Physica C 357±360 (2001) 406±409 www.elsevier.com/locate/physc Ferromagnetism and superconductivity in RuSr2RCu2O8 (R Sm, Eu, Gd) D.P Hai a,b,*, S Kamisawa a, I Kakeya a,c, M Furuyama a, T Mochiku d, K Kadowaki a,c a Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan b Faculty of Physics, Hanoi National University, Km 8, Quanhoa, Caugiay, Hanoi, Viet Nam c CREST, Japan Science and Technology Corporation (JST), Japan d National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Received 16 October 2000; accepted 15 December 2000 Abstract Polycrystalline single phase RuSr2 RCu2 O8 (R Sm, Eu, Gd) samples have been synthesized by a new solid state reaction We found superconductivity in two rare earths R Sm and Eu with Ts R0 12 and 17 K, whereas ferromagnetism occurs at Tc 146 and 139 K in these samples, respectively, in addition to the previously reported RuSr2 GdCu2 O8 with Ts 36 K and Tc 136 K Huge resistivity broadening phenomena common to the high Tc superconductors were observed in magnetic ®elds, which suggest that strong enhancement of superconducting ¯uctuations due to magnetic ®eld, suppresses the occurrence of superconducting phase coherence in these compounds Ó 2001 Published by Elsevier Science B.V PACS: 74.72.Àh; 74.62.Bf Keywords: Ferromagnetism; Superconductivity; Magnetoresistance; RuSr2 GdCu2 O8 Introduction Recently, much eort has been focused on the hybrid rutheno-cuprate 1212 compounds, especially on the RuSr2 GdCu2 O8 compound, since superconductivity was unexpectedly discovered at Ts 3050 K, which coexists with ferromagnetism * Corresponding author Address: Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Tel.: +81-298-53-6901; fax: +81-29855-7440 E-mail address: hai@ims.tsukuba.ac.jp (D.P Hai) occurring below Tc $ 136 K The ferromagnetism was considered to be due to Ru5 with 4d3 state by the nominal chemical charge balance and indeed the high temperature magnetic susceptibility shows a ferromagnetic exchange interaction with TH 138 K The recent neutron scattering study, however, clearly demonstrated that the ground state is in fact antiferromagnetic with a small ferromagnetic component up to the limit of $0.1 lB /Ru below Tc The origin of such small spontaneous moment is not clear at this moment [1±4] The appearance of bulk superconductivity at low temperatures was also initially criticized [5], which later turned out to be only the problem of 0921-4534/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V PII: S - ( ) 0 - D.P Hai et al / Physica C 357±360 (2001) 406±409 407 synthesizing processes [5±8] It is peculiar that the superconducting sample can be made only after long time annealing (30±40 h) at high temperature in an oxygen ¯ow Moreover, it is rather dicult to synthesize single phase samples without possible impurity phase of SrRuO3 , which is known to be ferromagnetic at Tc 165 K [9] Here we introduce our new solid state reaction route which has made possible to achieve superconductivity in not only the case of Gd and Eu but also Sm for the ®rst time according to our knowledge Sample preparation and experimental Polycrystalline RuSr2 RCu2 O8 (R Sm, Eu, Gd) samples have been synthesized by solid state reaction of stoichiometric ratio of the powders of RuO2 , SrCO3 , Gd2 O3 , Eu2 O3 , Sm2 O3 and CuO For the case of RuSr2 GdCu2 O8 , raw materials were ®rst reacted at 960°C in air for 24 h, then sintered at 1050°C, 1055°C, 1060°C and 1065°C in ¯owing oxygen gas for periods of 12 h with thoroughly regrinding and repressing between those steps Since for the cases of RuSr2 RCu2 O8 (R Sm and Eu) samples, it is much more dicult