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Sonochemically synthesized Fe2O3 nanoparticles were annealed in air or in vacuum while their magnetization was continuously recorded. Annealing in vacuum at temperatures Ta between 240 and 450 °C produced nanophases of γ-Fe2O3 with average particle size ranging from 4 to 14 nm, depending on Ta. Phase transformation into α-Fe2O3 occurred directly by annealing in air, or via an intermediate Fe3O4 phase by annealing in vacuum at temperatures higher than 450 °C. Mapping the correlation between the magnetic properties and the annealing conditions, enables control of the annealing process to obtain nanocrystals of γ-Fe2O3, α-Fe2O3, or Fe3O4 with different particle size and magnetic properties.

We report on the thickness dependence of the irreversible magnetization in superconducting Y1Ba2Cu3O7−δ films of thickness 350–3000 A. Our results reveal a nonmonotonous dependence of the persistent current density j on the film thickness, which is interpreted in terms of surface pinning and variations in the surface microstructure. Measurements of the time dependence of j show that under certain conditions relaxation curves of samples of different thickness cross each other, i.e., the sample with initially larger j exhibits after some time a lower j. The crossing point is shifted to shorter times as the temperature is increased. We propose a simple explanation to this effect and discuss its practical implications. Low dose heavy ion irradiation of the films has a modest effect on j and on the rate of its relaxation.

Local magnetization curves at different times, extracted from high-temporal resolution magneto-optical measurements in Bi2Sr2CaCu2O8+δ, demonstrate the absence of the second magnetization peak at short times, its appearance at longer times, and the movement of its onset toward higher-induction fields approaching the thermodynamic vortex order–disorder transition field. We relate this phenomena to metastable disordered states induced by edge contamination for inductions in the vicinity of the order–disorder vortex phase transition. We show that the time evolution of the second magnetization peak is governed by the relaxation times of the metastable disordered vortex states.

Bi2Sr2CaCu2O8 crystals were irradiated with heavy ions (Pb, Xe) to produce columnar defects along the c direction or at 45° with respect to it. We describe the dependence of the critical currents, irreversibility fields, pinning force, and magnetic relaxation rates on the dose and on the type of ions. The irreversible properties of the Pb‐irradiated samples are more enhanced than those of the Xe‐irradiated sample at low temperatures, and vice versa at high temperatures. At low temperatures the efficiency of the columnar defects is enhanced with the ion dose but, at high temperatures, the maximum dose (2.5×1011 ions/cm2) is less efficient than intermediate doses. The unidirectional nature of the columnar pinning centers serves to demonstrate that the vortices maintain linelike features. For most of the irradiated samples, the pinning force density data are reduced to a single curve below 45 K. The relaxation rates at low temperatures reflect thermal reduction of the effective pinning barriers.

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 voltage–current characteristics of a superconducting coil, made of multifilamentary silver-sheathed Bi2Sr2Ca2Cu3O10+δ tape, are investigated. We find that a small ac current Iac superimposed on a relatively large dc current Idc causes a significant increase in the coil dc voltage, approximately proportional to Idc, Iac2, and the ac frequency. We attribute this effect to the nonlinear magnetoresistance of the coil, and discuss its significance in power applications of high Tc superconducting coils, such as fault current limiters and superconducting magnetic energy storage devices.

We found that microcracks in thin Nd2/3Sr1/3MnO3 films create a series of intrinsic break junctions which are ideal for investigating tunneling phenomena in this system. A comparison of films with and without cracks, which have similar ferromagnetic Curie temperature (Tc) of 140–150 K, shows that the cracked film has a lower insulator to metal transition temperature TM (97 K vs 140 K), three orders of magnitude higher resistivity at TM, and two times larger magnetoresistance at 1 T near TM. At T>TM we observed that ln ραT−1/4 in the uncracked film while in the cracked film a ln ραT−1/2 dependence was found. This indicates that the conductivity in the first case is due to variable range hopping in three dimension, while in the second case it is dominated by thermally activated tunneling across the insulating barriers (the microcracks).

