bluuebunny/crossref_metadata_2025 huggingface dataset:
Macroscopic magnetic anisotropy and the vapor deposition process in amorphous RE-TM alloys (abstract)
Authors: Hellman F. , Rooney P. W. , Gyorgy E. M.
Published: July 2002

Thin films of amorphous rare earth–transition metal alloys are well known to possess perpendicular magnetic anisotropy. The vapor deposition process possesses factors that break the symmetry of an idealized, bulk, isotropic, amorphous material. These include the substrate, the incident atomic beams, incident energetic Ar atoms, and any applied field. Structural anisotropy such as columnar microstructure, nonperpendicular growth direction, stress and strain, and preferential growth of crystalline grains with low energy surfaces, are known consequences of the growth process. Evidence exists, however, that the anisotropy in these amorphous alloys is of a more subtle local nature. The perpendicular magnetic anisotropy in amorphous Tb-Fe in a broad range of compositions increases dramatically with increasing deposition temperature, including temperatures well above the Curie temperature. It is not strongly dependent on stress and is independent of film thickness. Upon annealing, the anisotropy vanishes. The nonrandomness in the amorphous structure thus does not appear to be related to nanocrystallites, which would presumably be enhanced by annealing, nor to kinetic effects of incident atomic beam directions which would be reduced by an increased deposition temperature. The dependence on deposition temperature can be fitted to a thermally activated form, implying an important role of surface diffusion and surface energy in the formation of the anisotropic local structure. We suggest that this process be generically thought of as a texturing of the amorphous phase, analogous to the preferred texturing of the low surface energy grains in polycrystalline growth. The dependence of the magnetic anisotropy of a-Tb-Fe on deposition temperature and rate will be presented and compared to a particular model involving reorientation of adatom configurations into lower energy orientations. Comparisons to other a-RE-TM alloys will be made, in particular to a-Gd-Fe and to a-Er-Fe (with the opposite sign Steven’s coefficient). The results of an extensive computer simulation of a competing model involving chemically driven preferential site occupation will also be presented.
Micromagnetic simulations of tunneling stabilized magnetic force microscopy
Authors: Oti John O. , Rice Paul
Published: July 2002

A micromagnetic model simulating the images produced by tunneling stabilized magnetic force microscopy is described. The images are related to the force interactions between the fringing fields from the imaged surface and stray fields from the sensing probe. The model, which allows for variations in the magnetization states of the probe, is used to examine the dependence of the interaction forces on varying probe-film separations for a magnetic garnet film. The results show that the images are sensitive to separation and that the changing probe magnetization plays an important role in determining the final image.
Investigation of the transition from ferromagnetic to antiferromagnetic order in the CeMn2(GexSi1−x)2 system
Authors: Hill P. , Ali Naushad
Published: July 2002

Samples in the alloy system CeMn2(GexSi1−x)2 exhibit a crossover from antiferromagnetic ordering for CeMn2Si2 to ferromagnetic ordering for CeMn2Ge2 as a function of concentration. The low temperature behavior of samples in the range 0.0≤x≤1.0 using the techniques of dc magnetization, ac susceptibility, and thermal expansion were studied. From this we have determined the magnetic phase diagram which shows four concentration regions of qualitatively different low temperature behavior. Increasing substitution of Ge for Si in CeMn2Si2 shows a depression of TN and the gradual development of ferromagnetic order at T≊305 K. For 0.42≤x≤0.60 there is a clear ferromagnetic transition at T≊305 K followed by a sharp drop in magnetization at lower temperatures. For all samples ac susceptibility shows a sharp peak near 300 K indicating the onset of magnetic order at that temperature. Neither ac susceptibility nor thermal expansion measurements show clear anomalies at TN below Tc. There is evidence of mixed magnetic behavior for these samples.
Magneto-optical waveguides of aluminum garnet
Authors: Gualtieri D. M.
Published: July 2002

Planar waveguides of high refractive index aluminum garnets containing magnetic rare-earth cations have been epitaxially grown on <111≳-oriented crystal wafers of yttrium aluminum garnet (Y3Al5O12, YAG). Epitaxy was by the liquid phase solution growth technique from supercooled mixtures of lead oxide and boron oxide, as is common for the epitaxy of the magneto-optical iron garnets. The liquid phase epitaxy technique is well established for aluminum garnets, so that mixed garnet compositions, such as (Ra,Rb)3Al5O12, can be prepared easily as crystal layers on YAG. The typical lattice constant mismatch which is tolerated for defect-free epitaxy is about 0.1%. Reduction of stress-induced birefringence demands a still closer matching of the epitaxial layer to the substrate, of the order of 0.01%. Such matching to a YAG substrate can be achieved in mixed garnet compositions, such as (Ra,Rb)3Al5O12, in which Ra and Rb are large and small cations which average to the cationic size of yttrium. One composition which is particularly suited for magneto-optical waveguides is Tb1.65Lu1.35Al5O12, which has a refractive index at 632.8 nm of about 1.8545, as compared with 1.8280 for YAG, and has no absorption bands in the visible and near-IR spectra. The room temperature Verdet constant of this composition has been measured over the wavelength range of 500–675 nm, and it is comparable to that of Tb3Al5O12. Magnetic field modulation of light intensity of 4.4 ppm/cm/Oe at 632.8 nm has been measured in a planar waveguide of the Tb1.65Lu1.35Al5O12 composition. Since aluminum garnets have melting points greater than 1900 °C, such waveguides should find application in high temperature magneto-optical sensors and isolators.
A SmCo5 field direct current linear motor with U-spring supports
Authors: Matsumura A. , Kano Y. , Basak A.
Published: July 2002

