Nuclear magnetic relaxation of F-19 in fluoride glasses

Temperature and frequency dependence of the F-19 nuclear spin relaxation of the fluoroindate glass, 40InF(3)-20ZnF(2)- 20SrF(2)-2GaF(3)-2NaF-16BaF(2) and the fluorozirconate glass, 50ZrF(4)-20BaF(2)-21LiF-5LaF(3)-4AlF(3); are reported. Measurements were undertaken on pure and Gd3+ doped samples, in the temperature range of 185-1000 K, covering the region below and above the glass transition temperature, T-g. The temperature and frequency dependence of the spin-lattice relaxation rate, T-1(-1), measured in the glassy state at temperature <300 K, is less than the observed dependence at higher temperatures. At temperatures >T-g, the fluorine mobility increases, leading to a more efficient spins lattice relaxation process. Activation energies, for F- motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluorozirconate glass. The addition of Gd3+ paramagnetic impurities;at 0.1-wt%, does not alter the temperature and frequency dependence of T-1(-1), but increases its magnitude more than one order of magnitude. At temperatures <400 K, the spin-spin relaxation time, T-2(-1), measured for all samples, is determined by the rigid-lattice nuclear dipole-dipole coupling, and it is temperature independent within the accuracy of the measurements. Results obtained for the pure glass, at temperatures >400 K, show that T-2(-1) decreases monotonically as the temperature increases. This decrease is explained as a consequence of the motional narrowing effect caused by the onset of the diffusive motion of the F- ions, with an activation energy around 0.8 eV. For the doped samples, the hyperfine interaction with the paramagnetic impurities is most effective in the relaxation of the nuclear spin, causing an increase in the T(2)(-1)s observed at temperatures >600 K. (C) 1999 Elsevier B.V. B.V. All rights reserved.

INPLANE RESISTIVITY IN GRAPHITE-INTERCALATION COMPOUNDS OBTAINED FROM CONDUCTION-ELECTRON-SPIN-RESONANCE MEASUREMENTS

We developed a procedure to take advantage of the magnetic-field-modulation-frequency effect on the line shape of conduction-electron-spin resonance of graphite intercalation compounds (GIC's) to extract the absolute value of the in-plane resistivity. We calculated the power absorbed in each slice of the sample normal to the wave penetration, multiplied by a factor to account for the magnetic-field-modulation-frequency effect. Room-temperature spectra of stage-I AlCl3-intercalated GIC in both H-0 perpendicular-to c and H-0 parallel-to c configurations were fitted to the theoretical line shapes and the value of in-plane resistivity (and also the value of c-axis resistivity) obtained from the fitting parameters are in reasonable agreement with those from the literature.

Differential scanning calorimetry, x-ray diffraction and F-19 nuclear magnetic resonance investigations of the crystallization of InF3-based glasses

Results of differential scanning calometry (DSC), x-ray diffraction (XRD), and F-19 nuclear magnetic resonance (NMR) of InF3-based glasses, treated at different temperatures, ranging from glass transition temperature (T-g) to crystallization temperature (T-c), are reported. The main features of the experimental results are as follows. DSC analysis emphasizes several steps in the crystallization process. Heat treatment at temperatures above T-g enhances the nucleation of the first growing phases but has little influence on the following ones. XRD results show that several crystalline phases are formed, with solid state transitions when heated above 680 K, the F-19 NMR results show that the spin-lattice relaxation, for the glass samples heat treated above 638 K, is described by two time constants. For samples treated below this temperature a single time constant T-1 was observed. Measurements of the F-19 spin-lattice relaxation time (T-1), as a function of temperature,made possible the identification of the mobile fluoride ions. The activation energy, for the ionic motion, in samples treated at crystallization temperature was found to be 0.18 +/- 0.01 eV. (C) 1998 American Institute of Physics.

