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Physica C 442 (2006) 33–38

Magnetic structure of RuSr2(Eu1.5Ce0.5)Cu2O10�d studied by119Sn–Mossbauer spectroscopy

Ada Lopez a,*, I. Souza Azevedo a, J.L. Gonzalez a, E. Baggio-Saitovitch a, H. Micklitz b

a Centro Brasileiro de Pesquisas Fısicas, CME, Rua Dr. Xavier Sigaud 150, 22290-180, Rio de Janeiro, RJ, Brazilb II. Physikalisches Institut, Universitat zu Koln, Zulpicher Strasse 77, D-50937 Koln, Germany

Received 2 February 2005; received in revised form 28 March 2006; accepted 6 April 2006

Abstract

We report on the results of 119Sn–Mossbauer measurements performed between room temperature and 4.2 K on a series of doped(Ru1�xSnx)Sr2(Eu1.5Ce0.5)Cu2O10�d samples (with x = 0.02, 0.04 and 0.06) (so called 1222-type). The spectral analysis suggests thatSn substitutes Ru5+ at the RuO2 layers, and enters as Sn4+. We observe for the 4.2 K Mossbauer measurements that an in-plane trans-ferred hyperfine field (hf) exists at the Sn(Ru) site in the (Ru,Sn) O2 layers. The value of the average hf field decreases with increasing Snconcentration from 3.64 T (for x = 0.02) to 2.62 T (for x = 0.06).� 2006 Elsevier B.V. All rights reserved.

PACS: 76.80.+y; 74.72.Jt; 74.25.Ha

Keywords: 119Sn-Mossbauer; High-TC cuprates superconductors; Ru-1222

1. Introduction

Many studies of high-TC superconductors have focusedon the modification of their magnetic properties as thenumber of carriers is varied. The study of the ruthenocup-rate systems RuSr2RCu2O8 (Ru-1212) and RuSr2(R1.4-Ce0.6)Cu2O10�d (Ru-1222) for R = Gd and Eu, hasattracted interest due to the possible coexistence of super-conductivity (SC), with TC � 32–42 K, and magneticordering, with TM � 122–180 K. The picture generally usedfor both systems is that magnetism is associated with theRuO2 planes, and SC is confined to the CuO2 layers [1,2].The crystalline structure of Ru-1212 is similar to that ofthe YBa2Cu3O7 compound, except for the replacementof the CuO chains by the RuO2 planes, resulting in a com-pound with a tetragonal structure. The RuO6 octahedra areconnected, through the apical oxygens, to two layers of

0921-4534/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2006.04.015

* Corresponding author. Tel.: +55 2121417503; fax: +55 2121417000.E-mail address: adalopez@cbpf.br (A. Lopez).

CuO5 square pyramids. On the other hand, in the Ru-1222, the adjacent RuO2 layers are shifted by (a/2, a/2),where ‘a’ is one of the lattice parameters of the crystallinestructure [3].

The magnetic structure of the ruthenocuprates remains atopic of discussion even after extensive investigations oftheir magnetization through recent neutron power diffrac-tion (NPD) and nuclear magnetic resonance (NMR) mea-surements. Even for the Ru-1212, which is considered asa relatively simple case, divergent experimental results havebeen found. The zero-field NMR data suggest that theordered magnetic moments of �1.6 lB/Ru are perpendicu-lar to the c-axis and that in addition the magnitude of themagnetic moment is comparable to that found in ferromag-netic (FM) materials [4]. Recent NPD studies, on the otherhand, show that the magnetic ordering is antiferromagnetic(AF) along the c-axis but with a canting component [5],resulting in a small FM moment (�0.1 lB/Ru) perpendicu-lar to the c-axis. This NPD result is in contrast to formerneutron scattering studies [6]. Further NMR measurements[7] show that the direction of the magnetic hyperfine-field

34 A. Lopez et al. / Physica C 442 (2006) 33–38

at the Ru-site is perpendicular to c-axis, i.e., parallel to thecanting component of the Ru moments. The magneticbehavior of Ru-1222, however, seems to be much morecomplicated: phase-separation and the formation of ferro-magnetic nanoclusters are well documented in this case[8,9]. Such a phase separation may also explain the seem-ingly conflicting NMR/NPD results mentioned above forthe Ru-1212.

