bluuebunny/crossref_metadata_2025 huggingface dataset:
Flux pinning and scaling laws for superconducting V3Ga
Authors: Hampshire D. P. , Clark A. F. , Jones H.
Published: July 2002

The electric field versus current density (E-J) characteristics of a multifilamentary V3Ga wire are presented for temperatures in the range 2.5 K≤T≤Tc in magnetic fields up to 15 T. The critical current density (Jc) is determined throughout the superconducting phase for this composite. The volume pinning force (Fp=Jc×B) obeys the Fietz–Webb scaling law which is of the separable form Fp=αBnc2(T)f(b), where Bnc2(T) is a function only of temperature and f(b) is a function only of reduced field b [b=B/Bc2(T)]. In addition, two other scaling laws of this separable variable form which characterize the variation of critical current densities throughout the sample and the flux flow state are explicitly determined. It is demonstrated that similarly to NbTi and Nb3Sn, the mechanism describing dissipation above Jc must incorporate the intermittent motion of fluxons or flux bundles across the superconductor such that only a fraction (∼10−5) of the flux-line lattice is in motion.
The charge pumping method: Experiment and complete simulation
Authors: Hofmann F. , Hänsch W.
Published: July 2002

We calculate the charge pumping current of a metal-oxide-semiconductor transistor in the time domain utilizing a transient two-dimensional device simulation. The dynamics of the interface states are included in the solution of the time-dependent problem with full self-consistency. The calculated charge pumping curve is in good agreement with the experiment, especially the rise and fall patterns of the signal, which are very sensitive to the source/drain profiles in small devices. The extraction of the density of states shows the range of validity of the analytical models and their restrictions. The influence of hot-carrier stress on the charge pumping signal, which relates to inhomogeneous spatial distribution of interface states and fixed oxide charges, is also discussed in experiment and simulation.
Fabrication of Bi-(Pb)-Sr-Ca-Cu-O mono- and multifilamentary superconductors and improvement in critical current density
Authors: Sekine H. , Inoue K. , Maeda H. , Numata K.
Published: July 2002

The Bi-(Pb)-Sr-Ca-Cu-O superconductor has been fabricated into tapes without sheathing and into multifilamentary wires and tapes with Ag sheathing. Metallurgical studies as well as measurements have been made for them. Tape specimens prepared by combination and repetition of cold work (or cold press) and sintering showed critical current density Jc as high as 5×103 A/cm2 at 77 K with good reproducibility and reliability, although these specimens are sufficiently thick (0.5–1.0 mm). (Generally, Jc at zero field is inversely proportional to thickness.) For these tape specimens (with Cu sheath), sintering and Jc measurement were performed after the Cu sheath was removed. X-ray analysis showed that most of these tape specimens consist purely of high-Tc phase (∼107 K). In these specimens, the c axis tends to align well. This c axis alignment as well as compacting effect may be a cause of the enhancement in Jc . On the other hand, the tape specimens cold-worked and sintered only once (not repeated) showed Jc as small as only 102 A/cm2 with the c axis showing no sign of alignment. A 1330-filament Bi-Pb-Sr-Ca-Cu-O wire with a Ag sheath has been successfully fabricated. This multifilamentary wire shows critical temperature Tc of ∼105 K. A multifilamentary tape specimen fabricated by a combination of cold work and sintering showed Jc of 1.05×103 A/cm2 at 77 K.
Development and quality measurements of cold relativistic electron beam for low-γ free-electron lasers
Authors: Kawai M. , Kawamura Y. , Toyoda K.
Published: July 2002

A relativistic electron beam source with low temperature (cold) using a field emission cathode and uniform electrostatic acceleration has been developed for use in low-γ free-electron lasers. An energy of 0.51 MeV and a current of 60 A (200 A/cm2) were obtained. The energy spread and the angular velocity spread were measured to be ΔE/E=0.14% and β⊥/β∥ =4×10−2, respectively.
Optical properties of epitaxial CoSi2 and NiSi2 films on silicon
Authors: Jimenez J. R. , Wu Z.-C. , Schowalter L. J. , Hunt B. D. , Fathauer R. W. , Grunthaner P. J. , Lin T. L.
Published: July 2002

We have measured the optical constants of epitaxial films of CoSi2 and NiSi2, grown by molecular beam epitaxy on Si(111), in the energy range of 0.9–4.0 eV. The behavior of the optical constants is characteristic of metals: Drudelike in the low energy region and deviating from Drude behavior as interband transitions set in. Interband transitions are found to have already set in at 1 eV. The absorption varies significantly with energy, which has implications for photoresponse studies of internal photoemission in these material systems.
The calibration of the strength of the localized vibrational modes of silicon impurities in epitaxial GaAs revealed by infrared absorption and Raman scattering
Authors: Murray R. , Newman R. C. , Sangster M. J. L. , Beall R. B. , Harris J. J. , Wright P. J. , Wagner J. , Ramsteiner M.
Published: July 2002

