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We analyse and compare properties of different models proposed so far for modelling the shapes of FB and JB . curves measured in RE-Ba Cu O RE-123, REs rare earth samples. The formulas following from the discussed 23 7 yd models allow for a direct fit of experimental data in various representations. The models divide in two categories, according to the pinning potential used, namely the power-law and the logarithmic potential. We show that none of the pinning regimes . proposed for low-T superconductors accounts for the fishtail effect FE observed in high- T materials. The conventional c c expressions allow a good fit of high-T experimental data, however, the resulting fitting parameters are much higher than c those predicted by the theory. The logarithmic pinning potential justified in high- T materials by both magnetic and transport c

Nd1+xBa2-xCu3Oy (Nd123ss) single crystals were isothermally grown under controlled oxygen partial pressure of 1% O2 at various growth temperatures (Tg) from 1000 to 1020 °C with a flux method. The flux composition of 20 mol% NdBa2Cu3Oy (Nd123) and 80 mol% Ba3Cu5O8 (035) was employed. Nd123ss single crystals grown at Tg≥1010 °C exhibited a transition temperature Tc≈95 K with transition width ≈1 K, the peak field (Bpk)≈2-3 T and the irreversibility field (Birr) at 77 K much higher than the experimental limit of 7 T. As the growth temperature is lowered, both the Bpk and Birr were reduced, which can be explained in terms of the chemical fluctuation which can occur at lower Tg.

Large Nd1+xBa2-xCu3Oy (Nd123ss) single crystal cubes of side 3 mm were fabricated isothermally by the flux method. The crystals exhibited a Tc of 95 K with a sharp superconducting transition. One crystal was cut into small pieces, and its magnetic properties were measured to study the spatial distribution of the superconducting properties. It was found that although Tc values were almost constant, a small but significant intrinsic difference in the Jc-B properties and irreversibility field was observed depending on the sample position. This suggests that the distribution of Nd-rich Nd123ss clusters are spatially different even within the same single crystal. Such differences may be caused by the time variation of Nd concentration in the liquid during crystal growth.

It is known that the amount of Nd ions substituting for a Ba site in the Nd1+xBa2−xCu3Oy solid solution (Nd123ss) tends to decrease with either decreasing oxygen partial pressure PO2 or increasing temperature, however, the precise solubility limit of Nd123ss has not been obtained. In the present study, we carefully studied the solubility limit of Nd123ss under low PO2 and for the first time could show the presence of x

The authors are investigating the possibility of using bulk magnets in the Maglev system. It is considered that bulk magnets for the Maglev system must be composed of numerous superconducting bulks arranged in rows and columns for each magnet pole because a bulk superconductor with a high critical current density doesn't have a sufficiently large size for Maglev magnet. Therefore, it is necessary to examine the flux-trapping characteristic of superconducting bulks arranged in array. Flux-trapping experiments are performed using the melt-processed YBaCuO superconducting bulks arranged in rows and columns, in which each bulk individually has a magnetizing coil, and the bulks are magnetized by field cooling. It is found that, when the superconducting bulks are arranged in rows and columns, the trapped flux density and the generated magnetic field are smaller than those when a superconducting bulk is used alone. The rate of decreases in the trapped flux density and the generated magnetic field becomes larger with increases in the number of superconducting bulks arranged in array. The trapped flux decreases conspicuously in particular at a superconducting bulk surrounded by other superconducting bulks.

The scaling of the volume pinning forces, Fp=jc×Ba, is performed versus the reduced fields bf=Ba/Bfp (where Ba is the applied field, and Bfp denotes the peak position of Fp,max) and h=Ha/Hirr (where Hirr denotes the irreversibility field) for two different NdBa2Cu3O7−δ superconductors, a single crystal and a melt-processed sample. In both cases, an excellent natural scaling is found to hold. Based on these good scaling results, we extend the scaling analysis towards lower temperatures. There, the scaling parameters are no longer directly accessible within our experimental field range, but can be deduced from the Fp(Ba) curves normalized using fitting values of Bfp and Fp,max so that these overlap with the high temperature curves. It is shown that such a procedure allows to obtain reasonably good estimates of Fp,max and Hirr, which leads in turn to an estimation of the irreversibility line well down to 0 temperature. At low temperatures, deviations from the analytical dependencies appear which are due to interference with the central and intermediate peak in the magnetization loops. The shape of the irreversibility line is discussed.

