bluuebunny/crossref_metadata_2025 huggingface dataset:
Frequency Conversion in the Sheath Capacitance of a Glow Discharge Plasma
Authors: Hilbish R. T. , Montgomery R. M. , Holmes R. A.
Published: February 2004

A study is made of large-signal, high-frequency phenomena occurring in a slightly ionized plasma contained within a pair of metallic coaxial cylinders. An rf equivalent circuit is developed wherein the sheath is represented by a voltage-dependent capacitor in parallel with a voltage-dependent resistor, the presheath by a series resistor, and the plasma region by a series resistor and inductor. It is found that when the sheath is thin compared to the radius of the inner cylinder, the voltage dependence of the sheath capacitor reduces to a form identical with that of an abrupt junction varactor diode, so that the plasma capacitor is properly called a plasma varactor. As an application of the plasma varactor, frequency doubling using standard semi-conductor varactor techniques is attempted. The resultant peak conversion efficiency is found to be 31.6% at a drive frequency of 142 MHz and input power of 0.42 W. It is shown that the sheath conductance is responsible for this low conversion efficiency relative to that obtainable from a semiconductor varactor.
Dispersion of Surface Elastic Waves Produced by a Conducting Grating on a Piezoelectric Crystal
Authors: Joshi S. G. , White R. M.
Published: February 2004

The dispersion relation for surface elastic waves propagating under a thin metallic grating on the basal plane of a hexagonal piezoelectric crystal is investigated theoretically. Purely reactive loads are assumed connected between adjacent electrodes of the grating. The effect of mass loading due to the grating is neglected. The presence of the grating imposes periodic boundary conditions on the piezoelectric fields associated with the surface wave. The resulting boundary-condition problem is solved to obtain the dispersion relation for surface elastic waves. For a material with a large electromechanical coupling coefficient an inductance connected between adjacent electrodes can cause a large reduction in the phase velocity of the surface wave. (For example, in PZT-4, this reduction is about 10%.)
Dipole Modes and the One-Wave Approximation for Trapped Modes in Magnetoplasma Cylinders
Authors: Weil A. J. , Klevans E. H.
Published: February 2004

An analysis of trapped mode wave propagation is given for a cold, uniform magnetoplasma cylinder embedded in free space. The full set of Maxwell's equations is used and an arbitrary magnetic field strength is allowed. New simplified forms are obtained for the dispersion relation and the fields. The simplified expressions are used to study the dipole modes in detail. Computer solutions are given for the dispersion relation and for the longitudinal fields when the cyclotron frequency is above and below the plasma frequency and for two different column diameters. The fields solutions are sums of terms with two different propagation constants. For the circularly symmetric and dipole modes, it is found that one wave dominates. This has led to a new approximation, the ``one-wave'' approximation, which gives dispersion curves accurate to within 1% except for the lowest-order dipole mode.
Decay of Resonance Radiation in a Planar Afterglow with Reflecting Walls
Authors: Ingold J. H.
Published: February 2004

The decay time of Doppler-broadened resonance radiation in a planar afterglow is calculated numerically and compared with the analytic approximation of Holstein. This comparison shows that Holstein's results are higher than the numerical results by as much as 18%, depending on the optical thickness. In addition, the effect of diffusely reflecting walls on the decay time is calculated numerically and compared with the analytic approximation of Weinstein where possible. The numerical results are rigorously valid in the optically thick limit and approximately valid in the optically thin limit.
Electron-Spin-Echo Envelope Modulation Induced by a Very Small Alternating Magnetic Field
Authors: Srivastava Vishnu C.
Published: February 2004

The effect of an additional very small alternating magnetic field h̄ on the electron spin echoes is reported. When single crystals of K3Co (CN)6 containing Cr3+, and CaWO4 containing Ce3+, as substitutional impurities were subjected to the extra field h̄, the echo envelope was modulated at the frequency of the extra field ranging from 100 to 2000 kHz. The magnitude of the additional field h̄ ranged from 0.1 to 0.85 G and the depth of the periodic modulation of the echo envelope was directly dependent on the magnitude of h̄. The maxima and minima in the envelope occurred when the period T of the additional field h̄ was related to the time interval between the microwave pulses by τ=nT and τ=(n+½)T, respectively. The results are discussed in view of the paper by M. Dupont and G. A. Woonton [Can. J. Phys. 46, 87 (1968)] which shows that the additional magnetic field h̄ contributes to these effects only at those instants when microwave pulse is also present, provided the field h̄ is homogeneous over the sample. A classical treatment of the problem is presented. The effect of additional field h̄ on the echo amplitude is decided by the magnitude of h̄ relative to microwave field in the cavity and not relative to steady magnetic field H0.
Radiation Annealing and Modification of the 35° and 65°K Defects in Germanium at Low Temperature
Authors: DeAngelis Henry M. , Penczer Rudolf E.
Published: February 2004

