bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theory of the Interaction of Vacancies with Stress Fields in Metals. II. The Interaction between Vacancies and Dislocations
Authors: Kuhlmann-Wilsdorf D. 
Published: April 2005

The theory developed in Part I is applied to the interaction energy between vacancies and dislocations for the case of elastic isotropy. In the elastic stress fields of dislocations, outside the dislocation cores, the vacancies are attracted into regions of compressional strain, as expected from the analogy with the Cottrell interaction between dislocations and substitutional atoms of a size smaller than the matrix atoms. Within dislocation cores, strains are much larger than in the elastic regions, but stresses are only moderately higher. Consequently, the two contributions to the binding energy which depend on normal strains rather than on normal stresses, namely, the electronic interaction energy and the change in the vibrational entropy, are much more important within cores than outside them and repel vacancies from regions of compressional strain. With the best possible estimates of core stresses and strains it is found that the electronic term is dominant in the cores at low temperatures, and the entropy term at high temperatures; therefore, vacancies are attracted into the dilated regions of dislocation cores with an interaction energy which rises linearly with the temperature.
In near-screw dislocations and in a range of mixed dislocations, the nominally ``compressed'' regions of the core are in fact dilated, because non-Hookean behavior, superimposed on the symmetric strains which may be extrapolated from conditions existing outside the core, results in an over-all dilatation. The interaction energy between vacancies and the cores of screw dislocations can therefore be significant, even though the contribution to it arising from the reduction of stored strain energy is small, and no Cottrell-type interaction energy exists. According to the above results, climb of edge dislocations and of a range of mixed dislocations is more difficult than expected on the basis of previous theories, inasmuch as the vacancies are repelled from the most compressed region of the dislocation cores, which they must traverse if a dislocation is to climb by the absorption or emission of vacancies. This repulsion is smaller and may even be replaced by an attraction in mixed dislocations with predominant screw character, depending on the angle between the Burgers vector and the dislocation axis, and would explain why climb of edge dislocations sometimes seems to be inhibited whereas mixed and near-screw dislocations climb. The interaction energies in the elastic stress field of dislocations at all temperatures, and in the cores at very low temperatures, is quite small, below 0.1 eV; at high temperatures in the cores, it may exceed 0.5 eV.
Magnetic and Chemical Properties of Europium Fluoride
Authors: Lee Kenneth , Muir Hugh , Catalano Edward 
Published: April 2005

The magnetic properties of divalent europium compounds have engendered considerable interest in recent years.1 The Eu2+ ion with an 8S ground configuration might be expected to exhibit magnetic ordering at low temperatures. The oxide, sulfide, selenide, and telluride all exhibit magnetic ordering. Magnetic susceptibility studies in powder samples indicated EuF2 to be apparently antiferromagnetic at 2°K.1 Interest in EuF2 is enhanced by the fact that it crystallizes in the fluorite structure and is stable toward both hydrolysis and oxidation below ∼500°K. The reported ordering, the structure, and the stability of this material prompted us to attempt single crystal studies of the magnetic properties of this compound.
Electron spin resonance at X-band frequencies was employed to detect the possible existence of antiferromagnetic resonance (AFMR) in a variety of single-crystal samples which were prepared under a number of different conditions. To our surprise europium difluoride is nonstoichiometric. At liquid helium temperatures, a number of the samples do show a weak AFMR, but always in the presence of a broad (peak-to-peak linewidth ΔH≃3–4 kOe) paramagnetic resonance. This broad paramagnetic resonance, which is typical for concentrated rare-earth compounds, broadens and remains down to the lowest temperatures investigated (≃1.6°K). In some of the samples, which came the closest to stoichiometry and purity, only the paramagnetic resonance was observed.
In the samples exhibiting AFMR, a transition temperature TN=19.5°K is observed. The strength of the AFMR signal was found to be proportional to the number of included crystal impurities. The sample having the largest concentration of included impurities had a mole ratio of impurity to host of the order of 10−5. The impurities are hexagonally shaped platelets with an average size of about 0.5×20×20 μ. They are oriented in the (111) planes of the host lattice. An observed uniaxial anisotropy and a direct dependence of the resonance field with frequency is compatible with the high-frequency mode of an uniaxial antiferromagnet. No demagnetizing factors are observed. An exchange field HE=4.84×104 Oe is calculated by assuming the fluorite structure and an ordering of the second kind. For T≪TN, (2HEHA)½=2510 Oe is determined and thus the anisotropy field HA=65 Oe is calculated. the 〈110〉 directions of the host lattice appear to be the easy directions.
These measurements indicate that the magnetically ordered inclusions are due to some form of divalent europium oxyfluoride and that the EuF2 host, independent of the exact composition, is paramagnetic down to 1.6°K. Although a simple uniaxial antiferromagnetic model gives good agreement with the experimental data, the possibility of a canted antiferromagnetic ordering cannot be excluded until magnetic susceptibility measurements are made on the inclusions.
The complete text of this investigation is being published elsewhere.2
Spatial Variation of the Magnetization across a Ferromagnetic Thin Film in the Bethe-Peierls-Weiss Approximation
Authors: Pearson J. J. 
Published: April 2005