The dc linear motor described here is a moving coil type using four permanent magnets (SmCo5) in the stationary unit. Four frictionless U-shaped springs are used as a new type of supporting mechanism for the mover. This linear motor can be controlled by either proportional controller or proportional, integral, and derivative controller. However, the springs have a natural frequency and it vibrates the motor. For this reason, an additional circuit is used in the feedback loop which eliminates the vibration of the springs and compensates for the delay of phase of the filter. This filter avoids the resonance of the springs and therefore the controller cannot only be used in this motor but also for the control of machines with some kind of vibrating mechanism. This dc linear motor has a new type Linear Variable Differential Transformer as a position detector.
Order from disorder in a kagome antiferromagnet
Authors: Chubukov Andrey
Published: July 2002

It will be discussed to what extent the large S approach to an antiferromagnet on a kagome lattice can explain the data of magnetothermal and neutron scattering experiments on a stacked kagome antiferromagnet Sr–Cr–Ga–O. In particular, it will be shown that quantum fluctuations lift the classical degeneracy related to a large amount of frustration in a kagome lattice and give rise to a finite velocity of spin-wave excitations which is by a factor S−1/3 smaller than for a conventional antiferromagnet.
Magnetism in a chromium jarosite Kagomé lattice KCr3 (OH)6 (SO4)2 (abstract)
Authors: Ramirez A. P. , Broholm C. , Lee S. H. , Collins M. F. , Heller L. , Kloc C. , Bucher E.
Published: July 2002

The S=3/2 magnetic Cr3+ ions in KCr3 (OH)6 (SO4)2 form a quasi-two-dimensional Kagomé lattice. Susceptibility measurements show a Curie–Weiss temperature of −70 K but specific heat data show a peak at 1.8 K. This depression of the ordering temperature is a result of the extreme frustration of the Kagomé lattice. The change in entropy up to 2.8 K is 0.15 K ln 4. Neutron scattering measurements on a powder specimen put an upper limit of 1 μB on the long-range ordered moments. Constant energy inelastic neutron scattering data with energy transfers of 1.0 and 1.4 meV show a broad peak centered at about 1 Å−1 indicative of strong spin fluctuations at both 0.3 and 10 K.
Re-entrant spin-glass behavior in the frustrated pyrochlore Y2Mn2O7 (abstract)
Authors: Greedan J. E. , Maignan A. , Simon Ch. , Raveau B. , Pedersen Jan Skov , Subramanian M. A.
Published: July 2002

Materials with the pyrochlore structure are subject to extreme frustration due to the topology of the magnetic sublattice which consists of a three-dimensional array of perfect corner sharing tetrahedra. The pyrochlore Y2Mn2O7 has been studied recently.1 The compound is crystalline and appears to have negligible crystallographic disorder, yet its magnetic properties resemble those of a re-entrant spin glass with a ferromagetic-like anomaly in the susceptibility at 17 K followed by a maximum at 7 K. In order to characterize this behavior dc and ac susceptibility measurements have been carried out along with small angle neutron scattering (SANS). Analysis of the spin dynamics through the frequency and temperature dependence of χ′ and χ″ show evidence for spin-freezing below 4 K. The SANS data can be fit by a cross section with both Lorentzian (L) and Lorentzian-squared (L2) terms. Both the magnitude and temperature dependence of the correlation lengths, ξ, of the L and L2 terms are different with ξ(L) growing to a finite size ∼25 Å at 15 K but ξ(L2) becomes resolution limited at the same temperature. In many respects the behavior of Y2Mn2O7 parallels that seen in magnetic metallic glasses.2
Magnetoresistance and magnetic properties of Co/Cu/Co sandwich film with thin Co layer insert (abstract)
Authors: McGuire T. R. , Plaskett T. S.
Published: July 2002

The enhanced magnetoresistance (MR), also called ‘‘spin valve,’’ found by Speriosu et al.1 in sandwich layers of the form Co(50 Å)/Cu(t)/Co(50) has been modified by addition of Co 2 and 4 Å insert layers midway in the Cu(t) layer of thickness t. The insert was suggested by Slonczewski2 as a method to influence the exchange coupling between the Co layers. In addition, we have also studied the effect of Cu overlayers on the MR and magnetic properties. The films are prepared by computer controlled magnetron sputtering from Co and Cu targets with Cu(t) varied from 20 to 36 Å. For MR values above 1% there is a pronounced plateau and hysteresis in the magnetization (M) versus applied field (H) curve and a corresponding plateau and hysteresis in MR versus H. The effect of the Co insert layers on MR is dependent on the Co insert thickness and also the Cu thickness. For example, for Co(x) in Co(50 Å)/Cu(18)/Co(x)/Cu(18)/Co(50) there is an increase in MR of 11%–14% for x=0–4 Å. In contrast, Co(50 Å)/Cu(14)/Co(x)/Cu(14)/Co(50) showed a decrease of 14%–6% for x=0–4 Å. Measurement of the MR temperature dependence for Co(50 Å)/Cu(28)/Co(50) showed a Brillouin-like decrease going from 14% at 4.2 K to less than 1% at 185 K. Another method to cause large changes in MR for the sandwich structures, either with or without the Co insert layer, is the addition of an overlayer of Cu of 10 or 20 Å. For the 10 Å overlay, there is up to a 50% decrease in the extent of the plateau as well as MR; for the 20 Å Cu both the plateau and MR have been reduced to almost zero. At this time, we cannot fully interpret the Cu overlay mechanism but its understanding should lead to improved control over the properties.
Mössbauer measurements of the static critical exponent β of the layered antiferromagnet Ba2FeF6
Authors: Brennan K. , Hohenemser C. , Eibschütz M.
Published: July 2002

