bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocations in GaAs17−xPx
Authors: Stringfellow Gerald B. , Greene Paul E.
Published: February 2004

Dislocations, their origins, and their effects on photoluminescence efficiency have been studied in GaAs1−xPx single crystals grown by vapor phase epitaxy. Dislocations were observed using etch pit, optical-transmission microscopy, and x-ray topography techniques. Two types of dislocations are grown into GaAs1−xPx epitaxial crystals. Near the GaAs (substrate)-GaAs1−xPx interface a high density (>108 cm−2) of pure edge dislocations with axes and Burgers vectors lying in the growth plane was observed. These originate from the lattice parameter mismatch between substrate and epitaxial layer. In the bulk GaAs1−xPx, a high density (∼106 cm−2) of dislocations was observed with 〈211〉 and 〈100〉 directions and Burgers vectors parallel to those of the mismatch dislocations. These dislocations originate from the mismatch dislocations at the substrate-epitaxial layer interface. The effect of dislocations on photoluminescence efficiency was studied by a novel technique. The photoluminescence was excited by focusing a He–Ne laser onto a 20-micron spot on the polished sample surface. The photoluminescence was observed through a microscope at ×400 magnification. It was found that the dislocations grown into the GaAs1−xPx are not intrinsically deleterious to the photoluminescence efficiency; however, in impure material, impurities segregate at dislocations causing a lowering of the luminescence efficiency in a region within one micron of the dislocation core.
The Time-Dependent Magnetic Fields of an Injected Helical Current
Authors: Cooper Richard K.
Published: February 2004

The electromagnetic field as a function of time in a conducting, infinitely long cylindrical tank into which a helical current is injected is considered. The longitudinal component of the magnetic field is treated in complete detail, and a plot of magnetic field vs time is given for the circularly symmetric case with parameters closely approximating the Astron configuration.
Infrared Detection by Optical Mixing
Authors: Kleinman D. A. , Boyd G. D.
Published: February 2004

A theoretical discussion is given of infrared detection systems employing an optically nonlinear crystal, a laser in the visible, and photomultiplier to detect the light produced at the sum or difference frequency. Three optical mixing systems are considered in detail and compared with direct detection: (a) cinnabar (HgS) in a single-pass optical system with the He–Ne 0.6328 μm cw laser, (b) the same crystal and laser with a ring resonator and narrow-band output filter, and (c) an ideal resonant system with a crystal as nonlinear as HgS but without absorption or double refraction. The noise output consisting of up-converted thermal noise and (in the case of the difference frequency) optical parametric noise is computed quantitatively. These systems have too small a quantum efficiency to compete with a heterodyne system employing an ir laser and a detector of high quantum efficiency such as a Ge:Cu-cooled photoconductor. The Ge:Cu detector has however a large dark noise compared to a good photomultiplier, and consequently optical mixing can surpass nonheterodyne direct detection if the required value of B≡(S/N)Δf is sufficiently small. The range of superiority of optical mixing over nonheterodyne direct detection extends high enough in B for Morse code for system (a) (if sum frequency is used), nearly high enough for a telephone channel for (b) and up to the television level for (c).
Zero-Crossing Statistics of 1/f Noise
Authors: Brophy James J.
Published: February 2004

Probability density distributions of the interval spacings between zero-crossings of band-limited Nyquist and 1/f noise signals are examined using an interval-to-pulseheight converter and a multichannel pulse-height analyzer. The Nyquist noise distributions follow a simple exponential law, as expected. In the case of 1/f noise, the distributions are approximately proportional to the inverse square of the spacing interval. The probability density is statistically stationary, that is, independent of sample length and independent of the sample selected. For signals in a 1 Hz to 5 kHz band the most probable spacing interval is 1.2×10−4 sec, which occurs with a maximum probability density of 2.2×103 sec−1. The most probable value is inversely proportional to the highest signal frequency present and the peak probability density is proportional to this frequency.
Transient Double Injection in Trap-Free Semiconductors
Authors: Dean R. H.
Published: February 2004

When a step voltage is applied to injecting contacts on the ends of a long trap-free semiconductor, such as p-type germanium, an electron-hole plasma forms at the minority-carrier injecting end and propagates down the length of the bar. When the recombination time (τ) is much longer than the small-pulse transit time (t0), the leading edge of the propagating plasma arrives at the far end at ta=(⅚)t0. The arrival is marked by a cusp in the time derivative of the current, which can be used to measure the minority-carrier mobility. After ta, the current acquires an exponential form, with a time constant equal to the recombination time. Approximate analytic solutions and numerical results indicate that the important recombination effects are small (<2%) for τ/t0 greater than about 2. Early-transient diffusion effects are small when (applied voltage/thermal voltage)1/2 is greater than about 40. All the prominent features can be described in terms of simple physical concepts, and the theoretical predictions are verified by experiment.
Transient Double Injection in Semiconductors with Traps
Authors: Dean R. H.
Published: February 2004

