bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferroelectric liquid-crystal light valve addressed by laser heating
Authors: Nakamura Kenichi , Nanataki Hideo , Arimoto Masashi
Published: July 2002

A ferroelectric liquid-crystal light valve for laser addressing is studied. Surface-stabilized ferroelectric liquid-crystal cells are employed, where the liquid crystals have bistable states with respect to molecular alignment. The threshold width of an applied voltage pulse required for ferroelectric switching (polarization reversal) decreases with raising cell temperature for a given pulse height. Laser writing and erasure are successfully performed with this effect. An important feature of this light valve is that the overall erasure can be done certainly, readily, and quickly by applying an appropriate pulse voltage.
Analytic solutions to the electrostatic problems of two dielectric spheres
Authors: Lin Weigan , Jin Hang
Published: July 2002

The electrostatic problems of the configurations of two dielectric spheres are solved with the aid of the inversion transformation. The potentials due to a unit point charge located on the symmetry axis or that excited by a uniform external field are obtained. In the case of conducting spheres, our results are in agreement with the previous results obtained by the method of images.
Photorefractive intermode space-charge fields in volume holographic interconnections
Authors: Sik Baek Woon , Lee Hyuk
Published: July 2002

Steady-state space-charge fields generated by a pair of light interference patterns, each of them formed by a pair of optical plane waves, are studied. Analytic solutions of the steady-state space-charge fields in photorefractive crystals are derived for small laser intensity at large modulation depth. Numerical solutions for the photorefractive crystal Bi12SiO20(BSO) are also obtained by utilizing Powell’s method without the assumption of small laser intensity, and they are compared with the analytic solutions in both cases of stationary light interference patterns and moving light interference patterns. The intermode space-charge fields, which may cause the second-order cross-talk problem in volume holographic interconnections, are described. The data regions are obtained in which the effect of intermode space-charge fields are suppressed and the criteria for optimal implementation of volume holographic interconnections are satisfied.
Enhancement of deposition rate by adding Si2F6 in low-pressure chemical vapor deposition of W using WF6 and H2
Authors: Nishikawa Satoshi , Matsuhashi Hideaki , Ohno Seigo
Published: July 2002

The effects of addition of Si2 F6 to low-pressure chemical vapor deposition of W using WF6 and H2 was studied. Adding sufficient Si2 F6 gas enhances the deposition rate (RD ) of W film in the WF6 and H2 system by a factor of 2 over a wide range of deposition parameters, that is, the substrate temperature and the partial pressures of H2, WF6, and Si2 F6. W films deposited under the conditions where the enhancement of RD occurs have a resistivity as low as that of films deposited in the WF6 and H2 system, except for some W films exhibiting β-W structure, and have no Si contamination. The origin of the enhancement of RD by adding Si2 F6 is discussed.
Observation of a new Al(111)/Si(111) orientational epitaxy
Authors: Yapsir A. S. , Choi C.-H. , Lu T.-M.
Published: July 2002

A new Al(111)/Si(111) orientational epitaxy using x-ray pole figure analysis is reported. The new structure has a 19° rotation with respect to the parallel epitaxy. The results are explained using a geometrical lattice matching concept.
Effect of annealing on the electrical properties of polycrystalline intermetallic compound HgTe
Authors: Nath Tirlok , Roy Savita , Saxena P. , Mathur P. C.
Published: July 2002

Polycrystalline HgTe samples have been prepared by taking Hg and Te in stochiometeric compositions. The effect of annealing in mercury has also been studied. The effect of temperature on the electronic conduction mechanism and grain-boundary states has been observed through the measurement of Hall coefficient RH resistivity (ρ) and Hall mobility μH=(RH/ρ) on the polycrystalline HgTe in the temperature range ∼77–350 K. It is found that in electronic conduction in the low-temperature range, the conduction is dominated by the thermionic emission of electrons over the grain boundaries. At a critical temperature Tc where the barrier height Φb and, hence the width of the space-charge region near the grain boundaries tends to zero, a crossover from the barrier limited to the mobility limited resistivity occurs. RH is found to be thermally activated over the temperature range where the conduction mechanism is governed by the grain-boundary barrier. The experimental results are explained on the basis of a grain-boundary trapping model combined with two-phase model. It is found that the mobility and the grain size increased with annealing time. We achieved the carrier mobility of polycrystalline HgTe at 270 K to as much as 11 000 cm2/V s.
Observation of telegraph noise in the reverse photocapacitance, photocurrent, and forward dark current of a quantum-well diode
Authors: Cavicchi R. E. , Panish M. B.
Published: July 2002

We report measurements on transport perpendicular to a single quantum well located on the n side of a p-n junction in a InP/InGaAs device. Zero-bias admittance spectroscopy and photocapacitance transient measurements are used to characterize electron and hole emission rates. Both rates show a crossover from a thermally activated regime to a regime of weak temperature dependence. The barrier dimensions for electron and hole emission preclude direct tunneling so that, in the low-temperature regime, transport must be defect assisted. In this regime we observe discrete fluctuations in the reverse-bias photocapacitance and photocurrent and in the forward-bias dark current. The results are explained with a model in which transport of electrons in or out of the well is regulated by the occupancy of a single electron trap associated with a defect.
Picosecond real-space electron transfer in GaAs–n-AlxGa1−xAs heterostructures with graded barriers: Monte Carlo simulation
Authors: Moško Martin , Novák Ivo
Published: July 2002

