bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic properties of a quantum wire with arbitrary bending angle
Authors: Wu Hua , Sprung D. W. L. , Martorell J.
Published: July 2002

The electron transmission and bound state properties of a quantum wire with a sharp bend at arbitrary angle are studied, extending results on the right angle sharp bend (the L-shaped wire). These new results are compared to those of a similar structure, the circular bend wire. The possibility of using a bent wire to perform transistor action is also discussed.
Current-voltage characteristics of Na/p-GaP(110) Schottky diodes
Authors: Emde M. von der , Zahn D. R. T. , Schultz Ch. , Evans D. A. , Horn K.
Published: July 2002

The Schottky barrier of Na deposited onto ultrahigh vacuum cleaved p-type GaP has been determined by in situ current-voltage measurements. The Schottky barrier height of 1.10 eV for this low work function metal contacts is consistent with a general weak metal dependence for barrier formation on GaP(110).
Characterization of geometric effects for the guide/antiguide intensity modulator
Authors: Huang T. C. , Chung Y. , Dagli N. , Coldren L. A.
Published: July 2002

The characteristics of geometric dimensions, such as the modulation length L, the central guide width W, and the gap G between the central guide and antiguide regions for the performance of the guide/antiguide intensity modulator have been examined. Both theoretical predictions and experimental measurements are reported here.
(100) metal alloy films containing body-centered cubic and face-centered cubic metals: Relation between structures and lattice spacings
Authors: Chang Chin-An
Published: July 2002

(100) oriented metal alloy films containing both body-centered cubic (bcc) and face-centered cubic (fcc) metals have been deposited on (100) Si using Pd/Cu seed layers. These include FeCo, FePt, FePd, FeCu, FeAu, FeAg, FeNi, VNi, and CrNi with different compositions. All the alloys show (200) spacings close to those of either the bcc or fcc component metals. To compare with Vegard’s rule, a correction factor of (1.5)1/2 is needed between the fcc and bcc lattices, with the fcc spacing being 22% larger. Twenty alloy spacings are compared, assuming that both the lattice structures and spacings of the alloy films are mainly determined by the component metals whose spacings are close to those of the alloys. Good agreements with the Vegard’s rule are obtained for the thirteen (100) alloys with lattice spacings close to those of their respective bcc component metals, and for the seven (100) alloy films with lattice spacings close to their fcc component metals.
High-intensity laser beam attenuation based on two-step absorption mechanism
Authors: Payne M. G. , Chen C. H. , Garrett W. R. , Templeton D.
Published: July 2002

Due to rapid progress in the development of high-power tunable visible lasers, it is expected that eye protection from tunable lasers in the open field will be needed in the near future. A nonlinear method is proposed that will transmit low-intensity light, but absorb light at high intensities. This high-intensity attenuator is based on the use of a liquid or solid made up of molecules having the property of undergoing two-photon photodissociation through most of the visible part of the spectrum. This material must also have the additional property that one of the products of the photodissociation is a one-photon absorber throughout the same wavelength region. It is suggested that the laser beam intensity can be attenuated by a large factor through these two-step absorption mechanisms, and that if the one-photon absorbing product is quenched by collisions with some component of the liquid in a time that is small compared with the laser pulse length, very large attenuation can be achieved from the built-up concentration of one-photon absorbers. Thus, the early part of a laser pulse is attenuated by two-photon absorption only, but the later parts of the pulse can be attenuated by factors as large as 105. Using a double-pass geometry, the leading edge of the pulse can be absorbed by linearly absorbing species formed on the first pass. The double-pass method, with an optical delay line, can even work well with picosecond pulses.
Application of a triple-well superlattice emitter structure to GaAs switching device
Authors: Guo Der-Feng , Liu Wen-Chau , Lour Wen-Shiung , Sun Chung-Yih , Liu Rong-Chau
Published: July 2002

A new GaAs switching device with a triple-well superlattice emitter structure, prepared by molecular beam epitaxy, has been fabricated and demonstrated. An S- and N-shaped negative-differential-resistance (NDR) phenomenon, attributed primarily to the avalanche multiplications process and resonant-tunneling effect, were observed simultaneously when a proper collector-emitter voltage (VCE) was applied. The operation temperature is known from the experimental results to play an important role on the influence of the NDR behaviors. A transistor action with a common-emitter current gain of over 36 was also achieved at 300 K when a control current was employed to the base electrode. This device exhibited a significantly regenerative switching phenomenon both at room temperature and low temperature if a −VCE voltage was used. The proposed structure consequently has good potential for switching and quantum functional device applications.
High power, 0.98 μm, Ga0.8In0.2As/GaAs/Ga0.51In0.49P multiple quantum well laser
Authors: Mobarhan K. , Razeghi M. , Marquebielle G. , Vassilaki E.
Published: July 2002

We report the fabrication of high quality Ga0.8In0.2As/GaAs/Ga0.51In0.49P multiple quantum well laser emitting at 0.98 μm grown by low pressure metalorganic chemical vapor deposition. Continuous wave operation with output power of 500 mW per facet was achieved at room temperature for a broad area laser with 130 μm width and 300 μm cavity length. This is an unusually high value of output power for this wavelength laser in this material system. The differential quantum efficiency exceeded 75% with excellent homogeneity and uniformity. The characteristic temperature, T0 was in the range of 120–130 K.
Lateral spread effects in the implantation of Ar+, Xe+, and Hg+ in Si3N4 films
Authors: Wang Ke-Ming , Qu Bao-Dong , Shi Bo-Rong , Wang Zhong-Lie , Liu Xiang-Dong , Liu Ji-Tian , Zhao Qing-Tai
Published: July 2002