n-type silicon-doped epitaxial layers of gallium arsenide grown by molecular-beam epitaxy (MBE) or metal-organo chemical vapor deposition (MOCVD) have been investigated by measurements of the Hall effect and the strengths of the localized vibrational modes (LVM) of silicon impurities using both Fourier transform absorption spectroscopy and Raman scattering at an excitation energy of 3 eV close to the E1 band gap. Lines from Si(Ga) donors, Si(As) acceptors, Si(Ga)-Si(As) pairs, and Si-X, a complex of silicon with a native defect, were detected and correlated for the two techniques. The maximum carrier concentration [n] found for samples grown under standard conditions was 5.5×1018 cm−3. At higher doping levels Si-X becomes dominant and acts as an acceptor, so reducing [n]. An integrated absorption of 1 cm−2 in the Si(Ga) LVM line corresponds to 5.0±4×1016 atoms cm−3: a similar calibration applies to the Si(As) line, but for Si-X, an absorption of 1 cm−2 corresponds to only 2.7±1.0×1016 defects cm−3. Possible structures for Si-X are discussed but a definitive model cannot yet be proposed. MBE samples grown at 400 °C had values of [n] close to 1019 cm−3, and a negligible concentration of Si-X. On annealing, [n] decreased and Si-X defects were produced together with site switching of Si(Ga) to Si(As). These results are important to the understanding of the mechanism of silicon diffusion at low temperatures. The infrared absorption and Raman measurements are complementary. Absorption measurements made at a resolution of 0.1 cm−1 require layers greater than or equal to 1 μm in thickness doped to a level of 3×1017 cm−3 but require the prior elimination of free-carrier absorption. Raman measurements can be made on as-grown layers only 10 nm in thickness doped to a level of 2×1018 cm−3, but with a spectral resolution of only 5 cm−1.
Atomic layer epitaxial growth of ZnSe, ZnTe, and ZnSe-ZnTe strained-layer superlattices
Authors: Dosho Shiro , Takemura Yasushi , Konagai Makoto , Takahashi Kiyoshi
Published: July 2002

ZnSe and ZnTe thin films and ZnSe-ZnTe strained-layer superlattices (SLS) have been successfully grown by atomic layer epitaxy (ALE) using molecular-beam epitaxy. The ALE growth of ZnSe and ZnTe was achieved at substrate temperatures between 250 and 350 °C and between 240 and 280 °C, respectively. The Zn, Se, and Te beam intensities necessary to achieve ALE growth were evaluated. Under optimum growth conditions, the surfaces of ZnSe and ZnTe thin films were very specular without any surface structure. Modulation-doped n-type SLSs have been prepared by ALE using Cl as a n-type dopant. The carrier concentration of superlattices grown by ALE (3.8×1016 cm−3) was three orders of magnitude higher than those of superlattices grown by conventional molecular-beam epitaxy.
Oxygen complexes in III-V compounds as determined by secondary-ion mass spectrometry under cesium bombardment
Authors: Gauneau M. , Chaplain R. , Rupert A. , Le Corre A. , Salvi M. , L’Haridon H. , Lecrosnier D. , Dubon-Chevallier C.
Published: July 2002

Recent studies related to the p-type doping of GaInAs epilayers, grown by molecular-beam epitaxy, mentioned erratic results concerning the electrical activity of beryllium (Be). This work reports on the secondary-ion mass spectrometry analysis of moderately Be-doped (between 5×1017 and 5×1018 cm−3 ) GaInAs/InP layers. Two of them were n type while they should have been p type. The comparison of Be depth profiles, obtained under either oxygen or cesium primary-ion bombardment, and detailed study of the secondary-ion mass spectra reveals that Be+ secondary-ion useful yields are largely enhanced in n-type samples when using cesium primary ions. Such behavior is attributed to the presence of varying oxygen contents in the samples, as confirmed by quantitative analyses. Comparison of Be and O concentrations indicates that the two elements probably form complexes inside the GaInAs matrix, providing a mechanism for the observed Be electrical inactivity. The use of positive secondary-ion yield enhancement under cesium bombardment is generalized to other elements (magnesium) in other semiconductor materials, in particular to the case of Au-Mn/GaAs ohmic contacts. Once again such ions reveal the presence of oxygen. Paradoxically, BeCs+ or MgCs+ molecular ions are not sensitive to the presence of oxygen. It is hypothesized that MCs+ species are more stable than MO+ . Such a differential effect is very helpful in characterizing the presence of oxygen in semiconductor materials.
Inhomogeneous Biot waves in layered media
Authors: Allard Jean F. , Depollier Claude , Rebillard Pascal , Lauriks Walter , Cops André
Published: July 2002

A description, using transfer matrices, is given of the propagation of the three inhomogeneous Biot waves in layered porous media. This is applied to predict the surface impedance of porous layered materials at oblique incidence, and an example is presented.
New double-heterostructure indium-tin oxide/InGaAsP/AlGaAs surface light-emitting diodes at 650-nm range
Authors: Ishikawa Joji , Funyu Yasuhito , Yonezawa Rumiko , Takagi Kazuo , Takahashi N. Shin-ichi , Kurita Shoichi
Published: July 2002

New double-heterostructure indium-tin oxide/InGaAsP/AlGaAs surface light-emitting diodes have been fabricated by liquid-phase epitaxy and rf sputtering methods. In this structure, indium-tin oxide acts as both an n-type cladding layer and a transparent conductor. Peak wavelength and full width at half maximum of the surface emitting spectrum were 653 and 17 nm, respectively. An output power of 1 mW was achieved at a current level of 66 mA, corresponding to a current density of 22 A/cm2 under pulsed operation for the diode with a 400 μm×450 μm emitting area. The optical emission was distributed over the entire emitting area.
Modeling magneto-optical domain erasure without cylindrical symmetry
Authors: Savage C. M. , Watson M. , Meystre P.
Published: July 2002