Anomalous transport effects on magnetic field profile in a motional current sheath

The radial magnetic field profile during implosion of a reversed field current sheath in a theta-pinch was investigated through local measurements and simulation of hybrid code. The actual profile was defined by Hermite interpolation polynomial through mean value of the field at discrete radial position of measurements. Simulation profile was provided by the numerical code with appropriate initial conditions. Classical and anomalous collision process were taken in account in the theoretical model. The results indicated that anomalous effects play major role during the implosion phase of current sheath in a slow rising theta pinch device.

Small-angle X-ray and nuclear-magnetic resonance study of siloxane-PMMA hybrids prepared by the sol-gel process

Transparent siloxane-polymethylmethacrylate (PMMA) hybrids were synthesized by the sol-gel process through hydrolysis of methacryloxyproyltrimethoxysilane (TMSM), tetramethoxysilane (TMOS) and polymerization of methylmethacrylate (MMA) using benzol peroxide (BPO) as catalyst. These composites have a good chemical stability due to the presence of covalent bonds between the inorganic (siloxane) and organic (PMMA) phases. The effects of siloxane content, pH of the initial sol and BPO content on the structure of the dried gels were analyzed by small-angle X-ray scattering (SAXS). SAXS results revealed the presence of an interference (or correlation) peak at medium q-range for all compositions, suggesting that siloxane groups located at the ends of PMMA chains form isolated clusters that are spatially correlated. The average intercluster distance - estimated from the q-value corresponding to the maximum in SAXS spectra - decreases for samples prepared with increasing amount of TMSM-TMOS. This effect was assigned to the expected increase in the number density of siloxane groups for progressively higher siloxane content. The increase of BPO content promotes a more efficient polymerization of MMA monomers but has no noticeable effect on the average intercluster distance. High pH favors polycondensation reactions between silicon species of both TMOS and TMSM silicon alcoxides, leading to a structure in which all siloxane clusters are bonded to PMMA chains. This effect was confirmed by Si-29 nuclear-magnetic resonance (NMR) measurements.

Magnetic-field and confinement effects on the effective Land, g factor in AlxGa1-xAs parabolic quantum wells

The magnetic-field and confinement effects on the Land, factor in AlxGa1-xAs parabolic quantum wells under magnetic fields applied parallel or perpendicular to the growth direction are theoretically studied. Calculations are performed in the limit of low temperatures and low electron density in the heterostructure. The g factor is obtained by taking into account the effects of non-parabolicity and anisotropy of the conduction band through the 2 x 2 Ogg-McCombe Hamiltonian, and by including the cubic Dresselhaus spin-orbit term. A simple formula describing the magnetic-field dependence of the effective Land, factor is analytically derived by using the Rayleigh-Schrodinger perturbation theory, and it is found in good agreement with previous experimental studies devoted to understand the behavior of the g factor, as a function of an applied magnetic field, in semiconductor heterostructures. Present numerical results for the effective Land, factor are shown as functions of the quantum-well parameters and magnetic-field strength, and compared with available experimental measurements.

A Non-Destructive Testing Based on Electromagnetic Measurements and Neural Networks for the Inspection of Concrete Structures

This paper presents a new non-destructive testing (NDT) for reinforced concrete structures, in order to identify the components of their reinforcement. A time varying electromagnetic field is generated close to the structure by electromagnetic devices specially designed for this purpose. The presence of ferromagnetic materials (the steel bars of the reinforcement) immersed in the concrete disturbs the magnetic field at the surface of the structure. These field alterations are detected by sensors coils placed on the concrete surface. Variations in position and cross section (the size) of steel bars immersed in concrete originate slightly different values for the induced voltages at the coils.. The values for the induced voltages were obtained in laboratory tests, and multi-layer perceptron artificial neural networks with Levemberg-Marquardt training algorithm were used to identify the location and size of the bar. Preliminary results can be considered very good.

