We reviewed our various results on rutheno-cuprate magneto-superconductors RuSr2GdCu2O8(Ru-1212) and RuSr2(Gd0.75Ce0.25)2Cu2O10(Ru-1222). It is observed, that it is difficult to control the oxygen content of Ru-1212, though the same is possible up to some

MAGNETO–SUPERCONDUCTIVITY IN RUTHENO-CUPRATES RuSr2GdCu2O8-δ (Ru-1212) AND RuSr2(Gd0.75Ce0.25)2Cu2O10-δ (Ru-1222):

A CRITICAL REVIEW

V.P.S. AWANA, M. KARPPINEN, AND H. YAMAUCHI

Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, JAPAN

1. INTRODUCTION

According to a long-term common sense superconductivity and magnetic long-range order do not mutually exist within a single (thermodynamical) phase. The topic has been widely discussed in condensed matter physics over decades. Nevertheless, coexistence of high-Tc superconductivity and magnetism was reported for a rutheno-cuprate of the Ru-1222 type, i.e. RuSr2(Gd0.7Ce0.3)2Cu2O10-δ [1], and more recently for RuSr2GdCu2O8-δ (Ru-1212) [2]. These reports have renewed the interest in the possible coexistence of superconductivity and magnetism. It is believed that in rutheno-cuprates the RuO6 octahedra in the charge reservoir are mainly responsible both for magnetism and for doping holes into the superconductive CuO2 plane.

The structures of both RuSr2GdCu2O8-δ and RuSr2(Gd,Ce)2Cu2O10-δ are derived from that of RBa2Cu3O7-δ or CuBa2RCu2O7-δ, with Cu in the charge reservoir replaced by Ru such that the CuO1-δ chain is replaced by a RuO2-δ sheet [3]. In the RuSr2(Gd,Ce)2Cu2O10-δ structure furthermore, a three-layer fluorite-type block instead of a single oxygen-free R (= rare earth element) layer is inserted between the two CuO2 planes of the Cu-1212 structure. The RuSr2(Gd,Ce)2Cu2O10-δ phase with a layer sequence of SrO-RuO2-δ-SrO-CuO2-(Gd,Ce)-O2-(Gd,Ce)-CuO2 is an m = 1, k = 1, s = 2 member of “category-B” cuprates, MmA2kBsCu1+kOm+4k+2s±δ or M-m(2k)s(1+k), i.e. Ru-1222, while RuSr2GdCu2O8-δ is an m = 1, r = 2, n = 2 member of “category-A” cuprates, MmArQn-1CunOm+2+2n±δ or M-mr(n-1)n, i.e.