The Mössbauer effect has been used to measure the hyperfine field of 57Fe nuclei in the layered antiferromagnet Ba2FeF6. The temperature dependence of the sublattice magnetization was measured an order of magnitude closer to the Néel temperature than previous studies. A range of fit analysis to the power law equation h(t)=Btβ, yields β=0.135(3) and TN=47.96(4) K for 4.31×10−3<t<0.7, where t=(1−T/TN). This value for β is substantially closer to the theoretical value (β=0.125) than previous work on Ba2FeF6.
Relaxation in the insulating spin glass Co1−xMnxCl2⋅2H2O (abstract)
Authors: Dell K. D. , DeFotis G. C.
Published: July 2002

Previous magnetization and susceptibility measurements on the mixed magnet Co1−xMnxCl2⋅2H2O disclosed a spin glass transition near 2.45 K over a wide composition range.1 Recent heat capacity and NMR measurements have confirmed and extended this finding.2 The time dependence of the thermoremanent magnetization (TRM) below Tg for an x=0.452 mixture was studied in some detail1 and was found to conform approximately to a decay of stretched exponential type, or perhaps slightly more accurately to the product of a stretched exponential and a power law. Small systematic deviations of data from fitted curves were apparent however. Recently a percolation model for relaxation in random systems was proposed,3 and yielded significantly improved fits to the TRM decay in an Au:Fe spin glass. The model assumes dispersive excitations within fixed finite domains, and includes as parameters the fastest and slowest relaxation rates characterizing the spectrum of domains. We find that this model also permits much better fits to be obtained for the TRM decay in Co1−xMnxCl2⋅2H2O, x=0.452. Systematic deviations that were present when using more traditional decay functions are virtually eliminated. The variation of fitted parameters with cooling field and temperature is also explored.
Magnetoresistance in rf-sputtered (NiFe/Cu/Co/Cu) spin-valve multilayers
Authors: Lottis D. , Fert A. , Morel R. , Pereira L. G. , Jacquet J. C. , Galtier P. , Coutellier J. M. , Valet T.
Published: July 2002

A study of the variation of the magnetoresistance in (Ni80Fe20/Cu/Co/Cu) multilayers with the thicknesses tNiFe, tCo, and tCu of each type of component layer has been performed. The magnetoresistance (MR), which at 4.2 K is larger than 20% for many samples, has been measured for fields applied both parallel and perpendicular to the current. This allows a direct measurement of the anisotropic magnetorestistance as well as an estimate of the spin-valve contribution to the total MR. The dependence of the MR on tCu indicates the presence of an oscillatory interlayer exchange coupling through the Cu layers with a period of about 12 Å. The dependence of the MR on tNiFe and tCo was studied at tCu=50 Å, for which the coupling is negligible. In this limit, the variation of the MR is dominated by the thickness dependence of the NiFe and Co component layer coercivities, which determine the degree of antiparallel alignement obtained during magnetization reversal.
Two-dimensional eddy current computation on the dewar’s thermal shields for a superconducting magnet in a magnetohydrodynamic device
Authors: Geri Alberto , Veca Giuseppe Maria
Published: July 2002

In this paper the authors applied an integral-differential method to compute the eddy current losses on the dewar’s shields of a magnetohydrodynamic device during a fast deenergization of the superconducting dipole magnet (i.e., during the magnet quenching). A brief description of the method and the calculation code developed by the authors are given. The analysis has been made on the cross plane at the dewar’s center of gravity, disregarding the edge effects of the frontal shields: that is, using a two-dimensional approximation. Only a quarter cross section has been modelled because of the geometrical symmetry of the device. The eddy current computation is made by integrating a time-dependent differential system of equations using the numerical backward difference scheme. The coefficient matrix has been derived from the general formulation of the vector potential algorithm referring to an infinitely long rectangular conductor. The method requires discretization only in the conductive regions of the dewar’s shields and does not necessitate any special treatment of the boundary conditions. The results obtained have been compared with those computed by means of a two-dimensional finite element analysis executed by the same authors using a general purpose code. From the comparison of the results emerges a good agreement between the finite element and integral-differential simulations, though quite different computational resources are required.
Magnetic properties and phase transitions in molecular based materials containing rare earth ions and organic radicals (invited)
Authors: Benelli Cristiano , Caneschi Andrea , Gatteschi Dante , Sessoli Roberta
Published: July 2002

The magnetic properties of materials containing rare earth ions coupled to the stable organic radicals nitronyl nitroxides are reported. When the rare earth ions are nonmagnetic, like yttrium(III) and europium(III), the materials behave as one-dimensional antiferromagnets, the coupling between the radicals being determined by superexchange interactions through the metal ions. With the isotropic gadolinium(III) ion the magnetic properties are dominated by the next nearest neighbor interactions between radicals and rare earth ions respectively, the nearest neighbor interactions between gadolinium and radical being ferromagnetic. All these materials do not order magnetically above 1.2 K. With the anisotropic dysprosium(III) the one-dimensional materials order antiferromagnetically with spin canting at 4.4 K. The unusually high transition temperature is related to the anisotropy of the rare earth ions with the aid of magnetic anisotropy measurements on single crystals. Similar results are obtained with other anisotropic rare earth ions.
Silicon nitride thin film production on Si(111)
Authors: Colaianni M. L. , Chen P. J. , Nagashima N. , Yates J. T.
Published: July 2002