Damage introduced in n-type germanium at ∼7°K with 1.0 MeV electrons undergoes radiation annealing and conversion when subjected to 0.5 MeV electron irradiation at ∼7°K. The radiation annealing with only 65°K and permanent defects present appears to be a first-order process with an estimated cross section of 10−15 cm2, while radiation annealing with 35°, 65°K, and permanent defects present appears to be a second-order process. It appears that only the 35° and 65°K defects are recovered and 65°K defects are converted to 35°K defects as part of the radiation-annealing process. The radiation anneal of converted damage seems to be the rate-limiting process in the radiation anneal of 65°K defects. Our results show evidence for another type of 65°K defect as suggested by Zizine and by Whan's work. If this is the case, then our results imply that this second type is not subject to radiation annealing.
Theory of the DeBlois Experiment. I. The Effect of the Small Coil
Authors: Aharoni Amikam
Published: February 2004

The nucleation field, as measured in the DeBlois experiment, should in principle be corrected for demagnetization, and for the field falloff. The correction for the demagnetization has been shown before to be negligibly small in practical cases and this is proved again here. The falloff, on the other hand, implies a correction of about 20% to the field at the center of the reversing coil. This correcting factor is calculated as a function of the coil dimensions, assuming, for the meantime, a perfectly smooth whisker.
Continuously Tunable Picosecond-Pulse Organic-Dye Laser
Authors: Soffer B. H. , Linn J. W.
Published: February 2004

Picosecond-pulse outputs, tunable over a broad spectral range, have been demonstrated in an organic-dye laser. Evidence is presented that no more than a small fraction of the available bandwidth cooperates in mode locking to produce individual short pulses.
A High-Resolution Scanning Transmission Electron Microscope
Authors: Crewe A. V. , Wall J. , Welter L. M.
Published: February 2004

A simple scanning transmission electron microscope has been built using a field-emission electron source, a new electron gun, and one lens to produce a high-contrast picture with 30 Å resolution. The final spot size is limited by the properties of the lens, in the same manner as a conventional transmission microscope. The field-emission tip requires a pressure below 10−9 Torr for stable operation and can have a lifetime of several months. The intensity of the source is such that high-quality pictures can be obtained in 10 sec. Specimen contamination or damage is small, as would be expected in view of the good vacuum conditions. The theory and design of the instrument are discussed, and experimental results are shown.
Factors Influencing the Acoustic Properties of Vitreous Silica
Authors: Fraser David B.
Published: February 2004

The acoustic properties of various types of commercial vitreous silica have been studied as functions of thermal treatment and impurity-ion concentrations. The properties studied include density, shear-wave attenuation, shear and dilatational wave velocities, and elastic constants. The measurements of acoustic properties are made on small sphere-shaped samples using primarily a technique that sets the sphere into a resonant mode of vibration and then allows the vibration to decay freely. Both frequency and the decrement of the decay are measured. Use of this resonant sphere technique is particularly appropriate to the present study because it permits precise measurements on samples small enough to conveniently undergo laboratory modification, and it permits the study of velocity variations in a single block of material by measurements on many localized samples. The major findings of this study are the following: (1) An increase of one degree in fictive temperature (a parameter assumed to be related to the equilibrium internal configuration of the glass) causes a decrease in the shear velocity of 13–16 ppm. (2) Gross differences of OH content result in differences in shear velocity. Although shear velocity differences could be induced by changing fictive temperature and the gross OH content, shear velocity fluctuations (of the order ± 125 ppm) observed in single large pieces of commercial vitreous silica were not correlated with variation of either fictive temperature or OH content. Attempts to relate fluctuations of shear velocity to differences of Na content were inconclusive.
Measurements of Demagnetizing Factors and Their Application to Magnetic Thermometry
Authors: Anderson A. C.
Published: February 2004

Shape corrections for several cerous magnesium nitrate magnetic thermometers have been measured, both for powders and for composites of oriented single crystals, in an attempt to establish a suitable geometry for a magnetic thermometer accurate to ∼1% at temperatures down to 0.02°K. The results indicate that this accuracy is available with either a spherical composite of oriented single crystals. or with a powder in the form of a right circular cylinder with ratio (length/diameter)=0.92. In either case it is imperative that the density be uniform within the sample. The results are compared with theoretical computations, where applicable, and with the results of other techniques.
Fluorescence Decay Measurements of Eu2+ in CaF2:Eu+Ho
Authors: Ewanizky T. F.
Published: February 2004

Measurements have been made of the submicrosecond fluorescence decay times of Eu2+ in the mixed dopant, Eu2++Ho3+ system. Significant decrease of Eu2+ decay time as a function of Ho concentration confirms effective Eu2+→Ho3+ nonradiative energy transfer. A system using analog techniques for direct measurement of short decay times is described.
Power Flow in Plasma-Filled Waveguides. II
Authors: Bevc Vladislav
Published: February 2004

Power flow in cyclotron and plasma modes propagating in a waveguide filled with a cold collisionless axially magnetized plasma studied in an earlier paper is reexamined. It is shown that proper evaluation of field expressions at special points on the Brillouin diagram removes difficulties contained in the earlier presentation. Comparison of suitably normalized total power flow as a function of the propagation constants for various representative examples of mode types is made. Interaction or coupling impedances relating transverse and longitudinal electric fields to power flow are calculated.
Secondary Twinning in Electron-Diffraction Patterns of Nickel Films Epitaxially Grown
Authors: Leibovich Harry A. , de Zimmerman Rosa S.
Published: February 2004

