Physica C 433 (2005) 14–20

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Existence of superconductivity in the magnetic stateof oxygen deficient RuSr2GdCu2O8� d (1212) system

P.K. Mishra a,*, R. Mishra b, C.L. Prajapat a, T.V. Chandrasekhar Rao a

a Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiab Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

Received 27 July 2005; received in revised form 31 August 2005; accepted 6 September 2005Available online 2 November 2005

Abstract

We have investigated the superconducting properties of an oxygen deficient RuSr2GdCu2O8� d system. We bring outa new feature from the dc magnetization measurement seen both in the field-cooled as well as in the zero-field-cooledstate. We propose to link this feature with the Meissner state of the superconducting phase. This stems from the com-plex situation arising from the simultaneous presence of signals corresponding to two different magnetic orders and ofthe diamagnetic signal of the superconducting phase. By separating out the contributions from the magnetic ordering,we find the visible presence of diamagnetic signal corresponding to the Meissner phase in this magnetically ordered sys-tem. However, signature of superconductivity, thus derived, is absent in a fully oxygenated system.� 2005 Elsevier B.V. All rights reserved.

Keywords: Meissner state; Magnetic superconductors; Rutheno–cuprates

1. Introduction

Quest for the coexistence of superconductivityand magnetic order has been there for a long time.The curiosity stems from the antagonistic natureof the two phenomena that tend to kill each other.Matthias, during his search for new superconduc-tors, pondered over this in solid solutions ofCeRu2 and GdRu2 and discovered the possibility

0921-4534/$ - see front matter � 2005 Elsevier B.V. All rights reservdoi:10.1016/j.physc.2005.09.003

* Corresponding author. Tel.: +91 22 25591660.E-mail address: pkmishra@barc.ernet.in (P.K. Mishra).

of an overlap of ferromagnetism and superconduc-tivity in them [1]. Long strides, since then, havebeen made in understanding the interplay betweenthe two phenomena in many ternary superconduc-tors like ErRh4B4, HoMo6S8 and others. Recentlydiscovered hybrid ruthenate–cuprate compoundslike RuSr2GdCu2O8 [2] possibly belong to the typewhere the magnetism and superconductivity coex-ist on a truly microscopic scale. The crystal struc-ture having a close resemblance with that ofYBCO-high-Tc superconductor is perovskite innature with the CuO chain replaced with RuO2

ed.

P.K. Mishra et al. / Physica C 433 (2005) 14–20 15

sheet. It shown that the RuO2 layer is magneticand it remains normal at all temperatures unlikehigh-Tc superconductors [9].

Though neutron scattering, muon-spin-reso-nance (lSR) and magnetic resonance studies pro-vide convincing evidence for the presence of bulkmagnetic order in this material [3,4,7], there is somekind of apprehension as regard to the presence ofbulk superconductivity in this system. In theabsence of a convincing Meissner signal, results ofdc magnetization studies do not conclusively favorthe presence of superconductivity in this system.In most of the reports, support for the presence ofsuperconductivity mainly relies on the weak dia-magnetic response observed mostly in zero-field-cooled (ZFC) state and occasionally in field-cooled(FC) dc magnetization measurements [4–6]. Inorder to account for the inconspicuous Meissnersignal, other scenarios viz., filamentary supercon-ductivity named as crypto-superconductivity [8],formation of spontaneous vortex phase have beenproposed [5]. There, it has been explained in termsof a crossover from a spontaneous vortex phase toa Meissner state. Nevertheless, a consensus on thisissue continues to be elusive.