We describe a technique for modeling domain dynamics in the context of thermomagnetic magneto-optical recording. The new feature of our model is that no assumption of cylindrical symmetry is required. We illustrate the potential of our technique by applying it to domain erasure on a moving disk.
Lithium amide (LiNH2) discharges
Authors: Drummond D. L. , Schlie L. A.
Published: July 2002

Results are presented showing the use of lithium amide (LiNH2) in place of pure lithium vapor produces a strong lithium excimer and dimer emission via discharge excitation. The use of this amide greatly reduces the required temperature necessary to generate the 1016/cm3 lithium densities. Temperature reductions from 800 to near 200 °C are possible. Such lower temperatures should enable potential alkali metal vapor excimer and dimer laser studies to be much easier.
Growth studies of molecular-beam epitaxial ZnSe using reflection high-energy electron diffraction oscillations
Authors: Turco F. S. , Tamargo M. C.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) intensity oscillations are often used to investigate in situ the growth of III-V materials by molecular-beam epitaxy (MBE). In this work, we have used RHEED oscillations to perform a quantitative study of the growth mechanisms of ZnSe, a II-VI semiconductor. Our experiments illustrate that the RHEED pattern of ZnSe is far less intense than that of III-V materials grown by MBE, and no specular spot is observed over a wide range of growth conditions. We have, however, been able to record up to 25 oscillations allowing a quantitative study of the growth of ZnSe by MBE. Thus we have used RHEED oscillations to make an in situ systematic study of the influence of the three main growth parameters (substrate temperature and Zn or Se impinging fluxes) on the ZnSe growth rate. We observed that the variation of the ZnSe growth rate is due to a nonunity sticking coefficient of both Zn and Se species at the interface in the standard growth conditions used. Our observations can be described using a thermodynamic model and enable us to control the desired growth conditions. Our work demonstrates the utility of RHEED oscillations to understand the MBE growth mechanisms of II-VI compounds.
Fabrication and spectral characteristics of a 1.55-μm distributed feedback laser with a tunable integrated external cavity
Authors: De Cooman B. C. , van der Hofstad G. L. A. , Kuindersma P. I. , Wong C. K. , Scheepers W. , Tiemeyer L. F.
Published: July 2002

The fabrication and lasing characteristics of a 1.5-μm distributed feedback laser with a butt-coupled tunable integrated resonator are described. An additional modulation of the distributed feedback laser spectrum introduced by the external cavity is clearly demonstrated. Optical bistability has been observed in the power versus tuning current characteristic. The device has a limited continuous tuning range of 4.2 Å. Tuning current change allows for the selection of compound cavity mode and one of the distributed feedback laser modes, while maintaining single-mode operation. Linewidth reduction due to the integrated cavity is clearly demonstrated, and linewidths <10 MHz have been obtained.
CO2 laser patterning of plasma-deposited high T
c superconducting thick films
Authors: Varshney Usha , Churchill R. J. , Groger H. P. , Kingon A. I.
Published: July 2002

Recent advances in the development of high Tc superconducting thick films have lead the present work to optimize laser etching parameters for patterning thick films for device fabrication. Plasma-deposited superconducting thick films (200–400 μm) of Y-Ba-Cu-oxide materials on polycrystalline alumina and copper substrates were laser etched using a CO2 laser (10.6 μm) in the enhanced pulse mode. Significant spatial nonuniformity of the surface elemental distribution of Y, Ba, Cu, and Al was observed in the underlying laser-etched area. The laser fluence for threshold etching was observed to be 25 J/cm2. An etch rate of 4.0 μm/scan was calculated at an optimized laser fluence of 292 J/cm2 for patterning plasma-sprayed thick films having an estimated heat affected zone of 15 μm. An absorption length of 52 μm for the CO2 laser was determined to be suitable for patterning thick films for device fabrication.
The influence of many-body effects on the gain and linewidth of semiconductor lasers
Authors: Bardyszewski Witold , Yevick David
Published: July 2002

We employ quantum-mechanical many-body theory to determine both the radiative recombination spectra and the linewidth broadening factor in III-V semiconductor diode lasers. We conclude correlation effects and the full k ⋅ p band structure in our calculation. Our results clearly illustrate the manner in which many-body effects relax momentum conservation in the recombination process.
Optical and thermal analyses on multilayered thin films of a phase-change optical recording disk
Authors: Koyanagi H. , Ito T. , Miyamoto N. , Sato Y. , Ando H.
Published: July 2002

A computer simulation method to study a laser-induced transient temperature profile of a multilayered phase-change optical recording disk (PCORD) has been developed. The simulation program consists of two parts. One part is an optical analysis program based on the optical characteristic matrix method, and the other is a thermal analysis program based on the finite-element method. A new estimation method for thermal conductivity of recording films was proposed. The estimated thermal conductivity of the thin-film recording media was found to be 50% lower than that of the bulk. Reflectivity, absorbance, and temperature values obtained with this simulation method were compared with those measured by a spectrophotometer and an infrared radiation thermometer when the samples were irradiated in an Ar+ laser beam. The accuracy using this simulator was within 8% for reflectivity and absorbance and within 10% for temperature. A method of optimizing the laser-absorbing layer was studied. With this method simulations showed that the PCORD had a maximum contrast ratio and minimum recording time when thickness, thermal conductivity, and complex refractive index of the laser-absorbing film were 60 nm, 4.2×10−3 W/cm °C, and 2.6−i⋅0.0, respectively. This simulation could provide better film designs for PCORDs and the other optical disks of heat mode recording.
Uniformity of 3-in., semi-insulating, vertical-gradient-freeze GaAs wafers
Authors: Look D. C. , Walters D. C. , Mier M. G. , Sewell J. S. , Sizelove J. S. , Akselrad A. , Clemans J. E.
Published: July 2002