Magnetic field dependent magnetization of a conducting plasticized poly(aniline) film

We report magnetic data of free standing films of poly( aniline) (PANI) protonated with a plasticizing di-ester of succinic acid. The data have been obtained using the electron spin resonance (ESR) technique at two different frequencies, X-band (9.4 GHz) and Q-band ( 34 GHz), on one hand, and by magnetization measurements in broad ranges of temperatures and magnetic fields on the other hand. All the data can be explained assuming a transition as a function of temperature from delocalized magnetic moments in the valence band to localized positive polarons in several antiferromagnetically correlated bands. By increasing the magnetic field, the magnetic properties are affected in several ways. An intra-band admixture of states occurs; it contributes to increase the spins' localization and finally promotes an antiferromagnetic-metamagnetic transition.

Bulk and high-energy ball-milled Gd5Si2Ge2: Comparative study of magnetic and magnetocaloric properties

High-energy ball milling was employed to produce small particles of Gd5Si2Ge2. Magnetic and magnetocaloric properties of the ball-milled and bulk Gd5Si2Ge2 samples were investigated through the magnetization measurements. When compared to the bulk material, a significant decrease in saturation magnetization and magnetocaloric effect (-Delta S-max = 4 vs. 20 J/kgK for Delta H = 0-5 T) is observed even after the relatively short ball milling time of 4 h which produced particles with an average size of ca. 0.5 mu m. The ball-milled samples appear to loose a first-order structural transition, present in bulk Gd5Si2Ge2, and display a superparamagnetic behaviour below the corresponding Curie temperatures. (C) 2010 Elsevier Masson SAS. All rights reserved.

Electric and magnetic properties of polymer electrolyte/carbon black composites

Measurements of 1H Nuclear Magnetic Resonance (NMR) relaxation times, Electron Paramagnetic Resonance (EPR) and AC Impedance Spectroscopy (IS) are reported for composites based on PEO8:LiClO4 and carbon black (CB), prepared by two methods: solvent and fusion processing. Three nuclear relaxation processes were identified for 1H nuclei: (i) belonging to the polymer chains in the amorphous phase, loosely bound to the CB particles, whose dynamics is almost the same as for unfilled polymer, (ii) belonging to the polymer chains which are tightly attached to the CB particles, and (iii) belonging to the crystalline phase in the loose polymer chain. The paramagnetic electronic susceptibility of the composite samples, measured by EPR, was interpreted by assuming a contribution of localized spin states that follow a Curie law, and a Pauli-like contribution of delocalized spins. A significant change of the EPR linewidth was observed at 40 K, which is the temperature where the Curie and Pauli susceptibilities equally contribute to the paramagnetic electronic susceptibility. The electrical properties are very sensitive to the preparation methods of the composites, which conditions the interaction between carbon particle-carbon particle and carbon particle-polymer chain. Classical statistic models to describe the conductivity in these media were not satisfactory. © 1998 Published by Elsevier Science B.V. All rights reserved.

Crystal structures and magnetic properties of CuX2(pdmp)2 complexes (X = Br, Cl)

We report the synthesis and the structural and magnetic characterization of two new compounds: dibromobis-(pdmp)copper(II), CuBr2C22H24N4 (1), and dichlorobis(pdmp)copper(II), CuCl2C22H24N4 (2), where pdmp = 1-phenyl-3,5-dimethylpyrazole. The structures were refined by full-matrix least-squares techniques to R1 = 0.0620 and 0.0777, respectively. Compound 1 belongs to the space group P21/n with a = 8.165(5) Å, b = 10.432(3) Å, c = 13.385(4) Å, β = 100.12(4)̊, and Z = 2. Compound 2 belongs to the space group P21/c with a = 8.379(2) Å, b = 22.630(2) Å, c = 12.256(2) Å, β= 98.43(3)°, and Z = 4. It has the same molecular formula as a compound reported previously but a different crystal structure. Detailed single-crystal EPR measurements were performed for single-crystal samples of 1 and 2 at 9 and 35 GHz and at room temperature. The positions and line widths of the EPR lines were measured as a function of the magnetic field orientation in three orthogonal planes. The data were used to study the electronic properties of the copper ions and to evaluate the exchange interactions between them. Our results are discussed in terms of the electronic pathways for superexchange between copper ions, which are provided by the stacking of pyrazole and phenyl rings of neighboring molecules and by hydrogen-halogen bonds. © 1999 American Chemical Society.

