Resistivity study of RuSr2GdCu2O8 superconductor

Phys. Status Solidi A 207, No. 2, 411–416 (2010) / DOI 10.1002/pssa.200925465 p s sa

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Resistivity study of RuSr2GdCu2O8

superconductor

Rajneesh Mohan*,1, Kiran Singh1, Nupinderjeet Kaur1, Naresh Kumar1, Shovit Bhattacharya2,Neeraj K. Gaur1, Shiv K. Gupta2, and Ram K. Singh1,3

1Department of Physics, Barkatullah University, Bhopal-462 026, India2Technical Physics and Prototype Engineering Division, B.A.R.C., Mumbai-400 085, India3MATS University, MATS Tower Pandri, Raipur-492 002 (CG), India

Received 26 August 2009, revised 6 September 2009, accepted 7 September 2009

Published online 13 October 2009

PACS 74.25.Fy, 74.62.Bf, 74.62.Dh, 74.72.Jt

* Corresponding author: e-mail [email protected], Phone: þ91 755 2491821, Fax: þ91 755 2491821

The superconducting properties of RuSr2GdCu2O8 (Ru-1212)

superconductor strongly depend on the processing conditions.

In this report we have made an investigation on the influence of

processing method and heat treatments on the superconducting

properties, as well as normal state resistivity of RuSr2GdCu2O8

(Ru-1212) compound. The samples were prepared using solid-

state reaction of high purity oxides and carbonates of

constituents cations through direct method, precursor method,

and standard method used for the synthesis of Ru-1212

compounds. These samples were then subjected to different

annealing treatments in different atmosphere (oxygen and air)

for 120 h. They have been investigated by X-ray diffraction, dc

resistivity and scanning electron microscopy. The normal state

resistivity has been studied on the basis of various conduction

mechanisms.

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction It is commonly believed that super-conductivity and magnetism are two phenomena thatgenerally do not coexist together. The magnetic field causethe superconductivity to disappear. However, co-existenceof superconductivity and magnetism can be achievedthrough spatial separation of the superconducting andmagnetic sub-lattices. In recent years, special attention hasbeen devoted to compounds, in which magnetism andsuperconductivity coexist. The cuprate compound thatexhibit this exotic behaviour is RuSr2RCu2O8 (Ru-1212),with R¼ Sm, Eu or Gd, was first synthesized and recognizedas superconductors in 1995 [1]. In 1997 Felner et al. [2]discovered a related superconductor, RuSr2R1.4Ce0.6-Cu2O10-d (R¼Eu or Gd). Ru-1212 exhibit ferromagneticorder at Curie temperatures well above the superconductingtransition temperature (Tcurie� Tc) [3, 4]. The magnetismpersists in the superconducting state in these compounds.The tetragonal structure of the Ru-1212 is similar to that ofYBa2Cu3O7-d, with Ru replacing the Cu atoms located in thecharge reservoir layer in their crystallographic sites [5].

In these compounds the establishment of the super-conducting state strongly dependent on the sample synthesisand annealing conditions [6–12]. A range of values

(15–50K) has been reported in literature for Tc and therehave also been reports on non-occurrence of superconduc-tivity in this compound even following the similar methodfor sample preparation [8–10]. The properties of Ru-1212superconductor are extremely sensitive to the detail thepreparation conditions. Different studies have reporteddifferent type of resistivity behaviour in normal state. Asresistivity is fundamental property of a superconductingmaterial. Thus, we have made an attempt to study resistivitybehaviour of Ru-1212 compound by synthesizing it throughdifferent techniques as well as changing the processingconditions like annealing and duration of annealing. Theresistivity mechanism is studied by fitting the observedresistivity data to various conduction mechanisms.

