bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutron-diffraction study on the structure of Nd(TiFe)12Nx and Nd(TiFeCo)12Nx alloys
Authors: Pan Shu-Ming , Chen Hong , Xu Zu-Xiong , Ma Ru-Zhang , Yang Ji-Lian , Zhang Bai-Sheng , Xue De-Yan , Ni Qiang
Published: July 2002

The preference of nitrogen atoms in Nd(FeTi)12Nx and Nd(FeTiCo)12Ny alloys have been studied by neutron diffraction. The results show that Ti atoms are preferentially to occupy the 8i sites, but Co atoms enter the 8f rather than the 8i site. Nitrogen atoms occupy the 2b sites in Nd(FeTi)12Nx and Nd(FeTiCo)12Ny.
Low field giant magnetoresistance and oscillatory interlayer exchange coupling in polycrystalline and (111)-oriented permalloy/Au multilayers (abstract)
Authors: Parkin S. S. P. , Rabedeau T. A. , Farrow R. F. C. , Marks R.
Published: July 2002

The existence of oscillatory interlayer exchange coupling of ferromagnetic layers via (111)-oriented noble metal spacer layers is controversial. We present evidence from magnetic and giant magnetoresistance studies for well-defined antiferromagnetic interlayer coupling in single crystalline (111) permalloy/Au multilayers. Four oscillations in the coupling are observed as the Au spacer layer thickness is increased. The oscillation period is ≂10 Å which is significantly shorter than the period of ≂11.5 Å predicted in Ruderman–Kittel–Kasuya–Yosida based models. Similar oscillatory interlayer coupling is found in polycrystalline permalloy/Au multilayers prepared by dc magnetron sputtering. The interlayer coupling strength is significantly weaker in the polycrystalline as compared to the (111)-oriented crystalline samples. In both cases the coupling strength is weaker than in comparable structures containing Ag, for which the coupling is weaker than in similar structures containing Cu. The weakness of the antiferromagnetic interlayer coupling via Au leads to very low saturation fields, lower than for all other noble and transition metals. Indeed, the saturation fields are as low as just a few Oersted for sufficiently thick Au layers. Consequently, we find giant magnetoresistance values of ≂1%/Oe or greater at room temperature in polycrystalline permalloy/Au multilayers. These values are the highest values yet reported in multilayer structures and are comparable to or greater than those recently reported in discontinuous permalloy/Ag multilayers.
Study of permanent magnetic properties of the 1-12 nitrides with Nd and Pr
Authors: Yang Ying-Chang , Pan Qi , Cheng Ben-Pai , Zhang Xiao-Dong , Liu Zun-Xiao , Sun Yun-Xi , Ge Sen-Ling
Published: July 2002

The permanent magnetic properties of isotropic and anisotropic powders of R(Fe,Mo)12Nx compounds, where R=Pr and Nd, were investigated by using mechanical alloying and a conventional milling process. For the isotropic magnetic powders, a coercive force of up to 9 kOe and a maximum energy product of 6.6 MGOe were obtained in Nd compounds, while with the anisotropic powders a maximum energy product of 12 MGOe was achieved. In the isotropic Pr(Fe,Mo)12Nx compounds the best result were 4.5 MGOe for (BH)max and 6.0 kOe for iHc. The temperature dependence of coercive force has been investigated in a temperature range between 300 and 500 K, and the results are reported in a comparison with that of Nd-Fe-B magnets.
Structural and magnetic properties of Ce(Fe,M)12Nx interstitial compounds, M=Ti, V, Cr, and Mo
Authors: Pan Qi , Liu Zun-Xiao , Yang Ying-Chang
Published: July 2002

Stabilizing the 1-12 phase with cerium was possible using a special heat treatment. The structural and magnetic properties of Ce(Fe,M)12 and their nitrides, for M=Ti, V, Cr, Mo, etc., were investigated using x-ray-diffraction and magnetic measurements. The effects of interstitial nitrogen atoms on lattice parameters, Curie temperature, saturation magnetization, and magnetocrystalline anisotropy are reported. A profound change was found on the magnetocrystalline anisotropy. As an example, in the case of CeMo2Fe10Nx, the easy magnetization direction changes from c axis to the basal plane depending on the nitrogen content x.
Magnetoresistance of ultrathin Co films grown in UHV on Au(111): Crossover from granular to continuous film behavior versus Co thickness (abstract)
Authors: Dupas C. , Kolb E. , Renard J. P. , Vélu E. , Galtier M. , Mulloy M. , Renard D.
Published: July 2002

Co/Au(111) is a model system for magnetoresistance (MR) studies due to abrupt interfaces without intermixing between Co and Au and perpendicular magnetization of Co films for Co thickness below nine atomic monolayers (AL). The high quality of Co/Au(111) multilayers is revealed by highly contrasted MR oscillations versus Au spacer thickness. We report here a detailed study of the effect of cobalt thickness done to 0.2 AL on the MR of various Au/Co structures. Experiments were performed on single Co layer sandwiched between two Au(111) films, Au/Co/Au, and on double Co films, Au/Co/Au/Co/Au, in the T range 1.5–300 K and in magnetic fields up to 10 T. For the sandwiches, two series were studied, each including eight samples grown in UHV in a single run, with Co thicknesses tCo from 0.2 to 1.6 and from 1.4 to 4 AL. The MR in the perpendicular and transverse configurations show that the easy magnetization axis is perpendicular to the films, even for low tCo. The MR value increases with tCo to the high value of 5% at low temperatures for a noncontinuous Co film and decreases afterwards, clearly evidencing the transition from islandlike to continuous-like character of the Co film. In the islandlike regime, the magnetic domain size is limited by the island size and is thus smaller than in the continuous Co films, leading to higher MR values. In the double Au/Co/Au/Co/Au samples, different Au spacer thicknesses were chosen in order to ensure either FM or AFM coupling between the magnetic layers, and tCo was varied up to 10 AL. We did not observe coupling oscillations versus tCo. Moreover, the lack of any significant variations of the MR value shows that the electronic mean free path in Co is larger than 10 AL at any T.
Magnetic properties of Fe clusters in NaY zeolite
Authors: Cowen J. A. , Tsai K. L. , Dye J. L.
Published: July 2002