We have evaluated the uniformity in [EL2], dislocation (or etch-pit) density (EPD), resistivity, mobility, and carrier concentration for 3-in., semi-insulating GaAs wafers grown by the vertical-gradient-freeze (VGF) technique. Although slight W or U patterns were observed in [EL2] and EPD along the 〈110〉 directions, for the first time, nevertheless the overall uniformity was excellent, and comparable to that in the best In-doped and whole-boule-annealed ingots grown by the liquid-encapsulated Czochralski (LEC) technique. Based on results from implant-activation studies on LEC wafers, it is estimated that the measured nonuniformities in EPD and [EL2] for the VGF wafers would contribute only about 1% to implant-activation-efficiency nonuniformities in Si-implanted wafers designed for field-effect transistor applications.
Defect identification in semiconductor alloys using deep level composition dependence. II. Application to GaAs1−xPx
Authors: Bylander E. G. , Myles Charles W. , Shen Yu-Tang
Published: July 2002

We predict the x dependencies of deep levels produced by vacancy-impurity complexes in GaAs1−xPx. These predictions, along with those obtained earlier for the x dependencies of deep levels due to impurities, show that the slope of a deep level with x depends strongly on the site of the impurity atom for both complexes and isolated impurities. Furthermore, we find that the slopes of some of the levels produced by the vacancy complexes are very different than those associated with the corresponding point defects. We thus suggest that the theory can be used to obtain site information about the defect producing an observed level and, in favorable cases, to distinguish between levels produced by isolated impurities and those produced by complexes. We also present photoluminescence data on two unknown centers in GaAs1−xPx and compare some of our theoretical slopes with those of the levels extracted from the data. The results show that the theory can be useful as an aid to defect identification in GaAs1−xPxx.
Photoemission from strained epitaxial magnetic layers (abstract)
Authors: Morra R. M. , Almeida F. J. D. , Willis R. F.
Published: July 2002

The important question of the effect of lattice strain on the electronic band structure of strained magnetic layers is addressed. Specifically, we compare high-resolution angle-resolved (k-dependent) photoemission from metastable fcc cobalt films with those from metastable fcc iron films, both epitaxed to a Cu(001) substrate as a function of film thickness over a range of 1–10 monolayers. The cobalt films show the effect of a stable magnetic moment in a homogeneously strained film. The iron films show more complex behavior indicative of inhomogeneous strain associated with an unstable magnetic moment and strong surface magnetic anisotropy effects. The photoemission measurements are compared with in situ magnetometry measurements using the surface magneto-optic Kerr effect (SMOKE).1
Neutron scattering investigation of the ordered state of the nuclear spins in Cu at 60 nK
Authors: Siemensmeyer K. , Kakurai K. , Steiner M. , Jyrkkiö T. A. , Huiku M. T. , Clausen K. N.
Published: July 2002

We have studied the magnetic structure of the nuclear spin system in Cu below 60 nK by means of neutron scattering. The observation of the nuclear (100) superlattice reflection proves the theoretically predicted antiferromagnetic arrangement of the nuclear spins. The critical field at the lowest temperatures was 0.24 mT. Around 0.1 mT the (100) reflection could not be observed. This is taken as an indication for, possibly, a reorientation phase transitition at this field. Therefore, the phase diagram contains at least two phases. For 0.1 mT<B<0.24 mT strong time dependencies of the ordering were observed. At low fields (B<0.1 mT) nucleation times of the order of 10 s have been observed. These results are discussed with respect to the cooling technique: Adiabatic demagnetization requires constant entropy, a rather unusual way to bring a system from the paramagnetic to the ordered phase.
Comparison between various hysteresis models and experimental data
Authors: Ossart F. , Phung T. A. , Meunier G.
Published: July 2002

We propose a comparison between results predicted by two mathematical models of magnetic hysteresis and experimental data. The first model is based on the Preisach model, the second consists in solving a differential equation between B and H. Those models are very different in their principles and the properties they exhibit. In particular, the second one models the accommodation of minor loops and the anhysteresis curve when the first does not. The magnetic material we study is a CoNiCr alloy for longitudinal digital recording. We give some results showing that, for this material, the Mayergoyz model fits better the experimental behavior.
Thermal properties of γ-Mn (abstract)
Authors: Acet M. , Stamm W. , Wassermann E. F. , Zähres H. , Pepperhoff W.
Published: July 2002

Mn can exist in a fcc environment in many alloys with other elements of the 3d-transition series. Some of these alloys, which can contain more than 50 at. % Mn, are classified as Invar. They show large magnetovolume effects and at the same time have ferromagnetic, antiferromagnetic, or mixed magnetic ground states. In order to gain a better understanding of Invar systems incorporating Mn, a closer examination of this element in a fcc environment is necessary. The γ state of Mn (fcc or fct Mn) is relatively little known, since its stability range lies only between 1364 and 1410 K. However, it is possible to stabilize γ-Mn in a temperature interval ranging from low temperatures up to about 600 K by the addition of small amounts of Cu. We have measured the specific heat (300<T<600 K) and thermal expansion (4<T<600 K and 1100<T<1300 K) of three MnCu alloys containing 4.42, 6.75, and 8.55 at. % Cu. The specific heat shows a first-order transition at Tt, which is the temperature below which the lattice undergoes a tetragonal distortion. This distortion is accompanied by an abrupt appearance of antiferromagnetism. The same effect is seen in the thermal expansion which shows a volume increase associated with the lattice distortion and the magnetic ordering. The high-temperature value of the thermal expansion coefficient is the largest so far observed in any fcc 3d system (45×10−6 1/K). From this data we estimate the thermal expansion behavior of pure γ-Mn. We obtain the atomic volume in the nonmagnetic and antiferromagnetic state and compare it to theoretical calculations.
Overwrite performance for high-coercivity particulate media
Authors: Tanabe M. , Fukui H. , Uesaka Y. , Sugita Y.
Published: July 2002

