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

in terms of the superconducting coherence length and the long-range magnetic ordering are important. Also in HTSC compounds, large anisotropy within the unit cell and microscopic compositional variations due to different cation intermixing or the variation in oxygen content are often seen. In such a situation the possibility of magnetic but not superconducting solid solutions coexisting microscopically with superconducting but not magnetic material cannot be ruled out without ambiguity. Such a possibility was indicated in the very beginning of the research on the “possible coexistence of superconductivity and magnetism in rutheno-cuprates” [11,14,25]. The concern of phase purity at the microscopic level in both Ru-1212 and Ru-1222 still remains unresolved. Also, we should look more carefully at the existing contradictions in the reported literature on rutheno-cuprates. Nevertheless, results of recent NMR (nuclear magnetic resonance) experiments on Ru-1212 were interpreted in terms of the coexistence of superconductivity and magnetism [26].

In the current review, we not only access the existing literature critically, but also present our own data in terms of phase formation and structural, thermal, magnetic, electrical, spectroscopic and microscopic characterization for both Ru-1212 and Ru-1222. We further conclude that the coexistence of superconductivity and magnetism, in particular the ferromagnetism at microscopic level in both these phases is far from conclusive.

2. EXPERIMENTAL DETAILS

Samples of RuSr2GdCu2O8-δ and RuSr2(Gd0.75Ce0.25)2Cu2O10-δ were synthesized through a solid-state reaction route from stoichiometric amounts of RuO2, SrO2, Gd2O3, CeO2 and CuO. Calcinations were carried out on mixed powders at 1000 oC, 1020 oC and 1040 oC for 24 hours at each temperature with intermediate grindings. The pressed bar-shaped pellets were annealed in a flow of oxygen at 1075 oC for 40 hours and subsequently cooled slowly over a span of another 20 hours down to room temperature. These samples are termed as “as-synthesized”. Part of the as-synthesized samples were further annealed in high-pressure oxygen (100 atm) at 420 oC for 100 hours and subsequently cooled slowly to room temperature. These samples are termed as “100-atm O2-annealed”. Though the heat treatments used for the samples in our study are in general similar to those as reported in literature [1,2,8,9,21-23], minor differences do exist from one laboratory to another in terms of annealing hours and the temperatures used. Also, it has been reported that not always all samples of the same batch with similar heating schedule show superconductivity [14,27]. Our general experience is also the same particularly for Ru-1212, in which achieving superconductivity seems to be a tricky job. Worth mentioning is the fact that for both Ru-1212 and Ru-1222, single-phase samples are achieved only for R = Gd, Sm and Eu, with the normal heating schedules mentioned above. For R = Y and Dy, etc., one needs to employ the HPHT (high-pressure high-temperature) procedure for attaining the Ru-1212 phase [7,13].

Thermogravimetric (TG) analyses (Perkin Elmer: System 7) were carried out in a 5 % H2/95 % Ar atmosphere at the rate of 1 oC/min to investigate the oxygen non-stoichiometry. X-ray diffraction (XRD) patterns were collected at room temperature (MAC Science: MXP18VAHF22; CuKα radiation). Magnetization measurements were carried out on a superconducting-quantum-interference-device (SQUID) magnetometer (Quantum Design: