bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental analysis of a ferroelectric domain layer wave
Authors: Li Xing-jiao , Pan W. Y. , Cross L. E. , Cao Huaze , Zhou Feng , Feng Hanhua , Li Yibing , Wu Zhiming , Wei Juan
Published: July 2002

The ferroelectric domain layer wave (FDLW) guide, B-G surface waveguide, and ferroelectric domain wall (M-T) waveguide were made by poled PZT ceramics. The waveguide properties of these waveguides were measured and compared. The FDLW waveguide has the lowest transmission attenuation in the frequency range (200–800 kHz) of these measurements. Due to its low transmission loss and function of modulation of amplitude, the FDLW waveguide has a better ratio of the realizable minimum insertion loss to relative maximum bandwidth than the other two structures in the frequency range of the measurements. Experimental dispersion curves measured by a phase shift method have been correlated with the theoretically predicted dispersion. The analysis demonstrates that the nature of the changes in dispersion is associated with the changes in the dielectric and piezoelectric properties of PZT.
Etch rates and surface chemistry of GaAs and AlGaAs reactively ion etched in C2H6/H2
Authors: Pearton S. J. , Hobson W. S. , Jones K. S.
Published: July 2002

The etch rates of GaAs and AlxGa1−xAs (x=0.09–1) in C2H6/H2 were investigated as a function of time (1–12 min), gas flow rate (5–25 sccm), total pressure (4–30 mTorr), plasma power density (0.56–1.32 W cm−2), and percentage of C2H6 in the discharge (10%–50%). The etch rates are constant with time, and decrease with increasing Al content in the AlGaAs. The maximum etch rates occur at 25% by volume C2H6 in H2 and increase linearly with increasing power density. Increasing the total pressure at constant gas composition reduces the etch rates by approximately a factor of 2 between 4 and 30 mTorr. The etched surfaces have smooth morphologies for C2H6 concentrations less than ∼40% of the total gas volume. A layer of subsurface dislocations approximately 40 Å deep were observed in GaAs by transmission electron microscopy for the highest-power density discharges, while the surfaces for all samples are As-deficient to a depth of ∼30 Å after reactive ion etching. Polymer deposition is not significant for C2H6 volumes less than 40% of the total gas volume.
Suspension of a magnet below a superconductor
Authors: Burton Wade D. , Ouseph P. J.
Published: July 2002

An experimental arrangement to suspend a magnet below a superconductor is discussed in this article. The suspension of the magnet is achieved with the help of an external force to overcome the repulsive force between the magnet and the superconductor. A simple experimental setup to measure the force between a magnet and a superconductor as a function of distance between them is also discussed.
Graphoepitaxial growth of germanium by laser recrystallization
Authors: Kanata T. , Takakura H. , Miyagi K. , Hamakawa Y. , Kariya T.
Published: July 2002

A systematic experimental approach has been performed to achieve orientation-controlled nucleation and graphoepitaxial growth of germanium on the replica substrate with a textured single-crystalline silicon (001) surface. The recrystallization of germanium was successfully obtained by scanning a cw argon-ion laser on a germanium/metal eutectic alloy system. It was found that the crystallographic orientation of germanium is strongly dependent on the recrystallization conditions such as laser power, scan direction of laser beam, size of surface relief, and wetting condition to crystallized material. The 〈001〉 directions of the recrystallized germanium are normal to the substrate, and the 〈110〉 directions are parallel to the valleys among the pyramidal relief, i.e., the crystallographic orientation coincides with one of original silicon (001) single crystal. It has been confirmed that the crystallite sizes range from 2 to 10 μm, and the deviation of the [001] crystallographic orientation distribution is less than 7°. A possible growth mechanism of graphoepitaxy is also discussed.
The effect of aluminum gate thickness on charge trapping in metal-oxide-semiconductor devices
Authors: Berger M. , Avni E. , Shappir J.
Published: July 2002

Metal-oxide-semiconductor devices with variable aluminum gate thickness were electrically stressed by constant-current Fowler–Nordheim tunneling, until intrinsic oxide breakdown was achieved. I-V and C-V measurements were used to detect oxide-bulk trapping and interface-state generation rates during the stressing process. It was found that these rates were affected by the Al thickness only in the initial stages of the stressing, and that these rates are not a simple monotonic function of the Al gate thickness, but reach their maximum value between 500 and 1000 Å of Al thickness.
Observation of anomalous far-field intensity distributions in semiconductor lasers and their explanation
Authors: Dutta N. K. , Piccirilli A. B.
Published: July 2002

The observed variations in the far-field intensity distributions of index-guided lasers correlates well with variations in the near-field intensity distributions. The far- and near-field nonuniformities (large intensity variations generally attributed to scattering) arise from variations in the active region dimensions along the length of the optical cavity. Model calculations are presented which show that 20% variation in active width can cause 20%–40% intensity modulation in the far field. The magnitude of the observed far-field nonuniformities depends on the laser structure and the details of the fabrication process. For example, the capped mesa buried heterostructure lasers generally show less scattering-type far-field nonuniformities than the etched mesa buried heterostructure lasers.
An investigation into the output characteristics of a discharge-heated copper vapor laser
Authors: Wang Tieh C. , Yang Ching Y.
Published: July 2002

