bluuebunny/crossref_metadata_2025 huggingface dataset:
Improving the External Efficiency of Electroluminescent Diodes
Authors: Galginaitis S. V.
Published: April 2005

To maximize the external radiant output from noncoherent electroluminescent diodes, it is necessary to reduce or eliminate total internal reflections and absorption. Total reflection can be eliminated by appropriate shaping of the diode. A reflecting, plane-paraboloid structure is described, and an evaluation of its performance is given. It is shown that the high self-absorption in Ga(As,P) alloys can be reduced by using material in which the As/P ratio varies across the thickness of the structure. The radiation emitted from a junction fabricated on the As-rich surface will then be absorbed less because of the higher band gap of the rest of the structure. When these corrective measures have been applied, the output of noncoherent diodes approaches that of semiconductor lasers made from similar material.
Effects Due to Absorption of Laser Radiation
Authors: Ready J. F.
Published: April 2005

This paper describes methods by which one may calculate the effects of high-power pulses of laser radiation of specified shapes absorbed at opaque surfaces. First the heating effects at levels where no phase change is produced are treated by ordinary thermodynamics. The vaporization produced by Q-switched lasers should proceed differently from that produced by ordinary lasers and the two processes are treated separately. The amount of material vaporized by an ordinary laser pulse is calculated using conventional methods, while we adopt a phenomenological model to treat the case of the higher peak power pulses from Q-switched lasers. The results are in agreement with experimental data.
Saturable Optical Absorption in Phthalocyanine Dyes
Authors: Armstrong J. A.
Published: April 2005

Strong optical absorption lines have been almost totally saturated in two phthalocyanine molecules. A giant pulse ruby laser served as the source of intense resonance radiation. The qualitative behavior of the nonlinear absorption is in good agreement with theory.
GaAs Injection Laser with Novel Mode Control and Switching Properties
Authors: Nathan Marshall I. , Marinace J. C. , Rutz R. F. , Michel A. E. , Lasher G. J.
Published: April 2005

The effects of nonuniform current densities on the properties of GaAs injection lasers are investigated. The structure studied is an injection laser with a channel etched on the p side of the junction parallel to the reflecting ends. It is found that the threshold current is higher for nonuniform currents than for uniform currents. The mode in which the laser oscillates depends on the distribution of current. A simple model of the transition and the energy vs density of states for the semiconductor is presented to explain these effects. Bistable operation of the structure has been observed.
New Approach to the Perturbation of Cavity Resonators by Homogeneous, Isotropic Spheres
Authors: Brodwin M. E. , Parsons M. K.
Published: April 2005

An analysis is presented for calculating the perturbation of a resonant cavity by a homogeneous sphere characterized by arbitrary radius and scalar values of σ, ε, and μ. This method involves the expansion of the cavity fields in spherical vector potential functions. The solutions are subject only to the restrictions that the perturbation be small, and that the sphere be located so that reflection effects in the cavity walls are negligible. The solutions are, in general, series which converge very rapidly for sphere diameters less than λ0/(2π). The first term gives sufficient accuracy in most instances. Other existing approximate solutions are shown to be limiting cases of the solutions presented in this paper. A numerical comparison is presented for the case of a semiconductor sphere at the location of a field maximum.
Shock-Induced Luminescence
Authors: Coleburn N. L. , Solow M. , Wiley R. C.
Published: April 2005

When a 270-kbar shock wave emerges from an illuminated aluminum surface in a vacuum, blue light is emitted. This phenomenon is affected significantly by surface conditions. A chemically formed aluminum oxide surface layer on the metal luminesces brightly for at least 0.64 μsec. Normally oxidized aluminum surfaces luminesce less brightly for a much shorter time (∼0.01 μsec). The effect of nonoxidized material on the phenomenon is shown in that shocked gold does not luminesce. The luminescence from aluminum is attributed to electrons, dragged by the shock (acoustoelectric effect). The electrons reach the aluminum oxide layer and, falling into ionized F′ centers, emit blue light.
Measurements of Interaction in an Assembly of γ-Iron Oxide Particles
Authors: Torre E. Della
Published: April 2005

