bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of the Characteristics of Detonation Waves in Real Materials
Authors: Boyer M. H.
Published: February 2004

The physical model and mathematical methods used in computing the behavior of detonation waves in real materials are described and used to study the detonation behavior of TNT. Data needed as input to the problem are the equation-of-state parameters of the reacted and unreacted phases and the reactionrate parameters. The equation-of-state data are derived from experimentally obtained Hugoniots of the explosive and from experimentally determined steady-state detonation velocities. Values for the rate parameters are obtained by matching the growth trajectories of computed waves with experimentally determined curves over a range of initiation pressures. Good agreement is found between calculated and observed reaction-zone thicknesses, and a trajectory computed for an initiation pressure significantly removed from those at which the rate parameters were evaluated showed fair agreement with experiment.
Theory of Strain-Induced Anisotropy and the Rotation of the Magnetization in Cubic Single Crystals
Authors: Bartel L. C.
Published: February 2004

A theory for strain-induced magnetic anisotropy energy in cubic single-domain single-crystal materials based on the usual magnetoelastic coupling model is given in this paper. The application of uniaxial strain such as produced by shock-wave loading induces an anisotropy field which can rotate the magnetization vector. Calculations based on the theory were made for various choices of the crystalline anisotropy constants and magnetoelastic constants; in addition, specific applications were made to nickel ferrite and YIG.
Electric-Field Distribution in Pulsed High-Current-Density Discharges in Helium and Helium-Hydrogen Mixtures
Authors: Allario Frank , Wainfan Nathan
Published: February 2004

The results of a study on high-current-density, pulsed glow discharges in He and He–H2 mixtures are presented. These results trace the temporal development of the electric-field and wall-potential distribution appearing between the electrodes of a discharge tube as the discharges approach dynamic equilibrium. These discharges, of 1.3 microsecond duration, are sustained in 2.5-cm-diameter pyrex tubes with interelectrode gaps variable between 10 and 40 cm. The pressure range runs from 0.50–5.0 Torr. As the discharge approaches dynamic equilibrium late in the discharge pulse, current densities of the order of 15 A/cm2 are maintained. The buildup of the electric field in the vicinity of the cathode is determined by time-resolved measurements of the Stark splitting of lines in the helium and hydrogen spectrum. The axial component of the electric-field and wall-potential distribution in the rest of the discharge are obtained from signals induced on a series of externally placed electrostatic ring probes. Fields as high as 78 kV/cm were detected at the cathode face, while uniform fields of the order of 10 V/cm Torr were found in most of the rest of the discharge. Comments are made on the transition region between high- and low-field regions in the discharge.
Influence of Lower Cutoff Frequency on the Measured Variance of 1/f Noise
Authors: Greenstein Larry J. , Brophy James J.
Published: February 2004

Previous experimental comparisons of 1/f and Nyquist noise in carbon resistors reveal that both are normally distributed in any time interval but that 1/f noise appears to have a nonstationary variance. The present paper examines the possibility that this fluctuation is caused by insufficient sampling of the 1/f noise signal. It is hypothesized that the 1/f noise is wide-sense stationary and the number of independent samples per measurement is found and shown to be far less than that for Nyquist noise. An experiment is then described in which the number of samples is sufficiently large to eliminate this source of variance fluctuation. The results exhibit the same spread in measured 1/f noise variance reported earlier, suggesting that an intrinsic nonstationarity does exist in this noise process when produced in carbon resistors.
Ionization Expansion of Pressure-Compacted Vitreous Silica
Authors: Primak William , Kampwirth Robert
Published: February 2004

In vitreous silica compacted by pressure at room temperature, a large proportion of the compaction is gradually released by ionization. This ionization expansion is shown to be a cooperative effect involving both radiation and thermal processes and is therefore termed a radiation-annealing effect. Data are given for soft x-ray radiation annealing at 15° and 100°C, and for radiation annealing after prior thermal annealing for 2 h at 200°C. The amount of compaction released (dilatation) is proportional to the radiation dose rate, nearly proportional to absolute temperature, rises rapidly from 0%–3% compaction, and then falls a little with increasing compaction. For vitreous silica compacted at higher temperature, the radiation-annealing rate is much reduced. The initial efficiency of the radiation annealing can be very high, ∼10 ionic oxygen volumes for each ionization. The effect seems to be related to transient ionization, which seems capable of releasing compaction over a radius of ∼8 ionic oxygen diameters. Accordingly, it is concluded that the infolding of the loops of Si–O tetrahedra in pressure-compacted vitreous silica is locked-in by overlapping strain fields which are unbalanced by transient ionization, and thermal annealing follows. This long-range locking-in may be absent in vitreous silica compacted by other means (e.g., ionization), and then the spectacular release of compaction by ionization does not occur. Other peculiarities of the effect are described and explained.
A Study of Charge Transport in a High-Pressure rf Discharge
Authors: Schwab H. A. , Manka C. K.
Published: February 2004