80–100 keV Ar+, 200 keV Xe+, and 200 keV Hg+ were implanted into Si3N4 films under angles of 0°, 45°, 60°, and 75°. Both normal and oblique incidence Rutherford backscattering of 2.1 MeV He ions have been used to study tilted angle implantation, i.e., the lateral spread of Ar, Xe, and Hg ions in the Si3N4 films. The results extracted are compared with the simulation of transport of ions in matter (TRIM’89 code). The experimental lateral spread seems to be in good agreement with the TRIM’89 prediction.
High dose, heavy ion implantation into metals: The use of a sacrificial carbon surface layer for increased dose retention
Authors: Clapham L. , Whitton J. L. , Ridgway M. C. , Hauser N. , Petravic M.
Published: July 2002

Ion implantation into metals has the potential of producing metastable compounds and solutions that cannot be achieved using conventional processing methods. However, when high doses of heavy ions into metals are necessitated, the technique is often limited by sputtering effects which dictate a maximum achievable implanted ion concentration in the target. Sputtering of light materials (such as C) by heavy ions is much less significant, however. This study investigates the feasibility of ‘‘protecting’’ a metal target surface from sputtering during a heavy ion implant by using a thin ‘‘sacrificial’’ C layer deposited on the target surface. Uncoated and C-coated (∼1000-Å-thick C) Cu targets were bombarded with 600–1000 keV I− ions to a total dose of ∼2×1017 I/cm2. Uncoated samples displayed typical saturation behavior, retaining between 17% and 42% of the dose, depending on the energy. The maximum I concentration achieved in the uncoated samples was about 6 at. %. Excellent results were achieved with the C-coated samples, with retentions of 100% and peak I concentrations between 18 and 33 at. %. Significant mixing of C was found to occur at the C/Cu interface, however as the implantation energy was increased the I concentration profile shifted more deeply into the sample and away from the mixed C/Cu region. This suggested that higher energies and careful tailoring of the implant parameters can eliminate any problems with C mixing.
Thermo-optical switching in Si based etalons
Authors: Feng S. T. , Irene E. A.
Published: July 2002

Thermo-optical switching in Si based etalons has been demonstrated in two device structures. In one experiment the switching time of a Si etalon has been reduced from milliseconds to microseconds by choosing a probe beam of shorter wavelength in an external switching configuration we reported previously [Appl. Phys. Lett. 58, 2073 (1991)]. The switching time has further been improved to the ns range by the use of a 1.06 μm Nd:YAG laser pump which is presumed to give rise to a thermo-refractive change in the Si etalon and at the same time the switching threshold energy has been reduced to ∼1 μJ as compared to ∼1 mJ for a CO2 laser pump. In comparing Si etalons with thicknesses of 400, 72 and 1.5 μm, we find that the 72 μm etalon exhibits the best behavior in terms of low threshold power, high speed and contrast. In addition, effects of the pump beam intensity on the signal pulse shape has been investigated which indicates a multiple interference fringe shift and transverse thermal relaxation dynamics. The second experiment used a Si/Si3N4 film structure in which the index of the Si3N4 is approximately the square root of the index of the Si, and the optical thickness of the Si3N4 is an odd quarter wavelength multiple of the probe beam. This yielded a minimum in surface reflection. Based on an increase of the optical thickness with pump beam heating, a probe beam reflection has been switched out with a high contrast ratio (switched on/off) of 61:1. Different structures using different pump beams are discussed and the restriction of the film optical thickness has been investigated.
Regrowth rates and dopant activation of Sb+-implanted Si-Ge alloys
Authors: Hong Stella Q. , Hong Q. Z. , Mayer James W.
Published: July 2002

Epitaxial regrowth and dopant activation of Sb-implanted Si and Si0.93Ge0.07 were investigated with ion channeling techniques and transmission electron microscopy. The presence of Sb greatly enhanced the regrowth rate of both Si and Si0.93Ge0.07. The initial crystallinity of Si0.93Ge0.07 was fully recovered after furnace annealing at 500–550 °C. Approximately 95% of Sb atoms were found on substitutional sites and most of them were electrically active.
Fundamental and harmonic mixing at 500 GHz using a superconductor-insulator-normal metal junction
Authors: Chernin L. M. , Blundell R. , Tong C. Y.-E. , Miller R. E.
Published: July 2002

We have demonstrated fundamental and harmonic mixing using a PbInAu/InOxPbOx/Ag superconductor-insulator-normal metal tunnel junction mixer designed to operate over the 450–550 GHz frequency range. Direct mixing experiments at up to 1.8 times the gap frequency of the tunnel junction have shown that the mixer works in both fundamental and second harmonic modes, with the highest conversion from the fundamental mode obtained at bias voltages near the gap voltage and the highest conversion from the harmonic mode obtained at zero bias. The measured mixer conversion curves are consistent with our five-port model simulations based on Tucker’s quantum mixer theory [IEEE J. Quantum. Electron. QE-15, 1234 (1979)].
Numerical evaluation of the stopping power for a proton in a strongly coupled electron gas
Authors: Ma Teng-Cai , Wang You-Nian , Cui Tao
Published: July 2002