2 Experimental procedure The polycrystallinesamples of RuSr2GdCu2O8 (Ru-1212) system have beensynthesized through three routes (i) direct solid state reactionmethod (sample A), (ii) precursor method (sample- B) and(iii) method reported in Ref. [3] (sample- C). The sample Awas prepared from stoichiometric mixture of RuO2, SrCO3,Gd2O3, and CuO. The mixture was heated at 960, 1020,1040, and 1060 8C each for 24 h with intermediate grindings


412 R. Mohan et al.: Resistivity study of RuSr2GdCu2O8 superconductorp

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Figure 1 XRD patterns of RA0, RA1 and RA2 samples.

and pellatization. This sample was designated as RA0. Thesample B was prepared following a two-step procedure.Firstly, Sr2GdRuO6 (Sr-2116) was prepared from stoichio-metric mixture of SrCO3, RuO2 and Gd2O3, which werecalcined at 950 8C in air, ground, pressed into pellets andfired for 16 h at 1250 8C in air. In the second step, theobtained Sr-2116 was mixed with CuO in the molarproportion of 1:2. After thorough grinding, this mixturewas pressed into pellets of 12mmdiameter. This sample wasdesignated as RB. In the synthesis of sample C an additionalheat in argon gas flow at 1010 8C was done after thecalcinations at 960 8C for the suppression of SrRuO3

impurity phase [3]. The pellets of sample C samples werefired at 1050 8C for 48 h and named as RC. The samples RA0,RBwere then heated at 1060 8C for 120 h and were named asRA1 and RB1, respectively. For studying the effect ofannealing duration on Ru-1212, we have heated some pelletsof sample A andB at 1060 8C for 120 h in air andwere namedas RA2 and RB2, respectively. The various heat treatmentsof samples are summarized in Table 1.

All the samples in the present investigation weresubjected to the structural characterization by X-ray powderdiffraction technique (XRD, Phillips PW 1710, Cu Ka), lowtemperature electrical transport measurement using thestandard four-probe method using a closed cycle heliumcryostat. The grain morphology of the fractured surface ofthe samples was analyzed by scanning electron microscopy.The results thus obtained are presented and discussed in thenext section

3 Results and discussion The X-ray patterns of thesamples of Ru-1212 prepared through direct solid-statereaction method (RA0, RA1 and RA2), precursor route(RB1 and RB2) and standard method (RC) are as shown inFigs. 1–3, respectively.

All the samples can be indexed with tetragonal structurein P4/mmm space group using lattice parameters a¼ 3.838(A) and c¼ 11.55 (A) [1, 13]. As can be seen, no evidentdifference, except a small impurity peak of SrRuO3 at 31.58in samples RA0 and RA1 is observed in the XRD patterns.This peak has also been observed for samples RB1 and RB2,while in XRD of sample do not show such peak. The smallimpurity peak has also been reported by other [1]. Thesample RA2 does not show any impurity peak, which impliesthat thermal treatment for a long duration in air suppresses

Table 1 Summary of preparation conditions used for preparedsamples.

Sample Heat treatments Final heat treatment

RA0 960 8C, 1020 8C, 10408C for 24 h each 1060 8C for 24 h

RA1 960 8C, 1020 8C, 1040 8C for 24 h each 1060 8C for 60 h

RA2 960 8C, 1020 8C, 1040 8C for 24 h each 1060 8C for 120 h

RB1 950 8C/24 h, 1250 8C for 16 h 1060 8C for 60 h

RB2 950 8C/24 h, 1250 8C for 16 h 1060 8C for 120 h

RC 960 8C, 1010 (in Argon) for 24 each 1050 8C for 48 h


the impurity phases. The calculated lattice parameters of thesamples RA0, RA1, RA2, RB1, RB2 andRC are presented inTable 2. The lattice parameters of these samples are in goodagreementwith the results obtained by other authors [13, 14].

It can be noted that these thermal treatment in differentatmosphere have very slight effect on the lattice parametersofRu-1212 compound.With the duration of annealing the ‘a’parameters decreases slightlywhile ‘c’ parameters increases,irrespective of the annealing atmosphere.