Deep traps can have important effects on the transient behavior of an electron-hole plasma injected into a long semiconductor. If the minority-carrier transit time is much shorter than all capture times (high voltage), trapping effects modify the trap-free transient after the first transit time by stretching out the transient and increasing the steady-state current. If the minority-carrier transit time is longer than the initial minority-carrier trapping time but shorter than all other capture times (intermediate voltage), trapping effects enter immediately. They reduce the velocity of the propagating plasma and cause the current to exhibit a long delay followed by a sharp rise. When the applied voltage is decreased further, the plasma velocity drops sharply toward zero because of recombination through the trapping centers. Properties for a typical bar of p-type InSb at liquid-nitrogen temperature are inserted in the appropriate formulas, and the results agree qualitatively with experimental data in the literature.
Step Motion on Crystal Surfaces. II
Authors: Schwoebel Richard L.
Published: February 2004

A theory for step motion on crystal surfaces undergoing growth, evaporation, and annealing is described. The capture and detachment of atoms at surface steps is considered to occur with arbitrary probability. It is shown that differences in the capture and/or detachment probabilities lead to important characteristics of the step motion. Surfaces with sinusoidally distributed steps will smooth or roughen during growth or evaporation in the same manner as predicted by macroscopic capillarity theory. The conditions under which the kinetic and quasiequilibrium considerations apply are given in terms of the capture and detachment rate constants.
Dielectric Properties of Unsintered Barium Titanate
Authors: Goswami A. K.
Published: February 2004

Dielectric properties of pure, unsintered and pressed barium titanate were investigated. The initial nonlinear variation of dielectric constant and dissipation factor with temperature observed is attributed to the water vapor adsorbed on the surface exposed to the pores. The absence of ferroelectric properties and the linearity in dielectric constant and dissipation factor with temperature found in a heat-treated sample are explained on the basis of a nonferroelectric surface layer. This was supported by the experiment showing the decrease in dielectric constant with decreasing particle size.
Surface and Bulk Waves on Axially Magnetized Plasma Columns
Authors: Bevc Vladislav
Published: February 2004

The dispersion relation and field solutions of slow electromagnetic waves propagating along a cold, collisionless, cylindrical plasma in an axial magnetic field of finite magnitude are studied numerically. Field distribution and gradual metamorphosis of surface waves into bulk waves along the dispersion characteristics are investigated. It is shown that both surface waves and bulk waves belong to the same set of plasma modes. Criteria concerning conditions under which surface waves occur are extended in include transitional situations. Power-flow density along the plasma may in certain cases be in opposite directions inside and outside of the plasma, both for forward surface waves as well as for backward cyclotron waves, implying transfer of energy between propagation media.
Evaporation of NaCl Cleavage Planes in Vacuum
Authors: Budke Jürgen
Published: February 2004

Cleavage planes of NaCl (99.999%) were produced in air and heated in vacua of 10−5−10−6 Torr between 454° and 593°C. Reproducible variations in pit densities were observed between 106 and 102 cm−2. Evidence is presented showing that these variations are due to changes in surface contamination. Matching cleavage planes showed no correlation in their pit sites; slip lines and small-angle boundaries showed no perferred pitting. It is concluded that dislocation emergence points per se are not sufficient for the formation of thermal pits in vacuum of 10−6 Torr. The evaporation process caused the development of (1) pits with different slope angles and (2) slope-dependent variations of their shape and faceting behavior. There was no rounding of leading or trailing edges between surface sections of different inclinations. These observations cannot be explained in terms of evaporation theories which have been suggested before. A different interpretation in terms of surface energy is given. A numerical estimate for the required variations in surface energy is included.
A Method for Growing Single Crystals of Metallic Indium Antimonide under Pressure
Authors: Dorer Gary L. , Bömmel Hans E.
Published: February 2004

A method is described for growing single crystals of the metallic β-tin phase of InSb. These crystals were grown from the melt at a pressure of 26 Kbars and recovered, in their metastable state, at liquid-nitrogen temperature. They were cylindrical in shape with lengths ranging from 6–20 mm and diameters ranging from 3–6 mm.
Impact Ionization of Filled Traps in Cadmium Sulfide
Authors: Simhony M. , Williams R.
Published: February 2004

Pulse measurements have been made of space-charge limited currents in thin layers of insulating CdS. These were prepared by controlled diffusion of a compensator into conducting n-type single crystals. Trapping centers were found at 0.4 eV below the conduction band in concentrations around 1015 cm−3 and at 0.6 eV in concentrations around 1014 cm−3. As the electric field was increased above 104 V/cm there was a change in the shape of the current transients. This effect was studied in detail and is attributed to impact ionization of carriers trapped in the 0.6 eV center.
Accurate Determination of Carbon-Foil Surface Densities
Authors: Stoner John O.
Published: February 2004

We have developed a method of accurate measurement of the surface densities of carbon foils prepared by vacuum evaporation, and have studied some of the properties of such foils in the range 0–70 μg/cm2. The method consists of burning a sample foil in a stream of pure oxygen, trapping the carbon dioxide thus formed, and measuring the pressure of the trapped gas in a known volume. We find that T, the optical transmittance at λ546 nm of a foil on a microscope slide substrate, indicates the foil's carbon content to within about 10% over the range 5–60 μg/cm2 and is given approximately by the relationship T=0.687 [exp(0.0911 σ) + 0.0273 exp(−0.0911 σ) −0.330 cos2π (0.089+0.055 σ)]−1, where σ=relative surface density with 1 μg/cm2 taken as unity. Such foils behave nearly as simple conducting layers having a complex refractive index 2.75+0.720i at λ546 nm. Our unbacked carbon foils have a noncarbon component with a surface density of 1.6±0.5 μg/cm2, approximately independent of foil thickness. This component may cause significant errors in other kinds of measurements of the thicknesses of foils prepared by evaporation.
Oxygen-Octahedra Ferroelectrics. I. Theory of Electro-optical and Nonlinear optical Effects
Authors: DrDomenico M. , Wemple S. H.
Published: February 2004

