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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.

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.

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 magnetization properties of a flux-grown (Nd123) single crystal were investigated by measuring DC magnetization hysteresis (M-H) loops in a wide temperature range (10-92 K) and for applied fields parallel and perpendicular to the c-axis of the sample. For both field directions, temperature-dependent fishtail peaks in the M-H loops are clearly observed at high temperatures above 70 K, and all the data could be scaled to a single master curve with the hysteresis loop width versus H plot when they are normalized with the peak field and its magnetization value , respectively. This indicates that the origins of the peak effect and the pinning are identical for this temperature range. We have also found that and the irreversibility field scale with the reduced temperature, (: critical temperature), and follow the relationship with a power n of 1.1-1.3. The anisotropy parameters of and of were deduced from the angular dependence of and at 88 K. We emphasize that all the present results support our proposal that the peak effect in the Nd123 originates from the localized Nd-Ba substituted regions with depressed .

Critical current densities of Nd123 crystals flux grown by the oxygen-controlled-melt-growth (OCMG) process have been studied with DC magnetization measurements. The superconducting transition temperature ( T c ) of the Nd123 crystal flux grown in 0.1%O 2 Ar gas was 94 K with the applied field of 10 Oe parallel to the c -axis. The sample was found to be homogeneous without weakly linked regions through magneto-optical measurements. DC magnetization-hysteresis curves measured with a SQUID magnetometer exhibited a peak effect at various temperatures above 70 K. The peak effect is attributable to field-induced pinning by finely distributed low- T c phases. The sample exhibited high critical current density ( J c ) valuesof 4 × 10 4 A/cm 2 in 2 T and over 1 × 10 4 A/cm 2 in 4 T at 77 K for fields parallel to the c -axis.

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.

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.

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.

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.

Flux pinning properties were studied for NdBa 2 Cu 3 O 7−d (Nd123) single crystals flux-grown in low oxygen partial pressures. In the high temperature ( T ) region roughly above 70 K, apparent fishtail peaks were commonly observed on the magnetization hysteresis ( M-H ) curves or the critical current density vs magnetic field ( J c -B curves). For this high T region, while J c (B,T ) curves in low B followed an empirical equation from Matsushita et al. [1], those in high B obviously deviated from the equation, however, those could be well scaled to a single curve by plotting the normalized flux pinning force density f p vs the reduced B/B irr where B irr is the irreversibility field, indicating that dominant pinning mechanisms are different between low and high B region.

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.