Nickel films epitaxially grown were obtained by high-vacuum evaporation on rocksalt faces cleaved in air. They were partially oriented and their thickness was 800 Å. The electron-diffraction pattern was interpreted assuming the presence of primary and secondary twins and the orientation (001) parallel to the substrate. The composite reciprocal lattice, containing points from the matrix and from primary and secondary twins, was calculated by means of a computer program. The symmetry elements of this lattice permit the deduction of a selection rule to determine the location of the secondary twinned points with respect to those of the matrix and a cubic supercell with nine unitary cells along each orthogonal axis. The double diffraction points, arising from beams rediffracted in secondary twins, explain the origin of the rings interior to the (111).
Molecular Motion in 2-Chlorostyrene-Styrene Copolymers from Dielectric Measurements
Authors: Leffingwell J. , Bueche F.
Published: February 2004

The complex dielectric constant of several poly(2-chlorostyrene-styrene) copolymers was measured in the range 0.1–105 Hz from 27° to 150°C. Using the WLF equation, the relaxation frequency at the glass temperature was found to be about 10−4 Hz for each copolymer, within the uncertainty in the dilatometric glass temperatures. Normalized loss peaks are compared and their shapes are found to be nearly independent of Cl concentration in the range of this study. From these curves it is concluded that in this case the observed non-Debye behavior is not the result of dipole-dipole interaction nor steric hindrance between chlorine atoms, but is related to the structure of the polystyrene molecule itself. Loss peaks below the glass temperature (associated with the hindered rotation of the phenyl group) were observed for the copolymer having the highest Cl concentration.
Noise and Equivalent Circuit of Double Injection
Authors: Bilger H. R. , Lee D. H. , Nicolet M-A. , McCarter E. R.
Published: February 2004

Measurements of the high-frequency noise of a silicon double-injection diode result in 〈i2〉=α·4kT(1/r)Δf with α=1.04 and in agreement with the literature. A new interpretation demands Nyquist noise with α≡1 in these devices at high frequencies. This is in accord with an equivalent circuit derived for the double-injection process. Speculations are made on the general validity of Nyquist noise in nonlinear devices at high frequencies. In addition, generation-recombination noise is suggested as the prime source of the low-frequency noise.
Resonant Frequencies and Fields in a Cavity Containing a Magnetoplasma Dielectric
Authors: Kent Gordon
Published: February 2004

The problem of a microwave cavity filled with a uniform, cold magnetoplasma is solved exactly. The results are a set of three nonlinear simultaneous equations for the frequency and two spatial parameters. Explict expressions for all field components for oscillations of a circular cylindrical cavity are given. Numerical results are given showing frequencies and spatial parameters of eight resonant modes in a particular cavity with the cyclotron frequency equal to the unloaded TE111 frequency. Four of the modes correspond to the TE111 resonance and four correspond to the TM111 resonance. In addition, there is a plasma type of resonance at a frequency below the plasma frequency and below the whistler resonance.
Dynamic Behavior of a Passive Q-Switched Ruby Laser
Authors: Asvazadurian M. , Riva S. , Soncini G. , Svelto O.
Published: February 2004

The risetime, fall time, and peak power of the output light pulse of a single-mode passive Q-switched ruby laser have been experimentally measured vs the measured ratio nt/nai, where nt and nai are the threshold inversions, respectively, without and with the saturable absorber. The results are compared with the fastswitch theory. As far as pulse risetime and fall time, the agreement is fairly good (within 20%). The experimental output power has been found 60% smaller than that theoretically expected.
Observations of the Flow in an Electromagnetic Shock Tube
Authors: Pert G. J.
Published: February 2004

Observations of the flow of plasma in an electromagnetic shock tube are presented. Three regimes of flow are found in hydrogen. At high initial gas pressure (>1 Torr) and low speeds (<2 cm/μsec) a cold shock-heated region of gas followed by hot turbulent expanding driver gas was found. At low initial gas pressure (<0.5 Torr) and high speeds (>4 cm/μsec) a single hot turbulent body of gas moved down the tube. An intermediate flow was found in the intermediate pressure-speed regime. The onset of turbulence in the flow was found to be described by the hydrodynamic Reynolds number.
Emission Processes Accompanying Megavolt Electron Irradiation of Dielectrics
Authors: Watson Alan , Dow Julian
Published: February 2004

Experiments are described in which thick dielectric samples were irradiated with electrons of about 1 MeV energy. The temporal growth of the external electric field from the trapped charge was punctuated by either total or partial collapses associated with electron emission and followed by pauses before the process repeated itself. Secondary electrons are thought to be heated by the growing internal space charge field until thermionic emission induces a regenerative growth of positive surface charge sufficient to oppose it. Electron emission then ceases until recombination can restore the outside surface of the dielectric to a field-free condition from which it grows again. Analysis of this model shows that it appropriately describes the observed phenomena.
Formation of Echoes in Type-II Superconducting Powders
Authors: Ehrenfreund E. , Goldberg I. B. , Weger M.
Published: February 2004

A mechanism which accounts for the observed echoes in various systems, such as type-II superconducting powders, is proposed here. The model proposed accounts for the dependence of the echo amplitude on the exciting power, but does not specify the origin of the echoes.
Nonlinear Interaction of Cyclotron Harmonic Waves
Authors: Harker K. J. , Crawford F. W.
Published: February 2004

