Search

Studies of nanoscale superconducting structures have revealed various physical phenomena and led to the development of a wide range of applications. Most of these studies concentrated on one- and two-dimensional structures due to the lack of approaches for creation of fully engineered three-dimensional (3D) nanostructures. Here, we present a ‘bottom-up’ method to create 3D superconducting nanostructures with prescribed multiscale organization using DNA-based self-assembly methods. We assemble 3D DNA superlattices from octahedral DNA frames with incorporated nanoparticles, through connecting frames at their vertices, which result in cubic superlattices with a 48 nm unit cell. The superconductive superlattice is formed by converting a DNA superlattice first into highly-structured 3D silica scaffold, to turn it from a soft and liquid-environment dependent macromolecular construction into a solid structure, following by its coating with superconducting niobium (Nb). Through low-temperature electrical characterization we demonstrate that this process creates 3D arrays of Josephson junctions. This approach may be utilized in development of a variety of applications such as 3D Superconducting Quantum interference Devices (SQUIDs) for measurement of the magnetic field vector, highly sensitive Superconducting Quantum Interference Filters (SQIFs), and parametric amplifiers for quantum information systems., Fabrication of superconducting 3D nanoarchitectures, using standard nanofabrication methods, is challenging. Here, the authors demonstrate the fabrication of a nanostructured 3D superconducting array of Josephson junctions, exploiting self-assembled DNA origami lattices as a template.

Analysis of superconducting ladders consisting of rectangular loops, yields an Ising like expression for the total energy of the ladders as a function of the loops vorticities and the applied magnetic field. This expression shows that fluxoids can be treated as repulsively interacting objects driven towards the ladder center by the applied field. Distinctive repulsive interactions between fluxoids are obtained depending on the ratio l between the loops length and the common width of adjacent loops. A ‘short range’ and a ‘long range’ interactions obtained for l ≳ 1 and l ≪ 1, respectively, give rise to remarkably different fluxoid configurations. The different configurations of fluxoids in different types of ladders are illustrated by simulations.

Magneto-optical imaging of Nb rings, open rings, and strips reveals that the topology affects the dendritic flux avalanches in the samples. In particular, dendrites crossing the entire width of the sample appear only in the rings. Such dendrites appear when the difference between the applied field and the average field inside the central hole reaches a certain threshold level $\mathrm{\ensuremath{\Delta}}{H}_{\mathrm{th}}$. With increasing applied field, this condition is reached quasiperiodically as a crossing dendrite creates momentarily a hot channel through which flux flows into the central hole, balancing the field inside the hole with that outside the ring. The threshold $\mathrm{\ensuremath{\Delta}}{H}_{\mathrm{th}}$ differs in magnitude from the onset field ${H}_{\mathrm{th}}$ of magnetic instability, and exhibits qualitatively different dependence on the rim width.

The hard magnetic material M-type strontium hexaferrite (SrFe12O19) was prepared by a facile surfactant-free sonochemical strategy and subsequently by a calcination process. The surface morphologies, structures, and Curie temperature of the obtained final product was investigated by powder X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), BET (Brunauer–Emmett–Teller) specific surface area (SSA) and thermogravimetric analysis (TGA). XRD confirmed the formation of single-phase strontium hexaferrite with space group P63/mmc (194) and hexagonal structure and the mean primary crystallite size was found to be 44 nm. The morphology of synthesized SrFe12O19 product was in spherical shape with good crystalline nature. The measured Curie temperature (Tc) of the nanoparticles was around 462 °C. The magnetic property of the sample was measured by using a Superconducting Quantum Interference Device (SQUID) magnetometer at temperatures between 4 and 300 K. The measured saturation magnetization (Ms) at 300 K, 33 emu/g, is significantly lower than that found for bulk SrFe12O19, probably reflecting a size effect of the nano-particles.

Utilizing self-assembled DNA structures in the development of nanoelectronic circuits requires transforming the DNA strands into highly conducting wires. Toward this end, we investigate the use of DNA self-assembled nanowires as templates for the deposition of a superconducting material. Nanowires formed by the deposition of superconducting NbN exhibit thermally activated and quantum phase slips as well as exceptionally large negative magnetoresistance. The latter effect can be utilized to suppress a significant part of the low temperature resistance caused by the quantum phase slips.

Magnetoresistance measurements in a granular Nb nanoring reveal current-induced crossover between two distinct quantum coherence effects. At low bias currents, Cooper-pair coherence is manifested by Little-Parks oscillations with flux periodicity of h/2 e. At high bias currents, magnetoresistance oscillations with flux periods of h/ e are observed and interpreted as Aharonov-Bohm oscillations, reflecting the phase coherence of individual quasi-particles. The model explaining these data views the ring as a chain of superconducting grains weakly coupled by tunnel junctions. Low bias currents allow coherent tunneling of Cooper pairs between the grains. Increasing the current above the critical current of all the junctions creates a quasi-particles conduction channel along the ring, allowing for quantum interference of quasi-particles.

