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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.

Magnetization of a number of oxide glasses containing Pr3+, Dy3+ and Nd3+ ions has been measured as a function of the magnetic field (0–50 kOe) and temperature (5–300 K). Magneto-optical Faraday rotation (FR) in the same glasses has been measured as a function of the light wavelength (240–1000 nm) and temperature (80–350 K). Dispersion of FR has permitted us to find the positions of the effective transitions responsible for FR and their magneto-optical activities. Temperature dependencies of both FR and the magnetization at high temperatures obey the Curie–Weiss law with negative Weiss constants. High temperature magnetic moments of the rare-earth ions in the glasses are rather close to those of the free ions. At low temperatures, all studied glasses reveal downward curvature of the reciprocal magnetization variation with decreasing temperature. The origin of this behavior is discussed. Formation of antiferromagnetically ordered clusters in Dy glasses is supposed.

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.

An aqueous solution of AgNO 3 in the presence of ammonia and Fe(CO) 5 is sonicated under a H 2 /Ar mixture, yielding a nanostructured homogeneous phase of Ag/Fe 2 O 3 . This composite material is further reduced at 300°C under hydrogen to produce the nanophased Fe/Ag solid mixture. The as-prepared material, as well as the reduced mixture, is analyzed by various conventional methods. Magnetization loops, ESR, Mossbauer, and magnetoresistance measurements are also conducted to determine the magnetic properties of the products.

Magnetization of Nd2Fe17−xGax, where 0⩽x⩽3 , has been measured at temperatures between 5 and 300 K and in fields between 0 and 50 kOe. These samples have the rhombohedral Th2Zn17 structure. Magnetization studies at 5 K indicate that the saturation magnetization decreases with increasing Ga concentration while at 300 K it increases for x >0 reaching a maximum at x =1, and decreases for x >1. Samples annealed for different times are found to have different magnetization for the same value of x . The low-field magnetization measurements show irreversible behavior which is a reflection of hysteretic effects in this system. The peak in the magnetization shifts to higher temperatures upon increasing the Ga amount due to the increase in the Curie temperature.

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.

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.

Global transport measurements and local Hall-probe array measurements of a section of silver clad BSCCO tape, are reported. Transport measurements are in good agreement with the `brick wall' model. The induction profiles obtained from magnetic measurements agree with the assumption of a uniform bulk current at low (40–60 K) temperatures. At higher temperatures and relatively high fields (700 G and up) local measurements seem to reveal the existence of surface currents. These currents are the result of small current loops, flowing within the grains, which result in a net current only at the edges of the sample. Thus, the local magnetic measurements do not agree with global transport ones, and we contend that the `brick wall' model aptly describes the flow of transport currents, but fails to describe the flow of currents induced by an external magnetic field.

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.

The relationship between the solubility parameter, x, in the solid solutions Nd 1+x Ba 2−x Cu 3 O 6.5+x 2+δ (Nd123SS) and XRD patterns, powder surface area (SA), particle size, morphology and melting points was investigated. An efficient way to determine the value of x and residual BaCuO2 content during Nd123SS powder synthesis is presented. The method is based on calculation of the orthorhombic splitting (OS) factor from the unit cell parameters obtained from XRD data. The final phase in the Nd123SS system is formed through a diffusion controlled reaction between BaCuO2 and Nd123SS and a kinetic model is developed to describe the formation of the Nd123 (x=0) superconducting powder. Finally, high values of Tc (measured by DC magnetization) were found in the powders. The highest Tc of 98.7 K was measured for the x=0 case. This value is the highest ever reported for the NdBaCuO system. The Tc was found to be insensitive to the value of x, ranging between 98.7 and 94 K for x between 0 and 0.25, respectively. This data is contrast to published results on samples synthesized at higher temperatures, where Tc fell to 40 K at x = 0.25.

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.}

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.

We study the angular dependence of the magnetic properties of Y-ba-Cu-O thin films irradiated with 0.86 GeV Pb and 5 GeV Xe ions. For the Pb irradiated samples the critical current has a maximum when the direction of the external magnetic field coincides with the direction of the columnar defects induced by the irradiation. For the Xe irradiated samples such anisotropy in the critical current was not found. The pinning force density obeys a scaling law of the form b p (1 − b ) q , where b = H ext / H irr ( T ), and H ext and H irr are the external and the irreversibility fields, respectively. The exponents p and q depend on the type and direction of the irradiation.