The purpose of this theoretical analysis is to show that certain properties of high-pressure rf gas discharges can be largely accounted for by a relatively simple model. An experimental investigation of rf glow discharges in air at atmospheric pressure in the frequency range 1–25 MHz has been made and a summary of the measurement results pertaining to this paper is presented. An analysis is made of the processes in an rf glow discharge which take place during approximately the first quarter of each half-cycle, while a cathode fall is built up in front of the negative electrode. The computations are based on a model plasma between two electrodes to which an external voltage is applied. This voltage is assumed to increase linearly with time, starting at zero. Thus it approximates the voltage increase in an rf discharge at the beginning of each half-cycle. The application of this voltage results in a current which can be both experimentally observed and computed. A numerical method to perform these computations on a digital computer is presented. The results of the computations are plotted and compared to the experimental values. It turns out that both the amplitude and the frequency behavior of the current through the plasma are in good agreement with experimental values, which the time dependence is not. Experiment shows the current to be constant while the computation yields a decrease with time. This decrease, however, is less than 1/t1/2.
Thermally Induced Strains in Diamond Cubic, Tetragonal, Orthorhombic, and Hexagonal Films
Authors: Witt F. , Vook R. W.
Published: February 2004

When thin films adhere to a substrate which has a different thermal expansion coefficient, changing the temperature of the composite will induce strains in both film and substrate. Calculations of strain and strain energy have been made for various polycrystalline diamond cubic, tetragonal, orthorhombic, and hexagonal films on substrates which are assumed to be undeformable and to have isotropic thermal expansion and elastic properties in directions parallel to the surface of the substrate. Computer programs were developed to calculate the above quantities for the diamond cubic semiconductors Ge and Si, the tetragonal metals Sn and In, the orthorhombic metal Ga, and the hexagonal metals Be, Cd, Co, Mg, Ti, Zn, and Zr. Each film was assumed to have been deposited on glass or on sodium chloride at room temperature and cooled to approximately liquid air temperature. The induced thermal strains and strain energies for many different crystallographically oriented grains were calculated. The particular (hkl) orientations chosen corresponded to the lowest order x-ray reflections from crystal planes parallel to the surface of the substrate. These calculations extend the authors' earlier work on adhering cubic metal films. Finally, in the case of nonadhering films having more than one thermal expansion coefficient, strains due to thermal expansion alone were calculated as a function of crystal orientation. Various applications of these results are discussed in the text.
Late-Stage Equivalence in Spherical Blasts as Calculated by the Method of Characteristics
Authors: Chou Pei Chi , Huang Shih Lien
Published: February 2004

The flow field of a high-pressure sphere expanding into atmosphere is solved numerically by the method of characteristics. In addition to the main shock, an inward traveling second shock is found to exist. This shock, together with the initial singularity, is treated successfully by special numerical techniques. Results from numerical calculations indicate a ``late-stage equivalence,'' i.e., approximately the same shock path and the same peak-pressure distribution at late times result from two different blasts, provided the initial total energy is the same for both.
Carrier Pulse Propagation under Double Injection Conditions
Authors: Fazakas A. B. , Friedman A. , Novacu V.
Published: February 2004

The behavior of a small carrier pulse in a semiconductor is investigated under space-charge conditions (double injection) with trapping and recombination effects, using the zeroth and first-order moments of pulse carrier densities. The general result—corroborated by a detailed study of two simplified models—shows a two-stage motion of the pulses: during a short time, they move in the drift direction of the corresponding individual carriers. Next, as in the (quasineutral) ambipolar case, the pulses move in a common direction, that of the individual minority carriers.
Elastic Surface Waves Guided by Thin Films
Authors: Tiersten H. F.
Published: February 2004

An isotropic, elastic solid permits the propagation of one straight-crested, nondispersive surface wave, called a Rayleigh wave. If a thin film of a different material is deposited on the substrate, an infinite number of straight-crested surface waves are possible, all of which are dispersive, including the one corresponding to the Rayleigh wave. Since the solution of the three-dimensional equations of elasticity is extremely unwieldy and the films may be considered thin in the frequency range of interest, the approximate equations of low-frequency extension and flexure of thin plates are employed in the solution to account for the motion of the platings. These approximate equations enable the entire effect of the plating to be treated as a boundary condition at the surface of the substrate. The accuracy of the approximation is shown to be excellent in the frequency range of interest. It is shown that platings which load the substrate reduce the velocity of straight-crested surface waves, while platings which stiffen the substrate increase the velocity. A condition is derived which tells whether a given combination of materials will lower or raise the surface-wave velocity. The solution for straight-crested waves in isotropic solids is extended to include waves with trigonometric and hyperbolic crests. An approximation technique is introduced, which enables the use of the dispersion curves for straight-crested waves in connection with the extended solution for waves with variable crests, in order to obtain approximate solutions for surface waves guided by thin films of finite width. The results of the calculation are shown to compare favorably with experiment. The analysis shows that strips of finite width may be used to guide surface waves only when the film loads the substrate and that slots of finite width in the film must be used when the film stiffens the substrate. It is also shown that while the strip configuration has a highly dispersive surface-wave velocity, the slot configuration has a large frequency range around which the surface wave velocity is essentially nondispersive. A formula for the coupling length between two weakly coupled, adjacent strips (or slots) is obtained by means of a perturbation technique, and shows that the coupling length increases rapidly with decreasing wavelength.
Optimum Second-Harmonic Generation in Nondegenerate Semiconductors using dc Electric Fields
Authors: Kaw Predhiman K.
Published: February 2004