An experimental study of the statistical stability of the Preisach function is performed. The shape of the sample is varied in order to introduce an adjustable demagnetizing field to simulate various degrees of statistical instability. By choosing the proper shape it is possible to obtain a statistically stable distribution of interaction fields. Measurements of the standard deviation of interaction field show, just as calculations of the critical fields of the particles have shown, that the particles do not appear to switch as signal units.
Nature of Rectifying Junctions in α-Silicon Carbide
Authors: Griffiths L. B.
Published: April 2005

Rectifying junctions have been prepared in α-SiC during crystal growth, from chromium solution, over a temperature range from 1700°–1900°C. From an observed systematic variation of junction capacitance at zero bias, with growth temperature, an effective diffusion coefficient Deff was computed. The temperature dependence of Deff yields an activation energy of 4.8 eV. This value is compared to an independently determined value of 4.9 eV for the diffusion of aluminum in αSiC. It is concluded that the structure of these junctions is dependent upon interdiffusion of the major dopants and therefore upon the temperature of crystal growth.

The form of the experimentally determined forward current—voltage characteristics is shown to be in general agreement with the foregoing model.
Crystallization in Thin Films of Carbon and Silicon Monoxide
Authors: Fuschillo N. , Gimpl M. L. , McMaster A. D.
Published: April 2005

Thin films of carbon or of silicon monoxide were observed while heated to 1000°C inside the electron microscope. Spherical graphite nuclei, approximately 200 Å in diameter, appear at 200°C in the carbon films. These nuclei maintain their spherical form to a temperature of 600°C while growing to 1800 Å in diameter. At this size the spheres become unstable and convert to crystals of hexagonal symmetry. At 700°C the crystals revert to spheres 3600 Å in diameter. The silicon monoxide films form 150-Å nuclei at 350°C; no further changes are noted until at 800°C the film recrystallizes. The recrystallized silicon monoxide film is identified as β crystobalite. For both materials, the nucleation temperatures and the sizes and shapes of the crystallites at a given temperature depended on the thicknesses of the films.
Ultrasonic Attenuation in Al2O3
Authors: Desilets Brian H. , Meister Robert
Published: April 2005

Ultrasonic attenuation was measured in single-crystal aluminum oxide (Al2O3) using the pulse technique. Measurements were made at frequencies from 100 to 3000 Mc/sec and at temperatures 4° to 300°K. The observed ultrasonic losses are explained by a scattering from microscopic bubbles in single crystals grown by flame fusion.
Anion Diffusion in Crystals of NaCl
Authors: Barr L. W. , Morrison J. A. , Schroeder P. A.
Published: April 2005

Diffusion coefficients of the chloride ion in NaCl have been measured between 300° and 700°C using an isotopic exchange method. The concentrations of cation impurities were deduced from electrical conductivity measurements over the same temperature range. In the extrinsic region, the diffusion coefficients do not decrease with increasing amounts of cation impurity to the extent required for diffusion by single vacancies.

The general conclusion that diffusion occurs both by single vacancies and by vacancy pairs agrees with that reached earlier by Laurance. Analyses of the experimental results in the intrinsic region yield for single vacancy anion diffusion Da=1.1×e−1.92/kT cm2 sec−1 and for vacancy pair diffusion Dp=363×e−2.37/kT cm2 sec−1, and the activation energies, which are given in electron volts, agree well with theoretical estimates.

Large variations were observed in the anion diffusion coefficients for crystals doped with Ca++; the cause cannot be established definitely.
Stability of the Shape of a Solid Cylinder Growing in a Diffusion Field
Authors: Coriell Sam R. , Parker Robert L.
Published: April 2005

The stability of the shape of an infinite cylinder undergoing radial growth controlled by diffusion is studied by a method originated by Mullins and Sekerka (MS). It is found that the circular cross section of a cylinder is stable when its radius is less than and unstable when its radius is greater than a certain radius Rc. This result is analogous to the MS result that a sphere is stable below and unstable above a certain radius Rc, which is seven times the critical radius R* of nucleation theory. However, in the present case, the ratio Rc/R* is not equal to seven, but is a function of S=(c∞-cs)/(C-cs), where c∞, cs, and C are the concentrations of the solute at infinity, at the surface of the cylinder, and in the precipitate, respectively.