The laser output characteristics, such as the pulse width, peak power, pulse energy, and average power of a discharge-heated copper vapor laser, which has a discharge tube of inner diameter 4.2 cm and length 150 cm, are investigated under such conditions as the charging voltage of 14.5 kV, laser tube temperature of 1450 °C, pulse repetition rate range from 0.5 to 9.5 kHz, and buffer gas pressures of 20 and 75 Torr. Comparing the widths of the laser pulses obtained at the buffer gas of 20 Torr with that obtained at 75 Torr, it is found that the rates of relaxation of metastable atoms at these two buffer gas pressures are comparable, even though the cooling mechanisms of the electrons, the collisional deexcitation partners of metastable atoms, are different. From the dependencies of the pulse energy on the pulse repetition rate, it is found that the prepulse electron density should be higher than the value of 1013 cm−3 for efficient pumping of this laser. The dependencies of the average power on the pulse repetition rate also indicate that the output power can be increased if the pulse repetition rate is increased and the thermal insulation is optimized. This points to the possibility of improved laser output power at elevated pulse repetition rate.
Growth of diamond thin films by spiral hollow cathode plasma-assisted chemical vapor deposition
Authors: Kung P. J. , Tzeng Y.
Published: July 2002

A direct current spiral hollow cathode which combines hot-filament, electron-beam, and plasma-assisted chemical vapor deposition mechanisms has been developed to grow diamond crystallites and continuous thin films from CH4/H2 [/Ar] gas mixtures. A comprehensive study of system parameters such as deposition time, gas pressure, susceptor temperature, methane concentration, distance between the cathode and the substrate, substrate material, and surface treatment indicates that diamond deposition in such a reaction is a composite process. By varying the susceptor temperature and gas pressure during deposition, the deposition process can be tailored in favor of the synthesis of structurally uniform diamond thin films. The deposits, obtained at a growth rate of about 1 μm/h, have been characterized using scanning electron microscopy, Raman spectroscopy, Auger spectroscopy, x-ray diffraction, reflection high-energy electron diffraction, infrared absorption spectroscopy, and current-voltage curve plotting.
Irreversible magnetization reversal in some Co-based alloy thin films
Authors: Admon U. , Dariel M. P. , Grunbaum E. , Lodder J. C.
Published: July 2002

Irreversible magnetization reversal occurs either by coherent or incoherent spin rotation or by wall displacement. In electrodeposited Co-W, Co-Fe, and Co-P 300–500-Å films, vibrating sample magnetometer hysteresis loop analyses indicate that magnetization reversal takes place by wall displacement. The formation and movement of domain walls has been put in evidence by Lorentz electron microscopy.
Transverse acoustoelectric voltage measurements of GaAs grown directly on (100) Si substrates
Authors: Abedin M. N. , Schowalter L. J. , Das P.
Published: July 2002

Transverse acoustoelectric voltage (TAV) measurements have been analyzed as a function of incident photon energy, dc bias voltage, temperature, and surface acoustic wave (SAW) frequency of GaAs layer (∼1.4 μm thick) grown on a vicinal Si(100) substrate by molecular-beam epitaxy. The TAV technique is used to show that the band gap and impurity level transitions are of slightly lower energies, compared to the bulk GaAs. This might be due to the residual strain changes in the band structure of GaAs grown on Si. It is also noticed that the presence of an interface considerably changes the shape of the experimental TAV versus bias voltage at low temperatures. Drastic variations of TAV as a function of SAW frequency and temperature are also observed. The cause of variations is not yet clear, but possible explanations are discussed.
Hot-phonon and electron-hole scattering effects on the transient transport of photogenerated electrons in GaAs
Authors: Joshi R. P. , Grondin R. O.
Published: July 2002

We investigate the transient response of photogenerated carriers to an external electric field in bulk GaAs. The results of our Monte Carlo simulations indicate that the initial velocity rise times are a strong function of the carrier density. This is caused by a combination of the hot-phonon effect and the enhanced electron-hole scattering within the plasma. Contrary to some previous suggestions, the hot-phonon effect alone is insufficient to explain the initial velocity behavior seen experimentally. The steady-state velocity is limited by the electron-hole scattering.
Optically pumped titanium vapor laser at 625.8 nm
Authors: Ninomiya H. , Hirata K. , Yoshino S.
Published: July 2002

Laser action has been observed on the titanium 625.8-nm line [(3d3(4F)4p3G04 −3d3(4F)4s 3F3 transition)]. A nitrogen laser is used to produce the titanium vapor by irradiating a metal plate, and the titanium atoms are optically pumped by a dye laser.
Preparation and characterization of superconducting phases in the Bi(Pb)-Sr-Ca-Cu-O system
Authors: Sarkar Asok K. , Maartense I. , Peterson T. L. , Kumar Binod
Published: July 2002

The superconductive properties in the Bi(Pb)-Sr-Ca-Cu-O system with nominal composition of Bi2−x Pbx Sr2 Ca2 Cu3 Oy (x=0, 0.2, 0.4, and 0.6) and annealed at 860 °C for 240 h were studied using dc resistivity and ac susceptibility measurements. For x=0.2 and 0.4, the bulk zero resistivity temperatures were found to be ∼103 K. For x=0 and 0.6, these temperatures were found to be ∼73–75 K. The ac susceptibility data show enrichment of the volume fraction of the high-Tc phase in compositions with x=0.2 and 0.4. Tc of this phase is constant at 107 K for all values of x. The enhancement of the bulk zero resistivity temperature for x=0.2 and 0.4 is due to the improved intergranular coupling of the high-Tc phase, although the resulting weak-link behavior reflects a low current carrying capacity in these materials. The presence of the low-Tc phase (∼65 K) is still visible in the susceptibility data. For x=0.6, the intergranular coupling of the high-Tc phase is almost destroyed; however, the coupling of the low-Tc phase is better than when x=0. Powder x-ray diffraction and scanning electron microscopy data show the formation of large grains of the calcium plumbate, Ca2PbO4 phase with increasing Pb up to x=0.6. The large grains of the Ca2 PbO4 phase may incorporate other impurity phases within it when x=0.6, cleaning up the remaining grain boundaries, thereby improving the intergranular coupling within this material. Energy dispersive x-ray analysis indicates that some Pb is incorporated into the structure of both superconducting phases.
A model calculation for surface plasma-enhanced internal photoemission in Schottky-barrier photodiodes
Authors: Cazeca M. J. , Chang C. C. , Karakashian A. S.
Published: July 2002