Using Monte Carlo simulation combined with iterative solution of the Poisson equation the real-space electron transfer (RSET) and negative differential conductivity (NDC) in GaAs–n-AlxGa1−xAs heterostructures with electric field applied parallel to the layer interfaces have been investigated. The original Hess RSET model has been modified by considering graded AlxGa1−xAs layers with proper spatial dependence of the Al composition. Simulation results confirm that this modification enables one to avoid undesirable effects due to space-charge fields: (1) Graded AlxGa1−xAs layers can be depleted in thermal equilibrium at higher donor concentrations than layers without compositional grading; (2) it is sufficient to use doped, but not compensated, AlxGa1−xAs layers because electron velocity in graded layers is low mainly due to electron transfer to L and X valleys; and (3) there is no confinement of cold electrons in graded AlxGa1−xAs layers due to space-charge fields when the RSET occurs. The RSET-induced NDC and characteristic time constants have been found close to 1 ps at 77 and 300 K. Similar results have been obtained in the RSET model without compositional grading; however, highly compensated AlxGa1−xAs material was needed and the total number of free electrons was ten times lower.
The superconducting properties of YBa2Cu3O7−x-gold composites
Authors: Imanaka N. , Saito F. , Imai H. , Adachi G. , Yoshikawa M. , Okuda K.
Published: July 2002

Gold powder was mixed with YBa2Cu3O7−x superconductors for the purpose of increasing the critical current density (Jc). The critical transition temperature at zero resistance (Tzeroc) gradually decreased by the Au addition. However, the depression of Tzeroc was as low as 2 K for the YBa2Cu3O7−x -Au composite with Au mixing up to 40 wt.%. A maximum Jc of 307 A/cm2 was obtained for the Au 5-wt. % mixed composite. The Jc was about four times as high as that for YBa2Cu3O7−x without Au mixing. In addition to the Jc enhancement, the weak-link problem was appreciably improved by the Au addition.
Characteristics of Ga0.51In0.49P/GaAs heterostructures grown on Si substrates by organometallic epitaxy
Authors: Horng R. H. , Wuu D. S. , Huang K. C. , Lee M. K.
Published: July 2002

We have demonstrated the feasibility of heteroepitaxial growth of Ga0.51In0.49P/GaAs layers on Si substrates by low-pressure organometallic vapor phase deposition. The growth parameters of the GaAs buffer layer were confirmed to be the controlling factors in obtaining higher quality Ga0.51In0.49P on Si. Under the optimum growth conditions, specular single-crystal Ga0.51In0.49P layers can be reproducibly obtained. The room-temperature electron mobility of the undoped Ga0.51In0.49P epilayer can reach 1000 cm2 /V s with a carrier concentration of 2×1016 cm−3. The efficient photoluminescence indicates that the Ga0.51In0.49P grown layer is of high optical quality. In addition, the GaAs intermediate layer is also effective in reducing the residual tensile stress in the Ga0.51In0.49P film on Si. The results presented can compete with those reported previously for the OMVPE-grown GaxIn1−xP on GaAs substrates.
Space-charge-neutralized proton beam transport in an axial magnetic field
Authors: Kerslick G. S. , Golkowski Cz. , Nation J. A. , Roth I. S. , Ivers J. D.
Published: July 2002

The generation and transport of a 1-MV, 5-kA, 50-ns annular proton beam has been studied. The diode is magnetically insulated with a full-cusp field geometry. Beam transport in a 6-kG axial magnetic field is measured for injection into vacuum and neutral gas. The neutral gas background is supplied by a fast puff valve system. The gas background is found to enhance the space-charge neutralization of the beam. Under these conditions the beam divergence is <1° and the annular profile is maintained throughout the 30-cm transport region. We discuss a simplified model of the beam injection process to explain the experimental results.
Vacancy self-diffusion in a screw dislocation in a face-centered-cubic lattice
Authors: Stark J. P.
Published: July 2002

Correlated self-diffusion in a screw dislocation in a face-centered-cubic crystal is treated with a vacancy mechanism using matrix functions that are generated to permit the length of the dislocation to approach infinity for the tight-binding approximation. The correlation factor f for positive vacancy binding to the dislocation is found to lie in the range 0.819≥f≥0.516. Previous calculations for a screw dislocation in a simple cubic structure are in the range 0.74≥f≥0.52, and are viewed as representing the differences associated with the core structure. Comparable calculations of the diffusion coefficient suggest that it becomes indefinitely large in the limit where the vacancies are tightly bound to the dislocation core but with a finite concentration in the surrounding lattice which is in agreement with similar limits for other dislocations.
The theoretical basis of application of fractal to fracture physics
Authors: Wu Xinhua , Zhu Ziyong , Ke Wei
Published: July 2002

A theoretical basis to support the application of the fractal approach in fracture physics is put forward. According to the theory, if Griffith’s criterion is approved, the fractal approach can be used.
Medical x-ray imaging in infrared to visible upconverting materials
Authors: Breteau J. M. , Ayral J. L. , Micheron F. , Auzel F.
Published: July 2002

In some crystalline and glass ceramic IR upconverting materials, the visible light intensity resulting from the IR excitation can be reduced by a prior x-ray exposure. Such behavior is studied for medical x-ray imaging applications. A model is developed to describe the interactions between the microscopic defects responsible for the quenching of the visible emission and the luminescent ions. At last, a radiograph of a tooth is obtained by using such upconverting materials.
A simple model of light transmission through metachronally moving cilia
Authors: Rabinovitch A. , Rabinovitch R.
Published: July 2002