Within the linear-response dielectric theory, the stopping powers for a proton moving in a uniform electron gas are calculated in the low and intermediate velocity regions. Taking into account the exchange-correlation of electron gas, the local-field correction dielectric function is used. A comparison of our calculations with other results based on the random-phase approximation dielectric theory is made and a fit expression to the direct numerical results for stopping number is given.
Orientation dependence of intersubband absorption in AlAs/Ga1−xAlxAs X valley superlattices
Authors: Xie H. , Katz J. , Wang W. I.
Published: July 2002

We present the first systematic studies of infrared absorption from interconduction subband transitions for AlAs/Ga1−xAlxAs X-valley superlattices grown in the [001], [115], [113], [112], [111], and [110] directions. In the AlAs quantum well material, electrons occupy X valleys with ellipsoidal constant energy surfaces. Due to the effective mass anisotropy of electrons in the ellipsoidal valleys, these structures can absorb normally incident radiation when the superlattice growth direction is not collinear with the principal axes of at least one of the ellipsoids (i.e., not grown along the <001≳ directions). For both parallel and normal incidence radiation at wavelengths of 12–20 μm, peak absorption coefficients of 3000–6000 cm−1 were obtained for the [113] and [112] superlattices with well widths in the range of 30–50 Å and sheet doping concentrations of 1012 cm−2. Their ability to detect normally incident light and to obtain absorption comparable to that in the GaAs/Ga1−xAlxAs superlattice detectors makes these novel structures promising for use as normal incidence infrared photodetectors.
Acoustic quality factor of molybdenum and tungsten at low temperatures
Authors: Duffy William
Published: July 2002

The amplitude-independent acoustical mechanical quality factors Q of 99.95% pure polycrystalline molybdenum and tungsten have been measured as a function of temperature and heat treatment from 50 mK to 300 K. Data were obtained from 1 kHz torsional modes of resonators designed to minimize extraneous loss effects. Thermal annealing at temperatures above the recrystallization temperatures yielded strongly enhanced Q. Maximum Q values of 4.38×107 in Mo and 1.23×107 in W were observed in annealed material at the lowest measurement temperatures. Internal friction peaks and backgrounds are discussed. The temperature dependence of the resonator frequency is reported. Mo and W are compared with Al-5056 as suitable materials for fabrication of cryogenic resonant-bar gravity-wave antennas.
Frequency selection and transient dynamics in single-mode lasers with optical feedback
Authors: Hernández-García E. , Abraham N. B. , San Miguel M. , De Pasquale F.
Published: July 2002

It is demonstrated that a closed delayed equation for the phase of the electric field can be used to describe accurately the transient switch on of a laser with external feedback when it can be described by field evolution equations. In contrast, it is shown that several more severe approximations to the dynamics, including an adiabatically evolving potential for one-dimensional relaxation dynamics (which had previously been used to accurately predict the laser linewidths for the steady-state solutions) fail to reliably describe the final state selection during transient switch on of the laser, the delay-induced oscillatory approach to the chosen state, and features of the amplitude and frequency spectra near multiples of the external cavity mode spacing which may be important for the stability and switching dynamics of the steady states that correspond to excitation of different external cavity resonances.
Subband effective mass and mobility of two-dimensional electrons in uniformly Si-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum wells
Authors: Yamada Syoji , Asai Hiromitsu , Kawamura Yuichi
Published: July 2002

Subband effective mass and mobility of a two-dimensional electron gas in uniformly Si-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum wells are estimated. In three samples with well widths of 130, 200, and 300 Å and a fixed barrier width of 68 Å, up to three two-dimensional subbands are found. Maximum enhancement of electron effective mass is 25% over the bulk value at a Fermi energy of 108 meV. These estimated effective masses are slightly smaller than previously reported values, and this is explained by recent theories which assume only nonparabolic correction. When the temperature ratio, (Dingle temperature)/(mobility temperature), is assumed to be identical in different subbands in each sample, the electron mobility of the first excited subband is at most twice that of the ground subband in the sample with the widest well. The effects of intersubband scattering and of screening on the subband mobilities are also discussed.
Bonded silicon-on-sapphire wafers and devices
Authors: Imthurn George P. , Garcia Graham A. , Walker Howard W. , Forbes Leonard
Published: July 2002

Silicon-on-sapphire (SOS) has been prepared by direct wafer bonding. The silicon layer was thinned to about 10 μm by mechanical grinding and chemical etching. P-N junction diodes were fabricated in the bonded SOS and compared with epitaxially grown SOS. The reverse bias leakage current was almost 15× less in the bonded SOS. A generation lifetime of 10 μs can be estimated from the junction leakage. The effects of processing temperatures on the bonded SOS were also studied.
Deep levels of vanadium and vanadium-hydrogen complex in silicon
Authors: Sadoh T. , Nakashima H. , Tsurushima T.
Published: July 2002