In this paper the author has analytically investigated the use of dc electric fields of arbitrary strength, for the generation of second harmonics of high-frequency electromagnetic waves in a nondegenerate semiconductor. Boltzmann equation techniques have been employed to describe the motion of free carriers inside the semiconductor; the only form of carrier scattering considered is acoustic phonon scattering. The curve of second-harmonic intensity vs dc field exhibits a maximum, thus illustrating that there is an optimum value of the dc field for which the generation can be most efficient.
Micromagnetic Solutions for Ferromagnetic Spheres
Authors: Stapper Charles H.
Published: February 2004

Computer solutions are described for micromagnetic equilibrium conditions (Brown's equations). Computations relate to spherical particles and are made possible by assuming a constraint on the magnetization. Magnetic moment vectors are constrained to rotate horizontally with their angle of rotation varying in a vertical direction. This produces micromagnetic solutions of domains separated by Bloch walls. The study demonstrates that domains exist as equilibrium solutions and that the magnetization in each domain is not constant. Although these solutions do not produce uniformly magnetized regions, domains are identifiable because of their separation by sharp magnetization reversals. These transitions can be identified unambiguously as Block walls. Solutions exist for particles of different size, and the energies for double and triple domain states are calculated. The results of the computer study show the existence of critical radii. All computer solutions are checked for stability with a special stability matrix computed for this purpose.
Energy Distribution of Atoms Sputtered from Polycrystalline Metals
Authors: Stuart R. V. , Wehner G. K. , Anderson G. S.
Published: February 2004

The energy distributions of atoms sputtered from various polycrystalline metals by Kr+ ions of energies up to 1200 eV were determined using a spectroscopic time-of-flight method. Measurements of the Doppler shift of light emitted in the direction of travel by sputtered Mo and Ag atoms have verified the time-of-flight measurements. Average ejection velocities range from 4×105 (Ag) to 11×105 (Be) cm/sec; average ejection energies increase with the atomic number of the target atoms, while average velocities decrease. Higher sputtering yield materials tend to exhibit lower ejection energies. The crystal structure seems to play no important role in ejection energies. No evidence for focusing collisions could be derived from ejection energy data.
Microscopic Effects of Electric Breakdown in MgO
Authors: Shapiro Louis , Offenbacher Elmer L.
Published: February 2004

Lattice damage in MgO caused by electrical breakdown is investigated using electron-spin-resonance, photoelastic, and dislocation-etching methods. ESR line broadening is related to localized elastic stress distributions ``frozen'' into the crystal at the time of breakdown. The magnitude of this stress is limited by generation and movement of dislocations. The stressed region is generally cylindrically symmetrical and extends about 0.5 mm from the site of the breakdown puncture. The Mn2+ (32) and (52) lines typically show increases in line width of up to 50% from the initial values of approximately 2 and 3½ G, respectively.
Formation of Beaks at Dislocation Sites in Sodium Chloride Crystals
Authors: Babu V. Hari , Bansigir K. G.
Published: February 2004

Beaks, solution channels, etc., can be obtained on NaCl cleavage faces by an etchant consisting of methanol and cadmium chloride. The beak formation is described in detail and the mechanism for beak formation is discussed. Etching parameters, such as the concentration of the poison and understuration of the etchant, have been varied, and their effect on the beak formation is studied. At higher concentrations of the poison, as well as at low undersaturations of the etchant, beak formation was found to be predominant. Vigorous agitation of the etchant was found to suppress the beak formation. Beaks are always associated with the aged dislocations. The etch pits due to fresh dislocations do not have beaks, and therefore it is concluded that segregated impurities at the aged dislocations help in the rapid dissolution along the dislocations. If the dissolution rate along the aged dislocation VAD exceeds the velocity of steps VS across the surface, beaks can be formed. Etch pits associated with the beaks appear like funnels when observed from the side.
Implantation and Annealing Behavior of Group III and V Dopants in Silicon as Studied by the Channeling Technique
Authors: Eriksson L. , Davies J. A. , Johansson N. G. E. , Mayer J. W.
Published: February 2004

The implantation and annealing behavior of several Group III (Ga, In, Tl) and Group V (As, Sb, Bi) dopants in silicon has been investigated by studying the orientation dependence of the scattering yield of a 1.0-MeV He+ beam. This orientation dependence (``channeling'') provides quantitative information on the location of the foreign atoms in the lattice, and also on the amount of lattice disorder introduced by the implantation. The behavior of the Group V elements is relatively simple. In hot (350°–450°C) implants and also in annealed room-temperature implants, the dopant atoms are located mostly on substitutional sites, even for implant doses exceeding equilibrium solubility values by 1–2 orders of magnitude. Upon subsequent anneal, no evidence of precipitation is observed below ∼900°C. The behavior of the Group III elements is much more complex. In addition to a substitutional component, a significant fraction of the implanted atoms are located in the regular interstitial holes along the (111) atomic rows. The substitutional/interstitial ratio in hot implants is usually 1:1, except in the case of Tl implants where, on annealing to ∼575°C, a pure interstitial component is observed. Furthermore, the subsequent annealing behavior is found to depend markedly not only on the particular Group III ion, but also on the implantation temperature. Various factors that may contribute to the Group III behavior are discussed. Preliminary ion implantation studies with other elements (Cd, Sn, Te, Xe, Cs, and Au) in silicon have been included.
Theory of Domain-Wall Mobility in Ferromagnetic Insulators
Authors: Huang Huei-Li
Published: February 2004