Clusters of up to 28 Fe0 atoms have been introduced into the ‘‘supercages’’ of NaY zeolite by ion exchange of Fe+2 for Na+ followed by reduction to Fe0 with solvated electrons and Na−1 ions. The temperature-dependent ac and dc susceptibilities, which exhibit Curie–Weiss behavior before reduction, change to something approximating superparamagnetic behavior afterwards. The blocking temperature, Tb shows a strong dependence on magnetic field but weak dependence on frequency. There is at best a very weak remanence and coercive field while the onset of irreversibility occurs at temperatures well above Tb. In addition, the real and imaginary parts of the ac susceptibility show essentially similar temperature dependence. Some of this anomalous behavior can be attributed to a distribution of particle sizes. To the best of our knowledge these preliminary data—while poorly understood—represent the first measurements of the temperature, frequency, and field dependence of the magnetic properties of such small clusters.
Spin-specific photoelectron diffraction using magnetic x-ray circular dichroism
Authors: Tobin J. G. , Waddill G. D. , Guo X. , Tong S. Y.
Published: July 2002

The first observation of spin-dependent photoelectron diffraction using circularly polarized x rays is reported for monolayer ferromagnetic fcc Fe films on Cu(001). Circularly polarized x rays are used to produce spin-polarized photoelectrons from the Fe2p spin-orbit split doublet, and intensity asymmetries in the 2p3/2 level of ≊3% are observed. The asymmetry is dependent on the relative orientation of the x-ray polarization vector and the Fe magnetic moment. This spin-dependent technique promises the direct, element-specific determination not only of local atomic structure, but magnetic structure as well.
Anisotropic giant magnetoresistance induced by magnetoannealing in Fe-Ag granular films
Authors: Na J. G. , Yu C. T. , Zhao X. G. , Lai W. Y. , Luo H. L. , Zhao J. G.
Published: July 2002

The effects of magnetoannealing on the giant magnetoresistance (GMR) in FexAg100−x granular films (X=15, 26, 29, 33, 37, and 60) were investigated. The thin films were annealed in a presence of magnetic field of 3 kOe at different temperatures of 300, 400, and 500 °C using various annealing times. It is found that the anisotropic GMR characteristics were developed when Fe-Ag granular thin films were annealed in the presence of a magnetic field. The anisotropic GMR characteristics of the thin films were closely related to the magnetic anisotropy developed along the field direction during magnetoannealing.
High sensitivity GMR in NiFeCo/Cu multilayers
Authors: Gangopadhyay S. , Hossain S. , Yang J. , Barnard J. A. , Kief M. T. , Fujiwara H. , Parker M. R.
Published: July 2002

Magnetoresistance and magnetic hysteresis in NiFeCo/Cu soft magnetic multilayers with a fixed number of bilayers (six) and magnetic and spacer layer thickness but varying composition has been studied. The highest value of the transverse GMR obtained is 6.8% in a saturation field of ≊40 Oe at room temperature. Very high sensitivity, around 1%-2%/Oe and unconventional easy-axis hysteresis and GMR loop shapes have been observed. The analysis of the GMR effects and the associated hysteretic behavior by using a model that includes biquadratic exchange coupling suggests that the samples are composed of at least two distinctively different parts.
Extended x-ray-absorption fine-structure studies of heat-treated fcc-Fe50Cu50 powders processed via high-energy ball milling
Authors: Crespo P. , Hernando A. , Escorial A. Garcia , Kemner K. M. , Harris V. G.
Published: July 2002

The local structure and chemistry of a ferromagnetic fcc-Fe50Cu50 solid solution obtained through high-energy ball milling were measured before and after heat-treatment-induced decomposition using extended x-ray-absorption fine-structure measurements. The decomposition is first evident with the phase separation of α-Fe after a heat treatment at 523 K. Analysis of the residual fcc component revealed that the Fe atoms were predominantly surrounded by other Fe atoms, suggesting that the Fe has coalesced within the fcc structure. The Fe atoms within the fcc phase likely exist in low-spin clusters which provide an explanation for the reduced values of low-temperature magnetization previously measured in annealed samples [P. Crespo et al., Phys. Rev. B 48, 7134 (1993)].
Experimental observation of magnetostochastic resonance
Authors: Grigorenko A. N. , Nikitin P. I. , Slavin A. N. , Zhou P. Y.
Published: July 2002

We report an observation of magnetostochastic resonance (MSR) in bistable magnetic systems. MSR manifests itself as a peak of a response of such a system to a periodic field as a function of the noise strength. Bi-substituted ferrite-garnet films with uniaxial anisotropy were used as simple examples of bistable systems. Noise was produced by (i) thermal fluctuations of an effective magnetic field and (ii) externally applied ‘‘noiselike’’ magnetic fields. The position of the peak depended dramatically on the magnitude of applied dc fields, so that dc fields of about 1 mOe in the spatial scale of 10 μm were measured. MSR applications to various tunneling phenomena such as tunneling of magnetization in small magnetic particles are discussed.
Monte Carlo simulation of Ising models with dipole interaction
Authors: Nowak U. , Hucht A.
Published: July 2002