The case of perturbations in the radius along the length of the cylinder is also treated. Potential application of the result to such problems as the growth of branches on dendrites is discussed.
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.
Energy Levels of the Terbium Ion in the Iron Garnet
Authors: Huber D. L.
Published: April 2005

The effect of the interplay of crystalline field and exchange interaction on the energy levels of the terbium ion in the iron garnet is investigated. It is assumed that a near-crossing of the two lowest levels of the terbium ion is reflected in the existence of a giant anisotropy peak in the resonance spectrum. A simple model for the energy levels is introduced which leads to two criteria which determine the locations of the peaks. At a microwave frequency of 20.2 kMc/sec peaks are found when either (1) the average iron spin makes an angle of 71° with respect to the x axis of a local rare-earth site or (2) the iron spin is along a cubic [110] direction. With one exception these criteria account for all the major anisotropy peaks reported by Dillon and Walker. The near-crossings predicted by the model are shown to be a consequence of the rhombic components of the crystalline field, rather than an anisotropy in the exchange interaction.
Magnetocrystalline Anisotropy of Gallium Iron Oxide
Authors: Schelleng J. H.
Published: April 2005

Magnetocrystalline anisotropy measurements have been made on a sample1 of ferromagnetic and piezo-electric Ga2−xFexO3 (x being near unity) using a torque method. Torque curves in the a-c plane on an x-ray oriented single crystal indicate a large uniaxial anisotropy of the form E = K sin2θ, where θ is the angle between Ms and the preferred (c) axis. As the anisotropy field (2K/Ms) is larger than the external field presently available (11 kOe), the anisotropy values were obtained from measurements in which the angle between the external field and the c direction was small. The results obtained with this method show, for example, that the ``effective'' anisotropy field at 77°K is about 40 kOe. This directly measured value is to be compared with the values of 300 kOe and 16 to 23 kOe deduced from energy gap2 and susceptibility3 measurements, respectively. The small-angle measurements also yield magnetization values which are in substantial agreement with those previously reported. Although exact theoretical treatment must await determination of its crystallographic and magnetic structure, it is believed that the observed anisotropy can be explained by the presence of strong crystal fields at the Fe3+ sites.
Exchange Interactions Between Chromium Ions in Ruby
Authors: Kisliuk P. , Krupke W. F.
Published: April 2005

Twenty-eight of the 100 or so sharp lines observed in relatively concentrated ruby (∼0.5%) are assigned to exchange-coupled pairs of chromium ions in two different relative positions. It is shown that the splitting of the 4A2 ground state of the Cr3+ ion is described by an isotropic exchange interaction in both cases, but that the splitting of the R levels (2E in an octahedral approximation to the crystal field) cannot be described so simply, probably because of the crystal field distortion due to the substitution of a Cr3+ ion for a nearby Al3+ ion. Values for the coupling parameters in the lower states are deduced, the criteria for the assignment discussed, and the pairs tentatively identified.
Determination of Exchange Constants from Relaxation Specific Heat Measurements on GdCl3
Authors: Boyd E. L. , Wolf W. P.
Published: April 2005

Measurements of the magnetic contribution to the specific heat have been made on polycrystalline samples of GdCl3 at temperatures between 20° and 77°K, using a relaxation method with frequencies up to 40 Mc/sec. The results, when combined with the Weiss constant determined by Marquard and Wyatt, give two sets of values for the nearest- and next-nearest-neighbor exchange interaction between Gd3+ ions. The first set gives ferromagnetic values for both Jnn(+0.071±0.005°K) and Jnnn(+0.011±0.005°K), while the other gives a small antiferromagnetic Jnn(−0.020±0.011°K) and a larger ferromagnetic Jnnn(+0.041±0.002°K). These results are discussed in terms of other experimental data.
Complex vs Band Formation in Perovskite Oxides
Authors: Goodenough J. B. , Raccah P. M.
Published: April 2005

