bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of Anomalous Infrared Attenuation in Neutron-Irradiated Compound Semiconductors
Authors: McNichols J. L. , Ginell W. S.
Published: January 2005

It is proposed that the anomalous infrared attenuation observed in neutron-irradiated compound semiconductors is the result of an induced local phase change to a metallic-like state. The total infrared scattering and absorption for a distribution of small metallic inhomogeneities in an otherwise uniform nonmetallic crystal is shown to produce attenuation which is proportional to the square of photon energy in the band gap. Application is made to fast-neutron-irradiated GaAs and agreement with reported data is found to be excellent. It is suggested that further investigation of the anomalous, structureless infrared attenuation observed for some irradiated compound semiconductors may lead to more quantitative understanding of spike phenomena in these materials.
Improved Room-Temperature Laser Performance in GaAs Diffused-Junction Diodes
Authors: Carlson R. O.
Published: January 2005

Significant reduction in room-temperature laser threshold and stimulated emission delay is obtained in diodes made from zinc-diffused wafers (850°C—3 hours in Zn–As atmosphere) if the single-diffusion process is modified with multiple diffusions, or, more controllably, by the addition of a post-annealing step (∼900°C in As atmosphere) following the initial diffusion. Incremental sheet-conductivity measurements show the zinc concentration gradient is reduced and etching studies show a lowered dislocation density near the junction after annealing. Without an annealing step, diffused-junction diodes show consistently larger emission delays (observed above ∼200°K) than epitaxial-junction diodes made from the same n-type substrates. The deliberate in-diffusion of high copper concentrations causes large delays even at 77°K. However, copper is probably not the center causing delays near room temperature because a mass spectrographic analysis did not reveal higher copper concentrations in diffused-junction wafers than in epitaxial-junction wafers when there was no deliberate copper doping.
Light Holes in Bismuth-Tin Alloys
Authors: Towle Laird C. , Cybriwsky Alex , Stajdohar Ralph E.
Published: January 2005

Galvanomagnetic measurements have been made on several polycrystalline bismuth alloys containing up to 0.15 at.% tin. Data obtained at 77.4°K have demonstrated the existence of a light-hole band in the bismuth band structure. The effective mass of the new carriers is ∼0.002m0.
Effects of Strain, Preferred Orientation, and Local Crystallite Size Variation on Néel and Cross-Tie Domain Walls
Authors: Walker G. A. , Heinzel C. O.
Published: January 2005

The effects on shape and coercive force, Hc, of Néel and cross-tie domain walls due to variation in crystallite size, strain, and preferred orientation have been studied. Experiments were designed to try to separate the different effects in order to better understand their respective contribution to domain wall motion. The breaking or joining of domain walls during motion has been shown to be caused either by crystallite size variation greater than 50 Å, or by local areas of preferred orientation plus strain. In order to assess the effects of strain or preferred orientation in these films, it is imperative to eliminate crystallite size effects. Once this is done, anisotropy terms are used to describe the experimental results. A crystallite size variation of 200 Å is shown to make a difference of 0.9 Oe in Hc. Preferred orientation plus strain in our films has been shown to reduce Hc by 2.6 Oe. The possible effects of intrinsic and extrinsic stacking faults are indicated.
Generalized Physical Optics Approximation for the Scattering of Electromagnetic Radiation from Imperfect Conductors of Irregular Shape
Authors: Von Roos Oldwig
Published: January 2005

The scattering of electromagnetic waves from plasmas of arbitrary shape (underdense and absorptive, overdense and/or collision-dominated) is calculated in the physical optics limit (wavelength short compared to a radius of curvature of the surface of the body). For infinitely conducting plasmas, the results for perfect conductors are recovered, and it is shown that the radar cross section for plasmas with a finite conductivity is, in general, polarization dependent, in contrast to the case of a perfect conductor.
Surface Self-Diffusion and Surface Energy of Nickel
Authors: Maiya P. S. , Blakely J. M.
Published: January 2005

The relaxation to flatness of sinusoidal single-crystal nickel surfaces produced by a photoetching technique has been measured under high-vacuum conditions. Results are presented on the variation of the surface diffusion coefficient Ds with temperature, orientation, and direction on certain low-index planes. In the temperature range studied, the diffusion coefficient is found to vary only by a factor of about two for orientations close to (100), (111), and (110). For the (110) plane, Ds is anisotropic with Ds (110)[001]> Ds(110)[11̄0]. From surface relaxation measurements in the wavelength range where volume diffusion contributes significantly, the surface free energy has been determined for the (100) and (110) planes. The values are 1821±182 erg/cm2 and 1900±190 erg/cm2, respectively.
Effect of Grip Stresses on Dislocation Configuration after Plastic Deformation
Authors: MacCrone R. K.
Published: January 2005

Berg—Barrett x-ray micrographs of strained single crystals of aluminum have revealed that the macroscopic dislocation configuration is inhomogeneous. Two kinds of inhomogeneity are recognised: short-range inhomogeneity in any one ``small'' area due to dislocation subboundary formation and long-range inhomogeneity in which both subboundary type and density depend upon the position along the specimen gauge length and also on the position within the cross-sectional area.