Deep levels in vanadium-doped n- and p-type silicon have been investigated using deep level transient spectroscopy (DLTS) and concentration profile measurements. The DLTS measurement reveals two electron traps of EC−0.20 eV and EC−0.45 eV, and a hole trap of EV+0.34 eV. These three levels correspond to the transitions between four charge states of interstitial vanadium. Furthermore, an electron trap of EC−0.49 eV is observed near the surface region of n-type samples etched with an acid mixture containing HF and HNO3. The origin of the trap has precisely been investigated by isochronal anneals and various chemical treatments. From these investigations, it is found that the trap is due to a complex of interstitial vanadium with hydrogen introduced by chemical etching.
Dynamical model of microscale electromechanical spatial light modulator
Authors: Wetsel Grover C. , Strozewski Kirk J.
Published: July 2002

A model based on classical physics describing the nonlinear dynamics of a microscale spatial light modulator has been developed. The coupled electromechanical equations of motion have been solved for steady-state, transient, and harmonic excitation. Comparison of theoretical predictions with the results of experiment for representative devices shows good agreement for small-amplitude deflections, large-amplitude deflections, and unstable transitions.
Characterization of a micromechanical spatial light modulator
Authors: Strozewski Kirk J. , Wang Chih-Yu , Wetsel Grover C. , Boysel R. Mark , Florence James M.
Published: July 2002

The optical-beam-control dynamics of torsion-beam deformable-mirror-device spatial light modulators has been characterized to determine properties important to their operation. The responses of individual picture elements to steady-state, transient, and harmonic excitation were measured using optical-beam-deflection sensors. The steady-state angular deflection is characterized by a stable, reversible regime for applied voltages less than a critical value, Vc (∼16 VDC), an unstable transition to stable maximal deflection at Vc, and hysteresis as the applied voltage is reduced to zero. Representative devices with 50 μm×50 μm pixels exhibit full-deflection rise times of tens of microseconds and small-deflection resonant frequencies of the order of 10 kHz.
Spin-wave resonance investigations of Ni-ion implanted Permalloy films
Authors: Kurowski D. , Meckenstock R. , Kordecki R. , Soliman A. , Pelzl J. , Brand K.
Published: July 2002

Spin-wave resonance (SWR) has been applied to investigate the influence of Ni-ion implantation onto the magnetic properties of Permalloy films. The implantation has been carried out at room temperature with different doses at Ni-ion energies of 95 keV. In films of 30–60 nm thicknesses, which are thin compared to the implantation range, with increasing dose the uniform mode is shifted to higher resonance fields in the parallel configuration. In the perpendicular configuration one obtains a shift to lower fields. The corresponding variation of the effective magnetization as a function of the implantation dose could be reproduced by assuming a change of the average volume magnetization. In thicker films (150–250 nm) standing spin waves are excited which are only slightly modified by implantation compared to the as-prepared samples. The observed effects can be attributed to changes of the surface anisotropy.
Propagation of antiferromagnetic order across paramagnetic layers in CoO/NiO superlattices
Authors: Borchers J. A. , Carey M. J. , Berkowitz A. E. , Erwin R. W. , Majkrzak C. F.
Published: July 2002

Using neutron-diffraction techniques, we have probed the magnetic structure of nickel-oxide/cobalt-oxide superlattices grown by reactive sputtering. Collinear antiferromagnetic order propagates through several bilayers despite the short-range nature of the spin interaction in the components. The magnetic order for 36 Å period superlattices develops simultaneously in the NiO and CoO layers at a Néel temperature between the values for bulk CoO and NiO. When the superlattice periodicity is increased to 72 Å, the Ni and Co spins appear to order separately at temperatures shifted from their bulk TN. The magnetic coherence length decreases as the temperature is increased, but remains greater than the width of a single bilayer above TN for the CoO interlayers. Mean-field calculations demonstrate that exchange coupling at the interfaces is responsible for the anomalous magnetic behavior near the phase transition.
Anomalies in the low velocity motion of the domain wall of yttrium orthoferrite (abstract)
Authors: Didosyan Y. S.
Published: July 2002

Investigations of the domain wall (DW) dynamics in weak ferromagnets, orthoferrites, have revealed a whole number of interesting effects, including the quasirelativistic motion of the DW, formation of periodic structures on the DW, moving in uniform along the wall magnetic field, and emission of phonons by the DW.1–3 These effects were observed at high velocities of the DW motion, equal to or exceeding the sound velocity (4.1 km/s). It was generally considered that below this value the dependence of the DW velocity V on external magnetic field H is linear. In this work the nonlinearities were found in the initial part of the V(H) dependence. Measurements were carried out on samples of yttrium orthoferrite cut perpendicular to the optic axis. A single DW, stabilized by a nonuniform magnetic field, was put in motion by a pulsed field. The intervals of time, during which the dark field image of the DW passed the given distances, were measured. At the DW velocity 1000 m/s the resolution of the time interval measurements was of the order of 0.1 ns. The distances covered by the DW were determined with a resolution of about 0.3 μm. The sequence of levels of constant velocity was found in the measured range 200+1500 m/s. Fast transitions of DW velocity between the levels take place. Investigation of these nonlinearities may provide more information about interaction of the DW with the magnon subsystem.
Demagnetizing effect for conducting disks and their conductivity determination from ac susceptibility
Authors: Chen D.-X. , Hernando A.
Published: July 2002

An eddy-current technique for determining conductivity of disks from complex ac susceptibility measurements is developed. Different from most other techniques, this technique can be used for both nonmagnetic and magnetic materials since there is a magnetic correction factor in the formula obtained from eddy-current and demagnetizing effect analyses.
Analytic model calculation of magnetic field in a magnetic half-space due to surface magnetic charge
Authors: Chen Z. J. , Govindaraju M. R. , Jiles D. C.
Published: July 2002