A microscopic tensor theory of the electro-optical and nonlinear optical effects in oxygen-octahedra ferroelectrics is presented. The theory stresses the importance of the basic BO6 octahedron building block in this class of materials. This common structural unit leads to similarities in band structure and similarities in polarization-induced, Stark-like energy band shifts. Using the polarization-potential tensor concept to describe these shifts, we relate the quadratic electro-optic g coefficients and the nonlinear optic δ coefficients to static and optical-frequency polarization-potential tensors, respectively. These tensors are found to be nearly the same in all oxygen-octahedra ferroelectrics, leading us to conclude that these materials possess the same fundamental electro-optical and nonlinear optical properties. The physical origin of both effects is shown to be related to polarization-induced modulation of the (pdπ) energy overlap integral. The resulting static and optical-frequency polarization potentials are found to be almost, but not exactly, equal. We also show that in the ferroelectric phase the linear electro-optic effect is fundamentally a quadratic effect biased by the spontaneous polarization, which enables us to calculate the important r coefficients. An analysis of optical-refractive-index-dispersion data shows that oxygen-octahedra ferroelectrics, and many other materials as well, have nearly the same dispersion behavior described by the parameter ε0/S0=6±0.5×10−14 eV·m2 where ε0 is an average interband-oscillator energy in eV and S0 is an average interband-oscillator strength defined by a single-term Sellmeier description of optical index data.
Oxygen-Octahedra Ferroelectrics. II. Electro-optical and Nonlinear-Optical Device Applications
Authors: Wemple S. H. , DiDomenico M.
Published: February 2004

Using the common polarization-potential tensors and the common refractive-index-dispersion parameter which characterize the electro-optical and nonlinear optical properties of all oxygen-octahedra ferroelectrics, we analyze in detail the device behavior of electro-optic modulators, light deflectors, and optical secondharmonic generators. General expressions for device characteristics are presented, existing materials are compared, and fundamental design limitations are emphasized. Important practical constraints related to drive circuitry and material-fabrication difficulties are also considered. It is shown that thermal considerations impose serious constraints on the power and bandwidth capabilities of modulators and deflectors. Based on the common refractive-index-dispersion behavior of oxygen-octahedra ferroelectrics, we show that in second-harmonic-generation applications there exists a general relationship between the crystal birefringence and the refractive index at the fundamental for collinear phase matching. This result, when combined with the common polarization-potential tensor, allows us to obtain a single expression for the harmonic power in which the only important material quantities are the refractive index at the fundamental and the crystal symmetry.
Medium-Coercive-Force Permanent-Magnet Alloys Based on the Co–Fe–Ti System
Authors: Chin G. Y. , Nesbitt E. A. , Wernick J. H. , Mendorf D. R.
Published: February 2004

The magnetic properties of Co–Fe–Ti alloys in the vicinity of 80% Co-11% Fe-9% Ti were investigated. Saturation magnetization decreases nearly linearly with Ti additions, extrapolating to a zero value at about 15% Ti. Values of coercive force up to about 60 Oe could be developed by precipitation aging of arc-melted alloys containing 8% Ti. Ductile alloys with Hc up to 45 Oe, however, were limited to 6% or less of Ti. For the latter, the coercive force was increased by cold-working prior to the aging treatment. Wire specimens of an 85% Co-12% Fe-3% Ti alloy exhibited excellent squareness and low magnetostriction.
Equilibrium Configurations of Dislocations under Stress: Comparison of Experimental Observations with Theory
Authors: Paré V. K. , Crump J. C.
Published: February 2004

Single crystals of copper were compressed and irradiated with fast neutrons while under load to pin dislocations in their stress-applied positions. Thin electron-microscope samples were prepared with surfaces parallel to the primary slip plane, and micrographs were taken of dislocations lying in this plane. The equilibrium shapes of these dislocations were compared with those predicted by the theory of DeWit and Koehler. A computer and digitally controlled plotter were used to generate curves representing the theoretical dislocation shapes, and these curves were plotted on transparent film for superposition onto micrographs of dislocations. The stresses thus measured on dislocations observed in samples compressed to 40, 100, and 500 gm/mm2 resolved shear stress were found to agree closely with the applied stress, even through a majority of the dislocations studied were in the vicinity of other dislocations, which might be expected to modify the stress acting on the dislocations studied.
Impurity Nucleation of Dislocations in Zinc
Authors: Deo P. G. , Marya S. K.
Published: February 2004

