bluuebunny/crossref_metadata_2025 huggingface dataset:
Mapping of Electromagnetic Fields by Photochromics and Their Application in Microwave Holography
Authors: Iizuka Keigo
Published: February 2004

The temperature dependence of the rate of color fading of an activated photochromic film was utilized in the mapping of a microwave field pattern. The minimum microwave power required was 5 mW/cm2. Using this mapping technique, a microwave hologram of a lamp bulb was recorded. The scaled down hologram was illuminated by a He–Ne laser beam in order to obtain an optical reconstruction of the microwave image of the bulb. Internal as well as the surface structure of the bulb was recognizable in the reconstructed optical image.
Radiative Recombination at Isoelectronic Point Defects in Ge
Authors: Feldman J. M. , Hergenrother K. M.
Published: February 2004

The photon energy and the conversion efficiency for radiative recombination in Sn-doped Ge are reported for temperatures ranging from 22 to 180 K. The spectrum of the emission from Pb-doped Ge at 77 K is shown. A model of the emission process based on the trapping of excitons at the Sn centers is proposed.
Microstructure and Electrical Properties of Thin Epitaxial Films of PbTe on NaCl
Authors: Myers J. H. , Morriss R. H. , Deck R. J.
Published: February 2004

Epitaxial thin films of PbTe have been prepared by vacuum evaporation onto the (001) cleavage face of NaCl with substrate temperatures of 100 and 300 °C and deposition rates of approximately 100 and 1000 Å/min, respectively. Electron diffraction showed the films to be strongly oriented with the (001) planes of the deposit parallel to the substrate. Transmission electron microscopy of thinner films, ≈ 0.2μ thick, showed no significant variation in microstructure with changes in substrate temperature and deposition rate. Such films showed a high density of dislocations, ≈ 5×1010/cm2, the majority of which ran normal to the plane of the film. Many were arranged in low-angle grain boundaries, the most prominent microstructural feature of the films. These exhibited rotations up to about 2° and the average boundary spacing for these thin samples was estimated at 2000 Å. Surface replication of a number of films grown with comparable conditions but varying thicknesses show features that could be indicative of internal crystalline boundaries. A decided increase in the spacing of these boundaries with increasing film thickness was noted. dc Hall-effect measurements from room temperature down to 77 °K showed the Hall mobilities exhibited a decreased temperature dependence as compared to that of the bulk. This could be ascribed to the additional scattering influence of the crystal-line boundaries. The spacing of the boundaries was calculated from the electrical data and a comparison with the spacings measured by electron microscopy showed satisfactory agreement.
dc Field-Induced Anisotropy of Microwave Conductivity in Nonparabolic Semiconductors at Low Temperatures
Authors: Sodha Mahendra S. , Sharma Satish K. , Dubey Prabhat K.
Published: February 2004

dc field-induced anisotropy of small-signal microwave conductivity has been theoretically studied and some calculations are presented in the microwave frequency region at low temperatures (77°K) in an n-InSb sample. Above a certain minimum value of the electric field, the spontaneous emission of optical phonons in the active region (ε>ℏωl), and elastic scattering by ionized impurities and by acoustic phonons in the passive region (ε>ℏωl) have been assumed to be the dominant scattering mechanisms. Comparison is made with the theoretical results at high temperatures (300°K) and, qualitatively, with the experimental results of Bonek and Richter. It is seen that the effect of electric field is much more appreciable at low temperatures than at high; many of the qualitative features of the experimental results are explained by the theory. An interesting reduction is made to the case of a parabolic semiconductor, which reveals that the anisotropy effects are greater for a nonparabolic semiconductor than the parabolic one in analogous situations.
Relation between Residual Polishing Agent and Material Hardness Determined by a Narrow Nuclear-Reaction Resonance Technique
Authors: Bernett Marianne K. , Butler J. W. , Wolicki E. A. , Zisman W. A.
Published: February 2004

Clean smooth solid surfaces needed in certain investigations are often prepared with the aid of fine polishing powders, which may remain on the surface as residual contaminant. The present paper reports on the alumina and magnesia retentivity of twelve different metal surfaces as measured by a novel technique involving the use of strong sharp resonances in the nuclear reactions 27Al(p,γ) 28Si and 26Mg(p,γ) 27Al. This technique has high sensitivity and, because of the sharpness of the resonances, also provides information on the penetration depths of the residual polishing agents as well as on the amounts present. A plot of the amount of residual polishing agent vs the hardness of the surface showed a general trend for the retentivity to be less with increase in solid hardness. Divergence from this trend was noted for metals which have other than cubic crystal structures; zinc and cadmium, two of the three softest metals, retained not only very small amounts of residue but also tended to keep this residue ``at'' the surface (little or no penetration of the base metal). The results are also discussed in relation to metal oxide formation and the adherence of such oxides to the metal. The nuclear-activation techniques and the conclusions should be applicable to all solid surfaces and to other surface-finishing procedures where the problem of residual or foreign material arises. In such cases, for each contaminant element present, a narrow-resonance nuclear reaction specific to that element would be used for the measurements.
Comments on Thermochemical Calculations of Tungsten Halogen Lamps
Authors: Gupta Suresh K.
Published: February 2004

Some aspects of the recently published thermochemical calculations by Yannopoulos and Pebler on the W–O– Br– C–H and allied systems are in discord with the available thermochemical data on tungsten oxybromides. Calculations are presented in an attempt to explain the discrepancy. The existence of condensed phases in some of these systems is also considered.
Electroluminescence Characteristics and Efficiency of GaAs:Si Diodes
Authors: Ladany I.
Published: February 2004

