A characterization of the magnetic superconductors RuSr 2 GdCu 2 O 8 ͓Ru-͑1212͔͒ and RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10 ͓Ru-͑1222͔͒ through resistance measurements as a function of temperature and magnetic field is presented. Two peaks in the derivative of the resistive curves are identified as intra-and intergrain superconducting transitions. Strong intragrain granularity effects are observed, and explained by considering the antiphase boundaries between structural domains of coherently rotated RuO 6 octahedra as intragrain Josephson junctions. A different field dependence of the intragrain transition temperature in these compounds was found. For Ru-͑1212͒ it remains unchanged up to 0.1 T, decreasing for higher fields. In Ru-͑1222͒ it smoothly diminishes with the increase in field even for a value as low as 100 Oe. These results are interpreted as a consequence of a spin-flop transition of the Ru moments. The large separation between the RuO 2 layers in Ru-͑1222͒ promotes a weak interlayer coupling, leading the magnetic transition to occur at lower fields. The suppression rate of the intragrain transition temperature is about five times higher for Ru-͑1222͒, a result we relate to an enhancement of the two-dimensional character of the vortex structure. A distinctive difference with conventional cuprates is the sharp increase in amplitude of the intergrain peak in both systems, as the field is raised, which is ascribed to percolation through a fraction of high quality intergrain junctions.

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The 119 Sn Mössbauer spectra for the (Ru 1Ϫx Sn x )Sr 2 GdCu 2 O 8 system, with nominal compositions xϭ0.05-0.30, were measured from room temperature down to 4.2 K in samples exhibiting magnetic order followed by a resistive superconducting transition on cooling. Two contributions to the spectra are observed in the samples at room temperature: a doublet, dominating at low x, and a singlet, probably coming from the impurity phase SrSnO 3 . A part of the doublet evolves into a magnetic structure at low temperatures and is identified to sense the matrix properties. The isomer shift indicates that Sn enters into the RuO 2 layers as Sn 4ϩ . The appearance of a magnetic phase, in the spectra, matches with the onset of the Ru-sublattice long-range order, as detected by susceptibility measurements. The area of this component exhibits a smooth decrease with the increase in the Sn content, which is well described by a random distribution in the (Ru, Sn)O 2 layers. An average in-plane transferred hyperfine field of 4.29 T was sensed at 4.2 K for the sample with xϭ0.05, gradually decreasing with an increase in x. This high value may be indicative of the emergence of a -phase state for the superconducting order parameter across the magnetically ordered planes, with low pair-breaking effects.

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