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

indicate such a possibility [6-9]. Non-linearity in M-H appears at high fields above 3 T. The M-H loop for the sample is shown in the lower inset of Fig. 9.

χ [em u/gram-Oe]

1.2x108.0x104.0x10

-2

-3

-3

0.0-4.0x10

-3

T [K]

Fig. 9. χ-T plot for an as-synthesized, superconducting RuSr2GdCu2O8-δ sample. The insets show

the M-H plots for the same.

C. Ru-1222 samples

Figure 10 shows the magnetic susceptibility χ-T behaviour in the temperature range of 5 to 160 K for an as-synthesized RuSr2(Gd0.75Ce0.25)2Cu2O10-δ sample under applied fields of 5, 10 and 100 Oe, measured in both ZFC and FC modes.

In an applied field of 5 Oe, the ZFC and FC curves start branching at around 140 K with a sharp upward turn at 100 K. The ZFC branch shows further a cusp at 85 K and a diamagnetic transition around 30 K. This is in agreement with earlier reports [1,20,21,23].

The ZFC curve does not show any diamagnetic transition in applied fields of 10 and 100 Oe, but the transition is marked with a change in the slope of the ZFC curves. As the field strength exceeds a certain threshold value the positive contribution from both Gd and Ru moments overcomes the negative contribution from superconductivity to the magnetic susceptibility. Interestingly the ZFC - FC branching temperature of 140 K in 5 Oe field decreases to around 60 K in an applied field of 100 Oe. This can be considered as a weak