The peak wavelength, spectral half-width, and efficiency of liquid-phase epitaxial GaAs:Si diodes were determined as a function of silicon concentration in the melt. The peak of the emission spectrum could be shifted from about 9200 to 10 000 Å by varying the silicon concentration. For all wavelengths investigated, uncoated diodes with 6% efficiency could be made, with some going as high as 10%. External room-temperature dc efficiencies up to 32% were measured on diodes provided with high-index glass domes. The spectral half-width was found to increase with silicon concentration, and was especially large for growth on the 111B plane. The internal absorption loss was found to be significant for all peak wavelengths. These findings are discussed in the light of current models of recombination processes in heavily doped semiconductors.
Orientation and Temperature Dependence of the Photoplastic Effect in ZnO
Authors: Carlsson Lennart
Published: February 2004

The photoplastic effect in ZnO is observed as a strong reversible increase in flow stress, during illumination with white light, when single crystals are deformed in basal slip at room temperature. An earlier model based on Read's theory for the space-charge region near a charged dislocation has been confirmed by further measurements. Basal dislocations in the wurtzite lattice are expected to show different electrical behavior than prismatic dislocations. The basal dislocations contain atoms of the same kind in the dislocation line and are expected to be electrically active through electron trapping to dangling bonds. The prismatic dislocations have alternating kinds of atoms in the dislocation line and are expected to be electrically inactive by a possible interlock of the broken bonds. Therefore, dislocations in prismatic slip should be uncharged and show no photoplastic effect. This was confirmed by experiments showing that there is no observable effect of illumination on the flow properties when crystals are deformed in prismatic slip. The different electrical activity of the dislocations in the two slip systems is also reflected in the resistivity and photoconductivity behavior of deformed samples.
Mössbauer Characteristics of ε, χ, and θ Iron Carbides
Authors: Mathalone Z. , Ron M. , Pipman J. , Niedzwiedz S.
Published: February 2004

The Mössbauer effect in 57Fe has been used to study carbide phases appearing during the tempering process of an alloy steel containing 1.33 wt.% Si, 1.13% wt. Mn, and 0.38 Wt.% C. Carbides were chemically extracted from the samples which had been isothermally tempered for 2 h at various temperatures. Carbides obtained from samples tempered at 300° and 400°C were identified as ε and χ phases, respectively. The ε carbide has an isomer shift (I.S.) of 0.18 mm/sec relative to α iron and an effective hyperfine field (Hn) of 162 kOe at 300°K. The χ carbide has an I.S. of 0.28 mm/sec relative to α iron and Hn of 179 kOe at 300°K. Carbides precipitated at and above 500°C were found to have the orthormbic cementite structure, with a composition of (Fe1−xMnx)3C. As the tempering temperature increases from 500° to 700°C the Curie temperature changes from 440° to 270°K, and x increases from 0.02 to 0.10. Curie points and extrapolated values of Hn(0) were found for all the carbides in question. The early stages of tempering in which transition carbides appear are explained as being controlled by the elastic strain. The elastic binding energy between an interstitial carbon atom and a substitutional silicon atom can then be estimated. Other phases which were found in the chemically extracted samples after tempering the alloy at a low temperature, were identified as oxyhydeoxides and iron gel. In a sample tempered at 700°C the carbide was detected in the steel matrix and compared with the same carbide after chemical extraction.
Excitation and Doping Dependences of Electron Diffusion Length in GaAs Junction Lasers
Authors: Hwang C. J.
Published: February 2004

The dependence of electron diffusion length on the excitation and impurity compensation in the p region of a GaAs junction laser has been calculated from the band theory of impure semiconductors. Using a screened potential for each impurity atom, the average room-temperature diffusion length in a typical diffused diode is found to be 1.4 μ at low injection limit. The diffusion length increases with increase in excitation because of the enhancement in carrier screening and carrier diffusion energy. This effect can be used to explain the much lower threshold current in an AlxGa1−xAs–GaAs heterostructure laser and an abnormally high threshold current in a shallow diffused homostructure laser. Furthermore, the electron diffusion length is found to be larger for the region near the junction due to longer electron lifetime in this region. This also explains the observation that in a normal diffused junction laser, the active region occurs somewhere in the p region but not right next to the space-charge region.
Threshold Currents of Large Cross-Section Superconductors
Authors: Iwasa Y. , Montgomery D. B.
Published: February 2004

The threshold currents of a variety of multifilament, composite Nb–Ti superconductors, both twisted and untwisted, have been measured and the results analyzed. The threshold current is defined as a minimum level of transport current which results in a sample quench following a flux jump. The twisted samples are able to maintain much higher levels of threshold current than the untwisted samples. Flux-jump-induced quenches existed in all samples, but analysis of the results reveals that different heat-transfer regimes were operative in the twisted and untwisted materials. For the case of twisted samples, the magnetic energy released in a flux jump is relatively small, and heat transfer is by nucleate boiling; in untwisted samples the energy released is much greater, and heat transfer is by film boiling. The threshold current levels computed for both the twisted and untwisted samples agree satisfactorily with the experimental results.
Magnetic Hysteresis in Simple Materials. II. Experiment
Authors: Baldwin John A. , Bayne Jay S. , Calkins Thomas R.
Published: February 2004