Single crystals of Zn containing 0.1% Bi were grown at different rates by Bridgman's method. The as-grown crystals were cut chemically with 10% solution of nitric acid to avoid plastic deformation. Chemical polishing of the crystals revealed cellular segregation and pox distribution of impurities. Crystals containing impurity substructures were chemically etched to investigate dislocation correlation with impurity substructures. Etching with Méléka's reagent gave oriented precipitates along cell boundaries. The formation of these precipitates is envisaged to be due to high local strains. This has been confirmed by etching with 5% solution of bromine in methanol. Individual etch pits were revealed at cell caps and cell boundaries. Dislocation density along the cell boundaries was more than at cell caps. Etching of the (0001) surface containing pox distribution of impurities revealed preferential clustering of dislocation etch pits around impurity-segregation sites. Dislocation content of inhomogeneous alloy crystals was 102–103 times larger than the dislocation content of pure Zn crystals. The role of impurities in dislocation nucleation is discussed along with other competitive dislocation-nucleation sources. Sufficient evidence of dislocation nucleation to relieve stresses set up at the composition-fluctuation boundaries has been obtained. Prolonged annealing of the crystals at higher temperatures produced partial homogenization of impurities. Chemical etching on the partially homogenized specimens revealed the dislocation arrays in homogenized regions and dislocation clustering at segregated boundaries. The array formation has been discussed on the basis of low strain energy of this configuration.
Mobility of Dislocations in Aluminum
Authors: Gorman J. A. , Wood D. S. , Vreeland T.
Published: February 2004

The velocities of individual dislocations of edge and mixed types in pure aluminum single crystals were determined as a function of applied-resolved shear stress and temperature. The dislocation velocities were determined from measurements of the displacements of individual dislocations produced by stress pulses of known duration. The Berg-Barrett x-ray technique was employed to observe the dislocations, and stress pulses of 15 to 108 μsec duration were applied by propagating torsional waves along the axes of [111]-oriented cylindrical crystals. Resolved shear stresses up to 16×106 dynes/cm2 were applied at temperatures ranging from −150° to +70°C, and dislocation velocities were found to vary from 10 to 2800 cm/sec over these ranges of stress and temperature. The experimental conditions were such that the dislocation velocities were not significantly influenced by impurities, dislocation curvature, dislocation-dislocation interactions, or long-range internal stress fields in the crystals. The velocity of dislocations is found to be linearly proportional to the applied-resolved shear stress, and to decrease with increasing temperature. Qualitative comparison of these results with existing theories leads to the conclusion that the mobility of individual dislocations in pure aluminum is governed by dislocation-phonon interactions. The phonon-viscosity theory of dislocation mobility can be brought into agreement with the experimental results by reasonable choices of the values of certain constants appearing in the theory.
Crystal-Lattice Images of End-On Dislocations in Deformed Aluminum
Authors: Parsons J. R. , Hoelke C. W.
Published: February 2004

Of particular interest in the theory of dislocations is the magnitude and extent of the perturbation to the atomic structure in the center of a dislocation and in its immediate vicinity. In the present work, two-beam crystal-lattice images have been used to directly examine the perturbing influence of end-on dislocations on the 2.34 Å spacing of (111) planes in deformed aluminum. The crystal-lattice images were obtained using a high-vacuum, short-focal-length (2.1 mm) Siemens Elmiskop 1 operated at 100 kV with pointed filaments. The experimental observations very clearly illustrate several fundamental properties of dislocations in crystals. As for the core region of a dislocation, the strains were observed to be nonuniform and high with a maximum of ∼0.15. A detailed examination of two end-on edge dislocations in a low-angle tilt boundary revealed dislocation core widths of 6.8 and 8.5 b. These widths agree favorably with the 7.3 b width predicted by isotropic-elasticity theory. Outside the dislocation core, the observed strain field agrees with that expected from elasticity theory, except for the existence of small-scale expansions and contractions of the lattice which are observed over a distance of 20 to 30 Å from the center of a dislocation.
Saturation Moments of Mixed Ferrites: A Simple Theory
Authors: Nowik Israel
Published: February 2004

The substitution dependence of the magnetic moment at 0°K [M (0, x)] of the ferrimagnetic system (ZnxFe1−x) [Me1−xFe1+x]O4 (Me stands for Mn, Co, or Ni) is calculated using a simple statistical local-molecular-field model. The model is able to reproduce quantitatively the experimental results for M (0, x) in the range 0.0≤x<0.7. The model has no continuously adjustable parameters but assumptions are made on the relative strength of A-B and B-B pair exchange interactions. Very good agreement with experimental observations is obtained if the following assumptions are made. For Mn and Fe in the B site, the antiferromagnetic A-B exchange JAB is more than six times larger than the average antiferromagnetic B-B exchange J̄BB. For Co, the inequality 3J̄BB<JAB<6J̄BB holds, and for Ni, the inequality 2J̄BB<JAB<3J̄BB holds.
Dislocation Velocity on the {1̄21̄2}〈12̄13〉 Slip Systems of Zinc
Authors: Blish R. C. , Vreeland T.
Published: February 2004

Dislocation velocity on the {1̄21̄2}〈12̄13〉 slip systems of zinc monocrystals was deduced from the rate of growth of slip bands. Near 77°K dislocation velocity is directly proportional to stress, and screw dislocations move more rapidly than edge dislocations. The difference between edge and screw dislocation velocity can be interpreted in two ways. The pre-exponential factors in a thermal activation model may differ by a factor 4 while the common activation energy is 0.21 eV, or the pre-exponential factors are the same, but the activation energy for edge dislocations (0.22 eV) exceeds that for screws by 5%. Other experiments will be required to establish the appropriate model. The authors favor the second alternative since extra activation energy might be needed to change the core structure of the edge dislocations (which lie on the basal planes) to make them glissile. Near room temperature, dislocation velocity decreases and cross-glide increases with increasing temperature. It is suggested that dragging dipoles and debris caused by their dissociation are responsible for the decrease in dislocation velocity. Finally, it is shown that the temperature dependence of both the yield strength and the plastic modulus is similar to the temperature dependence of the stress required to produce a constant dislocation velocity.
Anomalous and Ordinary Hall Effect in Terbium
Authors: Rhyne J. J.
Published: February 2004

