On the effect of heterovalent substitutions in ruthenocuprates

Abstract We discuss the properties of superconducting derivatives of the RuSr 2 GdCu 2 O 8 (1212-type) ruthenocuprate, for which heterovalent doping has been achieved through partial substitution of Cu ions into the RuO 2 planes (Ru 1− x Sr 2 GdCu 2+ x O 8− δ , 0⩽ x ⩽0.75, T c max =72 K for x =0.3–0.4) and Ce ions into the Gd sites (RuSr 2 Gd 1− y Ce y Cu 2 O 8 , 0⩽ y ⩽0.1). The measurements of XANES, thermopower, and magnetization under external pressure reveal an underdoped character of all compounds. Muon spin rotation experiments indicate the presence of magnetic order at low temperatures ( T m =14–2 K for x =0.1–0.4). Properties of these two series lead us to the qualitative phase diagram for differently doped 1212-type ruthenocuprates. The difference in temperature of magnetic ordering found for superconducting and non-superconducting RuSr 2 GdCu 2 O 8 is discussed in the context of the properties of substituted compounds. The high pressure oxygen conditions required for synthesis of Ru 1− x Sr 2 RECu 2+ x O 8− δ , have been extended to synthesis of a Ru 1− x Sr 2 Eu 2− y Ce y Cu 2+ x O 10− δ series. The Cu → Ru doping achieved in these phases is found to decrease the temperature for magnetic ordering as well the volume fraction of the magnetic phase.