By analogy with electrostatic field, a simple analytic model is presented on the distribution of magnetic field inside a test material as a result of surface inspection with a magnetic probe. According to this model, the penetration depth of the magnetic field is defined and the permeability of the material at different depths is calculated. Finally, the magnetic flux inside the inspection head when placed on samples of different thicknesses is calculated and compared with experimental results.
Shimming of yokeless permanent magnets designed to generate uniform fields
Authors: Bertora F. , Trequattrini A. , Abele M. G. , Rusinek H.
Published: July 2002

Recently introduced yokeless permanent magnets are very suitable for generating uniform fields. Practical yokeless magnets must be shimmed or compensated for fabrication and magnetization tolerances. The particular shimming approach described exploits the magnetic transparency of yokeless structures. The method is based on the use of active elements of high-energy-product material. The logic for determining the distribution of active elements is based on measuring the departure from uniformity of the field sampled on a sphere surrounding the region of interest. The magnetic moments of the elements are determined by solving a system of linear equations representing the field generated by active elements on each sampling point.
Investigation of the dependence of BLS frequencies on angle of incidence for thin iron films
Authors: From M. , Cochran J. F. , Heinrich B. , Celinski Z.
Published: July 2002

Brillouin light-scattering experiments have been done at various angles of incidence, θ, for four specimens prepared by molecular-beam epitaxy. The specimens were single ultrathin films of Fe deposited on single-crystal Ag substrates. Dependence of magnon frequency on θ is easily resolvable in all specimens. We find that the magnitude of this dependence is in good agreement with a theoretical calculation that takes into account magnetic anisotropies, dipole-dipole, and exchange interactions. Our results imply that magnetic excitations in these specimens are correlated over distances of at least 5000 Å.
Arithmetic for evaluating three-dimensional h vs p finite-element refinement strategies
Authors: Kotiuga P. R.
Published: July 2002

For pth degree scalar interpolation polynomials on an n-dimensional simplicial finite-element mesh R̄n, exact formulas are derived for the number of nonzero entries Xp(R̄n) in the finite element ‘‘stiffness’’ matrix associated with the Dirichlet integral and Dp(R̄n), the number of degrees of freedom. For n=1,2,3 these results are rephrased in terms of familiar data so that useful rules for h vs. p refinement can be considered.
A new approach for the calculation of nonlinear magnetic circuits
Authors: Heitbrink A. , Beyer A.
Published: July 2002

In the last few years, a various number of calculation methods have been presented that deal with nonlinear magnetic circuit design. Most of them are based on numerical models for the description of hysteresis loops. A fundamental problem exists in the adaptation of these models to actual magnetic materials. This counts particularly for high permeability NiFe alloys, as they are implemented in circuit breakers. In this application area, input signals with higher frequency components are used, which can be described with existing hysteresis models only insufficiently. To be able to calculate such networks, a combined CAD system has been developed. It combines a real-time measurement system with modern numerical methods to a productive total concept for the exact calculation of nonlinear magnetic circuits.
Numerical modeling of a conducting ferromagnetic disk in transient regime
Authors: Jellali F. , Quichaud G. , Saadi M.
Published: July 2002

This article presents both finite element and simplified methods to calculate the diffusion of the electromagnetic field in a conducting saturable ferromagnetic disk under transient excitation. Knowledge of only the tangent component of the magnetic field on the disk surface is sufficient to deduce approximately the two characteristic penetration depths of the diffusion as well as the induced current and fields. The two methods are tested on a system consisting of a solenoid through which a 400-μs current pulse is applied, and a nearby conducting saturable ferromagnetic disk. Interesting results, including fields and forces, are discussed and compared with measurements.
Optimization of electromagnetic devices: Circuit models, neural networks, and gradient methods in concert (abstract)
Authors: Ratnajeevan S. , Hoole H.
Published: July 2002

Optimization in designing electromagnetics is now increasingly better understood. As opposed to classical circuit models of magnetic circuits, today, gradient techniques for mathematical optimization have been proposed and are used. These techniques, while being expensive, are exact. More recently, artificial neural networks have been suggested, but they work best only if the data set of parameter-set, performance pairs for training the network is close to the optimal solution we seek. In this paper, it is shown how all three methods may be used in concert to increase efficiency. The circuit model is used to generate an approximate inverse solution. Then direct finite element solutions are used to generate the required training set and this is used with the Neural network to get a better solution. This solution is finally used as a starting point for the gradient optimization scheme which converges quickly because the starting point is close to the actual solution.
Fingers of auto-oscillation in a circular YIG film with pinned surface spins
Authors: Peterman D. W. , Shields P. J. , Wigen P. E.
Published: July 2002

