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Structure and its defects, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ magnetoresistive ceramics were investigated by x-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR and magnetoresistance methods. It was found that isovalent substitution of lanthanum by europium A-cation of a smaller ionic radius increases the structural imperfection and leads to a symmetry change from the rhombohedrally distorted perovskite structure of R 3 ¯ c symmetry to the pseudocubic type. It was shown that the real structure contains anionic and cationic vacancies, the concentrations of which increases with the Eu concentration and the sintering temperature tann. A decrease in the temperature of the metal–semiconductor Tms and ferromagnetic–paramagnetic TC phase transitions as well as an increase in the resistivity ρ and the activation energy Ea with increasing x are due to an increase in vacancy concentration, which weakens the high-frequency electron double exchange Mn3+ ↔ ...

Ceramic samples of Pr0.6−xNdxSr0.3Mn1.1O3−δ (x = 0−0.6) were studied by x-ray diffraction, resistive, magnetic (χac and 55Mn NMR), magnetoresistive and electron microscopy methods. It was shown that with increasing the concentration x, the type of unit cell distortion changes from orthorhombic (x = 0–0.2) to pseudo-cubic (x = 0.4–0.6), and the imperfection of the structure, which contains anion and cation vacancies, is increased. A decrease in the temperatures of metal–semiconductor (Tms) and ferromagnetic–paramagnetic (TC) phase transitions and an increase in the resistivity and activation energy with increasing x was explained by an increase in the concentration of vacancies, which weakens high-frequency electronic double-exchange Mn3+ ↔ Mn4+. It was found that the compositions with a higher content of neodymium exhibit a transition to the antiferromagnetic state at temperatures below 130 K. Two types of magnetoresistive effects were observed. The magnitude of the first effect, which occurs near the pha...

Comparative investigations of the nuclear magnetic resonance and nuclear spin-spin relaxation residents of Mn55 nuclei of nanopowders of the manganite La0.7Sr0.3MnO3 with different particle sizes 50–200nm (NP1) and 12nm (NP2) have been performed. It has been found that two local ferromagnetic phases (FgM and FbM) with different magnetic spin dynamics coexist in a wide temperature interval in the NP1 sample. The phase FgM phase is identified as a ferromagnetic metallic phase which is due to double exchange and the FbM phase is identified as a ferromagnetic phase with slower hole motion and weaker double exchange. It is established that only one ferromagnetic state, corresponding to the FbM phase, exists in the NP2 sample. The results of these investigations show that the stability of the nonuniform ferromagnetic state of lanthanum manganites can be limited below by a characteristic minimum scale.

Experiments on the Extrap T1 reversed‐field pinch (RFP) [Phys. Scr. 49, 224 (1994)] have shown that the formation of the RFP configuration is quite sensitive to the relative programming of the toroidal field and Ohmic heating circuits. In this paper, new measurements of the evolution of the current density profile and of the spectral structure of the fluctuations during the setup phase of RFP plasmas in the T1 experiment are presented. These measurements improve the understanding of the role of different spectral components in the dynamics of RFP formation. Under unfavorable (slow) setup conditions, comparatively high energy is accumulated in m=1 internal kinks prior to reversal of the edge toroidal field. At reversal, nonlinearly driven m=0 modes trigger a rapid broadening of the m=1 spectrum. This behavior is associated with a violent suppression of the current density in the core, leading to an overrelaxation of the discharge involving a hollowing of the parallel current density profile. The setup conditions are found to affect the volt‐second consumption and plasma/wall interaction during RFP formation, as well as the flat‐top discharge performance.

The effect of aspect ratio on magnetic field fluctuations in reversed‐field pinches is investigated using a three‐dimensional magnetohydrodynamic code. Configurations with aspect ratios of 1.1, 2.2, and 4.4 are modeled. The results are extrapolated to aspect ratio 8.8 for comparison with the Extrap T1 experiment [Nucl. Fusion 34, 427 (1994)]. It is found that the average modal amplitudes decrease with aspect ratio. However, the spectrum broadens correspondingly, resulting in negligible effect on the magnetic fluctuation level. The computed spectrum dynamics are found to be in good agreement with experimental observations on the T1 experiment. Quantitative evaluations of the field line stochasticity indicate no dependence of the mean magnetic field diffusion rate on aspect ratio.