It is argued that there is a critical cation-anion covalent mixing parameter λc such that ligand-field theory is appropriate for λ<λc, but band theory must be used for λ>λc. This provides, therefore, a criterion for distinguishing metallic vs magnetic compounds in those structures, like perovskite, where cation-cation interactions are negligible. It is also argued that λσ>λc can be anticipated where the cations are in a low-spin state. The fact that LaNiO3 contains low-spin NiIII and exhibits no Jahn-Teller distortion suggested that λσ>λc in this compound. Metallic conductivity from −200° to 300°C and Pauli paramagnetism from 4° to 300°K seem to confirm this suggestion. Where λ≈λc, there is the possibility of a phase change in which λ<λc in some directions, λ>λc in others. LaCoO3 seems to illustrate this situation. It undergoes a transition at 1210°K, the cobalt ordering into alternate (111) planes of high-spin Co3+ and planes containing low-spin CoIII. Below 400°K the latter planes contain only CoIII ions. The magnetic Co3+ ions couple antiferromagnetically via Co3+-``diamagnetic CoIIIO6 complex''-Co3+ superexchange to give TN≈90°K.
Cooperative Jahn-Teller Distortions in the Systems ZnMn2O4–Cu2GeO4 and ZnMn2O4–ZnCuGeO4
Authors: Robbins M. , Baltzer P. K.
Published: April 2005

Ionic distributions and cooperative J-T (Jahn-Teller) distortions are studied in the systems ZnMn2O4–Cu2GeO4 and ZnMn2O4–ZnCuGeO4 which contain both Cu2+ and Mn3+ ions. Cation distributions were determined by x-ray diffraction for Zn0.5Cu0.5[Ge0.5Cu0.5Mn]O4 and by neutron diffraction for Zn[CuGe]O4. The ionic distributions for the two systems were then taken to be Zn1−xCux [GexCuxMn2−2x]O4 and Zn[GexCuxMn2−2x]O4. In the first system, the axial ratio c/a was found to depend essentially on the Mn3+ content only. The equal distribution of the Cu2+ ions over the A and B lattice sites results in the apparent neutralization of contributions from Cu2+ ions to cooperative J-T distortions. In the second system J-T distortions at room temperature were observed to much lower concentrations of Mn3+ ions than for the first system. However, in spite of most of the Cu2+ ions being on the B sites, the Zn[CuGa]O4 end of the system was found to be cubic at room temperature. The cation distribution was determined with x-ray diffraction for Ga[CuGa]O4 which is also cubic.
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
Lattice Vibration and Cubic-to-Tetragonal Transformation in Spinel-Type Chromites
Authors: Siratori K. , Tsuchida A. , Tomono Y.
Published: April 2005

Infrared transmission of NiCr2O4 and MnCr2O4 powder was measured from 15 to 500 μ and at 100°–400°K. Four absorption peaks were discovered at ∼600, ∼500, ∼400, and ∼200 cm−1. The peak near 200 cm−1 of NiCr2O4 showed a shift with temperature which is apparently proportional to the change of c/a, while the width remained constant. This fact seems difficult to interpret by the localized limit theory of cubic-to-tetragonal transition of crystals.
Determination of the Ferromagnetic Exchange Energy Constant in Ferromagnetic Ni and Ni–Cu Films by Means of Domain Wall Width Measurements
Authors: Hemenger P. M. , Weik H.
Published: April 2005

A method for measuring the ferromagnetic exchange energy constant A is being proposed. From A the exchange integral J, which is the energy due to exchange coupling between nearest neighbors, can also be determined. The method depends upon relating A to the image width of a Néel-type domain wall observed in an electron microscope by Lorentz microscopy. An expression was found which relates the constants of the film (the exchange energy constant A, the saturation magnetization Ms and the anisotropy constant K) to the domain wall width. Measurements were made in a Philips EM100B electron microscope on vacuum-deposited films of 200 Å thickness. Values of A and J for Ni were found to be A = 0.46×10−6 erg/cm and J = 5.0×10−14 erg. Due to a rapid decrease in wall image contrast as Cu was added, observations were made only up to 3.7 wt% Cu. In the range of zero to 3.7 wt% Cu, A shows an initial sharp increase up to 0.3 wt% Cu then decreases fairly gradually. J also has an initial sharp increase, but then increases only quite slowly above 0.3 wt% Cu.
Magnetization Creep in Thin Nickel—Iron Films
Authors: Olson A. L. , Torok E. J.
Published: April 2005