A ‘‘matchbox’’ model of a ciliary field under the influence of metachronal waves is analyzed. The optical absorption response of an optical absorbing system is calculated. Results show a characteristic double-peaked wave train.
Period-doubling and period-quadrupling for an actively mode-locked laser diode with extended cavity
Authors: Chesnoy J. , Klein M. C. , Chusseau L. , Lourtioz J. M.
Published: July 2002

Active mode-locking of a laser diode in an external cavity has led to the observation of new instabilities. We report period-doubling manifested by the alternation of weak and strong pulses in the mode-locked laser pulse train. We also report period-quadrupling where four successive pulses have different amplitudes. These results are attributed to the imperfect antireflection coating of the semiconductor chip, the latter constituting a regenerative amplifier with a time-dependent delay for amplification. A qualitative analysis is proposed and validated by numerical simulations from a diode laser rate equation model.
Room-temperature optical absorption in undoped α-Al2O3
Authors: Innocenzi M. E. , Swimm R. T. , Bass M. , French R. H. , Villaverde A. B. , Kokta M. R.
Published: July 2002

Optical absorption over a large dynamic range obtained by two different experimental techniques is reported for an undoped α-Al2 O3 single crystal. Absorption data are presented in two energy ranges: 0.94–3.5 eV and 4.5–8.6 eV. Vacuum ultraviolet (vuv) absorption measurements were performed on three boules of Czochralski-grown single-crystal Al2 O3 with differing starting material purities. The initial powder purities were 99.99%, 99.999%, and 99.999999%. In addition to the vuv measurements, calorimetric absorption results obtained in the visible and near-infrared (ir) are presented. An empirical formula is obtained that provides an estimate of the absorption coefficient from the near-ir to the vuv for undoped Al2 O3 . Detailed impurity analyses and sample histories are given for the optical-quality α-Al2 O3 utilized herein.
Analysis of determining factors in the kinetics of anisotropic photoetching of GaAs
Authors: van de Ven J. , Nabben H. J. P.
Published: July 2002

The possible factors which can determine the localized anisotropic photoetching of n-GaAs are analyzed. Mass transport to and from the reaction zone is considered. It is shown that diffusion of the oxidizing agent can become limiting when photoetching is performed by illumination of a masked pattern. Photogalvanic effects, leading to a delocalized dissolution mechanism, play a vital role under almost all practical conditions. Heating effects and local photostimulation of the dissolution reaction are of minor importance.
Room-temperature creep of tantalum tritides
Authors: Schober T. , Trinkaus H.
Published: July 2002

We report on long-term creep experiments on dilute tantalum tritides at room temperature. Significant deviations of the recorded strain rates from isotropic swelling are found above approximately 30 MPa. We attribute this room-temperature creep to a stress-induced preferential dislocation loop punching by bubbles in crystallographic directions close the stress axis. Quantitative estimates show that this mechanism can indeed account for the observed creep rates.
Investigation of the normal zone along a layer of thermally insulated superconductors
Authors: Devred Arnaud
Published: July 2002

During the transition of a superconducting winding, most of the dissipated power heats the conductors causing the propagation of the normal zone. Some part of the dissipated power is also transferred into the surrounding insulation. Due to the weak thermal conductivity of the insulating material, a certain amount of time is required for this heat to diffuse. The propagation front thus carries with it a wake of thermal diffusion, which raises the question of how much insulator should be taken into account in calculating the propagation velocity. Postulating the existence of such a velocity, we derive a model for the thermal behavior of an insulated layer of composite multifilament superconductors during the transition phase. The two-way Laplace transform is used to solve the equations for temperature diffusion/propagation in the conducting and insulating media. This yields an implicit equation for propagation velocity, which is then written in a dimensionless form. We next investigate two limiting cases, deriving in each case a simplified formula for propagation velocity. Last, we complete the analytical solution of our problem by computing the originals of the temperature profiles.
Physical and electromagnetic properties of Y-Ba-Cu-O superconductors synthesized with peroxides
Authors: Peacor S. D. , Uher C. , Francis A. H. , Shewchun J.
Published: July 2002

The physical and electromagnetic properties of Y-Ba-Cu-O superconductors synthesized with BaO2 (barium peroxide) have been studied. A comparison of reactions with BaCO3 and BaO indicates that the peroxide promotes formation and strong alignment of platelet grains in planes normal to the c axis, yields material with larger grain sizes and aspect ratios, reduces the synthesis reaction time by over half, increases the volume fraction of material that goes superconducting (Meissner effect), and facilitates metallic conduction in the normal state. Critical current densities of 1.1×104 A/cm2 at 77 K have been observed in peroxide-synthesized material. These beneficial effects may be due to the low melting temperature of BaO2 (450 °C), and the elimination of residual unreacted BaCO3, BaO, and/or C.
Spin waves in a semi-infinite Heisenberg ferromagnet with an impurity layer
Authors: Chen Niu-Niu , Cottam M. G.
Published: July 2002

A Heisenberg model is employed in a theoretical study of the effect of a magnetic impurity layer embedded in a semi-infinite ferromagnet. Using a Green’s function formalism we find that, in addition to bulk spin-wave modes, there may be localized spin-wave modes associated with the impurity layer and with the surface. The spin correlation functions are also calculated as a function of the impurity-layer distance from the surface. Results are also deduced for the temperature dependence of the magnetization in different layers of the ferromagnet.
Study of excitation of low energy for the low-temperature internal friction in the metallic glass Co35Y65
Authors: Jin Zhun-Zhi , Zou Xian-Wu , Liu Fu-Sui
Published: July 2002