Attenuated total reflection is commonly used to excite the surface plasma wave (SPW) on a metal film. The SPW traveling on the surface of the metal will enhance the emission of photoelectrons from the metal into the semiconducting region of a Schottky barrier photodiode. A model consisting of an aluminum film on n-type gallium arsenide forming a Schottky barrier photodiode is used to predict the enhanced emission of photoelectrons over the barrier between the aluminum film and the gallium arsenide. The quantum efficiency for this model shows a strong dependence on parameters such as the electron escape depth, film thickness, and the spacer thickness between the prism coupler and the photodiode. The model proved to be effective when used to fit quantum efficiency and reflectivity data resulting from an angle scan experiment.
Magnetic properties of Co-Cr films and the effects of heat treatment
Authors: Fartash A. , Oesterreicher H.
Published: July 2002

We have studied the effects of low-temperature sputter preparation and subsequent heat treatment of Co79Cr21 thin films with emphasis on their microstructure and magnetic properties. Preparation at −196 °C resulted in textured films which did not show the presence of magnetic Co-rich regions. The saturation magnetization moment, Ms, of these films was similar to their homogeneous alloy phase. Conventionally sputtered films (substrate temperature>room temperature) had higher values of Ms than films prepared at −196 °C. A 1-h heat treatment at 500 °C in vacuum reduced the Ms of both films. The results suggest rearrangement of atoms through thermal diffusion. For the films sputtered at −196 °C (liquid-nitrogen temperature) an atomistically ordered phase can be proposed based on hexagonal Co which minimizes the presence of nearest Cr-Cr neighbors. This maximizes the moment quenching activity of Cr on Co and reduces the saturation moment over the one of a fully random material for segregated films prepared at higher temperature reductions in Ms can also be due to dilution of Co concentration in Co-rich regions.
Epitaxial Y-Ba-Cu-O thin films on MgO deposited by high-pressure reactive magnetron sputtering
Authors: Tanabe K. , Lathrop D. K. , Russek S. E. , Buhrman R. A.
Published: July 2002

Y-Ba-Cu-O thin films have been prepared on MgO(100) substrates by rf reactive magnetron sputtering from a single ceramic target. By adopting high total pressures, typically 280 mTorr, and relatively low substrate temperatures of approximately 650 °C, epitaxial films with the relations (001) YBa2Cu3Oy∥(001)MgO and [100]YBa2Cu3Oy∥[100]MgO can be reproducibly obtained. A high degree of epitaxy is confirmed by x-ray pole figure measurements and ion channeling. These films require a brief rapid thermal oxygen anneal at typically 850 °C, to exhibit sharp superconducting transitions with zero resistance around 75 K. Films deposited at higher temperatures above 700 °C show transitions as deposited with zero resistance near 80 K. The quality of the transition is correlated with expanded lattice constants with the best transitions occurring in films whose lattice constants approach that of the bulk. The films have high critical current densities of 1–5×106 A/cm2 at 4.2 K. They also show good uniformity and excellent surface morphology with a roughness less than 10 nm and can be readily patterned to micrometer and submicrometer dimensions.
Physical properties of alkoxy-cyanotolane liquid crystals
Authors: Wu Shin-Tson , Nguyen Huu Tinh
Published: July 2002

A simple method for synthesizing alkoxy-cyanotolanes (nOCDP) is presented. Optical and electro-optical properties of three nOCDP liquid crystals and an eutectic mixture were evaluated in their nematic range. Birefringence and viscoelastic coefficient results of nOCDP are compared with alkyl-cyanotolanes at reduced temperatures. Molecular conformation is found to make an important contribution to the observed birefringence, and so does clearing temperature to the measured viscoelastic coefficient.
Investigation on the phosphorous interstitials in phosphorus-implanted copper indium disulfide by a simplified total energy calculation
Authors: Hsu Y. J. , Hwang H. L.
Published: July 2002

This paper presents a simplified total energy calculation for the phosphorus interstitials in P+ -implanted CuInS2. The calculation was based on the tight-binding approximation and the atomic structure calculation scheme. Two types of interstitials were considered. One involved P and S2−, the other involved P3+ and S3−. Results showed that the latter would be more possible than the former, which confirms the results of our previous electron paramagnetic resonance study on the p-type doping of P+ -implanted CuInS2 .
Radio-frequency linac-driven free-electron laser configurations
Authors: Tang C. M. , Sprangle P. , Ting A. , Hafizi B.
Published: July 2002