This paper presents an analysis of nonlinear interaction between cyclotron harmonic waves propagating perpendicular to the static magnetic field in a warm magnetoplasma. It is demonstrated that the synchronism conditions for traveling wave parametric amplification, and passive mode conversion can easily be satisfied for such waves, and an analysis is carried out for a large-signal pump wave interacting with two small-signal waves. The theory employs the Vlasov equation and Maxwell's equations, and provides expressions suitable for numerical computation. Growth rates and corresponding pump powers are computed, and it is concluded that the effect should be observable in typical laboratory plasmas. The paper concludes with a discussion of possible device applications.
Néel Ferrimagnets in Large Magnetic Fields
Authors: Clark Arthur E. , Callen Earl
Published: February 2004

In this paper we analyze the high-field magnetization of Néel ferrimagnets and apply this analysis to our measurements of YbIG and DyIG. We show that in some ferrimagnetic systems, as the temperature is increased at fixed field strength, the magnetization can decrease in one temperature interval, be temperature independent in another, and rise or fall in a third. Conversely, at fixed temperature, as the field is increased, there can be intervals of small and large susceptibility with abrupt discontinuities between them, even in the absence of anisotropy. This behavior is ascribed to the formation of angles between the various sublattice moments and the external field. The angled region can be described by a phase diagram in the H-T plane. There are second-order phase transitions at all boundaries, along which the order parameter (the angles) undergoes critical fluctuations. Two features appear technologically promising for some ferrimagnets: (a) a temperature range over which the moment of the ferrimagnet is temperature independent, with discontinuities in dM/dT at both ends of the range, and (b) cooling by adiabatic magnetization, i.e., cooling by increasing the magnetic field strength. We have measured the magnetization of YbIG and DyIG from 1.4°–296°K in fields up to 130 kOe. At low temperatures, the YbIG magnetization is directly proportional to the magnetic field for 3.5 kOe < H < 130 kOe, while at the same time it is nearly temperature independent. Below a critical field of ∼3.5 kOe, sublattice-moment rotation does not take place, and the magnetization is not linearly related to the magnetic field. The 4.2°K spontaneous magnetization of YbIG is 0.069 μb. The DyIG magnetization, on the other hand, is large and highly anisotropic at low temperatures. At 4.2°K technical saturation is achieved near 40 kOe, but a large volume susceptibility (∼0.4×10−3 emu) persists above 130 kOe.
Effect of Neutron Irradiation on the Acoustic Performance of Evaporated CdS Films
Authors: Duncan W. , Hutchins R. , Richter J. , Stewart P. A. M.
Published: February 2004

Investigations have been carried out on the effects of neutron irradiation on the insertion loss of evaporated longitudinal CdS transducers operating in the frequency range 0.9–2.0 GHz. The one-way transducer loss was markedly reduced by irradiation and subsequent annealing, and the transducer bandwidth was also increased. Attempts were made to distinguish between the effects produced by thermal neutrons, fast neutrons, and gamma rays. The results suggest that neutron bombardment induces two opposing processes in a transducer. There is a significant improvement in the insertion loss due to the increase in resistivity, and an increase in the loss due to the lattice damage caused by irradiation, which effectively reduces the electromechanical coupling constant. The latter mechanism was investigated in several bulk samples of CdS. The effect of neutron irradiation on the film resistivity was investigated and it was found that the resistivity increased with irradiation. The measured resistivity was found to be a function of the thickness of the film, and this result was interpreted on the basis that the resistivity measured was an average or effective value of the mosaic of amorphous and crystalline CdS in the film, and was probably not a measure of the resistivity of the crystalline CdS.
The Precise Determination of Thermal Conductivity of Pure Fused Quartz
Authors: Sugawara Akira
Published: February 2004

The temperature dependence of the thermal conductivity of pure fused quartz is measured precisely over the temperature range 0° to 500°C. The results obtained are among the most reliable since the experimental accuracy is estimated to be ±1.5% and the purity of the fused quartz sample was 99.97%, exceeding the purity of the more than fifteen investigations previously published. A newly designed apparatus is used, employing a ``dual combined method'' in which thermal conductivity values are simultaneously obtained by both absolute and comparative methods. The thermal conductivity of fused quartz increases smoothly with increasing temperature.
Properties of Inductively Excited Ar+ Ion Lasers in High-Current Regions
Authors: Hattori Shuzo , Goto Toshio
Published: February 2004

Argon gas was filled in wide-bore tubes of 6 and 10 mm in diameters and excited inductivity with dc pulses of about 10 and 40 μsec durations. For five and two transitions in the 6 and 10 mm bore tubes, respectively, laser pulse waveforms and peak laser intensity vs peak current characteristics have been studied in detail. Laser pulses of the 4880 Å transition were obtained only in 40 μsec pulsed excitation at pressures above 30 mTorr. At a pressure of 15 mTorr, 10 μsec pulsed excitation was more favorable. The peak laser intensity of the 4765 Å transition was saturated but not quenched even at high currents and pressures in the 6 mm bore tube, although those of the other four transitions all were quenched. The phenomena are discussed on the basis of single- and multistep excitation processes.
Temperature Dependence of the Elastic Constants of Calcite between 160° and 300°K
Authors: Dandekar Dattatraya P. , Ruoff Arthur L.
Published: February 2004