Theory is compared with experiment. The applicability of the theory to a sample which is magnetically sufficiently simple is demonstrated by comparing certain qualitative predictions of the theory with experiment. In particular it is shown that the ascending branches of all minor loops coincide if they are all started at the same point. The ascending branch of a minor loop coincides with the ascending virgin curve if they both start at the same point and if the gain of the flux and mmf channels of the minor loop are reduced by a factor of 2. The virgin curve is the locus of apexes of centered minor loops. The relation between height and width of minor loops is shown to agree quantitatively with theory. The shape of a branch of a single minor loop is also found to agree. The first three terms of the Fourier analysis of mmf for sinusoidal flux variation are found to be in good agreement with theory except for the third harmonic which at least has the correct order-of-magnitude. Theory and experiment are shown to be in good agreement in the Rayleigh region. However, the applicability of the present theory is not confined to the Rayleigh region, but applies right up to saturation for a specimen meeting the criteria of simplicity. The predicted relation between the Rayleigh constants and the coercive force is shown to hold. Finally the distribution function is calculated from a minor loop and shown to be an exponential with reasonable accuracy.
Torsion of a Titanium Elastic-Visco-Plastic Shaft
Authors: Kratochvil J. , De Angelis R. J.
Published: February 2004

The thermodynamics of elastic-visco-plastic materials is used to obtain a solution of a simple boundary-value problem: the torsion of a solid circular titanium shaft. Constitutive relations for titanium have been deduced from the experiments and theory given in the series of the papers by Conrad et al. Attention is paid to the importance of temperature changes connected with inelastic deformation.
First Pressure Derivative of Bulk Modulus for Porous Materials
Authors: Walsh Joseph B.
Published: February 2004

Two equations are derived giving the effect of porosity in the form of spherical pores upon the pressure derivative dK/dp of bulk modulus. In the first equation, dK/dp is expressed in terms of porosity and the second-order elastic constants for the solid material, whereas in the second equation, the third-order constants are needed as well. The expressions are evaluated for thirteen materials, showing that dK/dp for porous material may be either larger or smaller than the value for the solid matrix.
Photoconductivity in Photochromic Strontium Titanate
Authors: Williams Richard
Published: February 2004

Photoconductivity transients accompany the coloring and bleaching of photochromic SrTiO3 (Fe:Mo). It is shown that there is a free-carrier step in the photochromic process. Analysis of the transients leads to a model from which the important factors determining the performance of this material are readily identified.
Transient Ion Sheath Effects on Probe Admittance
Authors: Bunting W. D. , Heikkila W. J.
Published: February 2004

We have investigated the transient behavior of the ion sheath around a spherical metallic probe in a laboratory plasma. The transients were initiated by step-function changes in the probe potential; only negative potentials were used in order to minimize the disturbance to the ambient plasma. The ion sheath was detected through its effect on the complex admittance of the probe at two frequencies, 5 and 15 MHz. The plasma frequency was varied over the range 5–32 MHz. As expected, the admittance transients were quite rapid (<50 μsec except close to the plasma frequency) following the application of a large negative probe bias; under this condition the electrons are quickly repelled to form a new thicker ion sheath about the probe, and further slow changes are too small to be measured. The return to a smaller negative bias also has an initial rapid variation due to the return of the electrons, but this is followed by a much slower part due to the ambipolar diffusion back to the steady-state thin sheath configuration. Undershoots and overshoots were observed near the plasma frequency. An approximate theory for the transient sheath properties is described and numerical results of the theory for a typical plasma condition are presented. A few discrepancies are noted, but in general there is agreement between the numerical results and the experiment.
Electron Beam Modulated Optical Properties of Semiconductors
Authors: McCoy John H. , Wittry D. B.
Published: February 2004

The electron beam of an electron probe microanalyzer has provided a new method of modulating the reflectance and absorption of light by semiconductors. The beam is periodically gated and the modulated light detected synchronously. The relative reflectance spectra of Ge, Si, GaAs, and GaP exhibit features at photon energies above the bandgap which resemble thermoreflectance of critical point transitions. An analysis of the possible modulation mechanisms and experiments involving gating frequency and beam density indicate that the modulation is due to temperature variations from the electron-beam heating. Modulated reflectance of light with photon energies below the bandgap of Ge and GaAs was measured and found to be attributable to the temperature coefficient of the real part of the refractive index. Electronbeam modulated absorption of long-wavelength light was observed and found to be due to excess carriers generated by the beam.
Propagation and Amplification of Shear-Horizontal Waves in Piezoelectric Plates
Authors: Fischler Chava
Published: February 2004

Propagation of shear horizontal waves in class C6v piezoelectric plates with polar (or sixfold) axis in the transverse horizontal direction is analyzed for various boundary conditions: faces electrically free, shorted, or coupled to a thin homogeneous semiconductor plate. The case considered is that of identical physical conditions at the opposite faces, which permits the symmetric and antisymmetric waves to be treated independently. Of the above waves, the lowest modes behave as transverse surface waves at high frequencies, and the higher modes retain their oscillatory behavior in the thickness direction. Numerical solutions are obtained for amplification and phase velocity in PZT under different boundary conditions, and it is shown that the phase velocity can also be strongly dependent on the drift velocity of carriers in the semiconductor. The shapes of the dispersion and amplification-frequency curves are also drift-dependent, especially in the vicinity of cutoff.
Influence of Dislocations in CdS Crystal on Its Electromechanical Coupling Factors
Authors: Chubachi Noriyoshi , Iinuma Kazumoto , Kikuchi Yoshimitsu
Published: February 2004

