bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferromagnetic Resonance of Iron Whisker Crystals
Authors: Rodbell D. S.
Published: March 2008

The ferromagnetic resonance absorption characteristics of iron single crystals (grown by K. W. DeBlois of this laboratory) have been examined between −196°C and 850°C. The crystals are of filamentary growth habit and are commonly known as whiskers. The selected crystals have 〈100〉 growth directions, and their square cross sections are bounded by {100} crystal planes. They are typically 10 μ on a side, but sizes ranging from 2 to 40 μ have been examined. Resonance absorption is observed using standard techniques with the dc magnetic field parallel to the length of the whisker. The microwave energy penetrates only a small fraction of the volume due to eddy current limitation. Two resonance modes may be observed. One is driven by the uniform microwave magnetic field, the other by its curl component. The curl mode is equivalent to resonance in a flat plane of infinite extent and no edge corrections are required. The uniform mode subject to an additional field arising from the uncompensated magnetic pole distributions at the specimen edges. After identification, the uniform mode is uninteresting because of distortions that probably result from the inhomogeneities of the edge demagnetizing field. The curl mode on the other hand exhibits a line shape and width that, in some cases, may be evaluated from first principles. The line width in these cases is accounted for almost entirely by exchange and conductivity effects, no phenomenological damping mechanism being required. Because other crystals of iron exhibit much broader resonance line widths, these results suggest that crystal perfection, particularly surfaces in the case of metals, may be the determining factor of ferromagnetic resonance line widths in bulk crystals.

The data yield for the spectroscopic splitting factor, g, a temperature and frequency independent value of 2.05±0.01. The exchange stiffness parameter, A, determined in a self-consistent way with this splitting factor, is 25±5×10−7 erg/cm at 20°C.
Solubility of Carbon in Iron as Determined by the Magnetic Aftereffect
Authors: Singer Joseph , Anolick Eugene S.
Published: March 2008

A slightly lower ΔH of solution for carbide, presumably cementite, in pure iron, about 8000 cal/g atom, than is given by internal friction was obtained by measurements of the time decay of permeability in infused carbon-iron Epstein samples. The method is independent of any assumption of proportionality between the time decay and dissolved carbon. While the lower ΔH may not be significant, the carbon solubility, e.g., 0.006% at 600°, is claimed to be significantly lower than the accepted internal friction values, e.g., 0.01% at 600°. Causes of the disagreement are suggested. The data also show that time decay of permeability is proportional to dissolved carbon.
Influence of Plastic Deformation on the Time Decrease of Permeability in Transformer Steel
Authors: Smoliński A. K. , Kaczkowski Z. , Zbikowski M.
Published: March 2008

The paper gives the results of measurements of the influence of plastic deformations in the time-decrease of permeability in hot rolled transformer steel. In the first part measurements were performed on samples rolled in a mill. The measurements showed a drop of time decrease of permeability at large elongations and a maximum at small elongations. In the second part measurements were performed on samples stretched in a tensile testing machine. The results of measurements are characterized by typical wavy shape of time decrease of permeability. The obtained results are connected with the variations of lengths of the paths of impurities diffusing in the crystal lattice.
Room Temperature Decomposition of Austenite in Fifty Percent Nickel-Fifty Percent Iron Magnetic Alloy Tapes
Authors: Ananthanarayanan N. I. , Peavler R. J.
Published: March 2008

Upon room-temperature storage, austenitic, annealed fifty percent nickel-fifty percent iron magnetic alloy tapes have been found to show structural instability resulting in a slow and partial decomposition into an orthorhombic or tetragonal phase. The decomposition has so far been detected in 0.002-in.-thick tapes from only one experimental and one commercial alloy. To date, covering a period of over three years, the decomposition appears to have progressed only very slowly and remains restricted to the surface of the tapes. The decomposition that has occurred in these tapes does not seem to have any measurable effect on their magnetic characteristics. Further studies of the influence of tape purity and of tape thickness upon the transformation and of the effect of any transformation in ultrathin tapes on their magnetic properties are being made.
Mercury Process for MnBi Production
Authors: Goldman A. , Post G. I.
Published: March 2008

A method for producing manganese bismuthide has been investigated where in the manganese and bismuth are first “dissolved” in mercury by heating at atmospheric pressure. The mercury is subsequently vacuum distilled at a low temperature, leaving a strongly ferromagnetic residue of MnBi. The MnBi prepared by this method was shown to be identical with that prepared by other methods. X-ray diffraction showed only lines that could be attributed to MnBi. In addition, the lattice constants agreed with those reported by other investigators. The first anisotropy constant was determined for this material as well as for known MnBi. These anisotropies were found to be similar. The magnetic moment of the material produced by the mercury process was found to depend on the stoichiometry of the product, the maximum being obtained for that computed for the formula, MnBi.
Investigation of a Precipitation Hardening Elinvar
Authors: Hawkes F. C.
Published: March 2008