Superconducting transitions are measured in field-cooled cooling (FCC) and field-cooled warming (FCW) modes in external magnetic fields ranging between 1 mT and 7 T on various high-T c superconductors (single crystal and melt-processed YBa2-Cu3O7-δ, single crystals and melt-processed NdBa2Cu3O7-δ, and on various ternary 123-superconductors). Our measurements reveal that a two step transition is observed if the sample exhibits also the secondary peak effect in the magnetization loops.

One of the applications of a bulk magnet in which flux is trapped in oxide superconducting bulks is a superconducting magnet for Maglev systems. We are investigating the possibility of using bulk magnets in Maglev systems. It is believed that bulk magnets far Maglev systems must be composed of superconducting bulks arranged in rows and columns for every pole, because a superconductor made of melt-processed Y-Ba-Cu-O with a high critical current density is smaller in size than the pole. Therefore, it is necessary to examine the flux-trapping characteristics of superconducting bulks in an array. Flux-trapping experiments are performed using melt-processed Y-Ba-Cu-O superconducting bulks arranged in rows and columns, in which each bulk has a magnetizing coil (an individual magnetization method) or all bulks have one magnetizing coil centered at outer bulks in an array (unified magnetization method), and the bulks are magnetized by field cooling. The magnetic field on the superconducting bulks arranged in an array is smaller than that of the single bulk arrangement because the magnetic field generated by the surrounding superconducting bulks acts to weaken the field of the surrounded suerconducting bulk. Furthermore, in the case of the individual magnetization method, the magnetic field applied is decreased during field cooling so that the trapped flux is smaller too. The rate of decrease in the individual magnetization method is larger than the rate of decrease in the unified magnetization method. It is clear that the unified magnetization method is more suitable than individual magnetization for magnetizing bulk magnets for aglev systems, which require a high magnetic field and large-scale system.

The magnetic flux dynamics of Nd-Ba-Cu-O (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method has been investigated using the Magneto-Optical (M-O) technique at various low temperatures for an external magnetic field applied both parallel (H//c) and perpendicular (H⊥c) to the c -axis of the sample. While for H//c the flux motion was isotropic, for H⊥c anisotropic penetration and expulsion of the flux lines was observed directly. In the configuration H⊥c the flux lines were observed to move in and out the sample predominantly parallel to the a-b planes. The flux behaviour has also been correlated to the microstructure. Particularly, microcracks along the basal plane were found to provide a low resistance channel for flux motion.

Among various potential applications of melt-textured RE-Ba-Cu-O (REBCO, RE: rare earth elements) superconductors, we have examined the bulk application as the superconducting permanent magnet, especially for the magnetically-levitated (MAGLEV) train. Compared with Y-Ba-Cu-0 (YBCO), oxygen-controlled melt-growth (OCMG)-processed LREBCO (LRE: light rare earth elements) bulk superconductors are more promising for this application because of larger critical current density ( J c ) values in high field and higher irreversibility field ( B irr ) within the range of the liquid nitrogen refrigeration (63–77 K), implying that even higher trapped fields ( B t ) are achievable in principle. In this paper, material requirements of superconducting bulks for the MAGLEV train are first presented and then processing aspects for the fabrication of good LREBCO bulks are described.

In this paper, we present the results of various magnetic measurements on an NdBa2Cu3O7−δ (NdBCO) single crystal. Special features observed in the magnetization loops (MHLs) are discussed. From these loops, critical current densities, jc as a function of the applied field, Ha, and volume pinning forces, Fp=jc×B, are derived. The samples exhibit a strong fishtail or peak effect at temperatures above 30 K. In a small temperature window between 50 K and 74 K, even three peaks are seen in the jc(T,Ha) curves. A good scaling of the jc (T,Ha) curves is obtained, which allows the identification of the peaks. The scaled volume pinning forces (vs the reduced field, h=Ha/Hirr), in the temperature regime between 60 and 90 K give experimental evidence for a strong pinning at extended superconducting defects (interaction volume Vpin∼ξ2d). These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, Tc. Furthermore, the scaling analysis of Fp is extended towards low temperatures. It is shown that at 5 K another peak in Fp is found, which is due to pinning at twin boundaries. Using this scaling analysis of both jc and Fp, the irreversibility line (IL) is determined in the entire temperature range 0–92 K. Magneto-optic (MO) imaging is employed to investigate the flux distributions of various NdBCO single crystals. These images confirm the important role of twin boundaries as pinning sites at low temperatures and low applied fields. PACS numbers: 74.60 Ec, 74.60 Ge, 74.60 Jg