Recent developments in accelerator technology and the optical guiding property of the free-electron laser (FEL) radiation indicate that rf linacs may be a strong contender for driving high average power FEL amplifiers. We discuss two rf linac-driven FEL configurations: the master oscillator power amplifier (MOPA) and the single-state power amplifier (SSPA). Our numerical results are obtained using the source-dependent expansion (SDE) method in the computer code shera. The SDE method and the code shera are discussed and illustrations of the MOPA and SSPA configurations presented.
A new approach to calculate the substrate current and oxide injection in a metal-oxide-semiconductor field-effect transistor
Authors: Hänsch W. , Schwerin A. v.
Published: July 2002

We derive an analytical expression for the distribution function for high energetic carriers in an arbitrary two- or three-dimensional electric field utilizing Boltzmann’s equation. This distribution function is of Maxwell–Boltzmann type if the system attains thermal equilibrium. We apply this new model to calculate the substrate and gate currents in a metal-oxide-semiconductor field-effect transistor and compare it with measurements from various devices.
Domain and write–read characteristics for magnetic field modulated magneto-optical disk with high data transfer rate
Authors: Miyamoto Harukazu , Niihara Toshio , Sukeda Hirofumi , Takahashi Masahiko , Nakao Takeshi , Ojima Masahiro , Ohta Norio
Published: July 2002

Magneto-optical domain shape and write–read characteristics were investigated in an overwrite scheme using a flying magnetic head. Through domain observation and domain shape numerical simulation on magnetic film, it was found that ‘‘tails’’ of domains must be shortened to increase the carrier-to-noise ratio (C/N) at high-density conditions. To reduce the length of domain tails, isotherms on the magnetic film must be circular. For quick heat flow, we constructed a quadrilayered disk which has a thin metal film. C/N of the disk was about 50 dB at 7.4 MHz (0.75-μm domain length) with linear velocity 11.3 m/s. Compared with the light power modulation method, the magnetic field modulation method is a more suitable overwrite technique, since the thermal profile is the same for any recording mark length.
2
9Si hyperfine spectra and structure of E′ dangling-bond defects in plasma-enhanced chemical-vapor deposited silicon dioxide films on silicon
Authors: Warren W. L. , Lenahan P. M.
Published: July 2002

We report the observation of 29Si hyperfine spectra in a thin silicon dioxide film. The hyperfine observations allow us to establish with virtually absolute certainty that E′ centers reported earlier in plasma-enhanced chemical-vapor deposited oxides are silicon atoms bonded to three oxygen atoms with an unpaired electron residing in an sp hybridized orbital on the silicon. The hyperfine spectra additionally provide us with information regarding silicon bond angles, the amount of electron localization on the silicon site, and the relative amounts of s and p character of the electron-wave function.
Calculations on temperature profiles in optical recording
Authors: Holtslag A. H. M.
Published: July 2002

Calculations on temperature distributions in a multilayer thin-film stack of a disk as used in optical recording are discussed. It is shown that an analytical solution of the heat equations can be obtained if certain realistic approximations are applied. The analytical solution is compared with numerical calculations on the heat equations, and for the cases investigated, a good agreement is obtained: If the active layer of an optical disk has a high(er) diffusivity with respect to the protective layer and the substrate, the analytical expressions obtained predict maximum temperatures in the active layer, correct to within a few percent, and accurate trends for a variety of problems. Results of numerical and analytical calculations are shown in detail for a realistic magneto-optical thin-film stack, and some general trends are discussed. If only the temperature distribution in the active layer is of interest, an enormous reduction of the calculation time can be achieved, important in write-strategy calculations, for instance.
Strain relaxation and alloying effects in the GaAs/In0.52Al0.48As/InP(100) heterostructure
Authors: Williams M. D. , Chang T. Y. , Nolte D. D.
Published: July 2002

I n situ surface sensitive photoemission spectroscopy has been used to study the evolution of the strained layer GaAs/In0.52Al0.48As heterostructure as the GaAs overlayer is grown past the critical layer thickness in sequential steps by molecular-beam epitaxy. The variation in spectral features evident in the GaAs valence band is inconsistent with predictions of tight binding theory for strain relaxation. An analysis of the core level spectra suggests that In out-diffusion is responsible for the valence band modification.
Influence of the island structure on the magnetic properties of quasi-two-dimensional Fe films (abstract)
Authors: Qiu Z. Q. , Mayer S. H. , Gutierrez C. J. , Tang H. , Walker J. C.
Published: July 2002

Reported reductions of Curie temperature and a linear temperature dependence of magnetization in quasi-two-dimensional magnetic films has generated significant interest in the past decade. We have made several Fe(110)/Ag(111) superlattices by molecular-beam epitaxy with the Fe component 2 monolayers thick and the Ag component of various thicknesses. By taking Mössbauer spectroscopy measurement in an external field, we found that a central feature in the Mössbauer spectrum at high temperature may not relate to the reduction of Curie temperature, but instead comes from the thermal relaxation of small islands in the Fe film. The magnetic hyperfine field has a linear temperature dependence, and this is explained by a simple relaxation model based on the island structure. This model is also supported by studying superlattices grown with different conditions.
A topological invariant for the accessibility problem of micromagnetics
Authors: Kotiuga P. R. , Giles R.
Published: July 2002

A singularity-free three-dimensional micromagnetic configuration is accessible from a given initial state if there is a continuous family of interpolating states. This paper proposes an integral formula for the accessibility invariant which classifies (up to homotopy) the possible final states which are inaccessible from a given initial state. For infinite samples, we give explicit formulas describing initial states from which the final state of uniform saturation cannot be accessed.
Crossover from random field to spin-glass behavior in FexZn1−xF2
Authors: Montenegro F. C. , Leitão U. A. , Coutinho-Filho M. D. , Rezende S. M.
Published: July 2002