The electromechanical coupling factors are measured in the frequency range from 100 kHz to 260 MHz for CdS single crystals with various dislocation densities. These measurements show that each of the electromechanical coupling factors kt, k15, and k31 begins to decrease in a steep slope as the density of dislocation which terminates at c surface of the crystal exceeds an amount around 106 cm−2. However, these coupling factors come back toward the normal values when the density of dislocation is decreased by means of the annealing of the crystals in a sulfur vapor. No appreciable change of the elastic constants and the dielectric constants is observed. Therefore, the observed variation of the electromechanical coupling factors is attributable to the dependence of the corresponding piezoelectric constants on the dislocation density.
X-Ray Study of Faulting in BCC Metals and Alloys
Authors: Rothman R. L. , Cohen J. B.
Published: February 2004

Faulting on {211} planes has been reported in filings of many bcc materials based on differences in the broadening of various peaks in the x-ray powder pattern. These experiments have been repeated in this work using (a) recent techniques for correcting for various experimental errors and (b) new derivations of effects of faults on other planes. It is shown that the mean-square strains 〈εL2〉 are generally inversely proportional to the distance L normal to the reflection planes. As a result, it is possible to separate effective particle-size (including faulting) and elastic strains from several first-order reflections, rather than with multiple orders. With this approach, the results indicate that there are no faults produced by filing in Fe, β brass, and Mo–Re alloys. For Nb there are faults on {110}, and the concentration varies inversely with the interstitial content.
Ising Model with Antiferromagnetic Next-Nearest-Neighbor Coupling. III. Disorder Points, Energy Points, and Scattering from a One-Dimensional Model
Authors: Stephenson John
Published: February 2004

A general argument is presented that a change in the nature of the short-range order will occur above a precisely located temperature TD, the disorder point, for a class of magnetic models. The possibility of detecting a disorder point by neutron scattering is investigated by calculating the relative scattering intensity for a one-dimensional model.
Properties of the Ising Square Lattice with Next-Nearest-Neighbor Interactions
Authors: Landau D. P.
Published: February 2004

The thermodynamic properties of the N×N two-dimensional Ising square lattice have been studied using a Monte Carlo technique. The critical temperature and types of long-range order have been investigated over a wide range of ratios of nearest-neighbor to next-nearest-neighbor interaction strengths.
Measurement of Dynamical Scaling in FeF2
Authors: Schulhof M. P. , Hutchings M. T. , Guggenheim H. J.
Published: February 2004

We have made a complete measurement of the scattering function S(q, ω) in the critical region of the strongly anisotropic material FeF2. From the angular dependence of the scattering we have determined the temperature dependence of the staggered static susceptibility χ(T) and inverse correlation range κ(T). From the observed inelasticities we have found the wave vector and temperature dependence of the longitudinal spin-spin relaxation rates. These data provide the first direct evidence of the dynamical scaling behavior in anisotropic antiferromagnets, and are compared with the recent calculations of Riedel and Wegner.
Neutron Investigation of the Spin Dynamics in Paramagnetic RbMnF3
Authors: Tucciarone A. , Corliss L. M. , Hastings J. M.
Published: February 2004

Measurements of the inelastic magnetic scattering of neutrons from the Heisenberg paramagnet RbMnF3 are presented for the temperature range from TN to 3.55TN. A direct deconvolution procedure is applied to the data, yielding, for various temperatures in the above range and for values of q in the (1, 1, 1) direction, the spectral density χ(q) F(q, ω) of the spin relaxation function. The evolution as a function of temperature and q of the dynamical properties of this isotropic Heisenberg paramagnet is thus exhibited experimentally. In particular, the persistence of the three-peaked structure of the spectral density, previously reported at TN, is observed also above TN at the antiferromagnetic zone boundary, and the merging of the peaks into a single mode, as temperature is increased, is described. The static susceptibility χ(q), determined on an absolute basis, is in excellent agreement with the calculated values obtained from nearest-neighbor Heisenberg-model high-temperature expansions and provides, therefore, further incentive to the prospect of explaining, within the framework of this model, the paramagnetic dynamical behavior of RbMnF3. This dynamical behavior is experimentally described by the present measurement, also on an absolute basis, of the shape function F(q, ω).
Thermal Expansivity of RbMnF3 Near TN
Authors: Golding B.
Published: February 2004

The thermal expansivity of single crystal RbMnF3 has been measured in the vicinity of its Néel temperature, TN≃83 K. A three-terminal capacitive dilatometer was employed to measure length changes with a strain sensitivity ∼10−10. Above TN, the magnetic contribution to the expansivity, αm, exhibits a temperature dependence consistent with a logarithmic divergence at TN for over two decades of reduced temperature and to within 4 mK of TN. By utilizing previous magnetic specific heat (Cm) data, it is found that αm and Cm are not directly proportional in the critical region. An asymptotic magnetic Gruneisen ``constant'' is evaluated for T→TN+ with the result γm=−∂ log J/∂ log V=3.14±0.3. The pressure dependence of the Néel temperature dTN/dP is 0.37 mK/bar.
Magnetic Properties of an Induced Moment Antiferromagnet
Authors: Wang Yung-Li , Rao B.
Published: February 2004