Recently, a new memory effect was found in the metamagnetic domain structure of the diluted Ising antiferromagnet FexMg1−xCl2 by domain imaging with Faraday contrast. Essential for this effect is the dipole interaction. We use a Monte Carlo method to simulate the low-temperature behavior of diluted Ising antiferromagnets in an external magnetic field. The metamagnetic domain structure occurring due to the dipole interaction is investigated by graphical representation. In the model considered, the antiferromagnetic state is stable for an external magnetic field smaller than a lower boundary Bc1 while for fields larger than an upper boundary Bc2 the system is in the saturated paramagnetic phase, where the spins are ferromagnetically polarized. For magnetic fields in between these two boundaries a mixed phase occurs consisting of ferromagnetic domains in an antiferromagnetic background. The position of these ferromagnetic domains is stored in the system: after a cycle in which the field is first removed and afterwards applied again the domains reappear at their original positions. The reason for this effect can be found in the frozen antiferromagnetic domain state which occurs after removing the field at those areas which have been ferromagnetic in the mixed phase.
Exchange coupling, interface structure, and perpendicular magnetic anisotropy in Tb/Fe multilayers (abstract)
Authors: Tappert J. , Jungermann J. , Scholz B. , Brand R. A. , Keune W.
Published: July 2002

Tb/Fe multilayer films are known to show perpendicular magnetic anisotropy (PMA), which is interesting for potential applications in magneto-optical data storage. We demonstrate by using 57Fe Mössbauer probe layers and x-ray diffraction that part of the Tb-deposited-on-Fe (Tb/Fe) interface remains chemically sharp and crystalline bcc Fe. This part is associated with an unexpectedly strong antiferromagnetic exchange coupling of the Fe to the Tb layers. We find by using both conventional and circularly polarized radiation that part of the interface and core bcc Fe but none of the amorphous Fe is strongly coupled to Tb. This behavior is qualitatively explained by a structural model including regions with a sharp Tb/bcc-Fe interface, where PMA originates from Tb ions that experience the broken symmetry of the crystal electric field at the interface. The extrapolated isomer shift at T=0 K of the amorphous phase located at the Fe-deposited-on-Tb (Fe/Tb) interface was found to be different from that of the amorphous phase located at the Tb/Fe interface, suggesting a different amorphous structure at both interfaces.
The magnetization density profile of a grain boundary in nickel (invited)
Authors: Fitzsimmons M. R. , Röll A. , Burkel E. , Sikafus K. E. , Nastasi M. A. , Smith G. S. , Pynn R.
Published: July 2002

We report the measured atomic and magnetization densities of a nickel twist grain boundary averaged over its lateral dimensions as a function of distance from the interface plane. The presence of a reconstructed interface region (the grain boundary) sandwiched between two single-crystal nickel films that were hot-pressed together was confirmed with grazing incidence x-ray diffraction. From reflection data taken using unpolarized neutrons, the atomic density profile of the grain boundary was determined to be (85±5)% of the bulk density when averaged over twice the grain-boundary width of (8±1) nm. Using this information in conjunction with the reflectivity data taken from the nickel sample with polarized neutrons reflection, the magnetic moment of a nickel atom was found to be between 18% and 52% larger in the twist grain boundary than in the bulk. The enhancement of the magnetic moment at the grain boundary is believed to result from the reduced atomic density of the interface region. Owing to this reduction in density, the magnetization density of the nickel interface is only somewhat enhanced, about 10%, compared to that of bulk nickel.
Anisotropic magnetism and resistivity of an Al70Ni15Co15 decagonal quasicrystal
Authors: Markert J. T. , Cobb J. L. , Bruton W. D. , Bhatnagar A. K. , Naugle D. G. , Kortan A. R.
Published: July 2002

We report the results of dc magnetization and electrical resistivity studies of a single-domain Al70Ni15Co15 decagonal quasicrystal. The temperature dependence and anisotropy of the electrical resistivity are in excellent agreement with previous results for a similar sample. The magnetic properties were determined by superconducting quantum interference device magnetometry over the temperature range 1.8–300 K for magnetic fields between 10 Oe and 10 kOe. The magnetic behavior can be characterized by a weak ferromagnetic component M0(H) which saturates in moderate fields (H≊1 kOe) and a differential susceptibility, χ0=ΔM/ΔH, obtainable from the high-field data. The observed magnetization M=χ0H+M0 is slightly anisotropic and essentially temperature independent below room temperature. For H parallel to the quasicrystal axis, we observe χ0∥=7.1×10−7 emu/g and M0∥=2.0×10−3 emu/g. For H perpendicular to the quasicrystal axes, we obtain χ0⊥=8.2×10−7 emu/g and M0⊥=1.7×10−3 emu/g. An anisotropic and weak superconducting transition was observed at T=3.2 K, corresponding to a superconducting volume fraction of only ∼10−3.
Antiferromagnetic order in superconducting UPt3: An x-ray magnetic scattering study (invited)
Authors: Isaacs E. D. , Zschack P. , Ramirez A. P. , Oglesby C. S. , Bucher E.
Published: July 2002