It is well known that the diluted antiferromagnet FexZn1−xF2 under an external field is a prototype random field Ising model system (RFIM) when the concentration x is larger than ≂0.4. Here we show that as x decreases there is a crossover to spin-glass (SG) behavior. For x=0.25 the dc magnetization has irreversibility, metastability, and remanence typical of a canonical SG whereas for x=0.48 they are characteristic of a RFIM system. The ac susceptibility also shows completely diverse behavior for x=0.25 and 0.48, characteristic of SG and RFIM system, respectively. The SG nature of the x=0.25 sample is confirmed by the critical behavior of the nonlinear dc susceptibility above the SG freezing temperature T≂10 K. Finally, for x=0.31 there is a temperature dependent crossover from RFIM to SG behavior.
Reexamination of critical dynamic scaling in spin glasses
Authors: Geschwind S. , Huse D. A. , Devlin G. E.
Published: July 2002

The frequently used scaling form for χ″(ω), the imaginary part of the ac susceptibility, is χ″(ω)T=ωβ/zνf(ω/tzν), where t≡(T−Tc)/Tc. Comparison of data to this form has generally been done using a 3-parameter visual fit on a log-log plot, which often conceals good departures from scaling. By recasting the argument of the scaling function to t/ω1/zν, one can use a linear scaling plot of χ″T/ωβ/zν vs t/ω1/zν, which more nearly resembles χ″(ω) vs T for a more revealing display of departures from good scaling. This procedure suggests a new method to determine β/zν,zν, and Tc separately and independently of each other. Application is made to Cd0.6Mn0.4Te and other spin glasses. A method as also presented for determining Tc and distinguishing between activated versus conventional dynamics by considering (χeq−χ′)/χ″ vs T. Reexamination of previous work reveals a general tendency to underestimate zν and overestimate Tc.
Dimensionality crossover in CuMn spin-glass films
Authors: Granberg P. , Nordblad P. , Svedlindh P. , Lundgren L. , Stubi R. , Kenning G. G. , Leslie-Pelecky D. L. , Bass J. , Cowen J.
Published: July 2002

The dynamic properties of Cu (13.5 at. % Mn) spin-glass films of various thicknesses have been investigated in a superconducting quantum-interference device (SQUID) magnetometer. The films are produced in a dc-sputtering system and fabricated in the form of multilayer samples. Utilizing zero-field-cooled magnetization and ac-susceptibility measurements, the time-dependent susceptibility has been probed over eight decades in time (10−4–104 s). Drastic changes of the dynamics are found upon varying the film thickness from 104 to 20 Å. The most significant feature is a crossover from a behavior typical for bulk spin glasses for the 104-Å film, with a finite critical temperature and a critical slowing down that can be accurately described by a conventional power-law divergence, to a slowing down for the very thin films that obeys a generalized Arrhenius law with a zero-temperature critical point. Thus, these measurements indicate a crossover from three- to two-dimensional spin-glass dynamics when one spatial dimension is gradually diminished to a finite size.
Structural and magnetic properties of Co(AlV) alloys
Authors: Booth J. G. , Mankikar R. M. , Honeybourne R. , Saleh A. S.
Published: July 2002

Magnetization and magnetic susceptibility measurements have been made over a wide range of temperature for the alloy series CoAl1−xVx for 0≤x≤0.5. Neutron diffraction measurements have also been made at room temperature using the Rutherford Appleton Laboratory spallation source (ISIS). The observed intensities suggest that for x≤0.25 the structure is the singly ordered B2 (CsCl) structure with the V atoms occupying Al sites. Beyond this concentration a model in which double ordering of the L21 (Heusler) type with the V atoms favoring one of the two equivalent Al sites seems consistent with the results. The magnetization measurements indicate that the series becomes ferromagnetic for x>∼0.1 with the intrinsic magnetization σ rising to a maximum of ∼50 emu. g−1 at x=0.5 for which the Curie temperature TC is ∼320 K. The inverse susceptibilities are generally linear with temperature and almost independent of concentration for x>0.2, suggesting an itinerant interpretation. The maximum values of TC and σ reached are less than half those achieved in the corresponding CoAl1−xMnx system but comparable with those seen in the CoAl1−xCrx system. Comparisons with the isoelectronic CoGa system are drawn. The results have relevance to theoretical estimates of impurity moments in CoAl and CoGa.
The nature of the ground state of Ce systems: CeM3 alloying studies of local moment to mixed valent transitions
Authors: Rahman S. , Timlin J. , Crow J. E. , Mihalisin T. , Schlottmann P.
Published: July 2002

The often quoted view that the nature of the ground state of Ce systems is determined by a single parameter, namely Jsf, the conduction-electron f-electron coupling constant, is shown to be incorrect. In particular, the alloy series CeTl3→CePb3→CeSn3 and CeTl3→CeIn3→CeSn3 show ground state phase diagrams that are qualitatively different. Moreover, both differ drastically from the ground-state phase diagram obtained for the CeTl3→CeSn3 series which presumably spans the same range of Jsf. The CeTl3→CePb3→CeSn3 alloy series show local moment antiferromagnetism (LMAF)→itinerant heavy fermion antiferromagnetism (IHFAF)→mixed valent (MV) transitions. The other two series show only LMAF→MV transitions which differ significantly in their Jsf dependence. In addition, the concentration dependence of the ground-state evolutions in these systems is such that theoretical models which hold that the ground state is determined by two parameters, not one (namely the Rudermann–Kittel temperature TRKKY as well as Jsf), also appear to be inadequate to explain these findings. Corroborative evidence from studies of the CePb3→CeIn3 series will be presented. The possibility that substitutionally induced disorder plays an essential role in these systems, even though the alloying takes place on the non-Ce site, will be discussed.
Dynamical calculation of magnetization reversal in elongated particles
Authors: Uesaka Yasutaro , Nakatani Yoshinobu , Hayashi Nobuo
Published: July 2002