The variation of the magnetization across a ferromagnetic film, because of the fact that a spin at the surface of the film has an environment different from that of an interior spin, has been calculated as a function of temperature. A previous calculation of this effect by Valenta used an approximation equivalent to the molecular field approximation for the bulk case. He extended this approximation to the spatially varying case by allowing the molecular field to vary across the film. Since the molecular field approximation gives a Curie temperature which is 60% too high for the bulk problem, it cannot be expected to give quantitatively correct results for the film. The present calculation introduces a spatially varying molecular field into a higher-order approximation, the Bethe-Peierls-Weiss approximation. In the bulk case this approximation treats one spin and its interaction with its nearest neighbors exactly. The rest of the spins are then approximated by a molecular field. The present approximation introduces a separate Hamiltonian for each plane of spins parallel to the film surface. For an interior plane, i, of a simple cubic crystal, this Hamiltonian is Hi=−J[2Si,0·(Si−1+ ∑ j=14Si,j+Si+1)+hi−1Si−1z+hi ∑ jSi,jz+hi+1Si+1z],where hi represents a molecular field, J is the nearest-neighbor exchange integral, and subscript j labels the nearest neighbors in plane i of spin Si0. At a surface plane the terms involving Si−1 are omitted from the Hamiltonian. The thermal average of Si,0z and Si,1z are calculated using S̄iz=Tr[Sizexp(−Hi/kT)].The field hi is determined by requiring that S̄i1z = S̄i0z. For a given value of kT/J a self-consistent set of hi's and S̄iz's is obtained using a computer.
The results of these calculations indicate that the finite thickness of the film has little effect on the temperature variation of the magnetization, reducing the Curie Temperature by only a few percent from the bulk value even for films as thin as eight layers The spatial fall-off of the magnetization at the surface is even more abrupt than in Valenta's approximation and almost independent of film thickness. For a film of twenty layers the magnetization fell at the surface to sixty percent of its value at the center and almost the entire drop occurred within four layers. The temperature dependence was found to be sensitive to the method of determining the molecular field.
A more complete account of this work will be published elsewhere.
Cavity Sensing of Cryoelectric Memory Planes
Authors: Burns L. L. , Sass A. R. 
Published: April 2005

Continuous film cryoelectric memory planes can be sensed by means of a superconducting line or a normally conducting sense plane located beneath the memory film. These types of sensing, known as line sensing and cavity sensing, respectively, have certain attributes and limitations which are discussed in detail. An expression for the sense voltage output is derived based on the change of the thermodynamic free energy of the memory during the read cycle. With this expression the sense outputs for line sensing and cavity sensing are calculated and compared with experiment. The signal outputs from cavities are compared with simple line sensing. Structured cavities, i.e., arrangements wherein the sense plane is multiply connected are discussed.
It is shown that at cold temperatures (T≈3.0°K) and thick memory films (a&gt;5000 Å), useful sense signal outputs can be obtained for cavities large enough to be used in practical computer memories. However, for large sense signals (Vs&gt;1 mV) some form of line sensing or structured cavity will be necessary.
Low-Field Hard-Direction Resonance in Uniaxial Permalloy Films
Authors: Ngo D. T. 
Published: April 2005