Variation in the elastic constants of calcite in the range of 160° to 300°K has been investigated by the ultrasonic phase-comparison method. The values of the elastic constants and the elastic compliances of calcite vary linearly over the temperature range, and thus are in disagreement with the nonlinear behavior of the elastic compliances of calcite between 90° and 273°K reported by Ramamurthy and Reddy (R and R). It is suggested that this discrepancy in the two results is likely to be due to the inconsistent vibrational data of R and R and the impossible behavior of the compressibility of calcite with temperature as obtained from their estimates of the elastic compliance values of calcite. The present investigation also indicates that the variation in the volume compressibility of calcite is primarily due to the variation in linear compressibility of calcite perpendicular to its optic axis, and the temperature dependence of all the elastic constants except for C44 are explicitly due to the temperature change.
Impurity Effect in the Ionization Dilatation of Vitreous Silica
Authors: Primak William , Kampwirth Robert
Published: February 2004

The normal effect of ionizing radiation is to cause a contraction. Vitreous silica which colors in the visible (presumably because of its aluminum content) shows a very rapid radiation-induced expansion which soon saturates. Thus, depending on the impurity composition, the vitreous silica may show an initial decreased contraction or an initial expansion followed by subsequent contraction. Data are given for the optical absorption and the dilatation, for x-ray, gamma-ray, and electron irradiations. The dilatation was measured by interferometric and photoelastic methods, and for the latter, the method was further developed. The expansion was found to have a different distribution along the range of electrons than the contraction. Spontaneous annealing was found for the expansion, but not for the contraction. The thermal annealing of the dilatation and optical absorption were studied. In the coloring vitreous silica, the expansion annealed at a lower temperature than the contraction. The expansion and the contraction appear to be quite independent. The expansion is explained as caused by breaking Al–O links in the structure, the contraction by a compaction of the Si–O structure. Variation in absorption behavior associated with OH content is attributed largely to the kinds of centers formed in the vicinity of impurities rather than the number of radiation-induced atomic configurations which can form centers.
Reflectivity Enhancement of Semiconductors by Q-Switched Ruby Lasers
Authors: Birnbaum Milton , Stocker Tom L.
Published: February 2004

Reflectivity enhancement of single-crystal silicon, germanium, gallium arsenide, and indium antimonide and polycrystalline boron and cadmium selenide was observed. The reflectivity variations were produced by irradiating the semiconductor with the output of a Q-switched ruby laser (6943 Å) with pulses of 30 and 10 nsec. The reflectivity variations were monitored with a pulsed argon-ion laser (mainly 4880 and 5145 Å). The effect of surface damage at high power levels produced an irreversible reflectivity decrease. Polished samples of molybdenum and tungsten, when irradiated by the Q-switched ruby-laser light, exhibited irreversible decreases in reflectivity due to surface damage and reversible variations, which were attributed to elastic deformation. Reflectivity enhancement of semiconductors was attributed to the formation on the semiconductor surface of a liquid film with metallic properties.
Electrotransport of Tungsten and Life of a Filament
Authors: Peacock James C. , Wilson Alan D.
Published: February 2004

Small-diameter tungsten filaments operating at an initial temperature less than that for significant tungsten evaporation exhibit substantial electrotransport of tungsten which, it is shown, determines the life of the filament. It is observed that for small-diameter tungsten filaments heated by bidirectional current, the life exceeds the unidirectional current life by a factor of three or more. A 52 power scaling law is developed. The ``dc etch,'' constriction formation, and finger growth are interpreted by considering differential rates of migration on certain crystal planes.
Simple Demonstration of Voltages Generated by Vortex Motion in Type-II Superconductors
Authors: LeBlanc M. A. R. , Chang C. T. M.
Published: February 2004

A signal generated between the terminals of an open superconducting coil is made to vary by as much as ±50% by introducing a net temperature gradient when flux trapped in the body of the coil is thermally destroyed. The sign and magnitude of the variations are determined by the sign and magnitude of the net temperature gradient. The signal observed with no net temperature gradient present as T→Tc and the trapped flux is expelled arises from Faraday induction. The variations in the signal are attributed to preferential vortex migration towards the hot surface of the sample. A simple modification of the arrangement is described where the classical Faraday signals are made to cancel out and the flux motion signal alone is observed.
GeTe–GaAs Tunnel Junction: A Negative-Resistance Schottky-Type Barrier
Authors: Chang L. L. , Esaki L. , Stiles P. J.
Published: February 2004

Tunnel junctions are fabricated by evaporating GeTe on GaAs substrates. The current-voltage characteristics are similar to those in a Schottky barrier, except for the existence of a negative-resistance region. A simple model which predicts such behavior is analyzed to interpret the experimental results.
A Phenomenological Study of Orientation Fluctuations in Dipolar Media
Authors: Tewarson Shanti P.
Published: February 2004