Based on the unified theory of low-frequency fluctuation, dissipation, and relaxation processes, we studied the broad and asymmetric low-temperature internal friction peak of the metallic glass Co35Y65. This theory, which differs from that of distributed relaxation times, involves only a single relaxation time τP. By this theory, the calculated infrared-divergence exponent n=0.62, characteristic relaxation time τ∞=2×10−14 s, actual activation energy EA=0.2 eV, and apparent activation energy E*A =0.52 eV. They are in agreement with available experimental results (τ∞=2.2×10−14 s, EA=0.25 eV, and E*A =0.56 eV). Since the composition is very close to that of the intermetallics CoY2, the chemical short range order exists partly in the metallic glass Co35Y65. We notice from the behavior of this peak that it is not caused by the motion of gas atoms dissolved in the sample. From the much smaller radius of a Co atom compared with that of Y, we suggest this peak results from migration of Co atoms to neighboring vacancy with infrared divergence.
Exchange coupling in metals as understood then and now (invited)
Authors: Watson R. E.
Published: July 2002

The first U. S. magnetism conference occurred in 1952 and featured an invited session on exchange coupling in metals with Zener, Slater, Wohlfarth, and Van Vleck speaking. All were concerned with intra- and interatomic exchange with Zener discussing coupling via conduction electron polarization while Wohlfarth and Slater were preoccupied with itinerant ferromagnetism. Van Vleck verbalized the Hubbard Hamiltonian (prior to its derivation). That meeting may be viewed as the ultimate discussion of magnetism prior to modern day computation and many of the ideas expressed there underlie our computational activities today. Solutions of the Hubbard Hamiltonian and electronic structure calculations, as applied to magnetism, come to mind. We will concentrate on the latter here, since solving the Hubbard Hamiltonian will be of great concern elsewhere in this meeting, and we will attempt to juxtapose current findings with some of the thoughts of that pioneering meeting.
Haldane conjecture and elementary excitations in the XXZ spin-1 linear chain: A novel variational approach
Authors: Gómez-Santos G.
Published: July 2002

A physically motivated approach is used to study the linear chain XXZ spin-1 Hamiltonian in the antiferromagnetic regime. A variational Hamiltonian is obtained restricting the basis set to states with local antiferromagnetic environments separated by sites with Sz=0 acting as domain walls. Ground-state properties of this variational Hamiltonian are obtained and, in particular, an order–disorder transition is found with critical behavior corresponding to the two-dimensional Ising model universality class, in agreement with Haldane conjecture. Excellent agreement with finite chain data concerning the position of Haldane’s point, ground-state energy, and elementary excitations is found.
Spin-1 biquadratic chain: Magnetization curve (abstract)
Authors: Bonner J. C.
Published: July 2002

The antiferromagnetic (AFM) s=1 chain with biquadratic exchange coupling has recently been the subject of considerable attention, and some controversy. Theoretical arguments by Affleck that the model has dimerized character associated with a small spectral excitation gap have not received unanimous support from numerical calculations. For finite systems the field-dependent dispersion spectra are interesting and informative concerning the nature of the model. The excitations divide into two classes having very different character, corresponding to SzT even and SzT odd. Reflection symmetry about k=π/2 is apparent if states with a given total spin ST, rather than SzT, are plotted as a function of wavevector k, consistent with the Affleck prediction of dimerized character. The high degeneracy of the eigenstates reflects the existence of new symmetry operators which (a) shift the SzT value, (b) change wave vector k to π/2−k, and (c) shift the total spin quantum ST, of the energy eigenstates. Parkinson has recently observed that finite N states with SzT even and SzT≥2 map into states of the finite N s= 1/2 XXZ model at a special value of uniaxial anisotropy. Barber and Batchelor (unpublished) have shown the existence of an analytic mapping between states of the s=1 biquadratic chain and the 2D 9-state Potts model. They further show that all states of the Parkinson s= 1/2 XXZ model map into states of the s=1 biquadratic model. Hence, they find the ground state and first excited state energies exactly, and verify the Affleck picture. Their approach falls short of establishing complete integrability, however. Numerical studies on finite chains indicate that the T=0 magnetization curve is determined by the lowest-energy states for given SzT even, which are states of the s= 1/2 XXZ chain. Hence the magnetization curve of the s=1 AFM biquadratic chain is the same as the analytically known magnetization curve of the s= 1/2 XXZ chain!
Complex behavior of magnetic transitions in thin films
Authors: Soohoo R. F.
Published: July 2002

When a ferromagnetic sample is subjected to a rf and a quasi-static field with field values ranging from 0 to that required for magnetic saturation, the transitions near the coercive force Hc can be observed. If the specimen undergoes ferromagnetic resonance, the discrete changes in rf absorption by the sample when domain-wall motion occurs is sufficiently large to be observed with no noise interference. Experiments with Permalloy films using this resonance enhancement technique gave dramatic evidence of complex behavior of these magnetic transitions. It was found that the threshold distributions were sensitive functions of the initial conditions, i.e., of the degree to which the specimen was truly initially saturated. Tens of these threshold fields for domain-wall motion were experimentally observed for each specimen and the distribution of these transitions have been statistically analyzed. For example, two Permalloy film samples of the same nominal 80/20 compositions yielded Hc values of 1.63 and 2.08 Oe and a mean-square deviation of 0.063 and 0.071 Oe, respectively. The relationship between the magnetic transition threshold field distributions and imperfection distribution in the sample is discussed.
Spin-wave excitations on the surface of two coupled semi-infinite ferromagnets
Authors: Qiu Z. Q. , Walker J. C.
Published: July 2002