We study the magnetoresistance of aluminium 'double-networks' formed by connecting the vertexes of nano-loops with relatively long wires, creating two interlaced subnetworks of small and large loops (SL and LL, respectively). Far below the critical temperature, Aharonov-Bohm like quantum interference effects are observed for both the LL and the SL subnetworks. When approaching T c, both exhibit the usual Little-Parks oscillations, with periodicity of the superconducting flux quantum Φ 0 =h/2e. For one sample, with a relatively large coherence length, ξ, at temperatures very close to T c, the Φ 0 periodicity of the SL disappears, and the waveform of the first period is consistent with that predicted recently for loops with a size a < ξ, indicating a crossover to 2Φ 0 periodicity. published

Magneto-optical imaging of dendritic flux instability is reported for NbN films exposed to magnetic fields ramped at a fast rate (0.1–3.2 kT s−1 ). The results show that as the magnetic ramp rate increases, the temperature and field range of the instability extends significantly. In particular, the lower and upper threshold fields (Hth 1 and H , th 2 respectively) that bound the field range for dendritic instability are affected. The upper field is found to increase linearly with the applied field sweep rate, a behavior which is discussed in terms of a recent theoretical work (Vestgarden et al 2016 Phys. Rev. B 73 174511). The extended instability range should be taken into account in applications in which the superconducting films are exposed to rapid changes in the magnetic field.

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.

Magneto-transport measurements in YBa2Cu3O7 nanobridges, patterned on laser ablated ultra-thin films, reveal phenomena that are usually absent in the bulk of the material. These include broadening of the resistive transition, magnetoresistance oscillation, negative magnetoresistance at low fields, negative magnetoresistance slope at high fields, and V–I curves that exhibit voltage jumps at temperatures well below Tc. These phenomena, attributed to the granular nature of the bridges, should be taken into account in any future attempts to utilize such bridges in technological applications.

A phenomenological theory is developed to explain the recently observed negative magnetoresistance slope in ultra-thin granular YBa 2 Cu 2 O 7− δ films. Viewing this system as a two-dimensional array of extended Josephson junctions, we numerically solve the sine-Gordon equations including a viscosity term that increases linearly with the external field. The solution yields a negative magnetoresistance slope setting in at a field that is determined by the geometry and thus independent of temperature, in agreement with the experimental results.

We report on magnetoresistance measurements in 10 nm thick and submicron-wide granular YBa 2 Cu 3 O 7− δ bridges. The results show a strong dependence of the resistance on the magnetic field at low fields crossing over to a relatively weak field dependence at high fields. The field derivative of the resistance at high fields decreases as the temperature is lowered and eventually changes sign, exhibiting a negative slope in a wide field range in the Tesla regime. This negative slope is sensitive to the bias current, turning to be positive as the bias current increases. This complex magnetoresistance behavior is attributed to both phase slips in a distribution of strongly and weakly linked superconducting grains, and tunneling of quasiparticles between grains. The latter dominates at low temperatures and high fields, giving rise to the negative magnetoresistance slope.

Our recent success in triggering dendritic flux instabilities in $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) films by applying magnetic fields at ultrahigh rates is followed here by a detailed study of the effect as a function of the field ramp rate, ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{a}$, and temperature, $T$. We trace the borderline in the ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{a}\ensuremath{-}T$ plane separating regions of smooth, gradual flux penetration and dendritic flux avalanches. In addition, we describe the changes in the dendritic morphology in the instability region as a result of changes in either ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{a}$ or $T$. Our experimental results, showing a monotonic increase of the avalanche threshold field ramp rate with temperature, are discussed in the framework of existing theories. On the basis of these theories we also explain the high stability of YBCO to dendritic avalanches as compared to, e.g., $\mathrm{Mg}{\mathrm{B}}_{2}$, identifying the flux flow resistivity, rather than any of the thermal parameters, as the main parameter governing the film stability.

Measurements on nanoscale structures constructed from high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. To date, loops made from these compounds have had sizes of the order of hundreds of nanometres(8-11). Here, we report the results of measurements on loops of La(1.84)Sr(0.16)CuO(4), a high-temperature superconductor that loses its resistance to electric currents when cooled below approximately 38 K, with dimensions down to tens of nanometres. We observe oscillations in the resistance of the loops as a function of the magnetic flux through the loops. The oscillations have a period of h/2e, and their amplitude is much larger than the amplitude of the resistance oscillations expected from the Little-Parks effect. Moreover, unlike Little-Parks oscillations, which are caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally excited moving vortices and the oscillating persistent current induced in the loops. However, despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of h/e, as recently predicted for nanoscale loops of superconductors with d-wave symmetry, or with a period of h/4e, as predicted for superconductors that exhibit stripes.