In a nickel-iron-chromium alloy of the Elinvar type containing 2.5% titanium, it is possible to retain the titanium in solid solution by a quench from 1000°C. Aging at 600°C produces a nonmagnetic precipitate of an intermetallic compound (Ni3Ti) dispersed throughout the ferromagnetic matrix. The precipitate can be detected by changes in mechanical and magnetic properties. The internal friction, Curie point, and coercive force were measured during the course of precipitation. The effect of aging time and temperature, plastic deformation, and composition on Q was studied. Both continuous and discontinuous precipitation were observed. The decrease in strain-induced magnetostriction as a result of aging in this alloy raised the elastic modulus and changed the shape of the modulus versus temperature curve below the Curie point.
Electronic Structure of Transition Metals
Authors: Marshall W. , Weiss R. J.
Published: March 2008

Several distinct lines of experimental work now point toward a new conception of the electronic structure of the transition metals. Recent x-ray work by R. J. Weiss suggests that metals of atomic number less than Fe and Fe itself have few 3d electrons and many conduction electrons while for metals above Fe this is reversed. These results are discussed and the ideas to which they lead described. On this basis the magnetic properties of all the pure metals are easily understood and the variation of the saturation moments of all the binary ferromagnetic alloys can be explained. Results of neutron diffraction on ferromagnetic alloys also support the new ideas and further experiments should provide very clear tests of them. It is shown that these alloys can be divided into four types, each with a characteristic behavior which will be described. Further evidence, coming from recent unpublished work, is also briefly mentioned.
Nucleation of Ferromagnetic Domains in Iron Whiskers
Authors: DeBlois R. W. , Bean C. P.
Published: March 2008

Nucleation fields up to 483 oe have been experimentally observed for the reversal of magnetization in regions of nearly perfect iron whiskers about 5 μ thick. This is near the theoretical limit 2K1/Ms ( = 560 oe  for iron) for the coercive force resulting from magnetocrystalline anisotropy. In addition, conspicuous growth defects on the surface are observed to be sites of easy nucleation. A field of about one oersted, if applied to the damaged end of an iron whisker of this size, suffices to reverse the magnetization by domain wall motion.
Magnetic Contributions to the Elastic Constants of Nickel and an Fe-30% Ni Alloy at High Magnetic Fields
Authors: Alers G. A. , Neighbours J. R. , Sato H.
Published: March 2008

The possibility that the fundamental interatomic magnetic infraction makes an observable contribution to the elastic constants of a ferromagnetic metal has been investigated experimentally in nickel and an Fe-30% Ni alloy. The effects of a noncentral interaction were found to be too small to be observed or were masked by effects arising from magnetization direction changes induced by the measurement process. The central force part of the interaction which should produce changes in the elastic constants upon passing through the Curie temperature was clearly observed in both nickel and the Fe-30% Ni alloy. The interpretation of these results in terms of the magnetic exchange energy is straightforward for nickel but the large volume magnetostriction in the Fe-Ni alloy makes the analysis of the data difficult.
Theoretical and Experimental Analysis of the Ferromagnetic Explosively Shocked Current Pulse Generator
Authors: Johnson Jerome H.
Published: March 2008

To most effectively utilize the potential capability of the ferromagnetic explosively actuated current generator one must understand the effect of variation of parameters upon the device output into a given load. This report presents theories of operation of the device, an approximate theoretical analysis of the effect of variation of the basic circuit and device parameters and a comparison of theory with experimental results.

Theoretical and experimental data are presented and compared for peak current versus circuit load inductance, device load coil turns, and magnetic core width (for a fixed thickness). There is a gratifying similarity between theoretically predicted and experimentally determined data.

Guiding criteria are given for an optimum ferromagnetic current pulse generator system.
Design of Automatic Recording Instruments for Magnetic Measurements in a Hot Cell
Authors: Byrnes W. S. , Crawford R. G. , Hall R. C.
Published: March 2008

Two remotely controlled, automatic recording instruments have been designed and constructed for making magnetic measurements in a hot cell: A torque magnetometer for crystal anistropy measurements and a magnetostrictometer for magnetostriction measurements. The magnetometer automatically plots the torque exerted on a slowly rotating sample as a function of angular displacement with respect to the direction of the applied field. Its sensitivity is 2×103 erg/mv. The magnetostrictometer automatically plots strain as a function of applied field. Its maximum sensitivity is 0.2×10−6 in./in. mv.

Emphasis is placed on design features that enable these instruments to be operated by remote control. In this type of hot cell work, the handling of the samples to be tested requires the use of an expert operator at the manipulator. For this reason tools and jigs are described that compensate for operator inexperience. Such tools are an automatic screw driver, a vacuum pickup, jigs for bonding prewound strain gauges to the samples, etc.
Partial Switching of Thin Permalloy Films
Authors: Hagedorn F. B.
Published: March 2008

The switching processes in thin Permalloy films have been investigated by means of partial switching. The procedure was to interrupt the switching process before completion and then to complete the reversal with a second switching pulse. Comparison of partial switching with complete switching in thin films confirms the existence of nonuniform rotation, a reversal mode different from either wall motion or uniform rotation. The domain structure of the partially switched states has also been determined. Several characteristics of nonuniform rotation have been inferred from the domain structure and from the partial switching wave forms. However, it has not been found possible to correlate these characteristics with a detailed model of the nonuniform rotational process.
Stress Anisotropy in Ni-Fe Thin Films
Authors: Blades John D.
Published: March 2008