The magnetic order in this system has a verycomplex structure as it has contributions fromboth Ru and Gd sub-lattice that align differentlyat different temperatures. Weak ferromagnetismwith transition temperature at around 140 K,observed from the temperature irreversibility offield-cooled and zero-field-cooled branches of thedc magnetizations. A canting arrangement forRu moments is proposed to account for the smallferromagnetism observed in this system [3]. How-ever, the major component of canting results inG type antiferromagnetic ordering i.e., magneticmoments on the neighboring Ru sites order anti-ferromagnetically in all three crystallographicdirections. The ferromagnetic component is only10% of the Ru moment of the size lRu � 1lB, verysmall compared to Gd moment (lGd � 7lB). Gdsublattice also orders with antiferromagnetic orderat TN � 2.5 K. Above this temperature magnetiza-tion has contributions from the paramagneticresponse of Gd sublattice as well as weak ferro-magnetism originating from the canting of Rumoments. In such a scenario, magnetic character-

ization in simultaneous presence of magnetic andsuperconducting order turns out to be a non-trivialtask. The standard diamagnetic signal correspond-ing to the superconducting state will be hard toobserve as the appearance of any weak supercon-ducting response is overwhelmed by the abovecontributions and appears as an inflection in themagnetization or susceptibility plot.

In this paper, we present our results on magne-tization measurement on an oxygen-reduced Ru-Sr2GdCu2O8� d (Ru1212) sample and show thepossibility of Meissner signal in a competing mag-netic environment. We see an inflection-like fea-ture in magnetization versus temperature plotsthat could be linked to the presence of supercon-ductivity. After separating out the magnetic con-tribution, we could extract the diamagneticMeissner phase contribution to the magnetization.Starting from a parent structure, with no sign ofsuperconductivity, we see that this feature deve-lops only after oxygen reduction. Thus, we believethat proper oxygen stoichiometry is essential toany possibility of occurrence of superconductivityin this system.

2. Sample preparation and experimental details

The hybrid rutheno–cuprate superconductorRuSr2GdCu2O8 was prepared by heating a mix-ture of RuO2, SrCO3, Gd2(CO3)3 and CuO in themolar ratio 1:2:0.5:2, respectively. The thoroughlyground mixture was initially heated at 960 �C for48 h with two intermittent grindings. The pallet-ized sample was then heated at 1040 �C underdry flowing oxygen for 24 h and allowed to coolnaturally inside the furnace under similar condi-tion. To have proper oxygen stoichiometry, partof the sample was annealed at 991 �C temperaturein a thermo-balance (Setaram, simultaneous TG,DTA, model no. 92/16.18) for about 5.5 h inflowing high pure Argon atmosphere. Thereafterit was furnace-cooled (see Fig. 1). The reasonfor choosing this temperature will be discussedlater in detail. Samples were analyzed by powderX-ray diffraction technique using a Phillips X-raydiffractometer (Model PW 1729) with CuKa radia-tion. Fig. 2 gives the XRD pattern of both the

Fig. 1. Results of thermo gravimetry and differential thermalanalysis (TG and DTA). The vertical arrow marks thedecomposition temperature. The inset shows the weight lossduring the annealing process.

16 P.K. Mishra et al. / Physica C 433 (2005) 14–20

compounds i.e., before and after Argon annealing.The XRD pattern of both the samples show thatthe samples are crystalline with tetragonal struc-ture and there is slight change in the lattice para-meters after annealing in Argon atmosphere. TheX-ray spectra show a weak peak indicating thepresence of an impurity phase in small amount.Generally, during the sample preparation ofRuSr2GdCu2O8 (1212), there is a possibility offormation of SrRuO3, which has been minimizedduring the synthesis. The limit of this impurityphase is estimated to be <4%. Any small contribu-tion to magnetization from this impurity phasewill be absorbed by the constant term in the fitting

20 30 40 50 60 70 80

(204

)

*

(109

)

(206

)

(205

)

(213

)

(200

)

(113

)(1

04)

(103

)

(101

)

*

Inte

nsity

(ar

buni

t)

2θ (degree)

Fig. 2. XRD pattern for as prepared (top) and argon annealedsample (bottom).

function described in the next section. No evidenceof any other impurity phase was seen within theinstrumental resolution. The dc magnetizationmeasurements were carried out using a SQUIDmagnetometer (MPMS-5, Quantum Design).Before each set of measurement, the sample wasbrought to room temperature to wipe out any previ-ous history.