The anomalous and ordinary Hall coefficients Rs and R0 defined by the relation ρH≡R0B+Rs4πM have been obtained for single-crystal Tb in both ordered and paramagnetic states. Results for the component of the Hall resistivity ρH characterized by H (and M) in the basal plane 101̄0 easy axis in Tb) show that ρH is linear in H above TN and exhibits a strong temperature dependence which is almost entirely due to that of the magnetic susceptibility. Rs is found equal to −40.4×10−12 Ω·cm/G and temperature independent. Below TN the anomalous coefficient found from the saturation Hall resistivity reverses sign, passes through a maximum near 140°K (where Rs=+22.4×10−12 Ω·cm/G) and falls to 0 below 30°K. Possible mechanisms for this unique sign reversal in Rs are discussed. The T-independent nature of Rs above TN plus a linear dependence of Rs on the derived magnetic resistivity below 120°K indicates that thermal disorder of the 4f spin system is the dominant scattering mechanism in the Hall effect. The normal coefficient is essentially temperature independent above approximately 260°K (R0=−1.0×10−12 Ω·cm/G) and below 90°K. At intermediate temperatures R0 rises to a positive maximum of +8.8×10−12 Ω·cm/G at 205°K. Hall resistivity data above TN with H along 〈0001〉 directions (second independent component in hcp symmetry) give significantly larger values of Rs and R0 reflecting the Fermi surface anisotropy.
Pressure Dependence of Ferromagnetic Phase Transitions of Chromium Chalcogenide Spinels
Authors: Srivastava Vishnu C.
Published: February 2004

The shift in the Curie temperature Tc has been measured as a function of hydrostatic pressure up to 10 kbars for the chromium chalcogenide spinels CdCr2Se4, HgCr2Se4, CdCr2S4, and HgCr2S4, by measuring the mutual inductance of an astatic coil assembly containing the sample. Also the compressibility of these compounds has been measured for the same pressure range at three temperatures. The measured values of Tc and dTc/dP for CdCr2Se4, HgCr2Se4, CdCr2S4, and HgCr2S4 are 128°, 105.5°, 84°, and 36.1°K and −0.82, −0.95, −0.58, and +0.14°K/Kbar, respectively. Combining the compressibility measurements with dTc/dP the values of dTc/da=+66, +99, +46, and −10°K/Å are obtained for CdCr2Se4, HgCr2Se4, CdCr2S4, HgCr2S4, respectively. These values of dTc/da for CdCr2Se4 and CdCr2S4 are in qualitative agreement with the dilution studies for these compounds. However, the value of dTc/da for HgCr2S4 is of opposite sign from that predicted from the solid-solution studies for this compound. High-pressure measurements seem to provide a better picture of variation of magnetic exchange interaction than dilution studies, because for the latter, the covalency effects become different due to changes in the nature of atomic environments. Conclusions are also made about the volume dependence of exchange interactions in these materials, and again it is quite different for HgCr2S4 as compared to the volume dependence of exchange interactions for the other three compounds. This seems to be consistent with the recent experimental and theoretical studies for HgCr2S4.
Phase Transitions in the Uniaxial Antiferromagnet
Authors: Rohrer H. , Thomas H.
Published: February 2004

We have studied the transitions induced by fields of arbitrary direction between the antiferromagnetic, the spin-flop, and the paramagnetic phase of a two-sublattice uniaxial antiferromagnet at zero temperature in molecular-field approximation. We find that the phase boundary between the antiferromagnetic and the spin-flop phase in the H plane extends only to a maximum angle with respect to the easy axis of order (anisotropy energy/exchange energy), where it ends in critical points. This behavior explains the sensitive angular dependence of ultrasonic absorption and magnetic resonance experiments.
Optical Measurements on Ferromagnetic Tb(OH)3
Authors: Scott P. D. , Wolf W. P.
Published: February 2004

The principal parameters characterizing the low-temperature magnetic and thermal properties of Tb(OH)3 have been inferred from measurements of optical fluorescence and absorption spectra of single crystals. Zeeman effect measurements in fields up to 36 kOe have determined g-values for various single ion levels; in particular g∥=17.7±0.2 for the ground state. This compares well with the value obtained from magnetization measurements and with the value predicted by a crystal field analysis based on a fit of the observed zero field energy levels. The onset of magnetic order near the Curie point (3.7°K) produces shifts and splittings of the observed spectra. At 1.3°K in zero external magnetic field, the ground state is split by 6.4±0.2 cm−1, which together with the observed g-value implies an effective internal field of 7.8±0.2 kOe parallel to the c axis. This value is close to other estimates based on bulk measurements and shows that the interaction is predominantly, but not exclusively due to magnetic dipole coupling. The existence of a small, additional antiferromagnetic interaction is inferred, and this is supported by the zero-field splittings observed in other levels.
Specific Heat of the Ferromagnets Tb(OH)3, Dy(OH)3 and Ho(OH)3
Authors: Meissner H. E. , Wolf W. P.
Published: February 2004

