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Journal of Magnetism and Magnetic Materials 320 (2008) e522–e525

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Magnetism in (Ca,Sr)2RuO4 observed by 119Sn-Mossbauer spectroscopy

Ada Lopeza, I. Souza Azevedob,�, J.L. Gonzalezc, E. Baggio-Saitovitchb,A.M. Gomesd, A.D. Tavares Jr.a

aUniversidade do Estado do Rio de Janeiro, IF-Rua Sao Francisco Xavier 524, Rio de Janeiro, BrazilbCentro Brasileiro de Pesquisas Fısicas, Rua Dr. Xavier Sigaud 150, Rio de Janeiro, Brazil

cPUC-Rio, Instituto de Fısica, Rio de Janeiro, BrazildUniversidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil

Available online 23 February 2008

Abstract

We report on the magnetic behavior observed by 119Sn-Mossbauer spectroscopy in Ca2�xSrxRuO4 samples (x ¼ 0.09, 0.2, and 2.0).

The 119Sn-Mossbauer probe substitutes Ru4+ at the RuO2-layers as Sn4+. Mossbauer spectra at 4.2K show the presence of magnetic

correlations due to transferred hyperfine field (Bhf) at the Sn(Ru)-sites for two samples (x ¼ 0.09 and 0.20), while it was not observed for

Sr2RuO4 (x ¼ 2.0). The Bhf softly decreases with x going from 0.09 to 0.2, indicating remaining short-range antiferromagnetic

correlations at x ¼ 0.2 concentration, where a first structural transition has been reported. In both samples, the transferred Bhf shows a

canted magnetic structure.

r 2008 Elsevier B.V. All rights reserved.

PACS: 74.70.Pq; 74.25.Ha; 76.80.+y

Keywords: Superconductivity; Magnetic material; Ruthenate; 119Sn-Mossbauer

1. Introduction

The phase diagram of Ca2�xSrxRuO4 exhibits a richvariety of physical phenomena going from the antiferro-magnetic (AF) Mott-insulator Ca2RuO4 to the unconven-tional superconductor Sr2RuO4 [1–4], which is widelybelieved to be a spin-triplet pairing superconductor [1]. Theionic radius on the Ca/Sr-site varies along the series leadingto different structural transitions and magnetic behavior.The coupling between structural, magnetic and electronictransitions in these compounds is crucial to understand thephysics of this compound [5]. In this work, we report, forthe first time, the magnetic behavior observed through119Sn-Mossbauer measurements for the Ca2�xSrxRuO4

compound with low strontium concentration, namelyx ¼ 0.09, 0.2, and also for x ¼ 2.0 (without Ca).

- see front matter r 2008 Elsevier B.V. All rights reserved.

/j.jmmm.2008.02.098

onding author. Tel.: +5521 21417177; fax: +55 21 21417540.

ddress: izabel@cbpf.br (I. Souza Azevedo).

2. Experimental details

Polycrystalline samples of Ca2�xSrx(Ru0.99Sn0.01)O4,doped with three strontium concentrations (namelyx ¼ 0.09, 0.2 and 2.0) and with 1% in 119Sn-probe forRu-substitution, were prepared by the standard solid-statereaction. Details about the sample preparation can befound in elsewhere [6].The X-ray diffraction (XRD) patterns were obtained in

an X’Pert PRO (PANalytical) powder diffractometer,using Cu Ka adiation (l ¼ 1.5418 A). Data were collectedby step-scanning mode (101p2Yp801) and 2 s countingtime at each step at room temperature (RT). The XRDdata were fitted with the FULLPROF program (ThierryRoisnel and Rodriguez-Carvajal, 2006). AC-magneticsusceptibility measurements at 10Oe were performed in aQuantum Design SQUID magnetometer. The 119Sn-Mossbauer spectra (MS) were collected at RT and 4.2K,in transmission geometry, using a conventional spectro-meter in constant acceleration mode and with a BaSnO3

source, to which the obtained isomer shift (d) values are

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Table 1

Crystallographic data, symmetry group and lattice parameters obtained

for the (Ca2�xSrx)Ru0.99Sn0.01O4 samples

x 0.09 0.20 2.00

Space group Pbca Pbca I4/mmm

a (A) 5.3692(6) 5.3601(3) 3.8706(4)

b (A) 5.3530(6) 5.3420(1) 3.8706(4)

c (A) 12.268(5) 12.339(2) 12.7180(3)