It is well known that spin-wave excitations at low temperatures can result in a T3/2 law for magnetization: M(T)/M(0)=1−BT3/2. The magnetization at a surface of a semi-infinite ferromagnet still obeys the T3/2 law, but with the B factor twice as big as that of bulk material. In the present paper, we set up a model of two semi-infinite single cubic Heisenberg ferromagnets with a magnetic coupling between them. By carrying out the Green’s function calculation, we show how this magnetic coupling affects the spin wave excitations at the surface. We found that the surface magnetization still follows a quasi-T3/2 law and, as the coupling between the two semi-infinite ferromagnets increases, B decreases monotonically to the bulk rate.
Effects of localization on the magnetic transition of thin gadolinium wires
Authors: Chin Arthur , Masden J. T. , Ali Naushad
Published: July 2002

Thin wires of gadolinium have been fabricated through a step lithographic technique with cross-sectional areas from 1.3×10−11 to 58×10−11 cm2. The Gd thin films were deposited in a UHV system by thermal evaporation and have a room-temperature resistivity of 85 μΩ cm. The electrical resistance of thin films and wires was measured by the four-probe method in the temperature range from 1.5 to 300 K. We also studied the isothermal magnetoresistance at temperatures below 30 K in a magnetic field up to 5.5 T. As the temperature decreases, the resistivity ρ of Gd wires first decreases towards a minimum at ∼55 K, then rises to a maximum at ∼ 31 K, followed by a sharp drop below 31 K. The minimum in the resistivity at 55 K is associated with the localization effect. It is believed that the sharp drop in ρ below 31 K is due to a magnetic transition in the Gd wire. This magnetic transition is possible due to a large shift in the Curie temperature caused by a reduction in the density of conduction electrons, arising from localization effects in the thin wire. The magnetoresistance (MR) data below 20 K show a negative value, and the field dependence of MR is similar to that of ferromagnetic metals. This is the first study of thin magnetic wires, to our knowledge.
Thickness dependence of magnetic surface anisotropy in ultrathin amorphous films (abstract)
Authors: Hicken R. J. , Rado G. T. , Xiao Gang , Chien C. L.
Published: July 2002

In searching for a possible dependence of magnetic surface anisotropy on film thickness, the magnetic surface anisotropy constant Ks of compositionally modulated amorphous Fe70B30-Ag films1 was measured by means of a method2 involving the dependence of the ferromagnetic resonance (FMR) fields on the magnetic layer thickness 2L. The ratio of the Ag to the Fe70B30 thickness was 3 to 1 while 2L ranged from 1.6 to 90 Å. Measurements were made with the applied static magnetic field parallel and perpendicular to the plane of the sample, at both X- and K-band frequencies for at least 14 values of 2L in each case. The results of magnetization measurements, made by SQUID magnetometry, have already been reported.1 Using the measured values of the thin-film magnetization and the aforementioned method, the value of Ks has been determined over the entire range of 2L. The results are consistent with a constant value of Ks for 2L>16.5 Å, while Ks is found to decrease monotonically as 2L is reduced from 16.5 to 1.6 Å. A similar decrease of Ks is found even if the bulk value rather than the thin-film values of the magnetization is used throughout. In addition, the value of Ks has been deduced for 2L=4.1, 6.7, 10.2, and 16.7 Å, from the original SQUID data. These values are in good agreement with those determined by the FMR method.
Observation of intermediate spin states in Er/Y superlattices
Authors: Beach R. S. , Borchers J. A. , Salamon M. B. , Du R. , Flynn C. P.
Published: July 2002

Low temperature hysteresis measurements, performed on a series of Er/Y superlattices with Er interlayer thickness ranging from 37 to 90 Å, suggest the existence of intermediate spin states that are stabilized in a field. While bulk Er becomes ferromagnetic at 20 K, even at 10 K these samples transform to a state carrying (1)/(7) the saturation moment. This state, which is observed only at high temperatures in bulk Er, is stable at 10 K to 15 kOe, beyond which the magnetization saturates. The widths of hysteresis loops associated with the low and high field transitions, respectively, are 0.5 and 2.5 kG, many times that of Er. Just below the critical field the spin structure passes through a narrow field range where the magnetization is approximately Ms/2. No such state is observed in bulk Er in zero field. This intermediate state is more apparent on reducing the field after saturation, and becomes more stable as the Er interlayer thickness is decreased. The magnetization is not found to decay appreciably when the field is held for 10 h on the Ms/2 plateau. Preliminary data on thin films also show the existence of the Ms/7 state at low temperatures, but with the range of stability decreasing with increasing Er film thickness. We discuss these phenomena in terms of modifications of the exchange interaction due to epitaxial strains.
New phenomena of magnetoresistance in ferrimagnetic Fe/Gd multilayers
Authors: Fujimori H. , Kamiguchi Y. , Hayakawa Y.
Published: July 2002

A new type of magnetoresistance characterized by its anomalous field dependence has been found in artificially multilayered ferrimagnets of Fe/Gd and of modified double-structured {Fe/Gd}/{Fe/Gd}. The anomaly is attributed to the spin-flop which remarkably takes place at fields less than 1 kOe even at room temperature and is particularly pronounced in the double-structured multilayers. In coincidence with the spin-flop, the magnetoresistance shows an up and down change against field.
Magnetic and hyperfine interactions in amorphous Fe-P alloys
Authors: McCally R. L. , Moorjani K.
Published: July 2002