119Sn–Mossbauer spectroscopy measurements on theRu1�xSnxSr2Eu1.5Ce0.5Cu2O10�d system can give on-siteinformation and, therefore, may help for a better under-standing of the magnetic structure of Ru-1222. It is alsoof interest to see if the 119Sn nucleus, a nonmagnetic probe,can sense the in-plane hyperfine magnetic field in the RuO2

layers. Such information could be relevant for the under-standing of the kind of competition existent between thebulk superconductivity and the magnetism in thesecompounds.

In this work we studied the magnetic structure in theRu-1222 by using 119Sn–Mossbauer Spectroscopy. Ourresults shown that the magnetic ordering stars at low tem-perature (around 125 K). At very low temperatures, insidethe superconducting state, our results shown that there is aFM component in the RuO2 planes resulting from the

Inte

nsi

ty (

arb

itar

y u

nit

s)

22

01

01

9

10

17

21

7

11

14

21

311

10

00

14

11

8

20

0

11

0

10

7

10

3

20 30 40 50 60 702θ (degrees)

x = 0.04

x = 0.06

x = 0.02

x = 0

Fig. 1. X-ray diffraction powder diagram of Ru1�xSnxSr2Eu1.5Ce0.5-Cu2O10�d samples. Also are identified the characteristic hkl planes for theabove structure.

canted AF ordering. The angle between the c-axis of thecrystalline structure and the hyperfine field is about 73�.

2. Experimental

Ceramic samples of Ru1�xSnxSr2Eu1.5Ce0.5Cu2O10�d

with nominal composition for x = 0, 0.02, 0.04 and 0.06were prepared by the solid-state reaction of stoichiometricpowders of RuO2, SrCO3, Gd2O3, CuO and 119SnO2. Theywere ground and pressurized into pellets before preliminaryreaction in air at 970 �C for 24 h. Then, the cooled sampleswere reground, pressurized again, and reheated at 1065 �Cin oxygen flux for another 72 h, followed by cooling toroom temperature at a rate of 45 �C/h.

In order to characterize the structure of the samples,powder X-ray diffraction (XRD) measurements were per-formed at room temperature, using CuKa radiation onan automated Rigaku diffractometer in the step-scanningmode (20� 6 2h 6 70�).

Mossbauer spectra (MS) were collected from room tem-perature (RT) down to 4.2 K in transmission geometry,using a conventional spectrometer in the sinusoidal modewith a 119Sn/BaSnO3 source. The isomer shift (d) valuesare given with respect to BaSnO3.

-3 -2 -1 0 1 2 3VELOCITY (mm/s)

x = 0.06

x = 0.04

4%

2%

RE

LA

TIV

E T

RA

NS

MIS

SIO

N

x = 0.02

1%

Fig. 2. Mossbauer spectra at RT for the Ru1�xSnxSr2Eu1.5Ce0.5Cu2O10�d

system.

A. Lopez et al. / Physica C 442 (2006) 33–38 35

In addition we have performed ac-magnetic susceptibil-ity measurements with a quantum design SQUID magne-tometer in order to see how the behavior changes withSn-doping.

3. Results and discussion

The results of powder XRD measurements for all sam-ples are shown in Fig. 1. The diffraction patterns reveal adominant phase, characteristic of the tetragonal Ru-1222compound. However, the presence of a small amount ofthe secondary perovskite-phase SrRuO3 can be observedfor the samples with higher Sn-concentration. Tetragonalspace group I4/mmm was assumed for all samples, andthe data were fitted using the Rietveld method for struc-tural refinements. The x dependence of the lattice parame-ters runs from a � 3.839(5) to 3.844(5) A and from c �28.558(3) to 28.529(9) A, when x runs from 0 to 0.06, withno measurable change in the cell volume.

MS obtained at RT show two Sn-sites (Fig. 2), whichwere fitted considering the superposition of one singlet (site

-10 -5 0 5VELOCITY (mm/s)

2%1

%

RE

LA

TIV

E T

RA

NS

MIS

SIO

N

(a)

