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We demonstrate an improved YBa2Cu3O7−δ -based microwave kinetic inductance detector with a quality factor Q i ∼ 2.5 × 10 4 and noise equivalent power, ∼ 10 − 12 W Hz − 1 at 10 K. Zero field cooled (ZFC) and field cooled (FC) measurements of the magnetic field dependence of the resonance characteristics, show substantially different behavior, indicating that both the screening currents and vortices play a role. The ZFC measurements exhibit a sharp decrease of the resonance frequency, f r , and Q i at low fields, up to the full penetration field, revealing the dominant role of the screening currents. In contrast, the FC measurements exhibit a moderate decrease of f r and Q i with field, revealing the role of vortices and reflecting the field dependence of the penetration depth in a d-wave superconductor.

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.

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.

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.

A study of the DC I–V curves of bifilar BSCCO/Ag tapes at 77 K is presented. Tapes under study were carrying DC transport current and exposed to external AC (0–20 mT, 50–400 Hz) or DC (0–20 mT) magnetic fields perpendicular to the plane of the tape. The measurements were carried out with bifilar tape to avoid the influence of the self-field. The measured I–V curves are well described by power law in the range of electric field 0.1–10 μV/cm. The AC field affects the I–V curves in a much stronger way than the DC field. The application of the AC field results in a linear decrease of the critical current (criterion 1 μV/cm) independent of the frequency of the magnetic field. The exponent n also exhibits a strong dependence on the AC magnetic field decreasing with increasing AC field amplitude and increasing with increasing frequency. The results are interpreted by assuming a confinement of the DC transport current in the central AC field free region of the tape. A low current density is assumed to flow in the region to which the AC magnetic field has penetrated.

A fine strontium hexaferrite powder has been successfully synthesized by a sonochemical method employing Fe(CO)5 and SrCO3. SrCO3 was first synthesized using strontium nitrate and urea, and it was found that applying ultrasound radiation during this process results in the uniform formation of SrCO3 hexagonal rods. These rods were then dispersed with in situ generated amorphous Fe2O3, using Fe(CO)5 as the source. The resultant precursor was then calcined at 900°C, which is lower than the conventional solid-state reaction of applying 1300°C to get the strontium hexaferrite fine powder. This sonochemically derived ferrite exhibited an intrinsic coercivity of ∼4600 Oe and a saturation magnetization of ∼60 emu/g at 20 K and ∼32 emu/g at 300 K.

We report on measurements of I–V curves in microbridges of thin Y–Ba–Cu–O films of different thickness, in the presence of external magnetic fields up to 6 T. A discontinuity is observed at a critical voltage, V*, in the flux flow regime, reflecting an electronic instability, as predicted by Larkin and Ovchinnikov (LO), and in agreement with results reported by Doettinger et al. [Phys. Rev. Let. 73 (1994) 1691]. The critical voltage, V*, and the flux flow resistance, R0, in the limit V→0, are calculated by fitting the data to the LO model. We find that the vortex critical velocity, v*, at the instability, derived from V*, decreases with magnetic field and film thickness. These results, not predicted by the LO theory, reflect the dependence of the (spatially averaged) quasiparticle energy relaxation rate on magnetic field and film thickness.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.}

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

We 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.

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.

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.

A superconducting ${\text{La}}_{1.84}{\text{Sr}}_{0.16}{\text{CuO}}_{4}$ film patterned into a network of $100\ifmmode\times\else\texttimes\fi{}100\text{ }{\text{nm}}^{2}$ noninteracting square loops exhibits large magnetoresistance oscillations superimposed on a background which increases monotonically with the applied magnetic field. Neither the oscillations amplitude nor its temperature dependence can be explained by the Little-Parks effect. Conversely, a good quantitative agreement is obtained with a recently proposed model ascribing the oscillations to the interaction between thermally excited moving vortices and the oscillating persistent currents induced in the loops. Extension of this model, allowing for direct interaction of the vortices and antivortices magnetic moment with the applied field, accounts quantitatively for the monotonic background as well. Analysis of the background indicates that in the patterned film both vortices and antivortices are present at comparable densities. This finding is consistent with the occurrence of Berezinskii-Kosterlitz-Thouless transition in ${\text{La}}_{1.84}{\text{Sr}}_{0.16}{\text{CuO}}_{4}$ films.

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 investigated ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ and ${\mathrm{PrBaO}}_{3}$ by several complementary experimental techniques. ${\mathrm{PrBaO}}_{3}$ crystallize in an orthorhombic structure. dc and ac susceptibility measurements exhibit irreversibility phenomena below ${\mathit{T}}_{\mathit{N}}$=11.7 K and reveal the magnetic structure of ${\mathrm{PrBaO}}_{3}$, which consists of an antiferromagnetic ordering together with a small canting of the spins which produces the ferromagnetic component. Both magnetic and specific-heat studies show that ${\mathit{T}}_{\mathit{N}}$ is not affected by the applied magnetic field up to 9 T. ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ crystallizes in an orthorhombic structure where a=10.56 \AA{}, b=12.43 \AA{}, and c=3.617 \AA{} belongs to Pnam space group. dc and ac susceptibility and specific-heat measurements show that ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ is paramagnetic down to 2 K, with a vanishing electronic specific-heat coefficient \ensuremath{\gamma}. X-ray photoemission spectroscopy shows that Pr is mainly trivalent in ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ and close to being tetravalent in ${\mathrm{PrBaO}}_{3}$. The pronounced ferromagnetic-like behavior found below 11.7 K in the ${\mathrm{Pr}}_{\mathit{x}}$${\mathit{R}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{z}}$ system is due to the impurity of ${\mathrm{PrBaO}}_{3}$.

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.