High-power ferromagnetic resonance (FMR) experiments involving thin circular yttrium-iron-garnet (YIG) films producing fingerlike regions of auto-oscillations as a function of excitation power and static field have been reported. These fingers have been associated with the magnetostatic modes found in low-power FMR spectra for the situation that the spins at the film surface are free (unpinned). This report contains the first observation and model analysis of auto-oscillation fingers in a sample with surface boundary conditions that pin the surface spins. The sample geometry allows for the magnetoexchange branches associated with wave vectors kz≊π/L and kz≊3π/L (where z is perpendicular to the plane of the film and L is the sample thickness) to be separated in the low-power FMR spectra. Observation of the auto-oscillation fingers at high power reveals frequencies in the 2–8 MHz range and excitation powers of 5–15 dBm in the kz≊π/L magnetoexchange branch. The kz≊3π/L magnetoexchange branch exhibits frequencies in the range of 500 kHz–4 MHz and have comparable excitation powers. Our model, using input parameters from low-power measurements, compares favorably with the data.
The control of magnetic characteristics of BaM ferrite thin films by bias sputtering (abstract)
Authors: Naoe M. , Noma K. , Nakagawa S. , Matsushita N.
Published: July 2002

The thin films of a magnetoplumbite type of barium (BaM) ferrite have been deposited on SiO2/Si substrates at a rate of 100 Å/min in a gas mixture of 10% O2–90% Ar at a pressure of 2 mTorr by using the facing targets sputtering (FTS) apparatus. They possessed excellent chemical stability and high corrosion resistivity as well as perpendicular anisotropy as large as applicable for perpendicular magnetic recording media. They also seem to be applicable for millimeter wave isolator and circulator. In this study, the saturation magnetization Ms, the coercive force Hc, and the anisotropy field Hk of BaM ferrite films have been controlled by adjusting the rf-bias voltages to substrate Vb during sputtering, where Vb was always negative. The dependencies of Ms and Hc on Vb were shown in Fig. 1. Ms took the maximum value as large as 345 emu/cc at Vb of −30 V and decreased as Vb decreased in the range below −50 V. On the other hand, the perpendicular Hc decreased as Vb increased in the range above −100 V and took the minimum value as low as 910 Oe at Vb of −95 V. Hk and the magnetic anisotropy constant Ku also greatly depended on Vb. Such an apparent Vb dependence of these magnetic characteristics of BaM ferrite films seem to be attributed to the definite change in c-axis orientation of hexagonal crystallite and the different distribution of Fe3+ ions among several occupation sites. These results indicated that the c-axis orientation of BaM ferrite crystallites were promoted by applying proper Vb. Consequently, the bias sputtering technique with FTS method may be useful for preparing the BaM ferrite films composed of the multilayers with different magnetic characteristics, even if only one pair of targets are used without adjusting the other sputtering conditions.
Temperature-dependent prefactor
Authors: Klik Ivo
Published: July 2002

The saturation magnetization and anisotropy constants of a ferromagnet are, below the Curie point TC, decreasing functions of temperature T. This effect leads to a T-dependent barrier height for thermal activation and, also, to T-dependent prefactor f0 of the thermal decay rate κ, which is here studied in the underdamped limit. For a uniaxial single domain ferromagnet f0∼τ−1(1−τ)ρ, where τ=T/TC and ρε<2,11/3≳ depending on the chosen model of dissipative coupling. Ohmic and superohmic couplings are considered that are either isotropic or anisotropic in space. Estimates for cubic systems are in the range ρε<13/3,32/3≳. Measurements of the function f0(τ) may thus provide information about the dissipative mechanism in single domain particles. Its influence on measurable data is demonstrated on a model calculation of frequency-dependent coercivity.
Groundstate properties of the degenerate supersymmetric t−J model with crystalline field splitting in one dimension
Authors: Schlottmann P.
Published: July 2002

The one-dimensional t−J model is considered, consisting of electrons with four spin-components on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the sites, exchange J between neighboring sites and a crystal field splitting Δ. This model is integrable at the supersymmetric point. The exact solution is presented via Bethe’s ansatz and groundstate properties as a function of Δ are discussed. The model could correspond to a two-band situation with one band being the z2 and the other one the x2−y2 orbitals of Cu2+.
Permanent magnets of high-T
c superconductors
Authors: Weinstein R. , Chen I.-G. , Liu J. , Xu J. , Obot V. , Foster C.
Published: July 2002

Advances on permanent high-Tc superconducting magnets are reported. Materials are tested in the form of small tiles. An accurate phenomenological model of the currents in a magnetized tile predicts that the maximum trapped field BT,max∝Jcf(d), where Jc is the critical current density, d is the diameter of the single-grain tile, and f(d) is a function which increases monotonically with d. Results are reported of increasing Jc via chemical additives, and via bombardment by high-energy light ions and fission fragments. Increases in d via chemical and temperature gradients are also reported. Methods, data, and most recent results are presented. Present values are d=2 cm, and Jc=85 kA/cm2, at 77 K. A six tile minimagnet, 1.2×1.2×1.5 cm3, fabricated from earlier tiles with d∼1 cm, Jc∼45 kA/cm2, retains 1.52 T at 77 K. It is calculated that the more recent values of Jc and d will result in fields of 3 T at 77 K. BT,max also increases rapidly with T−1, and approximately doubles at 60 K.
A new expansion for generalized Gutzwiller wave functions: Antiferromagnetic case
Authors: Li Y. M. , d’Ambrumenil N.
Published: July 2002

We propose an alternative approach to the 1/D-type expansions for evaluating properties of generalized Gutzwiller wave functions (GWF’s) in low dimensions. Our expansion uses the sum rule and symmetries of the trial wave function explicitly. We apply the scheme to an antiferromagnetic generalization of the GWF for the 1D Hubbard model. We find good agreement with known results from variational Monte Carlo calculations and a significant improvement over previous approximations.
Studies of the magnetic phase diagram of holmium using neutron diffraction
Authors: Tindall D. A. , Steinitz M. O. , Holden T. M.
Published: July 2002