The experimental results are interpreted in terms of the effects of grip stresses on dislocation motion.
Residual Strains in Phosphorus-Diffused Silicon
Authors: Joshi M. L. , Ma C. H. , Makris J.
Published: January 2005

An x-ray line-broadening method is used to determine the residual strain level in a single-crystal silicon slice diffused with high amounts of phosphorus (surface concentration >1021 atoms/cm3). The strain level was found to be higher than that from the Prussin model. This is shown to be due to insufficient penetration of the diffusion-induced dislocations inside the diffused layers. Moreover, the strains along 〈220〉 directions were found to be considerably higher than those along other directions. This is interpreted as additional broadening along the Burgers vector direction.
Mössbauer Studies of Ordered and Cold-Worked Fe–Al Alloys Containing 30 to 50 at. % Aluminum
Authors: Huffman G. P. , Fisher R. M.
Published: January 2005

Fe–Al alloys containing 30 to 50 at. % Al were investigated by Mössbauer spectroscopy. Both ordered alloys, having the CsCl or B2 structure, and alloys severely cold-worked by crushing were studied. At room temperature, all of the ordered alloys exhibit only paramagnetic lines, whereas the crushed alloys exhibit both paramagnetic and ferromagnetic lines, the intensity of the former increasing from 0% to 100% as the Al concentration increased from 30% to 50%. The results can be explained by considering the nearest neighbor configurations of Fe atoms in the ordered and deformed alloys, with slight corrections for next-nearest neighbors. Plastic deformation induces ferromagnetism by creating large numbers of antiphase boundaries across which the number of Fe–Fe nearest-neighbor bonds is significantly greater than in the ordered alloy. The observed percentages of paramagnetic and ferromagnetic material and the average effective field values in the crushed alloys indicate that the average linear dimensions of the antiphase domains are about six atomic distances. In addition, the effects of antiphase boundaries and short-range order produced by heat treatment have been observed in alloys of the DO3 structure (BiF3 type) having less than 25% Al.
Torque on a Superconducting Torus in a Uniform Field
Authors: Wilson B. J. , King E. E.
Published: January 2005

The torques of superconducting lead tori immersed in various uniform magnetic fields are calculated as a function of angular position and compared to measured values. The calculations, based on ideal superconducting behavior, are found to agree with experiment for the closed torus if persistent current decrease due to critical surface fields is taken into account. The recognition of trapped flux phenomena is needed to explain measured results for the open torus when it has been cooled in a nonzero field or when a magnetic transition to superconductivity has been made. The analytic model utilizes magnetic moments for a torus in an axial and in a perpendicular field, both having been calculated rigorously elsewhere. In addition, methods of obtaining approximations to these moments are presented because of their possible application to more general toroidal bodies. The total magnetic moment (and thus the torque) for arbitrary angular position is obtained by superposition, a result not heretofore available.
Electrical Properties of β-Ag2Te and β-Ag2Se from 4.2° to 300°K
Authors: Dalven Richard , Gill Robert
Published: January 2005

The study of electrical properties of β-Ag2Te and β-Ag2Se has been extended to 4.2°K. The former compound was zone refined without decomposition. Both n- and p-type samples of β-Ag2Te were studied; all samples of β-Ag2Se prepared were n type to 4.2°K. Neither semiconductor showed any indication of extrinsic carrier freeze-out or of impurity banding. Study of oxygen as a possible acceptor in β-Ag2Te showed no significant effect; a selenium-doped sample of β-Ag2Se was n-type to 4.2°K.
Growth of ZnO Single Crystals
Authors: Park Y. S. , Reynolds D. C.
Published: January 2005

Single crystals of ZnO have been grown by subliming both ZnS and ZnSe and reacting them with oxygen in an argon gas flow. Using ZnS as the starting material, both platelet- and prism-type ZnO crystals were grown. Prisms having a cross-sectional area in the range of 1–5 mm2 and a length of the order of 10–20 mm were obtained. The platelets had an area as large as 40 mm2 and were the order of a few microns in thickness. The c axis was always contained in the plane of the plate. The prisms reflected the hexagonal geometry and were bounded, in general, by the (101̄0) and (112̄0) planes of the wurtzite lattice. When ZnSe was used as the starting material, hollow-type prisms and plates were the general forms taken by the resulting ZnO crystals. In these plates the c axis was normal to the plane of the plate.
Detection of Selenium Clustering in GaAs by Transmission Electron Microscopy
Authors: Abrahams M. S. , Buiocchi C. J. , Tietjen J. J.
Published: January 2005

Direct evidence has been found for the existence of coherent particles and small precipitates in bulk Se-doped GaAs (n=2×1018 cm−3) and in Se-doped, vapor-grown epitaxial GaAs (n=4×1018; 1×1019 cm−3). Data obtained from moiré fringe contrast indicate a probable particle composition of Ga2Se3. In samples with more than 2×1018 carriers cm−3, the major part of the total Se content is in the form of particles, and this accounts for the difference between the total Se concentration and the electrically active Se concentration. Diffusion in the solid can account for particle growth in the Bridgman-grown crystal, but not in the vapor-grown epitaxial material.
Ionizing Space-Charge Waves in Gases
Authors: Winn William P.
Published: January 2005

Luminous waves have been produced along the positive column of a glow discharge in N2 by applying a fast-risetime step voltage to an electrode at one end of the column. Such waves have been identified as a primary mechanism in the buildup of ionization in many kinds of electrical breakdown in gases. Voltage-sensing probes and a photomultiplier were used to observe how the waves were affected by the electron density in the positive column of the glow discharge. The most noticeable effect was that the velocity increased from 1×109 cm/sec at zero density to around 7×109 cm/sec at a density of 3×109 /cm3. The velocity also depended upon the magnitude of the voltage pulse, pressure, and the diameter of the coaxial shielding surrounding the glow-discharge tube. The shape of the wavefront also depended on the electron density, but it was more strongly affected by the pressure.