The SEM images of the samples RA0, RA1, RA2, RB1,RB2 and RC are shown in Fig. 4. These images show anobvious effect of heat treatments and processing route on thegrain morphology of the samples. Sample RA0 is porous,RA1 is granular and sample RA2 is dense. It can beconcluded from the SEM images that with the duration of theheat treatment the density and thus strength of the sampleincreases considerably.

The scanning electron micrographs of samples RB1 andRB2 reveal that the morphology of the two samples clearlydifferent, the sample RA1 exhibits well-defined grains andpronounced boundaries, while the sample RB2 showssponge type grain boundaries. The SEM images of sampleRC show granular nature of the sample.

The resistivity-temperature (r –T) behaviour of thesample RA1, RB1 and C is as shown in Fig. 5.

These samples show metallic behaviour down to 150Kand semiconducting between 150K and the onset of

Table 2 The calculated lattice parameters a, c and unit cellvolume of prepared samples.

Sample a(A) c(A) V(A3)

RA0 3.836 11.531 169.68RA1 3.834 11.535 169.56RA2 3.833 11.568 169.96RB1 3.835 11.541 169.74RB2 3.832 11.572 169.93RC 3.835 11.534 169.63

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Figure 4 SEM images of (a) RA0, (b) RA1, (c) RA2, (d) RB1,(e) RB2 and (f) RC samples on given scale of 20mm.

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Figure 2 XRD patterns of RB1 and RB2 samples.

superconductivity (Tco) at 46, 28 and 45K for the samplesRA1, RB1 and C, respectively. This behaviour is consistentwith the earlier reported results [3, 13] with slight differencein themagnitude of transition temperature. For analyzing thetemperature dependence of resistivity, we have plotted thedr/dT versus T for sample RA1 in the Fig. 6(a). In this figureone can clearly observe two superimposed maxima at 30 and23K, in the superconducting transition region and a smallpeak around 130–140K. For making the distinction, dr/dTversus T is plotted in two different regions 1–60 (super-conducting transition region) as Fig. 6(a) and 100–200K(magnetic transition region) as Fig. 6(b). The maxima inFig. 6(b) indicate that the transition proceeds in two steps andhave been interpreted as intergrain and intragrain SC-transitions, respectively [15–18]. The cause of this observa-tion is attributed to the granular nature of samples [17, 18]. Along tail of the dr/dT appears at the high-temperature sidecrosses zero around 50K, which may mark the thermody-namics Tc. The derivative plot in magnetic transitionregion clearly show as a pronounced peak with maximaat TM¼ 136K. This is anomaly attributed to the reduction

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of spin scattering, where a conduction electron scattersby exchanging spin with magnetic moment or spinexcitation [19].

In the r-T behaviour of Ru-1212 sample, one can see aresistivity minima around 60–70K, a small hump around130–140K, a linear behaviour at higher temperature. In the

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Figure 5 The temperature dependence of resistivity plots for sam-ples RA1, RB1 and RC.



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Figure 6 The derivative of resistivity (dr/dT) of sample RA1 intemperature range of 20K<T< 300K, (a) in superconductingtransition region and (b) in magnetic transition region.


temperature range 150 –270K the r-T behaviour of thesamples RA1, RB1 and RC can be best fitted with Eq. (1)

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r ¼ r0 þ r1T : (1)

Here r0 is the residual resistivity and r1 is the resistivityslope in the normal state. Eq. (1) presents the usualbehaviour of resistivity in the optimally or near to optimallydoped cuprates [20]. There are many different explanationsgiven for this behaviour in literature [21–23]. The observeddata along with the linear fit line for the samples RA1, RB1and RC are presented in Fig. 7. The values of fittingparameters r0 and r1 for samples RA1, RB1 and RC arepresented in the Table 3. One can observe from this figurethat at temperature above �270K, the experimentallyobserved values deviate from the linear behaviour. Below150K the samples could not be fitted with the Eq. (1). Thismean the origin of resistivity below this temperature isdifferent from that at higher temperature. The differentmechanism responsible for this could be variable rangehopping (VRH) [24], activation behaviour [24], Kondoeffect [25] etc. Our data for these sample could not be fitted

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Figure 7 The resistivity (r) versus temper-ature (T) plot for samples RA0, RA1, RB1 andRC in the temperature range (a) 60–130K and(b) 150–300K.The solid lines are the best fit ofthe experimental data to Eq. (1) in (a) andEq. (2) in (b), respectively.

