ELSEVIER Physica C 253 (1995) 357-366 PHYSICA Enhanced irreversibility field and current stabilization by Pb substitution in aligned T1-1223 high-T c superconductor Y.C. Kim a, J.R. Thompson ,,u,,, D.K. Christen b, M. Paranthaman c, E.D. Specht d a Department of Physics, University of Tennessee, Knoxville TN 37996-1200, USA h Solid State Division, Oak Ridge National Laboratory, PO. Box 2008, Oak.Ridge TN 37831-6061, USA c Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, PO. Box 2008, Oak Ridge TN 37831-6100, USA d Metals and Ceramics Division, Oak Ridge National Laboratory, PO. Box 2008, Oak Ridge TN 37831-6118, USA Received 19 May 1995; revised 17 July 1995 Abstract Using magnetic methods, we have studied the effects of Pb substitution on the irreversibility line and other features of c-axis aligned T1 based cuprate superconductors with the "1223" structure. Specifically, the properties of (Tlo.sPbo.5)Sr2CazCu309 are compared with those of grain-aligned T1Ba2CazCu309 and Tl(Ba-Sr)2Ca2Cu309 materials. The irreversibility line Hi=(T) is notably higher for the Pb substituted material (~ 1.7 T at 77 K with HIIc) and markedly anisotropic for all of the samples. Also, results for the persistent current density are reported. Flux-creep studies of the current-decay rate show that the current is the most stable in the aligned (T10.sPb0.5)Sr2Ca2Cu309, and that the decay rate of all three materials increases monotonically as the temperature increases. 1. Introduction The T1-Ba-Ca-Cu-O system encompasses mul- tiple superconducting phases [1]. These include the T!-1212 and 1223 phases containing a single T1-O blocking layer and the T1-2212 and 2223 phases containing double TI-O layers [2,3]. It has been suggested that the T1-1223 phase is well suited for practical applications, because its critical current density Jc in applied magnetic fields is high, com- pared with materials containing double blocking lay- ers [4,5]. In light of this potential, it is interesting to study the persistent current density, its stability in time, and the irreversibility line in T1-1223 com- pounds, and particularly to compare on the same * Corresponding author. basis the properties of materials modified by simple chemical substitutions. In this work, we report the results of a study performed on uniaxially aligned powders, which allows one to isolate the applica- tions-limiting orientation with magnetic field H ap- plied perpendicular to the CuO planes. Using two-dimensional and three-dimensional vortex models, Kim et al. [6] have considered the effects of the distance between the superconducting Cu-O layers. It is thought that a higher current density at high temperatures and fields in the T1-1223 systems (compared with T1-2223) comes from reduc- ing the separation between sets of CuO planes by incorporating only a single T10 layer. This increases the Josephson coupling energy between two-dimen- sional CuO layer sets, which in turn leads to an increase in the irreversibility field. 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00449-1