The production of silicon nitride films by ammonia decomposition on Si(111)-(7×7) has been studied by high-resolution electron energy loss spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. Silicon nitride films of ≤1 monolayer thickness exhibit a characteristic four mode vibrational spectrum after annealing to 1200 K, while multilayer films produce three vibrational modes at 495, 720, and 1020 cm−1. Upon continued heating of the submonolayer and multilayer nitride films, identical vibrational spectra are obtained, suggesting that both layers form Si3N4. These results are in excellent agreement with the vibrational spectra reported in the literature for Si3N4 layers grown using N atoms on the heated Si(111)-(7×7) surface, signifying that identical nitride films can be grown using ammonia. Si3N4 films grown on Si(111) are able to chemisorb ammonia at 300 K, showing that coordinatively unsaturated silicon sites are probably present on the film surface. This is in agreement with other studies which indicate that the growth mechanism for these silicon nitride films involves silicon enrichment at the film/vacuum interface.
Structural properties of silicon epitaxy grown at 200–600 °C by electron-beam evaporation in an ultrahigh vacuum system
Authors: Yew Tri-Rung , Lin Yung-Jen , Shieh Ming-Deng , Chen Cheng-Hsien
Published: July 2002

This article presents the structural properties of silicon epitaxy grown at 200–600 °C by electron-beam evaporation in an ultrahigh vacuum system. Reflection high-energy electron diffraction was used to inspect wafer surface. Cross-sectional transmission electron microscopy was used to observe the epitaxial microstructure. The impurity profiles of epitaxial layers and epitaxy/substrate interface were measured by secondary ion mass spectroscopy. Furthermore, structural characteristics of the epitaxial films grown at different process conditions were summarized to explore the key parameters that control the epitaxial quality. In addition to ex situ cleaning prior to loading, the wafer was in situ cleaned by thermal desorption in the growth chamber at 840 °C. The thickness of the silicon epitaxial layer is about 0.1 μm, which was grown at a rate of 0.020 nm/s or higher.
Deep-level delta doping of Ti in GaAs: Modeling of tunnel-assisted recombination
Authors: Piprek J. , Schenk A.
Published: July 2002

The tunnel-assisted recombination current of a thin GaAs pn junction delta doped with Ti deep-level defects is modeled by 1D device simulation using a generalized Shockley–Read–Hall rate. The tunneling between band states and multiphonon sublevels of the recombination center, and the Poole–Frenkel effect are studied, varying the position of the delta–doped sheet and the junction width. Both field effects decisively influence the calculated current-voltage characteristics.
Impact ionization coefficients for electrons and holes in strained In0.2Ga0.8As and In0.15Ga0.63Al0.22As channels embedded in Al0.3Ga0.7As
Authors: Chen Y. C. , Bhattacharya P. K.
Published: July 2002

We have measured impact ionization coefficients, α and β, in 150 Å pseudomorphically strained materials for the first time. The measurements were made on specially designed lateral p-i-n diodes. α and β in lattice-matched GaAs layers are found to be lower than those in strained In0.2Ga0.8As and higher than those in strained In0.15Ga0.63Al0.22As. β is larger than α in all the samples. The results are discussed in terms of the changes in the band structure due to biaxial strain.
Instability in surface emitting lasers due to external optical feedback
Authors: Chen H. M. , Tai K. , Huang K. F. , Kao Y. H. , Wynn J. D.
Published: July 2002

Instability in the output of dc-biased surface emitting lasers due to an external cavity effect was observed. The output power spectrum exhibited multiple peaks with spacing corresponding to exactly the round-trip delay in silica fibers with length ranging from 2 m to 2 km. The magnitude of the peaks was enhanced in the spectral region centered at the laser relaxation frequency. With increased feedback, the background of the output spectrum was found to increase, indicating the presence of optical chaos. Numerical simulation based on the rate equation analysis was found to agree with the experiment, indicating the surface emitting lasers are well described by the rate equation and are susceptible to feedback as the edge emitting lasers.
Mechanical strength of Czochralski silicon crystals with carbon concentrations at and below 1015 cm−3
Authors: Fukuda Tetsuo , Ohsawa Akira
Published: July 2002

Using a tensile strength test at 900 °C, we study the mechanical strength of Czochralski silicon crystals with a low carbon concentration (3.5×1014 cm−3), and a typical carbon concentration (1.8×1015 cm−3). After heat treatment of these crystals at 700 °C, we find that low carbon content Si crystals are more resistant to oxygen precipitation and that the upper yield stress is higher than in typical carbon content Si crystals. Observations using a transmission electron microscope reveal that the precipitate density in the low carbon content Si crystals are nearly one-fifth that of the typical carbon content crystals. Also, the precipitates in both types of Si crystals act as dislocation generation sources during the deformation. Our model is compatible with the observations that less dislocations are generated in the lower carbon content Si crystals during the deformation, resulting in higher strength compared to conventional carbon content Si crystals.
Trap behavior in nonintentionally doped AlGaAs/GaAs single quantum well structures
Authors: Jiao K. L. , Anderson W. A.
Published: July 2002