REBaCuO (RE: rare earth element selected from Nd, Sm, Eu, Gd) superconductors exhibit high T c and J c values when they are melt processed in a reduced oxygen atmosphere (oxygen-controlled-melt-growth: OCMG process). Large J c values in these systems can be attributed to finely dispersed RE rich 123 clusters, which can act as field-induced pinning centers, and thus leading to the second peak effect in M - H loops. In this paper, we first propose the mechanism how RE rich clusters are formed during melt processing. Second we show that flux pinning depends on oxygen annealing conditions, because the optimum oxygenation conditions are different depending on the range of NdBa substitution. We also report that the addition of ND422 is effective in raising J c , although the mechanism is rather complicated owing to the fact that ND422 also has a REBa solid solution toward Ba rich direction.

We investigated the magnetic flux dynamics of NdBaCuO (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method using the Magneto-Optical (M-O) technique at various low temperatures. An external magnetic field was applied both parallel ( H t | c ) and perpendicular ( H ⊥ c ) to the c -axis of the sample. While for H t | c the flux motion was isotropic, for H ⊥ c anisotropic penetration and expulsion of the flux lines were directly visualized. In the configuration H ⊥ c the flux lines were observed to move in and out of the sample predominantly parallel to the a - b planes. The flux behavior was also correlated to the microstructure. Particularly, the microcracks along the basal plane were found to provide a channel for easy flux motion. The anisotropic behavior of the flux front reflects the anisotropy in the critical current density J c . The different components of J c were independently calculated from the M-O data directly analyzing the magnetic induction profiles. These J c values were consistent with those estimated interpreting the magnetization hysteresis measurements within the framework of the extended Bean model.

The a.c. magnetization characteristics of NdBa 2 Cu 3 O y (Nd123) bulk superconductors were investigated using a.c. susceptometry. For the present study, an OCMG-processed Nd123 bulk sample (OCMG-Nd123; T c = 93.9 K ) and a flux-grown Nd123 single crystal (Nd123SC; T c = 93.2 K ) were used. An MPMG-processed Y123 bulk sample (MPMG-Y123; T c = 91.6 K ) was also studied for comparison. While the fundamental responses are basically the same for all samples, the third harmonic responses by sweeping the bias field, x 3 ( H dc ), show an interesting feature in that the x 3 ( H dc ) curves for Nd123 samples indirectly reflect the fishtail peak of their d.c. magnetization hysteresis ( M-H ) curves. The boundary lines of Nd123 samples, determined from both the peak temperature ( T p ) of the x 1 ″( T ) maxima and the offset of the x 3 ( H dc ), are commonly located at higher field-temperature region relative to that of the Y123. Using the scaling relationship of H ∞(1 − t ) n , where t = T / T c , all the boundary lines from the x 3 ( H dc ) and the x 1 ″( T ) maxima could be best fit with a power n of ~ 1.2 and ~ 1.4, respectively.

We have measured trapped field distributions by scanning a Hall probe on various rectangular YBCO bulk superconductors and studied the effects of field dependence of Jc bulk thickness and bulk size on the trapped field. The trapped field could be increased by increasing the bulk thickness and size. However, a saturation was observed for the maximum thickness. We also calculated the distribution of trapped field taking account of the field dependence of Jc. Experimental trapped field distributions were in good agreement with our calculated values when we used the Jc-B curves obtained from a d.c. magnetization measurement for samples.

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

Many tentatives have been made these last years to increase the pinning properties of melt-textured YBaCuO by secondary phases additions. The results are more or less positive depending on the processing conditions and the reactivity of the dopant, especially its action on the size and shape of 211 particles. In this paper, the effect of BaSnO3 additions up to 30 mol% on the microstructure and the superconducting properties of MPMG-processed YBCO is investigated. At high temperatures, BaSnO3 reacts rapidly with the liquid phases to form few microns size inclusions of a stable Y-Ba-Sn-Cu-Pt-O phase. This phase is found to consume platinum and to favor 211 coarsening at high temperature. For 30% doping, the entire microstructure is strongly affected and for low levels of doping (2% and 5%), we could not evidence any improvement in the superconducting properties, compared to undoped samples. But when the doping level is increased up to 10%, one can locally find a high density of submicron size 211 inclusions and Tc enhancement is observed. In this case, we could obtain improved Jc(B) properties up to 110 000 A/cm2, at 77 K in zero field.