There were three striking differences between positive waves (those produced by a positive step voltage) and negative waves: (1) The velocities of positive waves depended more strongly on the initial electron density. (2) The wavefront of positive waves had a more complicated shape than that of negative waves. (3) Negative waves had to be initiated from a special electrode.

The electron density resulting from the ionizing waves has been calculated.
Thermoelastic Behavior of Natural Rubber
Authors: Shen Mitchel C. , McQuarrie Donald A. , Jackson Julius L.
Published: January 2005

Stress-temperature measurements of natural rubber were carried out up to the elongation ratio, α, of 2,0. An automatic stress relaxometer was constructed for this purpose which can be completely enclosed in a controlled environment. Experiments were so conducted as to minimize possible chemical effects and nonequilibrium conditions. Relative internal energy contribution to stress, fe/f, is calculated as a function of α in terms of statistical and thermodynamic theories. Both of these yield similar results. It is shown that in the region of low strains (1.0<α<1.5), fe/f decreases rapidly with increasing α, but appears to remain constant at 1.5<α<2.0. This observation is not in agreement with the prediction of the current statistical theory of rubber elasticity, which stipulates that the energy effects are intramolecular and independent of deformation. Implications of these findings are discussed. It is suggested that perhaps at low strains the intermolecular interactions are large in comparison with intramolecular energies, but become relatively insignificant at higher elongation ratios. The temperature coefficient of unperturbed chain dimensions is also calculated from thermoelastic data. It is constant only in the region 1.5<α<2.0. Finally, a new, more exact derivation of the Elliott-Lippmann anisotropy factor in terms of the statistical theory is given in the Appendix.
Surface Barrier Junctions on Semiconducting Ferroelectrics
Authors: Wemple S. H. , Kahng D. , Berglund C. N. , Van Uitert L. G.
Published: January 2005

Data are presented for semiconducting BaTiO3 and KTN which show that measurements obtained from surface barrier junctions formed on semiconducting ferroelectrics can be used to study transition temperatures and nonlinear dielectric behavior of these materials. An analysis is given which relates in a simple way the dielectric properties as described by the Devonshire thermodynamic formalism to the results of measurements on surface barrier junctions. Values of the Devonshire coefficients so obtained differ from those obtained in bulk insulating KTN, presumably because of clamping effects.
Location of Inert Gas Atoms in KCl, CaF2, and UO2 Crystals by H+ and He2+ ``Channeling'' Studies
Authors: Matzke Hj. , Davies J. A.
Published: January 2005

The α-particle emission rate of 222Rn, embedded in single crystals of KCl, CaF2, and UO2 at a concentration of 10−4−10−3 at. %, exhibits no detectable anisotropy—thereby proving that the radon atoms do not occupy single lattice vacancies and also, in the case of the fluorite structures, CaF2 and UO2, that they do not occupy the big interstitial holes either.

The α-emission rate of the short-lived daughter isotopes (218Po and 214Po) also exhibits no anisotropy in the KCl, and CaF2 experiments. In UO2, however, the polonium α-peaks are attenuated markedly within 0.8°±0.2° of the 〈100〉 direction, indicating that the Po atoms are located along the 〈100〉 lattice row.

Similarly, wide-angle Rutherford scattering of protons has been used to show that Xe, embedded in KCl at a concentration of 0.2 at. %, does not occupy single lattice vacancies.
Amplification in Two-Valley Semiconductors
Authors: Hakki Basil W.
Published: January 2005

Microwave amplification and negative conductance are discussed theoretically and experimentally for those semiconductors that exhibit the Gunn effect, e.g., GaAs, InP, and CdTe. Field-induced transfer of carriers, from the high-mobility, main conduction band minimum to energetically higher, low-mobility valleys, gives the medium a negative differential bulk conductivity. The resultant growing space-charge waves impart to the semiconductor structure a calculated negative conductance which is in good agreement with experiment. The regime of stable amplification in n-GaAs is found to be (2×107/l)<−(1+γ)n1l<0.65×1011 cm−2,where n1 is the carrier concentration in the lower valley, l is the sample length, and γ is the intervalley field rate of transfer of carriers. Static field distortion imposes a lower limit at n1l=1010 cm−2 on the above relation. Amplification under conditions of strong field distortion is also discussed, as well as other modes of amplification that exist in the bulk semiconductor.
Equiatomic Binary Compounds of Zr with Transition Elements Ru, Rh, and Pd
Authors: Wang Frederick E.
Published: January 2005