The GaAs/AlGaAs single quantum well (SQW) samples with nonintentionally doped confining layers were studied using deep level transient spectroscopy (DLTS) and capacitance-voltage-temperature. A sizeable DLTS signal was observed and believed to be from the thermal emission of the well electrons. However, it was found that the major signal peak was accompanied by two subpeaks and thus the QW must be a multilevel trap state. Different combinations of reverse voltage and fill pulse height allowed a DLTS study of the region before, within, and beyond the well location. Such an observation, in conjunction with the use of undoped AlGaAs barrier layers, proved that the DLTS signal is indeed from the well because it was only significant when probed within the well region and the assumption of the DX centers in some previous studies can be excluded. The fact that classically derived activation energy is close to the estimated band-gap discontinuity value and the carrier distribution centered at the geometric QW at room temperature revealed that the quantization effect was of second order. However, the detected activation energy depends on the testing conditions that precludes the determination of the band offset using the DLTS technique.
Free carrier, free, and bound exciton photoluminescence of quantum well
Authors: Ping Er-Xuan
Published: July 2002

Free exciton and bound exciton photoluminescence (PL) of a quantum well (QW) in the presence of free carriers is studied in a model that includes the transfer of particles among free carrier, free exciton, and bound exciton states under an external excitation. It is shown that the free carrier state is important not only at high temperature but also at low temperature. General formulas for the free exciton, bound exciton, and free carrier PL are developed. The two-dimensional (2D) law of mass action for the free carrier and free exciton PL is reproduced at high temperature, and the model of the free exciton trapped by the impurities is obtained for the QW system with low defect concentration. Free exciton and bound exciton PL for high defect concentration is also discussed and the 2D law of mass action is obtained at high temperature as well. Experimental observations such as the thermal behavior of free exciton and bound exciton PL, the recently observed sharp temperature-induced reduction of the PL linewidth in coupled QW subjected to an electric field are discussed. An experiment is proposed to understand the origin of this sharp reduction.
Study of pinning energies in silver-sheathed bismuth superconducting tape by 90° rotating sample magnetic measurements
Authors: Li Guohong , Yu Qishan , Sun Yunxi , Yan Shousheng , Ye Bin , Zhou Yiru , Zeng Rong
Published: July 2002

Flux pinning energies and their distributions at 77 K in silver-sheathed and highly c-axis-oriented Bi2223 superconducting tape with a nominal composition of Bi1.8Pb0.4Sr2Ca2.2Cu3Oy have been determined by 90° rotating sample magnetic measurements. These measurements are done in three different configurations in which the relative orientations between the trapped flux and the depinning fields vary. Three peaks, located at 0.09–0.1 eV, 0.3–0.6 eV, and 1.4 eV, respectively, are found in the distribution curves. The relation between the peaks and pinning mechanisms as pinning of a single kink, kink formation, and intrinsic pinning is discussed.
Ab initio calculation of band structure, x-ray emission, quantum yield, and electron-energy-loss spectra of hexagonal boron nitride
Authors: Ma H. , Lin S. H. , Carpenter R. W. , Rice P. , Sankey O. F.
Published: July 2002

The band structure of hexagonal boron nitride (BN) has been calculated using an ab initio linear combination of pseudoatomic-orbitals method. The calculated band structure confirms a previous finding that this material is an indirect band-gap insulator and has two empty interlayer bands. Projected densities of states are compared with the experimental x-ray emission spectra of B and N K edges and the agreement is good. This good agreement between the present ground-state calculation and the experimental x-ray emission spectra supports our previous finding that there should be very little valence electron relaxation effect on x-ray emission spectra. A real-space Green’s function technique and the Z+1 approximation have been used to calculate the exciton spectra of B and N K edges. The first peak at 192 eV in B K edge is found to be a bound exciton with a binding energy of 1.7±0.4 eV. Only resonance is found for the N K edge. The calculated exciton spectra agree very well with the experimental quantum-yield and electron-energy-loss spectra.
Melt-grown p-type GaAs with iron doping
Authors: Tang R-S. , Saban S. B. , Blakemore J. S. , Gray M. L.
Published: July 2002

Optical and electrical properties are described for bulk GaAs, grown from a melt doped with iron to create FeGa deep acceptors in a sufficient amount (exceeding the EL2 defect concentration) to make high-resistivity p-type rather than semi-insulating material. Both iron photoionization and EL2+ photoneutralization contribute to the near-infrared optical absorption. This made it possible to deduce the concentrations (NAi and NAn) of ionized and lattice-neutral iron, and the ratio (NAi/NAn). Temperature dependent measurements of dc electrical transport yielded quantities such as the free hole density, and hence the Fermi energy, for the 290–420 K range. This information combined with (NAi/NAn) led to a determination of the iron acceptor’s free energy εA(T): about 0.46 eV above the valence band at 300 K, and ∼40 meV closer at 420 K. The temperature dependence of εA for iron is shown to differ from εv, εc, midgap, or the free energy for CrGa acceptors in GaAs.
Stabilization of Li acceptors in ZnSe by above-band-gap photoirradiation
Authors: Ichimura Masaya , Wada Takao , Fujita Shizuo , Fujita Shigeo
Published: July 2002