A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four‐probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M‐H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x

By measuring the magnetization hysteresis, we studied the critical current density, Jc, and irreversibility fields, Hirr, of a c‐axis aligned Hg‐Ba‐Ca‐Cu‐O sample doped with Pb containing the Hg‐1223 phase as the main phase. Anisotropy in ΔM was clearly observed in two different magnetic field directions. Even though the Hg‐1223 phase contains three CuO2 sheets, the Hirr vs [1‐(T/Tc)] curve was located at roughly the same positions as those reported for the Hg‐1201 and Hg‐1212 phases, indicating that the higher Tc phase, i.e., Hg‐1223, is advantageous to the other phases in practical uses.

By means of a scaling analysis of the volume pinning forces, Fp, normalized by the maximum value, Fp,max, versus the reduced field h = H a /H irr where H irr denotes the irreversibility field, we analyze the pinning properties of various Nd-based superconductors of the 123-type. NdBa2Cu3O7-δ (NdBCO) samples show a peak in the scaled Fp-graphs above 0.4, and the ternary compounds (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG) even at ≈0.5. This high peak position provides strong evidence for a dominant pinning caused by spatial variations of the transition temperature, T c , throughout the samples. In NdBCO single crystals, two peaks are observed in the magnetization loops. Angular dependent measurements clarified that this extra peak is due to twin plane pinning.

Nd-Ba-Cu-O superconductors forms aNd1+xBa2-xCu3Oy solid solution (Ndl23ss). In general the solubility limit is dependent both on the oxygen partial pressure (PO2) and temperature, and therefore an understanding of PO2 and temperature dependence of the solubility limit will be an important guideline for obtaining high T c and J c materials. We carefully studied the solubility limit of Nd l23ss under low PO2 and at high temperatures. X-ray diffraction, thermal analysis and DC magnetization measurements confirmed that Ba can substitute for Nd site and the solubility limit for the Ba rich direction lies on x < 0 in low P O2 at high temperatures. Furthermore, Demagnetization measurements showed thatNdl23ss withx

Volume pinning forces, F p , are measured in a wide temperature (5 ≤ T ≤ 92 K) and field range (0 ≤ μ0 H a ≤ 9 T) on different NdBa2Cu3O7-δ samples. Above 60 K a good scaling of F p , versus the reduced field, h = H a /H irr can be established. The scaled pinning curves are compared to several theoretical predictions. Experimental evidence for strong pinning at extended (Vpin ~ ξ2 d) defects is given. These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, T c . Such a pinning echanism is especially important for applications of high-T c superconductors operating at 77 K.

To identify the relationship between the flux pinning properties and the process conditions in the Nd1+x Ba2-x CU3O7-d (Ndl23SS) superconductors, single crystals were flux-grown in 1%O2-Ar atmosphere using an isothermal crystallization procedure at various undercoolings (∆T) from 10 to 35 °C The flux was composed of 20 mol% NdBa2Cu3O7-d (Ndl23) and 80 mol% Ba3gCu5O8 (035). For ∆T ≤ 25 °C, flux-grown Ndl23 single crystals exhibited aspect ratio w/t smaller than 1, where w and t correspond to the lengths along the a- (or b-) and c-axis respectively, transition temperature T c ≈ 95 K with transition width ≈ 1 K, the peak field (B pk) ≈ 2 – 3 T and the irreversibility field (B in) much higher than the experimental limit 7 T. In contrast, for ∆T ≥ 30 °C, Ndl23 single crystals exhibited a slight increase in w/t, a slight decrease in T c with broader transition width, and a reduction in B pk and B in.

The effects of the second phase (Nd4Ba2Cu2Ox; Nd422) on the pinning property of Nd-Ba-Cu-O crystals have been studied. Hysteresis loops and magnetic relaxation were measured with a SQUID magnetometer. Critical current density (J s ) is obtained from the hysteresis loops using extended Bean model. The normalized relaxation rate (S) and the apparent pinning energy (U*0) are deduced from the relaxation data using Anderson-Kim model. It was found that J c -B curves and U*0-B curves are strongly dependent on the volume fraction of Nd422 phase, like Y211 phase with Y-Ba-Cu-O system.