The temperature dependence of the antiferromagnetic order in superconducting UPt3 has been measured using x-ray resonance magnetic scattering. The magnetic Bragg intensity at Q=(1/2,0,2) grows linearly from TN=5 K to T≂0.6 K (TC+=0.53 K), where it becomes suppressed with temperature to a reduction of ∼6% at T=180 mK. These results demonstrate a coupling of the superconducting and antiferromagnetic order parameters and are consistent with a suppression in the magnitude of the ordered moments below Tc when compared with previously obtained neutron-scattering data.
Non-Fermi liquid ground states in strongly correlated f-electron materials (invited) (abstract)
Authors: Maple M. B.
Published: July 2002

Strongly correlated electron materials, especially the high transition temperature (Tc) cuprate superconductors and heavy fermion rare earth and actinide compounds, have been the focus of intense investigation in recent years. The unusual normal state properties of the high Tc cuprates have been ascribed by some researchers to a non-Fermi liquid (NFL) metallic state, while the anomalous properties of certain heavy fermion U-based superconductors such as UBe13 have been attributed to NFL behavior due to a two-channel quadrupolar Kondo effect. Although NFL behavior has not been established in any of the U-based heavy fermion superconductors, evidence for its occurrence in an f-electron material has been found in the U alloy system Y1−xUxPd3. Subsequently, NFL behavior was observed in a number of other f-electron alloy systems, including Sc1−xUxPd3, UCu3.5Pd1.5, Th0.1U0.9Be13, Th1−xUxRu2Si2 (x≤0.07), Th1−xUxPd2Al3, La0.9Ce0.1Cu2Si2, and CeCu5.9Au0.1. Evidence for NFL behavior in Y1−xUxPd3 and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity ρ∼1−aT with the coefficient a either ≳0 or <0, a logarithmically diverging specific heat C/T∼−ln T, and T1/2 asymptotic behavior of the magnetic susceptibility χ∼1−T1/2.
Magneto-oscillatory phenomena in highly correlated metals (invited) (abstract)
Authors: Julian S. R. , McMullan G. J. , Pfleiderer C. , Tautz F. S. , Lonzarich G. G.
Published: July 2002

A central question in the physics of interacting electron systems is whether conditions exist under which the conventional Landau description fails to describe the low energy excitations of the ‘‘normal’’ metallic state. Measurements of the temperature and magnetic field dependences of the magnetization and resistivity—in particular of the de Haas–van Alphen and Shubnikov effects which provide sensitive probes of microscopic processes in metals—are currently being used to examine this question. Advances in material preparation and detection techniques have made it possible to conduct exhaustive studies in selected examples of systems of highly correlated d and f electrons. Most of our results will be shown to be consistent with the broad predictions of the Landau model. In some extreme circumstances, however, near low temperature transitions as a function of hydrostatic pressure or applied magnetic field, evidence has been collected which appears difficult to reconcile with the standard model of the metallic state.
Two-dimensional current transient and trap-state filling of amorphous silicon thin-film transistors
Authors: Huang J. S. , Wu C. H.
Published: July 2002

Two-dimensional simulations of amorphous silicon thin-film transistors are presented for the case when source-drain voltage is turned on long before gate voltage is turned on. Discrepancies between these results and the one-dimensional results of M. F. Willums, M. Hack, P. G. LeComber, and J. G. Shaw [MRS Symp. Proc. 258, 985 (1992)] are discussed. Valid reasons for drain current decay are provided, and occupation dynamics for the trap states are shown in order to distinguish these from the one-dimensional results of C. van Berkel, J. R. Hughes, and M. J. Powell [J. Appl. Phys. 66, 4488 (1989)] where a two-fluid model occupation function was assumed. The invalidity of such approximation is explicitly demonstrated. The mean trap-filling energy level moves up in three stages: First, the level varies with log t, then varies linearly with t, and finally, with log (log t) to a steady-state level.
Compensation of shallow donors in dimethylaluminum methoxide-doped GaAs
Authors: Park Y. , Skowronski M.
Published: July 2002

Hall-effect and deep-level transient spectroscopy measurements have been performed on GaAs epilayers grown by organometallic vapor-phase epitaxy and intentionally doped with diethyltellurium and dimethylaluminum methoxide (DMAlMO). The DMAlMO doping was observed to compensate shallow tellurium donors for concentrations as high as 1019 cm−3, producing fully compensated high-resistivity epilayers. Seven DMAlMO-induced deep levels have been observed in the GaAs band gap with activation energies of 0.25, 0.30, 0.37, 0.40, 0.58, 0.74, and 0.93 eV. Less than 5% of all oxygen atoms were present in the layer in the form of isolated off-center substitutional oxygen corresponding to the 0.58 eV level. The major traps located at 0.74 and 0.93 eV below the conduction band have been attributed to complexes of aluminum and oxygen involving two and one aluminum atoms, respectively. Both of these levels are deep acceptors responsible for the compensation of shallow donors.
Accurately determining the composition and thickness of layers in a GaAs/InGaAs superlattice
Authors: Jones K. A. , Cole M. W. , Cooke P. , Flemish J. R. , Pfeffer R. L. , Shen H.
Published: July 2002