We predict by theoretical calculation that the concentration of interstitial Li (Liint) in ZnSe can be decreased, i.e., the Li acceptor (LiZn) can be stabilized by above-band-gap photoirradiation. Creation of excess electrons by the irradiation increases the occupation probability of the Liint donor level and thus the concentration of neutral Liint (Liint0). Consequently, the LiZn concentration increases because of the reaction Liint0+VZn0→LiZn0, where VZn0 and LiZn0 are a neutral Zn vacancy and a neutral Li acceptor, respectively. The carrier injection through an electrical junction will have similar effects as the photoirradiation, and thus our results indicate that the Li acceptors tend to be stable in active regions of light-emitting diodes and laser diodes.
Influence of surface charge on the cathodoluminescence of Ce:YAG
Authors: Feng X. D. , Warde C.
Published: July 2002

Cathodoluminescence of surface-charged Ce:YAG crystals was carried out. It was found that the familiar YAG defect emission band at 300 nm had a stronger dependence on the incident electron beam energy and accumulated surface charge than the cerium emission band at 550 nm. Luminescent relaxations, observed upon charging and discharging, appear to be due to charge migrations within the active region of crystal bulk. The experimental results are consistent with the association of the defect luminescent center with electrons trapped at oxygen vacancies.
Microlithographic patterning of polythiophene films
Authors: Kutsche Carl , Targove Jim , Haaland Peter
Published: July 2002

Lithographic patterning of micrometer scale structures in films of polythiophene, an organic material with good linear and nonlinear χ(3) optical properties, has been accomplished by two methods: direct deposition of polythiophene in SiO2 trenches and an oxygen plasma etch of polythiophene through a spin-on glass mask.
Evidence for quantum tunneling of vortices in ultrathin superconducting films (abstract)
Authors: Liu Y. , Haviland D. B. , Glazman L. I. , Goldman A. M.
Published: July 2002

The creep and flow of vortices have been subjects of study because they give rise to nonzero electrical resistance even below the superconducting transition temperature. In the classical model of thermally activated creep of vortices the resistance vanishes exponentially at low temperatures with a dependence given by an Arrhenius form. Non-Arrhenius behavior at low temperatures can result from an alternative to the thermal processes involving macroscopic quantum tunneling. Unusual temperature dependence of electrical resistance has been observed in ultrathin superconducting films of Pb, Al, and Bi in small magnetic fields. At low temperatures, film sheet resistances varied with temperatures as R≊R0 exp(T/T0), where T0 and R0 are constants. The parameter T0 is independent of the sheet resistance at high temperatures. These results are not expected in conventional models of vortex motion involving thermal activation, but may be explained using a quantum tunneling picture.1
Stability of superconducting La1.8Sr0.2CaCu2O6 with respect to oxygen partial pressure
Authors: Sakurai Takeshi , Yamashita Toru , Yamauchi H.
Published: July 2002

The thermodynamical stability of a superconducting La-326 compound, La1.8Sr0.2CaCu2O6, was studied by observing the phase formation at high pressures and decomposition at high temperatures. The superconducting La-326 compound, which was synthesized by means of an oxygen hot isostatic press annealing was found to be stable up to 800 °C at ambient pressure. When post-annealed at temperatures between 600 and 800 °C, the amount of superconductive volume fraction was increased and TcR=0 was also enhanced. When post-annealed at temperatures higher than 800 °C, La-326 started to decompose and the amount of secondary 214(T-structure) phase increased. It was confirmed that high-pressure annealing at an oxygen pressure of 10 atm was required to form the superconducting La-326 phase, which exhibits Tc of higher than 40 K. The high-pressure annealing conditions, such as Ar-to-oxygen ratio and annealing temperature, were optimized in order to make the La-326 phase superconductive. It was concluded that an oxygen pressure of at least 10 atm was required to ‘‘superconductor-ize’’ the La-326 phase, regardless of the magnitude of total pressure.
Plasma-polymerized C60/C70 mixture films: Electric conductivity and structure
Authors: Takahashi Noboru , Dock Henrik , Matsuzawa Nobuyuki , Ata Masafumi
Published: July 2002

Polymerization of a mixture of C60/C70 in rf plasma is reported. The electric dark-current conductivity of the plasma-polymerized mixture of C60/C70, which is approximately 10−7 S/cm in the atmosphere, does not depend on the applied voltage at least in the range of −25 to 25 V. A semiconductor-type temperature dependence of the conductivity in the higher-temperature domain was observed, and the band-gap energy was estimated to be 2.1 eV. The conductivity increased with increasing temperature from 25 to 230 °C in vacuum, whereas in the atmosphere the conductivity increased upon decreasing the temperature below 80 °C. It is supposed that in this temperature domain the electric conductivity is enhanced by the existence of water molecules on the film, the surface of which is characterized by a high hydrophilicity. The surface morphology of the polymerized film was characterized by the presence of aggregates with diameters of about 300 Å and the surface was highly hydrophilic, polar, and heterogeneous. The surface has a completely amorphous molecular structure. When the C60 molecules polymerize each other, it has been shown that the polymerization of C60 molecules proceeds, at least primarily, by the formation of 1,2-cyclobutane structures between the cyclohexatrienyl parts of neighboring C60 molecules. The addition of hydrogens, hydroxyl groups, carbonyl groups, and aryl peroxide groups to the radical sites occurs but the free radicals of the order of magnitude 1017 spins/g were found to remain without quenching.
Large stable second-harmonic coefficients in an azo-dye attached polymer oriented by corona poling
Authors: Martinez David R. , Koch Karl , Ratsavong Felix K. , Carlisle Gene O.
Published: July 2002

