Hysteresis of critical currents of superconducting bridges in low perpendicular magnetic fields

Hysteresis of critcal currentsI _c of superconducting bridges with In, Nb, and NbN has been studied in low perpendicular magnetic fields. Influences of bridge geometry, small field sweep, trapped flux, and bombardment of argon ions on the hysteresis were made clear. The experimental results suggest that the edge pinning and trapped flux in the bank of bridges are associated with the hysteresis. The peak value ofI _c of NbN bridges, as well as granular Al and In bridges reported before, in decreasing fields agrees with the calculated pair-breaking current. The origin of the hysteresis is discussed.     

Vortex Dynamics in a Superconducting Film with a Kagome and?a?Honeycomb Pinning Landscape

The vortex dynamics in a superconducting thin Al film with a periodic Honeycomb or Kagome array of antidots has been investigated by electrical transport measurements. The large values of the superconducting coherence length and penetration depth of the Al films guarantee a maximum of one flux quantum trapped per pinning site. This allows us to directly compare the experimental results with previous theoretical investigations based on molecular dynamics simulations. For the Kagome lattice, two submatching features not anticipated theoretically at H/H ₁=1/3 and 2/3, where H ₁ is the field at which the number of vortices coincides with the number of pinning sites, are observed. Possible corresponding stable vortex patterns are suggested. For the Honeycomb pinning landscape, the commensurability effects are in agreement with the theoretical expectations. A preliminary analysis of the vortex mobility in this lattice shows the presence of a weak vortex guidance.     

Magnetic field orientation dependence of critical current in industrial Nb3Sn strands

In usual superconducting devices such as magnets for NMR, the magnetic field is perpendicular to the superconducting strand axis. But in some special devices, such as magnets for the toroidal field system of fusion machines, the strands can experience any field orientation. For NbTi strands, the pinning force is dependent on the field orientation because of the drawing process (Takacs, S., Polak, M. and Krempasky, L., Critical currents of NbTi tapes with differently oriented anisotropic defects, Cryogenics, 1983, 23, 153–159). In the case of Nb₃Sn strands, the draw and react process suggests that the pinning force is isotropic. In fact, preliminary experiments have shown the contrary, which is why the magnetic field orientation dependence of the critical current for two types of industrial Nb₃Sn strands has been measured. These measurements have been performed for seven field orientations at field strengths up to 20 T. A clear anisotropic effect has been observed, which cannot be explained by Kramer's pinning law. The results are in very good agreement with an empirical law proposed in a recent study by Takayasu et al. (Takayasu, M., Montgomery, D.B. and Minervini, J.V., Effect of magnetic field direction on the critical current of twisted multifilamentary superconducting wires, Inst. of Phys. Conf. Ser., 1997, 158, 917–920). The parameters to be used in this law could be specific to the manufacturing process.     

Pulsed magnetic field losses and critical current densities of V3 Ga and Nb3Sn multifilament superconductors

Pulsed magnetic field loss measurements have been performed on special V₃Ga filament superconductors, which are known to have very high critical current densities, and on a NB₃Sn filament superconductor.The dependence of the hysteresis losses on the diffusion heat treatment parameters for the V₃Ga layer formation is reported. To analyse the results, the hysteresis losses of hollow superconducting filaments in parallel and perpendicular magnetic fields were calculated.From E.J. Kramer's theory of flux pinning we postulate that, for a given material, upper and lower intrinsic limits of the hysteresis losses are due to the flux line shearing effect. A part of the results was presented at the ICEC 7.     

Effect of Micro-Phase Separation on Magnetic Response in Pr-Doping YBa2Cu3O7−δ Single Crystals

Pr-doping Y_0.928Pr_0.072Ba₂Cu₃O_7−δ superconducting single crystal was studied systemically by the magnetic response measurement below T _c . DC magnetization shows a possible existence of micro-phase separation caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The magnetic field dependence of critical current density J _c and pinning force density F _p had been obtained from the magnetization hysteresis loop. Flux pinning mechanisms in different magnetic fields had been discussed using the scaling function as well as the ratio of the pinning force field at H _on and H _peak. We find that the peak effect is influenced by the micro-phase separation which is caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The interface between the different T _c superconductivity areas can be the effective pinning center which is useful to the appearance of peak effect. The value of the ratio of the maximum pinning force field to the irreversibility field is discussed. The conclusion is consistent with the model constructed by Wen et al.; we finally draw out the thermomagnetic phase diagram for our samples.      