The motion of a 180° domain-wall in pure yttrium iron garnet has been investigated, using the Winter-Janak domain-wall excitation spectrum. In particular, the domain-wall mobility as a function of temperature has been thoroughly studied. It is shown that the demagnetization energy due to the surface roughness of the sample where the domain-wall plane crosses the sample surface is the main source of resistance to domain-wall motion; the two-body part of the surface roughness causes transitions from a uniform translational domain-wall magnon mode to a degenerate domain-wall translational magnon mode, and is the predominant process that leads to the domain-wall motion relaxation. The four-body interaction intrinsic process contributes very little even at higher temperatures (about 10% of the two-body interaction at 300°K, much less at lower temperatures). The two-body process being considered here is, in some sense, similar to the Sparks-Loudon-Kittel calculations in which each pit is assumed to scatter independently; in the present case, however, we find it essential to take into account the mutual energy between pits. For a checker-board like surface corresponding to grit diameter R≈0.25 μ−0.30 μ(μ=10−4 cm), it is found that the domain-wall mobilities are 720, 740, 1200, and 5000 cm/sec-Oe at temperatures 4.2°, 77°, 195°, and 300°K, respectively. These numbers are of the same order of magnitude as the experimental results reported by Wanas and by Hagedorn and Gyorgy. We also show that the domain-wall mobility depends upon the size of the sample; the bigger the sample, the larger the mobility. Pseudodipolar fluctuation effects are negligible in YIG, but are important in ferrites with an inverted spinel structure. Assuming the pseudodipolar fluctuation field is about 105 Oe at 0°K, we show that this along with the surface roughness effect yields the same order of domain-wall mobilities as Dillon and Earl experimentally observed in manganese ferrite.
Voltage Dependence of Electroluminescence from GaP Diodes Prepared by Liquid Epitaxial Techniques
Authors: Kennedy D. I. , Koteles E. S. , Webb W. A.
Published: February 2004

Investigations are reported on the voltage dependence of red and green electroluminescence in GaP diodes prepared by the growth of an n-type liquid-epitaxial layer on a p-type solution-grown substrate. The green-emission intensity is found to be proportional to exp (eV/kT) for all the devices studied. The voltage dependence of the red-emission intensity for devices exhibiting a predominantly red visual appearance is found amenable to interpretation using the theoretical model proposed by Morgan, or that of Nelson. The anomalous behavior of the red-emission intensity in diodes with a predominantly green visual appearance permitted classification of the devices into three groups. The first group contained devices whose red intensity voltage dependence followed Nelson's theory. The dependence of the diodes in the second could be interpreted using Nelson's theory with some modifications. The third group of diodes possessed a red-intensity voltage dependence which was not amenable to interpretation in terms of available theoretical models. The green peak in diodes with a predominantly green visual appearance is attributed to the so-called ``A'' line resulting from excitonic transitions via impurity levels introduced by the isoelectronic substitution of nitrogen atoms on phosphorus sites.
Spectrum Analysis of Spin-Echo Signals in Iron-Doped Rutile
Authors: Bozanic D. A. , Mergerian D. , Minarik R. W.
Published: February 2004

X-band electron spin echoes have been obtained at 4.2°K in iron-doped rutile. The frequency spectrum of these echoes was observed on a conventional spectrum analyzer and compared to the spectrum of the input pulses. Not only is it possible for the echo frequency response to be a good replica of the input pulse response, but there is no observed distortion of the echo response vs delay time over the time interval investigated (5–50 μsec). This latter result of no frequency distortion versus delay time for the time interval investigated is in agreement with the fact that the spectral diffusion time constant (as measured by the three-pulse stimulated echo technique) was found to be no less than one millisecond. It is shown, however, that for two identical input pulses the turning angle can be no greater than 120° if good spectral reproducibility is desired. Echoes were also observed simultaneously at two different frequencies within the same resonance line and the same microwave cavity mode with no distorting effect of one echo upon the other; and at two different frequencies in different resonance lines and two different microwave cavity modes.
Influence of Irradiation Rate on the Production of F Centers in LiF (TLD Grade)
Authors: Ehrlich Margarete
Published: February 2004