The induced moment system with a singlet crystal-field ground state has been recently studied extensively for ferromagnetic exchange interaction between ionic moments.1 We report here the calculations for an induced moment antiferromagnet in the presence of an external magnetic field. The behavior of the sublattice magnetization as a function of the external magnetic field shows a remarkable difference from a fixed spin antiferromagnet, although a similar ``matamagnetic transition'' occurs at some critical field. The magnetic susceptibility and specific heat are also calculated in molecular field approximation and by the Green's function technique.
Low-Temperature Behavior of a Planar Ferromagnet
Authors: Semura J. S. , Huber D. L.
Published: February 2004

We have examined the low-temperature properties of a simple cubic planar Heisenberg ferromagnet with nearest-neighbor interactions H=−  ∑ ij JijSi·Sj−  ∑ ij (Kij−Jij)SixSjxwhere −J≤K≤J. At zero temperature the spin deviation per spin increases from zero in the isotropic limit to 0.022 when K=0 and to 0.078 when K=−J. The ground state energy E0=−6N JS(S+β/6) is depressed by the anisotropy from β=0 in the isotropic limit to β=0.16 when K=0 and β=0.58 when K=−J. The lowest-order correction to the harmonic spin-wave frequency in the long-wave-length limit scales approximately as [U(T)-U(0)], where U(T) is the internal energy. The renormalization shifts the zero-temperature spin-wave frequency by less than 4S−1% (at K=0). We also calculate the spin-wave damping using a relaxation function formalism and find that in the hydrodynamic limit the decay rate is proportional to k2 in agreement with the predictions of macroscopic hydrodynamics.
Phase Transition of an Anisotropic Ferromagnet
Authors: Wang Yung-Li , Yang David , Khajehpour M. R. H.
Published: February 2004

The phase transition of an Ising Ferromagnet with uniaxial single ion anisotropy with spin one has been investigated extensively by Capel. Here we consider a similar but a more general system with anisotropic exchange interaction. The Hamiltonian can be written as H=D  ∑ i (Siz)2−  ∑ ij JijSizSjz−  ∑ ij Kij(SixSjx+SiySjy),where J, K, and D are positive parameters. We show that in addition to the more familiar phase transition from an ordered phase to the paramagnetic phase, as temperature is raised, a first-order phase transition can occur. Depending upon the relative magnitude of D, J, and K, a phase transition from the ferromagnetic state with spin ordering along (perpendicular to) the z axis to a ferromagnetic state with spin ordering perpendicular to (along) the z axis can take place at a critical temperature lower than the Curie temperature where a transition to the paramagnetic phase occurs. The boundaries of the three phases have been calculated in the molecular field approximation.
Two-Dimensional Stress Measurement in Permalloy Thin Films by Moiré Method
Authors: Chiang Fu-Pen , Faber Charles S. , Wang Franklin F. Y.
Published: February 2004

A modified Ligtenberg's reflective Moiré method may be used to measure stress in Permalloy film (∼1000 Å) on glass substrate. At four points on a sample film, stress values ranged between 109 and 1011 dyn/cm2. Angle φ between the principal stress S1 and the x axis varied from 178.5° to 0°. The extent of such variations differed with each individual film on substrate. Their causes were not determined in this study. This Moiré technique can be very useful in establishing these causes.
Anisotropy, Inhomogeneity, and Magnetization Creep in Plated Wire
Authors: Lund R. E. , Oredson H. N. , Torok E. J.
Published: February 2004

Plated wire, unlike vacuum-deposited films, can be made to have a reversible limit five times higher than predicted by the Stoner-Wohlfarth theory. To examine this problem the inhomogeneity in the magnitude of the anisotropy was measured via a variation of the transverse biased permeability method; the measurements indicate the presence of small regions with very high anistropy embedded in a matrix with low anisotropy. HC was found to increase about 20% with increasing field, indicating that these tiny regions have coercivity many times that of the rest of the film. However these regions can be switched with a small easy-axis field if the magnetization in the rest of the film is rotated with a transverse field; this creates a stray field at the domain boundaries that aids switching. Measurements of creep switching and of switching using a single hard-axis pulse are presented supporting this view.
Irreversible Photoinduced Changes in Optical Absorption of YIG(Si4+) and YIG(Ca2+)
Authors: Gyorgy E. M. , Dillon J. F. , Remeika J. P.
Published: February 2004

Irradiation by white light at low temperatures produces drastic changes in the near-infrared absorption coefficient of YIG doped with divalent Ca or quadrivalent Si. For Ca2+ doping, α increases; for Si4+ doping, α decreases on irradiation. In both cases the magnitude of the effect varies sharply with concentration. For YIG(Si) the results are interpreted in terms of the different properties of Fe2+ in sites that are near or far from the Si4+ ions. A similar model holds for YIG(Ca) with Fe4+ rather than Fe2+ being the important impurity. In both cases irradiation enhances occupation of the far sites.
High-Frequency Measurements of the Effective Spin-Spin Interactions in Tb(OH)3
Authors: Skjeltorp A. T. , Wolf W. P.
Published: February 2004