Intrinsic Pinning in the Layered Superconductor YBa2Cu3O7 Studied by the Conversion of Ultrasonic to Electromagnetic Waves

Intrinsic pinning in a single-crystal YBa₂Cu₃O₇ superconductor has been investigated through the conversion of ultrasonic to electromagnetic waves. The interaction with a longitudinal ultrasonic wave was dramatically enhanced as a result of the intrinsic pinning of the vortex so as to form a sharp peak when a dc magnetic field was applied nearly parallel to the superconducting plane. Not only the height but also the angle width of the peak decreased with the temperature increase because of the weakening two-dimensionality. The crossover between the intrinsic pinning and the point-defect pinning is discussed by taking into account the pinning strength.     

Studies of magnetic flux penetration in a chemically synthesized Bi(Pb)SrCaCuO superconductor

Magnetic measurements were conducted on a sample of chemically synthesized Bi_1.5Pb_0.5Sr_1.5Ca_1.75Cu₂O _x using an a.c. susceptometer and a vibrating sample magnetometer. The sample showed a large fraction of the phase with a superconducting onset temperature of 110K. This high-temperature superconducting phase appears to have extremely narrow hysteresis loops above liquid-nitrogen temperatures, showing a small number of eflective flux pinning sites. Below 40 K, a dimpling can be observed in the hysteresis loop under certain conditions.     

Temperature and Current Dependent Matching Fields in?Superconducting NbN Film

Patterned superconducting thin films having a periodic array of submicrometric pinning centers have been of great interest due to their excellence for the studies of the vortex pinning mechanisms in the type-II superconductors. Square hole array has been fabricated over a micro-bridge 60 mm??60 mm of NbN thin film by electron beam lithography. Previous works have been carried out in Nb, Pb and Al thin films where the vortex pinning effect is assumed to be small. In this work, we study the matching pinning effect by the artificial hole array in superconducting NbN thin films. We observed the interplay between the vortex quantization and the artificial hole array. Magneto-resistance minima at integer matching fields up to five times of H ₁ (the first matching field corresponding to one vortex inside each hole) and fractional matching fields at 1/2H ₁, 3/2H ₁ and 5/2H ₁ have been observed.     

Anomalous flux pinning of UPd2Al3

The ac magnetic susceptibility of UPd₂Al₃ in the superconducting state has been measured to test of the existence of the Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) state. A dip in the susceptibility has been observed in the vicinity of the upper critical field and the temperature range from zero to T_c. This dip is very sensitive to the amplitude of the ac magnetic field. The anomaly is explained by the increase of the flux pinning effect. Recent theories suggest that the pinning force becomes stronger in the FFLO state, and the existence of the nonuniform state due to the order parameter mixing in a d-wave superconductor is possible near T_c.     

Effect of laser irradiation on the superconducting properties of high-T c SmBa2Cu3O x

Studies of the effect of high power laser (Q-switched Ruby laser, 694 nm, 30 ns) irradiation on the critical current density (J _c ) and magnetic hysteresis at 77K and temperature variation of microwave induced d.c. voltage on SmBa₂Cu₃O _x ceramic samples have been performed. Irradiation did not substantially changeT _c but caused a strong increase inJ _c and magnetic hysteresis at 77K. The microwave-induced d.c. voltage at 77K showed appreciable decrease after irradiation. SEM studies showed grain growth due to sintering which improves the interconnectivity among the superconducting grains. These are attributed to physical densification and consequent reduction in the number of weak links. The increase of magnetic hysteresis after laser irradiation is presumably connected with the creation of defects which act as pinning centres. Thermal modelling suggests that on irradiation the surface melts up to a depth of 1μ and laser-induced evaporation occurs at energy density of 2·5 J/cm².     