A model of overwrite based on anisotropy field distributions of particles has been proposed for γFe2O3 and Co-γFe2O3 high-coercivity particulate media. In this model, overwrite is determined by the ratio of the number of nonreversed particles to the total number of particles. The magnitude and angle distributions of the anisotropy field were obtained by torque magnetometer measurements. From these observed distributions, switching field distributions of the particles were derived using the curling model for an infinite cylinder. The write field of thin-film heads was calculated using a finite element method. The nonreversal ratio in the medium was given by both the switching and the write fields. The calculated overwrite values were obtained from this ratio and were found to be in good agreement with the measured ones. This model was valid for the media which differed in the distributions of the anisotropy field. The overwrite performance can be improved by narrowing the anisotropy field distributions of the particles in the media.
On remanence loops and recording performance
Authors: Speliotis D. E.
Published: July 2002

Magnetization transitions in recording media have frequently been modeled by a distribution of the form Mr arctan (x/a), where the transition length ‘‘a’’ is presumably determined by the Mrt/Hc ratio of the medium and the head field gradient. We propose that the remanence loop parameters are more relevant to the magnetic recording process than the hysteresis loop parameters. Measured from the remanence loop in the direction of orientation, the important parameters for recording include the switching field distribution (SFDr), the initial slope of the remanence curve, and, of course, the remanence (Mr) and remanence coercivity (Hr). In this respect, medium perfection is reflected in perfectly rectangular remanence loops. Furthermore, a large angular dependence of Hr (from the easy to the hard axis of the media) is also very desirable in minimizing sidetrack writing and recording demagnetization. This proposal is justified by comparing the hysteresis, remanence, and recording properties of various advanced particulate (Co-modified iron oxide and Ba-ferrite longitudinally oriented media) and metal thin-film media. A rather surprising result was the attainment of superior high density response and a high signal output in BaFe media with extremely small SFDr, even though the half-pulsewidth is not commensurately small because it is masked by the contributions of effective spacing, media thickness, and gap length. These thicker BaFe media provide equivalent recording performance to that of much thinner and higher coercivity metal thin-film media.
Critical behavior in the two-impurity Kondo problem (abstract)
Authors: Jones B. A. , Kotliar B. G. , Millis A. J.
Published: July 2002

The antiferromagnetic two-impurity Kondo problem, involving two magnetic moments coupled antiferromagnetically to each other and also to a band of itinerant electrons, has been found to exhibit nontrivial behavior.1 We show analytically2 that the presence of a type of particle-hole symmetry guarantees a nonanalyticity in the ground-state energy as the antiferromagnetic RKKY coupling is varied relative to the Kondo temperature. This symmetry is not present in the generic antiferromagnetic two-impurity model. Previous work on the symmetric model1 has shown that the nonanalyticity is a second-order T=0 phase transition with diverging specific heat coefficient and staggered susceptibility. We have studied the effect of breaking the symmetry via an analytic large-N expansion and via Wilson’s numerical renormalization group technique. The analytic work suggests that there may be no transition if the symmetry is broken. The preliminary numerical work suggests either that the transition survives, or that there is a well-defined crossover signalled by an order of magnitude or more enhancement of the specific heat coefficient. Further analytic and numerical work is in progress to resolve the dilemma.
Ground states of hcp vector antiferromagnets: Zn1−xMnxSe
Authors: Larson B. E.
Published: July 2002

Type III ground states of hcp vector antiferromagnets—appropriate to the wurtzite magnetic semiconductors (MS) such as Zn1−xMnxSe —are shown to be classically unstable at long wavelengths. The Hamiltonian includes antiferromagnetic nearest-neighbor (NN) and next-nearest-neighbor (NNN) isotropic exchange (J1,J2), and NN anisotropic Dzyaloshinsky–Moriya exchange (D). The hexagonal symmetry allows Ja1 for NN in planes perpendicular to the c axis to be different from Jc1 for NN between planes. For three-component spins a five-dimensional degenerate manifold of type III ground states is found (Ja1=Jc1, D=0). Their instability is investigated through a continuum formulation of the exchange energy, treating D and ΔJ≡(Ja1−Jc1) as small compared with J1. ΔJ is found to induce a twist of a noncollinear ground state, stabilized by D to zeroth order. By fitting the shift δQ of magnetic peaks seen in neutron scattering experiments on Zn0.45Mn0.55Se the required ΔJ/J1 (∼δQ) is found to be 0.06. This value agrees with estimates of higher order superexchange processes for the two types of pairs.
Magnetic excitations in thulium metal
Authors: Fernandez-Baca J. A. , Nicklow R. M. , Rhyne J. J.
Published: July 2002

