The macroscopic magnetizations of a RuSr 2 (Eu 0.7 Ce 0.3 ) 2 Cu 2 O 10ϩ␦ sample were investigated. A ferromagnetlike transition occurs around T M in the low-field magnetization. Highly nonlinear M (H), non-Curie-Weiss susceptibility, and slow spin dynamics, however, were observed up to T 1 Ϸ(2 -3)T M . In addition, an antiferromagnetlike differential-susceptibility maximum of Ru appears around a separate temperature T AM between T 1 and T M . The data are therefore consistent with a phase-separation model: superparamagnetic clusters ͑or short-range spin orders͒ are first precipitated from the paramagnetic matrix below T 1 , followed by an antiferromagnetic transition of the matrix at T AM and an apparent ferromagnetic ͑FM͒ transition around T M , where the long-range spin order is established in the FM species embedded in the matrix.

The hole concentration p(␦), the transition temperature T c , the intragrain penetration depth , and the Meissner effect were measured for annealed RuSr 2 (Gd,Ce) 2 Cu 2 O 10ϩ␦ samples. The intragrain superconducting transition temperature T c varied from 17 to 40 K while the p changed by only 0.03 holes/CuO 2 . The intragrain superfluid density 1/ 2 and the diamagnetic drop of the field-cooled magnetization across T c ͑the Meissner effect͒, however, increased more than 10 times. All of these findings are in disagreement with both the T c vs p and the T c vs 1/ 2 correlations proposed for homogeneous cuprates, but are in line with a possible phase separation and the granularity associated with it. underdoped nature ͑i.e., the hole concentration pӶ0.16 holes/CuO 2 ) or the competition between the SC and the coexisting FM through the Fulde-FerrellLarkin-Ovchinnikov ͑FFLO͒ phase. 1-5 A verification was difficult until the recent reports that the transition temperatures T c of Ru1222 and Ru1212 can be adjusted over a broad range by oxygen annealing and Cu substitution, respectively. [6][7][8] The reported data, unfortunately, show that the T c enhancement is accompanied by both a p increase and a suppression of the ferromagnetic spin order and offer no clear distinction. To explore the topic, we measured the intragrain T c , the p, the intragrain superfluid density 1/ 2 , and the Meissner effect in several annealed Ru1222 samples. The data were then compared with both the T c vs p correlation 9 and the T c vs 1/ 2 line proposed. 10 We observed that a twofold enhancement of T c ͑from 17 to 40 K͒ is accompanied by a relatively small change of p ͑from 0.09 to 0.12 holes/CuO 2 ), but a 20-fold increase in 1/ 2 ͑from 0.3 to 6 m Ϫ2 ). Together with an extremely large field effect of dT c /dHϾ100 K/T and a linear increase of the Meissner fraction with 1/ 2 , the data suggest that those ruthenocuprate grains are actually Josephson-junction arrays ͑JJA's͒, in agreement with the phase-separation model suggested. 11 Further investigations of local magnetic structures are needed to solve the problem.Ceramic RuSr 2 (Gd 0.7 Ce 0.3 ) 2 Cu 2 O 10ϩ␦ samples were synthesized following the standard solid-state-reaction procedure. Precursors were first prepared by calcined commercial oxides at 400-900°C under flowing O 2 . Mixed powder with a proper cation ratio was then pressed into pellets and sintered at 900°C in air for 24 h. The final heat treatment of the ceramics was done at 1090°C for 60 h after repeatedly sintering and regrinding at 1000°C. Powders with different particle sizes were prepared according to the procedures previously reported. 12 The structure of the samples was determined by powder x-ray diffraction ͑XRD͒, using a Rigaku DMAX-IIIB diffractometer. The XRD pattern of a typical sample is shown in Fig. 1. Refinement was done based on a space group of I4/mmm with lattice parameters of aϭ3.839(1) and cϭ28.591(5) using the RIETAN-2000 program. 13 There are no noticeable impurity lines in the x-ray diffraction pattern ...