1% x = 0.0

x = 0.0

x = 0.0

Fig. 3. Mossbauer spectra at 4.2 K (a) and hyperfine field distribut

I) and one doublet (site II). The doublet, which is the maincomponent, is presumably associated to Sn in the Ru-sitein a distorted oxygen-octahedral configuration. The isomershift (d), obtained for the doublet, indicates that Sn entersinto the RuO2 layers as Sn4+. The corresponding quadru-pole splitting (DEq) is bigger than that for SnO2, and is sim-ilar to those obtained for the Sn-doped conventionalcuprates [10], reflecting a distortion of the oxygen coordi-nation. The singlet accounts for a local electronic configu-ration of higher symmetry for Sn. It is interpreted as beingdue to the presence of the impurity phase SrSnO3, the pseu-docubic perovskite, with an amount not detectable byXRD. In order to confirm this assignment, we have pre-pared a pure SrSnO3 sample and obtained the expectedXRD and 119Sn MS data. We exclude the possibility thatthe singlet line results from Sr(Ru1�xSnx)O3 since this com-pound orders below T � 165 K [11], and we do not see anymagnetic splitting of the singlet line at low temperatures(see below). Fig. 3(a) shows the MS obtained at 4.2 Kfor the Ru1�xSnxSr2Eu1.5Ce0.5Cu2O10�d samples with x =0.02, 0.04 and 0.06. These spectra were fitted assuming

10 0 1 2 3 4 5 6 7

P (

Bhf

)

Bhf (T)(b)

2

4

6

x = 0.02

x = 0.04

x = 0.06

ion P(Bhf) (b) for the Ru1�xSnxSr2Eu1.5Ce0.5Cu2O10�d system.

36 A. Lopez et al. / Physica C 442 (2006) 33–38

the superposition of site I, the singlet, and site II, whichshows now, in addition to the quadrupole splitting, a mag-netic hyperfine field (hf) distribution. The magnetic hf fielddistribution results from the ordered Ru moments togetherwith an inhomogeneity in the local surrounding of the Snatoms sitting at the Ru-site. The fit of all Mossbauer spec-tra was done with a fixed linewidth of 1.14 mm/s, a value,which was obtained from the measurement using a naturalSn-foil and the same 119Sn/BaSnO3 source for comparison.The hf field distributions resulting from these least-squaresfits are shown in Fig. 3(b) for the three different samples.One can clearly see the decrease of Bhf with increasing x.These results, summarized in Table 1, suggest that theSn-atoms are progressively replacing the Ru in the mainphase, i.e., in Ru-1222. The values for the average hyper-fine field are similar to the ones reported by Felner et al. [9].

Taking the relation Deff = DEq (3cos2H � 1)/2 (H is theangle between the magnetic hf field and the main compo-nent Vzz of the electrical field gradient tensor) with theaverage values for Deff obtained from the field distributionfittings, and the DEq values obtained from the RT spectra,

Table 1Mossbauer hyperfine parameters of (Ru1�x

119Snx)Sr2(Eu1.5Ce0.5)Cu2O10�d sam

x Site T = 300 K

d (mm/s) DEq (mm/s) A (%)

0.02 I 0.00(3) – 10(2)II 0.200(7) 1.36(2) 90(2)

0.04 I 0.02(3) – 19(2)II 0.210(7) 1.42(2) 81(2)

0.06 I 0.01(3) – 32(2)II 0.200(7) 1.42(2) 68(2)

d = isomer shift relative to 119Sn/BaSnO3 source; DEq = quadrupole splitting;magnetic hyperfine field; A = relative absorption area.

0 40 80

0.0

0.5

1.0

-20

-10

χ' A

C (

arb

. un

its)

Fig. 4. Temperature dependence of ac-susceptibility

we calculated an average angle of Hav ffi 73� ± 10� for Sn atthe Ru-site. On a lattice site with tetragonal point symme-try the electric field gradient is axial and the main compo-nent Vzz is directed along the fourfold tetragonal c-axis[12–14]. Therefore, we can conclude that the ferromagneticcomponent of the Ru moments, resulting from the cantedAF structure, is oriented along the a,b-plane (RuO2-plane).This finding is consistent with that of our previous 119Sn–Mossbauer studies in the Ru-1212 system [15].

The measurement of the temperature dependence of thenormalized ac-susceptibility for the Ru1�xSnxSr2Eu1.5-Ce0.5Cu2O10�d system is shown in (Fig. 4. The Meissnereffect for the undoped sample is observed below TC(onset) =14 K, which is far below the onset temperature as obtainedfrom resistivity measurements (the results in Ref. [16]showing the resistivity measurements of the undoped Ru-1222 sample, which give TC = 35 K, have been obtainedwith our sample). The large difference between TC (onset)from susceptibility measurements and that from resistivitymeasurements, typical for Ru-based compounds, has beenexplained as caused by the formation of a spontaneous

ples

T = 4.2 K

d (mm/s) Deff (mm/s) hBhfi (T) A (%)

0.00(3) – – 10(2)0.19(2) �0.50(2) 3.64(11) 90(2)

0.01(3) – – 19(2)0.19(2) �0.56(2) 3.20(06) 81(2)

0.01(3) – – 32(2)0.19(2) �0.50(2) 2.62(09) 68(2)

Deff = effective quadrupole shift in magnetically split spectra, hBhfi = mean

120 160 200 240

60 90 120 150

χ' A

C (

arb.

un

its)

T(K)

x = 0x = 0.02x = 0.04x = 0.06

T(K)

of the Ru1�xSnxSr2Eu1.5Ce0.5Cu2O10�d system.