The layer thicknesses and composition of molecular beam epitaxy grown four period 200 Å/100 Å GaAs/InGaAs superlattice structures with nominal indium concentrations of 10%, 15%, and 20% were determined by transmission electron microscopy, Rutherford backscattering spectroscopy, double crystal x-ray diffraction (DXRD), photoreflectance (PR), and photoluminescence (PL). The results show that the indium concentration obtained by DXRD is a little low and that obtained by PR and PL is a little high, and that the discrepancies are larger for the larger indium concentrations. We show that both discrepancies can be accounted for by relaxation of the lattice, elastic relaxation as represented by a radius of curvature, and/or plastic deformation as represented by mismatch of dislocations. For the case of elastic relaxation the tetragonal distortion is less than it would be if the sample were perfectly pseudomorphic. The fractions by which it is reduced for the 10%, 15%, and 20% samples was 0.91, 0.86, and 0.77 as determined by DXRD and 0.80, 0.78, and 0.85 as determined by PR/PL.
Magnetic susceptibility and electronic specific heat of Anderson lattice with finite f-band width
Authors: Panwar Sunil , Singh Ishwar
Published: July 2002

We study an extension of the periodic Anderson model by considering finite f-band width. A variational method recently developed, has been used to study the temperature dependence of the average valence of magnetic susceptibility χs and electronic specific heat Cv for different values of the f-band width. As f-band width increases, the low-temperature peak in χs and Cv becomes more broad and shifts towards the high-temperature region.
Comparison of generation and recombination lifetimes in high-resistivity silicon
Authors: Fontaine Jean-Charles , Barthe Sylviane , Ponpon Jean-Pierre , Schunck Jean-Pierre , Siffert Paul
Published: July 2002

Generation and recombination lifetimes have been theoretically studied for the case of high-resistivity silicon on the basis of the Shockley–Read–Hall model. Thanks to the high resistivity of silicon, it has been possible to take into account only the generation-recombination levels which are situated between the Fermi level EF and the level E′F symmetrical to EF with regard to the midgap. First, a simple approach based on the presence of a single generation-recombination level in the band gap has been considered; however, its validity is fair because chemical analyses of high-resistivity silicon show that several levels with energy close to midgap or to the boundaries EF–E′F are generally present. This means that levels which are located at different positions in the gap may influence the generation and recombination lifetimes τg and τr but are not taken into account by such a model. In order to overcome this difficulty, a two-level model has been developed. The main result is that the ratio of τg/τr should range between 1 and 10. Experimental confirmation of this later model has been obtained by measuring the two lifetimes τg and τr whose resulting ratio was found to be within the limits 1–4.
Stabilization of the hexagonal close-packed phase of cobalt at high temperature
Authors: Barradas N. P. , Wolters H. , Melo A. A. , Soares J. C. , da Silva M. F. , Rots M. , Leal J. L. , Melo L. V. , Freitas P. P.
Published: July 2002

The structure of thin cobalt films sputtered on a rhenium buffer was analyzed from room temperature to 963 K, using the perturbed angular correlation technique with the 111In/111Cd probe. The hyperfine field corresponding to the fcc phase, which is the stable one above 704 K, is never observed, and the measured hyperfine field follows closely a theoretical prediction for the hcp phase. This is a consequence of the good match between the cobalt and the Re hcp lattices that leads to the stabilization of the hcp phase up to 963 K.
Changes in the trapping and recombination process of hydrogenated amorphous silicon in the Staebler–Wronski effect
Authors: Kounavis P.
Published: July 2002

The constant photocurrent method and the modulated photocurrent method (MPC) indicate that the defect density remains constant during the first minutes of the optical degradation of a-Si:H or during its thermal annealing at temperatures lower than 150 °C. Under these conditions of exposure or annealing the optical bias dependence of the MPC (OBMPC) indicate that significant changes are induced to the trapping and the recombination process of the photocarriers. These changes are interpreted by a light induced increase of the capture probability of the deeper localized states of the energy gap for the photocarriers, increasing the recombination rate. The conclusions drawn are discussed with the predictions of various existing models concerning the Staebler–Wronski effect. Finally, a model is proposed which is combined with the bond breaking model and explains the present experimental results.
Electrical characteristics and thermal stability of ohmic contacts to p-type In0.47Ga0.53/As/InP
Authors: Leech Patrick W. , Reeves Geoffrey K.
Published: July 2002

The electrical characteristics and thermal stability of Pd/Zn/Pd/Au, Pd/Au, Zn/Pd/Au, Au/Zn/Au, Ni/Zn/Ni/Au, and Pd/Mn/Sb/Pd/Au contacts to p-type In0.47Ga0.53As/InP have been investigated. For all of the as-deposited contacts, the specific contact resistance, ρc, was within the range between 1 and 3×10−5 Ω cm2. The thermal annealing of the contacts between 250 and 500 °C produced a differing effect on ρc for each of the metallization schemes. Based on ρc measurements, the thermal stability of the contacts at 400 °C showed an initial regime of low degradation rate with a subsequent transition to a higher rate regime. The exception to this trend was the Pd/Mn/Sb/Pd/Au contact for which no threshold was evident, and for which the dependence of degradation rate on time, t0.15, was lower than for the other configurations with t0.5. During aging at 500 °C, a single regime of high degradation rate was present. In both the low rate and high rate regimes, the type of interfacial metal was not a significant factor in determining the dependence of degradation rate on time.
Intragrain pinning strength depth dependence of 2223 (Bi,Pb)-based high critical T
c superconducting ceramics made by a vitreous route
Authors: Dang An , Godelaine P. A. , Vanderbemden Ph. , Cloots R. , Ausloos M.
Published: July 2002