Vortex pinning and magnetic peak effect in Eu(Eu,Ba)2.125Cu3Ox

Eu–Ba–Cu–O composition was synthesized by solid state reaction technique. To determine optimum growth temperature, heat treatment was examined on the material at 880–1,100 °C. Microstructural evolution, phase formation and elemental distribution depending on heat treatments were examined by using X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectroscope analysis. Optimum fabrication conditions were determined as 1,020 °C for 24 h under oxygen atmosphere and detailed characterization of corresponding compound was performed. The magnetization hysteresis loops are expounded to be the product of superconducting Eu-123 grains and magnetic Eu²⁺ ions. The peak effect on the magnetization curves was described by the extended critical state model. Scaling of the pinning force was found such that the peak position is proportional to the irreversibility field H _irr and the maximum pinning force is proportional to H _irr ² .     

Unusual Hysteresis Loop of (Tb0.2Pr0.8)(Fe0.4Co0.6)1.9−x C x in Pulsed High Magnetic Field

The magnetization properties of (Tb_0.2Pr_0.8)(Fe_0.4Co_0.6)_1.9?x C _x ( $x=0$ –0.5) were investigated in a pulsed high magnetic field. Obvious unsymmetry magnetic hysteresis loops were observed, in which the magnetization measured in the second(negative) magnetic field route was larger than that measured in the first(positive) one. Meanwhile, the unsymmetry of the hysteresis loops was C doping levels, magnetic fields and temperatures dependent. The experimental results suggest that the unusual hysteresis phenomenon result mainly from the collapse of the pinning center and the formation of ferromagnetic(FM) pseudo-domain.     

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi_1.7Pb _0.3-xGd_xSr₂Ca₃Cu₄O_12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd³⁺ doping for Pb²⁺ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_c, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_c decreases with increasing temperature and/or Gd concentration, as expected.     

DC magnetization studies in Nb/Fe superconducting multilayers

The behavior of superconducting transition temperature T_C in superconducting/ferromagnetic (S/F) multilayers as a function of different layer thicknesses and for varying magnetic moment μ_B of the F layer atoms is studied. Multilayer structures consist of five bilayers of constant superconducting Nb layer thickness of 400 Å and Fe of 6 and 24 Å each. The analysis of the magnetization data revealed that for t_Fe=6 Å, the Fe layer is non-magnetic. The interpretation of the observed T_C behavior is attributed to the change of the interaction of the cooper pairs with this layer at the onset of ferromagnetism for t_Fe=6 Å. The hysteresis curve recorded under isothermal conditions at 4.5 K for [Nb (400 Å)/Fe (6 Å)]₅ multilayers shows the usual M–H hysteresis behavior which is typical of a hard type-II superconductor exhibiting an irreversibility field H_irr of 3.5 kOe with substantial pinning at lower field. In addition, [Nb(400 Å)/Fe(6 Å)]₅ multilayer displays anomalous behavior in the form of paramagnetic peak in the superconducting state just below the transition temperature T_C=6.25 K.     

Effect of microstructure on superconducting properties of Mo/34 at.% Re alloy

The effect of deformation and heat-treatments on magnetisation of superconducting Mo/34 at.% Re alloy was investigated. The microstructure was studied using optical and electron microscopy. The values of the parameter of the GLAG theory were determined by using three different methods, and they are found to be in accordance with some of the predictions of the Maki theory. The results show that in deformed specimens the magnetic hysteresis is mainly due to the interaction of Abrikosov supercurrent vortices with dislocation tangles; both twin boundaries and grain boundaries are relatively ineffective in causing flux hindrance. The hysteresis observed after annealing has been explained in terms of the occurrence of dislocation rearrangement and the precipitation of the sigma phase. Flux-pinning by dislocation tangles has been treated as a special case of pinning by normal particles and a magnitude of the resulting pinning force is obtained. The experimental results are compared with the predictions of a theoretical model of flux-pinning.     

Influence of Interfacial LSMO Nanoparticles/Layer on the Vortex Pinning Properties of YBCO Thin Film