By means of single-crystal x-ray diffraction methods, an underlying structural similarity among ZrRu, ZrRh, and ZrPd has been established. Further, based on the high-temperature x-ray photographs, the existence of an ``Ms'' transition for ZrRh at 380°C and an ``Mf'' transition for ZrPd at 550°C has been confirmed. Based on this evidence, the existence of the unique ``martensitic'' transition ``bands'' can be visualized in the TiNi and ZrPd series of equiatomic binary compounds.
Concerning the Possibility of Observing Lifetime-Gradient and Dember Photovoltages in Semiconductors
Authors: Esposito R. M. , Loferski J. J. , Flicker H.
Published: January 2005

The photovoltage associated with a lifetime gradient, which was first analyzed by Buimistrov, is investigated. The analysis is extended to the high-illumination case, and it is shown that the theory predicts substantial photovoltages under suitable conditions. Attempts to measure this photovoltage across a ``lifetime junction'' produced by electron irradiation and by changing the surface-recombination velocity of part of the semiconductor sample are described. It is concluded that neither the lifetime-gradient nor the Dember effect can be measured if the specimen is terminated by ``Ohmic'' contacts. These effects were, however, measured by using capacitative coupling to the semiconductor. The materials on which the experiments were performed were n- and p-type germanium and silicon of various resistivities.
Space-Charge Effects on Emission-Limited Current Flow in Insulators
Authors: Frank R. I. , Simmons J. G.
Published: January 2005

The effect of electron traps on the current-voltage (J-V) relationship and the charge distribution in insulators, under conditions of space-charge-limited current flow, is well understood. This paper extends the consideration of trapping effects into the region of emission-limited current flow. Trapped space charge distorts the field in the insulator so that the usual J-V relationship for emission-limited current in a vacuum, J ∝ exp [(constant) V½], which is often applied to insulators, will not be observed initially. It is shown that at sufficiently high voltages, however, the trapped space charge becomes negligible and the above J-V relationship is obtained. A calculation is done to obtain the J-V characteristic in an insulator throughout the emission-limited region, for typical values of trapping depth, trapping density, and barrier height. The resulting curves show that the transition from space-charge-limited to emission-limited current occurs at reasonable voltages, and that at these voltages, space-charge effects are large enough to be readily observed in practice. It is shown that the absence of space-charge effects at higher voltages is a consequence of field lowering of the barriers around trapping sites. In addition, several general results are obtained concerning field and charge distributions in insulators containing traps.
Dislocation Climb Forces
Authors: Lothe J. , Hirth J. P.
Published: January 2005

With a definition of standard state in accord with common usage in thermodynamics, the chemical potential of vacancies is shown to yield the Peach-Koehler, Bardeen-Herring formulas, without any modifications, for the total force on a dislocation. The theory includes a derivation of the effect of internal stresses on the chemical potential of vacancies. The stability of dislocations under various conditions of stress, external and internal, and various conditions of vacancy equilibration, is discussed.
Mechanical Hysteresis in Deformed Copper Crystals
Authors: Bilello J. C. , Metzger M.
Published: January 2005

Half-cycle loops measured at room temperature in copper crystals of an easy glide strained into stage I represented decrements of the order of 10−1 at strain amplitudes in the 10−5 range. Coating the crystals with chromium was useful in extending the amplitude range over which closed loops could be studied. The energy loss was attributed to unpinning of primary-dislocation-forest-dislocation interactions. The large modulus reduction could be accounted for by using the density of excess primary dislocations inferred from pertinent etch-pit studies.
Oblique-Incidence Magnetic Anisotropy in Codeposited Alloy Films
Authors: Cohen Mitchell S.
Published: January 2005

Alloy films produced by simultaneous deposition from two separate sources, one evaporating NiFe and the other evaporating a nonferromagnetic metal X, can exhibit large oblique-incidence ferromagnetic anisotropy even for small-incidence angles. For vapor-beam angles of NiFe high compared with those of X, the simultaneous deposition causes a large increase in the value of the magnetic anisotropy over that produced by oblique incidence of the NiFe alone; for vapor-beam angles of X high compared with those of NiFe the oblique incidence of X controls the production of the anisotropy. The values of the magnetization dispersion and the wall coercive force Hw increase with the X concentration. For high X concentrations magnetic-viscosity phenomena may appear, while for sufficient X concentration the magnetic anisotropy of the film is destroyed. These effects were exhibited by films of NiFe with Ag, Al, Au, Pb, Sn, Zn, and Cu, but NiFeCu films were most extensively studied. Various measurements showed that the presence of X did not promote the growth of the crystallite chains which cause the oblique-incidence anisotropy in pure NiFe films. Instead, it is suggested that X metal is distributed at the crystallite interfaces in a layer which is so thin that it is not directly detectable, but which is thick enough to weaken or destroy the intercrystalline exchange bonds. Oblique incidence causes the interfacial layer of X around a given crystallite to be thicker in the direction of that vapor beam having the higher incidence angle, thus leading to magnetic anisotropy. When the X concentration and/or the deposition temperature is sufficiently high, X becomes isotropically distributed around each crystallite and each crystallite tends to become magnetically independent. This causes high magnetization dispersion, magnetic-viscosity effects (thermally induced magnetization fluctuations), and eventual destruction of the anisotropy.
Reflection and Transmission of Electromagnetic Waves by a Moving Uniaxially Anisotropic Medium
Authors: Lee S. W. , Lo Y. T.
Published: January 2005

