A substantial increase in flux pinning has been reported in Li doped YBCO grains following thermal neutron irradiation. This effect observed in sintered YBCO was attributed to the increased number of defects in the crystal lattice produced by the particles emitted from the fission of Li-6. In this study we report preliminary measurements on the effect of irradiating Li doped melt grown YBCO with thermal neutrons. A single grain was divided into two samples, one of which was then irradiated with approximate to 10(17) neutrons. VSM measurements have revealed that the irradiated sample is far superior at low fields (at best a factor of five increase in J(c) at 77 K), but at high fields the magnetic moments become very similar and the irreversibility field remains the same. The absence of fishtail irradiation suggests the introduction of an additional low field pinning mechanism. (C) 1998 Published by Elsevier Science S.A. All rights reserved.

We present data from a.c. susceptibility and resistivity measurements in an applied d.c. magnetic field on clean detwinned YBa2Cu3O6.92 single crystals. Our investigations have revealed an extremely sharp amplitude and frequency independent onset of both the in phase and out of phase a.c. susceptibility components. For fields below H*=6T the position of this onset in H-T plane agrees with the melting transition obtained from resistivity measurements on the same crystal. At H≥H* this transition becomes criteria dependent. We found that H* accurately correlates with the multi-critical point obtained from resistivity measurements.

Magnetic studies have been carried out in high quality detwinned YBa2Cu3Oδ single crystals. We report for the first time that the maximum in magnetisation shifts to higher fields when the oxygen concentration of the samples increases. Also, the magnetisation peak or “fishtail” peak as commonly referred at, shifts to higher field values as the temperature raises, contrary to previous experimental results. In addition the peak effect which appears in the magnetic hysteresis loops is a two step process, being separated at high temperatures into two peaks, one of which consistently correlates to the multicritical point. Furthermore, the position of these two peaks in the phase diagram is disorder independent. Our findings strongly suggest the importance of in intrinsic parameter unique in YBa2Cu3Oδ, namely the in-plane anisotropy.

We present magneto-resistance data from a high quality detwinned YBa2Cu3O7−δ single crystal, taken using the conventional four-point method with the current applied along the ab plane and the field along the c-axis. Measurements were taken in fields of up to 12T. At temperatures just below the melting line, a sharp feature (a ‘peak effect’) appeared in the resistance when sufficiently high current densities were applied. We saw this feature as both a function of temperature, ϱ(T), and field, ϱ(H). At higher fields, above the multi-critical point H*=6T, the peak effect in ϱ(H) splits into two and spreads apart in field. This forms two branches in the H-T phase diagram, and their implications for the dynamics of the vortices are discussed.

The amplitude of the χ″ peak is found to be strongly temperature dependent in a wide temperature region below the melting line. We explain this effect by a crossover from a hysteretic (at low T) to non-hysteretic (at high T) mechanism of the a.c. losses.

Magnetic and transport measurements have been performed in high quality YBa2Cu3O7-delta single crystals. We demonstrate for the first time that the magnetization peak line in the phase diagram of YBa2Cu3O7-delta, H-p(T) exhibits an impressive similarity with the equivalent line for Bi2Sr2CaCu2O8. Our results show not only a similar temperature dependence but also an almost identical response to oxygen doping and a correlation to the multicritical point. At high temperatures we observe a previously unreported splitting of the magnetization peak.

Resistivity-temperature and voltage-current characteristics of clean detwinned YBa2Cu3O7−δ single crystals have been measured using a SQUID picovoltmeter in magnetic fields of up to 5 T. We have observed hysteretic behaviour near Tm in the ϱ(T) dependence: the resistivity when warming the sample up (the “superheated” state) was lower than when cooling it down (the “supercooled” state). The V-I characteristics for the sample in the “superheated” and “supercooled” states were also found to be different. We have found that the “superheated” state is unstable and can be switched to the “supercooled” state by applying a comparatively low probing current density (j ∼ 0.4A/cm2). We relate the current-induced resistivity hysteresis to the rearrangement of the solid vortex domains due to the probing current.

The dynamic behaviour of a driven vortex lattice in the vicinity of the melting line has been investigated. We have shown that a square wave driving current with asymmetric period or amplitude can induce a slow periodic variation in the voltage response. We also show that this effect can be observed for current modulations with sinusoidal and triangular wave forms thereby demonstrating that these oscillations are not due to the sharp fronts of the probing current which are a feature of the square wave modulation.

Electrochemical synthesis has been demonstrated in the production of quality high-temperature superconducting films. Major influences upon the deposition process, and electrochemical mechanisms are discussed. Such controlling factors include: solution water content, cell geometry, temperature stability, and relative metallic ion concentration. Finally, the synthesis and characterisation of thick films of the high temperature superconductors Bi2Sr2CaCu2O8+delta and (Tl,Pb) Sr2Ca2Cu3O9+gamma with excellent c-axis alignment, produced via this route, are reported. (C) 1997 Academic Press Limited.

Films of TI-Pb-Sr-Ca-Cu superconductor precursor material have been synthesised. The films were electrodeposited onto polished Ag substrates, with and without the presence of an ultrasonic field in the solution, by applying a pulsed potential sufficiently large to cause reduction of the appropriate metallic cations. The deposition solution was composed of dehydrated metallic nitrate salts dissolved in dimethylsulfoxide. The morphology, and the compositions, of the films were analysed using scanning electron microscopy and emission dispersive spectroscopy respectively. Deposition in the presence of the ultrasonic field showed an increased deposition current, by a factor of 4, caused by mass transfer enhancements as a result of cavitation effects produced by the ultrasonic irradiation of the solution. These films were denser compared with those deposited without the presence of the ultrasound, displaying a void percentage of around 45%, and also displayed greater morphological homogeneity.

