Crystal Growth of a Lamellar Sr3Ru2O7–Sr4Ru3O10 Eutectic System Rosalba Fittipaldi,* Daniela Sisti, Antonio Vecchione, and Sandro Pace CNR-INFM SuperMat Regional Laboratory Salerno and Department of Physics “E. R. Caianiello” UniVersity of Salerno Via S. Allende I-84081 Baronissi (Sa), Italy
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2495–2499
ReceiVed February 22, 2007; ReVised Manuscript ReceiVed August 1, 2007
ABSTRACT: Lamellar eutectic crystals of Sr3Ru2O7–Sr4Ru3O10 have been successfully grown in a double-mirror optical floating zone furnace in the flux feeding floating zone configuration with Ru self-flux. The synthesis procedure of the polycrystalline rod is found to be a critical factor for the crystal growth. The eutectic dark-brown crystals grow in the [100] direction and cleave in the a–b plane as the single phase crystals. The morphology, structure, phase purity, and composition of the samples were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results from backscattered electron (BSE) imaging as well as from energy dispersive spectrometry (EDS) were combined with the electron backscatter diffraction (EBSD) data to obtain complementary chemical and crystallographic information. Moreover, XRD analysis indicated that the Sr3Ru2O7 and Sr4Ru3O10 phases match the same crystal with parallel in-plane and out-of-plane lattice parameters. The resistance measurements showed a Fermi liquid behavior at low temperatures analogous to the one found in the single phases. We grew eutectic crystals representing natural junctions made of metamagnetic Sr3Ru2O7 and ferromagnetic Sr4Ru3O10 that may be useful in studies of possible novel quantum critical phenomena specific to itinerant magnetism systems. Introduction Understanding itinerant ferromagnetism and metamagnetism is a longstanding challenge in condensed matter physics.1,2 In this context, a very rich phenomenology is exhibited by the strontium ruthenates of the so-called Ruddlesden–Popper (RP) series, described by the formula Srn+1RunO3n+1.3–9 Since the discovery, nearly 40 years ago, of the distorted perovskite SrRuO3 as a ferromagnetic metal with Tcurie ) 160 K,4 many studies of the RP compounds have provided clear evidence that their physical properties strongly depend on the number of RuO6 octahedral layers. Sr2RuO4, the n ) 1 member, shows an unconventional superconductivity with a spin-triplet pairing.3,6,10 The n ) 2 bilayered member, Sr3Ru2O7, is an enhanced Pauli paramagnet7 close to magnetic order,5,11–13 which at low temperatures undergoes a metamagnetic quantum phase transition induced by the application of moderate applied magnetic fields, ranging at the transition from 4.9 T (for H | ab plane) to 7.9 T (for H | c-axis).11,12 Moreover, Sr3Ru2O7 exhibits induced ferromagnetism upon the application of a hydrostatic pressure.7 On the contrary, the magnetic behavior of the third RP series member (n ) 3), Sr4Ru3O10, is not well understood.8,9,14–16 Several magnetization studies of this system provide evidence for anisotropic ferromagnetism with Tcurie ) 105 K,14,15 with an additional metamagnetic transition at T* ≈ 50 K induced by a magnetic field applied in the a–b plane.14,15 To explain these features, a canting of the Ru magnetic moments has been proposed. Nevertheless, a ferromagnetic transition with an easy axis lying in the a–b plane has very recently been found at Tcurie ) 100 K by neutron scattering measurements.16 This experiment suggests that the complex magnetization and the metamagnetic behavior result from a phase separation process with magnetic domain formation. Several studies on ruthenate compounds pointed out the importance of crystal purity.17,18 The superconductivity in Sr2RuO4 crystals survives only for residual resistivities, Fres, lower than 1.0 µΩ cm.19 Moreover, extremely pure single crystals of Sr3Ru2O7 with Fres as low as 0.4 µΩ cm have enabled * To whom correspondence should be addressed. E-mail: fittipaldi@ sa.infn.it; phone: +39 089 965256.