We report on a study of the magnetic properties of the intermetallic superconductor YNi 2 B 2 C in a wide range of magnetic fields (up to 8 T) and temperatures (4.4−16 K). The results yield the characteristic length scales and fields for this compound: λ (0) ≈ (3.5 ± 0.5) × 10 −5 cm, ξ(0) ≈ (10 ±2) × 10 −7 cm, H c1 (0) ≈ 83 G and H c2 (0)≈ 47600 G. We compare our results with those obtained by other groups and add new information on thermodynamic properties, in particular on the lower critical field, and on irreversible magnetic features such as the critical current, pinning force and magnetic relaxation. We conclude that YNi 2 B 2 C is a type-II superconductor with relatively weak pinning centers.

Magnetization curves were measured for Bi2Sr2CaCu2O8 crystals which were irradiated with 5.8 GeV Pb ions along the c direction or at 45° with respect to it. Each ion produces continuous columnar that crosses the sample and yields a unidirectional pinning center. The density of the defects in the various samples ranges from 1010 to 2.5 X 1011 ions/cm2. In all the irradiated samples the width of the magnetization curves is found to be the largest for fields along the defects indicating that when the field is at other directions the vortices are tilted away from the defects. This manifests the important role of the Josephson coupling between the CuO2 planes. We also report on accelerated flux escape around zero magnetization and around zero field, particularly in samples with a high dose of irradiation.

Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals were irradiated with 5.8-GeV Pb ions to produce columnar defects either along the c direction or at 45 o with respect to it. Magnetization curves of the irradiated BSCCO crystals were measured with the applied field at different directions relative to the defects. The width of the magnetization curves, which reflects the pinning strength, is found to be the largest for fields along the defects. This unidirectional enhancement clearly indicates the line nature of the vortices in BSCCO

Pinning by columnar defects in irradiated YBa 2 Cu 3 O 7 (YBCO) and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals is most efficient when the field is parallel to the defect. However, in the low field limit the remanent magnetization is independent of the direction of the field, indicating that the fluxons flop towards the columnar defect. The flux-flop is almost temperature independent in YBCO and strongly temperature dependent in BSCCO due to the different nature of the vortices

Defects in the form of cylindrical amorphous tracks were introduced into ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystals by irradiation with 5.3-GeV Pb ions. The columnar defects formed provide maximum possible pinning of flux lines parallel to the tracks and induce giant enhancement of magnetic irreversibility. However, even for this strong pinning, magnetic decay was measured at high temperatures. Relaxation is nonlogarithmic and exhibits an increase of the effective barrier for flux creep with decreasing persistent current. This behavior is interpreted in the framework of the nucleation creep model.

We present magnetic measurements in a single crystal of the newly discovered superconducting iron-pnictide $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$. The magnetization loops exhibit a second magnetization peak (SMP) similar to that observed in most high-temperature superconductors (HTSs). Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, $S=d\text{ }\text{ln}\text{ }M/d\text{ }\text{ln}\text{ }t$, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$ the onset and peak fields, as well as the minimum point in $S$, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$ is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of $S$. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.

We have investigated the irreversibility line (IRL) of a ceramic YBa 2 Cu 3 O 7 sample by probing the onset of irreversibility in four DC and AC techniques. All techniques yield a line which obeys a power law H=A( 1−T T c ) n but A and n depend on the technique. Also, the location of the peak in the AC signal obeys this power-law but it does not scale with the onset of irreversibility. We have also investigated the effect of two doses of 2.5 MeV electron irradiation on the IRL and find that after irradiation both the transition temperature T c and the irreversibility temperature T irr decrease. The decrease in T irr scales with the decrease in T c . We propose that this scaling indicates that electron-irradiation does not induce pinning centers stronger than those present in the unirradiated sample.

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.

DC magnetic measurements on the high- T c superconductors YBa 2 Cu 4 O 8 (124, T c =80 K) and Y 0.9 Ca 0.1 Ba 2 Cu 4 O 8 ( T c =87K) are reported. The irreversible lines of 124 and ceramic YBa 2 Cu 3 O 7 (123) are quite similar. The results show that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase. Conversions of 124 into 123+CuO does not change the grain size of the material but enhances its critical current density.