The effective domain anisotropy energy of Baltzer has been extended to a simple, randomly oriented polycrystalline, single domain thin film under the influence of an isotropic planar and linear stress system. Energy equilibrium conditions lead to the definition of an average effective rotational anisotropy field, Hs. Utilization of the monocrystal data of Hegg, and Bozorth, and Walker for Ni-Fe with the linear stress determination of MacDonald permits a qualitative prediction of Hs. Measurements of Hc, and Hk on evaporrated Ni-Fe thin films for three substrate temperatures at various compositions indicate the existence of a stress anisotopy below possibly a recrystallization temperature, which varies with composition.
In-Pile Measurements of Radiation Effects in Magnetic Materials
Authors: Alley R. E.
Published: March 2008

Experiments have been performed at Brookhaven National Laboratory to determine the effects of nuclear reactor radiation upon the characteristics of ferrites and metallic magnetic materials. Photographs of 60 cycle hysteresis loops and bridge measurements of 1000 cycle inductance were made before irradiation, periodically during irradiation, and after reactor shutdown. The samples were irradiated for 12 days, resulting in an integrated fast flux of 1.6×1017 neutrons/cm2.

No permanent effect was observed in any ferrite. In-pile changes in ferrite properties are identified with changes due to temperature variations resulting from gamma ray absorption.

Permanent damage was observed in all metallic samples tested. Supermendur and Deltamax were least affected, showing increases of the order of 50% in coercive force and small decreases in initial permeability. Various samples of 4–79 Mo-Permalloy were affected differently, but all showed change in shape of hysteresis loop, several hundred percent increase in coercive force, and about 80% decrease in initial permeability. Supermalloy was most damaged by radiation, showing marked effects after 24 hr, and progressively greater effects throughout the exposure period.
Some New Magnetic Phenomena of Hematite Single Crystal
Authors: Lin S. T.
Published: March 2008

Three sets of magnetic measurements of pure natural hematite single crystals have been carried out in the temperature range from 488°K down to 4.2°K. The first set is the magnetization isotherms along a certain direction in the basal plane which show nothing new but confirm Néel's magnetic model of a superposition of a weak ferromagnetism on a normal antiferromagnetism. The second set is the isotherms along the ternary axis which display very unusual form. The analysis of the isotherms shows: (1) that the antiferromagnetic susceptibility-temperature curves, χ-T, are in good agreement with those obtained by Néel and Pauthenet, (2) that the spontaneous magnetization-temperature curves, σ0-T, indicate that there is no evidence of isotropic ferromagnetism, and that the weak anisotropic ferromagnetism in the basal plane above transition and along the ternary axis below transition seems to have the same nature and origin, (3) the wide transition over a hundred degrees takes place gradually and continuously. The third set of the magnetic measurements is the remanent magnetization along the ternary axis. The σR-T curve is very similar to the σ0-T curve. A more general magnetic model has been proposed which explains the experimental data very satisfactorily. From the present model Haigh's data of remanent magnetization of hematite powder seems to be explained naturally.
Recent Advances in Magnetic Devices for Computers
Authors: Looney Duncan H.
Published: March 2008

Major advances in computer technology have been made by the use of magnetic devices. The speed of operation, storage capacity, and reliability of computers has been significantly increased by the development of the memory core. The present paper will describe some of the newer magnetic elements which may replace ferrite toroids in memory matrices. Ferrite materials are being used as sheets and multiaperture devices. The sheets show great promise in achieving a cost reduction. The multiaperture devices in memory applications offer the advantages of increased speeds, a wider operating temperature range, and nondestructive readout. The ferromagnetic materials are used in the form of thin films and wrapped wire. The films are capable of increased switching speeds at nominal current drives. The use of magnetic wire as a storage element offers new fabrication technology, a wider operating temperature range, and new functional structures. A comparison is made to highlight the device characteristics of the various structures.
Reversible, Diodeless, Twistor Shift Register
Authors: Bobeck Andrew H. , Fischer Robert F.
Published: March 2008

A twistor shift register has been built and operated successfully. The design utilizes interaction effects which exist between magnetized regions on a magnetic wire. Only a single magnetic wire is required for a complete register. The information is stored as magnetically polarized zones which can be moved along the wire by means of a five phase pulse source.

No diodes are required. Therefore, drive powers can be greatly decreased since the only threshold consideration is the magnetic material itself. Bi-directional operation is easily secured. The upper frequency limit has not been established; however, a several hundred kilocycle bit rate should be possible. Physically, the register could be made of no more than magnetic and copper wire. This should make fabrication considerably cheaper than conventional shift registers.
Operating Characteristics of a Thin Film Memory
Authors: Raffel Jack I.
Published: March 2008

An experimental prototype memory with 32 ten bit words has been designed, built, and tested. Circular spots 116 in. in diameter about 600 A thick are used. These are evaporated on two pieces of glass each comprising a 16×16 spot array. An operating cycle time of less than one-half microsecond appears possible.

The circuitry for driving and sensing is transistorized and the memory uses external register selection from a core-diode matrix. Word selection is provided by a transverse field, and a digit winding conditions the information written by applying a longitudinal field in the “one” or “zero” direction.