Continuing a study of the magnetic rare-earth hydroxides,1 we have measured the specific heats of hydrothermally crystallized powder samples of Tb(OH)3, Dy(OH)3 and Ho(OH)3 between 0.6° and 18°K. Sharp anomalies associated with paramagnetic-ferromagnetic phase transitions were found at 3.71° Tb(OH)3; 3.48° Dy(OH)3; and 2.54°K Ho(OH)3, in good agreement with the Curie temperatures deduced from magnetic measurements.1 Analysis of the specific heats for Dy(OH)3 and Ho(OH)3 is complicated by Schottky contributions from low-lying crystal-field levels, and in the latter compound also by a large hyperfine contribution, Chf. However, for Tb(OH)3 the ground state doublet is far below the first excited state which is at 170°K,2 and the magnetic specific heat Cm could therefore be isolated by a simple analysis of the ``high'' temperature measurements to determine the lattice contribution, and a small calculated correction for Chf at the lowest temperatures. The corresponding total entropy change is estimated to be (0.69±0.05) R, in good agreement with the expected value R In2. The highly anisotropic ground state1−3 makes Tb(OH)3 an unusually good approximation to an Ising ferromagnet, and this was confirmed by an analysis of the low-temperature tail of Cm. A value for the energy of a single-spin reversal at T=0°K is deduced: Δ0=8.4±0.3°K, in reasonable agreement with the value 9.2±0.8°K estimated from the integrated specific heat and with the value 9.4±0.2°K obtained spectroscopically.2 The reason for the apparent small discrepancy is not clear. Comparison of the experimental Δ0 with the calculated magnetic dipole contribution (Δ0)dip=11.8°K shows that the ordering in Tb(OH)3 is predominantly, but not exclusively, due to dipolar interactions. A similar situation was found for Dy(OH)3, where Δ0=8.8±0.3°K was determined from the low-temperature specific heat while the calculated (Δ0)dip=13.3°K. For Ho(OH)3 an accurate value for Δ0 could not be estimated, but it seems likely that the interactions are similar to those in Tb- and Dy(OH)3. Full details of this work are being published elsewhere.
Magnetic Forces for Medical Applications
Authors: Montgomery D. Bruce , Weggel R. J.
Published: February 2004

The force that a magnetic field gradient can exert on a ferromagnetic material can be utilized in a number of medical applications. This force is proportional to the spatial gradient of the ambient magnetic field times the magnetic moment of the material; the magnetic moment is a function of the magnitude of the ambient field and the geometry and material used. This paper discusses the importance of each of these factors and discusses methods for calculating magnetic fields and gradients produced by permanent magnets, electromagnets, and superconducting magnets. Examples of an electromagnet and a superconducting magnet for medical applications are presented.
High-Degree Electrostatic and Exchange Interactions in Rare-Earth Compounds
Authors: Birgeneau R. J. , Hutchings M. T. , Baker J. M. , Riley J. D.
Published: February 2004

In both rare-earth and transition-metal salts there may be appreciable interactions of high degree in the orbital or total spin angular-momentum operators acting between the ions. These interactions may arise either from the exchange interaction between the electrons or from the electrostatic coupling between the electric multipole moments of the two atoms. The latter in turn may arise either directly through the multipole coulombic field or indirectly through the coupling of the ions to the lattice. In transition-metal compounds the orbital moment is usually quenched, so that for interactions involving only the ground multiplet, the anisotropy introduced by the orbital effects is quite small. However, in rare-earth insulators the rare-earth magnetic moment has appreciable orbital content, so that the high-rank interactions are of considerable importance. In this paper we give a brief discussion of the theory of these interactions, and we then review recent experimental work on rare-earth compounds in which these effects have been shown to be important. Particular emphasis is given to: (a) experiments on YIG which indicate the presence of appreciable high-degree exchange; (b) experiments on the EPR of pairs of coupled Ce3+ and Nd3+ ions in LaCl3 and LaBr3; in these experiments it was possible both to measure the electric quadrupole-quadrupole interaction and to isolate contributions to the interaction tensors from high-degree exchange of the form J+5(1) J−5(2) and Jxl(1) J±5(2); (c) a series of experiments and calculations on UO2 which indicate the presence of appreciable electric multipole coupling via the lattice.
Effect of Correlation on Occurrence of Local Magnetic Moments in Metals
Authors: Roth Laura M.
Published: February 2004

A new Green's function decoupling scheme has been applied to the problem of how correlation affects the occurrence of local moments in metals. Using Anderson's one orbital model, the decoupling level is above Hartree but presumably too low to include the Kondo effect. The scheme is similar to Hewson's but includes certain energy shifts which make magnetic states more likely to occur. For very weak transfer interaction between the impurity and conduction electrons, both magnetic and nonmagnetic solutions are found. The nonmagnetic state usually has lower energy in agreement with Schrieffer and Mattis and others, but when the intra-atomic Coulomb energy is sufficiently large, the magnetic solution has the lower energy. For strong enough transfer interaction the magnetic solution, of course, disappears.
Magnetically Coupled Impurities in a Linear-Chain Heisenberg Ferromagnet
Authors: Erdos P. , Rhodes E.
Published: February 2004