We present a study of the field dependence of the remanent nonlinear magnetic response induced in sintered Y-Ba-Cu-O samples by momentary application of a dc magnetic field H. The results show that, contrary to theoretical predictions, the remanent nonlinear response depends strongly on H. We attribute this behavior to weak links subjected to internal fields induced by flux trapped within the grains. To support this view, we present an extensive comparison between ac and dc data in the same samples. This comparison clarifies the role of grains and of weak links in various field regimes, and demonstrates the relationship between the remanent nonlinear response and the trapped flux.

Vector magnetization measurements in ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$ under fields slightly tilted from the crystallographic $ab$ plane reveal flux rotation with distinctive behavior around ${T}^{*}\ensuremath{\approx}15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of the onset-field ${H}^{*}$ for flux rotation and the dependence of ${H}^{*}$ upon the tilting angle are markedly different below and above ${T}^{*}$, exhibiting pronounced anomalies in the crossover regime. Possible mechanisms for these anomalies, such as dimensional crossover in superconductivity or the anomalous behavior of the in-plane penetration depth due to coexistence of two superconducting gaps, are discussed.

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

Magneto-optical measurements of transient vortex states in Bi2Sr2CaCu2O8+delta show enhanced effects of metastability in prism-shaped as compared to platelet crystals including a significant shift of the second magnetization peak and qualitatively different dynamics. In contrast to platelets, where dislocations are generated only at the sample edges, we propose that in prism samples the dislocations are generated dynamically in the entire sample due to distributed surface barriers. As a result, a dynamic phase transition from a Bragg glass to a metastable disordered phase may occur well below the thermodynamic transition field.

A theory of structural phase transition of the vortex lattice in tetragonal superconductors is constructed based on the self consistent harmonic approximation for lattice anharmonicities, within the fourfold-symmetric generalization of the London model. Thermal fluctuations on the mesoscopic scale are strong enough to affect the location of the square to rhomb transition line in the $T\text{\ensuremath{-}}H$ plane. We find that the slope of the transition line is generally negative: thermal fluctuations favor the more symmetric square lattice. The calculated transition line is concave, and fits the experimental line deduced in LaSCO crystals from the onset of the second magnetization peak. Near the transition line the ``squash'' modulus ${C}_{\mathit{sq}}=2({C}_{11}+{C}_{12})\ensuremath{-}{C}_{66}$ is softened leading to enhancement of the critical current, born out in the experiment as a second magnetization peak.

Time resolved magneto-optical technique is employed to trace the time evolution of the second magnetization peak (SMP) in Bi2Sr2CaCu2O8+δ at different temperatures. The results show that the external field and induction marking the onset of the SMP exhibit entirely different temperature and time dependence. The differences are attributed to variations of the local magnetization with time and temperature. The implications of these findings on measurements of the thermodynamic order-disorder vortex phase transition line are discussed.

Magneto-optical snapshots of the induction distribution across a Bi 2 Sr 2 CaCu 2 O 8 + δ crystal subjected to ramped magnetic fields reveal strong effects of transient disordered vortex states near the disorder-driven vortex phase transition. These transient states, predominantly generated by edge contamination during field ascending and by supercooling of the disordered vortex lattice during field descending, cause smearing of the transition and shift of its onset towards lower inductions. With decreasing temperature, these effects are more pronounced as the lifetime of the transient vortex states increases. Similar effects are observed with increasing field sweep rates, as transient states with shorter lifetime play a role. These observations clarify previously reported puzzles, e.g., the absence of the second magnetization peak at low temperatures, its appearance at longer times, and the shift of its onset to lower inductions for higher field sweeping rates.

We report on magnetization measurements in $({\mathrm{La}}_{0.937}{\mathrm{Sr}}_{0.063}{)}_{2}{\mathrm{CuO}}_{4}$ crystals with the field either parallel or perpendicular to the c axis. A second magnetization peak (``fishtail''), interpreted as indicating a vortex order-disorder transition, is observed in both directions. Differences in the details of the fishtail anomaly in the two directions (width, temperature and time dependence, and history effects) are attributed to the anisotropy and the twin boundaries in the ab plane. The transition fields in both directions, although different in magnitude, exhibit similar qualitative behavior, namely, a strong decrease with temperature in the entire measured range. This unique behavior is explained postulating that both thermally and disordered induced fluctuations contribute to the destruction of the vortex lattice. The resulting ``pinned liquid'' disordered state is characterized by large thermal fluctuations and irreversible magnetic behavior.