The initial magnetization creep threshold of vacuum-deposited thin nickel—iron films has been investigated as a function of film thickness, magnitude of the anisotropy field, and the amount of ``negative'' anisotropy present. The creep threshold, normalized to the transverse field where the probability density function is a maximum, shows a large improvement as the anisotropy field is decreased by uniformly straining the film; this process makes ``negative'' anisotropy regions appear. For isotropic films this normalized creep threshold is inversely proportional to the film thickness for thicknesses above 200 Å. The creep threshold for anisotropic films also decreases with increasing thickness, except for a dip in the thickness range where crosstie walls appear, 250–900 Å. Because the Bloch—Néel—Bloch transition occurs at larger transverse fields for thicker films, this mechanism is rejected. Another theory is proposed for a cause of creep that yields the correct thickness dependence: When the transverse field is zero, the net magnetic charge on the wall is zero. When a transverse pulse is applied, domains with M antiparallel to the applied dc field rotate more than do those with M parallel, creating a net magnetic charge on the wall. The stray field from this charge together with the applied field causes wall motion.
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.
Magnetization Ripple and Arctic Foxes
Authors: Callen H. B. , Coren R. L. , Doyle W. D.
Published: April 2005

A model of a polycrystalline magnetic film is considered in which the film is made up of transverse strips of (N) grains, with the magnetization M everywhere parallel within a strip but varying in direction from strip to strip. It is shown that the effective crystalline anisotropy in each strip can be represented by a transverse field Ht which varies randomly from strip to strip, where Ht (rms) = K/[2M(2N)½] (K is the crystalline anisotropy constant). It is recalled that random number sequences appear to have a periodicity of about 3.5 units, a phenomenon now recognized as having led to the assignment of a periodicity in the population of arctic foxes. A similar apparent periodicity of the transverse anisotropy field leads to an apparent periodicity of the magnetization ripple. Exchange forces, however, modify the amplitude and wavelength λ of the magnetization ripple when λ<λ0, where λ0=2π (A/Ku)½ is the ``relaxation length'' (A is the exchange constant, Ku is the induced anisotropy constant). When ripple is observed in the electron microscope, an additional instrumental factor must be taken into account. A simple treatment is presented which shows that for grain sizes a<λ0, the angular amplitude of the magnetization ripple θ(rms)≃(πa/λ0)½× {K/[Ku(2N)½]}∼1° and the apparent wavelength λ≃4.6a. Both of these values are in agreement with experiment. When a∼λ0/2π the approximation fails and the linearity between λ and a is destroyed, as observed.
Diffuse Paramagnetic Neutron Scattering in Chromium Spinels
Authors: Dwight K. , Menyuk N. , Kaplan T. A.
Published: April 2005

This paper describes an investigation of the diffuse neutron diffraction attributable to the paramagnetic state in normal, cubic spinels. The elastic neutron scattering is calculated from the high-temperature expansion of the correlation function 〈Si·Sj〉 as a power series in inverse temperature, the pertinent coefficients being evaluated elsewhere for a Heisenberg Hamiltonian with both nearest-neighbor A-B and B-B interactions. After normalization to the nuclear diffraction pattern, the computed diffuse scattering is compared with the experimental findings for both MnCr2O4 and MnCr2S4. Good agreement exists in both cases, despite the striking qualitative difference between these two patterns. This agreement supports the calculational model, and thereby answers earlier speculation concerning the origin of the liquid-type diffraction peaks observed in the room-temperature diffraction patterns of most chromium spinels. It is shown that these peaks are not directly related to any spin canting in the ground state. They arise from the basic antiferromagnetic nature of the exchange interactions, would persist in the absence of any B-B exchange, and can be suppressed by sufficiently strong ferromagnetic B-B interactions, as evidenced in MnCr2S4.
Spin Arrangement of the High-Temperature Magnetic Phase in Chromium
Authors: Brown P. Jane , Wilkinson C. , Forsyth J. B. , Nathans R.
Published: April 2005

Experiments with unpolarized neutrons on random domain single crystals are unable to distinguish between a simple spiral model and a transverse sinusoidal spin arrangement for the Cr magnetic structure above the low-temperature transition. Using a Cr single crystal which had been cooled through the Néel point in a strong magnetic field, we analysed the polarization of the diffracted beams from the {100} satellites on the three-crystal polarized beam spectrometer. The polarizations of the reflected beam in the various satellites predicted by the simple spiral model, independent of domain distribution were not observed. On the contrary our observations are consistent with each domain containing two transverse sinusoidal spin arrangements with unequal populations each oriented along a {100} direction. These inequalities, however, were far smaller (≈10%) than the inequalities in domain population (2:1) produced by the cooling field. We also observed that the preferred spin direction was that 〈100〉 direction most nearly perpendicular to the magnetic field direction, whereas the induced domain appeared to be related only to the shape of the specimen.
Some New Thin-Film Shift Register Designs
Authors: Spain Robert J. , Jauvtis Harvey I. , Fuller Harrison W.
Published: April 2005