Utilizing the technique of Green functions, the states of the linear-chain Heisenberg ferromagnet with a single substituted impurity are studied in the linear spin-wave approximation. The most important features are the appearances of localized states outside the spin wave band and resonant states in the band. For a ferromagnetically coupled impurity, the problem has been extensively treated. An approximate spin-wave analysis for the antiferromagnetically coupled impurity yields ground state energies up to 50% higher than those obtained in an exact solution of single-spin deviation states for the case of impurity spin ½. Allowing arbitrary host and impurity spins, exchange constants, and Landé factors, and an external magnetic field of arbitrary magnitude, we obtain the exact single-spin deviation states. This is achieved by using the fully aligned state as the state of no excitations. Because the spin-deviation excitation energies are negative for states below the spin-wave band for an extended range of parameters, a state of higher spin deviation can be the ground state. Considering the mixing of higher spin deviation states, a variational calculation of the ground state is being performed.
Neutron Diffraction Investigation of the Spiral Magnetic Structure in Cr2BeO4
Authors: Cox D. E. , Frazer B. C. , Newnham R. E. , Santoro R. P.
Published: February 2004

Cr2BeO4 is orthorhombic with the olivine (Mg2SiO4) type of crystal structure. Previous magnetic susceptibility, neutron diffraction and AFMR measurements have revealed that the compound undergoes an antiferromagnetic transition at about 28°K, probably to a noncollinear spin arrangement. More accurate neutron powder data have now been obtained at 4.5°K with 1.1 and 2.5 Å neutrons which reveal that the magnetic structure is a cycloidal spiral having a propagation vector directed along the c axis (Pbnm orientation). The spiral periodicity is about 65 Å, which is roughly 12 times the unit cell length in this direction. The observed intensities are accounted for reasonably well by a simple model in which there is modulation of a basic antiferromagnetic arrangement consisting of a G mode on the Cr(I) inversion sites coupled with a G mode on the Cr(II) mirror sites. The moments lie in the ac plane, and there is an additional phase factor of 90° between the moments of the Cr ions on the two kinds of site.
Magnetic Structure of CrAs and Mn-Substituted CrAs
Authors: Watanabe H. , Kazama N. , Yamaguchi Y. , Ohashi M.
Published: February 2004

A neutron diffraction study has been made of powdered samples of CrAs and Mn-substituted CrAs, namely, Cr0.7Mn0.3As, Cr0.4Mn0.6As, and Cr0.3Mn0.7As. Although CrAs shows a kink in susceptibility at 796°K, the anomaly was found to be due to a crystallographic transformation, from MnP-type structure (low-temperature side) to NiAs type. The liquid-nitrogen temperature diffraction pattern contains a number of magnetic peaks. An analysis of the data gives a double-layered helical structure, with the propagation vector along the c axis (the space group Pnma), like that found in MnP. The Néel temperature as determined from the temperature dependence of (101)− intensity is about 280°K. The magnetic structure of Cr0.7Mn0.3As is much the same as that of CrAs, but with the Néel temperature at 263°K. Cr0.4Mn0.6As is ferromagnetic with a Curie temperature at 160°K, and has a magnetic transition in the paramagnetic range, which separates the low-temperature low-spin state from the high-temperature intermediate-spin state. Cr0.3Mn0.7As is metamagnetic at low temperatures.
Magnetic Structure and Spin-Density Distribution in UFe2
Authors: Yessik M.
Published: February 2004

A study has been made by the diffraction of polarized neutrons of a single crystal of the ferromagnetic compound UFe2 at 84°K (T/TC = 0.49). The flipping ratios of all reflections out to a sinθ/λ value of 0.41 A−1 have been measured. These measurements show that, at this temperature, the Fe atom has a magnetic moment of 0.38 μB and that there is a small but significant moment of the U atom, parallel to the Fe moment, of about 0.03 μB. The spin density distribution at the U atom position is very nearly spherically symmetric, but the spin density at the Fe site is seen to have large deviations from spherical symmetry, being elongated in the direction of the cube edges and contracted along the nearest-neighbor connecting line. Good agreement between observed and calculated structure factors is obtained by using a shallow form factor of the 5f3 type for the U moment, and by using an Fe form factor calculated on the basis of 65% of the magnetic electrons having Eg symmetry and 35% T2g symmetry.
EPR Determination of the Nearest-Neighbor Exchange Constant for Mn2+ Pairs in KZnF3
Authors: Krebs J. J.
Published: February 2004

The electron paramagnetic resonance of exchange-coupled pairs of Mn2+ ions has been observed in Mn2+ doped KZnF3, a cubic perovskite. The Mn/Zn ratio used was approximately 0.01. The angular dependence of the pair spectrum indicates that the pair axes are along the three equivalent 〈100〉 directions. Thus the pairs are nearest magnetic neighbors and are expected to be antiferromagnetically coupled. At 4.2°K, only the S = 1 and S = 0 states are significantly occupied and the spectrum which arises from pairs in the S = 1 state is adequately explained by the spin Hamiltonian H=gβH·S+DSz2+AS·(I1+I2),with g = 2.004, | D | = 2.996 kG. From the linewidths of the unresolved hyperfine sets, A∼50 G is found. For a nn Mn2+ pair separated by the normal Zn–Zn distance (4.06 Å) in KZnF3, the dipolar contribution is expected to be D = −3.11 kG, indicating that any local distortion is small. From the temperature dependence of the S = 1 lines in the region 1.5°–25°K, the exchange constant Jnn (Hex=JS1·S2) is found to be +9.9°±0.6°K. This is compared with macroscopic measurements of J in antiferromagnetic KMnF3 and optical measurements in KZnF3:Mn.
Calculation of Some Ytterbium-Oxygen-Iron Superexchange Integrals
Authors: Pearson J. J. , Herrmann G. F.
Published: February 2004