to suppress the magnetic impurity SrRuO3 , which not only aects on the magnetic measurements but also hinders the superconductivity, a new synthesizing process has been developed Firstly, RuSr2 RO6 (R Sm and Eu) compounds were synthesized at 820°C in air, then the product was mixed with 2CuO and sintered at 1040°C, 1045°C, 1050°C and 1055°C in ¯owing oxygen gas In order to achieve superconductivty, all the as-prepared samples have been further annealed in ¯owing oxygen gas for several days X-ray powder diraction was carried out on RIGAKU RINT-2500 X-ray diractometer Samples were cut into the bar shape for resistivity measurements by the standard four-probe method Magnetic measurements were performed from to 300 K by a SQUID (Quantum Design) magnetometer Results and discussion Fig shows an X-ray powder diraction pattern of our as-prepared samples The Rietveld Fig The Rietveld X-ray pattern of one as-prepared RuSr2 EuCu2 O8 samples All lines observed are indexed to be the proper tetragonal crystal structure with space group P4/ mmm The lattice parameters for a and c axes are 3.8394 and respectively 11.5697 A, re®nements revealed that annealing in ¯owing oxygen gas for a long time improves the phase purity and the impurity had ®nally been reduced to less than 1% level with Rp value of 4.14% The and c lattice parameters were a 3:8376 A and c 11:5694 A for R Gd, a 3:8394 A and 11:5697 A for R Eu and a 3:8402 A c 11:5701 A for R Sm From the analysis of the X-ray patterns, our superconducting and nonsuperconducting samples are indistinguishable and superconductivity could only be obtained in samples annealed for a long time at high temperatures We speculate that oxygen order may play an important role for superconductivity in Ru1212 samples This compound has the isostructure with the well-known 123 compound (typical one may be YBa2 Cu3 O7Àd ) Since Ru has higher valency than Cu, no oxygen vacacies in the Ru1212 structure would be expected, forming the stacking of layers of the CuO2 GdCuO2 superconducting block and the SrORuO2 SrO block The distance between CuO2 plane and RuO2 and the oxygen O(2) at the plane is only 4.1 A SrO layer is corner shared by the CuO5 pyramid 408 D.P Hai et al / Physica C 357±360 (2001) 406±409 Fig Atomic coordination of part of the crystal structure along the c-axis with CuO5 pyramid and RuO6 octahedron The Ru atom is located almost at the center of the octahedron of the six oxygens and the RuO6 octahedron The local environment as well as the interrelation between CuO5 pyramid and RuO6 octahedron determined in the present study is shown schematically in Fig It is noted from this result that the Ru5 atom is octahedrally surrounded by six oxygens separated almost equally, resulting in the nearly cubic symmetry, while Cu2 atom is surrounded by ®ve oxygens with slightly elongated pyramid arrangement, resulting in the tetragonal symmetry We also performed neutron powder diraction at JAERI (Japan Atomic Energy Research Institute) in order to determine local structure, especially to determine local oxygen arrangement of this compound The sample is the RuSr2 EuCu2 O8 compound with Ts 9:5 K, which is isotope substituted of 153 Eu instead of natural Eu, because of high absorption rate of natural Eu Both measurements agree well each other and will be reported elsewhere [10] Fig shows the magnetoresistance of the RuSr2 GdCu2 O8 sample, while the inset is the ®eld dependence of Ts of RuSr2 EuCu2 O8 sample With Fig Magnetoresistance of RuSr2 GdCu2 O8 sample up to 7.75 T The irreversibility line Hirr de®ned by q 3:0 Â 10À6 mX cm is shown for RuSr2 