Optically polished samples of extruded plaques of LiF (TLD grade) were given 60Co gamma-ray exposures ranging from about 35 000–450 000 R, at exposure rates between about 1.5×103 and 5×106 R/h. Over this range of exposures and exposure rates, the resulting absorption in the F band was found to be independent of exposure rate, and to increase with exposure, even for exposures for which the thermoluminescence exhibited saturation effects. After thermoluminescence readout at about 235°C, practically all the absorption in the F band created by the 60Co gamma-ray exposures had disappeared. These results are compatible with the dominant role given F centers in some of the current models for the thermoluminescence process in LiF (TLD grade).
Magnetic Structure Properties of Gd–Y and Gd–Sc Alloys
Authors: Child H. R. , Cable J. W.
Published: February 2004

The magnetic structure properties of several specimens in the gadolinium-scandium and gadolinium-yttrium alloy systems have been determined by neutron diffraction using the low-absorbing isotope 160Gd. Neutron patterns were taken from annealed powder samples at temperatures from 4° to 300°K. In both systems, the alloys show ferromagnetism at high gadolinium concentrations and antiferromagnetism at low concentrations. The concentration at which this change from ferro- to antiferromagnetism occurs is about 80 at.% for Gd–Y and about 70 at.% for Gd–Sc. The observed antiferromagnetic structures are spiral type with turn angles between the moments in adjacent layers which approach 50 deg. per layer at low Gd concentrations. The ordering temperatures decrease with decreasing Gd concentration more rapidly in the Sc than in the Y alloys. Thus these alloys behave similarly to the other rare earth Y and Sc alloys studied previously. Dilution of Gd by Y and Sc affects the anisotropy of gadolinium differently. An 80 at.% Gd in Sc alloy has a temperature variation of the angle the ferromagnetic moment makes with the c axis similar to pure Gd in which this angle is 0° at Tc, increases to 65° as the temperature is lowered, and then returns to about 32° at very low temperatures. An 80 at.% Gd in Y alloy, on the other hand, has its ferromagnetic moment about 70° from the c axis at all temperatures below Tc. In both systems, the antiferromagnetic spiral structure has the moments in the base plane perpendicular to the c axis.
Thermal Conductivity of Gadolinium, Terbium, and Holmium Single Crystals from 5° to 300°K
Authors: Nellis W. J. , Legvold S.
Published: February 2004

The thermal conductivities of gadolinium, terbium, and holmium single crystals have been measured as a function of temperature from 5° to 300°K. The steady-state heat flow method was used. For each element measurements were taken in the 〈112̄0〉 (a axis) and in the 〈0001〉 (c axis) directions of the hcp crystal structure. Electrical resistivity measurements were made on the same samples to obtain Lorenz functions. Anomalies near magnetic transition temperatures are interpreted in terms of magnetic superzones and spin-disorder scattering. High-temperature anisotropy is interpreted in terms of Fermi surface anisotropy. Lorenz numbers appreciably larger than L0 suggest that, in addition to electrons, phonons and magnons can carry heat.
Magnetic Properties of Er–Dy Alloys
Authors: Millhouse A. H. , Koehler W. C. , Child H. R.
Published: February 2004

Single-crystal neutron-diffraction measurements have been made on samples in the Er–Dy alloy system. In the region from 0%–30% Dy, the temperature at which a sinusoidally-modulated c-axis moment appears decreases and the temperature at which a helically-ordered basal-plane moment appears increases with increasing Dy content. These two transition temperatures become equal at ∼7% Dy. The temperature at which the cone structure appears increases from 0%–20% Dy and levels off at 34°K in the region from 20%–30% Dy. At 5% Dy, third-order harmonics appear before the transition to the cone structure, but these are relatively weaker than in pure Er and do not appear at all in an alloy containing 10% Dy. This weakening of the tendency for the c-axis component of the moment to square-up is associated with the decreasing magnitude of the c-axis component of the moment. At 4.2°K, the c axis and basal-plane components of the ordered moment extracted from intensity measurements are in good agreement with those calculated assuming that the Er3+ and Dy3+ moments are parallel within hexagonal layers. This would suggest that multimoment anisotropies dominate in this alloy system. At 4.2°K, the semicone angle increases linearly from 29° at 0% Dy to 43° at 30% Dy and is 90° at 50% Dy. Alloys containing 50% and 75% Dy show no ferromagnetic transitions to 4.2°K. An alloy containing 90% Dy transforms to a simple-ferromagnetic structure at 65°K.
Configuration of Sm in SmB6
Authors: Menth A. , Buehler E. , Levinstein H. J. , Geballe T. H.
Published: February 2004

SmB6 has been found to show no magnetic ordering down to a temperature of 0.35°K, in spite of the fact that the other rare earth hexaborides showed ordering in the neighborhood of 10–20°K.1,2 We interpret this to mean that the Sm ion is in its nonmagnetic divalent configuration at low temperatures. Direct evidence for the changing electronic configuration of Sm from trivalent to divalent with decreasing temperature has now been found by the observation that the size of the unit cell goes through a minimum near 150°K. The magnetic susceptibility, which has been measured between 1° and 800°K shows evidence for the presence of primarily Sm+3 above 300°K and Sm+2 below 100°K. Below 40°K the resistivity of SmB6 increases as T decreases with an activation energy of about 0.002 eV. Such semiconducting behavior is to be expected from divalent Sm in the hexaboride lattice.
Partial Quenching of Rare Earth Moment in Cubic Laves Intermetallic Compounds
Authors: McDermott M. J. , Marklund K. K.
Published: February 2004