Amorphous alloys FexP100−x have been fabricated by rf sputtering in the composition range 50≤x≤75. Magnetic ordering temperatures were determined using thermogravimetric analysis (for x=75) and by zero-velocity thermal scanning (for 50≤x≤71). Ordering temperatures are depressed from those of the corresponding crystalline phases and range between 550 K for a-Fe75P25 and 10 K for a-Fe50P50. The Mössbauer spectra at 8 K of alloys with x≤71 all had a central unsplit line and broad, essentially featureless wings, indicating the coexistence of magnetically ordered and nonmagnetically ordered regions. The spectrum of a-Fe75P25 consisted of six broad lines, indicating that this composition is magnetically more homogeneous. Hyperfine-field distributions P(H) and mean isomer shifts (IS), determined using Window’s method, show that mean hyperfine fields have an essentially linear dependence on concentration and, by extrapolation, vanish at x∼46. The IS values at 293 K, however, have only small variations with composition and a weak maximum at x=67, whereas at 8 K the composition dependence is larger and there is a minimum at x=67. This behavior suggests a structural alteration whose nature is, as yet, unclear.
Impact of solid modelers on electromagnetic device design
Authors: Chia B. T. , Coulomb J. L. , Vérité J. C.
Published: July 2002

The many methods and formulations available in field computation have made possible a wide range of applications. However, this success is partially limited by the absence of suitable preprocessors in field packages. This discrepancy is particularly acute in three-dimensional (3D) field analysis where processed objects may take on very complicated shapes. The lack of efficient interactive user interfaces and design environments renders the manipulation of 3D geometry and meshes very tedious. This is all the more surprising because computer support in 3D geometry design and mesh generation is an established engineering science. Indeed, it will be interesting to create and mesh 3D electromagnetic devices using standard solid modeling and mesh generation systems and then couple the output to a field package. This paper explores the plausibility of integrating solid modelers in an electromagnetic device design environment and describes the experience gained in interfacing a commercial solid modeler with established field codes. A brief overview of solid modeling techniques will be presented, followed by discussions on the pros and cons of using a separate solid modeler. The paper will be amply illustrated with examples.
Computation of fields and inductance using a three-dimensional boundary-element technique (abstract)
Authors: Shah M. , Salon S. , Wendling P.
Published: July 2002

Three-dimensional techniques are often required to compute fields and inductances in devices in which leakage flux plays an important role. One such example is the interphase transformer shown in Fig. 1. There are two modes of operation. For currents in the flux additive sense, the flux crosses the small gap and the inductance is relatively high. For currents in the flux opposing sense, the flux is forced into the leakage paths and the inductance is much lower. The device is thus used as a filter, blocking harmonics which are ‘‘in phase’’ and allowing harmonics which are ‘‘out of phase’’ to pass. The inductance computations were done with the PHI3D boundary-element package. The method of computing the fields and inductance is discussed in the paper along with comparison with test results.
Linear-chain ordering in a spin-3/2 system: Magnetic susceptibilities of [Cr(py)3Cl3]; magnetic susceptibilities and crystal structures of several S=3/2, three-dimensional antiferromagnets (abstract)
Authors: Merabet K. E. , Burriel R. , Carlin Richard L.
Published: July 2002

The magnetic susceptibility at zero applied field of single crystals of [Cr(py)3Cl3] has been measured over the temperature interval from 40 mK to 4.2 K. Magnetic ordering is found at 190 mK. This spin S=3/2 material appears to behave as a magnetic linear chain; the parameters resulting from the data analysis are J/kB=−0.044 K, 2D/kB=−0.14 K and 〈g〉=1.96. The zero-field ac susceptibilities of single crystals of two isomorphous S=3/2 trigonal compounds, [Cr(en)3]Cl3⋅3H2O and [Na(H2O)6][Cr(en)3]2Br6Cl, are reported. Antiferromagnetic ordering is found at, respectively, 124(5) and 112(5) mK. The data on [Cr(en)3]Cl3⋅3H2O may be fitted with the parameters 2D/kB=−0.091(8) K, g∥=1.994(3), g⊥=1.988(3), and zJ/kB=−0.061(2) K, while the bromide may be described by 2D/kB=0.064(8) K, g∥=2.001(3), g⊥=1.991(3), and zJ/kB=−0.066(6) K, where longitudinal (∥) and transverse (⊥) refer to the unique crystallographic axis. The easy axis for the chloride lies parallel to the longitudinal axis, while the easy axis for the bromide lies in the transverse plane. The crystal-structure analysis of the bromide is also reported.
High-resolution magnetic force microscopy (abstract)
Authors: Grütter P. , Wadas A. , Meyer E. , Heinzelmann H. , Hidber H.-R. , Güntherodt H.-J.
Published: July 2002