YBa₂Cu₃O_7?δ (YBCO) thin films have been deposited on bare and La_0.67Sr_0.33MnO₃ (LSMO) modified single crystal SrTiO₃ (STO) substrates. The effect of randomly distributed ferromagnetic LSMO nanoparticles and a complete LSMO layer, present at STO/YBCO interface, on the superconducting properties of YBCO thin films has been investigated by temperature dependent magnetization studies. The YBCO thin film on LSMO nanoparticles decorated STO substrate shows significant improvement in the critical current density and pinning force density as compared to the YBCO thin film deposited on bare STO substrate and this improvement is more significant at higher applied magnetic field. However, the LSMO/YBCO bilayer showed the improved flux pinning properties only up to a magnetic field of 1.5 T above which it deteriorates. In the case of LSMO/YBCO bilayer, the underlying LSMO layer gives rise to magnetic inhomogeneities due to domain structure, which leads to improved flux pinning properties limited to lower field. However, in the case of LSMO nanoparticles decorated substrate, the presence of LSMO nanoparticles at YBCO/STO interface seems to introduce magnetic inhomogeneities as well as structural defects, which might be acting as correlated pinning sites leading to improved flux pinning properties of the YBCO thin film over a wide range of applied magnetic field.     

On the Scaling Law of the Pinning Force in MgB2 Superconducting Composites with Magnetic Nanoinclusions

Dense superconducting composites with magnetic nanoparticles were prepared by spark plasma sintering. In addition to structure and magnetic properties, we focused on the pinning properties as described by the field and temperature dependence of the scaled pinning force analyzed. We found that the pinning force does not obey any scaling law and attributed this effect to the anisotropy of the MgB₂ and the almost random distribution of the orientation of the superconducting grains within the polycrystalline sample.     

Magnetic Nanorod—Superconductor Hybrids

A novel type of magnet–superconductor hybrid is discussed: an array of magnetic nanorods/nanowires embedded into a superconducting film. The general behavior of the phase diagram of this system can be understand in the framework of mean field theory and the hysteresis curve of the magnetic nanostructure. Magnetic nanorods/nanowires can strongly increase vortex pinning and, consequently, the critical current and critical field. The results of experimental and theoretical studies of this new class of superconducting systems are discussed.     

Asymmetry of the Pinning Force in Thin Nb Films in Parallel Magnetic Field

The pinning force, F _p, is studied in Nb films of different thickness in parallel magnetic field H. The asymmetry in the magnetic field dependence of F _p has been observed for two opposite directions of the transport current. The effect is less pronounced for thin and thick films where, respectively, single vortex pinning and pinning of the internal vortices, is relevant. At intermediate thickness, where the pinning mechanism is mostly caused by surface effects, an asymmetry in the F _p(H) dependence is clearly visible. The different surface barriers that vortices should overcome to enter the sample from opposite sides of the film explain the effect, as confirmed by numerical calculations. These have been obtained by solving the Ginzburg?CLandau equations with asymmetric boundary conditions which take into account the different superconducting properties of the film?Csubstrate and film?Cvacuum interface. Such difference can also explain the reduction of the critical current usually observed in thin films as a function of their thickness.     

Detailed Elaboration Method and Magnetic Study of?Nanometrical YBCO/YIG System

The blending of magnetic elements or compounds into a superconducting matrix has a direct effect on the physical properties of the latter and provides precious information on the superconductivity mechanisms, the spin/vortex interaction and the vortex pinning. To our knowledge, no study of the addition of nanoparticles of yttrium iron garnet Y₃Fe₅O₁₂ (called commonly YIG) in the yttrium, barium and copper oxide YBa₂Cu₃O_7??? (well known under the YBCO appellation) nanometrical matrix has been performed up to the present time. The YBCO and YIG powders have been elaborated by a conventional ceramic process. The obtained products have been characterized by magnetization measurements, X-ray diffraction and optical microscopy. The used ways to reduce their grain sizes below 50 nm and to mix them avoiding the formation of grain crowds have been explained. Their nanometrical sizes have been confirmed by atomic force microscope analysis. Eight pellets have been elaborated. The thermal variations of zero field cooled magnetizations, in a residual field of 44 G, have shown that the introduction of 5 and 9% in weight of a non-superconductor material (YIG) into a superconducting matrix (YBCO) has reduced the T _C only by 1 K for a ybcoyig5% pellet and 1.57 K for a ybcoyig9% one, comparatively with the ybcopur reference pellet??s T _C. In another way, our results have shown that the incorporation of YIG in very low quantities, between 0 and 0.5% in weight of YIG, increases the value of the critical current J _C of a ybcopur pellet (and, so, the vortex pinning). By fitting our experimental points starting from 0.5% in weight of YIG, an empirical law of the J _C variation versus added YIG percentage is suggested.