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We report on a new type of correlated nanometer-scale pinning structure observed in a melt-processed (Nd0.33Eu0.38Gd0.28)Ba(2)Cu(3)O(y) (NEG-123). It consists of NEG/Ba-rich clusters in the stoichiometric NEG-123 matrix forming a lamellar array with a period of a few nanometers. These lamellas appear within regular twins, thus representing their fine substructure-sometimes straight, sometimes wavy. This new material structure correlates well with the significant enhancement of pinning at high fields, represented by irreversibility field above 14 T at 77 K (B parallel c). We believe that the new pinning medium enables one to significantly broaden the limits for high-field applications.

The effects of (NEG-211), Pt and additions on OCMG-processed (`NEG') composites have been investigated in detail using a combination of scanning electron microscopy and measurements of critical current densities, . Microstructural observations clearly indicated that the platinum addition was effective in refining the NEG-211 secondary-phase particles, which led to a dramatic increase in in the low-field as well as in the high-field region. TEM-EDX analyses revealed that the small secondary-phase particles (diameter ) consist mainly of Gd in the rare earth site, while the large particles contain Nd, Eu and Gd in the rare earth site with an even ratio, which is identical to the nominal composition of the precursor powder. For an NEG sample with 40 mol% NEG-211 additions, a large critical current density of 60 000 A at 77 K was observed in a magnetic field of 3 T applied parallel to the c-axis of the sample. The critical current density measurements showed that the zero field and peak values are similar in samples with 30 and 40 mol% of NEG-211 added. In contrast to this, the combination of and Pt addition was not as effective in refining the NEG-211 particles, and also the values were found to decrease in the entire field region.

We report the effect of mixing three rare-earth (RE) elements in the RE site on the superconducting properties of melt-processed REBa 2 Cu 3 O y , in which RE elements are selected from the group of Nd, Sm, Eu and Gd. The heat treatment profiles for melt processing were determined according to the peritectic decomposition temperatures obtained from the thermal analysis measurements. Microstructural observations and compositional analysis showed that the main phase was (R1 0.33 R2 0.33 R3 0.33 )Ba 2 Cu 3 O y , in which the ratio of three RE elements (R1, R2, R3) was equal to that of the nominal composition, indicating that mixing of RE elements was uniform. All the samples showed relatively high T c in the range 93.1-95.3 K and exhibited the secondary peak effect in the magnetization curves at 77 K with the irreversibility field higher than that of melt-processed Y-Ba-Cu-O. A high J c value of 50 000 A cm −2 at 77 K and 2 T was achieved in (Nd, Eu, Gd)Ba 2 Cu 3 O y for field parallel to the c-axis.

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The flux pinning characteristics of ternary melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG) superconductors are studied in the temperature range 60⩽T⩽90 K. NEG samples exhibit a strongly developed peak effect in the dependence of the critical current densities on the external field, Ha. The scaling of the pinning forces versus the reduced field h=Ha/Hirr (where Hirr denotes the irreversibility field) yields a peak at h0=0.5 which is an indication of pinning provided by a spatial variation of the transition temperature. The presence of a weaker superconducting second phase is demonstrated by means of field cooling and warming experiments in fields up to 7 T. Furthermore, we discuss the possible effect of the magnetic moments of Gd and Nd on the flux pinning.