Koichiro Waki

Superconducting transitions of Nd-based 123 superconductors in fields up to 7 T

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.

E–J characteristics of NdBa2Cu3O7−δ bulk superconductor determined with ac susceptibility

Abstract Electric field ( E )–current ( J ) characteristics of the NdBa 2 Cu 3 O 7− δ (Nd-123) bulk superconductor have been investigated by measuring the third harmonic ac susceptibility. They were compared with E–J characteristics determined with the dc magnetization measurements and magnetic relaxation methods. The n value was estimated on the assumption that E follows the power law: E ∼ J n . We found that the sample, which has the larger secondary peak on the J c – B curve, exhibits the more remarkable peak structure on the magnetic field dependence of the n value.

Flux pinning properties of NdBa2Cu3O7-d single crystals flux-grown in low oxygen partial pressures

Abstract 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.

Magneto optical characterizations of flux dynamics in NdBaCuO bulk superconductors

Abstract 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.

E–J characteristics of melt-textured RE123 (RE: Nd, Y) superconductors

Abstract We have investigated the E–J characteristics for melt-textured NdBa2Cu3O7−δ (Nd123) with the secondary peak and Y123 with no secondary peak over a wide range of the electric field by measuring the ac susceptibility and the dc magnetization. In the case of Y123, the E–J characteristics exhibited normal behavior in that the convex curve changed to the concave with increasing magnetic field. In contrast the E–J characteristics of Nd123 were complicated and the convex curve changed to the concave with decreasing electric field near the secondary peak field. This shows that the E–J characteristics are affected by the field-induced pinning.

Superconducting properties of Nd1+xBa2-xCu3Oy single crystals isothermally grown at various temperatures

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.

AC susceptibility measurement for Nd–Ba–Cu–O superconductor

Abstract Nd–Ba–Cu–O sample fabricated by the oxygen-controlled-melt-growth method exhibits large J c values in a high field region and also shows a secondary peak effect. In the present study, the Nd–Ba–Cu–O sample was characterized by using the flux profile technique, in which the χ ′ signal is plotted against the amplitude of AC field. J c values determined with the flux profile technique also showed a clear secondary peak effect. In addition, such J c values were higher than those measured with DC susceptibility, presumably due to a difference in the electric field to determine J c .

Ac susceptibility in (Nd, Eu, Gd)- Ba-Cu-O superconductors

We investigate the complex ac susceptibility of the melt-processed (Nd, Eu, Gd)- Ba-Cu-O and Nd-Ba-Cu-O. We observe the anomalous structure in the temperature dependence of the complex ac susceptibility. This anomaly may be related to the superconducting transition of the second phase.

Spatial variation in the magnetic properties of single crystal Nd123

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.

A.C. magnetization characteristics of Nd123 bulk superconductors

Abstract 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.