REBa2Cu3Oy superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm, Eu) exhibit relatively low T c due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1−x,REx)2Cu3Oy phase in a good superconductive matrix.

An estimation of an intragranular critical current density Jc for a large YBCO bulk sample prepared by the MPMG method was performed by a trapped field measurement using a Hall probe. A single-domained rectangular YBCO sample (10mm×10mm×lmm) was field-cooled in liquid nitrogen under the field of 4000G. After the applied field was turned off, the field distribution in the area of l0mm×10mm was mapped by scanning the Hall probe at the interval of 0.5mm. The trapped field distribution was compared with the distributions calculated by numerical analyses assuming square current loops inside the sample. First, assuming a field-independent Jc, the measured profile was in good agreement with the calculated profile except in the high field region near the center. Therefore, the intragranular Jc was estimated to be 2.6×104A/cm2 (77K, 0T). Second, using the Irie-Yamafuji model taking into account a field-dependent Jc, the measured profile was in better agreement with the calculated profile even in the high field region. The material parameters in the relationship of J c =αB-n were found to be α=1.4×104A/cm2 and n=0.20. Finally, it is found that a trapped field measurement using a Hall probe is applicable to a Jc estimation of a large superconductor without destroying the sample.

Magnetic shielding properties were measured at 77 K in YBaCuO superconducting plates prepared by melt processes. Melt processed YBaCuO has a high Jc value, hence the material is promising for the application to magnetic shielding in a relatively high magnetic field. The shielding plate with 1 mm in thickness reduces an external magnetic field of 7.1 mT to a minimum value of less than 0.1 mT. A plate which consists of small samples in a tile shape is considered to be realistic to design a large shielding plate, and a laminated plate can enhance the shielding properties. The touching part in one layer does not extend from the bottom to the top of the shielding plate in three layers. The laminated plate in three layers reduces an applied magnetic field of 14.4 mT to an average value of about 8 mT.

Since High-Tc superconductors such as Y-system and Bi-system show perfect diamagnetism at above 90 K, it is expected that a magnetic field will be shielded by superconductors cooled with liquid nitrogen. In this study, magnetic shielding effect of superconducting YBaCuO disks prepared by melt processes was measured at 77K. The results indicate that disk samples with ~16 mm in diameter and ~ 1 mm in thickness can reduce an external magnetic field in intermediate field range, but that the magnetic flux comes in through grainboundaries and cracks of the sample.

Magnetization properties by a pulsed magnetic field were studied in YBaCuO superconductors prepared by the Melt-Powder-Melt-Growth (MPMG) process. The sample size was 10 mm × 10 mm × 1 mm. A pulsed magnetic field of up to 2.3 T was applied perpendicular to the sample surface at 77 K. Two-dimensional distributions of the trapped magnetic field over the YBaCuO sample surface were measured at 77 K by a scanning Hall sensor. A maximum magnetic field of 0.15 T was trapped. The maximum trapped magnetic field increased with an increasing applied magnetic field, and showed a peak. Then the trapped field decreased with a further increasing pulsed magnetic field.

REBa 2 Cu 3 O y (RE: Nd, Sm) superconductors exhibit relatively high J c values even at 77K when melt-processed in a reduced oxygen atmosphere. The J c values of these superconductors are higher than those of melt processed YBa 2 Cu 3 O y with fine Y 2 BaCuO 5 inclusions in a high field region. The irreversibility field with H ∥ c axis was dramatically increased to 8T at 77K. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba 1−x , RE x ) 2 Cu 3 O y phase in a good superconductive matrix.

The effects of various oxygen partial pressures (P O2) during melt processing on the superconducting properties of fabricated NdBa2Cu3O y (Nd123) superconductors have been systematically investigated. Unlike in air or O2, samples melt-processed in reduced oxygen atmospheres exhibited unprecedentedly high T c's of 95–96 K with a sharp superconducting transition. Especially, for a sample melt-processed in 0.1% O2 in Ar, the magnetization hysteresis ( M-H ) loop was kept open up to 7 T at 77 K and for the field parallel to the c-axis of the sample (H//c), implying that very effective flux pinning can be achieved in high magnetic fields.

REBa2Cu3O y superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T c due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa2Cu3O y with fine Y2BaCuO5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1- x , RE x )2Cu3O y phase in a good superconductive matrix.