The Landau–Lifshitz–Gilbert (LLG) equation is directly solved to investigate squareness and time-dependent magnetization changes of elongated particles. Squareness scarcely changes until the particle size exceeds some critical value. The critical value increases with increasing aspect ratio. It was found that there are three kinds of magnetization reversal mechanism in elongated particles: flower1, flower2, and vortex particles. Some time interval is necessary for the irreversible transition to occur in all cases. In a flower1 particle, the transition occurs from the top and bottom planes. In flower2 and vortex particles, the irreversible transitions occur from vortex states. In a flower2 particle, during the irreversible transition process, all magnetic moments at the top and bottom planes rotate to the same direction; consequently, some magnetic moments rotate to the antiapplied-field direction and then rotate to the applied-field direction. In a vortex particle, each magnetic moment at the top and bottom planes rotates to the applied-field direction.
Anisotropy field distributions in magnetic recording particles
Authors: Parker F. T. , Berkowitz A. E.
Published: July 2002

First-quadrant moment versus field data for several types of recording media based on γ-Fe2O3 are least-squares fit to anisotropy field distributions within the Stoner–Wohlfarth model. For γ-Fe2O3, the mean anisotropy field is less than expected from the nominal axial ratios of the particles. All media show broad distributions in anisotropy fields, especially those containing Fe2+. Coercivities calculated assuming coherent rotation in γ-Fe2O3 are much smaller than predicted from the nominal axial ratios, but larger than experimental. Predicted coercivities for the other samples are relatively closer to experimental, reflecting broader anisotropy field distribution widths relative to the mean values.
The magnetic aftereffect in CoCr films: A model
Authors: Lottis D. , White R. , Dahlberg E. Dan
Published: July 2002

We have previously reported experimental studies of the magnetic aftereffect in CoCr films. In that work data were compared to phenomenological models, and it was recognized that the decay of the magnetization was driven by the demagnetizing field. The difficulty of incorporating such fields into phenomenological models, which rely on a broad distribution of relaxation times or activation energies to account for the quasilogarithmic decays, motivated the present study. Here we present a simple alternate model which does not assume a distribution of energies. Instead, we focus on the time dependence of the local magnetic field h which drives the magnetization. We approximate h by the average internal field Hi =Ha−4πM, where Ha is the applied field, M is the magnetization averaged over regions large compared to domains, and 4π is the demagnetizing factor appropriate for a planar geometry. The magnetization is found to relax quasilogarithmically with time in this model. We have also made a more detailed comparison with experimental results. We conclude that this simple model is more transparent to the essential physics of the problem than previously used models, and may be useful in analyzing decays in other materials.
Fermi liquid character in the photoemission/inverse photoemission spectra of highly correlated electron systems
Authors: Riseborough Peter S.
Published: July 2002

We calculate the photoemission/inverse photoemission spectrum for an N-fold degenerate Hubbard model, in the 1/N approximation. The spectra are broadened, and for sufficiently large Coulomb interaction strengths the spectra show satellites both in the photoemission and the brehmstrahlung isochromat spectroscopy portions of the spectra. The intensity of the spectra at the fermi level are equal to the noninteracting values, in accordance with Luttinger’s theorem. We show that this can result in a temperature-dependent peak at the Fermi level; the width of the peak is governed by the quasi-particle lifetime. We relate the temperature dependent peak to the Fermi-liquid properties.
Absolute evaluation of magnetic ordering in correlated f-electron systems
Authors: Sheng Q. G. , Cooper B. R. , Wills J. M. , Kioussis N.
Published: July 2002

For partially delocalized correlated f-electron systems, the key aspect of the electronic behavior is the hybridization of f electrons with the non-f-band electrons. This gives unusual properties including suppressed crystal-field splitting and highly anisotropic ordered magnetism. To improve the general understanding and to make the theory materially predictive, a technique is being developed to evaluate absolutely the parameters of the correlated electron model Hamiltonian, and then to use these to predict observed phenomenology including details of magnetic ordering such as magnetic structures and transitions between structures. The most difficult quantity to predict is the magnetic ordering temperature, among other reasons because it depends on the hybridization strength in a highly nonlinear way. Previously Wills and Cooper have reported on a technique involving a nonconventional electronic structure calculation based on treating the f electron as a resonant state in a solid-state environment to evaluate the hybridization. As an independent check on the evaluation of hybridization, here a conventional tight binding parametrization scheme was used to evaluate the hybridization. These results are compared both with previous results and with experiment for the magnetic ordering temperature and crystal-field dressing of Ce compounds, and the situation introduced by the relative degrees of nonlinearity for the crystal-field and magnetic ordering response to the hybridization strength are commented on.
Pressure dependence of the electrical resistivity in the magnetic Kondo lattice systems CePtGe and CePt2
Authors: Rebelsky L. , Reilly K. , Horn S. , Borges H. , Thompson J. D. , Caspary R.
Published: July 2002