Extension to sizes of the order of 1000 words is planned using these techniques. The memory constructed here will soon be installed in the control element of the TX-2 computer.
Initial Permeability Processes in Nickel-Cobalt Ferrites of Various Densities
Authors: Pippin J. E. , Thiess G. H.
Published: March 2008

Initial permeability has been measured at a frequency of 10 Mc/sec on ferrites of composition Ni1−αCoαMn0.02Fe1.9O4± with varying densities. Here α = 0, 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.04, and  0.06, and density is varied for each composition between 3.8 g/cm3 and 5.1 g/cm3.

The data are presented in a family of curves of permeability, μ′, versus density with α as a parameter, and in a family of curves of μ′ versus α with density as a parameter. For α = 0, the extremely rapid increase of μ′ at high density is indicative of an increase in wall motion, the increase being much too sharp to explain by a rotational model. For α≥0.04, this rapid increase is not seen, suggesting that the walls are almost completely pinned at these cobalt contents.

For high densities, the curve of μ′ versus α shows a sharp decrease with the first cobalt addition, then rises to a maximum near α = 0.03, and decreases for further cobalt additions. But ferromagnetic resonance line width measurements show that the crystalline anisotropy is decreased by the first cobalt addition, reaches a minimum near α = 0.03, and subsequently increases again. Thus it is concluded that rotational contributions to μ′ must have been increased by the first cobalt addition; therefore some other mechanism of magnetization, presumably domain wall motion, existed in the beginning, and its contribution to μ′ was decreased by more than the rotational contribution was increased.
Internal Structure of Cross-Tie Walls in Thin Permalloy Films through High-Resolution Bitter Techniques
Authors: Moon Ralph M.
Published: March 2008

The use of a water immersion objective with the optical microscope for Bitter pattern studies has resulted in a significant improvement in resolution. A discussion of technique in using such an objective is presented.

Applied to the study of cross-tie domain walls in thin Permalloy films, this technique has revealed details of internal wall structure. In the proposed model for such walls the magnetization within the wall is thought to be predominantly in the plane of the film, with periodic transition regions in which the magnetization is normal to the film. This configuration reduces the large magnetostatic energy which would be associated with a conventional 180° wall. Areas of low particle density are observed at the intersection of the main wall and cross-ties and in the main wall midway between cross-ties. From conventional ideas of colloid-pattern formation, these areas might be expected to have a high particle density because they are the transition regions in which the magnetization is normal to the film. The apparent conflict is resolved by assuming that these regions are large compared to the colloid particle size so that the fringing field at the edge of the region governs the particle distribution. In support of the model, convincing evidence is presented of the reported shift of colloid particles when an external field is applied normal to the film.
Magnetic Contribution to the Anomalous γ-Loop Shear of Fe-Al Alloys
Authors: Kikuchi Ryoichi , Sato Hiroshi
Published: March 2008

The free energy of the Fe-Al system is calculated using the cluster-variation method of cooperative phenomena on the Ising-type scalar spin model under the assumptions that there are a direct positive (ferromagnetic) interaction between nearest neighboring Fe atoms and an indirect negative (antiferromagnetic) superexchange interaction between a pair of Fe atoms which are separated by an Al atom. These assumptions were proved to be quite successful in explaining the magnetic properties over the entire range of Fe-Al system. It is found that the magnetic free energy of the system can be lowered by adding Al atoms while the Curie point is also lowered. This kind of situation is required for explaining the anomalous upward shear of γ loop in Fe-Al system, but this type of conclusion cannot be obtained from the nearest neighbor interaction model or from the concept of the corresponding state.
Saturation Magnetization and Curie Points in Dilute Alloys of Iron
Authors: Arrott Anthony , Noakes J. E.
Published: March 2008

A report of the initial stages of an investigation of the intrinsic properties of dilute alloys of iron is given. Methods of determining spontaneous magnetization and Curie temperatures with precision of one part in 30 000 are discussed. The results for Fe-Ti alloys are singled out for attention. The decrease of spontaneous magnetization at a rate of more than 1.6 times that of simple dilution implies either a negative moment on the Ti atoms or more probably that the occupation of states contributing to the moment on the iron is decreased by the addition of Ti. The increase in the Curie temperature at a rate of 3.7°C per atomic percent Ti is discussed on the basis of the Bethe-Slater curve of exchange interaction as a function of atomic separation.
Some Problems of Phenomenological Theory of Ferro- and Antiferromagnetism
Authors: Vonsovsky S. V. , Turov E. A.
Published: March 2008

This paper presents a general review of the modern state of phenomenological theory of ferro- and antiferromagnetism and the results of some investigations of the authors in this field.
Temperature dependent sign change of the organic magnetoresistance effect
Authors: Bloom F. L. , Wagemans W. , Koopmans B.
Published: March 2008