3. Results

We measured zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature oncoarse powdered sample at different applied mag-netic fields viz., 10, 50, 75, 100, 150 and 250 Oe.We see irreversibility of FC and the ZFC branchesat around 160 K and a magnetic transition at tem-perature Tm nearly 140 K. The magnetic transitionshows a weak ferromagnetism superimposed by astronger paramagnetic contribution. In M–Tplots, at extremely low temperature below 2.7 Kthe antiferromagnetic transition corresponding tothe Gd sublattice is clearly seen. In addition, theannealed sample shows an extra low-temperaturefeature, a vertical dip (marked by circle) in thebackground of positive moment (in both FC andZFC cases) to be followed by rise in magnetizationbelow this (Fig. 3). This feature develops at around20 K and is magnetic field dependent. The dipdecreases in depth and slowly disappears with theincrease in the applied magnetic field. The samefeature is completely absent in case of the parentsample without annealing. We surmise that thereis a link between the Meissner signal and thisfeature. We retrieve the weak superconductingcontribution, overwhelmed by strong magneticcontributions, in the following way.

We fit experimental data in the temperaturerange lying above this feature and below Tm, toa functional form given below.

M=H ¼ C1=ðT þ TNÞ � C2T þ C3

where C1, C2 and C3 are the fitting parameterswith C1, C3 � C2. TN is the Neel temperature(2.7 K) determined from the data. The first termcorresponds to the induced magnetization from

0 50 100 150 200 2500.00

0.02

0.04

0.06

0.08

0.10

0.00

0.01

0.02

0.03

0.04

0.05

H=50 Oe

m(e

mu

/gm

)

T (K)

H=10 Oe

m(e

mu/

gm)

0.00

0.04

0.09

0.13

0.17

0.22

0 50 100 150 200 2500.00

0.10

0.20

0.30

0.40

0.50

0.60

H = 100 Oe

m(e

mu

/gm

)

H = 250 Oe

m(e

mu

/gm

)

T (K)

(a)

(b)

(c)

(d)

Fig. 3. M (T) plot for FC (top) and ZFC (bottom) for the annealed sample at magnetic fields (a) 10 Oe, (b) 50 Oe, (c) 100 Oe, (d)250 Oe. The solid curve in ZFC of (a) is an extrapolation of fitting to the data. The detail is described in the text. The dip-like feature inboth FC and ZFC plots at 10 Oe field is marked by circle.

0 20 40 60 80 100-0.008

-0.006

-0.004

-0.002

0.000

H= 10 OeH= 100 OeH= 250 Oe

χ v

T (K)

Fig. 4. Extracted diamagnetic susceptibility versus temperatureat (d) 10 Oe, (j) 100 Oe and (m) 250 Oe external magneticfields.

P.K. Mishra et al. / Physica C 433 (2005) 14–20 17

the alignment of the Gd moments under the influ-ence of external and local field in the paramagneticregime. The last two terms correspond to the spon-taneous magnetization from the weak ferromag-netic component that develops due to the cantingof Ru moments and is assumed to rise almost lin-early with decreasing temperature.

We extend this fitted curve to low temperaturerange above TN and subsequently separate thisfrom the data to extract the superconducting con-tribution to magnetization (see the ZFC curve ofFig. 3a). The indicative plots showing the super-conducting diamagnetic signal versus temperatureare shown in Fig. 4. We clearly see the diamagneticsignal corresponding to superconducting screeningeven at the applied magnetic field of 250 Oe, wherethe above-mentioned feature is not prominent.Secondly, the diamagnetic signal did not attainsaturation down to the lowest experimentallyachievable temperature in our system. The M–Hdata extracted from it at different temperatures(for the magnetic fields at which M–T wasrecorded) shows regions of Meissner regime as wellas mixed regime (Fig. 5).

4. Discussion

As mentioned previously, below Tm, magnetiza-tion of this material has signals from differentsources: canted Ru moments that provide a weakferromagnetic contribution and there is contribu-tion from paramagnetic spin lattice of Gd. Sotherefore, the appearance of superconductivity, in

-3.0x10-3

-2.0x10-3

-1.0x10-3

0.00 50 100 150 200 250

H (Oe)

5 K10 K14 K15 K

m(e

mu)

Fig. 5. ExtractedM (H) at few temperatures. The solid lines arejust guides to the eye.

