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By implementing a unique magneto-optical system with ultrafast magnetic-field ramping-rate capability (up to 3 kT/s), we have been able to routinely generate and image dendritic flux instabilities in YBa2Cu3O7-x films. In the present work we study the effect of the film thickness on the dendritic instability. Dendritic avalanches in 50 - 600 nm thick films were magneto-optically imaged at 7 K, after ramping the magnetic field from zero to 60 mT at different rates. The data reveal a remarkable change in flux morphologies between the thin and the thicker films. While the former (50-250 nm) display well-developed dendritic patterns, the latter (350-600 nm) exhibit few avalanches with favored branch directions parallel to the film's edges. Several possible explanations for this behavior are discussed.

Nb nano-rings connected serially by Nb wires exhibit, at low bias currents, the typical parabolic Little-Parks magnetoresistance oscillations. As the bias current increases, these oscillations become sinusoidal. This result is ascribed to the generation of Josephson junctions caused by the combined effect of current-induced phase slips and the non-uniformity of the order parameter along each ring due to the Nb wires attached to it. This interpretation is validated by further increasing the bias current, which results in magnetoresistance oscillations typical of a SQUID.

Effects of sample size on the second magnetization peak (SMP) in Bi2Sr2CaCuO8+δ crystals are observed at low temperatures, above the temperature where the SMP totally disappears. In particular, the onset of the SMP shifts to lower fields as the sample size decreases —a result that could be interpreted as a size effect in the orderdisorder vortex matter phase transition. However, local magnetic measurements trace this effect to metastable disordered vortex states, revealing the same order-disorder transition induction in samples of different size.

Magnetic flux instability limits potential applications of superconductors such as MgB2 in practical devices. Previous studies in MgB2 films exposed to magnetic fields revealed the occurrence of dendritic flux avalanches at temperatures below T ~ 10 K. In the present work it is shown that films of MgB2 exposed to a fast-ramped magnetic field display a dendritic flux instability at elevated temperatures, up to 23 K. Such instability can therefore cause malfunctioning of practical devices based on MgB2 films even when operating at liquid hydrogen temperature.

We calculate the distribution of the current density $j$ in superconducting films along the direction of an external field applied perpendicular to the film plane. Our analysis reveals that in the presence of bulk pinning $j$ is inhomogeneous on a length scale of order the inter vortex distance. This inhomogeneity is significantly enhanced in the presence of surface pinning. We introduce new critical state model, which takes into account the current density variations throughout the film thickness, and show how these variations give rise to the experimentally observed thickness dependence of $% j$ and magnetic relaxation rate., RevTex, 9 PS figures. To appear in Phys. Rev. B

We report on measurements of the angular dependence of the irreversibility temperature {ital T}{sub irr}({theta}) in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films, defined by the onset of a third-harmonic signal and measured by a miniature Hall probe. From the functional form of {ital T}{sub irr}({theta}) we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle. {copyright} {ital 1996 The American Physical Society.}

Local magnetic relaxation measurements in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the {open_quote}{open_quote}fishtail{close_quote}{close_quote} magnetization. In this range the activation energy {ital U}{sub pl}, which decreases with field, drops below the elastic (collective) creep activation energy, {ital U}{sub el}, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}. {copyright} {ital 1996 The American Physical Society.}

This paper investigates the interaction of long-rod penetrators with thick ceramic tiles, sandwiched between steel plates, through several model experiments and 2-D simulations. Experimental data from low velocity penetrations have been used to calibrate the relevant properties of the ceramic specimens. The influence of increasing impact velocity on tile performance was then investigated through data and simulations of shaped charge jets penetrating the ceramic. We found that the ballistic efficiency of the ceramic tile is lower against high velocity (5 km/s) long-rods, in contrast with the common thesis that their improved performance against shaped charge jets is the result of their enhanced strength. On the other hand, our simulations clearly show that, for high strength ceramics, there is a radial motion of metal and ceramic debris towards the penetration axis. This effect is, probably, the main reason for the considerable improvement in the performance of ceramic tiles against shaped charge jets.

We report measurements of the magnetic susceptibility of single crystals of Mn$_{12}$-acetate-MeOH, a new high-symmetry variant of the original single molecule magnet Mn$_{12}$-acetate. A comparison of these data to theory and to data for the Mn$_{12}$ acetate material shows that Mn$_{12}$-acetate-MeOH is a realization of a transverse-field Ising ferromagnet in contrast to the original Mn$_{12}$ acetate material, in which solvent disorder leads to effects attributed to random field Ising ferromagnetism., 6 pages, 4 figures

We report the structural and magnetic properties of SmCo7−xCux, where x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7. X-ray diffraction shows that these alloys from the disordered hexagonal TbCu7-type structure. For large values of x(x⩾0.8) the hexagonal TbCu7-type structure cannot be formed. X-ray diffraction on magnetically aligned samples show that these samples have uniaxial anisotropy. The lattice parameters (a and c) are dependent on the Cu concentration, and the unit cell volume is found to increase with x. The saturation magnetization decreases with x at both room temperature and 25 K. The Curie temperature increases with x for small values of x while it decreases with x for large values of x. A maximum value of TC=852 °C is found in these alloys.