Thin-film shift registers have been designed which utilize smooth controlled domain wall motion that occurs in tapered-edge magnetic films. Films used are long and narrow with an easy axis parallel to the short dimension. Information in the form of a sequence of domains of reversed magnetization is introduced serially at one end of an initially saturated film and propagated along the film by sidewise motion of the domain boundaries. Two shift registers are described which differ in the method of bringing the domain walls to predetermined positions during the shifting sequence. One method, denoted as graded coercive force, utilizes a pattern of local, rapid variations in the wall motion coercive force and a time sequence of uniformly applied drive fields to correctly position the domain walls at specific locations. The other method uses the nonuniform drive fields produced by a set of specially shaped conductors to step domains along a film of uniform wall motion coercive force. At present, the operating principles of graded coercive force devices have been verified and working models of shift registers utilizing nonuniform drive fields have been operated.
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>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>1 mV) some form of line sensing or structured cavity will be necessary.
Thickness Dependence of Creep Switching in Magnetic Films
Authors: Gagnon Gilbert P. , Crowther Thomas S.
Published: April 2005

The dependence of creep switching on film thickness has been measured in Permalloy magnetic films 200 to 2000 Å thick. No creep was observed in films thinner than 400 Å. With a 1-Oe (0.6 Hw) dc easy-axis field present, the transverse threshold field for many-pulse disturbing decreases an order of magnitude between 400 and 600 Å. Between 600 and 2000 Å, this creep threshold remains essentially constant. The practical significance in limiting the word line density in magnetic film memories and providing a writing mode for NDRO or band-switched memories are discussed.
Efficiency of High-Frequency Mixing in a Permalloy Toroid
Authors: Webber Ronald J. , Battarel Claude P.
Published: April 2005

A theoretical harmonic analysis is made of the output voltage from a small, etched Permalloy sheet core which is energized by the sum of two low-amplitude currents of different frequencies. The magnitudes of the two fundamentals, the sum and the difference frequencies, are predicted by the analysis. The two-frequency analysis of a hysteretic device is complicated by the multivaluedness of the input—output relation. The small signal behavior of a Permalloy core is further complicated by the lack of a model for high-frequency drive (∼1.0 Mc/sec). The analysis of mixing begins, therefore, with an experimentally measured high-frequency minor loop of a saturated core. The amplitude and frequency of the applied current is selected to insure nonswitching behavior. Once the flux—current relation is established, multiple Fourier analysis is used to calculate the output voltage waveform. The theoretical amplitudes of the frequency components are compared with measured values from a small Permalloy core (o.d.=23 mil, i.d.=17 mil, thickness=0.25 mil).
Precise Long-Range Linear Transducer Using Magnetic Techniques
Authors: Trechsel H. W. , Moe J. E. , Johnson C. E.
Published: April 2005

By precisely recording a magnetic thread on a magnetic bar such as Vicalloy, a linear standard of high precision can be fabricated. Accuracies of ±0.00001 in. are possible over several feet. Since there is no wear, recalibration is never necessary. Readout is done by a rotating, noncontacting nut, with an interspersed coil, of the same pitch as the magnetic thread. A phase detector compares this signal to the angular position of the rotating bar. This transducer can be used not only for precision linear measurement, but also as an element in a servo-control system.
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.
Determination of Magnetic Ordering in Heisenberg Magnets from High-Temperature Expansions
Authors: Kaplan T. A. , Stanley H. E. , Dwight K. , Menyuk N.
Published: April 2005