18 P.K. Mishra et al. / Physica C 433 (2005) 14–20

the form of a diamagnetic signal, in the tempera-ture regime below Tm, will not be so apparent.In other words, incipience of a superconductingstate in a collective ferromagnetic and paramag-netic environment should be visible as a deviationfrom the monotonic increase in the magneticmoment with decreasing temperature. In this sce-nario, the attribution of the vertical-dip in ourdata to the reminiscence of Meissner signal, in acompeting ferro/paramagnetic environment suitswell. This will be further justified in our discussion.As the applied magnetic field is increased progres-sively, the dip gets shallower and ultimately is notvisible at around 250 Oe (Fig. 3d). However, thevanishing of this feature does not completelynegate the presence of superconductivity. The vis-ibility of the feature depends on the relativestrength of the magnetic and superconducting sig-nal. Higher the applied magnetic field stronger isthe magnetic signal and there is a possibility of itcompletely overwhelming the weaker diamagneticcomponent of the signal. We shall reason thisout from the following.

In order to extract the contribution from thesuperconducting state we followed the fitting exer-cise described in the previous section. By subtract-ing the fitted curve from the data for temperaturesabove TN, we retrieve the Meissner signal corre-sponding to superconducting state of the sample.In Fig. 4, we show some representative plots for10, 100 and 250 Oe applied magnetic field. It isto be noted that diamagnetic signal does not satu-rate down to the lowest temperature in our exper-iment. The other observation is that the onset

superconducting transition temperature (Tc) varieslittle from 22 K to 16 K as the applied magneticfield changes from 10 Oe to 250 Oe. Here it canbe seen that though at 250 Oe applied magneticfield the dip-like feature is not so prominent as atlow fields, nevertheless the superconducting transi-tion from the corresponding diamagnetic signal isclearly visible. Thus, we can conclude that super-conductivity survives up to a field of 250 Oe. How-ever, we do not understand the possible cause forthe double transition-like structure that slowlyvanishes at higher applied field values. One possi-bility could be a two-component response likeintergrain and intragrain effects in granular high-Tc superconductors.

At some fixed temperatures, we picked few datapoints from Fig. 4 corresponding to six appliedmagnetic fields and showed them as M–H plotsin Fig. 5 (for clarity in Fig. 4 only three fieldsare shown). The low temperature behavior is sim-ilar to the Meissner effect whereas close to thesuperconducting transition temperature showssigns of deviation from it. Interestingly the initialslopes, corresponding to the Meissner phase, inthe plots are not same at all the temperature. Atlow temperatures, the M–H slopes are significantlylarger than those at higher temperatures are. Thisis unusual and can be explained considering theeffective field in place of just the external magneticfield. The superconducting response is decided notonly by the external magnetic field but has addi-tional influences of induced and spontaneous mag-netization. As mentioned previously, towards thelower temperature, the induced magnetizationoriginate from the alignment of the Gd momentsunder the influence of external and local field inthe paramagnetic regime and the spontaneousmagnetization from the weak ferromagnetic com-ponent, due to the canting of Ru moments. Itcan be noted in Fig. 3, at low temperature, a siz-able enhancement of the induced magnetization.Because of the induced positive magnetization,the extracted Meissner moment actually corre-sponds to a field that is larger than the externalmagnetic field plotted on the x-axis of the graph.In other words, in the above M–H plot the Meiss-ner phase is a collective response of the externalmagnetic field and induced positive magnetization.

P.K. Mishra et al. / Physica C 433 (2005) 14–20 19

Hence, the x-axis here should actually representthe net effect of the two.