Campbell’s method for measuring the critical current in superconductors has been used to obtain the critical current density and the pinning strength in Bi1.7Pb0.3Sr2Ca2Cu3O10−y ceramics synthesized by a vitreous route. The intragrain critical current is much higher than 105 A/cm2 at 40 K in zero dc magnetic field. A large increase of the pinning strength is observed near the grain surface. The decrease with depth is hyperbolic. The role of the precursors in the synthesis route is emphasized for introducing specific pinning centers. The analysis takes into consideration the ceramics granular nature, i.e., the existence of intergrain and intragrain currents.
Thermal relaxation, textural and sweep rate effects on reversible transverse susceptibility of magnetic particles
Authors: Huang Huei Li , Lu Jing Ju
Published: July 2002

We investigated the effect of thermal relaxation on reversible transverse susceptibility (RTS), χ, of single domain Stoner–Wohlfarth particles. We studied such an effect as a function of the orientation β of the easy axis with respect to the drive field (the z axis) within a non-interacting bi-state model. At β=00 one prominent cusp occurs for both χt1 and χt2 along the direction perpendicular to the drive field at H≂HC<HN (nucleation field) at high values KV/kBT (the ratio of the anisotropy energy versus thermal energy). The height of the cusp varies with β and reduces with decreasing values of KV/kBT. Correspondingly, the switching field distribution (SFD) becomes broadened with decreasing values of KV/kBT. The RTS may exhibit one, two, or three prominent cusps at high values of KV/kBT depending upon the textural distribution of the easy axes. For cos2 θ distribution of the easy axis about the x axis, y axis, and z axis there exists two, three, and one cusps, respectively. For random distribution of the easy axis in 2-D in the XZ and YZ plane there are one and three cusps, respectively, while there are three for random distribution in 3-D. Finally, we studied coercivity, RTS and SFD as a function of sweep rate. All these quantities are found to increase with sweeping frequency of the drive field. The peak field value of RTS is consistently the smallest of the three, while that of SFD is equal to or a shade larger than HC depending upon whether the hysteresis loop is rectangular or severely sheared.
Fe-57 Mössbauer study of the (FeCo)75SiB metallic alloy series
Authors: Plazaola F. , Orúe I. , Fernández-Gubieda M. L. , Barandiarán J. M.
Published: July 2002

Liquid-quenched metallic amorphous alloys (FeCo)75(SiB) have been studied by 57Fe Mössbauer spectroscopy at room temperature. Direct insight in the short-range order (SRO) and the bonding properties involving Fe atoms in these alloys is presented, taking apart clearly the metal and metalloid influence over such properties. A large increase of the SRO is found in the Co-rich alloys which is also affected by variations in the relative concentration of metalloids Si and B. Such SRO changes influence the bonding of Si and B with Fe. The substitution of B for Si decreases the Fe 3d polarization in the Co-rich alloys with large SRO.
Strain, alloy composition, and lattice relaxation measured by optical-absorption spectroscopy
Authors: Clark S. A. , Roura P. , Bosch J. , Pérez-Rodríguez A. , Morante J. R. , Westwood D. I. , Williams R. H.
Published: July 2002

The validity of optical absorption (OA) as a technique for the measurement of strain e11, alloy composition x, and relaxation in InxGa1−xAs epilayers on InP has been examined by comparison with similar measurements by double-crystal x-ray diffraction (DCXD). Provided that the strain arising from differences in the thermal contraction of the substrate and epilayer are taken into account, measurements of strain by OA show good agreement with DCXD results, with a dispersion of Δe11=±0.27×10−3. Comparison of alloy compositions given by the two techniques shows similarly good agreement, with a dispersion in the values of x of less than Δx=±0.7%. OA may also be used to determine lattice relaxation. The degree of uncertainty in the measurement of this parameter increases as lattice match is approached and decreases as the lattice relaxes. Our studies indicate that OA may be used as an independent technique to evaluate strain, alloy composition, and the degree of lattice relaxation in InxGa1−xAs epilayers.
Nonlinear optical properties of epitaxial lithium tantalate thin films
Authors: Xie Huyang , Hsu Wei-Yung , Raj Rishi
Published: July 2002

Epitaxial lithium tantalate thin films were grown on prismatic sapphire substrates by a specially designed ultrasonic nozzle assisted liquid injection metalorganic chemical vapor deposition system. The films were characterized by x-ray diffraction and transmission electron microscopy. They contained two crystallographic iso-variants. The relative volume of two variants in the as-grown films was nearly equal. The optical loss in the films, measured by photographic method, was 2 dB/cm. Second harmonic properties of the films were measured in transmission. Compared to bulk single crystals, the as-grown thin film had a weak second harmonic response in the range of 1 pm/V, which was ascribed to the antiparallel domains in the films. Mathematical equations are derived to correlate this microstructure to second harmonic generation. The films were successfully poled by the application of in-plane electric field. After poling the second harmonic generation coefficient increased to 12 pm/V.
Chevron structures in surface-stabilized ferroelectric liquid crystals
Authors: Sabater J. , Pena J. M. S. , Otón J. M.
Published: July 2002

The computation of director profiles and energy parameters in surface-stabilized ferroelectric liquid crystal structures requires the evaluation of many terms accounting for different contributions to the general solution. This evaluation is usually performed using several simplifications and assumptions adapted to the specific phenomenon under study. In this work, a comprehensive expression including all possible terms not avoiding a computable solution is presented. The effect of every term on the general expression is analyzed.
Quantum bound states in a double-bend quantum channel
Authors: Wang Chuan-Kui , Berggren K.-F. , Ji Zhen-Li
Published: July 2002