Measurements have been made of the field dependence of the differential susceptibility of Tb(OH)3 at temperatures between 4.5° and 16°K and dc fields up to 1 kOe, using measuring frequencies between 2 and 4 MHz. Following the method of Casimir and du Pré the results were combined to derive the temperature variation of the zero field magnetic specific heat per spin CM/R and this was fitted to an asymptotic expansion of the form CM/R = C2/T2+C 3/T3+···. Subtracting small contributions to C2 from hyperfine and crystal field effects, it was thus possible to find the leading specific heat term due to interactions between the Tb3+ ions: C2int = 0.93±0.03(°K)2 and C3int = 1.6±0.6(°K)3. Combining these values with earlier optical, thermal, and magnetic estimates of the low temperature excitations and Curie-Weiss constant, the predicted Ising form for the interactions could be verified and values for the nondipolar interaction constants between the first, second, and third nearest neighbors determined. The analysis indicates that the ferromagnetic ordering in Tb(OH)3 is dominated by the nondipolar next-nearest-neighbor interactions, but there are a number of other terms both ferromagnetic and antiferromagnetic in sign which are comparable in strength. Possible consequences of the competing interactions are discussed.
High-Low-Temperature Static Magnetic Properties of FeF2
Authors: Libelo L. F. , Petrone F. J.
Published: February 2004

The cluster variation method was applied to study static properties of the anisotropic uniaxial antiferromagnetic insulator FeF2. In particular the range of temperatures extending down to the spin-wave region was investigated. Theoretical results for spontaneous magnetization, longitudinal, and transverse susceptibilities, for example, were found to be in surprisingly good agreement with experiment for quite low temperatures on up.
Critical Fields Determining Magnetic Coercivity in Microparticles of SmCo5 and LaCo5
Authors: Zijlstra H.
Published: February 2004

Magnetic hysteresis curves measured by a specially designed magnetometer on a single particle taken from a ground SmCo5 powder show that the coercivity is determined by pinning of domain walls. Heat treatment at 100°C in air reduces the pinning force. Magnetizing the particle in a strong field increases the pinning force. A wall-pinning model is proposed to explain this effect. The coercivity of a particle taken from the powder after electrolytic etching seems to be determined by nucleation of domain walls. Heat treatment at 100°C in air does not significantly influence the field at which nucleation occurs. The coercivity of an LaCo5 particle is determined by pinning. The pinning force is reduced by hydrogen absorption.
Interactions between Small Magnetic Clusters in Copper-Rich Cu–Ni–Fe Alloys
Authors: Bennett L. H. , Swartzendruber L. J. , Watson R. E.
Published: February 2004

Traces of iron in ``nonmagnetic'' Cu-rich Cu–Ni alloys produce small magnetic clusters largely limited to an Fe site and its Ni nearest neighbors, as observed in Mössbauer source experiments. The present work concerns 57Fe Mössbauer absorber data for higher Fe concentrations (¼%–3%), in the 10%–30% Ni range, which demonstrates the existence of a magnetic-moment distribution centered about two distinct moment values. For the lower Fe concentrations, the smaller and more abundant of these moments is identical to that observed in the source experiments and is associated with isolated small Fe–Ni clusters. The other moment is about twice as large and can be ascribed to coupled pairs of these small clusters. These results are almost free of effects of metallurgical clustering. By appropriate heat treatment, clustered samples are obtained with different moment distributions and higher Curie temperatures.
Hydrothermal Growth of Bubble-Domain Memory Materials
Authors: Kolb E. D. , Laudise R. A.
Published: February 2004

Tm and Y orthoferrite crystals with areas on the (001) face larger than 1 in.2 were grown. Optimum conditions were 20M (molal) KOH, 89% fill, 5% baffle opening, 385°C crystallization temperature, temperature differential 25°, and seeds {001} hydrothermally grown Y and TmFeO3 plates. Growth rates were about 1–4 mil/day. Bubble size, mobility, and other properties compared favorably with flux-grown crystals. Magnetically uniaxial garnets were crystallized from starting materials including (Gd, Tb)3Fe5O12, (Gd, Tb, Er)3(Al, Fe)2[(Fe, Al)O4]3, (Eu, Er)3(Ga, Fe)2[(Fe, Ga)O4]3 and (Y, Gd)3(Al, Fe)2[(Fe, Al)O4]3. Seeds were flux-grown crystals of similar composition. Orientation of the easy magnetic axis was usually similar to that observed in flux growth and properties compared favorably. Heteroepitaxial growth of (Eu, Er)3(Ga, Fe)5O12 on (Nd0.2Gd0.8)3Ga5O12 was achieved. Optimum conditions for homoepitaxial garnet growth were generally similar to those for orthoferrite. In garnet growth most experiments were conducted at 87% fill and with a ΔT (temperature difference between dissolving and growth zone) of 40°C. Ga2O3 was added to reduce the magnetic moment of the crystallized material.
Mössbauer Study of Local Molecular Fields, Cation Distributions, and Recoilless Fractions in Sb-Substituted Lithium Ferrite
Authors: Evans B. J. , Swartzendruber L. J.
Published: February 2004

Previous studies of the B-site hyperfine field distribution in spinel ferrites utilizing a local-molecular-field model have demonstrated that A-site cation disorder has a pronounced effect on Heff(B) and virtually no influence on Heff(A). The influence of B-site disorder on these hyperfine fields is less well known. We have made 57Fe Mössbauer measurements on Li1.2Fe4.6Sb0.2O8 in which B-site disorder greatly predominates over A-site disorder. From the near natural linewidths for the A and B-site patterns we conclude that, in this material, B-site cation disorder is not nearly as effective as A-site cation disorder in producing inhomogeneities in Heff at either site. The area ratio of the two hyperfine field patterns, which are well resolved in an applied field, yields equal recoilless fractions for the two sites.
Magnetostriction Constants of Cubic Ferrites Containing Only Fe+3 as Magnetic Ions
Authors: Tsuya N. , Arai K. I.
Published: February 2004