Table 2

Mossbauer parameters at RT for (Ca2�xSrx)Ru0.99Sn0.01O4 samples

x d (mm/s) DEq (mm/s) G (mm/s)

0.09 0.07(6) 0.38(2) 0.85(0)

0.20 0.07(2) 0.35(8) 0.86(4)

2.00 0.02(4) 0.49(4) 0.85(1)

d, isomer shift; DEq, quadrupole splitting; G, linewidth.

x = 0.204%

ve T

rans

mis

sion

x = 0.095%

A. Lopez et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e522–e525 e523

referred. The experimental details used here are similar tothe ones reported in a previous study of 119Sn-MS onRuSr2(Eu1.5Ce0.5)Cu2O10 [7].

3. Results and discussion

The results of powder XRD measurements are shown inFig. 1 for Sr2(Ru0.99Sn0.01)O4, Ca1.8Sr0.2(Ru0.99Sn0.01)O4

and Ca1.91Sr0.09(Ru0.99Sn0.01)O4 samples. The diffractionpatterns reveal a dominant phase characteristic of thecompound, and the data were fitted using the Rietveldmethod for structural refinements. All the peaks wereconfirmed as belonging to the compound structure,showing very small differences between their experimentalposition and the theoretical ones from the fitting. Thelattice parameters and the symmetry groups are presentedin Table 1, where Pbca is the space group related to theorthorhombic symmetry (low-Sr concentration) and I4/mmm to the tetragonal symmetry as in the phase diagram.

The MS at RT were analyzed with only one symmetricline doublet, and their hyperfine parameters are summar-ized in Table 2. The doublet is presumably associated to Snin the Ru-site in a distorted oxygen-octahedral configura-tion. The isomer-shift (d) values, obtained for the doubletindicate that Sn enters into the RuO2 layers as Sn

4+, sincethese d values are different from those corresponding toSnO2 (d ¼ 0.53) and also from the natural Sn-layers(d ¼ 2.5). This fact and our X-ray measurements, whichdid not detect any signal of secondary phases, indicate thatSn4+ really enter into the Ru-site. The Sr2RuO4 sampleshows a quadrupole splitting (DEq) value lower than forthe samples with x ¼ 0.09 and 0.20, confirming that thesubstitution of Sr by Ca (smaller radius) in the Sr2RuO4

compound strongly distorts its ideal crystalline structure(K2NiF4) in concordance with detailed studies which

10

Inte

nsity

(arb

. uni

ts)

x = 0.20

x = 2

x = 0.09

20 30 40 50 60 70 802� (°)

Fig. 1. XRD measurements for the (Ca2�xSrx)Ru0.99Sn0.01O4 samples.

-10

x = 2.00

2%

Rel

ati

velocity (mm/s)-8 -6 -4 -20 2 4 6 8 10

Fig. 2. Mossbauer spectra at 4.2K for (Ca2�xSrx)Ru0.99Sn0.01O4 samples.

argues that the low Sr-concentration phase is rotatedalong the c-axis, and also strongly tilted along the b-axis[5].The MS at 4.2K for the Ca2�xSrx(Ru0.99Sn0.01)O4

samples with x ¼ 0.09, 0.20 and 2.0 are shown in Fig. 2,

ARTICLE IN PRESSA. Lopez et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e522–e525e524

where it can be seen that the spectra for x ¼ 0.09 and 0.2samples are magnetically splitted.

The spectra were fitted using the total Hamiltonian, inorder to correlate the magnetic hyperfine field (Bhf) withthe Y-angle (Y is the angle between the magnetic hyperfinefield and the main component Vzz of the electrical fieldgradient tensor). These two samples show different Bhf andY-angle (for x ¼ 0.09, Bhf ¼ 1.2470.02T and Y ¼42.871.51 while for x ¼ 0.2, Bhf ¼ 1.1370.02T andY ¼ 47.672.051). The MS obtained for the third samplewithout Ca (x ¼ 2.0) is only a doublet; there is not evidenceof a hyperfine magnetic field.