Second-harmonic generation and spectroscopic absorption measurements were made over a 15-month period to study the nonlinear-optical thin-film properties of an azo-dye attached polymer in which the dye chromophores were oriented by corona poling. Stable, resonance-enhanced values of the nonlinear optical coefficient d33 for the fundamental of 1.064 μm were as high as 68 pm/V. The stabilized order parameter, calculated from absorbance measurements, was found to be 0.51 for the chromophores.
Determination of the reduced mass of the exciton ground state in a quantum well
Authors: Lee Kyu-Seok , Lee El-Hang
Published: July 2002

A variational method employing two variational parameters is proposed for the calculation of the magnetoexciton ground state in a semiconductor quantum well. One parameter is to take into account the effective oscillator potential and the other is related to the effective coupling constant of the Coulomb potential, both under the influence of a magnetic field. The in-plane reduced mass of the heavy-hole exciton and the effective coupling constant of the Coulomb interaction at zero magnetic field are treated as adjustable parameters, to be determined from the comparison of the present calculation and experimental results. This provides a useful method for obtaining the in-plane mass of a heavy hole in a quantum well.
Ion size effect on normal state resistivity in (R0.8Pr0.2)Ba2Cu3O7−δ (R=Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb) systems
Authors: Chen Y. , Lai T. S. , Wu M. K. , Guan W. Y.
Published: July 2002

Measurements of the temperature dependence of the normal state resistivity, ρn (T), for (R0.8Pr0.2)Ba2Cu3O7−δ (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb) are reported. We found that ρn is linearly dependent on the temperature (except R=Nd). At a constant temperature, ρn is linearly dependent on the ionic radius of R. The results are interpreted in terms of the hybridization between the local states of the Pr ion and the valence-band states of the CuO2 planes.
Insufficient oxidation in pulsed laser deposition
Authors: Hiratani Masahiko , Imagawa Kazushige , Takagi Kazumasa
Published: July 2002

The influence of the ratio of oxygen atoms to ablated species in the ablation plume on the oxidation state of thin films was examined through the film growth of SrTiO3 by pulsed laser deposition. The amount of the ablated species was adjusted by changing the laser spot size on the target while keeping the oxygen pressure and laser energy density constant. The oxidation state is not defined by the background oxygen pressure during deposition. Instead, it is dominated by the oxidation which occurs when a burst of deposited species interacts with the oxygen atoms in the plume and incorporates oxygen ions into the lattice.
Nanometric crystallization of Fe73.5Cu1Nb3Si13.5B9 by laser annealing
Authors: Lanotte Luciano , Iannotti Vincenzo
Published: July 2002

The possibility of producing nanocrystallites by laser heat treatment of a metallic glass is investigated. Structural and magnetic properties, after various annealing conditions, are reported. Laser annealing is demonstrated to be an effective, alternative technique for control of nanometric crystallization. This technique also shows a number of advantages with respect to conventional heat treatments.
Characterization of C coimplanted GexSi1−x epitaxial layers formed by high dose Ge ion implantation in (100) Si
Authors: Lombardo S. , Larsen K. Kyllesbech , Raineri V. , La Via F. , Campisano S. U. , Lagomarsino S. , Kazimirov A.
Published: July 2002

We have investigated electrical and structural characteristics of C coimplanted GexSi1−x layers formed by high dose Ge implantation in (100) Si followed by rapid thermal annealing at 1100 °C for 10 s. In the absence of C the layers, with Ge peak concentrations of about 15 at. %, are epitaxial and fully strained but show extrinsic dislocation loops in the Ge end of range region. In C coimplanted GexSi1−x the crystalline quality of the materials is improved since the formation of defects at the end of range is suppressed and the strain is reduced. From an electrical point of view the situation is reversed. The electrical characterization of p+/n diodes shows that, without C coimplantation, in GexSi1−x the hole mobility is larger than in Si, and the bandgap narrowing results in an increase of the minority carrier concentration. Moreover, the diffusion of minority carriers does not appear to be influenced by the presence of the secondary defects at the end of range. Vice versa, in C coimplanted GexSi1−x, even if the extended defects are not present, we observe a high concentration of minority carrier recombination centers, which strongly reduce the carrier diffusion length.
Determination of space-charge dynamics and transport parameters in hydrogenated carbon layers by time-resolved electroluminescence measurements
Authors: Foulani A. , Laurent C. , Mebarki B. , Segui Y.
Published: July 2002

A new way is presented for deriving space-charge dynamics and transport parameters in thin films sandwiched between two metallic electrodes. The measurement method uses the detection of electroluminescence excited by square voltage pulses. The space-charge buildup time is derived from the electroluminescence time lag when the structure is excited by a single square voltage pulse. The relaxation time of the space charge is investigated by using a double pulse excitation. Carrier mobility is determined by an optical transit-time technique. Measurements are carried out on hydrogenated amorphous carbon layers with different composition and electrical properties. The films display space-charge limited conduction which is due to the existence of trap states. Their concentration is derived from the current–voltage characteristics and the mobility measurements. The trap concentration increases with a decreasing graphite content in the films. Film properties are weakly temperature dependent in graphite-rich layers as opposed to the behavior of layers with a lower graphite concentration. In this later case, the structural features of the films change at low temperature as suggested by the evolution of the trap concentration which decreases by three orders of magnitude, going from an exponential to a uniform distribution.
Exchange coupling in spin-valve structures containing amorphous CoFeB
Authors: Fujita M. , Yamano K. , Maeda A. , Tanuma T. , Kume M.
Published: July 2002