The resonance experiment for hard-direction bias fields below Hk has been hindered by the change in coupling between rf field M and pick-up loop as the bias field varies. To solve the problem, a new measurement technique is used. For each value of hard-direction bias field (HT), M is rotated to an angle θ with respect to the easy axis. A bias field Hθ is then applied parallel to M. For the above biasing conditions, the following resonance relation is derived: (1/4πM) (ω/γ)2=Hk cos2θ+Hθ which, for a fixed θ, is the equation of a straight line whose ω2 axis intercept corresponds to the solution of the standard resonance equation at the corresponding value of HT less than Hk.
An experimental apparatus is designed so that the coupling between rf field M and pick-up loop is constant throughout the experiment. For each value of HT, the orientation of M can be located precisely with a newly developed technique. Using the above techniques, the theoretical hard-direction resonance curve is experimentally verified for the first time.
Phase Transitions and Antiferromagnetic Resonance in Uniaxial Antiferromagnets
Authors: Callen Herbert B. 
Published: April 2005

In collaboration with F. B. Anderson and Yung Li Wang, we have investigated the spin-flop transition, the transition to the paramagnetic state, AFMR, sublattice magnetization, and susceptibility in the various phases for a simple Heisenberg antiferromagnet with uniaxial anisotropy. The various effects are sensitive to the temperature-dependent renormalization of spin-wave energies, and this renormalization is analyzed in some detail by a Green function analysis. The second-order transition from spin-flop to paramagnetic phases is directly proportional to the renormalization factor, and it is found that this varies as (1−2aT32+⋯) at low temperatures. In contrast to the ferromagnet (in which the magnetization varies as 1−aT32) the kinematical and dynamical—kinematical interactions add rather than cancel; the physical interpretation is discussed. In the antiferromagnetic phase two renormalizations (of spin-wave energies and of spin-wave moments) compete. This competition again gives a physical interpretation of the temperature dependence of the spin-flop transition and of the AFMR field. Temperature dependence of transverse susceptibility in the antiferromagnetic phase, field dependence of susceptibility in the spin-flop phase, and the spin-flop excitation spectrum are also discussed.
A full account of this work appears in the Physical Review1 and in the J. Phys. Chem. Solids.2
Preparation and Crystal Chemistry of Divalent Europium Compounds
Authors: Shafer M. W. 
Published: April 2005

The discovery that the europium chalcogenides are a particularly simple and important group of magnetic materials has resulted in considerable work being done on the synthesis of these materials. Several methods of preparing pure EuO have been investigated and the results are reported. The method which resulted in the best EuO is when an excess of europium metal is reacted with Eu2O3 at 800°C and then distilled off at 1150° in a high vacuum. Chemical analyses, microstructures, and magnetization measurements were used to determine the stoichiometry of EuO as a function of temperature. The results indicated that the rock salt phase can accommodate about 4 mole% trivalent europium in solid solution at 1300°C. Very little (&lt;1%) europium metal was found to go into the EuO phase, but by the addition of 5% to 7% the paramagnetic Curie temperature θ was increased from 73° to 79°K.
A number of new divalent europium compounds have been prepared and their magnetic properties investigated. Ferromagnetism was found in the silicates Eu2SiO4 and Eu3SiO5, the aluminates Eu3Al2O6 and Eu5Al2O8, and the phosphate Eu3(PO4)2.
Other divalent europium compounds in which the europium concentration is lower but which had simple well-known crystal structures were also investigated. These included EuTiO3, EuZrO3, and CsEuF3 with the perovskite structure, EuF2 with the fluorite, EuWO4 with the scheelite, Eu2CaWO6 and Eu2SrWO6 with the cryolite, and Eu2ZrO4 with the K2NiF4 structures. None of these compounds were shown to be ferromagnetic despite the fact that the nearest-neighbor Eu—Eu distances were less than 4.5 Å, the distance determined from the chalcogenide series to be the critical one for a positive ferromagnetic Eu—Eu interaction. The absence of ferromagnetism in these compounds is discussed in relation to the fact that, except in CsEuF3, the co-ordination number of the Eu+2 ion is not six as it is in the chalcogenides. The assumption that the ferromagnetic interaction takes place via overlap of 5d orbitals is considered and discussed in relation to their structures.
Superconducting ac and dc Properties of Lead-Bismuth Films
Authors: Garbuny M. , Gottlieb M. , Conroy J. 
Published: April 2005

