Influence of Re substitution on the flux pinning in (Hg,Re)Ba2Ca2Cu3O8+δ single crystals

Flux pinning in Hg0.77Re0.23Ba2Ca2Cu3O8 + delta single crystals (T-c = 130 K) and in HgBa2Ca2Cu3O8+delta crystals with T-c equal to 130 and 120 K was studied. In the investigated crystals Re atoms substitute at the Hg site of HgBa2Ca2Cu3O8 + delta and alter the defect structure of the host layer by pulling in four new oxygen atoms at (0.34, 0.34, 0) position to form an octahedron around Re. As a result a significant improvement of the irreversibility Line position at the H-T phase diagram for Hg0.77Re0.23Ba2Ca2Cu3O8 + delta crystal is observed only at low temperatures (below 80 K). However, magnetization measurements performed at temperatures above 100 K show insignificant change in the irreversibility line position only, as compared to that one of the parent compound with T-c = 130 K. This indicates insignificant influence of new pinning centers, introduced by Re substitution, in the temperature range, where thermal activation energy becomes significant in comparison with pinning energy. Furthermore, obtained data indicate that no improvement of the coupling between superconducting layers was achieved as a result of the expected increase of electrical conductivity in the blocking layer. Since only a small change in the c-axis lattice constant was observed in Re substituted crystals, we can conclude that a larger shortening of the blocking layer in the material is necessary to improve the irreversibility Line position in HgBa2Ca2Cu3O8 + delta crystals at high temperatures. (C) 1998 Elsevier Science B.V. All rights reserved.