We have performed inelastic neutron scattering measurements on a single-crystal specimen of Tm at wave vectors κ=(1, 1,ζ) and (0,0,2+ζ) (ζ=0,...,1). Most of the measurements have been made at T=5 K, where Tm exhibits a seven-layer ferrimagnetic-antiphase-domain structure (four moments up, parallel to the c axis, followed by three moments down). At this temperature the excitation spectra consist of three peaks. The two lower energy excitations have been identified as originating from magnetovibrational scattering from the TA phonon, while the higher energy excitation is magnetic and exhibits only a weak dispersion (between 8.3 and 9.6 meV). At T=50 K, a temperature at which the system exhibits a c-axis sinusoidally modulated structure, the magnetic mode shows significant softening and broadening. The magnetovibrational scattering vanishes above the Néel temperature (TN=58.5 K) while the magnetic mode persists at least up to T=70 K. These results suggest that the Hamiltonian in this system is dominated by the crystal-field-anisotropy energy and that the exchange interaction is relatively weak.
Reversal mechanisms and the temperature dependence of σR/σS, H
C, and H
K in particle assemblies
Authors: Berkowitz A. E. , Parker F. T. , Slade S. B. , Podolsky G.
Published: July 2002

Hysteresis loops of randomly oriented assemblies of acicular particles were measured at temperatures from 4 to 296 K in maximum fields of 24 kOe. The compositions were γ-Fe2O3, γ-Fe2O3 with 7% Fe2+ (precursor for Co surface treatment), Co surface-treated precursor with 4, 8, and 16 wt. % Co, and Fe. Mean HK ’s were determined from first quadrant data and from susceptibility at remanence. The precursor and the Co surface-treated particles had a similar temperature dependence for HC, σR/σS, and HK. This suggests that the effective Co2+ ions replace Fe2+ cations on the surface. The utility of the different mean HK determinations for indicating the HK distribution width is discussed, as well as the sensitivity to superparamagnetic fractions of the HK determined from susceptibility.
Predicted thermal dependence of barium ferrite dead layers and exchange parameters
Authors: Victora R. H.
Published: July 2002

Theoretical predictions are made for the thermal dependence of the macroscopic exchange parameter AM2s and the magnetic dead layer thickness observed on the surface of barium ferrite particles. The calculations are performed within mean field theory and are based on atomic exchange integrals taken from bulk data. It is found that the exchange parameter is approximately 20% smaller along the c axis than within the basal plane. The dead layer calculations require knowledge of the surface structure: An assumption that the surface is near the middle of the spinel block leads to a layer thickness in an undoped sample at 300 K of 3 Å, in close agreement with experiment.
Measurement of magnetic fields inside EB-cured barium ferrite coatings
Authors: Shao Xiaoping , Alexander Chester , Artman J. O.
Published: July 2002

Molecules of l-alanine, irradiated in an electron-beam-processed barium ferrite coating, have been used as probes of local magnetic fields inside the coating. These probes have been used to measure the average magnetic fields HD and HL, and the field variation ΔH, in the coating. For these thin-film samples, the HD, HL, and ΔH values are proportional to the volume fraction of the barium ferrite and are given in (Oe) by HD =−3573 Vf cos θ, HL =1247 Vf and ΔH=1200 Vf. The value of HD was dependent on the angle θ between the external field direction and the sample normal, and was interpreted as a demagnetizing field. The value of HL was independent of direction, had a magnitude of (1)/(3) of HD, and was interpreted as a Lorentz field. The field variation ΔH was determined from changes in ESR spectra linewidth and is consistent with an inhomogeneous line broadening model.
Magnetic excitations in CePd2Si2
Authors: Steeman R. A. , Mason T. E. , Lin H. , Buyers W. J. L. , Menovsky A. A. , Collins M. F. , Frikkee E. , Nieuwenhuys G. J. , Mydosh J. A.
Published: July 2002

The results of inelastic neutron scattering measurements on a single crystal of CePd2Si2 are presented. A broad magnetic contribution was observed near 20 meV at 20 K. Two different models were used to analyze this response: a single crystal-field excitation with a slightly Q-dependent energy, and two nearly degenerate crystal-field excitations without dispersion. In both cases the excitations have an appreciable intrinsic width due to the strong coupling between the 4f electrons of Ce and the conduction electrons. It is remarkable that, despite the presence of this strong coupling, the reduction of the Ce moment to 0.66μB at 4.2 K seems to be solely due to crystal-field effects.
Magnetism and crystal structure of Fe/Al/Si films prepared by dual-ion-beam sputtering
Authors: Kubota K. , Nagakubo M. , Naoe M.
Published: July 2002

Fe/Al/Si multilayered films have been deposited using an ion-beam sputtering apparatus. The effects of ion bombardment onto growing film surfaces and the effects of annealing after deposition on their crystal structure and magnetic properties have been investigated. The thicknesses of Fe, Al, and Si layers were adjusted at the value corresponding to the Sendust composition. Then the Fe layer thickness was set at 10 Å. Sharp peaks in small-angle x-ray diffraction diagrams of the as-deposited films clearly revealed the multilayered structure. Such peaks for films bombarded by argon ions at the acceleration voltage (Vb) of above 100 V during deposition were not observed. In addition, the films annealed at the temperature above 300 °C for 3 h showed the DO3 and/or B2 type of crystal structure and exhibited coercivity (Hc) below 3 Oe. It has been found that the Fe/Al/Si multilayered films can be alloyed at relatively low substrate temperature by the aid of ion bombardment during deposition. Annealing after deposition is effective for reducing internal stress and crystalline irregularity, leading to a decrease in Hc.
Magnetic ordering in (Y1−xPrx)Ba2Cu3O7 as evidenced by muon spin relaxation
Authors: Cooke D. W. , Kwok R. S. , Jahan M. S. , Lichti R. L. , Adams T. R. , Boekema C. , Dawson W. K. , Kebede A. , Schwegler J. , Crow J. E. , Mihalisin T.
Published: July 2002