Measurements of the low-temperature (0.1° to 1.0°K) heat capacities of several cubic Laves intermetallic compounds containing praseodymium indicate the presence of a substantial nuclear hyperfine contribution to the specific heat. It is possible to relate the magnitude of this contribution to the magnetic moment carried by the Pr3+ ion in each of the materials studied. The moments deduced in this way are PrNi2, 0.582±0.006; PrCo2, 2.18±0.05; PrAl2, 2.65±0.03 μB. These are each significantly less than the free Pr3+ ionic moment of 3.20 μB. The results are discussed in terms of a theory of Bleaney and it is concluded that the crystalline electric field is the same in PrCo2 and PrNi2, the large moment in the former being due to a larger exchange interaction. In the case of PrAl2, the data indicate an accidental degeneracy in the crystalline field ground state.
Magnetic Structure and Exchange Interactions in Cubic Gadolinium Compounds
Authors: McGuire T. R. , Gambino R. J. , Pickart S. J. , Alperin H. A.
Published: February 2004

The two groups of metallic compounds Gd3+X3− (X=N, P, As, Sb, Bi) and Gd3+Z2− (Z=S, Se, Te) have the rocksalt structure and except for ferromagnetic GdN they are antiferromagnetic. Powder neutron diffraction measurements on GdS, GdSe, GdSb, and GdBi show them to have order of the second kind. The exchange interactions are discussed on the basis of the magnetic structure and susceptibility measurements.
Theory of the Susceptibility of CeB6
Authors: Nickerson J. Charles , White Robert M.
Published: February 2004

Cerium hexaboride is an antiferromagnetic metal with a Néel temperature of 3°K. The high-temperature inverse susceptibility of this material is observed to decrease with decreasing temperature more rapidly than would be expected from a Curie-Weiss law. Since the temperature at which this deviation appears is much higher than the eventual ordering temperature, short-range ordering effects should be negligible. Also, it can be shown that crystal-field effects alone cannot produce such a decrease in χ−1. In this paper we offer a possible mechanism which explains this behavior. Basically, we assume that the Ce–Ce exchange arises through an indirect exchange involving the conduction electrons. We visualize the conduction electrons as moving in the spaces between the boron complexes so that the magnitude of the s-f exchange will be different for the different crystal-field states of cerium. In a cubic field the 2F5/2 state of Ce3+ splits into a Γ7 and a Γ8. Low-temperature measurements of the saturation moment indicate that the Γ7 state lies lowest. Therefore, as the temperature decreases, the Γ8 state, which protrudes into the space between the borons and, hence, contributes strongly to the s-f exchange, is frozen out, leaving only the weakly coupled Γ7 state.
Single-Crystal Growth of Some Chalcogenide Spinels
Authors: Von Neida A. R. , Shick L. K.
Published: February 2004

A new technique which has been useful in growing single crystals of a number of the chalcogenide spinels is described. Crystals ACr2X4, where A = Zn or Cd, X = S or Se, are grown by reacting compound AX and CrCl3 in a sealed and evacuated silica vial. The compounds, which are separated to suppress an interface reaction, are heated to 800°–1000°C maintaining about a 50°C gradient along the vial. Spinel crystals form at the low-temperature end of the vial embedded in a liquid phase which consists principally of ACl2. Commonly a number of crystals grow, usually separated by the liquid phase, but sometimes as a cluster of crystals. This technique yields larger crystals than have been obtained by previous methods (for CdCr2Se4 and CdCr2S4) in which the maximum size was reported to be 2–3 mm. The antiferromagnetic ZnCr2Se4 and ZnCr2S4 crystals have, to our knowledge, been grown for the first time. The growth mode is principally octahedral with well-developed (111) faces, although the larger crystals tend to be flat, truncated along [100] and [110]. Microscopic examination shows the crystals to be sound, containing few internal voids. The complex growth mechanism is discussed in general terms involving an initial vapor transport of the system components and the resultant liquid-vapor evironment required for crystal formation. The effect of some substitutional (A-site) doping experiments in CdCr2S4 crystals is reviewed.
Unique Behavior of Seebeck Coefficient in N-Type CdCr2Se4
Authors: Amith A. , Gunsalus G. L.
Published: February 2004

Electrical resistivities ρ, normal Hall coefficients R0, and Seebeck coefficients Q, are reported for a series of samples of the semiconducting ferromagnetic compound CdCr2Se4, doped with varying amounts of indium. The behavior of ρ and of R0 indicates that from room temperature down to 150°K the electrons deionize onto the donor centers. From 150°K down to about 55°K, the carrier concentration increases by approximately four orders of magnitude, accompanied by a proportional drop in ρ; this can be attributed to the known decrease in bandgap with lowering of T, if the donor level is assumed to remain constant with reference to the valence-band edge. The rise of ρ below 55°K is ascribed to impurity conduction. The mobilities reach a maximum near Tc, and fall off on both sides. Strong dependence of R0 on the magnetic field H is found above 110°K. The magnetoresistance is negative, and peaks sharply at 130°K; it obeys different power laws on the two sides of its peak. The Seebeck coefficient Q shows unique dependence on T, H and on In content: it displays local extrema at 150°K and at 125°K. Between these two temperatures, H has a strong effect on Q, depending on the In concentration. No simple relation between the behavior of Q on the one hand, and of ρ and R0 on the other, has been deduced. The model suggested by the data requires two bands, with charge carriers whose concentrations and mobilities are very different and have strong dependencies on T.
NMR in HgCr2S4: Dependence on Magnetic Structure
Authors: Berger S. B. , Budnick J. I. , Burch T. J.
Published: February 2004