EuCu2 O8 sample in the inset increasing applied magnetic ®eld, the resistance in the superconducting state shows a broadening phenomenon as commonly observed in other high Tc superconductors Despite of polycrystalline sample, this resistance behavior may represent qab and the nature of superconducting critical ®eld corresponding to the irreversibility line for the weakest direction, perhaps, for the c-axis could be deduced As it is clearly seen, in the RuSr2 GdCu2 O8 sample, superconductivity is not destroyed even at 7.75 T at low temperatures, where the magnetization data (not shown here) show the full polarization of Ru moment resulting in the induced ferromagnetic state This striking phenomenon poses a serious question as to how the supercurrent can ¯ow through the ferromagnetic RuO2 layers in this compound In contrast to several reports [5,7,8] that the Meissner phase is absent in the RuSr2 GdCu2 O8 sample, the large diamagnetic signals due to superconductivity and the Meissner eect are clearly seen in all of our RuSr2 RCu2 O8 (R Sm, Eu and Gd) samples (Fig 4) The zero ®eld cooled (ZFC) and ®eld cooled (FC) susceptibility curves start to split up at TCurie 136 K in RuSr2 GdCu2 O8 D.P Hai et al / Physica C 357±360 (2001) 406±409 409 Conclusion By applying a new synthesizing process, we have successfully achieved the bulk superconductivity in RuSr2 RCu2 O8 with all three cases of rare earth R Sm, Eu, Gd with Tcoffset 12, 17 and 36 K, respectively, and their ferromagnetic transition temperatures are 146, 139 and 136 K References Fig ZFC and FC magnetic susceptibility curves of RuSr2 RCu2 O8 (R Sm, Eu, Gd) samples FC: (a) RuSr2 GdCu2 O8 , (b) RuSr2 EuCu2 O8 , (c) RuSr2 SmCu2 O8 ; ZFC: (d) RuSr2 GdCu2 O8 , (e) RuSr2 EuCu2 O8 , (f) RuSr2 SmCu2 O8 sample due to the ferromagnetic order of Ru sublattice and this magnetic transition temperature slightly increase to 139 and 146 K for the other two cases of rare earth R Eu and Sm, respectively More detail of our magnetic study of three samples will be reported elsewhere [11] [1] C Bernhard, J.L Tallon, Ch Niedermayer, Th Blasius, A Golnik, E Brucher, R.K Kremer, D.R Noakes, C.E Sronach, E.J Ansaldo, Phys Rev B 59 (1999) 14099 [2] J.W Lynn, B Keimer, C, Ulrich, C, Bernhard, J.L Tallon, Phys Rev B 61 (2000) 14964 [3] D.J Pingle, J.L Tallon, B.G Walker, H.J Trodahl, Phys Rev B 59 (1999) 11679 [4] A.C Mc Laughlin, W Zhou, J.P Att®eld, A.N Fitch, J.L Tallon, Phys Rev B 60 (1999) 7512 [5] C.W Chu, Y.Y Xue, R.L Meng, J Cmaidalka, L.M Dezaneti, Y.S Wang, B Lorenz, A.K Heilman, condmat/ 9910056, October, 2000 [6] C Bernhard, J.L Tallon, E Brucher, R.K Kremer, Phys Rev B 61 (2000) 14960 [7] I Felner, U Asaf, S Reich, Y Tsabba, Physica C 311 (1999) 163 [8] R.W Henn, H Friedrich, V.P.S Awana, E Gmelin, Physica C 341±348 (2000) 457 [9] J.M Longo, P.M Raccah, J.B Goodenough, J Appl Phys 39 (1968) 1327 [10] T Mochiku, D.P Hai, I Kakeya, K Kadowaki, Phys Rev B, in press [11] D.P Hai, S Kamisawa, M Furuyama, I Kakeya, K Kindo, K Kadowaki, in preparation ... X-ray patterns, our superconducting and nonsuperconducting samples are indistinguishable and superconductivity could only be obtained in samples annealed for a long time at high temperatures... then sintered at 1050°C, 1055°C, 1060°C and 1065°C in ¯owing oxygen gas for periods of 12 h with thoroughly regrinding and repressing between those steps Since for the cases of RuSr2 RCu2 O8 (R. .. Firstly, RuSr2 RO6 (R Sm and Eu) compounds were synthesized at 820°C in air, then the product was mixed with 2CuO and sintered at 1040°C, 1045°C, 1050°C and 1055°C in ¯owing oxygen gas In order to