Soleillet's equations of dipolar scattering have been found to support the concept of nonrandomness of orientation fluctuations. It has been found that the second parameter (m) in Soleillet's matrix, characterizes the nature of orientation correlations, which are random, rodlike, or disklike, depending on whether m is zero, positive, or negative, respectively. A correlation factor ρ=I+′/I∥′ has been proposed, for direct experimental determination of the nature of correlations, from the ratio of observed scattered intensities corresponding to crossed and parallel polarizer-analyzer pair. Comparison with similar results derived from Van de Hulsts' scattering matrix, for a cloud of very small particles, shows that Soleillet's second parameter (m) is a function of the polarizability components of the medium and therefore, can have both real and complex values. The experimental method has been shown to be useful in the determination of particle size, concentration, and polarizability, besides the determination of correlations as was done by Stein and others.
Numerical Analysis of Deformation Twin Behavior. Small Static Twin Lamellas
Authors: Marcinkowski M. J. , Sree Harsha K. S.
Published: February 2004

A detailed numerical analysis has been made of the energies and equilibrium configurations of deformation twin lamellae under various conditions of applied stress and twin boundary energies. Exact expressions have been used for the self-energies and interaction energies associated with the various partial dislocations comprising the twin lamellae using isotropic elasticity theory developed for circular glide dislocation loops. It has been found that as the applied stress and/or twin boundary energy is increased, the size of the stable twin lamella as well as the number of dislocation loops comprising it decreases. Accompanying this size decrease is a correspondingly smaller energy for activation. In this respect it is found that the critical step in the formation of a twin lamella is the activation of the first loop which requires the formation of a stacking fault. Subsequent loop nucleation on adjacent planes, which increases the twin thickness, occurs spontaneously.
Two-Stream Instability in Hot-Electron Systems of Many-Valley Semiconductors
Authors: Kakihana S.
Published: February 2004

Feasibility of observing growing two-stream instability arising from the interaction of hot electrons in many-valley semiconductors (germanium and silicon) has been investigated by the use of Boltzmann equation. The anisotropy of the band structure has been incorporated in the dispersion relation by successive coordinate transformations and by the Herring-Vogt transformation in crystal momentum space. Drift velocities, temperatures, and densities of individual valleys have been calculated by modifying the theory of Reik and Risken and using these values the dispersion relationships have been evaluated for the (1, 0, 0), (1, 1, 1), and (1, 1, 0) directions for germanium and for the (1, 1, 1) and (1, 0, 0) direction for silicon. The calculation based on the collisionless Boltzmann equation shows a positive growth rate for the (1, 1, 1) direction in germanium, but collision damping is so severe in the hot electron region that no net growth of instability may occur. In all other cases, both in germanium and silicon the waves are damped at all wavevectors even without collision damping because of Landau damping.
Auger Recombination in GaAs
Authors: Weisberg Leonard R.
Published: February 2004

Calculations have been made of both band-to-band and band-to-impurity Auger (nonradiative) recombination, using the formulations of Beattie and Landsberg, and Bess, respectively. It is shown first that the band-to-band Auger process cannot be neglected for high bandgap materials such as GaAs when heavily doped. Bess' formulation is applied for the first time to shallow traps. The calculations indicate that unavoidable Auger recombination processes dominate in heavily doped GaAs.
Mössbauer Spectra of 57Fe in Superparamagnetic Nickel
Authors: Lindquist R. H. , Constabaris G. , Kündig Walter , Portis A. M.
Published: February 2004

The Mössbauer effect is used to study the superparamagnetic behavior of ultrafine particles of nickel dispersed on a high-area substrate. In zero applied field, a characteristic hyperfine spectrum is observed at 78°K. As the sample is warmed to room temperature, the spectrum narrows to a single line as the result of rapid fluctuations in the magnetization direction. A fluctuation time of the order of 2 nsec is indicated from the room-temperature data. With the application of a magnetic field the hyperfine spectrum is developed at room temperature. Although it is not possible to assign a single relaxation time to the magnetic-field data, there is some indication that the fluctuations are slowed by the application of an external magnetic field.
Point Singularities in Micromagnetism
Authors: Döring W.
Published: February 2004

Solutions of the micromagnetic equations for the equilibrium distribution of magnetization may have singular points, at which no spontaneous magnetization exists, because the limit of the magnetization vector for an approach to this point on different rays has different values. The boundary conditions at these points are considered. The exchange-energy density goes to infinity at such a point, but the condition that its integral should be a minimum leads to no definite boundary condition. For a special class of such singularities, namely those at which every direction of the magnetization is accepted once on a small sphere around it, the conditions for a minimum of the field energy are derived.
Chlorine NQR in Antiferromagnetic PrCl3
Authors: Hessler J. P. , Carlson E. H.
Published: February 2004

Colwell and Mangum1 have observed a sharp peak in the specific heat of PrCl3 near 0.4°K which they interpreted as a transition to an AF state. We have observed the 35Cl nuclear quadrupole resonance line2 at 4.567 MHz to split below this temperature into two lines. The behavior in a small magnetic field is consistent with an Ising AF with the magnetic moment along the C3 direction, and implies a symmetry of P2c3̄ (antiferromagnetic chains). At the lowest temperature reached, 0.31°K, the splitting is 80 kHz. The projection of the local field at the Cl site on the xy plane of the principle axis system of the electrostatic field gradient is approximately along the bisection of y and z. The angle between the local field and x, C3, is approximately 30°. The magnitude of local field is about 500 G for a splitting of 80 kHz. Lineshape changes in the short range ordering region above TN are observed.
Proton Magnetic Resonance in Partially Deuterated Antiferromagnetic CuCl2·2H2O
Authors: Benoit H. , Drocourt J-M. , Legrand J-P. , Renard J-P.
Published: February 2004

We have studied the magnetic resonance of protons in antiferromagnetic CuCl2·2H2O when hydrogen is replaced by deuterium in a proportion c.