A practical method was devised for measurements of the diamagnetic response of high temperature superconductor specimens. The method is useful for the study of the temperature and field dependencies of the local magnetic induction and its two-dimensional visualisation B(T,Ha,x,y) with a resolution of 0.1 gauss and spatial resolution of 0.1 mm in the plane of the specimen surface. The proposed method facilitates quite simple and quick measurements at liquid nitrogen temperature and in applied Ha magnetic field in excess of 1 T. Cross-shaped miniature Hall probes (HPs) of thin film n-InSb on semi-insulating substrate i-GaAs with a small magnetically sensitive zone of 0.1 × 0.1 mm2 were used as measuring transducers. The high degree of doping of n-InSb layers (n ~ 1018 cm−3) resulted in a sensitivity of 5–7 μV mT−1 (with nominal current up to 10 mA through HPs) and thermosensitivity of less than 0.03% K−1, and made it possible to operate without the need for thermal stabilization of transducers. This method enabled us to study anisotropy of the critical current densities (Jc) of elt-grown YBa2Cu3O7 − x pellets. Also, we have been able to determine local cracks and grain connectivity in the YBa2Cu3O7 − x pellet samples.

We present magneto-resistance data for a high quality, twinned YBa2Cu3O7−δ crystal, taken with the current applied along the ab plane. The crystal was examined at a number of angles to an applied magnetic field, of up to 20T, in order to observe the influence of correlated and point-like disorder on the vortex dynamics. When the applied field was orientated at 15° to the crystalline c-axis (θ=15°), for fields below H*=12T, we observed the kink in ϱ(T) associated with vortex lattice melting. We found that when the field exceeded this value, there was a complete suppression of this kink and the ϱ(T) curves resembled those of crystals with extensive point disorder. This suppression in melting occurs abruptly between 11T and 12T. The melting transition is recovered when the angle between the c-axis and the field is increased. An analysis of features around 12T in ϱ(T) and ϱ(θ) has been performed for a number of fields and angles.

Precursor films for Bi-Sr-Ca-Cu superconductors have been prepared by employing a reproducible electrochemical technique. Thick films (> 1 μm ) were routinely electrodeposited, within a controlled environment, onto polished Ag substrates, from a non-aqueous solution containing the relevant dehydrated nitrate salts. The as-deposited films were analysed using energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). Heat treated as-deposited films yielded almost single phase, highly c -axis aligned, Bi 2 Sr 2 CaCu 2 O 8 + δ superconductors, with a superconducting onset transition temperature, T c , of 87 K.

Magnetoelastic stress measurements in Ho/Y and Ho/Lu superlattices were performed by using a capacitive cantilever technique. The magnetic phase diagrams obtained from in-field transitions observed in the stress isotherms show helical, fan and ferromagnetic phases. At zero-field, the ferromagnetic phase is suppressed in Ho/Y and stabilised in Ho/Lu. Magnetoelastic coupling coefficients corresponding to the breaking of the cylindrical symmetry of the basal plane have been determined. Surface magnetostriction is observed for {Ho31Lu19}50.

Magnetization measurements have been used to study the Bose-glass to vortex-glass crossover in YBa2Cu3O7-x, crystals by varying the angle theta between the field and the twin planes. Two critical angles theta(L) and theta(K) are found: For theta < theta(L), vortices lock in to the twin planes; for theta(L) < theta < theta(K), a kinked vortex structure occurs; for theta > theta(K), hysteresis loops reproduce the magnetic response of the untwinned regions where pinning is produced by random point defects. The angles theta(K) and theta(L) depend only on the disorder in the untwinned regions and scale with [Delta M(theta(X))](-0.5), where Delta M(theta(K)) is the hysteresis width for theta > theta(K).

We demonstrate, by studying the magnetic hysteresis of twinned and detwinned YBa2Cu3O7-delta single crystals, that twin planes, contrary to the usual belief, can greatly decrease the effective pinning in the field regime near the hysteresis maximum. Twin planes act as flux channels: an easy way for vortices to enter into the sample. By varying the angle of the twin planes with the magnetic field, we have found that the hysteresis and, thus, the critical current is greatly affected in a wide field and temperature range.

MBE grown Ho/Lu superlattices, investigated by vibrating sample magnetometry, are found to have simple phase diagrams, with a helical phase analogous to that in bulk Ho, and a low temperature ferromagnet-like phase. This is stabilised by the strain of the lattice mismatch between Ho and Lu.

The field-cooled magnetization of Bi2Sr2CaCu2Ox, Tl2Ba2CaCu2Oy, and YBa2Cu3O7-z single-crystalline superconductors has been studied as a function of thermal history. A dependence of the Meissner fraction on the sample cooling rate was found. After a more rapid cooling, the temperature dependence of the magnetization during warming showed a pronounced minimum just before the transition into the normal state. This behavior has been analyzed within a generalized critical-state model, taking into account nonlinear flux diffusion. In the case of linear temperature dependences of H-cl and conductivity, an exact solution was found which qualitatively reproduces the experiment.

Magnetic measurements are presented for MBE grown superlattices of Ho/Tm. For Tm blocks with ≥ 20 Tm layers the ground state is a ferrimagnetic structure, as in pure Tm. The compressive strain due to the lattice mismatch between Ho and Tm stabilises a ferromagnetic state in the Ho layers, for blocks ≤ 20 Ho layers thick.