We report measurements of the intensity and position of the satellites arising from the helimagnetic structure of holmium in magnetic fields applied along different crystal axes, concentrating on lock-in effects at commensurate values of the spiral wave vector τ. We have made the first observations of the lock-in at τ=1/5 near 42 K in a 3 T c-axis field as well as the lock-in at τ=1/4 in a 3 T b-axis field. We have also observed a lock-in at τ=5/18 in a b-axis field near the Néel temperature of 132 K, in contrast to an earlier reported value of 8/29. This illustrates the origin of the splitting previously reported by us in magnetization and dilatometric measurements.
Self-consistent crystal-field description of tetragonal Pr and U compounds with Van Vleck induced magnetism
Authors: Santini P. , Amoretti G. , Blaise A. , Caciuffo R.
Published: July 2002

Many U and Pr compounds presenting localized electron behavior are conveniently described in terms of crystal electric field (CF) acting on the J=4 ground multiplet. In particular, compounds having the ThCr2Si2- or CaBe2Ge2-type structure are characterized by a tetragonal CF at the magnetic site. We have developed a method allowing us to estimate the five CF parameters in the case of a singlet CF ground state, a situation in which a Van Vleck induced magnetism can exist. This is done by imposing a priori some experimental constraints, in particular a self-consistent condition on the ordered moment and the ordering temperature. We apply the method to UPt2Si2, PrFe2Si2, and PrFe2Ge2. In the U compound we find that the value of the ordered moment can be accounted for in the localized model. For the two Pr compounds, we find that details of the CF splittings can produce the two very different values of the ordered moment observed.
FeN phase generation effects for high moment FeTaN films
Authors: Takeshima Yoshio , Ishiwata Nobuyuki , Korenari Takahiro , Urai Haruo
Published: July 2002

FeN phase generation has been observed by x-ray analysis in FeTaN films, which normally show α-Fe, TaN phase, and no FeN phases. With a rise in substrate temperature Ts above 200 °C, FeN phases are generated in nitrogen reactive sputtering process with columnar structure formation, and remain after annealing. With FeN phase generation, coercivity Hc and magnetostriction λ values increase markedly. Moreover, internal stress σ for the films increases drastically. The FeTaN films, sputtered on low temperature substrate, generate no FeN phases, and show low Hc, about 0.1 Oe, and low λ. Consequently, FeN phase generation with columnar structure formation has negative effects for FeTaN films to be used as magnetic head core materials. For preparation of FeTaN films, the substrate temperature has to be controlled to a sufficiently low value for suppression of FeN phase generation.
The influence of quenching rate on magnetic properties of amorphous ribbons
Authors: Knobel M. , Sinnecker J. P. , Sato Turtelli R. , Rechenberg H. R. , Grössinger R.
Published: July 2002

Five sets of amorphous ribbons of the composition Fe73.5Cu1Nb3Si13.5B9 produced with different quenching rates (QR) were studied in order to verify the influence of the topological disorder on magnetic properties. For this purpose several magnetic methods were applied. Significant differences have been found between the results of magnetic disaccommodation He⋅Δμ/μ, pinning field Hp and stress dependence of the magnetic permeability μ/μ0(σ) of as-cast materials produced at different QR. The values of He⋅Δμ/μ and Hp are directly proportional to the disorder degree of the samples. The μ/μ0(σ) curves display broad maxima, which are found to be related to the quenched-in stresses. In contrast with these results, no significant effects of the amorphous structure on the Curie temperature and Mössbauer spectra were found, because other parameters are dominating.
Structural and magnetic characterization of amorphous Gd2Fe14B thin films
Authors: Bushnell S. E. , Dorsey P. C. , Nowak W. B. , Vittoria C.
Published: July 2002

Thin amorphous films of nominal composition Gd2Fe14B were fabricated on Si (100) substrates using ion-beam sputtering for the purpose of establishing their structural and magnetic properties at room temperature. X-ray-diffraction scans performed on as-deposited and annealed films revealed an amorphous structure. The as-deposited saturation magnetization was found to be 8101 G at room temperature suggesting ferrimagnetic ordering with anti-parallel coupling similar to other heavy rare earth-transition metal systems. Results from in-plane and perpendicular ferromagnetic resonance experiments conducted at 9.108 GHz showed the films to be inhomogeneous. This was evident from the presence of two in-plane resonance lines and spin-wave spectra that were not quadratic. Annealing at 300 °C for 30 min significantly reduced the absorption of the second in-plane resonance and restored classical n2 law spin-wave behavior. It is speculated that the origin of the inhomogeneities is due to the kinetics of the deposition process where the increased mobility of the condensing Fe atoms leads to interspersed amorphous regions which are rich in Fe and Gd, respectively.
Shock compaction of Fe-Nd-B
Authors: Chandramouli Mahesh , Thomas Gareth , Nellis W. J.
Published: July 2002