A sign change of the organic magnetoresistance effect is observed as a function of temperature. There is a large difference in the IV behavior when the sign of the magnetoresistance (MR) is positive compared to when the sign of the MR is negative, pointing to the possibility that the sign change of the MR is due to a change in the charge transport mechanism. The positive and negative MRs show different characteristic field widths B0 in the MR versus magnetic field curves. Also, the traces with positive MR show a clear temperature dependence of B0 while no systematic dependence on temperature is seen in the traces with negative MR. This behavior can be qualitatively explained by the recently proposed bipolaron model.
Relation between surface magnetization reversal and magnetoimpedance in Co-rich amorphous microwires
Authors: Chizhik A. , Garcia C. , Zhukov A. , Gonzalez J. , Gawronski P. , Kulakowski K. , Blanco J. M.
Published: March 2008

We report the investigations on the giant magnetoimpedance (GMI) effect and the surface magnetic structure performed in the series of glass covered Co-rich amorphous microwires with different thicknesses of glass covering. The calculation of the surface hysteresis loops has been performed in the frames of the model which takes into account the helical magnetic anisotropy. The correlation between the value of the GMI ratio, the jump of the circular magnetization, and the value of the angle of helical anisotropy has been established. It was found that the angle of the surface helical anisotropy, for which the highest value of GMI, is close to 60°.
Effects of annealing on the magnetic properties of Fe–6.5%Si alloy powder cores
Authors: Jang Pyungwoo , Lee Bonghan , Choi Gwangbo
Published: March 2008

Powder compressed cores can overcome poor workability of Fe–6.5%Si alloys. In this study, effects of annealing temperature on the losses of Fe–6.5%Si powder compressed cores were investigated. The hysteresis loss was two to three times larger than the eddy current loss. With the increasing annealing temperature, the hysteresis loss decreased due to a residual stress relief. With increasing the temperature, on the contrary, the eddy current loss increased even though the resistivity increased abruptly. An increase of the eddy current loss with the temperature can be understood by coarsening of DO3 phase. It was thought that controlling the microstructure of the powders is more important than that of resistivity. It is also very effective to anneal the powders at a temperature higher than 1000°C in order to reduce the total loss further.
Nanoring magnetic tunnel junction and its application in magnetic random access memory demo devices with spin-polarized current switching (invited)
Authors: Han X. F. , Wen Z. C. , Wei H. X.
Published: March 2008

Current-driven magnetization switching was observed in the nanoring-shaped magnetic tunnel junctions (NR-MTJs) with key stack layers of both spin-valve-type antiferromagnetic/ferromagnetic/insulator/ferromagnetic and sandwich-type hard ferromagnetic/insulator/soft ferromagnetic structures. We successfully fabricated a series of ring-shaped MTJs with different ring-outer diameters of between 80nm and 4μm and different ring width of between 25nm and 2μm. Tunneling magnetoresistance ratio between 20% and 80% with different thickness of thin Al–O barrier was measured at room temperature as we apply a magnetic field or a pulsed current. When the electric current density exceeds a critical value of the order of 6×106A∕cm2, the magnetization of the two free and reference magnetic rings can be switched back and forth between parallel and antiparallel onion states. The experiments show that the spin transfer torque plays a main switching role in the magnetization reversal and the current-induced circular magnetic field plays an assisted-switching role in such NR-MTJs.
Enhancement of stochastic oscillations by colored noise or internal noise in NO reduction by CO on small platinum surfaces
Authors: Gong Yubing , Xie Yanhang , Xu Bo , Ma Xiaoguang
Published: March 2008

In this paper, based on the stochastic model of NO reduction by CO on Pt crystal surfaces and taking Gaussian colored noise as external fluctuations of the NO partial pressure, we study the effect of the colored noise on the internal noise-induced stochastic oscillations (INSOs) and the effect of internal noise on the colored noise-induced stochastic oscillations (CNSOs). It is found that the INSO can be enhanced by the colored noise with appropriate correlation time or noise strength and, interestingly, the CNSO can be enhanced by the internal noise as well and, moreover, the enhanced CNSO can reach the best oscillatory states repetitively via proper internal noises. This effect of the internal noise is different from its effect on the stochastic oscillations induced by the external Gaussian white noise, which probably results from the interaction of the correlated colored noise and the internal noise.
Kinetic Monte Carlo modeling of chemical reactions coupled with heat transfer
Authors: Castonguay Thomas C. , Wang Feng
Published: March 2008

In this paper, we describe two types of effective events for describing heat transfer in a kinetic Monte Carlo (KMC) simulation that may involve stochastic chemical reactions. Simulations employing these events are referred to as KMC-TBT and KMC-PHE. In KMC-TBT, heat transfer is modeled as the stochastic transfer of “thermal bits” between adjacent grid points. In KMC-PHE, heat transfer is modeled by integrating the Poisson heat equation for a short time. Either approach is capable of capturing the time dependent system behavior exactly. Both KMC-PHE and KMC-TBT are validated by simulating pure heat transfer in a rod and a square and modeling a heated desorption problem where exact numerical results are available. KMC-PHE is much faster than KMC-TBT and is used to study the endothermic desorption of a lattice gas. Interesting findings from this study are reported.
An improved procedure for determining grain boundary diffusion coefficients from averaged concentration profiles
Authors: Gryaznov D. , Fleig J. , Maier J.
Published: March 2008