Using the zero-field-muon-spin-relaxation (μSR) technique clear evidence has been found for antiferromagnetic ordering of Cu moments within the CuO planes of (Y1−xPrx)Ba2Cu3O7. The Néel temperatures are approximately 285, 220, 35, 30, and 20 K for x=1, 0.8, 0.6, 0.58, and 0.54, respectively. For x=0.50 we observe a fast-relaxing component of the muon polarization in addition to a long-time tail, reminiscent of spin-glass behavior. This region of the phase diagram (0.5≤x≤0.54) corresponds to the existence of both superconductivity and magnetism. The fully developed local magnetic field for x>0.54 is found to be ∼16 mT, but decreases to ∼12 mT at T=17 K for the x=1 sample, presumably due to the onset of Pr-ion ordering. Magnetic ordering also occurs in PrBa2Cu3O6; the Néel temperature is ∼325 K.
Microwave absorption in the presence of dc transport current in polycrystalline YBCO
Authors: Karim R. , How H. , Seed R. , Widom A. , Vittoria C.
Published: July 2002

Low-field microwave absorption in polycrystalline YBa2Cu3O7−x was measured as a function of dc current applied through the sample at various temperatures below Tc. The peak-to-peak value of the magnetically modulated microwave absorption (MMMA) signal increases with dc current at a fixed temperature. Above a threshold value of the current, the absorption signal level rapidly drops to zero, indicating quenching of superconductivity. If this critical current is plotted as a function of temperature, the results can be explained on the basis of the flux creep model. The MMMA signal obeys a scaling rule as predicted by the flux creep model. We conclude that flux creep limits the critical current density in these materials; also, this presents a new way of measuring critical current densities while using only a two-contact method. It gives some insight into the relationship between microwave properties and dc transport phenomena in these superconductors.
SQUID and magnetically modulated microwave absorption observations of the superconducting transition in single crystals of YBa2Cu3Ox
Authors: Bennett L. H. , Swartzendruber L. J. , Kaiser D. , Bohandy J. , Kim B. F. , Adrian F. J. , Moorjani K.
Published: July 2002

Two melt-grown single crystals of YBa2Cu3O7−x, one (crystal A) in the shape of a nearly perfect parallelepiped of dimensions 200×200×70 μm3 and with x≂0, the other (crystal B) with somewhat less oxygen content and of less geometric perfection, with dimensions≂750×900×10 μm,3 were measured by both SQUID magnetometry and by magnetically modulated microwave absorption (MAMMA). The two methods respond to the change in magnetic properties at the superconducting transition temperature, but in different ways, giving complementary information. For crystal A both methods show a single sharp transition near 90 K. For crystal B the SQUID measurements show a broad transition near 80 K, implying a material with a range of oxygen concentrations, while the MAMMA reveals considerable structure, with details of this structure depending on the direction in which the magnetic fields are applied.
Magnetostriction behavior of Co-Fe-Si-B amorphous alloys
Authors: Gomez-Polo C. , Pulido E. , Rivero G. , Hernando A.
Published: July 2002

It is well known that the saturation magnetostriction constant of nearly-zero-magnetostriction amorphous alloys exhibits a dependence on both magnetic field and applied stress. Therefore the anisotropy field induced by the applied stress does not depend linearly on the stress strength. Experiments carried out on Co-rich amorphous alloys show a stress dependence of the anisotropy field as that expected by assuming long-range fluctuations of the magnetoelastic anisotropy. In this report the existence of local fluctuations of saturation magnetostriction is shown to be a reasonable cause of the stress dependence of magnetostriction.
Effects of gas exposure on the surface-acoustic-wave attenuation and magnetoresistance of nickel thin films
Authors: Wiegert R. F. , Levy M.
Published: July 2002

We present experimental data showing that both the magnetic bias field H-dependent attenuation response to microwave (0.6 GHz) surface acoustic waves A(H) and the dc magnetoresistance ΔR(H) of vacuum-evaporated nickel thin films (100–250 Å) can significantly decrease or ‘‘age’’ when the samples are maintained in vacuum after film deposition. However, these losses in A(H) and ΔR(H) response can be recovered by exposing the samples to air. These effects are reversible, that is, if the samples are again placed in vacuum; their A(H), ΔR(H) responses again decrease while subsequent exposure to air again results in a recovery of sample response. Exposure of vacuum-aged samples to Ar results in no change in sample response and a passivation of the sample against the subsequent effects of other gases. These phenomena are apparently due to reversible physiabsorption processes that affect the magnetostriction-stress contribution to film coercivity. Apart from their significance for the study of the impact of physical absorption processes on thin film micromagnetic structure, these ‘‘gas effects’’ may have practical applications for modifying the response characteristics of magnetoelastic surface-wave devices and sensors.
High field magnetization of aluminum-doped cobalt ferrite single crystals
Authors: Ostoréro J. , Guillot M.
Published: July 2002

