Department of Physics

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Giant magneto-impedance (GMI) behavior have been studied in melt spun Co71-XFeXCr7Si8B14 (X = 0, 2, 3.2, 4, 6, 8, and 12 at%) alloys. The addition of Fe in the system changed the saturation magnetostriction constant from negative to positive values. The GMI property was measured with a driving current of 5 mA and 4 MHz frequency using a spectrum analyzer. The GMI ratio increased with the increase of the applied dc field and became maximum at a field Hk known as the anisotropy field of the materials. The maximum of GMI ratio (GMImax) depended on the Fe content and was maximum (12%) for Fe = 4 at% for the alloys in the as-melt spun state. Substantial increase of GMImax to 66% for Fe = 4 at% was observed after annealing the sample at 673 K.

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Materials and Manufacturing Processes

Study of giant magneto-impedance behavior in Co-Fe based amorphous/nanocrystalline materials

A comparative study of complex dielectric properties has been carried out at the X-band of microwave frequencies of composites of barium titanate (BaTiO3) with two different polymer matrices: insulating polyaniline (PANI) powder (emeraldine base) and maleic resin. From these studies, it is observed that the composites of BaTiO3 with maleic resin show normal composite behavior and the dielectric constant follows the asymmetric Bruggeman model. In contrast, the composites of BaTiO3 with PANI show an unusual behavior wherein even at a low concentration of PANI (5 wt.%) there is a drastic reduction in the dielectric constant of BaTiO3. This behavior of the dielectric constant is explained on the basis of coating of BaTiO3 particles by PANI which in turn is attributed to the highly surface adsorbing character. The materials have also been characterized using Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy and optical microscopy studies. © 2006 Acta Materialia Inc.

Acta Materialia

Study of the dielectric properties of barium titanate-polymer composites

The magnetic properties of heat-treated, copper (Cu)-containing high-strength low-alloy (HSLA-100) steel and the variations of the magnetic properties during tensile deformation were studied. For samples aged at temperatures below 775 K, precipitation of nanosized coherent Cu particles occurred, causing an increase in hardness but a decrease in coercivity. For higher aging temperatures, the Cu precipitates coarsened and then were able to pin domain walls. As a result, magnetic coercivity increased. During tensile deformation of selected heat treated samples, the interaction of dislocations with the coherent and incoherent Cu precipitates influenced the magnetic properties. Systematic variation of coercivity with plastic strain was found for brittle and ductile types of fracture of the materials. © 2006 IEEE.

IEEE Transactions on Magnetics

Evaluation of deformation behavior of HSLA-100 steel using magnetic hysteresis technique

The author study conduction mechanism in two conducting states of a bistable device at 10-300 K range. They find that in the electrical bistable devices, electrical switching is associated with a change in the conduction mechanism. Device current in the low-conducting state follows an injection-limited mechanism. The current in the high-conducting state conforms a bulk-dominated mechanism, namely, space-charge limited conduction with an exponential distribution of traps. The bistability has an associated memory phenomenon. The devices exhibit read-only and random-access memory applications for several hours. © 2006 American Institute of Physics.

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Applied Physics Letters

Electrical bistability in a xanthene class molecule: Conduction mechanisms

Alloying with Fe caused a non-monotonic change in the soft magnetic properties of Co71-X Fe X Cr7Si 8B14 alloys. Hydrogenation caused a progressive deterioration in the soft magnetic properties. The differences in the response of the alloys is possibly due to a non-systematic variation in the inter-atomic exchange interactions which modulates the dipole moment, and due to introduction of internal stresses on hydrogen charging.