We describe an experimental technique for simultaneous measurement of both the normal (Bz) and the in-plane (Bx) components of the magnetic induction field near the surface of a superconducting sample. This technique utilizes a novel design of a double-layered Hall sensor array fabricated from a GaAs/AlGaAs heterostructure containing two parallel layers of a two-dimensional electron gas. The effectiveness of this technique is demonstrated in measurements of Bx and Bz and the current distribution at the surface of a thin YBa2Cu3O7 crystal.

The details of a new method for studying thermally activated flux creep in thin superconducting samples is described. The method employs a linear array of microscopic Hall sensors to measure the time and spatial dependence of the magnetic induction across the sample. These data are analyzed on the basis of the local rate equation for thermally activated flux motion, taking into account both the in-plane and out-of-plane components of the induction field. Following this analysis, flux creep parameters, such as the flux-line current density, flux-line average velocity, and the activation energy for flux creep, can be directly determined as a function of position and time. New experimental data in a superconducting Nd1.85Ce0.15CuO4−δ crystal demonstrate this method.

Superconducting films of YBa2Cu3O7–δ are shown to become thermomagnetically unstable when experiencing a time-varying perpendicular magnetic field. Using magneto-optical imaging and ramping the applied field at rates up to 3000 T/s, dendritic flux avalanches were observed in two different films, one grown by evaporation on sapphire and one by laser ablation on SrTiO3. The unstable behavior occurs over a wide temperature range limited by an upper threshold value of 40 K for the film on sapphire, and 20 K for the one on SrTiO3. At 7 K for the same films, the threshold ramping rates are 1000 T/s and 3000 T/s, respectively. The avalanches are causing permanent damage by leaving a micron wide track where the superconductor melted during the thermomagnetic runaway.

We report a ‘‘memory’’ effect in the response of sintered Y‐Ba‐Cu‐O to alternating magnetic fields. The application of a steady bias field, above a certain threshold, causes the generation of harmonic components in the response. These harmonics persist after the field is removed, exhibiting a logarithmic decay with time. We explain these phenomena in the framework of the Bean model, taking into account the granularity of the sample. The memory phenomenon offers a principle for a new recording mechanism which may allow extremely high storage capacity.

We report on generation of spatiotemporal oscillations of magnetic flux in a Bi2Sr2CaCu2O8+δ crystal irradiated in part with 2.2 GeV Au ions. Flux oscillations are spontaneously excited after exposing the sample to a steady magnetic field near the order-disorder vortex phase transition line. The oscillations originate at the border between the irradiated and nonirradiated parts of the sample and propagate into the nonirradiated region toward the sample edge. Previously reported flux oscillations were observed in the vicinity of undefined defects in as grown Bi2Sr2CaCu2O8+δ crystals. Observation of spontaneous oscillations in partially irradiated samples present the first attempt to generate such oscillations in a controlled manner.

ac magnetic measurements were employed in the study of the vortex phase diagram in Bi2Sr2CaCu2O8+x. The Bragg-glass to vortex-glass (solid-solid) and the Bragg-glass to liquid (melting) transitions are manifested in these measurements by an increase or a decrease in the screening signal, respectively. The ac measured solid-solid transition line extends to high temperatures where a melting transition is observed in dc measurements. Increasing the frequency of the ac field can further enlarge this region of the solid-solid transition. These results confirm that the solid-solid and the solid-liquid transition lines are two segments of the same order-disorder phase transition line.

Vector magnetization measurements in La1.85Sr0.15CuO4 under a steady field H tilted relative to the ab plane show that the induction B rotates toward H with time. The rate of rotation as a function of temperature exhibits two peaks separated by a dip around 12K. Measurements of the remnant induction show that for temperatures below ∼12K, B rotates towards the c axis, whereas for temperatures above ∼12K, B rotates towards the ab planes. We argue that these data are consistent with a two-dimensional and a three-dimensional behavior below and above the dimensional crossover temperature, ∼12K, respectively.