A study of the steady‐state magnetohydrodynamic dynamo power balance in the Extrap T1 reversed‐field pinch [Nucl. Fusion 34, 427 (1994)] is reported. The power transfer between the poloidal and toroidal magnetic fields is discussed in terms of a dynamo efficiency which is degraded with increasing pinch parameter Θ=Bθ(a)/〈Bφ〉 as a consequence of a larger fraction of the input power (up to 50% at high Θ) being dissipated by the fluctuations. It is shown that this anomalous (non‐Spitzer) input power scales specifically with the power in nonlinearly driven dissipative m∈[0,2] modes. This is consistent with numerical results indicating the dynamo arises primarily from the m=1 modes.

The structure of the magnetic fluctuations in the Extrap T1 reversed‐field pinch [Phys. Scr. 44, 358 (1991)] has been studied over the pinch parameter range 1.6

Edge electrostatic fluctuations, in the Extrap T1 reversed‐field pinch [Nucl. Fusion 34, 427 (1994)], are observed to be correlated to internal tearing mode activity. Bispectral analysis of the edge electrostatic fluctuations shows the occurrence of nonlinear coupling between the low frequency internal tearing‐mode‐related activity and the high frequency, external, electrostatic fluctuations. In addition, the fluctuation levels of both the edge electrostatic fluctuations and the internal tearing modes have comparable scaling with plasma current. These results suggest that suppression of the internal tearing mode activity may decrease the edge electrostatic fluctuations and the related particle loss in the reversed‐field pinch configuration.

We present the study of the local structure and magnetic inhomogeneity of nano-sized La0.7Sr0.3MnO3 manganites performed by x-ray absorption fine structure (XAFS) spectroscopy and nuclear magnetic resonance (NMR). Nano-powders with particle size of 50–200 nm (NP1) and 20 nm (NP2), and bulk samples (BS) were studied. EXAFS data show the difference between the local structure of core and outer shell of the nano-particle. The data evidenced that the shell of particles presents the disordered crystalline array. According to the NMR, at low temperature the magnetic state of NP1 sample is homogenous. However, with increase of temperature the two ferromagnetic phases, with strong and weakened double exchange, coexist. The NP2 powder is magnetically homogeneous and characterized by weakened double exchange at all temperatures of the existence of the magneto-ordered state. The results specify on possible existence of characteristic spatial scale below which the phase stratification becomes energetically unfavorable.

The magnetic, resonance, and electric properties of LaxMnO3+δ (0.815≤x≤1.0) polycrystalline samples have been studied in the temperature range of 77–370 K and at high pressures of up to 11.5 kbar. It is shown that the increase in the La/Mn ratio gives rise to a change in the low temperature magnetic state from ferromagnetic to cluster spin glass, as well as to a drastic transformation of electric properties. A peculiar double-peaked shape is characteristic of the resistivity versus temperature curves for the intermediate range of x values. Within this range, the application of high pressures drastically changes both the value of resistivity and the character of its temperature dependence. It is shown that the approach, which regards the state of LaxMnO3+δ polycrystals as a mixture of interpenetrating paramagnetic insulating and ferromagnetic metallic phases, is able to successfully describe the peculiarities of the temperature dependence of total resistivity, as well as the features of its transformation ...

Annealing at 220–420 °C and exposure to laser light were used to induce sintering of silver metal interlayers in polyimide films. The sintering process was followed by transmission electron microscopy, electrical conductivity, optical reflectivity measurements, and x‐ray scattering. We find that upon sintering at temperatures below Tg≂380 °C of the polymide film, sintering of the silver interlayer is induced. A dramatic increase in both the optical reflectivity (from 32% to 95%) and a hundredfold increase in electrical conductivity are observed upon sintering. Both the average size and the connectivity of the silver particles in the interlayer are increased in the process; however, there is no apparent change in the polymer. High‐speed (∼100 ns) high spatial resolution laser writing (∼1 μm) is feasible by a selective sintering process. It is, therefore, of potential interest as a basis for write‐once, high‐density optical information storage. It is suggested that a surface diffusion mechanism is responsible for the observed sintering process.