Yoshihiro
Published: July 2002

Distribution of light-induced defect states in undoped amorphous silicon has been studied by using the modulated photoconductivity spectroscopy technique. The experimental results show that a large increase of the neutral defect states occurs, and the positively charged defect states grow particularly in the midgap energy range. The qualitative features of the measured energy distribution agree well with the theoretical prediction from the current defect formation model, although a quantitative comparison with respect to the magnitude of density-of-states reveals a discrepancy between theory and experiment.
Numerical studies on the electromagnetic properties of the nonlinear Lorentz computational model for dielectric media
Authors: Abe Hirotada , Okuda Hideo
Published: July 2002

We present the results of a new computer simulation model developed to study the propagation of electromagnetic waves in a dielectric medium in both the linear and nonlinear regimes. The model is constructed by combining a microscopic model used in the semi-classical approximation for the dielectric media and a particle model developed for the plasma simulations. It is shown that the model may be useful for studying linear and nonlinear wave propagation in the dielectric media.
Rapid migration of silver on roughened glass surfaces in a methanol/air mixture near 500 °C
Authors: Sears W. M. , Love D. A.
Published: July 2002

Experimental results presented here show that at elevated temperatures (about 500 °C) silver can be mobile on tin oxide, as well as other metal oxide surfaces. Because the migrating species seems to be Ag+ both diffusion and electrically assisted migration occurs. This movement is made visible in the form of tiny silver droplets on the glass surface by the presence of methanol vapor mixed in air. Without oxygen no migration occurs at all. It is clear that the movement requires a continuous or dynamic conversion of silver ions to silver oxide and back again, through the presence of both oxidizing and reducing agents. We show how it is possible to measure the diffusion constant and the electrical mobility constant by an appropriate analysis of the time evolution of a circular front of silver droplets measured with a video camera connected to a microscope zoom lens. Typical values on a roughened glass surface at 500 °C are for mobility, 42±13×10−6 cm2/V s, and for diffusion 12±6×10−6 cm2/s. Similar values for the electrical mobility on a smooth tin oxide surface have been measured. At this temperature diffusion on pure silica or even Pyrex glass powder is very slow, but the diffusion constant for MgO or CaO powder, annealed onto a sandblasted glass surface, is about 10−4 cm2/s. These oxides may be responsible for the movement seen on soda glass surfaces. The activation energy is about 4 eV and seems related to the work function of silver metal.
Electron irradiation of two-terminal, monolithic InP/Ga0.47In0.53As tandem solar cells
Authors: Cotal H. L. , Walters R. J. , Summers G. P. , Messenger S. R.
Published: July 2002

Results are presented for 1 MeV electron-irradiated, two terminal, monolithic InP/Ga0.47In0.53As tandem solar cells. These highly efficient prototype cells show radiation resistance that is comparable to single junction InP cells. A current mismatch between the subcells does not occur until high fluence levels, that is, near 3×1015 e−/cm2. This value for the onset of current mismatch and the measured remaining absolute efficiency of 9.4% at 1×1016 e−/cm2 are excellent results reported for a tandem cell designed for space applications.
Metalorganic molecular beam epitaxy growth characteristics of GaAs using triethylgallium and trisdimethylaminoarsenic
Authors: Liu X. F. , Asahi H. , Inoue K. , Marx D. , Asami K. , Miki K. , Gonda S.
Published: July 2002

Metalorganic molecular beam epitaxy (MOMBE) growth characteristics of GaAs using triethylgallium (TEGa) and trisdimethylaminoarsenic (TDMAAs) are studied in detail by using reflection high energy electron diffraction intensity oscillations. It is found that the GaAs growth rate variation with substrate temperature (Tsub) is similar to that in the MOMBE growth of GaAs with TEGa and elemental As (As4) except in the high Tsub region, despite the use of uncracked TDMAAs instead of As4. GaAs growth starts at about 350 °C and shows a maximum growth rate at about 500 °C. However, the absolute growth rate is about 15% lower than that using As4 with the same TEGa flow rate in the mass-transport limited growth region. Above 600 °C the growth rate shows a rapid decrease with increasing Tsub. Further, it is found that etching of GaAs occurs when only TDMAAs is supplied to the GaAs surface at Tsub above 500 °C. Unintentionally doped GaAs films show n-type conduction in the whole Tsub range investigated with the highest 77 K electron mobility of 22 000 cm2/V s and a low carrier concentration of 2×1015 cm−3 for the 650 °C grown samples. 4.2 K photoluminescence spectra show an exciton-bound-to-impurity emission peak at the wavelength of 819.3 nm with a full width at half-maximum of less than 3.5 meV indicating good optical qualities.
Investigation of the temperature dependence of the critical state in melt processed YBa2Cu3O7−∂ thick films
Authors: Dewhurst C. D. , Cardwell D. A. , Alford N. McN.
Published: July 2002

