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

Figure14 shows the R-T plot and magneto-transport behaviour for a 100-atm O2-annealed Ru-1212 sample. Interestingly no R = 0 is observed only a Tconset is observed around 20 K. Other characteristics in terms of Thump in R-T at around 140 K and the systematic changes in MR with applied field and T are the same as for the as-synthesized sample.

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Fig.14. R - T and magneto-transport for 100-atm O2-annealed Ru-1212 sample.The inset shows the

MR behaviour of the same.

B. Ru-1222 samples

Figure 15 shows the R-T behaviour in zero and 7 T fields for as-synthesized Ru-1222. The R-T behaviour in zero field is metallic down to 270 K and semiconducting below 270 K until superconductivity starts with Tconset at 30 K and TcR = 0 at 23 K. The R-T behaviour under an applied field of 7 T is nearly the same as that of 0 T above Tconset. However in 7 T field TcR = 0 is not observed down to 5 K. In an intermediate field of 3 T, Tconset and TcR = 0 were seen at 25 and 15 K, respectively. Also seen is a shoulder in the R-T curve in 3 T, the origin of which is not known.

In the inset of Fig. 15, the MR data of the same Ru-1222 sample is shown at various temperatures and fields, revealing a small negative MR effect in the whole temperature range. Below 100 K the degree of MR is nearly the same in all applied fields and the nature of the MR effect is of the tunnelling-magneto-resistance (TMR) type as judged from the