The pinning mechanism in La1.85Sr0.15CuO4 (LaSCO) was investigated by analyzing the critical current dependence on field and temperature. Unlike other high temperature superconductors such as YBa2Cu3O7, DyBa2Cu3O7−δ, and Nd1.85Ce0.15CuO4−δ, which exhibit a single pinning mechanism over wide temperature and magnetic field ranges, our data in LaSCO show a clear crossover from δl to δTc pinning with temperature or field. While δl pinning is effective at low fields or low temperatures, δTc pinning dominates at high fields or high temperatures. Implications of the pinning mechanism crossover on the unique behavior of the second magnetization peak in LaSCO are discussed.

The magnetic moment and relaxation rates in Bi2Sr2CaCu2O8+δ crystals exhibit remarkable dependence on the sample size in the vicinity of the vortex order–disorder phase transition line. Specifically, we find that: (1) the magnitude of the normalized magnetic moment increases as the sample size decreases, (2) the relaxation rate increases with the sample size, and (3) the difference between the relaxation rates in samples of different size, decreases with temperature and field. These phenomena are attributed to dynamic coexistence of quasi-ordered and metastable disordered vortex states in the samples, in the temperature and field range near the vortex phase transition.

Commercial manganin stress gauges are used in planar impact experiments to determine spall strengths of variously heat‐treated 2024A1 specimens. A one‐dimensional Lagrangian code is used to find the best fit of calculated versus experimental stress histories by varying the spall strength in the code and using a simple threshold criterion. We found values in the range of 1.2–1.75 GPa for spall strengths of the differently treated specimens. These values are in good agreement with published data obtained by other methods. We also confirmed the spall strength values by a series of low‐stress experiments resulting in only incipient spalls. A specimen of 2024T351 which was shocked perpendicular to the rolling direction has a lower spall strength (by about 0.3 GPa) than a specimen shocked along the rolling direction. This difference is very close to that predicted earlier.

Plate impact experiments were performed on commercial alumina disks (AD‐85, manufactured by Coors), with in‐material Manganin gauges, in order to determine the dynamic response of this material in the 0–100 kb range. The measured stress signals showed that the transmitted waves, at the high stress range, are composed of an initial elastic jump, to about 60 kb, followed by a dispersive rise to the final stress level. This behavior was found recently for pure alumina but not for AD‐85. We used the gauges to determine the Hugoniot curve of AD‐85 in the 0–100 kb range and its spall strength in this region.

We have measured the temperature dependence of magnetic relaxation and critical current in an YBaCuO single crystal. Unusually strong time‐logarithmic magnetic relaxation is observed. The relaxation rate increases initially with temperature, peaks at ≂25 K and slows down at higher temperatures. The results, interpreted with a thermally activated flux model, yield a low‐temperature pinning energy U0≂0.02 eV. This low pinning energy results in an unusually rapid decrease of the critical current as temperature is increased.

The shear strength of shock‐loaded commercial alumina (AD‐85 manufactured by Coors) is determined in the 0–140‐kbar range of shock stresses. Longitudinal and transverse manganin gauges were used to determine the principal stresses in the shocked specimens. Shear strengths were determined from the difference between the longitudinal and lateral stresses. It was found that the shear strength remains essentially constant at about 27 kbar for shock stresses between 60 kbar (the Hugoniot elastic limit) and the maximum shock amplitude tested in this series (142 kbar). The source for the high shear strength is attributed to the confining pressures that strengthen the comminuted ceramic. Evidence for this interpretation is obtained by considering the release profiles as recorded by the longitudinal gauges when the free‐surface rarefactions reach gauge location.

Plate impact experiments were conducted on AD‐85 alumina specimens (manufactured by Coors) in the low shock stress region. The experimental configuration consisted of impacting a thin flyer disc (either copper or alumina) on a relatively thick specimen disc (the target). A thin manganin stress gauge was placed at the back face of the specimen supported by a thick polymethylmethacrylate disc. After recording the initial shock at the specimen‐PMMA interface, the gauge showed an unusually fast compressional wave preceding the main release wave which originates at the flyer’s back surface. This wave appeared for all the shots with copper flyers and impact velocities resulting shock stresses in the 0–30 kb range. They did not appear when alumina flyers were used. Our explanation for the origin of this fast compressional wave is based on the possible occurrence of dilatancy in the specimen upon stress unloading.