The spinel HgCr2S4 has a spiral antiferromagnetic spin structure which can be ferromagnetically aligned by application of a moderate magnetic field.1 The presence of the nonmagnetic Hg and S ions affords an excellent opportunity to corroborate and investigate the details of the magnetic structure by study of the hyperfine interaction of these nuclei. We have observed the NMR of the 199Hg, 201Hg, and 53Cr nuclei in polycrystalline samples at 1.4°K. The Hg nuclei, which are situated in layers between aligned sheets of Cr spins, exhibit a magnetic anisotropy which disappears with ferromagnetic alignment. The spectral position and shape depend strongly on magnetic field. In zero magnetic field, the center of the Hg spectra is at 507 kOe, and that of the Cr spectrum is at 190 kOe. The intensities of the spectra are field dependent, and are observed to initially increase up to 5 kOe and then decrease in higher magnetic fields. An analysis of the Hg spectra has been made in which the magnetic anisotropy is shown to be a consequence of the spiral spin structure. The strong magnetic-field dependence of the spectra is found to be a result of the changing spin structure. A fuller account of this work will be presented elsewhere.
Exchange Interactions in Mn[Sc2]S4
Authors: Wojtowicz Peter J. , Darcy Lynne , Rayl Martin
Published: February 2004

One possible explanation for the rapid development of antiferromagnetism in the system MnCr2S4–MnInCrS4 is the existence of antiferromagnetic interactions among the A-site Mn. The normal spinel MnSc2S4 was prepared and studied to determine the actual magnitude of A-A interactions in sulfide spinels. The cubic lattice parameter is a=10.615 Å with u=0.381. Magnetic susceptibility measurements showed this material to be an antiferromagnet with a Néel temperature below 4.2°K and with a θ-parameter of about −21°K. The magnitude of the Mn–Mn exchange interaction was determined by fitting the susceptibility data from 15° to 295°K to the [3, 3] Padé approximant to the Rushbrooke- Wood series expansion. The value of the Mn–Mn exchange constant was found to be J/k=−0.80°K. The use of Padé approximants rather than the truncated series itself greatly improves the applicability of the power series expansions at temperatures comparable to or below the value of the θ-parameter.
Theory of the Metamagnetic Transition in FeCl2
Authors: Kenan R. P. , Mills R. E. , Campbell C. E.
Published: February 2004

A crystal-field calculation of the low-lying levels of a Fe2+ ion in FeCl2 shows that the lowest lying levels, a doublet and a singlet, are well separated from the remaining levels. The relative isolation of these levels suggest taking the effective spin as unity. The Hamiltonian incorporates a Zeeman term, an exchange term between spins S=1, and an anisotropy term. An RPA Green's function theory is derived, and, because of the highly anisotropic exchange, is reduced to an expansion about the molecular-field theory, in powers of the non-Ising exchange factors. It is found that even in MFT the metamagnetic-transition field has temperature dependence which follows well that of the zero-field sublattice magnetization.
Magnetothermal Studies of Phase Transitions in CuCl2·2H2O
Authors: Butterworth G. J. , Zidell V. S.
Published: February 2004

The antiferromagnetic spin flop transition in CuCl2·2H2O has been studied in the temperature region 1°–4.2°K by means of adiabatic measurements on a single crystal. The phase boundary obtained from isentropes in the H-T plane, which show a characteristic anomaly at the transition, conforms closely with that observed by NMR. Isentropic rotations of an external field in the ab and ac planes of the crystal confirm a flopping of the magnetic moments in the ab plane. When the applied field exceeds the critical spin-flop field by more than 50%, however, it appears that spin flopping no longer occurs in this plane. The forms of the isentropic curves are in general agreement with those predicted from the molecular-field model. Specific-heat measurements in applied fields yielded the paramagnetic phase boundary up to 22 kOe, the sharpness of the specific heat singularity being maintained in the highest field applied. The Néel temperature at H=0 was 4.357±0.010°K with the triple point at 4.31°K, 8.50 kOe. Comparison of the sublattice magnetization vs temperature curve derived from the specific-heat data with the predictions of the molecular field and Ising models suggests that neither of these models accurately represents the magnetic behavior of CuCl2·2H2O.
Perpendicular Magnetization of FeCl2 in High Magnetic Fields
Authors: Carrara P. , de Gunzbourg J. , Allain Y.
Published: February 2004