The development of advanced ceramics for high strain rate applications has generated an interest in the measurement of the dynamical mechanical properties of these materials. Planar impact experiments were performed on a hot-pressed particulate composite of an oxynitride glass matrix with 50 wt% SiC filler. The samples contained less than 2 vol% porosity. The SiC filler powder was chemically inert in the oxynitride glass but was nevertheless completely wetted and homogeneously dispersed. Static and dynamic mechanical properties were measured for both the monolithic glass matrix and the particulate composite. Hugoniot curves were determined in the shock pressure range from 2.5 to 10 GPa using manganin gauges. The shock impedance of the composite was approximately that expected from the law of mixtures while the Hugoniot elastic limit (HEL) of the composite, 9.2 GPa, was close to that of the parent glass. On the otherhand, the toughness and bend strength of the composite greatly exceeded those of the parent glass.

This paper presents an attractor neural network (ANN) model of recall and recognition. It is shown that an ANN Hopfield-based network can qualitatively account for a wide range of experimental psychological data pertaining to these two main aspects of memory retrieval. After providing simple, straightforward definitions of recall and recognition in the model, a wide variety of ‘high-level’ psychological phenomena are shown to emerge from the ‘low-level’ neural-like properties of the network. It is shown that modeling the effect of memory load on the network's retrieval properties requires the incorporation of noise into the network's dynamics. External projections may account for phenomena related with the stored items’ associative links, but are not sufficient for representing context. With low memory load, the network generates retrieval response times which have the same distribution form as that observed experimentally. Finally, estimations of the probabilities of successful recall and recognition are...

Evolution of the phase state of paramagnetic additions at various stages of synthesis and subsequent thermal treatment of glasses of the system Al2O3–K2O–B2O3 simultaneously doped with Fe2O3 and MnO is studied by means of a combination of experimental techniques: Faraday rotation (FR), electron magnetic resonance (EMR), transmission electron microscopy (TEM), Mossbauer spectroscopy, and magnetic measurements. Both FR and EMR show that magnetically ordered clusters occur already at the first stage of the glass preparation. In particular, for the ratio of the Fe and Mn oxides in the charge close to 3:2, fine magnetic nanoparticles are formed with characteristics similar to those of manganese ferrite. By computer simulating the EMR spectra at variable temperatures, a superparamagnetic nature of these nanoparticles is confirmed and their mean diameter is estimated as approximately 3.2 nm. In the thermally treated glasses larger magnetic nanoparticles are formed, giving rise to FR spectra, characteristic of ma...

Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry., Comment: 11 pages, 4 figures

Experimental methods are presented for the determination of fracture initiation toughness of ceramics and ceramic composites in pure tension under quasi-static and dynamic loading conditions over a range of temperature spanning 20° to 1300°C. Circumferentially notched and cyclic fatigue precracked rods of a variety of ceramic materials were subjected to quasi-static tensile fracture (rate of stress intensity factor loading, K1∼ 0.1 MPa ∼ m1/2∼ s−1) in an electroservohydraulic test machine and to dynamic tensile fracture (K1∼ 106 MPa ∼ m1/2· s−1) using a modified tensile Kolsky (split-Hopkinson) bar. For the quasi-static and dynamic fracture tests at elevated temperatures, the ceramic specimen was inserted into an air furnace where either friction grips or stress wave loading outside the furnace subjected the specimen to fracture. Dynamic finite-element analyses of the circumferentially notched cylindrical rod have been conducted to develop the optimum specimen dimensions and test procedures for the measurement of dynamic fracture toughness at ambient and elevated temperature. Experiments conducted on Al2O3, Si3N4, and SiC, and an Al2O3-25 vol% SiC whisker composite at room temperature indicate that the dynamic to quasistatic fracture initiation toughness ratio KId/KIc is in the range of 1.1 to 1.4. Elevated-temperature fracture tests for the polycrystalline Al2O3 of 3-μm average grain size reveal that KId is only mildly sensitive to temperature over a range of 20° to 1100°C, whereas it suffers a precipitous drop above 1100°C. Over the temperature range 20° to 1300°C, the ratio KId/KIc is found to be in the range 1.2 to 1.5. Scanning electron microscopy observations show that failure above 1100°C usually evolves by the nucleation, growth, and coalescence of cavities. The mechanisms of elevated-temperature quasi-static and dynamic fracture in polycrystalline Al2O3 are examined and possible causes for the apparently higher dynamic fracture initiation resistance are discussed. The significance and limitations of the proposed experimental techniques are highlighted.

Measurements of the nonlinear flux-flow resistivity $\ensuremath{\rho}$ and the critical vortex velocity ${v}_{\ensuremath{\varphi}}^{*}$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov (Z. Eksp. Teor. Fiz 68, 1915 (1975) [Sov. Phys. JETP 41, 960 (1976)]) are reported along the node and antinode directions of the $d$-wave order parameter in the $a\mathrm{\text{\ensuremath{-}}}b$ plane of epitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films. In this pinning-free regime, $\ensuremath{\rho}$ and ${v}_{\ensuremath{\varphi}}^{*}$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $\ensuremath{\rho}$ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$.