Measurements have been made on the superconducting properties of thin films of lead-bismuth alloys ranging in composition from pure lead to 70% bismuth. These were deposited in 300-Å-thick layers on glass and polypropylene substrates. At T=4.2°K, Hc rises with increasing bismuth concentration to a maximum of 23 kG at 50% bismuth and then falls abruptly to zero for greater bismuth concentrations. At 1.9°K this maximum increases to 33 kG at 50% bismuth. Current densities of 107 A/cm2 were reached, corresponding to 7 A in a 2-mm-wide sample.
Measurements were made of the low-frequency ac superconducting losses by the L-C circuit Q method. Frequencies in the order of a few hundred cps were achieved by vacuum deposition of circuit elements several hundred feet long. The only dielectric material present was polypropylene whose dissipation factor was found to be 5.5×10−6 (Q=1.8×105). With this background loss the superconducting losses were barely detectable in lead-bismuth, with a lower limit on Q of about 106. Losses became large when currents were sufficiently high for the formation of normal regions. The onset of these losses correlated very well with dc resistance measurements. For the long circuit elements this resistive onset occurs at low current densities, about 104 A/cm2. Present work is aimed at raising the current-carrying capacity of the long circuits to that of the short samples, and measuring ac superconducting losses at high current densities.
Control of Texture in Magnetic Material by Surface Energy
Authors: Walter John L. 
Published: April 2005

Different crystallographic planes, at a gas-metal interface, have different surface energies. Although the energy difference is small (less than 100 erg/cm2), it is sufficient to cause migration of boundaries between grains of different (hkl) surfaces provided there is a sufficiently small restraint to boundary migration. Since the (110) plane is the plane of lowest surface energy γ in 3% silicon-iron, grains with (110) planes at the gas-metal interface are favored to grow at the expense of other (hkl) grains (γ110&lt;γhkl) when the material and annealing atmosphere are free of impurities. In the presence of certain impurities, however, γ100 may be lower than γhkl and grains with (100) planes are favored to grow at the expense of all others.
With proper processing, sharp (110)[001] or (100)[001] textures may be obtained in sheets of high-purity silicon-iron.
Grain size and texture can be altered by deliberate additions of oxide film to the surface, by the amount of silicon in the alloy, and by the choice of annealing atmosphere.
The effect of improved texture and low impurity concentration on magnetic properties is discussed.
Magnetic Properties of Mixed Metal Oxides Containing Trivalent Cobalt
Authors: Blasse G. 
Published: April 2005

In oxidic lattices the Co3+ (3d6) ion has been found both in the diamagnetic low-spin state and in the paramagnetic high-spin state. The factors influencing the equilibrium between high- and low-spin Co3+ ions are discussed.
New compounds are presented in which the change of the equilibrium distribution among these two states is measurable because the energy difference between the two states is small. It is then possible to change the equilibrium by varying the temperature. The temperature dependence of the magnetic susceptibility of such a compound is anomalous as has been derived by Ballhausen and Liehr.
Causes of the Inhomogeneity in the Magnitude of the Anisotropy Field in Thin Nickel—Iron Films
Authors: Oredson H. N. , Torok E. J. 
Published: April 2005