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.

Magnetization measurements as a function of temperature are reported for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ crystal ${(T}_{c}=45.2 \mathrm{K})$ for fields between 0.2 and 3.5 T. All isochamps for $Hg1$ T intersect at ${T}_{2D}^{\ensuremath{\star}}\ensuremath{\simeq}42.8$ K, implying a fluctuation contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted ``two-dimensional (2D) scaling'' in the fluctuation regime. Surprisingly, the low-field curves also intersect, at ${T}_{3D}^{\ensuremath{\star}}\ensuremath{\simeq}43.4$ K, and they obey a 3D scaling. We provide a theoretical picture of the magnetization in the fluctuation regime based on the Lawrence-Doniach model. Within this model we calculate the field and temperature dependence of the magnetization. The two intersection points and the $2\stackrel{\ensuremath{\rightarrow}}{D}3\mathrm{D}$ crossover are consistent with the experimental observation.

The authors have investigated the effects of 2.5 MeV electron irradiation on the irreversibility line of a sintered YBa2Cu3O7 by applying DC and AC measuring techniques. They find that after electron irradiation, both the superconducting transition temperature Tc and the irreversibility temperature Tirr, determined by either technique, decrease; the decrease in Tirr scales with the decrease in Tc.

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

Local magnetization curves at different times, extracted from high-temporal resolution magneto-optical measurements in Bi 2 Sr 2 CaCu 2 O 8+ δ , demonstrate the absence of the second magnetization peak (SMP) at short times, its appearance at longer times, and the movement of its onset toward higher induction fields approaching the limit of the thermodynamic order–disorder transition field. We utilize theoretical analysis, based on the Landau–Khalatnikov dynamic equation adopted for the vortex matter order parameter, to simulate the measured time evolution of the SMP. We show that the observed time evolution is a manifestation of a dynamic coexistence of transient-disordered and ordered vortex phases.

Y-Ba-Cu-O crystals were irradiated with 5.3-GeV Pb ions. The induced columnar defects are either perpendicular or at 45 o to the Cu-O planes. Magnetization curves confirm an anisotropic, unidirectional enhancement of the critical current J c . However, a sharp crossover to isotropic enhancement is observed in the low-field limit. We interpret these results in terms of flux flop from a direction determined by the field to the direction of the defect. From this feature we determine the pinning energy of a flux line in a columnar defect

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

By comparing data of the angular dependence of the remanent magnetization for a ceramic sample and for a single crystal of YBa{sub 2}Cu{sub 3}O{sub 7} with similar demagnetization factors, we conclude that intrinsic anisotropy contributes significantly to anisotropic flux trapping in the crystal.

The laser ablation technique is used to grid-pattern thin Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O crystals to form isolated squares of length scale {ital R}. The effect of {ital R} on irreversible magnetization and deduced critical currents is studied. The remanent magnetization ({bold H}{parallel}{bold c}) at low temperature is linear within experimental error below a few hundred micrometers, and sublinear at larger sizes. The results are inconsistent with granularity on the scale of the twin boundaries but suggest the presence of some larger-scale defects that impede current flow. In a conventional critical-state model, a field (or flux-density) dependence of the critical current density {ital J}{sub {ital c}}({ital h}){proportional to}{ital h}{sup {minus}{ital n}} gives rise to a sublinear size dependence {ital D}{sup 1/({ital n}+1)} of the zero-field remanent magnetization. However, in our geometry, strong demagnetizing fields are expected to curve the flux lines into the plane and reduce the size of this effect. Further evidence for the flux line curvature comes from the fact that the field for the minimum in the virgin curve is almost independent of sample size, in contrast to models that ignore the curvature.

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.

Local magnetic measurements in Nd 1.85 Ce 0.15 CuO 4−δ crystals show a ‘fishtail’ anomaly in the magnetization curves together with anomalous relaxation behavior similar to that measured in YBa 2 Cu 3 O 7−x crystals, suggesting a universal flux dynamics in the field range of the fishtail peak.