The inter- and intra-grain critical current densities of melt processed YBa2Cu3O7−∂ (YBCO) thick films have been measured as a function of temperature using vibrating sample magnetometry. The width of the M-H hysteresis curve has been observed to scale with the cube of the sample width for temperatures up to ∼70 K and applied fields of greater than 1 T which implies that current flows on the length scale of the sample over this temperature and field regime. An exponential decrease of magnetic moment hysteresis with temperature up to ∼70 K and between ∼80 K and Tc has been observed and attributed to the dominance of inter- and intra-granular current, respectively. An empirical model is presented to account for the observed dependence of the critical current density on temperature, based upon weak link behavior within the ‘‘Hub and Spoke’’ like morphology characteristic of melt processed YBCO thick films.
Interaction of optical waveguide modes with dipole-exchange surface spin waves in a Lu2.14Bi0.86Fe4.94Mg0.06O12 film
Authors: Matyushev V. V. , Kostylev M. P. , Stashkevich A. A. , Desvignes J. M.
Published: July 2002

Collinear interactions of guided light with dipole-exchange spin waves in a 4.66-μm-thick Lu2.14Bi0.86Fe4.94Mg0.06O12 single crystal film has been experimentally investigated. For all four regimes of interaction TM5→TE5, TM6→TE6, TM7→TE7, TM8→TE8 diffraction peaks had a complex ‘‘fine structure’’ indicating the splitting of the spectrum of the spin waves. Frequencies of these peaks are in good agreement with the theoretical predictions based on the numerical estimations of the strong ‘‘hybridization’’ of a magnetostatic mode with seventeen spin wave resonance modes. The analysis of the amplitudes of the diffraction peaks corresponding to different hybrid spin wave modes has shown the presence of asymmetry of the pinning conditions on the surfaces of the film.
Analysis of spectroscopic-ellipsometry and thermoreflectance spectra of Si
Authors: Miyazaki Takayuki , Adachi Sadao
Published: July 2002

Spectroscopic-ellipsometry (SE) and thermoreflectance (TR) spectra of bulk single-crystalline Si are measured. Measurements are made on the same sample in the 2.5–6.0 eV range at room temperature. These data are analyzed on the basis of a simplified model of interband transitions, named the model dielectric function. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. The finding definitely links the temperature-induced change in the dielectric function (TR) to the first derivative of the dielectric function (SE).
Superconducting resonator circuits at frequencies above the gap frequency
Authors: de Lange G. , Kuipers J. J. , Klapwijk T. M. , Panhuyzen R. A. , van de Stadt H. , de Graauw M. W. M.
Published: July 2002

The frequency response of three superconductive niobium resonating circuits, formed by a Nb microstrip and a Nb tunnel junction, is measured and analyzed at frequencies above the superconducting gap frequency. The circuits are placed in a waveguide system and the frequency response is determined with a Fourier transform spectrometer. The calculated and measured resonance frequencies and bandwidths are in good agreement with the extreme anomalous limit of the Mattis–Bardeen theory on the anomalous skin effect in superconductors [D.C. Mattis and J. Bardeen, Phys. Rev. 111, 412 (1958)]. The observed loss is higher than predicted by this theory, in agreement with previous observations on Nb films. The use of other materials for striplines as tuning circuits for heterodyne superconducting tunnel junction mixers is analyzed.
Waveguides based on drift and diffusion
Authors: Hayata K. , Koshiba M.
Published: July 2002

A novel principle of waveguides for microwave transmission is proposed, which is based on drift and diffusion of carriers (electrons) in a semiconductor slab bounded by isotype homojunctions. In striking contrast to usual waveguides for electromagnetic radiation, the present waveguides support only the longitudinal electric field. The effective dielectric constant is proportional to the concentration of ionized donors.