We present high-resolution magnetic force microscopy1 (MFM) images of a Co80Cr20 thin film. Clearly a stripe domain structure with a domain width of 220 nm is observed, the highest resolution of a periodic magnetic structure measured by MFM to this date, and magnetic fine structure of typically 50 nm width and more than 200 nm length by measuring forces as small as 0.1 nN. The micromagnetic structure is compared to the morphology of the sample, which consists of vertical columns with an average diameter of 40 nm×60 nm, and to theoretical simulations of force traces, which are in reasonable agreement with experiment.2 These results are compared to the 10-nm resolution obtained on rapidly quenched FeNdB, a nonperiodic magnetic structure.3 If an effective tip domain structure is assumed, reasonable agreement of theoretical simulations and experiment is achieved. The sample morphology as determined by atomic force microscopy is very flat on a 500-nm lateral scale. In contrast, scanning-tunneling-microscope measurements in ultrahigh vacuum show a grain structure after removing the nonconducting surface layer by Ar-ion etching. The experimental resolutions are compared to theoretical expectations4; possibilities of improvement are discussed.
Rotational spin-glass properties of Tb0.3Y0.7Ag
Authors: Said M. R. , Kouvel J. S. , Brun T. O.
Published: July 2002

This cubic pseudobinary compound with antiferromagnetic order and spin-glass properties below Tg (36 K) was studied by magnetization-vector measurements on a field-cooled (FC) polycrystalline sample disk rotated in various fixed fields (H). At 4.2 K, the FC-induced anisotropy field HK is seen to turn rigidly with the sample for all rotation angles (θ≤180°) even at H=15 kOe, and since the rotational magnetization follows HK very closely, it is deduced that HK >100 kOe. At 20 K, HK turns rigidly, but only below a threshold field, above which HK rotates up to some critical angle relative to H, where it remains as the sample continues to turn. Thus HK rotates frictionally relative to the sample. Nevertheless, it is found that its magnitude continues to exceed 100 kOe.
Static strains and ferroelastic domains in orientational glasses
Authors: Galam Serge
Published: July 2002

A new model is presented to describe the low temperature part of the phase diagram of M(CN)xX1−x mixed crystals. The ferroelastic transition is lost at some threshold value xc greater than the percolation threshold pc. Between xc and pc no phase transition occurs. However, cyanides are found to form ferroelastic domains with random orientations generated by local static strains. These results are consistent with molecular dynamics calculations and supported by very recent shear torque experiments.
Evidence of clustering of magnetic ions in weakly doped MgO crystals (abstract)
Authors: Seehra M. S. , Dean J. C.
Published: July 2002

The temperature dependence of the magnetic susceptibility of CopMg1−pO and MnpMg1−pO systems has been measured in the temperature range of 1.7–300 K. The nominal compositions investigated are p=0.000 84 and 0.0048 for the MnpMg1−pO system, and p=0.000 31, 0.0099, 0.07, and 0.10 for CopMg1−pO. The data are analyzed in terms of contributions from the single ions (Mn2+ and Co2+) and the exchange-coupled pairs (dimers). For the MnpMg1−pO system, it is found that concentration of dimers determined from a theoretical fit to the data is nearly an order of magnitude larger than that expected from the random distribution of Mn2+ ions, suggesting clustering of the Mn2+ ions in MgO [B. L. Gordon and M. S. Seehra, Phys. Rev. B 40, 2348 (1989)]. For the CopMg1−pO system, the analysis is complicated by the effect of spin-orbit interaction and leads to a fictiously large (≂50 K) Curie–Weiss temperature even for the p=0.000 31 sample, in agreement with the experiment. Analysis of the data by including the effect of exchange interaction is now underway, and these results will be reported elsewhere.
Crystal structure and magnetism of Co(HPO3)⋅H2O: A novel layered compound of Co(II)
Authors: Marcos M. D. , Gómez-Romero P. , Amorós P. , Sapiña F. , Beltrán-Porter D. , Navarro R. , Rillo C. , Lera F.
Published: July 2002

The crystal structure and magnetic properties of Co(HPO3)⋅H2O have been determined. The solid crystallizes in space group Pca21, a=8.984(2) Å, b=7.918(2) Å, c=10.139(8) Å, V=721.2 Å3. Z=4 dcalc =2.89 g/cm3. The structure consists of layers only connected by hydrogen bonds. These layers can be viewed as formed by zigzag chains of edge-sharing Co(II)O6 octahedra interconnected by a three atom bridge (O-P-O) and a single oxygen bridge. ac magnetic susceptibility measurements show a sharp peak at Tc =(10.8±0.1) K, and are consistent with a transition from one- to two-dimensional magnetic order, in agreement with the structure. An analysis of the χT/C vs ε=(1−Tc/T) data in the critical region, for T>Tc, using a double-logarithmic plot, yields a critical exponent γ=1.75 for 0.01<ε<0.1, which correspond to a two-dimensional Ising model.
Magnetism and EPR spectra of the two-sublattice manganese chain Mn2(EDTA)⋅9H2O
Authors: Borrás-Almenar J. J. , Coronado E. , Gómez-García C. J. , Gatteschi D. , Zanchini C.
Published: July 2002

We report on the magnetic behavior and single-crystal EPR spectra of the chain complex Mn2(EDTA)⋅9H2O characterized by two different alternating sites for the manganese ions. Magnetic susceptibility data are indicative of weak antiferromagnetic exchange interactions between the manganese ions, showing a maximum of about 3 K. This behavior can be accurately described on the basis of a Heisenberg chain model that assumes classical spins, giving J/k=−0.72 K and g=2.0. The EPR spectra are typically low dimensional, with an angular dependence of the linewidth of the type (3 cos2 θ−1)n (n=4/3 or 2). Nevertheless, such a behavior cannot be reproduced from a one-dimensional model with dipolar broadening. An alternative broadening mechanism arising from single-ion zero-field splitting is then suggested.
Abnormal grain growth in sintered Nd-Fe-B magnets
Authors: Liu N. C. , Kim A. S.
Published: July 2002

