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

3.4 Superstructures: SAED results

A. As-synthesized Ru-1212 sample

Figure 5 depicts a selected-area electron diffraction (SAED) pattern of the a-b plane for a superconducting, as-synthesized RuSr2GdCu2O8-δ sample. Two superstructures are seen: one weak spot at the centre of the a-b rectangle and the other along the b direction. Earlier microstructural study on samples of the Ru-1212 phase showed only one superstructure along the a-b plane, that is, the spot at the centre of the a-b rectangle. This was interpreted as being due to tilting of the RuO6 octahedra [33]. For the sample presently studied showing the additional superspot along the b direction there is a possibility that either Ru/Cu or vacancy ordering of 2b periodicity is taking place along the b direction. It thus seems that superconducting clusters of composition (Ru1-xCux)Sr2GdCu2O8-δ may be present within the RuSr2GdCu2O8-δ phase giving rise to SNS/SIS (superconductor-normal metal-superconductor/superconductor-insulator-superconductor) junctions in the resulting material. This would explain the different type of broadening of resistive transition under a magnetic field for many Ru-1212 samples [10,11,14,17,25] along with the present one, see section 3.5. Though the possible presence of SNS/SIS junctions in various Ru-1212 samples could be strictly sample dependent, we argue that all reported Ru-1212 magneto-superconducting samples may not be homogenous in composition, but Ru/Cu ordering at the charge-reservoir cation site may be present in some of them. Also we would like to mention that we also get the b-direction clean a-b plane with only RuO6 tilting superstructure in the same sample. Electron diffraction pattern from the a-c plane of the same sample is seen clean without any superstructures.

B. As-synthesized Ru-1222 sample

Figure 6 shows a SAED pattern from the a-c plane of an as-synthesized RuSr2(Gd0.75Ce0.25)2Cu2O10-δ sample. No superstructures are seen. The c-parameter length

Fig. 5. SAED pattern for an as-synthesized Ru-1212 sample taken with incident beam // c.