-12 -8 -4 0 4 8 12

4.2K

VELOCITY (mm/s)

RE

LA

TIV

E T

RA

NS

MIS

SIO

N

300K

125K

2%

50K

Fig. 5. Mossbauer spectra as function of temperature for Ru0.98Sn0.02Sr2-Eu1.5Ce0.5Cu2O10�d sample.

A. Lopez et al. / Physica C 442 (2006) 33–38 37

vortex phase [16]. In addition a peak in the magnetic sus-ceptibility at T = 100 K can be seen in the parent com-pound. In contrast to this, two peaks were detected in theSn-doped samples, namely, a major one at 83(1), 56(1),and 43(1) K, and a minor one (Fig. 4 inset) at T =135(1), 114(1), and 95(1) K, for x = 0.02, 0.04 and 0.06,respectively. The temperature of the main peak for thedoped samples decreases with increasing x. This may indi-cate a decrease of the magnetic transition temperature withincreasing x, which would be in agreement with theobserved decreasing magnetization with increasing x. It

also should be remarked that none of these samples dis-plays a superconducting transition down to 4.2 K in ourac-susceptibility measurements. However, as has beenoutlined above, this does not necessarily mean that sucha transition would also not be seen in resistivitymeasurements.

A set of MS was collected for the x = 0.02 sample atRT, 125, 50 and 4.2 K, as can be seen in Fig. 5. Thebroadened spectrum obtained at 125 K is an indicationthat a transferred magnetic hyperfine field (Bhf) appearsbelow this temperature; on cooling below 125 K, there isa continuous evolution towards a defined magnetic struc-ture. The values of the transferred average magnetichyperfine field (hBhfi), as obtained from the fits to thespectra measured at the temperatures of 125 and 50 K,are 1.15(5) and 2.56(9) T, respectively. This result clearlyshows that the Sn nuclei are sensing the long-range orderof the Ru moments.

Our extrapolated value of �4 T at 4.2 K for the averagefield at the Sn-site for the undoped samples gives an on-siteestimation of the in-plane field in the RuO2 layers, whichcan be considered as a lower limit due to its transferred nat-ure. Since Sn does not carry a magnetic moment, the inter-pretation of the observed hf field is more straightforwardthan in the case of the 57Fe–Mossbauer studies, wherethe main part of the observed hf field results from the mag-netic moment of the Fe atom itself.

The in-plane magnetic field at the RuO2 layers is impor-tant for the coexistence of superconductivity (SC) and fer-romagnetism (FM) in the Ru-based ceramics [17].According to theory for SC–FM superlattices, the super-conducting order parameter will exhibit a node at theFM layer if the in-plane magnetization exceeds a highthreshold value [18]. The average hyperfine field sensedby the Mossbauer measurements with a magnitude ofabout 4 T, would favor the emergence of such a node. Thiswould lead to a low pair-breaking effect of the Cooperpairs moving along the c-axis, and thus allowing the onsetof bulk SC.

4. Conclusion

In this work we show the advantage of using 119Sn–Mossbauer spectroscopy in the investigation of Ru-1222ceramics. The effect of Sn-doping on the physical proper-ties of this Ru-compound could be detected at a micro-scopic level. Since the Sn nucleus does not have amagnetic moment, it can act as a very good and innocentprobe to study the behavior of the surrounding magneticstructure. The broadening of the Mossbauer spectraobserved at 4.2 K is due to a transferred magnetic hyper-fine field, induced by the ordered Ru magnetic momentsbelow the transition temperature. From the analysis ofthe spectra measured at 4.2 K, the existence of an in-planetransferred magnetic field (Bhf) in the (Ru,Sn)O2 planes isconcluded. The value of the Bhf decreases with increasingSn-doping.

38 A. Lopez et al. / Physica C 442 (2006) 33–38

Acknowledgments

The authors acknowledge W. Vanoni for XRD mea-surements. This work was, financially, supported by CNPqand CAPES (Brazil).

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