The problem of reflection and transmission of electromagnetic waves at the interface between a stationary isotropic medium and a uniaxially anisotropic medium moving parallel to the interface is investigated. Solutions are obtained for both incident E-wave and H-wave when the moving medium is either a non-dispersive uniaxial crystal or a dispersive uniaxial plasma. It is found that (i) total reflection is possible even if the wave is incident from a comparatively less ``dense'' medium into a ``dense'' medium; (ii) Brewster angles may exist even for incident E-wave; (iii) backward wave may be excited in the moving uniaxial plasma by an incident H-wave. Other interesting features which are not found in the case of two stationary media are also discussed in detail, and a few numerical examples are given.
Dislocation Velocities in Nickel Single Crystals
Authors: Rohde Richard W. , Pitt Charles H.
Published: January 2005

The velocities of dislocations moving in nickel single crystals were measured at various temperatures as a function of stress. The data were found to fit an equation obtained from reaction-rate theory.

The activated volume varied approximately linearly with temperature and was independent of the applied stress over the rather limited range of stresses investigated.

The enthalpy of activation was also independent of stress but changed abruptly with temperature from 1200 cal/mole at approximately 130°K to 2500 cal/mole at higher temperatures. This change is explained by a change in the Peierls stress, which is associated with a sudden increase in dislocation density at higher temperatures. The dislocations were believed to be emitted from sources which are thermally activated.
A Family of New Cobalt-Base Permanent Magnet Materials
Authors: Strnat K. , Hoffer G. , Olson J. , Ostertag W. , Becker J. J.
Published: January 2005

The magnetocrystalline anisotropy of several intermetallic phases of the type RCo5 (R = Y, Ce, Pr, Sm, Y-rich and Ce-rich mischmetals) has been investigated, and it is concluded that these alloys are promising candidates for fine-particle permanent magnets. They have extremely high uniaxial anisotropy (K = 5.4 to 7.7 × 107 erg/cm3), single easy axis, high saturation (Bs = 8500 to 11 200 G) and Curie point (tc = 464° to 747°C). Approximate upper limits for the possible energy product lie between 18 and 31.3 MGOe. Experimentally, coercive forces of over 8000 Oe and (BH)max = 5.1 MGOe have been observed in SmCo5 merely ground at room temperature. Grinding of YCo5 and (Ce-MM) Co5 produces an increase of MHc to 2200 and 2700 Oe, respectively, followed by a decrease as particle size continues to decrease.
The Formation of Uniaxial Anisotropy in Nickel by Plastic Deformation
Authors: Krause R. F. , Cullity B. D.
Published: January 2005

Previous plastic elongation, or compression, of polycrystalline nickel was found to produce a uniaxial anisotropy. The easy axis of magnetization was parallel to the direction of previous elongation and at right angles to the direction of previous compression. The maximum value of the uniaxial anisotropy energy constant Ku was about −2 700 erg/cm3 after 2.5% elongation.

The observed anisotropy is shown to be due to a particular distribution of residual microstress, in which most of the volume of the specimen is in residual compression, balanced by a small volume in tension, after plastic elongation. These stresses, although balanced, introduce magnetoelastic effects which do not cancel in a nonsaturated specimen and which thereby cause the observed anisotropy. Evidence for the assumed stress distribution was obtained by x-ray diffraction.
Magnetic Properties of Splat-Cooled Fe-Co-V Alloys
Authors: Nesbitt E. A. , Willens R. H. , Williams H. J. , Sherwood R. C.
Published: January 2005

Previous investigations showed that the magnetic properties of Fe-Co-V alloys were susceptible to the speed of quenching and therefore it was thought that splat cooling might give further insight into their magnetic behavior. Accordingly, alloys having contents from 0% to 20% vanadium with 52% cobalt and the remainder iron were splat-cooled. Coercive forces of 850 Oe which are high for this system were obtained on splat-cooled specimens having 20% V after an aging treatment. A rather puzzling phenomenon was observed on splat-cooled samples of alloys in the vicinity of 14% V; at 300°K, the specimens acted in a paramagnetic manner, but upon cooling down to 1.4°K, they became ferromagnetic. The increase in magnetization cannot be attributed to an increase in the α phase since this transformation is irreversible, and in this case the magnetic properties are reversible on cooling and heating. This behavior could be explained, however, if there were a large number of superparamagnetic particles present. Electron micrographs were taken which showed particles of less than 50 Å in diameter. Furthermore, upon aging at 500°C, particles of a much larger size were observed (several hundred angstroms), the specimens then became ferromagnetic at room temperature and the large increase in magnetization by cooling to low temperatures disappeared.
Magnetic Interactions in Assemblies of Single-Domain Particles. The Effect of Aggregation
Authors: Waring R. K.
Published: January 2005

A model is presented for describing the effect of magnetic interactions in assemblies of single-domain particles, such as occur in magnetic recording tape coatings. Bundles of particles are hypothesized within which interaction takes the form of a demagnetizing field, but between which interaction is weak or absent. The switching fields of particles within a specified bundle are equal. This approach yields both a finite initial slope for the anhysteretic characteristic and a stable Preisach diagram. Thus it combines the attributes of a demagnetizing field model and a simple pair model. Relations among assorted magnetization characteristics that have been derived for a pair model follow more generally from a stable Preisach diagram and, therefore, do not serve to distinguish between microscopic models.
Computer Simulation of the Magnetic Dipole Interaction Problem
Authors: Moskowitz Ronald , Della Torre Edward
Published: January 2005