Pressure-dependent electrical resistivity measurements on CePtGe and CePt2, which belong to the class of magnetically ordering Kondo lattice systems, reveal that the antiferromagnetic transition temperature TN, as determined from the position of the maximum of the derivative dρ/dT, depends only weakly on the applied external pressure. At higher temperatures the shape of the ρ(T) curve indicates the presence of crystal-field effects. Application of pressure does not affect the high-temperature behavior significantly. In view of these results, which suggest a characteristic temperature smaller than TN for these systems, we discuss a steep decrease of the resistivity at T≊10 K in CePtGe and CePt2 in terms of magnetic short-range order between local moments rather than in terms of formation of a coherent Kondo ground state.
f-electron localization/delocalization phenomena in PrBa2Cu3O7 and related compounds
Authors: Soderholm L. , Goodman G. L. , Loong C.-K.
Published: July 2002

In order to investigate possible mechanisms for the suppression of superconductivity in PrBa2Cu3O7 and CmBa2Cu3O7, the electronic properties of the Pr-compound have been examined using magnetic susceptibility and inelastic neutron scattering. These results are compared with those obtained on an insulator, PrScO3, chosen as a model compound. These experiments, together with cluster calculations, lead to the conclusion that an interaction between the f electrons and the Cu d electrons occurs in both PrBa2Cu3O7 and CmBa2Cu3O7.
Distribution of iron over copper sets in GdBa2(Cu1−xFex)3O7−δ (abstract)
Authors: Bornemann H. J. , Czjzek G. , Kmieć R.
Published: July 2002

Iron-doped compounds GdBa2(Cu1−xFex)3O7−δ with 0.03≤x≤0.25 and δ<0.1 as well as with δ>0.7 have been investigated by Mössbauer spectroscopy with 155Gd and 57Fe. For each iron concentration x, samples with different transition temperatures Tc (at constant oxygen content, δ<0.1) were prepared and investigated. Samples were reduced to an oxygen deficiency δ>0.7 either at 650 °C under vacuum or at 950 °C in a N2 atmosphere, followed by a quench to room temperature. A comparison of 57Fe Mössbauer spectra for samples treated by these different procedures has yielded an unambiguous determination of the fraction of iron atoms occupying Cu(2) sites. This fraction extrapolates to ∼4% for x→0. It increases monotonously with x to ∼17% for x=0.12. In samples reduced at 950 °C for 44 h, the fraction of iron on Cu(2) sites is doubled, and this large Cu(2) site occupancy is maintained when the material is reoxidized at 450 °C. From the systematic variation of the parameters associated with the remaining quadrupole doublets with iron concentration x and with Tc (for constant x), tentative assignments of the doublets to specific oxygen coordinations of Fe on Cu(1) sites were derived. For 155Gd, both the quadrupole splitting and the isomer shift vary smoothly with iron concentration. No influence of an extra charge associated with iron on Cu(2) sites in the vicinity of Gd nuclei is discernible.
Temperature dependence of the transmission of gigahertz sound through Invar
Authors: Carvajal F. , Dewar G.
Published: July 2002

The temperature dependence of the transmission of 9-GHz sound waves through polycrystalline samples of the Invar alloy Fe63Ni36Mn0.4Si0.2 has been measured over the temperature interval 77–300 K. Configurations in which the applied, static magnetic field as in the plane of the sample but either parallel of perpendicular to the incident microwave magnetic field were studied. The experimental results are compared to transmission calculations including both magnetoelastically and electromagnetically excited sound.
Tunneling measurements of the energy gap in Tl- and Bi-based oxide superconductors
Authors: Vieira S. , Rodrigo J. G. , Ramos M. A. , Agrait N. , Rao K. V. , Makino Y. , Costa J. L.
Published: July 2002

We discuss the behavior found in tunneling measurements obtained by us on two high-Tc superconductors—a ceramic sample of Tl2Ca2Ba2Cu3O10+δ and a Bi4Ca3Sr3Cu4O16+δ single crystal—using a scanning tunneling microscope. We compare some of the experimental curves with simple model calculations, assuming different tunnel-barrier shapes (i.e., different barrier heights, degree of asymmetry, thickness, etc.), and also different quasiparticle density-of-states profiles and tunnel-junction configurations.
Normal-state 63Cu Knight shift and hole-band modification in Y1−xPrxBa2Cu3O7
Authors: Reyes A. P. , MacLaughlin D. E. , Takigawa M. , Hammel P. C. , Heffner R. H. , Thompson J. D. , Crow J. E. , Kebede A. , Mihalisin T. , Schwegler J.
Published: July 2002

The 63Cu NMR Knight shift K has been measured in oriented powder samples of normal-state Y1−xPrxBa2Cu3O7, x=0.2 and 0.4. At plane Cu sites K is found to decrease with Pr doping, and a temperature dependence (dK/dT>0) develops. The variation of K with bulk susceptibility χ is considerably greater than can be explained using RKKY coupling between Pr spins and Cu nuclei, and the temperature variation of K resembles that of χ in high-Tc systems with reduced conduction-hole concentration. These results are consistent with the view that Pr impurities significantly modify conduction-band properties, rather than acting as a ‘‘conventional’’ pair-breaking mechanism.
First magneto-optical Kerr-effect study below 1 K on the superconducting Chevrel phase Eu1−xSnxMo6S8−ySey
Authors: Fumagalli P. , Schoenes J. , Decroux M. , Fischer O/.
Published: July 2002

