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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.

Strong magnetic fields can be generated for magnetization of the high-Tc superconductor (HTS) with pulsed magnetic fields using a small exciting coil. In the present study, magnetization using the pulsed magnetic field is performed on HTS bulk samples in the zero-field cooled state. In order to saturate the magnetization, it was necessary to optimize the time constant of the magnetic flux invasion and the flux flow during the increasing and decreasing processes of the pulsed magnetic field. Therefore, we measured the magnetic flux density on the pulsed field magnetization process and explained the relation between the pulsed magnetic field and the time constant of the magnetic flux invasion and flux flow. An empirical model of the pulsed field magnetization process was also proposed and optimum conditions were studied for pulsed field magnetization of the HTS. From this study, it was confirmed that an optimum pulsed waveform existed for pulsed field magnetization.

The angular dependence of the critical currents is investigated in a moderately twinned ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (NdBCO) single crystal in the temperature range $30l~Tl~90\mathrm{K}.$ In intermediate fields around the fishtail peak a reduction of critical current density due to twin structure is observed up to 90 K. This effect implies that vortices retain their three-dimensional character practically up to ${T}_{c}.$ The effect of twin planes disappeared at external fields declined more than $\ensuremath{\theta}\ensuremath{\approx}15\ifmmode^\circ\else\textdegree\fi{}$ from the $c$ axis (twin planes). In this case, the hysteresis loops exhibited a clear fishtail shape. The intermediate peak observed previously in NdBCO single crystal at $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}$ is identified with the low-field limit of the twin plane activity.

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

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 .

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.

Induced current densities {ital j}{sub {ital s}}({ital T},{ital B}) were measured in a wide temperature (5 {le}{ital T}{le} 90 K) and field range (0 {le}{mu}{sub 0}{ital H}{sub {ital a}}{le}7 T) on single crystals of NdBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} using a superconducting quantum interference device magnetometer. The samples exhibit the fishtail or peak effect at temperatures above 30 K. In a small temperature window between 50 and 74 K, three peaks are seen in the {ital j}{sub {ital s}}({ital T},{mu}{sub 0}{ital H}{sub {ital a}}) curves. The temperature and field behavior of the induced current densities is analyzed using a model describing a magnetization curve as composed of two independent contributions at low and high fields. By means of this analysis, it is demonstrated that the third peak corresponds to the fishtail peak, whereas the position of the second peak is practically independent of temperature. It is shown that the appearance of the second peak is due to the large values of the position of the fishtail peak. {copyright} {ital 1996 The American Physical Society.}

Induced current densities and magnetic relaxation rates were measured over a wide temperature and field range on a single crystal and a melt processed sample of using a SQUID magnetometer. The temperature dependence of the magnetic relaxation rate shows a pronounced minimum at elevated temperatures. This leads to very high values of the effective pinning energy and for the single crystal and the melt processed sample respectively, for an applied magnetic field . Furthermore, these samples show a well developed fishtail effect, leading to a current density . This demonstrates that the fishtail effect is closely related to the low relaxation rates observed here.

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.

YBCO samples were prepared using different types of precursor powders: quenched, or YBa2Cu3O7−x and Y2BaCuO5 commercial powders, with and without Pt addition. Two stoichiomctrics, corresponding to 0 and 40% molar excess of the Y2BaCuO5 phase, were adopted. The behavior of the Y2BaCuO5 phase through thermal treatments, typical of melt processes, was observed by quenching the samples at various stages. YBa2Cu3O7−x grain growth kinetics was enhanced and liquid phase losses were limited by optimizing the morphology of the Y2BaCuO5 phase in the partially melted state, with the simultaneous presence of round particles ≤1 μm and needle-like particles with very high aspect ratio. The optimized processing conditions were adopted, together with a seeding technique, to grow YBCO samples. At 77 K, a magnetic levitation force of 2.5 kg was measured for a sample with 18 mm diameter, and magnetization critical current densities over 104 A/cm2 were reached at 1 T.

YBCO MPMG samples, with 40% Y2BaCuO5, (Y-211) phase excess, were prepared using different thermal pretreatments during the temperature increase up to the partially melted state. No temperature gradient was applied during the slow-cooling growth process. Presintered samples turned out to be almost single grain, while all the others were characterized by the presence of a higher number of randomly oriented domains. Presintering favours compositional homogeneity, but, most important, enhances YBa2Cu3O7-x (Y-123) phase solidification kinetics. Though various samples show similar behaviours, concerning the values of magnetization critical-current densities (Jcm) measured at 77 K (Jcm approximately=104 A cm-2 with B//c, B=1 T), levitation forces are subject to noticeable variations related to granular macrostructures and reach a peak of over 2 kg, at 77 K, in the case of presintered material.

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

Critical current density J c of Nd1+xJBa2-xCu3Oy (Nd123) single crystals flux-grown under the Oxygen-Controlled-Melt-Growth (OCMG) process has been investigated using DC magnetization measurements. The superconducting transition temperature T c of a Nd123 single crystal flux-grown using a 0.1% O2 — Ar gas in a zirconia crucible was 94 K. Using a magneto-optical observation method, the sample was found to be free of weakly-linked regions. DC magnetization hysteresis curves exhibited an anomalous peak effect in the high temperature range. This behavior is attributable to a field-induced pinning by finely-distributed low T c phases in the high T c matrix. J c calculation using the extended Bean model resulted in high J c of about 4 × 104 A/cm2 under 2 T and over 1 × 104 A/cm2 under 4 T at 77 K.

The effects of the nominal batch composition on the NdBa2Cu3Oy (Nd123) single crystals grown by a flux method were investigated. The single crystals of Nd123 were prepared by the slow cooling at a rate of 1°C/h from 1100°C to 960°C with various nominal batch compositions. Thicker crystals along c-axis could be obtained for the batches with Ba-richer compositions. Although all Nd123 crystals were non-superconducting above 10 K in a SQUID magnetization measurement due to Al contamination from Al2O3 crucible, thicker crystals could be applied as good seed crystals for the fabrication of large textured YBa2Cu3Oy (Y123) bulks.

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.

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.