We show that the deuteration involves a diminution of the magnetization in the ground state and a diminution of the Néel temperature, both proportional to the deuterium concentration c.

We explain these effects by a diminution of the ferromagnetic exchange integral between next-nearest neighbors due to the difference between zero-point energies of hydrogen and deuterium.
Microwave Absorption near the Reorientation Temperature in Rare Earth Orthoferrites
Authors: LeCraw R. C. , Wolfe R. , Gyorgy E. M. , Hagedorn F. B. , Hensel J. C. , Remeika J. P.
Published: February 2004

Studies have been made on Tm, Er, and Ho rare earth orthoferrites in their spin-reorientation temperature ranges in an attempt to observe magnetic resonance. Magnetic loss peaks sharp with dc field H and temperature T (at constant H) have been observed. They form two well-defined linear branches when H at a loss peak is plotted vs T. One branch has H ∥ a and hrf ∥ c, and the other has H ∥ c and hrf ∥ a. In TmFeO3 the former branch extends from 82°K where H=0, to 86°K where H=7000 Oe (the maximum field available). The latter branch extends from 93°K where H=0 to 82.5°K where H=7000 Oe. The two branches thus cross each other. A remarkable feature is the almost complete lack of frequency dependence of the two branches from 5 to 54 000 MHz. Torque and magnetization measurements have shown that the two temperatures of the peaks for H=0 are essentially the same as the end temperatures of the spin reorientation range. It is proposed, preliminarily, that the loss peaks are associated with a maximum in the susceptibility which occurs when H is just large enough to pull the net moment parallel to the a or c axis.
Antiferromagnetic Resonance in Twinned Crystals of FeF3
Authors: Shane J. R. , Kestigian M.
Published: February 2004

Resonance studies at 70 GHz have been performed in crystals of FeF3 from liquid-helium temperature to 400°K. Below 367°K the field for resonance is anisotropic and temperature dependent. Because of a high-temperature cubic-to-rhombohedral phase transition, the observed spectra of the twinned samples consist of superpositions of the resonance lines resulting from single crystals with c axes parallel to each of the four 〈111〉 directions of the cubic unit cell. The angular dependence is well described by a two-sublattice model applied to each of the single-crystal orientations. The model contains symmetric and antisymmetric exchange interactions and a weak anisotropy field parallel to the c axis. Both high- and low-frequency modes have been observed. The effective fields used in the two-sublattice model have been obtained as functions of temperature from the resonance data at special applied field orientations. It is found that the c axis is a hard direction of magnetization between the Néel point and 251°K and changes to an easy axis at low temperatures. This effect is interpreted as a result of competing sources of anisotropy. Good agreement is found between the AFMR results and temperature dependences calculated using the molecular-field approximation.
Review of Magnon-Sideband Experiments
Authors: Sell D. D.
Published: February 2004

A new type of optical transition called a magnon sideband has recently been observed in several antiferromagnetic insulators. A general discussion of such transition is given and the experimental literature is reviewed. Sidebands have been identified by their line profiles, magnetic-field behavior, stress behavior, and temperature dependence. Examples illustrating the results of such experiments are presented. A number of unsolved problems and topics deserving further study are briefly noted.
Light Scattering by Spin Waves
Authors: Fleury P. A. , Porto S. P. S.
Published: February 2004

Light scattering by spin waves in the rutile-structure antiferromagnets MnF2 and FeF2 is reviewed. Emphasis is placed upon details of the experimental results and technique. The relations of first- and second-order spin-wave scatterings to the magnon dispersion relation are discussed. We show that for FeF2 the entire magnon dispersion curve can be inferred from results of a single light-scattering experiment.
Theory of Absorption and Scattering of Light by Magnetic Crystals
Authors: Moriya Tôru
Published: February 2004

Theories of absorption and scattering of light due to electric origins by magnetic crystals are surveyed. Electric dipole moments associated with ionic spins are discussed from symmetry point of view. Two-magnon absorptions are treated by using this spin-dependent electric dipole moment. Relative importance of various possible mechanisms giving rise to this moment are discussed. Light scattering by spin systems is treated by using the spin-dependent electric polarizability and a general expression is given for the scattering intensity. Symmetry properties and microscopic mechanisms of this polarizability are discussed systematically. One- and two-magnon scatterings as well as critical scatterings by spin systems are discussed in terms of this theory. Discussions are also extended briefly to the case of magnon-phonon absorption and the magnon sidebands or the magnon-exciton absorptions.
The de Haas-van Alphen Effect in Ferromagnetic Nickel
Authors: Stark R. W. , Tsui D. C.
Published: February 2004