(Co 0.9 Fe 0.1 ) 80 B 20 /Cu/(Co 0.9 Fe 0.1 ) 80 B 20 /Ni 80 Fe 20 /Fe 50 Mn 50 spin-valve structures with a different Ni80Fe20 layer thickness were prepared by ion-beam sputtering. Varying the thickness of the Ni80Fe20 layer made it possible to control the exchange coupling between the Fe50Mn50 and amorphous CoFeB layers. When no Ni80Fe20 layer was inserted, the exchange coupling was responsible for increasing the coercivity of the a-CoFeB layers. With increasing Ni80Fe20 layer thickness, the biasing fields of the Fe50Mn50 layer increased from 0 to 125 Oe. The critical Ni80Fe20 thickness, above which the biasing effect was observed, was experimentally estimated to be 2 nm.
Structure of the optical phase change memory alloy, Ag–V–In–Sb–Te, determined by optical spectroscopy and electron diffraction
Authors: Tominaga J. , Kikukawa T. , Takahashi M. , Phillips R. T.
Published: July 2002

The structure of the optical phase change memory alloy, silver–vanadium–indium–antimony–tellurium (AVIST), has been investigated by the methods of optical reflectivity change, Raman spectroscopy, and electron diffraction. In order to identify each phase, Sb, AgSb, AgSbTe, and AgInTe films were also studied. The Raman spectrum of an AVIST thin film annealed at 523 K for 1 h in Ar gas, shows two main broad peaks around 116 and 150 cm−1. These peaks appear to be related to Sb vibration modes in the AVIST alloy, but their relative intensities are reversed for the AVIST films in comparison with those for a Sb film. Furthermore, in AVIST, the peak at 116 cm−1 shifts to lower wave number when the vanadium content is increased from 0.4 to 4.7 at %. The electron diffraction pattern of AVIST alloy indicates that the AgSbTe2 crystalline phase dominates rather than Sb. A model of the structure of AVIST consistent with these observations is proposed.
Fano factor and the mean energy per ion pair in counting gases, at low x-ray energies
Authors: Pansky A. , Breskin A. , Chechik R.
Published: July 2002

The mean energy per ion-pair (Wi) and the Fano factor (F) are provided with high accuracy (2% and 3%-4%, respectively), in C2H6, C3H8, i-C4H10, CH4, DME, Ar/C2H6(20:80), Ar/i-C4H10(20:80) Ar/DME (20:80) and Ar/Xe/i-C4H10(66.6/16.7/16.7), in the x-ray energy range of 0.11–1.5 keV. These parameters were extracted from precise measurements of the number and temporal distribution of x-ray induced electrons, accompanied by extended simulations of the detection process. A decrease in these parameters with increasing x-ray energy was observed, accompanied by sharp increases at x-ray energies just above some atomic shells. The effect is discussed in relation to Auger electron emission. A Penning process in Ar/C2H6(20:80) and Ar/i-C4H10 (20:80) is observed on the basis of comparative measurements of Wi and F in these mixtures and in the pure hydrocarbons. Ways are proposed for further improving the accuracy provided by the electron counting technique to better than 1%.
Chiral-homeotropic liquid crystal cells for high contrast and low voltage displays
Authors: Wu Shin-Tson , Wu Chiung-Sheng , Lin Kun-Wei
Published: July 2002

Computer simulations on the display characteristics of chiral-homeotropic liquid crystal cells are performed. This normally black display mode exhibits an excellent contrast ratio, nearly achromatic light transmission, and low operation voltage. The optimal twist angle for suppressing the wavelength and voltage dependencies ranges from 90° to 150°. Promising applications of this operation mode for both transmissive and reflective displays are discussed.
Nanometer-scale lithography in thin carbon layers using electric field assisted scanning force microscopy
Authors: Mühl T. , Brückl H. , Weise G. , Reiss G.
Published: July 2002

Resist patterning by scanning probe microscopy is a promising method to create structures in the nanometer range beyond the resolution of conventional electron beam or photo lithography. In conventional resist processing one has to remove either the exposed or unexposed resist in a solvent by an additional step. In this article we demonstrate the possibility of directly writing nano-scaled patterns in a thin amorphous carbon layer, which can be used as an etching mask, by a scanning force microscope. Above a threshold voltage between tip and sample small trenches can be created, whereby the carbon is completely removed from the exposed areas. Evidence is given that the mechanism responsible for the trench formation is a local field-induced oxidation of the carbon layer underneath the tip.
Enhanced Yb3+-related 0.98 μm emission in porous silicon and its time decay characteristics
Authors: Kimura Tadamasa , Nishida Yasuhiro , Yokoi Akinori , Saito Riichiro
Published: July 2002

A strong enhancement in the intensity of the ∼0.98 μm emission (2F5/2→2F7/2) of Yb3+-ions in porous silicon is obtained by adding a pre-annealing process to host porous silicon in O2 or H2 prior to Yb3+-ion incorporation and subsequent post-dope annealing. The luminescence intensity shows a small temperature quenching, decreasing from 20 K to 300 K by a factor of ∼10. The time decay measurements show that there are two major Yb3+-related luminescence centers in Yb-doped porous silicon. One is a fast decaying center with a decay time of ∼30 μs at 20 K which decreases rapidly with increasing temperature. The other is a slowly decaying center with an almost temperature independent decay time of ∼400 μs. The latter is responsible for the small temperature quenching of Yb3+-related 0.98 μm emission.