By use of the mode-matching technique the quantum bound states in a double-bend quantum channel of finite length connecting to two 2D electron gas reservoirs have been investigated in detail. The conductance G of the quantum system is calculated as a function of Fermi energy and the electron density associated with bound states. It is found that there exists one resonant peak in G corresponding to resonant tunneling via one quasibound state below the first conductance plateau for the quantum channel with double-bend continuity and two resonant peaks in G corresponding to resonant tunneling via two quasibound states which are symmetric and antisymmetric superposition of two local bound states localized at two right-angle bends below the first conductance plateau for the quantum channel with double-bend discontinuity. At finite temperature the results are compared with experimental results and are found to explain them well.
Optical characterization of ZnSe/ZnSxSe1−x superlattices pseudomorphically grown on GaAs(100) substrates by molecular beam epitaxy
Authors: Wang J. , Wang X. , Zhu Z. Q. , Yao T.
Published: July 2002

This article presents optical characteristics of ZnSe/ZnS0.12Se0.88 strained-layer superlattices (SLs) with ZnS0.06Se0.94 buffers pseudomorphically grown on GaAs(100) substrates by molecular beam epitaxy. The SL samples exhibit strong blue luminescence. The main emission peaks in photoluminescence spectra can be attributed to the free exciton transitions between lowest electron subband and ground heavy-hole subband of ZnSe wells. The temperature dependence of PL was investigated in detail. The experimental results of temperature dependence of peak positions and linewidths (FWHM) were fitted to the theoretical calculations, based on Varshni’s formula and broadening model. The activation energies of the samples were derived from the temperature dependence of PL intensities. The effects of strain and quantum confinement in ZnSe/ZnS0.12Se0.88 strained-layer SLs with different well thicknesses of 30, 60, and 120 Å were studied by experiments and theoretical calculations. Theoretical calculations and experimental observations are in reasonable agreement.
Switching mechanisms in layered CoCrPt thin films
Authors: Sinclair G. J. , Jones G. A. , Grundy P. J. , O’Grady K. , el Hilo M.
Published: July 2002

A series of thin CoCrPt layered structures has been studied with a view to understanding the nature and extent of any interactions present, both within and between the various constituent layers. In this investigation, we have utilized the principal remanence curves and their derivatives to characterize the bulk magnetic properties of the samples. In addition, we have employed a low energy broad beam Kaufman-type ion source to sputter etch some of the layers, enabling unambiguous determination of the reversal fields of the various constituent layers. For each layered sample the magnetic layer grown on the thick Cr underlayer has the highest in-plane coercivity. The second layer grown on a 100 Å Cr spacer has the next largest coercivity. All additional layers reverse at the same applied field strength. The coercivity of the layers is also insensitive to thickness reduction from 200 Å down to 80 Å.
Kinetic model for photoinduced and thermally induced creation and annihilation of metastable defects in hydrogenated amorphous silicon
Authors: Abdulhalim I.
Published: July 2002

A microscopic many-body model is proposed for the kinetics of metastable defects (MSDs) in hydrogenated amorphous silicon (a-Si:H). It is based on the existence of short-lived large energy fluctuations which induce transient traps for carriers that release their energy and enhance the creation or annihilation of MSDs. The expressions found for the photoinduced and thermally induced creation and annihilation rates’ coefficients explain the dependence on the variety of parameters.
A numerical study of the ζ2 parameter for random suspensions of disks
Authors: Greengard Leslie , Helsing Johan
Published: July 2002

The effective conductivity of two-component composites can be tightly bounded through the knowledge of structural parameters. While the first- and second-order parameters are known analytically for isotropic materials, the third and higher order parameters are generally not. Their evaluation has, therefore, become the subject of much research. In particular, the third-order structural parameter ζ2 has been computed many times. Interface methods, beginning with Rayleigh, have proven successful for periodic composites with simple unit cells. Statistical methods, involving three-point correlation functions, work well for dilute random suspensions. Composites consisting of complicated, dense suspensions have been much more difficult to treat. In this article, we illustrate how one can greatly accelerate the computation of structural parameters with interface methods, so that these methods can be applied to dense suspensions with tens of thousands of randomly placed inclusions per unit cell. We implement a numerical scheme, based on the fast multipole method, for which the amount of work grows linearly with the number of inclusions per unit cell and quadratically with the logarithm of the desired precision. By incorporating a Monte Carlo sampling technique, we have computed values of ζ2 for the random suspension of disks at 20 volume fractions between 0.50 and 0.69. These tabulated values are accurate to at least three digits and improve on the best previous estimates by a factor of between 30 and 100.
Proposal of step barrier structures to reduce hole localization in multiple quantum well structures
Authors: Tomita Akihisa
Published: July 2002

A simple model has shown that the ratio of carrier transit time to carrier capture time determines the transport properties in multiple quantum well structures. An analysis of carrier transport in the wave picture shows that the transit time is increased by the carrier reflection at the quantum well interface. The carrier reflection thus enhances the hole localization in multiple quantum well structures. Step barrier structures are proposed as antireflection layers to the quantum well interfaces. An optimally designed step barrier structure increases the hole transmittance from 57% to 92% in the lattice-matched InGaAs/InP interface. The transit time per well is also reduced from 0.88 to 0.28 ps, which correspond to the threshold current density of 650 and 430 A/cm2 for a four quantum well laser active layer.
Magneto-optical readout properties of Ba-ferrite particulate media recorded by a magnetic head
Authors: Kishimoto M. , Ajima T. , Sugiyama T. , Sakurai Y.
Published: July 2002

