Evidence for possible phase separations inRuSr2(Gd,Ce)

Abstract

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...

Magnetoresistance hysteresis and critical current density in granularRuSr2Gd2−xFelner1, Galstyan2, Lorenz3 et al. 2003Phys. Rev. BSelf Cite5412230View full textAdd to dashboardCite

“…Samples of M Sr 2 Gd 2Ϫx Ce x Cu 2 O 10Ϫ␦ (xϭ0.6 and 0.7, M ϭRu and Nb͒ were prepared by a solid-state reaction technique as described elsewhere 1,2,15 The as-prepared ͑asp͒ RuSr 2 Gd 1.4 Ce 0.6 Cu 2 O 10Ϫ␦ sample was reheated for a few hours at 600°C under 300-bar pure oxygen pressure. The dimensions of this bar shaped sample are 0.3ϫ0.154 ϫ0.08 cm 3 .…”

“…At T C ϽT irr , SC is induced and both SC and WFM states coexist intrinsically on a microscopic scale. ͑B͒ A detailed analysis of the magnetization under various thermal-magnetic conditions suggests a phase separation of Ru-1222 into ferromagnetic ͑FM͒ and antiferromagnetic 15 ͑AFM͒ nanodomains species inside the crystal grains. A minor part of the material becomes FM, whereas the major part orders the AFM part then becomes SC at T C .…”

Superconductivity, intragrain penetration depth, and Meissner effect ofRuSr2(Gd,Ce)2Xue1, Lorenz2, Baikalov3 et al. 2002Phys. Rev. BSelf Cite283331View full textAdd to dashboardCite

“…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.…”

“…1,25 We, therefore, favor a mesoscopic phase separation between FM and AFM species as the origin of the granularity. 11,26 The observation of a correlation between the magnetic transition temperature and the granularity (T c , 1/ 2 , . .…”

Effect of Ca substitution on crystal structure and superconducting properties of ferromagnetic superconductor RuSr2−Ca Gd1.4Ce0.6Cu2O10−δ

“…The first one measures the physical properties of a standard sample using different techniques, which should supply complementary information sufficient to support or discard a specific model. Significant results have been obtained in this way, although sometimes there have been contradictions in results between different groups [8][9][10][11][12]. A second approach is the investigation of chemically altered compounds, where by proper chemical substitutions one can control relevant parameters that affect the sample's properties and, in this way, get a better understanding of the mechanisms involved in it [13][14][15][16][17][18][19][20].…”