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

ferromagnetic behaviour. In fact no ZFC - FC branching is observed down to 5 K in 1,000 and 10,000 Oe fields where both the anomaly and the irreversibility in ZFC and FC branches look to be washed out, see inset in Fig. 10. The down-turn cusp at 85 K in low fields is indicative of antiferromagnetic or spin-glass nature of Ru spins. The FC curve is seen increasing or saturating due to the contribution from paramagnetic Gd spins.

6050

χ [emu/mol Oe]

403020100

T [K]

Fig. 10. χ-T plot for an as-synthesized RuSr2(Gd0.75Ce0.25)2Cu2O10-δ sample. The inset shows the

same at higher fields.

To further elucidate the magnetic property of as-synthesised Ru-1222 we show its isothermal M-H behaviour at various temperatures in Fig. 11. The isothermal magnetization as a function of magnetic field may be viewed as previously given equation (1), where χH is the linear contribution from antiferromagnetic or spin-glass Ru spins and also from paramagnetic Gd spins, and σs(H) represents the ferromagnetic component of Ru spins. The appearance of the ferromagnetic component at low temperatures within antiferromagnetic/spin-glass Ru spins could happen due to a slight canting of the spins, as seen from neutron diffraction data for another similar magneto-superconducting Ru-1212 [6-9]. The contribution from the weak ferromagnetic component starts to appear below 150 K. At this temperature the M-H plot remains purely linear. The saturation of the isothermal