We report on the first magneto-optical Kerr-effect measurements at temperatures below 1 K on a magnetic superconductor. We have measured the complex polar Kerr effect at temperatures as low as 0.4 K and in magnetic fields from 0 to 12 T on sintered samples of Eu0.75Sn0.25Mo6S7.6Se0.4 and EuMo6S8. In addition, the near-normal reflectivity has been determined at room temperature in the energy range of 0.03–12 eV. Both compounds exhibit a Kerr spectrum dominated by intra–Eu  f→d transitions. A small conduction-band contribution is detectable only in the Kerr rotation and not in the ellipticity. The maximum absolute Kerr rotation is very small and amounts to 0.19° and 0.13° at 2.25 eV for EuMo6S8 and Eu0.75Sn0.25Mo6S7.6Se0.4, respectively. In both compounds the crystal field splitting of the Eu-5d state is clearly visible and yields 0.9 eV. The total weight 〈σ2xy〉 per Eu2+ of the  f→d transitions is a factor 5 smaller than in the europium chalcogenides, possibly due to a smaller overlap of the radial  f and d wave functions. Field-dependent Kerr-rotation measurements up to 12 T at a temperature of 0.5 K show a substantial deviation from a Brillouin function.
Co-W films with an artificially layered structure (abstract)
Authors: Sato Noboru
Published: July 2002

Magnetic and crystallographic properties of Co-W films, deposited in an alternating multilayer structure by a magnetron dc-sputtering method, have been investigated as a function of CO composition and Co layer thickness. Regular periodic stacking without interdiffusion of Co and W atoms is made on all films with various periodicities (5–100 Å) and thickness ratios (8/1–1/4). Films with small periodicities (less than 30 Å) composed of large thickness ratios have an artificial superlattice structure with crystallographic coherence between Co and W layers, where they grow epitaxially on the closest planes.1 The crystallographic structures of films are hcp with the (001) plane parallel to the film for films with a Co layer thicker than the W layer, and bcc with the (110) plane parallel to the film for films with a Co layer thinner than the W layer. Films with small periodicities composed of small thickness ratios have an artificial superstructure with periodic stacking of constituent layers of an amorphous phase. Films with large periodicities have an artificial superstructure wih periodic stacking of constituent layers of a crystalline phase. Films are ferromagnetic with an easy magnetization direction in the film plane. The magnetization of Co in Co-W films is smaller than that of bulk Co. The paramagnetism or ferromagnetism of films with an extremely small magnetization are observed in films with small periodicities, especially composed of a Co layer thinner than the W layer. The small magnetization of Co is attributed to decreases in both the magnetization and the Curie temperature of Co due to lattice deformation. Films with a bcc structure exhibit a Curie temperature of about 600 K. The magnetization of Co in films with a bcc structure seems to be larger than that in films with a hcp structure.
Magneto-optical transitions in GaAs-AlGaAs coupled double quantum wells
Authors: Perry C. H. , Lee K-S. , Ma L. , Koteles Emil S. , Elman B. S. , Broido D. A.
Published: July 2002

Magnetophotoluminescence excitation spectroscopy has been employed to study GaAs-AlGaAs coupled double quantum wells. The investigations were undertaken at liquid helium temperatures and in magnetic fields up to 15 T applied parallel to the growth direction. In zero magnetic field, under flat-band conditions, the spectrum is dominated by ground-state excitonic transitions between electron and hole states with envelope functions of the same symmetry. In a magnetic field, the oscillator strengths of excited excitonic states are enhanced and four series of transitions are observed. The spectral features corresponding to the ground state and the excited states have been analyzed and these results were utilized to determine the exciton binding energies for several excitonic transitions of different subbands in this coupled double quantum-well structure. A number of additional features are visible at higher magnetic fields; some of these peaks are believed to be associated with 2p exciton transitions between hole and electron states with envelope functions of opposite parity due to exciton mixing. A comparison is made between the experimental results and theoretical calculations which include these effects.
Spin separation and type I/type II behavior in ZnSe/(Zn,Fe)Se quantum wells (abstract)
Authors: Liu X. , Petrou A. , Warnock J. , Jonker B. T. , Krebs J. J. , Prinz G. A.
Published: July 2002

Quantum well structures with 100-Å Zn0.9Fe0.1Se barriers and 100–200-Å ZnSe wells have been grown by molecular-beam epitaxy. The magnetic field splitting of the heavy-hole exciton was studied by magnetoreflectivity at 4.2 K and B≤8 T, and is observed to be strikingly asymmetric, in contrast to the behavior observed in thick single epilayers. This behavior may be quantitatively understood through a model which incorporates strain effects, quantum confinement in the conduction and valence bands, and the strong spin exchange interaction in the (Zn,Fe)Se layers. In this system the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. The valence-band (VB) lineup is thus determined primarily by the field-induced spin splitting of the heavy-hole level, which at modest fields (1 T) is an order of magnitude larger than the zero-field VB offset. This produces a field-induced spin-dependent band alignment, with the quantum well structure exhibiting a type I ‘‘straddling’’ alignment for the mj=+ (3)/(2) level, and a type II ‘‘staggered’’ alignment for the mj=− (3)/(2) level. This leads to a spatial spin separation of the holes, with the mj=+ (3)/(2) spin-‘‘up’’ holes localized in the wells, and the mj=− (3)/(2) spin-‘‘down’’ holes localized in the barriers. The observed exciton intensities and splittings are both consistent with such a model.