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

O2-annealed sample also with nearly same non-linearity field. Summarily, it has been possible to enhance the superconductivity in Ru-1222 by 100-atm O2-annealing.

40

χ [emu/mol Oe]

3020100T [K]

Fig. 12. χ-T plot for a 100-atm O2-annealed RuSr2(Gd0.75Ce0.25)2Cu2O10-δ sample.

A. (Superconducting) Ru-1212 samples

3.6 Electrical transport properties: PPMS results

Figure 13 shows the resistance (R) versus T for an as-synthesized (superconducting) RuSr2GdCu2O8-δ sample in zero, 3 and 7 T applied fields. The R-T behaviour without any applied magnetic field is metallic down to 150 K and semiconducting between 150 K and 25 K, with a superconducting transition onset (Tconset) at 25 K and R = 0 at 20 K. This behaviour is typical of underdoped HTSC compounds. Also observed is an upward hump (Thump) in R-T around 140 K, which indicates the possibility of antiferromagnetic ordering of spins. The R-T behaviour under an applied field of 7 T is nearly the same above Tconset, except that Thump is completely smeared out due to possible change in the magnetic structure. Also in 7 T applied field the Tconset decreases to around 10 K and R = 0 is not observed down to 5 K.

In an intermediate field of 3 T, both Tconset and TcR = 0 decreased, to 20 and 10 K respectively. For conventional HTSC, Tconset remains nearly the same under all possible applied fields, with decreasing R = 0 temperature and an increased transition width (Tconset – Tc R = 0). Therefore, a different type of broadening of the transition under a magnetic field is obtained for Ru-1212 from that reported for conventional HTSC. In earlier reports on Ru-