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Table 3 The fitting coefficients of Eqs. (1) and (2) for samplesRA0, RA1, RB1 and RC.

sample in temperaturerange of150–270K

in temperaturerange of 60–130K

r0(V cm)

r1(Vcm/K)

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(Vcm/K)

RA1 0.0463 2.03E-4 0.114 1.35 0.0136RB1 0.0445 1.88E-4 0.115 1.29 0.0141RC 0.0489 1.25E-4 0.08 1.12 0.0074

with VRH and activation behaviour. Thus, we can rule outthe possibility of VRH and activation behaviour for thesesamples. Our data could be best fitted with the equationgiven below in the temperature range of 60–130K.

FiguRA2resisregi

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r� ¼ r�0 þ r�1T2 � r�2 lnðTÞ: (2)

The T2 in Eq. (2) may be attributed to electro-electroninteraction. The presence of –ln(T) term in Eq. (2) indicatesthat the resistivity behaviour appears to be due to eitherKondo effect [25], weak localization [26, 27] or interactioneffect [28]. However, Kondo effect appears to be unlikelybecause according toWuyts et al. [20]. Kondo temperature incuprates is very small, of order of mK. The observedbehaviour may therefore most reasonably be attributed tolocalization arising due to weak localization [26, 27], orinteraction effect [28].

The temperature dependence of the electrical resistivity(r) for samples annealed in air, i.e. RA2 andRB2 samples areshown in Fig. 8. The sample RA2 shows a hint of onset ofsuperconductivity around 25K while sample RB2 shows nohint of superconductivity until 13K. The resistivity behav-iour of these samples is similar to that observed byAbtal et al.

re 8 Dependence of the electrical resistivity (r) for samplesand RB2 on temperature. Inset: comparison of derivative oftivity (dr/dT) of samples RA2 and RB2 in magnetic transitionon.

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[19] and Matsubara et al. [29]. There is a small kink in theresistivity at around 134K of both the samples, this anomalyis attributed to the reduction of spin scattering, where aconduction electron scatters by exchanging spin withmagnetic moment or spin excitation [19]. To explore thiskink we have plotted the derivative of the resistivity versustemperature curve, which shows a peak maximum at about134K in both the samples inset of Fig. 8. If we compare thelattice parameter of the superconducting sampleswith that ofthe non-superconducting ones (see Table 2), we find that thec axis of the non-superconducting samples is larger than thatof the superconducting samples. Therefore, the unit cellvolume of non superconducting samples is also larger thansuperconducting sample of the Ru-1212.

4 Conclusions The RuSr2GdCu2O8 samples havebeen synthesized through direct solid-state reaction, pre-cursor routes and standard method used by others. Thesamples synthesized through standard route are purely singlephase, while the other samples contains small amount ofimpurity phases. The resistivity– temperature behaviour isconsistent with the earlier reported results [4], although themagnitude of transition temperature is some what different.The sample annealed for 120 h in air is non-superconductingand their resistivity behaviour is similar to that as observed inRefs. [19, 29]. The non-superconducting sample has thelarger c axis and the unit cell volume than that of thesuperconducting sample of Ru-1212. The observed resis-tivity behaviour of all the samples at low temperature (60–130K) can be attributed to localization arising due to weaklocalization or interaction effect. The high temperature(150–270K) resistivity behaviour is consistant with othercuprate superconductors.

Acknowledgements The authors are thankful to UniversityGrants Commission, NewDelhi andMPCST, Bhopal for providingthe financial support for this work. These authors are thankful toDr. V. Shelke and Dr. S. Rayaprol for useful discussion.

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Resistivity study of RuSr2GdCu2O8 superconductor

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