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.

Time-resolved magneto optical measurements in Bi 2 Sr 2 CaCu 2 O 8+ δ show oscillatory relaxation of the local magnetization. The oscillatory behaviour is observed in a limited range of temperatures and fields below the vortex order–disorder phase transition line. We ascribe this phenomenon to coupled effects of thermally activated flux creep and annealing of transient disordered vortex states injected into the sample during field increase.

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.

We highlight interesting thermomagnetic history effects across the transition line between the quasi-ordered and disordered vortex states in single crystal YBa 2 Cu 3 O 7− δ (YBCO), and argue that these features are indicative of the first-order nature of the transition line. We suggest that the destruction of the ordered vortex state in YBCO leading to vortex liquid (at high temperatures and low fields) and amorphous vortex solid (at low temperatures and high fields), takes place along a unified first-order transition line. The non-monotonic behavior of this first-order transition line gives rise to the possibility of more than one Kauzmann point where the entropies of the ordered and disordered vortex states are equal. In the high temperature region, one may order the vortex lattice by warming it, giving rise to an inverse melting effect.

Magnetization measurements in La2−x Sr x CuO4 (LSCO) 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. We argue that the transition in LSCO is driven by both thermally- and disorder-induced fluctuations. The resulting “pinned liquid” disordered state is characterized by large thermal fluctuations and irreversible magnetic behavior. We extend these results and postulate that melting, solid-solid, and solid to pinned liquid transitions, are special cases of 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 transition lines. In order to test our numerical results we present studies on a series of irradiated LSCO crystals, and show that increase of pinning, causes a depression of the transition line as well as a decrease in the curvature as the transition line, in agreement with our calculations.

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.

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 report on measurements of the critical current in a single and bifilar multifilamentary Bi-2223/Ag tapes, at 77 K, under various DC magnetic fields applied perpendicular to the tape plane. At low applied fields the bifilar tape exhibits higher critical current, as expected. However, above ∼50 G the situation is reversed, i.e., the critical current of the single tape is higher. These results are attributed to the different spatial distribution of the self-field in these two configurations. Our data and analysis imply that, in terms of critical current, the bifilar configuration has over advantage over the single tape configuration only in a limited range of external magnetic fields.

The time evolution of the vortex structure in a Bi2Sr2CaCu2O8+δ crystal is magneto-optically recorded after a sudden change of the external magnetic field applied to the sample. The magneto-optical images reveal dynamic coexistence of a quasi-ordered vortex phase and a transient disordered vortex phase. The front of the quasi-ordered state moves with time toward the sample edges (center) when the field is suddenly increased (decreased), until a quasi-ordered state is established throughout the entire sample, as dictated by thermodynamics. Effects of magnetic relaxation on the growth rate of the quasi-ordered state are discussed.

Magnetization measurements in an untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystal reveal a range of temperatures and fields for which the magnetization exhibits unusual history-dependent behavior. This range envelopes the transition line, ${B}_{\mathrm{ss}}(T),$ separating between the vortex quasiordered and disordered phases. The observed history effects indicate that a disordered (quasiordered) vortex state can exist as a metastable state below (above) the ${B}_{\mathrm{ss}}(T)$ line. The fields, defining the phase transition and the borders of the metastability region, are correlated to sharp features in the magnetization loop.

A high temporal resolution magneto-optical system is employed to observe the time evolution of the vortex structure in Bi(2)Sr(2)CaCu(2)O(8+delta) crystals after a sudden application of a magnetic field. The magneto-optical images reveal dynamic coexistence of two vortex phases: a quasiordered phase in the sample interior and a transient disordered phase near the sample edges. The border between these two phases, marked by an abrupt change in the gradient of the local induction, moves with time. This motion enables tracing the decay of the transient state and the concurrent growth of the thermodynamic vortex phases. The growth rate is sensitive to the location in the field-temperature phase diagram.

Local magnetic measurements vs temperature in an untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystal reveal an abrupt increase in the local magnetization at a field-dependent temperature, well below the melting line. At the same field and temperature a pronounced kink is observed in the local magnetization vs field curves. The line ${B}_{k}(T)$ describing the locations of these anomalies in the field-temperature phase diagram divides the vortex solid phase into two regions characterized by weak and strong pinning. A recently developed model describing the vortex solid-solid disorder-induced phase transition explains quantitatively the observed behavior of ${B}_{k}(T).$ From this behavior we infer that the microscopic origin of pinning in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is fluctuations in the charge-carrier mean free path.