Past calculations of the anisotropic superexchange interaction between Yb3+ and Fe3+ in the garnets have made use of drastic assumptions about the form of the exchange potential. A more fundamental calculation has been impossible until now because of a lack of detailed knowledge of the relative magnitudes of the superexchange integrals. We have calculated the nineteen exchange integrals between the different combinations of O2− p orbitals and Yb3+ f orbitals. The O2− wave functions were taken from Watson's analytic Hartree-Fock calculation and the Yb3+ wave functions were calculated using a Hartree-Fock computer program of F. Herman. The values of the integrals span a range of three orders of magnitude. They exhibit a strong dependence on the azimuthal quantum numbers m and m′ of the oxygen p orbitals along the bond, and a much weaker dependence on the f-electron quantum numbers. The integrals involving m = 0 and m′ = 0 clearly predominate, and those involving m = 0 and m′ = ±1 are smaller by almost an order of magnitude. The transfer integrals and the implications of these results for the form of the exchange potential are discussed.
Low-Temperature Magnetization of Cu(NO3)2·2.5H2O
Authors: Myers B. E. , Berger L. , Friedberg S. A.
Published: February 2004

We have measured magnetization isotherms for single crystals of monoclinic Cu(NO3)2·2.5H2O between 1.2° and 4.2°K by a ballistic method. Fields up to 43 kOe were applied along the b axis, i.e., the long axis of the needlelike specimens. The character of the M vs H curves changes rapidly in this temperature interval. They are described in first approximation by a model of this salt in which Cu+ + ions are coupled by isotropic antiferromagnetic exchange, −JS1·S2, in isolated pairs. The best fit of the data with the isolated cluster model yields J = −5.23k and g = 2.23, in reasonable agreement with earlier susceptibility, specific heat, and proton resonance results. Several modifications of the model were considered in order to remove small systematic discrepancies. The addition of a very weak intercluster coupling in a molecular field approximation yields a quantitative fit of all the data. If H = H0+nM, we find n = −1.2 mole/cgs while J = −5.14k and g = 2.35, in even closer agreement with other observations. This weak antiferromagnetic coupling is large enough to explain the absence of hyperfine structure on paramagnetic resonances seen in the excited pair triplet state.
Magnetic Properties of the Oxygen Molecule in Solid Oxygen-Argon Mixtures
Authors: Blocker T. G. , Simmons C. L. , West F. G.
Published: February 2004

Recent neutron diffraction and crystallographic studies of the magnetically ordered phase (≤24°K) of solid oxygen have aroused new interest in magnetic exchange interactions between oxygen molecules. The interpretation of the magnetic ground state in the pure oxygen system is difficult because multiple solid-phase transitions and the narrow range of temperatures over which each phase is stable prevent a determination of the magnitude of the exchange interaction. We report here on magnetic susceptibility studies of O2-A solid mixtures which exist from 0°–70°K in a single hcp phase for O2 concentrations in argon ranging from 20%–50% and in a single fcc phase from 0%–20%. The measurements can be quantitatively interpreted using a statistical molecular field theory of disordered solid-solutions. We find θ̄/c≅60∘K, where θ̄ is the ``statistical'' Curie-Weiss temperature and c is the concentration of O2 in A. This is comparable to the paramagnetic Curie-Weiss temperature θ=50°–60°K observed in pure liquid and γ solid phase O2. The relatively large temperature range over which the data and theory agree contrasted to the narrow temperature range available for the liquid γ solid phases reenforces the interpretation that strong exchange interactions do occur between O2 molecules.
Distant-Neighbor B-B Interactions in Cobalt Chromite
Authors: Dwight K. , Menyuk N.
Published: February 2004

We have recently shown that five distinct distant-neighbor B-B exchange interactions can play significant roles in determining the ground-state spin configuration in chromium spinels having nonmagnetic A-site ions. This investigation is now extended to materials with magnetic A-site ions and is further generalized to include A-A interactions. The results are used to reinterpret our experimental findings for cobalt chromite. Reasonable values for the distant-neighbor interactions lead to an eight percent decrease in the theoretical wavelength of the minimum-energy ferrimagnetic spiral, which brings it into agreement with the observed value. Furthermore, inclusion of these values in the minimization of the free energy (in the molecular-field approximation) leads to the prediction of an additional magnetic transition (where a phase angle vanishes) at about one third of Tc. The resulting kink in the computed variation of magnetization with temperature is in close agreement with experiment. Our neutron-diffraction pattern is shown to be incompatible with contradictory results recently reported for a CoCr2O4 sample produced by conventional ceramic techniques. The overall agreement between theory and experimental results obtained with our sample indicates that the precursor method of preparation yields more nearly ideal CoCr2O4.