Whipple’s solution of the problem of grain boundary diffusion and Le Claire’s relation, which is often used to determine grain boundary diffusion coefficients, are examined for a broad range of ratios of grain boundary to bulk diffusivities Δ and diffusion times t. Different reasons leading to errors in determining the grain boundary diffusivity (DGB) when using Le Claire’s relation are discussed. It is shown that nonlinearities of the diffusion profiles in lnCav−y6∕5 plots and deviations from “Le Claire’s constant” (−0.78) are the major error sources (Cav=averaged concentration, y=coordinate in diffusion direction). An improved relation (replacing Le Claire’s constant) is suggested for analyzing diffusion profiles particularly suited for small diffusion lengths (short times) as often required in diffusion experiments on nanocrystalline materials.
In
As
∕
Ga
Sb
cascaded active region superlattice light emitting diodes for operation at 3.8μm
Authors: Koerperick E. J. , Olesberg J. T. , Boggess T. F. , Hicks J. L. , Wassink L. S. , Murray L. M. , Prineas J. P.
Published: March 2008

We report on the growth and characterization of InAs∕GaSb superlattice light emitting diodes (LEDs) operating in the midwave infrared at 3.8μm at 77K. Devices were grown by solid source molecular beam epitaxy on (100) GaSb substrates and were fabricated into 120×120μm2 mesa devices using wet etching. By employing an eight-stage cascaded active region design, output powers in excess of 1.5mW were achieved at 77K with 100mA peak drive current and a 50% duty cycle. Operating characteristics of the devices were examined from room temperature to 77K under quasi-dc excitation conditions.
Note on the role of friction-induced momentum conservation in the collisional drift wave instability
Authors: Vranjes J. , Poedts S.
Published: March 2008

The collisional drift wave instability is re-examined taking into account the ion response in the direction parallel to the magnetic field lines, which appears due to friction with electrons and which cannot be omitted in view of the momentum conservation. A modified instability threshold is obtained. In plasmas with dominant electron collisions with neutrals, the instability threshold is shifted towards higher frequencies, compared to the case of dominant electron collisions with ions. The difference between the two cases vanishes when the ion sound response is negligible, i.e., when the instability threshold disappears, and both ions and neutrals react to the electron friction in the same manner.
The structure of weak shocks in quantum plasmas
Authors: Bychkov Vitaly , Modestov Mikhail , Marklund Mattias
Published: March 2008

The structure of a weak shock in a quantum plasma is studied, taking into account both dissipation terms due to thermal conduction and dispersive quantum terms due to the Bohm potential. Unlike quantum systems without dissipations, even a small thermal conduction may lead to a stationary shock structure. In the limit of zero quantum effects, the monotonic Burgers solution for the weak shock is recovered. Still, even small quantum terms make the structure nonmonotonic with the shock driving a train of oscillations into the initial plasma. The oscillations propagate together with the shock. The oscillations become stronger as the role of Bohm potential increases in comparison with thermal conduction. The results could be of importance for laser-plasma interactions, such as inertial confinement fusion plasmas, and in astrophysical environments, as well as in condensed matter systems.
Formation of hydrogenated boron clusters in an external quadrupole static attraction ion trap
Authors: Ohishi Yuji , Kimura Kaoru , Yamaguchi Masaaki , Uchida Noriyuki , Kanayama Toshihiko
Published: March 2008

We report the formation of icosahedral B12H8+ through ion-molecule reactions of the decaborane ion [B10Hx+ (x=6–14)] with diborane (B2H6) molecules in an external quadrupole static attraction ion trap. The hydrogen content n of B12Hn+ is determined by the analysis of the mass spectrum. The result reveals that B12H8+ is the main product. Ab initio calculations indicate that B12H8+ preferentially forms an icosahedral structure rather than a quasiplanar structure. The energies of the formation reactions of B12H14+ and B12H12+ between B10Hx+ (x=6,8) ions, which are considered to be involved in the formation of B12Hn+, and a B2H6 molecule are calculated. The calculations of the detachment pathway of H2 molecules and H atoms from the product ions, B12H14+ and B12H12+, indicate that the intermediate state has a relatively low energy, enabling the detachment reaction to proceed owing to the sufficient reaction energy. This autodetachment of H2 accounts for the experimental result that B12H8+ is the most abundant product, even though it does not have the lowest energy among B12Hn+.
Formation of grown-in defects in molecular beam epitaxial Ga(In)NP: Effects of growth conditions and postgrowth treatments
Authors: Dagnelund D. , Buyanova I. A. , Wang X. J. , Chen W. M. , Utsumi A. , Furukawa Y. , Wakahara A. , Yonezu H.
Published: March 2008