The low-temperature experimental study of the antiferromagnet FeCl2 in pulsed fields up to nearly 500 kOe has given evidence of a nonlinear behavior of the magnetization when the field is applied perpendicular to the anisotropy axis, the magnetization versus field increasing more steeply near the saturation than one would expect from the usual molecular field theory alone. A simple microscopic explanation of this phenomenon (which has already been observed in many antiferromagnets) can be put forward in this particular case: the essential in FeCl2 lies in the existence of comparable orders of magnitude for crystal anisotropy and exchange interactions. At 0°K, by using a crystal-field approximation (strong cubic potential and weak trigonal distortion), and a molecular-field approximation (Heisenberg Hamiltonian with two exchange parameters |JF|≫|JAF|), we get a nonlinear magnetization law before the saturation. The calculation has been carried out thoroughly only in the case where the ferromagnetic exchange is large compared with the crystal anisotropy. The value of the exchange and crystal-field parameters of FeCl2 lead to a satisfactory quantitative agreement between experiment and theory.
Spin Structure as a Function of Magnetic Field and Temperature in MnP Single Crystal
Authors: Komatsubara Takemi , Shinohara Hajime , Suzuki Takashi , Hirahara Eiji
Published: February 2004

The b-axis magnetization has been studied at various temperatures under various values of external magnetic field applied along the b axis, which is the intermediate direction of magnetization. The transformations to the fan structure from the screw and ferromagnetic spin structures have been observed at various temperatures. From these results a spin structure diagram is made as a function of temperature and magnetic field applied along the b axis.
Magnetically Induced Function of Heart and Bladder
Authors: Grob D. , Stein P.
Published: February 2004

Normal function was simulated in nonfunctioning heart and bladder of dogs by implanting ferromagnetic material on or in the walls of these organs and applying a magnetic field to produce compression against the body wall. A field of 0.1 T and a 0.05-T/cm gradient applied to three strips of C1010 steel (each 8.5 g and 5.1×2.5×0.08 cm), sutured to the base of the right ventricle of the 165-g heart of a 17.5-kg dog, produced normal left and right intraventricular pressures. At higher field gradients, less metal was needed. A magnetic field of 0.17 T and 0.11 T/cm applied to a steel strip of 0.22 g, or to 0.48 g of sintered iron on or in the posterior wall of the bladder of a 19-kg dog containing 75-cc saline produced a rise in intravesical pressure of up to 16 mm Hg, adequate for complete emptying. The forces produced by application of high magnetic fields to ferromagnetic material on or in dog heart or bladder appeared to be sufficient to restore function in man.
Occlusion of Intracranial Aneurysms by Ferromagnetic Thrombi
Authors: Roth David A.
Published: February 2004

The rupture of intracranial aneurysms is a major cause of strokes, but present operations to repair aneurysms and to prevent their recurrent rupture are plagued by high morbidity and mortality rates. A simpler and safer operation is proposed utilizing two permanent magnet-tipped probes introduced through a small opening in the skull and guided under x-ray control to lie near the aneurysm. The magnets attract ferromagnetic emboli injected into the cerebral circulation through the internal carotid artery in the neck, eventually filling the aneurysm. Following the formation of a blood clot which would reinforce the aneurysm wall against rupture, the probes would be withdrawn. Simulated aneurysms were occluded with three-micron carbonyl-iron particles injected upstream from two 5-mm-diameter, bullet-shaped Alnico V magnets applied at the aneurysmal opening. The same particles injected into the intracranial circulation of animals did not obstruct the capillaries and produced no detectable clinical, physiologic, or pathologic changes in the brains. A clinical trial using this technique was successful in occluding an aneurysm in a patient.
POD Catheter: A Means for Small Vessel Exploration
Authors: Hilal S. K. , Michelsen W. J. , Driller J.
Published: February 2004

A new magnetic catheter was developed and used for the exploration of small vessels in animals and humans. The operation of the device is based on the application of both alternating and constant magnetic fields.1 The mechanism of operation is illustrated in glass models using high-speed cinematography. The ability of the device to negotiate multiple curves and bifurcations is demonstrated. The design of the outside magnets is simple and compact, allowing easy maneuverability and reasonable power consumption. The catheter is less than 1 mm in diameter and is very flexible, being constructed of silicon rubber tubing. The device was used to selectively catheterize the middle cerebral artery from a percutaneous puncture of the carotid artery in the neck of a patient. The construction of the device and its mechanism of operation are presented. The diagnostic and therepeutic applications are discussed.
Sub-Kondo-Temperature Properties of Localized Moments in Metals
Authors: Daybell M. D. , Steyert W. A.
Published: February 2004

Experimental properties of Kondo-type systems in the very low temperature regime are examined using as examples both old and new data on the specific heat, susceptibility, and resistivity, primarily of copper-based third-row transition series alloys. Information about the T equal 0°K limit of the entropy and susceptibility of the Cu(Fe) system has been obtained, as well as data showing a specific-heat peak at 0.45°K in a Cu(Cr) alloy of 32 ppm concentration. The low field susceptibility of Cu(Cr) has been measured down to 15 mK. An interesting systematic variation in the zero temperature limit of the resistivity is pointed out.