Shock compaction was performed using the 6.5-m-long two-stage light gas gun facility at LLNL on powders of Fe74Nd20B6, 10–20 μm in size which were prepared using conventional powder metallurgical techniques. The behavior of Fe-Nd-B powders and Fe-Nd-B powders blended with 10 vol. % Al was examined. X-ray-diffraction experiments detected no discernable difference in phase distribution or crystallinity when shock pressures below 100 kbar were applied to Fe-Nd-B powders alone and below 175 kbar for the case of the blended powders. After a 193 kbar shock was applied to Fe-Nd-B powder, only α-Fe was detected via x-ray diffraction. Scanning electron microscopy of shocked Fe-Nd-B indicated that a high degree of fracture occurred for pressures below 100 kbar. Indications of melting were observed in the 193 kbar sample along with a higher saturation magnetization as measured by vibrating sample magnetometry. Scanning electron microscopy of the Al-blended shocked specimens which are more dense revealed that very little fracture has occurred.
The effects of pinning magnetization with geometrical features in thin permalloy films
Authors: Tomlinson S. L. , Hill E. W.
Published: July 2002

This work considers the use of geometric alterations to the edges of a strip of thin magnetoresistive material used to make a magnetometer device. The geometric features in the form of narrow fins are intended to improve the performance of the device by pinning the magnetization at the strip edges at an angle corresponding to the high sensitivity region of the magnetoresistance curve, thus including this region in the contribution to total resistance change. Both theoretical calculations using a micromagnetic model and experimental work on a prototype device are described. Results show that the device behaves closely to the modeling predictions, exhibiting a large sensitivity of 17 μV/V/μT, 70% greater than a more conventional device, with the additional capability of a trimmable orthogonal field rejection. Differences between the predicted and measured devices are described in terms of the shortcomings of the model and to domain activity in the device.
Correlation of remanence and giant magnetoresistance data in sputtered Co/Cu and Fe/Cu multilayers (abstract)
Authors: Parker M. R. , Seale D. , Tsang E. , Barnard J. A. , Hossain S. , Richards D. A. , Watson M. L.
Published: July 2002

It is well established that magnetic interactions in particulate media and films can be assessed on the basis of the correlation between the normalized dc demagnetization remanence, Id(H), and the normalized isothermal remagnetization remanence, Ir(H). Departures of Henkel plots from the straight-line relationship of the Wohlfarth equation may be regarded as a measure of the strength of the intergranular field interactions in thin films. Delta-I plots are commonly used to manifest these interactions. These remanence techniques have been applied to both ordered and highly disordered (granular) sputtered multilayers of Co/Cu in which the giant magnetoresistance effect has been observed. Both the Henkel plots and the delta-I (MR) plots exhibit oscillatory characteristics, which correlate with magnetoresistance and critical field data. Figure 1 shows the amplitude of the delta-I plot as a function of Cu volume fraction in quasigranular Co/Cu films in good correlation with the amplitude of the giant MR effect. Similar results have been obtained even with relatively thick multilayers of Fe/Cu in which the giant MR is extremely weak. These results would seem to indicate that comprehensive remanence measurements involving Henkel and/or delta-I plots are effective in evaluating intergranular exchange coupling in giant MR film systems, given that these plots can be processed to provide quantitative interaction field data.
The magnetic and microstructural properties of Pt/Co multilayers grown on ZnO
Authors: Carcia P. F. , Li Z. G. , Reilly M. , Zeper W. B.
Published: July 2002

We measured the magnetic properties and examined the microstructure of Pt/Co multilayers sputtered at ambient temperature on ZnO underlayers grown between 25 and 500 °C. For ZnO grown at 150 °C, the perpendicular magnetic anisotropy energy of Pt/Co multilayers was a maximum, ∼1.1×107 erg/cm3, and then decreased monotonically to ∼3×106 erg/cm3 for ZnO grown at 500 °C. From measuring the angular dependence of magnetic coercivity, we deduced that magnetic reversal was by domain-wall motion in multilayers on low temperature (≤200 °C) ZnO, and predominantly by rotation on high-temperature ZnO. By cross-sectional transmission electron microscopy of Pt and Co layers on ZnO, we determined that all layers were rougher for high ZnO growth temperatures, although higher temperature promoted better multilayer (111)-fcc texture. The microstructure of Pt/Co multilayers on high-temperature ZnO was also particulate or granular, compared to more continuous on low-temperature ZnO. We concluded that the multilayer roughness reduced perpendicular magnetic anisotropy, and that particulate structural features caused magnetic reversal by rotation.
NQR study of local structure and cooling rate-dependent superconductivity in La2CuO4+δ
Authors: Reyes A. P. , Ahrens E. T. , Hammel P. C. , Thompson J. D. , Canfield P. C. , Fisk Z. , Heffner R. H. , Schirber J. E.
Published: July 2002

Preliminary results of structural studies in oxygen-annealed La2CuO4+δ(δ∼0.03) using 139La nuclear quadrupole resonance (NQR) spectroscopy are reported. Superconducting critical temperatures were found to depend on the rate of cooling through a narrow temperature range near 195 K. Analysis of the 139La NQR spectra reveal that lanthanum atoms in the oxygen-rich metallic region occupy two sites having different local environments. One site has a structural configuration closely related to the stoichiometric oxygen-poor compound. The second appears only below 200 K and exhibits a large shift and broad distribution of NQR frequency νQ. Shifts in νQ in the vicinity of the superconducting transition were also observed for both metallic sites. Possible origins of these anomalies and their connection to superconductivity are discussed.