The observation of the magnetostriction constants λ100 and λ111 were made for the single crystals (ZnxFe1−x) (Li(0.5−0.5x)Fe(1.5+0.5x))O4 (0≤x≤0.48) and (MnηFe1−η) (Mg1−ηFe1+η)O4 (0.10≤η≤0.22), by observing the shift of resonance field with and without a concentrated load. The analysis of the results shows that, besides the dipolar interaction among ferric ions, one Fe3+ ion per unit formula in A site contributes to λ100 and λ111, 8.6×10−6 and 12.6×10−6, while in B site contributes to λ100 and λ111, −21.4×10−6 and −2.5×10−6, respectively.
Excited State Exchange in MnF2
Authors: Freeman S.
Published: February 2004

The optical absorption spectrum of Mn++ salts is typically dominated by spin-wave sidebands of forbidden ``atomic'' transitions. The Mn++ excited states are properly described as Frenkel excitons. Their dispersion, as well as their exchange interactions with the surrounding ions, arise from the same sort of ``super-exchange'' processes which determine the ground-state exchange constants. It would be useful to have a unifying descriptive framework for such interactions. In the study of a particular case, the 4E(I) excitons in MnF2, such a phenomenological theory has been constructed. Using the familiar MO description of the weak covalent bonding characteristic of these materials, the exchange interaction can be expressed in terms of a small number of parameters. These are determined from the ground-state exchange constants and then employed to predict properties of the excited states. The order-of-magnitude of the exciton dispersion is correctly given for both Eθ (−74 cm−1) and Eε (≈0 cm−1). The predicted interaction parameters which determine the binding energies of the Eθ exciton-magnon bound states are also qualitatively correct.
Optical Absorption Spectrum of MnF2 at High Fields
Authors: Caird R. S. , Garn W. B. , Fowler C. M. , Thomson D. B.
Published: February 2004

Absorption spectra covering the range from 3800 to 6400 Å have been obtained for single-crystal samples of MnF2 cooled to 6.5°K in fields above 1 MG. The field and light propagation vectors were parallel to the c axis. Molecular field theory predicts that the magnetic moments of the sublattices should tilt toward the field direction as the magnetic field increases, achieving complete parallelism at twice the exchange field (1.10 MG). Drastic changes are to be expected in the absorption structure as the sample approaches the aligned state. The present data on MnF2 are the first obtained in this field region and show the expected behavior. Most of the absorption lines and diffuse bands fade out between 0.7 and 1.0 MG. The last of the changes occur at 1.01±0.02 MG, indicating that alignment is essentially complete at this field. The sharp line at 5410 Å moves toward the red, the shift amounting to 31 cm−1 at 830 kG.
Optical Absorption Induced by Ca Impurities in MnF2
Authors: Sell D. D. , Stokowski S. E.
Published: February 2004

We have measured the absorption spectrum of MnF2 doped with 0.1% of Ca in the 18 100–18 400 cm−1 spectral region at 2°K. There is considerable weak structure in this region induced by the presence of the impurity. We concentrate upon two lines, CaE1(I) at 18 122 cm−1 and CaE2(I) at 18 322 cm−1, which we identify on the basis of their polarization as pure electronic transitions of Mn2+ ions perturbed by first nearest-neighbor Ca2+ impurities. These two lines appear in π and σ, but not α polarization. Thus, the lines have both electric and magnetic dipole character and are consistent with a C2 site symmetry for the absorbing ion. Stress results for the impurity transitions for [001] and [110] directed uniaxial stress are presented. It is found that CaE1(I) and CaE2(I) have the properties that the intrinsic excitons would have for a large internal [001] stress.
Voigt Effect in Magnetic Films
Authors: Lissberger P. H. , Parker M. R.
Published: February 2004

The sizes of magneto-optical signals have been investigated theoretically for the case of transmission at normal incidence through a thin magnetic film and periodic hard-axis magnetization reversal in the plane of the film. The magneto-optical effect in this arrangement, termed Voigt effect, is of second order; it depends on the square of the basic magneto-optical constant Q and on the square of the magnetization. Consequently, in the permittivity tensor describing the macroscopic optical and magneto-optical properties of the material, it is necessary to take into account not only the off-diagonal elements but also the terms in Q2 in the diagonal elements which are justifiably ignored in evaluations of first-order effects. The presence of the film boundaries is also considered. The theory is used to reconcile published measurements of the Voigt effect in films of cobalt and iron in the near infrared with magneto-optical parameters obtained by Krinchik [J. Appl. Phys. 35, 2 (1964)] from measurements of first-order effects. A recent experiment carried out to test the theory further is also described.
Optical Properties of Cr–Mn and Cr–V Alloys
Authors: Crandell M. E. , Schröder K.
Published: February 2004

The optical reflectance of chromium alloys with up to 10 at.% manganese or 50 at.% vanadium was measured at room temperature for photon energies from 0.475 to 2.24 eV. The Cr–Mn alloy system shows a very slight shift of the chromium absorption edge to higher photon energies, whereas the addition of vanadium shifts the absorption edge to lower photon energies. For the alloy with 10 at.% V, the edge disappears to values below the measured photon energy range. The direction of the shift of the absorption edge can be explained qualitatively in a rigid band model with transitions from the t2g band to the Fermi level. This model was previously proposed to explain optical reflection data of chromium-iron alloys.
Mössbauer Investigations of Magnetic Phases of Anhydrous FeCl3 in High Fields
Authors: Stampfel J. P. , Oosterhuis W. T. , De S. Barros F.
Published: February 2004