The inhomogeneity in the magnitude of the anisotropy field of vacuum-deposited nickel—iron films has been determined, from the data of Oredson and Torok, and from new data presented here, as a function of film composition. From this data, the contributions from the magnetocrystalline anisotropy of the randomly oriented crystallites, and from local uniaxial strains has been determined for various substrate temperatures and sources. For example, for evaporation from a ring source onto a substrate at 300°C, (δHb)2=(2.87×10−10)η2+ (1.25×10−8) K12+0.133. The constant term is attributed to shape anisotropy due to film-surface roughness and to the edge of the film and to variations in the magnitude of the anisotropy field due to temperature variations across the film during deposition. The contribution due to magnetocrystalline anisotropy and that due to magnetostriction plus local strain are roughly equal.
Hoffmann recently proposed a theory which gives the size of a domain, and shows that the effect of the randomly oriented crystallite anisotropies can be characterized by an effective anisotropy constant Keff=N−½K1 where N is the number of crystallites in a domain. Extending this theory, a similar relationship is found for the effective uniaxial strain due to the random strains. From this, a theoretical curve of δHb vs composition is obtained which is a fairly good fit to the data.
Range of Ferromagnetic Exchange Interaction in Nonmagnetic Films on Iron Substrates
Authors: Trousdale W. L. , Lindgren R. A. 
Published: April 2005

An experiment has been performed to measure the penetration of conduction electron spin polarization into nonmagnetic metallic films deposited on an iron substrate. The technique was to evaporate the metal in question on the iron and then to probe the effective exchange field, produced by the electron spin polarization, as a function of the distance from the substrate by evaporating an intermediate partial layer of 57Co. The hyperfine field of the 57Fe was measured by the Mössbauer method and the exchange was calculated from the hyperfine field under the assumption that the hyperfine field is related to the exchange field through the Brillouin function of the local spin. Measurements were carried out at 300°, 78°, 4.20°, and 1.22°K.
Samples using palladium on an iron substrate were investigated with the 57Fe placed from 20 to 120 lattice spacings away from the substrate with the following results: (1) The size of the exchange field varies from 0° to ∼150°K in the thinnest sample and from 0° to 25°K in the thickest. (2) For samples of given thickness, an empirical probability distribution function for the exchange field was found which gave a good fit to the hyperfine spectra at all temperatures. (3) The probability function is maximum at zero exchange field, decays initially quite rapidly, and then tails off slowly.
A few measurements have been made with copper and silver as the intermediate metal. The indication is that the effective exchange field in copper is greater than in silver but considerably less than in palladium.
Double Barrier Transmission Characteristics
Authors: Hosack H. H. 
Published: April 2005

A general expression for the time-independent transmission coefficient of a double barrier is derived. Unity double barrier transmission is found possible for all energy levels above or below the double barrier potential energy levels. The conditions under which unity transmission occurs are obtained and discussed.
Application of the general theory to a piecewise constant potential model indicates that double barrier effects in thin film devices may be most noticeable for a majority of current carriers at energy levels near the maximum potential energy level of the double barrier. The small width of the transmission resonances in the examples considered indicates that double barrier effects would probably not be detected, and if they were detected would probably not be interpreted as such, unless investigation of this particular phenomenon was an objective of the research being conducted.
An Ionization Process in a Low-Energy Cesium Plasma
Authors: Witting Harald L. , Gyftopoulos Elias P. 
Published: April 2005

The mechanisms of ion formation in cesium plasmas at relatively low electron temperatures (3500°K) and pressures of a few Torr are investigated. It is concluded that most of the ions are molecular and that they are formed by collisions between cesium atoms in the first excited state. The excited atoms are predominantly produced by inelastic electron collisions.
The excitation cross section is computed by means of the impact parameter method. It has a maximum value of 100×10−16 cm2 at 7 eV. The de-excitation cross section is computed and found to be approximately constant at 50×10−16 cm2. Radiation trapping and diffusion of excited atoms are also studied.
The ionization cross section is calculated by means of statistical thermodynamics from the measured recombination coefficient for the inverse of the ionizing process. It is found to be 1450×10−16 cm2. The plasma is uniform throughout its volume except in the immediate vicinity of the bounding electrodes.
The proposed mechanism results in performance characteristics for cesium thermionic converters, operating in the ``ignited mode,'' which are in qualitative and quantitative agreement with experimental data.