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, Cau
Trang 1Ferromagnetism and superconductivity in RuSr 2 RCu 2 O 8
(R Sm, Eu, Gd) D.P Haia,b,*, S Kamisawaa, I Kakeyaa,c, M Furuyamaa, T Mochikud,
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 RuSr2RCu2O8 (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
RuSr2GdCu2O8 with Ts 36 K and Tc 136 K Huge resistivity broadening phenomena common to the high Tc su-perconductors were observed in magnetic ®elds, which suggest that strong enhancement of superconducting ¯uctua-tions 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 h; 74.62.Bf
Keywords: Ferromagnetism; Superconductivity; Magnetoresistance; RuSr 2 GdCu 2 O 8
1 Introduction
Recently, much eort has been focused on the
hybrid rutheno-cuprate 1212 compounds,
espe-cially on the RuSr2GdCu2O8 compound, since
superconductivity was unexpectedly discovered at
Ts 3050 K, which coexists with ferromagnetism
occurring below Tc 136 K The ferromagnetism was considered to be due to Ru5with 4d3state 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, how-ever, clearly demonstrated that the ground state is
in fact antiferromagnetic with a small ferromag-netic 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
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* Corresponding author Address: Institute of Materials
Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8573, Japan Tel.: 53-6901; fax:
+81-298-55-7440.
E-mail address: hai@ims.tsukuba.ac.jp (D.P Hai).
0921-4534/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V.
PII: S0921-4534(01)00456-7
Trang 2synthesizing 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
2 Sample preparation and experimental
Polycrystalline RuSr2RCu2O8 (R Sm, Eu,
Gd) samples have been synthesized by solid state
reaction of stoichiometric ratio of the powders of
RuO2, SrCO3, Gd2O3, Eu2O3, Sm2O3 and CuO
For the case of RuSr2GdCu2O8, 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
re-grinding and repressing between those steps Since
for the cases of RuSr2RCu2O8 (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, RuSr2RO6 (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
an-nealed in ¯owing oxygen gas for several days
X-ray powder diraction was carried out on
RIGAKU RINT-2500 X-ray diractometer
Sam-ples were cut into the bar shape for resistivity
measurements by the standard four-probe method
Magnetic measurements were performed from 2
to 300 K by a SQUID (Quantum Design)
mag-netometer
3 Results and discussion
Fig 1 shows an X-ray powder diraction
pattern of our as-prepared samples The Rietveld
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 lattice parameters were a 3:8376 A and c 11:5694 A for R Gd, a 3:8394 A and c 11:5697 A for R Eu and a 3:8402 A and
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 tem-peratures We speculate that oxygen order may play an important role for superconductivity in Ru1212 samples This compound has the iso-structure with the well-known 123 compound (typical one may be YBa2Cu3O7 d) 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 super-conducting block and the SrORuO2 SrO block The distance between CuO2 plane and RuO2 plane is only 4.1 A and the oxygen O(2) at the SrO layer is corner shared by the CuO5 pyramid
Fig 1 The Rietveld X-ray pattern of one as-prepared RuSr 2 EuCu 2 O 8 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 11.5697 A, respectively.
Trang 3and the RuO6octahedron The local environment
as well as the interrelation between CuO5
pyra-mid and RuO6 octahedron determined in the
present study is shown schematically in Fig 2 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
ar-rangement, 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 RuSr2EuCu2O8
compound with Ts 9:5 K, which is isotope
substituted of 153Eu instead of natural Eu,
be-cause of high absorption rate of natural Eu Both
measurements agree well each other and will be
reported elsewhere [10]
Fig 3 shows the magnetoresistance of the
RuSr2GdCu2O8sample, while the inset is the ®eld
dependence of Tsof RuSr2EuCu2O8 sample With
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 -GdCu2O8 sample, superconductivity is not de-stroyed 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 phe-nomenon 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 RuSr2GdCu2O8
sample, the large diamagnetic signals due to su-perconductivity and the Meissner eect are clearly seen in all of our RuSr2RCu2O8(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 RuSr2GdCu2O8
Fig 2 Atomic coordination of part of the crystal structure
along the c-axis with CuO 5 pyramid and RuO 6 octahedron The
Ru atom is located almost at the center of the octahedron of the
six oxygens.
Fig 3 Magnetoresistance of RuSr 2 GdCu 2 O 8 sample up to 7.75
T The irreversibility line H irr de®ned by q 3:0 10 6 mX cm
is shown for RuSr 2 EuCu 2 O 8 sample in the inset.
Trang 4sample due to the ferromagnetic order of Ru
sublattice and this magnetic transition
tem-perature 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]
4 Conclusion
By applying a new synthesizing process, we have successfully achieved the bulk superconduc-tivity in RuSr2RCu2O8 with all three cases of rare earth R Sm, Eu, Gd with Toffset
c 12, 17 and 36
K, respectively, and their ferromagnetic transition temperatures are 146, 139 and 136 K
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Fig 4 ZFC and FC magnetic susceptibility curves of RuSr 2
RCu 2 O 8 (R Sm, Eu, Gd) samples FC: (a) RuSr 2 GdCu 2 O 8 ,
(b) RuSr 2 EuCu 2 O 8 , (c) RuSr 2 SmCu 2 O 8 ; ZFC: (d) RuSr 2
-GdCu 2 O 8 , (e) RuSr 2 EuCu 2 O 8 , (f) RuSr 2 SmCu 2 O 8