The dynamics of transient disordered vortex states in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ was magneto-optically traced in three experiments: (i) during continuous injection of transient vortex states while ramping up the external magnetic field, (ii) during annealing of injected transient states while keeping the external field constant, and (iii) during annealing of transient ``supercooled'' disordered states while ramping down the external field. The results reveal frontlike propagation [experiment (i)] or retreat [experiments (ii) and (iii)] of the transient vortex states, at a rate governed by the rate of change of the external field, the annealing time $\ensuremath{\tau}$ of the transient states and the creep rate. The experimental results are theoretically analyzed in terms of competition between generation and annealing of transient disordered vortex states. Extraction of the annealing time $\ensuremath{\tau}$ from the above three experiments yields the same results for $\ensuremath{\tau}$ as a function of the induction B and temperature T. Knowledge of $\ensuremath{\tau}(B,T)$ allows for correct determination of the thermodynamic order-disorder vortex phase-transition line.

The diversity of vortex melting and solid-solid transition lines measured in different high-T c superconductors is explained, postulating a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of monotonic and nonmonotonic transition lines similar to those measured in Bi 2 Sr 2 CaCu 2 O 8 , YBa 2 Cu 3 O 7 - Φ , Nd 1 . 8 5 Ce 0 . 1 5 CuO 4 - Φ , Bi 1 . 6 Pb 0 . 4 Sr 2 CaCu 2 O 8 + Φ , and (La 0 . 9 3 7 Sr 0 . 0 6 3 ) 2 CuO 4 .

Magnetization measurements in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ crystals indicate vortex order-disorder transition manifested by a sharp kink in the second magnetization peak. The transition field exhibits unique temperature dependence, namely a strong decrease with temperature in the entire measured range. This behavior rules out the conventional interpretation of a disorder-driven transition into an entangled vortex solid phase. It is shown that the transition in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ is driven by both thermally- and disorder-induced fluctuations, resulting in a pinned liquid state. We conclude that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions are different manifestations of the same thermodynamic order-disorder transition, distinguished by the relative contributions of thermal and disorder-induced fluctuations.

Magnetic decoration of BaFe2(As1-xPx)2 revealed an hexagonal vortex lattice in overdoped single crystals (x>0.4, Tc=14-18 K). A disordered vortex lattice was observed in a crystals with optimal content of phosphorus (x ≈ 0.4, Tc = 25-30 K). Magneto-optical studies and global magnetization measurement of the optimally-doped crystal have led to an estimated critical current density of ~ 10 5 A/cm 2 at 5-12 K in fields up to 0.7 T. This high critical current density suggests strong pinning, probably due to defects, a result consistent with the observed disordered vortex lattice. The critical current density of the overdoped crystal is much smaller, i.e. ~ 10 4 A/cm 2 at 5 K in fields up to 0.1 T, decreasing to virtually zero at 0.3 T.

The Landau-Khalatnikov time-dependent equation is applied to describe the crystallization process of the ordered vortex lattice in high temperature superconductors after a sudden application of a magnetic field. Dynamic coexistence of a stable ordered phase and an unstable disordered phase, with a sharp interface between them, is demonstrated. The transformation to the equilibrium ordered state proceeds by movement of this interface from the sample center toward its edge. The theoretical analysis dictates specific conditions for the creation of a propagating interface, and provides the time scale for this process., Comment: 8 pages and 3 figures; to be published in Phys. Rev. B (Rapid Communications section)

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three $Fe_{2}O_{3}$ samples composed of regular amorphous, acicular amorphous and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time dependent barrier yields a natural explanation to the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define a new energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data., Comment: RevTex, 16 JPEG figures, to appear in PRB

Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions., 10 pages, 4 figures

Magneto-optical imaging was employed to study magnetic flux propagation through an interface separating between heavy-ion irradiated and non-irradiated parts of the same Bi2Sr2CaCu2O8+δ crystal. As the flux front hits the interface the local induction increases abruptly at a rate considerably larger than that of the external field. This induction jump is accompanied by the appearance of a finger-like flux pattern at the interface. At low temperatures or high field ramp-rates, the induction at the interface increases gradually, finger patterns do not form and the flux front crosses the interface smoothly. We explain the flux behavior at the interface on the basis of the magnetic diffusion equation, and propose that the induction jump triggers the formation of finger patterns.

Local nonlinear AC magnetic measurements were employed in the study of the vortex phase diagram in Bi2Sr2CaCu2O8+x. The melting and the Bragg-Glass to Vortex-Glass (solid-solid) transitions are manifested by a sharp peak and a sharp drop in the third harmonic response, respectively. The peak at the solid-liquid transition signifies the hysteretic nature of the first-order melting transition. The disappearance of the nonlinear response in the vicinity of the solid-solid phase transition line suggests the existence of an intermediate soft lattice vortex state in-between the ordered and disordered vortex phases.