Effects of growth conditions and post-growth treatments, such as presence of N ions, N2 flow, growth temperature, In alloying, and postgrowth rapid thermal annealing (RTA), on formation of grown-in defects in Ga(In)NP prepared by molecular beam epitaxy are studied in detail by the optically detected magnetic resonance (ODMR) technique. Several common residual defects, such as two Ga-interstitial defects (i.e., Gai-A and Gai-B) and two unidentified defects with a g factor around 2 (denoted by S1 and S2), are closely monitored. Bombardment of impinging N ions on grown sample surface is found to facilitate formation of these defects. Higher N2 flow is shown to have an even more profound effect than a higher number of ions in introducing these defects. Incorporation of a small amount of In (e.g., 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In; however, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai-related defects formed during the growth. In the alloys where the Gai-A and Gai-B defects are absent in the as-grown samples (i.e., GaNP grown at a low temperature of 460°C), the concentrations of the two Gai defects are found to increase after postgrowth RTA. This indicates that the defects originally introduced in the as-grown alloys have been transformed into the more thermally stable Gai-A and Gai-B during RTA. On the other hand, when the Gai-A and Gai-B are readily abundant (e.g., at higher growth temperatures (⩾500°C), RTA leads to a slight reduction of the Gai-A and Gai-B ODMR signals. The S2 defect is also shown to be thermally stable upon the RTA treatment.
Optimization of force produced by electromagnet needles acting on superparamagnetic microparticles
Authors: Xiang Yu , Miller Jacob , Sica Vincent , LaVan David A.
Published: March 2008

The design of an electromagnet needle (EMN) has been numerically optimized based on finite element analysis of the detailed interaction between an EMN and a typical superparamagnetic microparticle. The nonlinear magnetization curves of the core materials and particles are considered; the model calculates the force acting on the particle without far-field simplifications. The EMN optimized for maximum force per unit of heating power generates over 40 000 times the force per unit of heating power of typical optical tweezers. The EMN optimized for a fixed high power level produced 56.7nN, a ten-fold improvement over prior EMNs.
Theory of phase-modulation atomic force microscopy with constant-oscillation amplitude
Authors: Hölscher Hendrik
Published: March 2008

The recently introduced phase-modulation mode with constant oscillation amplitude is analyzed and compared with the conventional amplitude-modulation technique. We show that instabilities in the amplitude and phase versus distance curves are prevented by the additional feedback controlling the oscillation amplitude. Moreover, we demonstrate that the phase-modulation technique is capable to measure conservative and dissipative tip-sample forces by dynamic force spectroscopy without any jumps caused by instabilities. This feature is an important advantage compared to a spectroscopy technique based on the conventional tapping mode.
InN nanotips as excellent field emitters
Authors: Wang K. R. , Lin S. J. , Tu L. W. , Chen M. , Chen Q. Y. , Chen T. H. , Chen M. L. , Seo H. W. , Tai N. H. , Chang S. C. , Lo I. , Wang D. P. , Chu W. K.
Published: March 2008

Unidirectional single crystalline InN nanoemitters were fabricated on the silicon (111) substrate via ion etching. These InN nanoemitters showed excellent field emission properties with the threshold field as low as 0.9V∕μm based on the criterion of 1μA∕cm2 field emission current density. This superior property is ascribed to the double enhancement of (1) the geometrical factor of the InN nanostructures and (2) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reduced the effective electron tunneling barrier even under very low external field.
Effect of stress and free-carrier concentration on photoluminescence in InN
Authors: Song D. Y. , Holtz M. E. , Chandolu A. , Bernussi A. , Nikishin S. A. , Holtz M. W. , Gherasoiu I.
Published: March 2008

We report photoluminescence (PL) studies of InN epilayers grown by plasma-assisted molecular beam epitaxy with free-electron concentration ranging from 5.9×1017to4.2×1018cm−3. X-ray diffraction measurements are used to determine strains, which are best described as a combination of hydrostatic and biaxial. The PL energy is affected by both strains along with free-carrier concentration through band filling. PL spectra are used to estimate the dependence of the Fermi level on free-carrier concentration, taking strain into account. The fundamental energy gap is found to be ∼0.70eV. PL broadening is well described based on band filling.
Modeling the elastic moduli of fiber networks and nanocomposites: Transversely isotropic filler particles
Authors: Chatterjee Avik P.
Published: March 2008

A model is developed for the elastic moduli of networks composed of transversely isotropic elongated particles characterized by aspect ratio polydispersity. An effective medium approach is employed to integrate our treatment of elastic fiber networks with results from (i) the Mori–Tanaka model for dispersions of transversely isotropic inclusions and from (ii) percolation theory, and to describe fiber-reinforced nanocomposites. Model calculations are presented for the dependences of composite moduli on particle aspect ratios, volume fractions, and polydispersities, and on anisotropy in the fiber stiffness tensor.
Novel semiconducting materials for optoelectronic applications: Al1−xTlxN alloys
Authors: Souza Dantas N. , de Almeida J. S. , Ahuja R. , Persson C. , Ferreira da Silva A.
Published: March 2008

We proprose the ternary semiconducting Al1−xTlxN alloys as new material for optoelectronic applications. Ab initio calculations have been performed to study structural, electronic, and optical properties of the theoretically designed thallium-aluminum based nitride alloys. We found that the lattice constants vary linearly with thallium composition whereas the band gap and absorption edge span from ultraviolet to infrared energy region by increasing thallium content which make the predicted material interesting for infrared optical devices among other technological applications.
Optical limiting in lead magnesium niobate–lead titanate multilayers
Authors: Lu Yalin , Gao Chen
Published: March 2008