Now we correlate our Mossbauer results on thesesamples with their crystalline structure. The first oneconcerns to the Sr-0.09 sample where the Sn-probe detectsa transferred Bhf, which is inclined around 42.81 respect tothe c-axis. It is in accordance to previous reports, whichsuggest a canted AF structure with the spins approximatelyaligned along the tilt-axis (b-axis) in Ca2�xSrxRuO4 forxo0.2. In fact, in the ac–susceptibility curve (not shownhere), we have performed on this same sample, there is asharp peak indicating an AF ordering at low temperatures.We note that for samples with xo0.2 there has beendetected a weak ferromagnetic hysteresis ascribable to thisAF-canted magnetic structure [4]. The tilt of the RuO6-octahedron for xo0.2 diminishes the bandwidth, tuningthe electronic ground state of this system insulate andinducing a magnetic moment in the Ru-ions [8]. To the bestof our knowledge, this is the first report, which shows themicroscopic details of the magnetic structure at Ru-sitesinside the RuO2 layers. Our results for x ¼ 0.09 point to amagnetic structure where the spins are not totally alignedalong the b-axis. It is also consistent with the smallferromagnetic component detected in magnetic measure-ments for samples with low Sr concentration, just aroundthis region of the phase diagram [4].

The next result concerns about MS on the x ¼ 0.2sample. It should be noted that, for xo0.2 at lowtemperatures, the sample’s crystalline structure shows alarge tilt, however for x40.2 there is a structural changewith a sharp decrease in the tilt-angle [5]. This change isimportant and is responsible for the low-temperaturemetallic–insulator (M–I) transition in these materials [5].Since the verge of the M–I transition is around x ¼ 0.2, wehave also chosen this concentration to study. On the otherhand, the fact that around x ¼ 0.5 has been detected amaximum in the low-temperature susceptibility while forxo0.5 (where the structure first tilts) the system ismagnetic [5], suggests that the magnetism is stronglycoupled to the low-lying tilt modes. In our study, the Sr-0.2 sample showed a lower Bhf value in a canted AFconfiguration. In order to understand this result, we notethat the decreasing of the tilt angle above x ¼ 0.2 shoulddecrease the magnetic correlations in this compound. Thislatter picture is consistent with our results, insofar as thehyperfine field found for the Sr-0.2 sample is lower thanthat for the Sr-0.09 sample, the difference being larger than

experimental uncertainty. It is remarkable that the relativedecrease in Bhf is not so sharp as it seems to happen in thetilt-angle when the Sr-concentration is increased above 0.2(see Fig. 11 of Ref. [8]), where at x ¼ 0.2, there is a first-order structural transition. This suggests that the magneticorder can be influenced by other reasons (spin-orbitcoupling) besides the structural ones as it has beenproposed in others works [5].The picture, emerging from our results suggests that in

the low-Sr region (xo0.2) the system orders antiferro-magnetically, and upon increase of x above 0.2 short-rangeAF correlations can survive. We should also expect thatthese magnetic correlations continuously decrease, disap-pearing above x ¼ 0.5 (where the tilt disappears) as it hasbeen suggested in different reports [4,8]. Within thisscenario, the good metal Sr2RuO4 should not showmagnetic correlation inside the RuO2-planes, and this isverified with our MS results at 4.2K. Sr2RuO4 orders in theK2NiF4-type structure without tilting and with three Fermisurface sheets (xy, xz, and yz bands) due to the fourelectrons in the 4d-Ru orbital, where these bands arealmost equally occupied [8]. Thus, for the x ¼ 0.2compound there are no localization effects, and atT ¼ 4.2K it is a paramagnetic metal with no magneticorder, in accordance with our studies which did not showany local hyperfine field.

4. Conclusions

The 119Sn-MS of Ca2�xSrxRuO4 samples (doped with1% of 119Sn) show that the 119Sn-probe detects a localhyperfine field at low temperatures inside the RuO2-planesfor low Sr-concentrations, namely x ¼ 0.09 and 0.2. TheBhf value decreases little upon x increase from 0.09 to 0.2 inspite of the existence of a first structural transition aroundx ¼ 0.2. This result can indicate that other mechanisms(not only structural) can influence the magnetic structure ofthis compound. The transferred hyperfine magnetic field,Bhf, is canted along the c-axis, which also can indicate thatthe spins are not totally aligned along the tilt-axis (b-axis).Finally, the 119Sn-probe in the Sr2RuO4 sample did notdetect any Bhf. Our study is at an initial stage, and newmeasurements should reveal interesting aspects (like the Bhf

dependence on Sr) about the microscopic magneticstructure of this compound.

Acknowledgments

We are grateful to H. Micklitz for helpful discussions.This work was supported by FAPERJ.

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