Interacting systems of a large number of hysteretic bistable magnetic particles have been studied by direct calculation using digital computers. In this paper, we discuss calculations made on oriented dipole moments in a regular cubic array periodically extended parallel to the dipole axes in order to avoid demagnetization effects. Distributions of critical fields and moments are assigned to the array. The switching fields of the particles are modified by the interaction field. After the interaction fields are computed, the array is relaxed for the current applied field which may be cycled in any desired variation. The relaxation process is in order of decreasing particle vulnerability. The average interaction field has been found to be a linear function of the magnetization in agreement with the theoretical predictions of Della Torre. Distribution functions, n(Hi), have been determined. In addition a type of breakdown phenomena in these arrays suggested by Barker has been observed.

In application to fine particle systems as used in magnetic recording tapes, effects such as variation in particle orientation and nonuniform dispersions may be simply introduced into a subprogram of the simulation. In a similar fashion the model can be generalized to different types of interactions and applied to other magnetic and dielectric cooperative phenomena.
Isothermal Magnetic-Annealing Experiments on Substitutional Alloys
Authors: Lutes O. S. , Ulmer R. P.
Published: January 2005

Isothermal magnetic-annealing experiments were carried out by means of torque measurements on a Ni(Fe) alloy containing 16% Fe and on Fe(Si) alloys with concentrations between 2.7 and 7.9 at. %Si. The measured torques showed an approximate exponential time-dependence in approaching equilibrium. The measured time constants for Ni(Fe) were greater than those of Fe(Si) by a factor of about 40 in the vicinity of 400°C. In the case of Ni(Fe), fair agreement of the induced uniaxial anisotropy coefficient, Ku, with previous quenching techniques was obtained by assuming Ku to vary with measuring temperature as the square of the saturation magnetization. For Fe(Si) the Ku-results permitted a comparison with directional-ordering theory applied to dilute alloys. The pair-energy coefficient derived from Ku was higher than that calculated from magnetostriction and elasticity data. It is suggested that this discrepancy may be due to the effect of second-nearest-neighbor contributions.
New Figure of Merit for Ceramic Permanent Magnet Material Intended for dc Motor Applications
Authors: Ireland J. R.
Published: January 2005

In evaluating ceramic magnet materials as to suitability for permanent magnet motor use, it has been found that the normally accepted criterions, energy product, is of very little value.

A new figure of merit is proposed, which is BrHx. Br is the residual induction of the material, and Hx is defined as the H coordinate of a point on the intrinsic demagnetization curve, whose B coordinate is 0.8Br. For an imaginary motor with zero air gap, equations are developed to show an inverse proportionality between BrHx and the total volume of magnet material required to meet a given motor performance curve. For real motors, which contain air gaps, the figure of merit becomes; Br[Hx−0.8Br/(Pc +1)], where Pc is the permeance coefficient at which the magnet is operating. Since Pc does not vary widely in typical motors, and since this second figure of merit is not extremely sensitive to the value of Pc in typical ceramic magnet materials, an average value can be assigned to each material for comparison purposes. This second quantity is designated BrHa.

A tabulation of both figures of merit, and other data, is given for the materials designated by the Magnetic Materials Producers Association as Ceramic 2–6, inclusive.
Origin of Constricted Magnetization Loops in Hard-Rolled Nickel at Low Temperatures
Authors: Hirsch A. A. , Eliezer Z.
Published: January 2005

Constricted magnetization hysteresis loops were observed at 4.2°K in tapes of spectroscopically pure Ni, which were prepared by cold-rolling with a reduction in thickness of 95% when the driving field was applied perpendicularly to the rolling plane. The thickness of these samples was 0.003 cm. A pronounced anisotropy with respect to the rolling axis was detected in the slow relaxation effects when hysteresis loops of the magnetoresistance effect were displayed in hard-rolled samples at 20.3° and 4.2°K. The loops of the magnetization and of the magnetoresistance effect were cycled with triangular waveform fields, with very low frequencies varying from 0.005 to 1 cps when the drive amplitude changes from 0.5 to 7.2 kOe. The lower the frequency the larger the observed constriction of the loops. Because of the relaxation effects, one assumes that the hard-rolled metal has an intrinsic structure, which may be compared with that of thin films with densely spaced domain walls. In view of the theory of double walls in thin films, we suggest an interpretation for the magnetization process in the laminated material, supposing a mechanism of stabilization of domain double-walls of different types. This mechanism seems to be responsible for the observed constricted magnetization loops, as well as for the anomalous hysteresis of the magnetoresistance appearing in some samples at 4.2°K.
Angular Variation of Coercive Force in Permanent Magnet Materials
Authors: Becker J. J.
Published: January 2005

The following model is taken for a permanent magnet material: (1) A distribution of coercive forces is present, and (2) Hc is determined by a nonrotational process such as domain wall motion. This should be a good description of a powder material in which Hc is much less than 2K/M. From a given Hc distribution, one can then compute Hc as a function of peak magnetizing field for an aligned sample in various directions. One can also compute Hc as a function of angle at constant peak magnetizing field. This may show a maximum at 0° even in quite large fields. Thus, such a maximum need not indicate the onset of Stoner-Wohlfarth behavior. The computed curves may also show maxima in intermediate directions. The computed results are compared with experimental results on MnBi and Co5Y.
Magnetic Interactions of Superparamagnetic Particles
Authors: Brown William Fuller
Published: January 2005