Magnetization measurements performed on single crystals of aluminum-substituted cobalt ferrites CoFe2−xAlxO4 are presented under a high magnetic field up to 20 T in the 4.2–300 K temperature range. Three substitution ranges (x=0.27, 0.46, and 1.05) are studied. In each case [100] is found to be the easy axis of magnetization as in pure cobalt ferrite while an abrupt field-induced transition is observed along the [111] direction below 200 K. When the temperature is greater than 100 K, saturation along [100] cannot be attained for x=1.05. A net decrease of the spontaneous magnetization due to the aluminum substitution on the octahedral B sites of the spinel lattice is found. Magnetization results are interpreted on the basis of the ionic distribution of the Co2+ and Fe3+ ions.
Pressure effect on magnetic transition temperatures of Mn3Ga1−xMxC (M=Zn,Sn)
Authors: Kanomata T. , Kawashima T. , Yoshida H. , Kaneko T.
Published: July 2002

Pressure effects on magnetic transition temperatures of Mn3Ga1−xMxC (M=Zn,Sn) have been measured in order to study the nature of the order-order magnetic phase transitions. The magnetic phase diagrams for the transition temperatures versus concentration and for the transition temperatures versus pressure have been determined. For Mn3Ga1−xZnxC, the intermediate magnetic phase observed in Mn3GaC above 3 kbar appears even at normal atmospheric pressure. On the basis of these results, the relation between the effect of alloying and of pressure on the magnetic transitions of the Mn3Ga1−xMxC (M=Zn,Sn) systems is discussed.
Shielded MR gradiometer read pulse shape for perpendicular medium
Authors: Hesterman Vic , Gill H. S. , Bhattacharyya M. K.
Published: July 2002

Isolated output pulse shapes are calculated for magnetoresistive gradiometer read heads having various geometric and magnetic parameters. The two-dimensional finite-element and reciprocity methods are used. The ferromagnetic shield has a big effect on the pulse shape, especially for closely spaces shields. High shield reluctance causes pulse broadening and large positive tails. Addition of a medium underlayer decreases the pulse undershoots and increases pulse amplitude. A reluctance model is given that explains most of the observed effects on pulse shapes and illustrates the importance of reluctance balance to achieve optimum pulse shape.
Contour effect in metal-in-gap heads
Authors: Kobayashi H. , Inoue K. , Kawakami M. , Kurokawa H. , Uehara Y.
Published: July 2002

The pseudogap in a metal-in-gap head causes ripples in output spectra. This gap is a layer with poor magnetic properties at the boundary between sendust film and a ferrite core. The ripple amplitude reduces as the bonding temperature decreases. It also depends on the ferrite-core crystal orientation. We examined the diffusion at the boundary by Auger-electron spectroscopy and investigated the crystal structure of sendust films by x-ray diffraction. It is proved that diffusion causes the contour effect. The thickness of the diffusion layer at the boundary depends on bonding temperature. It is also found that the diffusion at the boundary and the crystal structures of the sendust film are independent of the crystal orientation of the ferrite cores. Computer-simulation results indicate that the contour effect is a function of the ferrite-core permeability. The permeability varies in accordance with the crystal orientation. The above explains that the contour effect depends on the ferrite-core crystal orientation.
Ellipsometric analysis of computer disk structures
Authors: He Ping , De Bhola N. , Chen Liang-Yao , Zhao Yong , Woollam John A. , Miller Mark , Simpson Edward
Published: July 2002

We have used variable angle spectroscopic ellipsometry (VASE) to analyze the materials surfaces and interfaces in multilayer sputtered media computer disks. Specifically, the system C/CoNiCr/Cr/NiP/Al has been investigated for layer thicknesses, interfacial and surface roughness, and radial and circumferential uniformity. By first characterizing the Cr/NiP/Al then CoNiCr/Cr/NiP/Al structures, we were able to fully characterize the complete disk structure. The interface width between the carbon layer and CoNiCr magnetic layer was determined to be approximately 260 Å. This is reasonable considering typical surface roughness present on magnetic disks, and that the carbon ‘‘fills’’ in this surface roughness. VASE is a nondestructive technique and used at atmospheric pressure, and is thus suitable for use in a production environment.
Calculation of local orbital moments of conduction electrons in Nd2Fe14B
Authors: Zhong Xue-Fu , Ching W. Y.
Published: July 2002

Based on the spin-polarized band structure of Nd2Fe14B using the OLCAO method and the inclusion of spin-orbit interaction in the Hamiltonian, the contribution to the local moments have been calculated from the orbital angular momenta of the conduction electrons. A site decomposition of partial density of states provides all the essential information about the local orbital moments on each site. The calculation involves diagonalization of complex band matrix equations for all wave vectors in several stars of k in the Brillouin zone. The calculated orbital moments on different Fe sites are in general agreement with other calculations and are much smaller than the spin magnetic moments. The calculation presented here also reveals that for the crystallographically equivalent sites, the local orbital moments may not be the same depending on the local environment of the site.
Influence of Mn substitution on the magnetic properties of Ho2Fe14C compounds
Authors: Verhoef R. , de Boer F. R. , Jacobs T. H. , Buschow K. H. J.
Published: July 2002

The crystallographic and magnetic properties of the compounds Ho2Fe14−xMnxC have been investigated by means of x-ray diffraction and magnetic measurements. Magnetization measurements made in fields up to 38 T showed that the saturation moment exhibits a minimum at a Mn concentration corresponding to about x=2. The occurrence of this minimum is explained by antiparallel coupling between the Ho and 3d sublattice magnetization, the latter becoming strongly reduced with increasing Mn concentration. Most of the Ho2Fe14−xMnxC compounds investigated have a break in the magnetization curves measured at 4.2 K. This break is interpreted as the onset of decoupling between the antiparallel coupling between the rare-earth and 3d sublattice. A mean-field analysis was used to derive the intersublattice molecular-field coefficients nRT.