We investigate the possibility of determining the type of magnetic ordering to be expected for a Heisenberg model by using high-temperature expansion methods. (The latter have been addressed in the past only to questions about the model given the type of ordering.) We take as a criterion for the critical temperature Tc that the static correlation function 〈Si·Sj〉 becomes long range as the critical temperature is approached from above. This criterion is applied by looking for the ``generalized Fourier amplitude'' λα of 〈Si·Sj〉 that will diverge at a finite temperature. This ``generalized Fourier amplitude'' is essentially like the usual Fourier amplitude—for Bravais lattices it is precisely the latter, and it is suitably generalized for lattices with more than one spin per unit cell. In the special case where ferromagnetism is expected, the divergent λα is essentially the susceptibility. In general, the divergent λα is to be estimated by extrapolation from the first terms of its expansion in powers of T−1, in the spirit of the usual high-temperature expansion methods. This approach has been applied to normal spinels with nearest neighbor AB and BB interactions. Our preliminary results suggest that the approach will give reasonably definitive answers to the question of the type of ordering to be expected. Furthermore, they suggest that correlation corrections to the predictions of molecular-field theory can have a very large effect on the qualitative properties of the model, i.e., the type of ordering. The possible relevance of these results to the problems of CoCr2O4 and MnCr2O4 is mentioned.
Renormalized Spin Waves in Ferrimagnets
Authors: Mills Roger E. , Kenan Richard P. , Milford Frederick J.
Published: April 2005

The two-time Green's function theories of Heisenberg ferromagnets have been generalized to treat ferrimagnets with arbitrary numbers of sublattices. The Tyablikov method of decoupling has been followed. A theorem due to Callen has been used to simplify the theory and to obtain a physical interpretation of the results. The renormalized sublattice magnetizations are expressed as the solutions to a coupled set of implicit equations. A second such set of equations determines a common Curie temperature for the sublattices of the ferrimagnet. Either set of equations can be solved numerically by iterative procedures which yield self-consistent solutions. Renormalized spin-wave frequencies are obtained which enter the theory in a manner which is consistent with the fact that the perfectly ordered spin state is not, in general, the ground state of a ferrimagnet. The sublattice magnetizations and Curie temperature have been calculated for magnetite and are compared with available experimental data.
Intrinsic Lifetime of Spin Waves in an Ideal Metal
Authors: Thompson Eric D.
Published: April 2005

The spin-wave state, being an eigenstate of the approximate Hamiltonian only within the adiabatic approximation, has a finite lifetime in a metal. This lifetime is here calculated within the molecular field model (local and nonlocal fields) for long-wavelength spin waves and is shown to be long. The dominant process in determining this lifetime is the forward scattering of the d electrons out of the Fermi sea with the consequent reduction in the momentum and energy of the spin wave. The Pauli exclusion principle is responsible for this long lifetime by restricting this scattering process to those d electrons with energies near the Fermi surface. The intrinsic lifetime is found to be proportional to the sixth power of the wavelength of the spin wave.
Paramagnetic Behavior of Nickel just Above the Ferromagnetic Curie Temperature
Authors: Arajs Sigurds
Published: April 2005

Magnetic susceptibility of two nickel spheres of different purity has been studied between TF and TF+12°K, TF being the ferromagnetic Curie temperature. The susceptibility is proportional to (T−TF)n with the average value of n=−1.29±0.03. This result is slightly lower than that (n=−1.33) recently predicted theoretically for a three-dimensional Heisenberg ferromagnet.
Spin-Wave Theory of MnF2 in the Random Phase Approximation
Authors: Carleton H. R. , Brout R. , Stinchcombe R.
Published: April 2005

In order to extend the validity of ferromagnetic spin-wave theory to higher temperatures, Englert used the random phase approximation (RPA), in which the spin-wave interactions are represented by a model in which the number of spin-wave excitations are related self-consistently to the magnetization through a sum rule. We have applied RPA to the two-sublattice antiferromagnet of arbitrary spin and range of exchange interaction, and have found a formal correspondence to the Green's function theory of ferromagnetism of Tahir-Kheli and ter Haar. Using this approximation, the sublattice magnetization is computed self-consistently for parameters appropriate to MnF2 and compared over a wide temperature range to the sublattice magnetization predicted by the NMR experiments of several investigators. We find that the theory represents the significant features of the sublattice magnetization at low temperatures, and exhibits features peculiar to the temperature dependence of the anisotropy in that the magnetization is double valued in the region of the Néel point.
Double Exchange for Polarons
Authors: Smit J.
Published: April 2005

The transition of a metallic type of electronic conductivity to a semiconducting one, observed at the Curie point in several ferromagnetic compounds with mixed valency, is explained in the limit of heavy ions. It is shown that above the Curie point the susceptibility should obey the Curie—Weiss law.
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 (<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.