Abnormal grain growth was found in sintered Nd-Fe-B magnets which are made from powder milled after hydrogen decrepitation. The abnormal grain size in the magnets sintered at 1000 °C for 1.5 h was as large as 400 μm. The formation of abnormally large grains reduced the magnetic properties. As sintering temperature increased, the number of large grains increased and coercivity reduced. However, abnormal grain growth was rarely found when the magnets were fabricated without hydrogen decrepitation. A grain growth mechanism is discussed in relation to the effect of hydrogenation and sintering process.
Near-band-edge photoluminescence from chemically treated CdTe surfaces
Authors: Amirtharaj P. M. , Dhar N. K.
Published: July 2002

The effects of chemical etching and aging under atmospheric conditions have been investigated in In doped, bulk CdTe using photoluminescence (PL) spectroscopy. The etchants studied included Br2/CH3OH, KOH/CH3OH, Na2S2O4 /NaOH, and K2Cr2O7 /HNO3. The results indicate a large enhancement of the 1.5896-eV excitonic feature due to chemical treatment and aging. The 1.5896-eV peak was previously shown to originate from a native defect involving a Cd vacancy. On this basis, we interpret the primary perturbation to be a small loss of Cd within the sampling region. Previous studies have reported substantial loss of Cd in the near-surface region, within ∼25 A of the surface, as a result of etching in Br2/CH3OH. In contrast, a much smaller loss over a possibly larger depth is observed here. The Cd loss is, most likely, substantial nearest to the surface and extends deeper in but with less severity. The implications of these results on interpreting PL spectra, device processing, and long term stability are considered.
Atmospheric pressure organometallic vapor-phase epitaxial growth of (AlxGa1−x)0.51In0.49P (x from 0 to 1) using trimethylalkyls
Authors: Cao D. S. , Kimball A. W. , Stringfellow G. B.
Published: July 2002

(AlxGa1−x)0.51In0.49P layers, lattice matched to (001)-oriented GaAs substrates, have been grown throughout the entire aluminum composition range from x=0 to 1.0 by atmospheric pressure organometallic vapor-phase epitaxy (OMVPE), using trimethylaluminum (TMAl), trimethylgallium (TMGa), trimethylindium (TMln), and phosphine (PH3) as source materials in a horizontal reactor. The growth temperature was held constant at 680 °C. Excellent surface morphologies were obtained over the entire composition range. Unlike previously reported results, neither high growth temperatures nor low pressures were needed in order to obtain good-quality, high-x (AlxGa1−x)0.51In0.49P alloys using trimethylalkyls. Photoluminescence (PL) was observed, even at 300 K, for all samples with Al solid compositions of x≤0.52. The results show that the energy band gap measured by PL at room temperature for this material varies as Eg=1.9 +0.6x, in accord with previous studies. It was found that the PL emission intensity was nearly constant at 10 K with increasing x in the range from 0 to 0.52. This contrasts with earlier published results which showed a decreasing PL intensity for the higher values of x. The 300-K PL intensity was almost a constant for x≤0.3 and gradually decreased with increasing Al content for x>0.30. The dependence is nearly that predicted from a simple calculation based on the relative occupancies of the Γ and X conduction bands using a constant minority-carrier lifetime. The PL full width at half maximum (FWHM) for x=0 was 7.2 meV at 10 K and 35 meV at 300 K. These are the narrowest reported results to date. For x=0.48, the FWHM was 31 meV at 10 K and 78 meV at 300 K. It was observed that at both 10 and 300 K, the FWHM increased slowly with increasing aluminum concentration.
Spatially resolved differential resistance of bulk superconductors by laser-induced heating
Authors: McLaren Robert , Dovichi Norman J.
Published: July 2002

Spatially resolved measurements of the change in resistance with temperature of bulk high-temperature superconductors are generated by heating a localized region of a sample with a modulated laser beam. The amplitude of the thermally induced resistance change is related to the heat capacity and derivative of resistance with temperature whereas the phase of the signal is related to the thermal diffusivity of the sample. By moving the sample with respect to the focused laser beam, spatially resolved measurements may be made. Images with ∼50-μm resolution have been used to locate defects in polycrystalline Bi-Sr-Ca-Cu-O sample and to estimate the thermal diffusivity of the bulk material.
Kinetic aspects of epitaxial silicon growth using disilane in a rapid thermal processing system
Authors: Pares G. , Regolini J. L. , Mercier J. , Dutartre D. , Bensahel D.
Published: July 2002

We have measured the epitaxial growth rate of Si in the disilane/hydrogen system for different experimental conditions in a rapid thermal processing–low-pressure chemical vapor deposition reactor. The measured activation energy is about 43 kcal/mol for temperatures below 700 °C. The growth rate dependence on disilane or hydrogen partial pressures is measured in order to verify the reliability of a proposed model for the decomposition of the disilane molecule in the torr pressure regime. The obtained crystal quality is comparable to that obtained with silane gas except for higher growth rate in the disilane system.
The topography effect on magnetic images in magnetic force microscopy
Authors: Wadas A. , Güntherodt H.-J.
Published: July 2002

This paper discusses the principles of topographic influence on magnetic force microscopy (MFM) response. We present in a theoretical quantitative way the effect from some chosen topographic features on MFM signal and conditions, which should be fulfilled to detect such an effect. We considered two cases of magnetization direction inside the sample, perpendicular and parallel to the surface.