The current results of our de Haas-van Alphen investigation of ferromagnetic nickel yield complete data for the [111] directed neck which is assumed to be associated with L2′ of the spin ↑ s band and for a set of X-centered pockets which have been assigned to the X5 spin ↓ d-band holes. The negative results of the experiment indicate that the X2 spin ↓ level is below the Fermi level and that the ordering of the L32 ↓ and L2′ ↓ levels is that recently proposed by Krinchik and Canshina.
Very Weak Itinerant Ferromagnets; Application to ZrZn2
Authors: Wohlfarth E. P.
Published: February 2004

The itinerant electron model is considered in the limit of very low exchange splitting energies. Magnetic isotherms giving M(H, T) are derived to cover a wide range of fields and temperatures. The isotherms are transformed so as to show that plots of M2 vs H/M at different temperatures give parallel straight lines. The differential susceptibility below and above the Curie temperature is derived as a function of T. The free energy corresponding to this model is calculated and several results concerning the specific heat of very weak itinerant ferromagnets are deduced. Some properties related to spin wave excitations are also considered. New experimental data on the material ZrZn2 are considered on the basis of the theory and shown to be in reasonable agreement with it. Several characteristic parameters of this material are deduced, for example a value of the exchange splitting energy 0.040 eV, and of the effective interation between the itinerant electrons 0.35 eV. The occurrence of very weak itinerant ferromagnetism in other substances is also discussed.
Temperature Variation of FMR Linewidth in Metals
Authors: Bhagat S. M. , Stevens H. O.
Published: February 2004

New measurements on FMR linewidths in high-purity single-crystal iron whiskers at 22 kMc/sec and 31 kMc/sec are reported at temperatures varying between 300° and 1000°K. It is shown that the observations can be accounted for by a relaxation parameter λ which is nearly independent of temperature. It is suggested that the low-temperature variation of the relaxation parameter in Ni and Fe may be qualitatively understood on the basis of spin-wave-conduction-electron interactions.
Spin-Wave Relaxation in Ferromagnetic CdCr2Se4
Authors: Bartkowski R. , Sage J. P. , Le Craw R. C.
Published: February 2004

The rather simple magnetic structure of the ferromagnetic insulator CdCr2Se4 makes it an attractive substance for the study of magnetic relaxation mechanisms in general, but in particular those mechanisms of importance in the vicinity of the ordering temperature. The ferromagnetic relaxation has been studied here by means of both FMR and high-power parallel-pump techniques. Using the latter technique, the intrinsic spin-wave linewidth was measured up to temperatures within a few degrees of the Curie temperature. Near the Curie temperature the linewidth diverges as (T0-T)−1 with T0=133°K. This value of T0 is in close agreement with our measured value of the Curie temperature 130°K. At low temperatures, up to 30°K, the relaxation appears to result from two-magnon coupling of the k=0 mode to modes of higher k, which relax by the three-magnon confluence process.
A New Mechanism of Linewidth Maxima in Ferrimagnetic Resonance
Authors: Kumar N. , Sinha K. P.
Published: February 2004

A new mechanism for magnon-phonon relaxation process in ferrimagnetic insulators (e.g., YIG) is proposed which involves direct coupling of the magnons of various spin-wave modes with phonons of the transverse optical modes. This direct interaction emanates from the near-zone magnetic field associated with the transverse optical modes of vibrations in ionic lattices. The dominant part of the interaction term has the form Hint=−i[ ∑ σtφσt+(bσt+βσ−h.c.)+  ∑ σt,λ ψσt,λ(bσt+βλαλ+σ−h.c.)],where φσt+ and ψσt,λ are appropriate coupling constants and bσt+, βλ and αλ the phonon-creation and magnon-annihilation operators of the respective branches. These mechanisms lead to a temperature-dependent linewidth of ferrimagnetic resonance exhibiting maxima at certain values of temperature. An order of magnitude agreement with experimental results on YIG is obtained for a reasonable choice of parameters involved. Spin-phonon interaction processes due to transverse optical modes have been shown to play a significant role in the paramagnetic relaxation of some doped ionic crystals.
Mechanism of the Uniform Mode Growth in Ferromagnetic Thin Films
Authors: Pascard Hubert , Berteaud André-Jean
Published: February 2004

It was experimentally observed that the rf pulse transmitted through a microwave cavity containing a ferromagnetic film with perpendicular polarization includes, in certain cases, a spike on the leading edge of the pulse. The presence of the spike cannot be ascribed to the growth of either thermal or parametric spin waves. The theoretical calculations of the rf magnetization during the transient period were made by taking into account the large values of the precession angle in a perfect disk shape and the time constant of the cavity-film ensemble. These calculations account for the experimental results and, in particular, for the absence of spike on the high-field side of the resonance curve.
Instability of Nonlinear Ferromagnetic Resonance
Authors: Monosov Ya. A. , Lisovsky F. V. , Surin V. V.
Published: February 2004

The results of a theoretical and experimental investigation of the problem of nonlinear ferromagnetic resonance (NFMR) are given. It is shown that the NFMR instability is a fundamental property and takes place with Bloch's mechanism of magnetic-moment relaxation. Characteristics of automodulation oscillations (which accompany the NFMR) are briefly described. It is shown that in parametric ferrite amplifiers (PFA) and limiters the NFMR instability leads to the occurrence of nonthermal noise, and in PFA causes the autosuppression of amplification. Observations are reported of ``spike'' electromagnetic emission and nonstationary retarded reradiation, which cannot be explained in the framework of previously developed theory.