Optical limiting in highly oriented lead magnesium niobate–lead titanate multilayers has been investigated at both 1.064 and 0.532μm wavelengths. Limiting thresholds for three such multilayers having periods of 20, 106, and 218nm, respectively, were determined at the level ∼220μJ/pulse for 1.064μm and ∼150μJ/pulse for 0.532μm. Nonlinear optical scattering from domain walls was used to explain the observed limiting behavior. Possible formation of nonlinear refraction grating was also discussed.
Temperature dependence of surface plasmon mediated emission from metal-capped ZnO films
Authors: Li J. , Ong H. C.
Published: March 2008

Surface plasmon (SP) mediated emission from Al-capped ZnO films has been studied by temperature-dependent photoluminescence from 10to300K. By comparing with bare ZnO, it is found that the SP induced emission enhancement ratio linearly decreases with increasing temperature. By taking into account temperature-dependent dielectric functions of both Al and ZnO, theoretical Purcell factor has been calculated for different temperatures. We find the experimental results are highly consistent with theory. From the calculation, we have found the increase of plasmonic density of states with decreasing temperature is the primary cause for the increase of SP coupling.
Bonding of GaN structures with Si(100) substrates using sequentially deposited NiAu metal layers
Authors: Arokiaraj J. , Leong Cheong Kee , Lixian Vivian , Yong Anna Marie , Xincai Wang
Published: March 2008

A process has been developed to bond gallium nitride structures grown on sapphire with silicon substrates, using sequentially deposited nickel-gold thin metal layers. Temperature and pressure treatment results in alloying of NiAu with robust interface bond strength of 7.1MPa between GaN and Si. Transmission electron microscopy showed bright and dark regions of varying nickel-gold composition with uniform layer thickness. Micro-Raman and x-ray photoelectron spectroscopy revealed compressive stress relaxation and Ga–Ni and Au–Si alloying phases, respectively. The transferred light emitting diodes exhibit a turn-on voltage of 2.5V and could sustain beyond 300mA, proving a way to have vertical structure light emitting diodes on Si substrates.
Ferroelectric PbTiO3 nanotube arrays synthesized by hydrothermal method
Authors: Yang Yang , Wang Xiaohui , Zhong Caifu , Sun Changku , Li Longtu
Published: March 2008

Lead titanate nanotube arrays with perfect nanotubular structure and excellent piezoelectric property were formed by hydrothermal method. The nanotubular structure proved to have strong influences on both Curie temperature and piezoelectric properties of PbTiO3 nanotube arrays. The Curie temperature of PbTiO3 nanotube arrays was detected to be 620°C. Perfect piezoelectric properties with a maximum displacement of 0.88nm and d33* of 120pmV−1 were observed in PbTiO3 nanotube arrays.
Modulated photoluminescence of shallow levels in arsenic-doped Hg1−xCdxTe (x≈0.3) grown by molecular beam epitaxy
Authors: Yue Fangyu , Chu Junhao , Wu Jun , Hu Zhigao , Li Yawei , Yang Pingxiong
Published: March 2008

Shallow levels in arsenic-doped Hg1−xCdxTe grown by molecular beam epitaxy have been investigated by temperature- and excitation power-dependent modulated photoluminescence spectroscopy. The ionization energies of the shallow levels of AsTe, AsHg, and the AsHg–VHg complex are preliminarily determined to be about 11.0, 8.5, and 33.5meV, respectively. Correspondingly, the forming energy of the AsHg–VHg complex has been deduced to be approximately 10.5meV. The results could be used as guidelines for the material growth or the fabrication of related devices.
Staying Competitive While Subsidized: A Governmental Policy to Reduce Production of Environmentally Harmful Products
Authors: Olsson Lars E , Gärling Tommy
Published: March 2008

A price-setting experiment was conducted to investigate whether a proposed governmental subsidy system to reduce sales and production of environmentally harmful products would maintain competition, as required by EU regulations. Dyads of participants played the role of producers, independently setting unit prices for their products in a sequence of trials with feedback. In experimental conditions a subsidy was paid for unsold units, a system which had previously been found to raise prices and reduce the number of units sold. For half of the dyads in each condition the payoffs were individual, in the other half they were split equally to motivate cooperation. Substantiating that the subsidy did not eliminate competition, the prices were set lower and incomes were lower when the payoffs were individual than when they were split equally. The participants also adjusted their prices to increase their own payoff, taking into account the prices set by the others when the payoffs were individual—which they did not when the payoffs were split equally. The results support the claim that the proposed subsidy system is effective in raising prices so that sales and production of environmentally harmful products can be reduced without eliminating competition.
Tackling Concentrations of Worklessness: Highlighting the Limits of Work-Focused Organisational Cultures in the UK
Authors: Fletcher Del Roy
Published: March 2008

UK policy makers have increasingly developed area-based employment initiatives. This has culminated in the Working Neighbourhoods Pilot, which targets concentrations of worklessness in twelve deprived communities across Great Britain. Drawing upon detailed case study work undertaken in the Sheffield pilot, I discuss the challenges faced by the Jobcentre Plus agency in tackling such problems. A key finding is that the creation of the agency has placed too much emphasis on the most visible and superficial manifestations of organisational culture. At a deeper level, I find that the culture of the agency limits its ability to respond imaginatively and flexibly to the needs of such communities.