Data on the susceptibility χ of superparamagnetic particles show deviations from the simple formula χ=c0β(β=1/kT, c0=NMs2v2/3; the particles, N per unit volume, have spontaneous magnetization Ms and volume v). An obvious explanation, magnetic interactions, has been questioned. To test the idea further, the following model has been examined theoretically: uniformly magnetized isotropic spheres of radius a, grouped in pairs with center-to-center distance R≥2a, with appreciable magnetic interactions only between the two members of a pair. The resulting formula gives, at high temperatures, χ=β(c0+c1β+...), with c1=(4π/3)c02 in accordance with Hahn's general theorem, and at low temperatures, χT=C1−C2T, with C1=(2/3)pvMs2/k and C2=(2/3)pR3/v; p=Nv is the packing fraction. A fit to Hahn's data on iron gives C1=1.924, C2=0.00962; with p=0.0042 and Ms=1710, this gives v=3.24×10−20 cm3, a=19.8 Å, R=48.1 Å>2a, physically reasonable values. It therefore appears that magnetic interactions can account for the observed deviations of χ from c0β in some cases, despite seemingly strong evidence against this explanation in others.
Lead Contamination of Flux-Grown Garnets and the Effect on Optical Properties
Authors: Sobon L. E. , Wickersheim K. A. , Robinson J. C. , Mitchell M. J.
Published: January 2005

One occasionally observes in flux-grown gallium and aluminum garnets a purple discoloration. Combined optical absorption, emission spectrographic and electron microprobe studies of crystals showing pronounced color banding indicate a direct correlation between depth of purple color and dissolved lead content. In addition to the broad visible absorption band leading to the purple color, a sharp absorption band is observed near 2.8 μ in the discolored crystals. Since the small lead impurity cannot by itself produce either of these absorptions, they must be attributed to associated charge-compensating mechanism—e.g., charged vacancies (color centers) in the one instance and OH− replacements for oxygen ions in the other. While it can be argued that the lead impurity must be partially self-compensating, the divalent state appears to predominate.
Linewidth Characteristics of Ce3+-Doped YIG
Authors: Nilsen W. G. , Hensel J. C. , LeCraw R. C. , Remeika J. P.
Published: January 2005

The linewidth characteristics of single-crystal Ce3+-doped YIG have been measured as a function of temperature and microwave frequency. The linewidth-vs-temperature curve exhibits a maximum at about 120°K. The temperature of this linewidth maximum is essentially the same both at 12 000 Mc/sec and 56 022 Mc/ sec. Of more direct interest in determining the linewidth mechanism is the observation that the linewidth increases linearly with microwave frequency both above and below the linewidth maximum. These results can be interpreted either in terms of a longitudinal or transverse relaxation mechanism. For the longitudinal mechanism, the microwave frequency ω and relaxation time τ enter the expression for the linewidth as ωτ/(1+ω2τ2), and the linear dependence of linewidth on frequency can only be accounted for by assuming ω2τ2≪1. In our experiment, this requires that the relaxation time be less than about 10−12 sec. For the transverse mechanism, the linewidth is proportional to ωτ in agreement with experiment, but for this mechanism to be operative τ must be sufficiently short so that there is overlap between adjacent energy levels above the ground state. With present infrared data, it is not possible to ascertain if this condition is satisfied.
Effects of Hydrostatic Pressure on the Compensation Temperatures of Iron Garnets
Authors: Bloch D. , Chaissé F. , Pauthenet R.
Published: January 2005

We have measured the variation of the temperature of the compensation point, θc, of several garnet ferrites as a function of hydrostatic pressure p, up to 6 kbar. One observes that dθc/dp is positive and decreases from 0.95×10−3 °K·bar−1 for GdIG to 0.38×10−3 °K·bar−1 for HoIG. The value dθc/dp yields information about the variation with pressure of the exchange interaction between the iron and rare earth ions.
Effect of Impurities on the Microwave Properties of Yttrium Iron Garnet
Authors: Sparks Marshall
Published: January 2005

Simplified physical models are given for the giant anisotropy theory, the longitudinal relaxation theory, and the rapidly relaxing impurity (i.e., transverse relaxation) theory for the ferromagnetic resonance linewidth ΔH, and field for resonance Hres. A classification of the types of temperature T and frequency ω dependence of ΔH and Hres which may result from the longitudinal relaxation mechanism is given. Recent experiments on giant anisotropy peaks in ΔH and Hres and on maxima in ΔH and Hres vs T are interpreted with reference to this classification. By observing both ΔH and Hres, it is possible to distinguish between the transverse and longitudinal mechanisms even when ΔHis proportional to ω above and below the maximum of ΔH vs T.