The magneto-optical readout properties of Ba-ferrite particulate media recorded by a magnetic head were investigated. Although the readout wave form reproduced using the Faraday effect contained a large noise component, the wave form with same bit period as that of the recorded signal was obtained. The domain pattern observed using a polarized microscope shows obscure boundaries between adjacent magnetized areas. The origin of noise is discussed from the standpoint of dispersion and the refractive index in particulate media.
Optical and crystalline properties of Yb implanted InP
Authors: Katsumata H. , Uekusa S. , Majima A. , Kumagai M.
Published: July 2002

The effect of the annealing temperature on the optical and crystalline properties of 2 MeV Yb+ ion-implanted InP are systematically studied by photoluminescence (PL), photoluminescence excitation (PLE), selectively excited photoluminescence (SPL), and Raman scattering measurements. PL measurements present that the recrystallization of the implanted layers and the optical activation of Yb3+ begin at 450 °C and 500–550 °C, respectively. It is demonstrated from the Raman experiments that there is a continuous restoration in radiation damages when the as-implanted samples are annealed at temperatures between 450 °C and 550 °C. However, beyond 550 °C, the crystalline quality near the surface is shown to be degraded. For an explanation of this point, a comparison with optical micrographs is conducted. From the PLE spectra of Yb-related luminescence, it is concluded that the highest crystalline quality of Yb+ implanted layers could be achieved by annealing at 750 °C for 15 min. In the PLE spectra, a band observed earlier at 1.409 eV (8800 Å) is recognized to be split into two peaks at 1.409 eV (8798 Å) and 1.408 eV (8806 Å), denoted by B1 and B2, respectively. A convictive assignment for these two peaks is proposed. Furthermore, an unusual broad band at 1.24 eV, which is observed only in SPL spectrum when the excitation is set on B1 energy, is described.
Relaxation in tensile-strained InAlSb/InSb heterostructures
Authors: Maigné P. , Dharma-Wardana M. W. C. , Lockwood D. J. , Webb J. B.
Published: July 2002

X-ray diffraction has been used to measure the residual strain in In1−xAlxSb (x=0.15–0.64) layers of thickness h=0.1 μm, grown epitaxially on (100) InSb substrates. The results for this tensile-strained system are compared with a well-studied compressive-strain system, viz., InGaAs/GaAs. Experimental evidence is presented which suggests that the strain relaxation mechanism in this system is different from that in InGaAs/GaAs. This includes an observed critical layer thickness (hc) which is about five times the hc given by the Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)] type models and a measured relaxation coefficient which is quasilinear in h/hc for 5<h/hc<35. The results have also been compared with predictions of the Dodson–Tsao (DT) model [Appl. Phys. Lett. 51, 1710 (1987)] which fits well the quasilinear behavior of the relaxation coefficient. In addition, it is argued that the DT equation provides a generic model that is not restricted to dislocation-mediated strain relief. Other possible mechanisms of strain relaxation in InAlSb/InSb are discussed.
Nonlocal and nonlinear transport in semiconductors: Real-space transfer effects
Authors: Gribnikov Z. S. , Hess Karl , Kosinovsky G. A.
Published: July 2002

The contributions of nonlocal mechanisms to nonlinear transport in semiconductors, with special emphasis on hot-electron emission at heterojunctions and its variations which are now commonly termed real-space transfer effects, are reviewed. The goal is to equitably account for and bring together the body of literature that has developed, often independently, in the U.S. and the former Soviet Union as well as in Europe and Japan.
Characterization of buried K-fulleride layers formed by ion implantation
Authors: Palmetshofer L. , Geretschläger M. , Kastner J. , Kuzmany H. , Piplits K.
Published: July 2002

The formation of buried K-fulleride layers by implantation of 30 keV K+ into C60 films at an implant temperature of 300 °C has been studied as a function of dose ranging from 1×1016 to 1×1017 cm−2. After implantation K depth profiles were measured by secondary-ion-mass spectrometry and Rutherford backscattering spectrometry. Phase characterization was done by Raman scattering and grazing-angle x-ray diffraction. Within the range of the implanted ions a transformation of the fullerene molecules to amorphous carbon (a-C) was found; however, due to the elevated implant temperature of 300 °C most of the K diffuses into the depth where doping of the undestroyed film occurs. The shape of the observed K profiles depends on the dose. A local maximum of K observed right underneath the a-C layer tends to saturate at a value of about 2×1021 cm−3 for high implant doses. Both Raman scattering and x-ray diffraction strongly indicate the existence of K3C60 regions in the implanted films. The buried K-fulleride layers are stable on air due to a passivation effect of the a-C surface layer. The formation of the K-fulleride is discussed on the basis of the phase diagram for the K-C60 system and various thermodynamic processes such as segregation, phase formation, and diffusion.
Correlation of mesoscopic structure, electronic structure, and current-controlled electrical switching of KH2PO4 and poly(p-phenylene vinylene) composite films
Authors: Yamamoto Hiromichi
Published: July 2002

The present study found a relationship among the mesoscopic structure, the electronic structure, and the current-controlled electrical switching of a composite film of KH2PO4 (KDP) and poly(p-phenylene vinylene) (PPV). A composite film with a 11.8 wt % of KDP only showed ohmic behavior with breakdown in a high field as did KDP and PPV alone. Composite films with 25–91.6 wt % of KDP containing segregated clusters and dendrites of KDP exhibited switching phenomena from high to low electrical resistivity state. These switching films at high resistivity showed non-ohmic behavior which is ascribed to hole trapping on oxygen sites on the surface of KDP. The switching appears to be caused by an abrupt carrier flow between the high density of hole states formed in an energy band gap and the π* electronic state of the polymer stabilized by surrounding KDP.