An array of miniature Hall sensors has been utilized to measure the spatial distribution and time evolution of the local induction in a thin YBa2Cu3O7−xfilm at the remanent state, before and after the application of a dc transport current. As a result of transport current the induction peak is shifted from the center of the sample, indicating an inhomogeneous current distribution. Transport current markedly changes the spatial distribution of the electric field and the relaxation rate, creating a “dynamic neutral line” where the electric field is maximum, and the relaxation rate is zero.

Local magnetic measurements as a function of temperature (mH(T)) in Nd1.85Ce0.15CuO4−d(NCCO) and untwinned YBa2Cu3O7−δ (YBCO) crystals reveal an abrupt increase in the local magnetization at a field dependent temperature, in a certain field range. Both crystals also exhibit a pronounced kink in their local magnetization vs field (mT(H)) curves. However, while in YBCO the anomalies in mT(H) and mH(T) curves occur along the same line in the field-temperature plane, in NCCO the (B,T) locations of the anomalies do not coincide. In both crystals the anomalies in mT(H) are identified as signifying vortex solid-solid disorder induced transition. However, we show that the anomalies in mH(T), although associated with the vortex solid-solid transition, do not necessarily indicate the location of the transition.

We exploit the magnetic flux rotation effect under tilted fields, and its associated magnetization peak, to study the temperature induced crossover from a 2D to 3D behavior in La 1.85 Sr 0.15 CuO 4 . Measurements of the peak field versus temperature show a pronounced anomaly around the theoretical 2D–3D crossover temperature (∼12 K). Analysis of the data on both sides of this anomaly shows two distinct regimes corresponding to a 2D and 3D behavior.

Magneto-optical measurements in Bi2Sr2CaCu2O8+d demonstrate enhanced effects of transient disordered vortex states (TDVS) in a prism-shaped sample. The existence of such transient states is indicated by the appearance of a sharp change (‘break’) in the slope of the induction profiles. As the external magnetic field is swept up or down, the break, which signifies the front of the TDVS, moves towards the sample center at a constant velocity determined by the rate of change of the external field. We discuss the results in terms of a competition between injection and annealing processes, the latter being ineffective in the prism due to direct injection of disorder into the bulk through surface barriers distributed along the tilted facets of the prism. � 2007 Elsevier B.V. All rights reserved.

The interplay between AC and DC currents in a High- T c Superconducting (HTS) coil, made of multifilamentary silver-sheathed Bi-2223 tape, was investigated. We observed that the application of a small sinusoidal current in the frequency range of 50–500 Hz into the coil, while it is already carrying a DC current in the range of 16–22.5 A, caused an increase in the coil DC voltage. The DC voltage increment due to the AC signal is found to increase linearly with frequency and quadratically with amplitude. The DC voltage increment increases as the coil current grows towards its critical value of 22.2 A. This result may be important in some power applications such as fault current limiters (FCL) and superconducting magnet energy storage (SMES) based on HTS coils.

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 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 magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an anomalous magnetization peak at a temperature-dependent field Bon. The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below Bon to Bean-like profiles above it. The temperature dependence of Bon and the flux distribution above and below Bon imply a disorder-induced transition at Bon from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above Bon show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure. [S0031-9007(97)04113-6]

We describe the details of a new method for studying thermally activated flux creep in superconductors. this method employs an array of microscopic Hall sensors to probe the time evolution of the field profile in the sample. We analyze these data on the basis of a continuity equation which takes into account contributions to the relaxation process from both the in-plane and out-of-plane components of the induction field. This analysis enables direct determination of flux creep parameters such as the flux-line current density, flux-line velocity and the activation energy for flux creep. We demonstrate this method by presenting experimental data for YBa2Cu3O7-x crystals in the remanent state and in the presence of a field.

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.}

We review experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation. This effect has its origin in motion of flux lines out of their pinning sites due to thermal activation or quantum tunneling. The combination of relatively weak flux pinning and high temperatures leads to rich properties that are unconventional in the context of low temperature superconductivity and that have been the subject to intense studies. The results are assessed from a purely experimental perspective and discussed in the context of present phenomenological theories. [S0034-6861(96)00403-5]

We review experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation. This effect has its origin in motion of flux lines out of their pinning sites due to thermal activation or quantum tunneling. The combination of relatively weak flux pinning and high temperatures leads to rich properties that are unconventional in the context of low temperature superconductivity and that have been the subject to intense studies. The results are assessed from a purely experimental perspective and discussed in the context of present phenomenological theories. [S0034-6861(96)00403-5]

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.

The onset temperature, Tirr, for magnetic irreversibility in high-temperature superconductors has been investigated as a function of frequency (0.2 Hz⩽f⩽65 000 Hz) and DC magnetic field (0