A new interpretation is given for observed linewidths at the angles for the giant anisotropy effect, and the possibility of having a longitudinal relaxation mechanism without maxima—such as T−1 sech2(ℏωn/2kBT) —in ΔH and −Hres from the population factor is discussed.
Effect of Low-Temperature Magnetic Anneal on the Linewidth of Garnets Containing Fe2+ Ions
Authors: Tchernev Dimiter I.
Published: January 2005

The linewidth of garnets containing Fe2+ ions was measured from 4.2° to 400°K at 10 and 25 kMc/sec for unannealed samples and samples annealed in the three major crystallographic axes. It was found that the linewidth at low temperatures depends very strongly on the magnetic history of the sample during the cooling cycle. For samples cooled in zero magnetic field, the linewidth at 4.2°K becomes very large and resembles the case of nonsaturated resonance where domains go through resonance at different fields. Magnetization measurements of such samples at 4.2°K confirm qualitatively the increase of field for saturation. The linewidth of samples cooled in a magnetic field is much smaller and exhibits the characteristic peak as a function of temperature. The wellknown electron diffusion at low temperatures, which causes induced anisotropy in these samples, affects both the value and position of the peak. In particular, the linewidth is reduced and the peak is shifted to higher temperatures when the sample is cooled and measured in the same direction. Any change of the direction of the applied magnetic field increases the linewidth and shifts the peak to lower temperatures. Above 100°K this effect disappears. This is in good agreement with induced anisotropy measurements which show that above this temperature the diffusion time constant of the electrons is too short to affect the linewidth. We conclude that at low temperatures electron diffusion affects the linewidth but is not the main relaxation mechanism. The latter seems to be intraionic relaxation.
Optical Studies of Exchange in Substituted Garnets
Authors: Wickersheim K. A. , Buchanan R. A.
Published: January 2005

The purpose of this study is to see if spectroscopic techniques can be extended to yield information about the strength of coupling between the rare earths and specific iron neighbors in the iron garnets. Trivalent cerium is used as the probing ion because of the simplicity of its spectrum and because of the unusually large exchange splittings which it exhibits in the garnets. Cerium is incorporated in low concentrations in single crystals of the garnets under investigation. Octahedral or tetrahedral iron ions are replaced selectively by other trivalent ions, the influence of these substitutions on the exchange-split 2.7-μ line of cerium being observed. For tetrahedral iron replacements in excess of 20 at.%, discrete new spectra are superimposed on the spectrum of unsubstituted YIG:Ce. At low substitution levels (replacements of less than 1% of the iron), the new spectra consist of a small number of additional weak but sharp lines. Such spectra obtained using low substitutions of aluminum and gallium can be interpreted in terms of reduced exchange splittings produced by the replacement of single tetrahedral iron neighbors only. From these preliminary experiments, the magnitudes of the contributions of the nearest (axial) and next-nearest (nonaxial) tetrahedral iron neighbors to the exchange field at the rare earth can be separately deduced. Both contributions are significant with the axial contribution dominating. No contribution from the octahedral iron neighbors is observed.
Boundary Conditions for Ferromagnetic Media
Authors: McIsaac Paul R.
Published: January 2005

The boundary conditions for plane wave propagation in a classically continuous model of a ferromagnetic medium are discussed. Inclusion of the exchange effect introduces spatial dispersion into the magnetic susceptibility. The spatial dispersion expresses the nonlocal relationship between the magnetic flux density and the magnetic field, and this leads to a reexamination of the boundary conditions. A boundary condition for the magnetization is derived by applying the physically reasonable requirement that the exchange energy for any finite volume of the medium (in particular, including the boundary) must be finite. The modified boundary conditions are found to depend on a parameter which involves the logarithmic derivative of the magnetic susceptibility with respect to the phase constant. The magnitude of this parameter is discussed for the case where the dc magnetic field is normal to the boundary, and this parameter is found to be important in the spin wave region. These boundary conditions are relevant to the interpretation of spin wave resonance experiments in thin magnetic films.
The Local and Nonlocal Magnetization of Impurities in Linear Antiferromagnetic Chains
Authors: Edelstein A. S.
Published: January 2005

Some organic free radicals represent linear antiferromagnetic chains in which the low-temperature susceptibility is dominated by impurities. The impurities appear to be coupled via exchange to their neighbors and yet still give rise to a paramagnetic contribution to the magnetization. Previously,1,2 an Ising model with ferromagnetically coupled, randomly distributed impurities was presented to interpret the magnetization. Simple assumptions permit this model to be generalized to other anisotropies. It is shown that for a small concentration of impurities there are two contributions to the low-temperature magnetization. One is a local contribution and is proportional to the number of impurities q. It is largest and least localized for Heisenberg coupling. The local contribution is given by gβ(2Σ lim i=0∞ Si)1/2/2where the Si's were defined in a previous work.3 The other contribution is nonlocal, depends upon the zero-temperature long-range order ωinfty, and probably can be represented as a single spin of magnitude gβ(ω∞q)1/2/2. Near the Ising limit the system acts like a superparamagnet. The magnetic field dependence of the magnetization can be used to separate the two contributions. For Heisenberg coupling, the magnetization will be paramagnetic and this paramagnetism persists as kT/ | J | → 0. This is a direct consequence or alternatively a measure of whether ω∞=0 in the case of Heisenberg coupling. The interaction between spins in the organic free radicals is better represented by Heisenberg coupling than Ising coupling. The low-temperature experimental results agree with the paramagnetism predicted in the case of Heisenberg coupling.