Journal of Materials Science

Effect of hydrogen on the magnetic behavior of CoFeCrSiB amorphous alloys

An ab initio study of the energetic, structural, electronic, and optical absorption properties of the 26 lead nanowires, Pbn (n=1,18) having different m -gonal (m=1-8) cross sections has been made in the density functional theory in local density approximation considering also the spin-orbit coupling (SOI). There are four groups of the stable structures: planar, caged, pyramidal, and helical. The binding energy of a nanowire, in general, increases with the coordination number except in those systems where the nearest neighbors are comparatively far away. A 14-Pb hexagonal helical configuration has maximum stability followed by the heptagonal, other hexagonal, and pentagonal wires. All the nanowires are metallic. The exceptions are the 2-Pb and 3-Pb semiconducting nanowires. A large number of the conduction channels leading to high quantum ballistic conduction are seen for a number of the m -gonal (m=4-8) configuration wires. The calculated optical absorption without and with the SOI are quite different in terms of the number of the absorption peaks which are enhanced approximately by a multiplying factor of 2 by the SOI. The m -gonal (m=4-8) nanowires reveal multipeaked, strong, and extended optical absorption over the whole visible region. Our analysis of the experimental data for the Pb samples that have been fabricated by Romanov points towards the occurrence of the 2-Pb ladder chains. © 2006 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Ab initio study of the structural, electronic, and optical properties of ultrathin lead nanowires

An ab initio study of the structural and electronic properties of carbon nanowires Cn with n = 1, 6 has been made within density functional theory (DFT) in the generalized gradient approximation (GGA). The present study reveals that all the wires are metallic. The binding energies (BE's) for all the optimized carbon wires are more or less similar. The C-C bond length increases with n up to n = 5 and decreases thereafter. The maximum electron density of states (DOS) is seen for the 4-C wire followed by 3-C and 5-C wires. The number of channels available for the electric conduction is large (7, 9) for n = 4-6. These features may be of great interest for achieving high conductivity. © 2006 Elsevier Ltd. All rights reserved.

Progress in Crystal Growth and Characterization of Materials

Ab initio study of the structural and electronic properties of carbon nanowires

We suggest that superbursts from some low mass X-ray binaries may be due to breaking and re-formation of diquark pairs, on the surface of realistic strange stars. Diquarks are expected to break up due to the explosion and shock of the thermonuclear process. After a prolonged accretion when almost all pairs get broken, the subsequent production of copious diquarks may produce sufficient energy to produce the superbursts. © 2006.

Advances in Space Research

Superbursts and long bursts as surface phenomenon of compact objects

The Wigner transform of the master equation describing the reduced dynamics of the system, of a harmonic oscillator coupled to an oscillator bath, was obtained by Karrlein and Grabert. It was shown that for some special correlated initial conditions the master equation reduces, in the classical limit, to the corresponding classical Fokker-Planck equation obtained by Adelman. However, for separable initial conditions the Adelman equations were not recovered. We resolve this problem by showing that, for separable initial conditions, the classical Langevin equations are somewhat different from the one considered by Adelman. We obtain the corresponding Fokker-Planck equation and show that they exactly match the classical limit of the evolution of the Wigner function obtained from the master equation for separable initial conditions. We also discuss why thermal initial conditions correspond to Adelman's solution. © 2006 The American Physical Society.

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

Classical limit of master equation for a harmonic oscillator coupled to an oscillator bath with separable initial conditions

We report fabrication and characterization of photodetectors based on organic semiconductors. Single and multiple heterostructures between electron donor and acceptor materials are used. Moreover, by varying the substitutes of the acceptor molecule, five different donor/acceptor combinations are presented. Photocurrent depends on the choice of the acceptor material, showing importance of energy mismatch between lowest unoccupied molecular orbital (and highest occupied molecular orbital) levels of donor and acceptor molecules. Additionally, the photocurrent also depends on the number of donor/acceptor interfaces. The results can be explained in terms of an energy band diagram of the devices. From the impedance spectroscopy, we show that the change in dielectric constant of the active material due to illumination has a direct relevance to the external current of the photodetectors. 2006 American Institute of Physics. © 2006 American Institute of Physics.