An unusual thermal-magnetic hysteresis was observed between a minor magnetic transition around 120 K and the main one at 80 K in superconducting RuSr 2 (R,Ce) 2 Cu 2 O 10−δ (Ru1222R) samples, where R = Gd or Eu, down to a submicron length-scale. The observation suggests a possible phase-separation and is consistent with the very small but universal demagnetizing factor observed, which is difficult to reconcile with the canted spin-structure previously proposed. In such a scenario, the unusual superconducting properties of the Ru-based cuprates can also be understood naturally. 75.70.Cn,74.72.Jt Typeset using REVT E X 1The neutron-powder-diffraction (NPD) observation that the Ru-spins in RuSr 2 GdCu 2 O 8 (Ru1212), which is superconducting below 20-40 K, are antiferromagnetically (AFM) aligned below a magnetic transition-temperature T M ≈ 133 K makes its physics more complicated. 1,2Although canted Ru-spins have been proposed, 1,4 the interpretation seems to be debatable. On one hand, the macroscopic moment of ceramic Ru1212 samples appears as that of a ferromagnet with a remnant moment of m r ≈ 0.14 µ B /cell.3,4 On the other hand, the NPD sets an upper limit of only 0.1 µ B /cell for the ferromagnetically (FM) aligned spins, M. 1,4Since M should be larger than 2m r in a ceramic, 5 the two measurements are difficult to reconcile with each other as canted spins. A possible way to accommodate these conflicting values is the existance of mesoscopic phase-separations. Previously, such a possibility has been disregarded based on a µSR measurement.3 We would like to point out, however, that the volume-fraction ≈ 2m r /m o of the possible FM species may be well below the experimental uncertainty of 20% with the Ru moment m o > 3 µ B /cell.3,6 A similar debate in manganites further demonstrates that it is difficult to rule out phase-separations with only a few measurements.7 A re-examination of the Ru-based cuprates, therefore, is warranted. It should be noted that the magnetic configuration of Ru1212 is directly related to its superconductivity (SC). While "ordinary" SC may coexist with canted AFM, a phase-separation will result in the appearance of Josephson-junction arrays as previously proposed.8 The severe granularity of Ru1212 could thus be understood, 3,9,10 similar to the heterogeneous magnetic structure of ErRh 4 B 4 . 11As part of our effort, the thermal-magnetic hysteresis of superconducting RuSr 2 R 2−x Ce x Cu 2 O 10−δ (Ru1222R) with R = Gd or Eu and x between 0.6 and 0.7, whose magnetic structure is expected to be similar to that of Ru1212, 10,12 is studied. Our data confirm that there are minor weak-ferromagnetic transitions at T M 2 ≈ 120 K and T M 3 ≈ 140 K above the main transition at T M 1 ≈ 80 K, as reported previously.10 A dipole-dipole AFM interaction is further detected between the T M 1 and the T M 2 FM-species with a universal strength down to a length scale far smaller than the grain size, i.e. these two FM-species should coexist in a single structural grain. Furthermore, the strengt...

We report resistivity data on ceramic RuSr 2 Gd 2Ϫx Ce x Cu 2 O 10Ϫ␦ which is the first Cu-O based system in which superconductivity ͑SC͒ in the CuO 2 planes and weak-ferromagnetism ͑W-FM͒ in the Ru sublattice are considered to coexist. Due to the granular nature of the materials the critical current density at 5 K is extremely small as compared to other high-T C superconducting materials. Below the superconducting transition (T C ), the magnetoresistance ⌬R(H)ϭR(H)-R(0) is positive and unexpected hysteresis loops are observed. The hysteresis loop is completely different than that observed in conventional homogeneous superconductors. ⌬R(H) on decreasing the applied field (H ext ) is much smaller than the ⌬R(H) for increasing (H ext ). The width of the loops depends strongly on the weak-link properties. Similar hysteresis loops are observed in the reference nonmagnetic NbSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10 which is SC at T C ϭ28 K, thus the possibility that the hysteresis phenomenom is caused by the coexistence of both SC and magnetic states is excluded.

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