This paper reports preliminary results obtained with the Mössbauer effect in 57Fe in single crystals of anhydrous FeCl3. Neutron diffraction studies reported that this antiferromagnet had a spiral spin configuration and a Néel temperature of 15 K. More recent work has shown a TN of 9.75±0.2 K. We have found a transition temperature of 9±1 K. Experiments in fields up to 50 kOe at 4.2 K with the field and gamma ray parallel to the crystalline c axis were carried out. No evidence of a spiral structure was found above 10 kOe but rather a uniform decrease of internal hyperfine field with increasing applied field.
Collapse of the Magnetic Hyperfine Field by Intense rf Perturbation
Authors: Pfeiffer Loren
Published: February 2004

By carrying the technique of rf enhancement to its extreme limit, the entire magnetic hyperfine field at an iron nucleus was made to oscillate rapidly through large angles in response to an applied rf field. When the frequency of these rf oscillations was increased to several times the nuclear Larmor frequency, the time average field seen by the iron nuclei was reduced from ∼300 kG to zero.
UHV Mössbauer Emission Spectra of Thin Iron Films
Authors: Varma M. N. , Hoffman R. W.
Published: February 2004

Mössbauer emission spectra are used to determine relaxation effects of 57Fe in thin iron films deposited at 10−9 Torr and measured at 10−11 Torr. The spectra from the radioactive films have been analyzed at room temperature, liquid nitrogen, and 15°K over a range of thickness from 3×1016 atoms/cm2 to 0.12×1016 atoms/cm2 (25–1 Å). Samples thinner than 7 Å show the following characteristics: a positively resolved doublet interpreted as quadrupole splitting of about 0.6 mm/sec independent of thickness and temperature; a normalized resonance area roughly constant with temperature; one line increases in width and area and the other decreases as the temperature is lowered. Samples thicker than 10 Å are approximately characteristic of bulk iron. The transition is ascribed to superparamagnetic fluctuations in the internal field but the detailed line shapes are not simply explained. A multilayer 25-Å sample has six lines in the normal positions at 15° and 80°K, but the outer lines show a decreasing intensity as the temperature increases until they vanish at room temperature.
CrO2-Based Thermomagnetic Information Storage and Retrieval Systems
Authors: Waring R. K.
Published: February 2004

Its high intrinsic moment (σ≅85 emu/g at room temperature) and its readily accessible Curie point (Tc≅134°C) make single-domain CrO2 particularly suitable for thermomagnetic recording. Because coercivity in CrO2 scales with intrinsic magnetization, the large, thermally induced changes in local magnetization that occur near but below Tc are reversible and can be used for nondestructive ``pyromagnetic'' readout of recorded information. System component demonstration indicates that a 1-W cw laser-powered digital storage and retrieval system can be operated with thermomagnetic recording and nondestructive pyromagnetic readout at 2×106 bits/sec. A flash lamp powered system for thermomagnetic storage of halftoned photographs has been built with readout by anhysteretic transfer to a thin ferromagnetic film and subsequent magneto-optic conversion to a visible image.
Dependence of the E1 Reflectivity Structure in EuO on Temperature and Doping
Authors: Grant Paul M.
Published: February 2004

The temperature dependence of the E1 reflectivity peak (4f7→4f65d transition) in both intrinsic and Gd-doped single crystal EuO over the range 20°–300°K has been measured. It is observed that this structure occurs consistently at lower energies in Gd-doped EuO vis-a-vis intrinsic for all temperatures covered. X-ray data indicate a smaller lattice constant for the doped material thus substantiating the direction of the shift on the basis of an overlap model. A strong Burstein-Moss effect would have shifted the peak oppositely to that observed. The onset of the ``red shift'' associated with magnetic ordering in the bulk occurs at a higher temperature for the doped material consistent with its higher Tc, but the shift is not as closely correlated with this temperature as it is for intrinsic EuO. These results suggest a localized model for the E1 excitation.
Magnetic Phase Transitions in EuTe
Authors: Oliveira N. F. , Foner S. , Shapira Y. , Reed T. B.
Published: February 2004

The phase transitions of two EuTe single crystals were investigated at 0.4 K<T<4.2 K by means of differential magnetization and magnetic moment measurements in fields up to 140 kOe. The two single crystals had very different electrical resistivities and showed differences in their magnetic behavior. The high-resistivity sample (∼108 Ω cm at 300 K) exhibited a rise in dM/dH at low fields (H≲103 Oe) corresponding to the in-plane spin-flop transition. At the transition from the canted phase to the paramagnetic phase this sample exhibited a λ peak in dM/dH. For the low resistivity sample (∼10−2 Ω cm at 300°K) dM/dH was large at H=0 and it decreased at low fields (H≲103 Oe). No λ peak in dM/dH was present in this sample although a small broad peak was observed near the high-field transition. The canted-to-paramagnetic phase boundary in either sample obeyed the theoretical relation Hc(T)=A-BT3/2 at 2.0 K≲T≤4.2 K. Deviations from this T3/2 law appeared at T≲2.0 K. The results for Hc(T) agreed with ultrasonic data on the same samples. The saturation magnetic moment for the high-resistivity sample was 7.0 μB per Eu ion.