Magnetic properties of spindle-type iron fine particles obtained from hematite

Spindle-type iron fine particles have been prepared by reduction of silica-coated-hematite particles. Hydrogen reduction of the coated-hematite cores yielded uniform spindle-type iron particles, which were stabilized by surface oxidation. Narrow particle distributions are observed from TEM measurements. X-ray, Mössbauer and magnetization data are in agreement with the presence of nanosized α-Fe particles, having surface layer of spinel structure oxide. Mössbauer spectra show that the oxide surface is superparamagnetic at room temperature. © 2001 Elsevier Science B.V. All rights reserved.

Visualizing the ac magnetic susceptibility of superconducting films via magneto-optical imaging

We have established a link between the global ac response and the local flux distribution of superconducting films by combining magnetic ac susceptibility, dc magnetization, and magneto-optical measurements. The investigated samples are three Nb films: a plain specimen, used as a reference sample, and other two films patterned with square arrays of antidots. At low temperatures and small ac amplitudes of the excitation field, the Meissner screening prevents penetration of flux into the sample. Above a certain ac drive threshold, flux avalanches are triggered during the first cycle of the ac excitation. The subsequent periodic removal, inversion, and rise of flux occurs essentially through the already-created dendrites, giving rise to an ac susceptibility signal weakly dependent on the applied field. The intradendrite flux oscillation is followed, at higher values of the excitation field, by a more drastic process consisting of creation of new dendrites and antidendrites. In this more invasive regime, the ac susceptibility shows a clear field dependence. At higher temperatures a smooth penetration occurs, and the flux profile is characteristic of a critical state. We have also shown that the regime dominated by vortex avalanches can be reliably identified by ac susceptibility measurements. © 2011 American Physical Society.

High-resolution thermal expansion measurements under helium-gas pressure

We report on the realization of a capacitive dilatometer, designed for high-resolution measurements of length changes of a material for temperatures 1.4K ≤ T ≤ 300K and hydrostatic pressure P ≤ 250MPa. Helium ( 4He) is used as a pressure-transmitting medium, ensuring hydrostatic-pressure conditions. Special emphasis has been given to guarantee, to a good approximation, constant-pressure conditions during temperature sweeps. The performance of the dilatometer is demonstrated by measurements of the coefficient of thermal expansion at pressures P ≃ 0.1MPa (ambient pressure) and 104MPa on a single crystal of azurite, Cu 3(CO 3) 2(OH) 2, a quasi-one-dimensional spin S = 1/2 Heisenberg antiferromagnet. The results indicate a strong effect of pressure on the magnetic interactions in this system. © 2012 American Institute of Physics.

Effect of structural anisotropy on electrical and magnetic properties of polyaniline conducting films

It is shown that highly conducting films of polyaniline protonated with di-esters of sulfosuccinic and sulfophthalic acids which contain alkyl- or alkoxy-type substituents exhibit highly anisotropic structural, electrical and magnetic properties. The layered-like structure of these films can be described as consisting of polyaniline chains which are mainly oriented parallel to the plane of the film and form regular out-of-plane stacks. These stacks are separated by bilayers of the dopant anions. Accordingly, the main anisotropy observed for solution cast films implies in-plane and out-of-plane measurements. An electrical anisotropy of about 80 is found for the in-plane and out-of-plane electronic conductivities at 5 K. The temperature dependences of the in-plane and out-of-plane conductivities are qualitatively similar and have been fitted as a series combination of variable-range-hopping-type and power law contributions. A maximum is observed in the temperature dependence of the electrical anisotropy at low temperature. The films also show a clear anisotropy of magnetization whose temperature and field characteristics depend on the chemical structure of the dopant anion. © 2013 Elsevier B.V.