As mentioned earlier [5,10], the Meissner phasewill be stable as long as the condition 4pM +Hext < Hc1(T) is satisfied. Here 4pM is the sum ofspontaneous and induced magnetization. In ourobservation, we see at 5 K, the Meissner phase tobe stable at external magnetic field up to 250 Oe.This is in contrast with the observation made inRef. [5] but in agreement with the finding by Awanaet al. [10] in a similar system of Ru0.9Sr2YCu2.1O7.9.However, it shows a tendency of transition to a vor-tex phase above 10 K temperature.

The non-quenching of spontaneous moment inthe superconducting state and hence a higher effec-tive field than the external magnetic field can beperceived from other experimental findings. In arecent paper, Pozek et al. have shown that thecoexistence of magnetic order and superconduc-ting order is facilitated through decoupled subsys-tems of RuO2 and CuO2 planes [9]. Here, it isfound that the magnetic order resides in the nor-mal RuO2 plane whereas the superconductingorder dwells in the CuO2 plane and the interactionbetween the two orders being very weak. TheRuO2 layer remains normal at all temperatureswithout any induced superconductivity. Thus, thesuperconducting current is not able to screen themagnetic moment of the RuO2 plane. In the samereference, the authors conclude that the intragran-ular screening current in powder sample is veryweak. This is a consequence of large penetrationdepth kL as a large fraction of the charge carriersremains normal below the superconducting transi-tion temperature. In such a scenario, superconduc-ting screening on a microscopic scale and hence acomplete Meissner state may not be possible.

Finally, we provide the rationale behind theheat treatment described in the sample preparationsection. Though this field-dependent dip-like fea-ture supports our attribution that this is associatedwith the Meissner phase of the superconductingstate, however, in the fully oxygenated parent sam-ple the magnetization data was monotonic and didnot show the above-mentioned feature or anythingelse to subscribe for the presence of Meissner state.This prompted us to look for a different oxygenstoichiometry. So therefore, a part of the parent

sample was heated up to 1200 �C temperature atthe rate of 10 �C/min in a thermo-balance (simul-taneous TG-DTA) in flowing high pure Argonatmosphere. The thermo-gravimetric data showsthat the mass change was small at temperaturebelow 1000 �C and very rapid with temperatureabove 1005 �C. Simultaneous differential thermalanalysis (DTA) confirms phase separation at thistemperature (Fig. 1).

Guided by this observation, we have annealedthe starting sample at 991 �C, just below thedecomposition-temperature, for nearly five andhalf hours (see the inset of Fig. 1). The correspond-ing oxygen stoichiometry was calculated to beRuSr2GdCu2O8� d with d being 0.345. The XRDpattern of this annealed sample did not show anystructural change as evident from Fig. 2. The latticeparameters calculated for samples before and afterannealing are a = b = 3.840(7) A, c = 11.549(5) Aand a = b = 3.832(2) A, c = 11.528(4) A respec-tively. The volume change of the unit cell is nearly0.6%. This sample was used for magnetizationstudies as described above. Thus, we believe thatthe fully oxygenated RuSr2GdCu2O8 compoundis not superconducting and proper reduction ofoxygen is necessary to make it superconducting.

5. Conclusion

In conclusion, we show the evidence of coexis-tence of superconductivity and magnetism fromM (T) data in oxygen deficient RuSr2GdCu2O8� d

system. This is brought out from a feature, likenedto Meissener signal, which develops both in field-cooled and zero-field-cooled states. By separatingout the magnetic contributions from the data, wecould show the diamagnetic response and henceof superconductivity in this magnetically orderedsystem in a more prominent manner. Meissner sig-nal was observed up to Hext = 250 Oe at 5 K tem-perature. In contrast, we see absence of anysignature of superconductivity in the parentRuSr2GdCu2O8 compound without oxygen reduc-tion. Thus, we conclude that proper oxygen reduc-tion is essential to superconductivity in this system.However, for the result to be very conclusive, themicroscopic mixture of a superconducting phase

20 P.K. Mishra et al. / Physica C 433 (2005) 14–20

and a magnetic phase needs to be addressed. Inthis system, such a possibility may arise by a par-tial substitution of Cu for Ru and X-ray detectionof a small replacement of Ru by Cu is difficult[10,11].

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