the observation of quantum oscillations in the resistivity both above and below the metamagnetic field.20 On the other hand, a deeper comprehension of the intrinsic magnetic behavior of the Sr4Ru3O10 phase requires the preparation of high-quality single crystals, free from grain boundaries, stacking faults, and point defects. Recently, interesting results were obtained21–24 from the study of Sr-based layered ruthenates eutectic systems. One of the main motivations for carrying out eutectic growth is the possibility to develop in-situ composite materials where the properties of the distinct constituents merge to a variable extent into those of a single material.25,26 The recent discovery of superconductivity at 3 K in Sr2RuO4–Ru metal eutectic crystals21,27–29 has stimulated investigations for other eutectics, such as Sr2RuO4– Sr3Ru2O7,22–24 where magnetic and superconducting properties appear to be different from those exhibited separately by the two phases.24,30 In this context, Sr3Ru2O7–Sr4Ru3O10 eutectic crystals, featuring a coexistence of metamagnetism and ferromagnetism, are likely to exhibit a magnetic behavior qualitatively different with respect to the corresponding single phase crystals. Guided by this motivation, we describe here the growth of this system realized by using a flux-feeding floating-zone (FFFZ) technique. The starting point was the analysis of the eutectic phase diagram of strontium ruthenate compounds and the methods used to grow high-quality crystals of Sr2RuO4,31 Sr3Ru2O7,32 Sr4Ru3O10,33 and Sr2RuO4–Sr3Ru2O7 eutectic crystals.22 Experimental Section Single crystals of eutectic Sr3Ru2O7–Sr4Ru3O10 were grown employing the FFFZ method with Ru self-flux. A commercial image furnace equipped with double-elliptical mirrors and two 2.0 kW halogen lamps (NEC Machinery, model SC1-MDH11020) was used. Details of the FFFZ crystal growth are described elsewhere.22,32 As for the growth of other strontium–ruthenate single crystals, because of the high volatility of Ru off-stoichiometric feed rods rich in Ru were prepared to compensate for the loss of RuO2 from the melting zone.32,33 The nominal ratio of the components in the feed rod is denoted as n ) 2N(Ru)/N(Sr), where N(Ru) and N(Sr) are the molar numbers. Following
10.1021/cg070180p CCC: $37.00 2007 American Chemical Society Published on Web 10/30/2007
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Table 1. Dependence of the Crystal Phase Constitution on the Growth Conditions for Sr3Ru2O7–Sr4Ru3O10 Eutectic Crystals using FFFZ Methoda sample
n
n′
bc01 bc02 bc03 bc04 bc05 bc06 bc07 bc08 bc09 bc10 bc11 bc12
1.8 1.8 2 1.8 1.8 1.8 1.8 1.8 1.7 1.7 1.65 1.6
1.55 1.55 1.71 1.58 1.58 1.58 1.55 1.59 1.50 1.48 1.44 1.43
v1 v2 P (mm/h) (mm/h) (bar) 12 15 15 12 13 14 12 11 13 13 13 13
35 29 37 33 34 35 35 35 35 35 35 35
9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 10 10 10
phases constitution of the crystal 113 + 4310 4310 + 113 113 + 4310 4310 + 327 4310 + 327 4310 + 113 4310 + 327 4310 + 113 4310 + 327 4310 + 327 327 + 4310 327 + 214
+ 327 + + + + + +
327 113 113 327 + 214 113 + 214 327
a All the crystal growth were performed in a mixed atmosphere of 10% O2 90% Ar. n ) 2N(Ru)/N(Sr) is the nominal ratio in the material feed rod; P is total pressure in the quartz tube; V1 indicates the crystal growth speed; V2 is the feed speed of material rod; 214 ≡ Sr2RuO4; 327 ≡ Sr3Ru2O7; 4310 ≡ Sr4Ru3O10 and 113 ≡ SrRuO3. Phase constitution of the crystal rod is reported in order of decreasing volume fraction as determined by powder XRD.
Figure 1. SEM image of a cleaved surface of the sample bc10.
the feed preparation method described in refs 22 and 32 and assuming that only RuO2 is lost during the growth, the following reaction takes place: SrRun/2O1+n f SrRun'/2O1+n' + (n - n')/2RuO2
(1)
Moreover, the parameter n′, equal to twice the average ratio of Ru to Sr for the grown single crystals, is given by n' ) n(1 – L) – 1.557L
(2) 32
where L is the fraction of mass loss during the crystal growth. The mass loss L can be evaluated by measuring the mass of the polycrystalline feed rod before and after the growth and the mass of the obtained crystal. It is important to underline that n′ is the average ratio over the whole crystal. In case of unstable crystal growth, many phases may be formed in the crystal, whose occurrence may not be reflected in n′. The optimal conditions to grow eutectic crystals Sr3Ru2O7–Sr4Ru3O10 were investigated changing n from 1.6 to 2. The ceramic feed rods were prepared by a standard solid-state reaction method. The starting powders were SrCO3 (99.99% purity; Ba < 8 ppm) and RuO2 (99.9%). The choice of an appropriate feed composition is crucial in growing Sr3Ru2O7–Sr4Ru3O10 eutectic crystals, since it affects the phase established during the growth. For this reason, to synthesize the proper polycrystalline feed rod, we optimized the sintering temperatures, dwell times, and atmospheres by thermogravimetric analysis/differential thermal analysis (TGA/DTA) and X-ray diffraction (XRD) measurements. The optimization procedure was intended to get a polycrystalline feed rod with complete reaction of the constituents and a small amount of SrRuO3 compared to the Sr3Ru2O7 and Sr4Ru3O10 phases. The optimal conditions were given by a preheating at 1450 °C for 18 h in air, followed by a sintering at 1420 °C for 2 h in air. After these thermal treatments, the feed rod is mainly constituted by Sr3Ru2O7 and Sr4Ru3O10 phases with a ratio depending on the chosen n value. For the crystal growth, we used a 6-mm-diameter feed rod and either Sr2RuO4 or Sr3Ru2O7 single crystal as seed. Since we wanted to grow a single crystal where Sr3Ru2O7 is embedded in a Sr4Ru3O10 matrix, the crystal growth speed, the atmosphere, and the pressure were chosen as appropriate for getting pure Sr4Ru3O10 single crystals.33 Seed translation speed, V1, that is, the crystal growth speed, and pressure, P, in the quartz tube were changed, while a gas mixture of 10% oxygen and 90% argon was used as atmospheric content. In Table 1, the crystal growth conditions as well as the phases identified in the crystals by XRD are reported. Similarly to the other strontium ruthenate crystals, the produced samples tend to cleave parallel to the growth direction. As shown by XRD, the cleaved surface is the a–b plane. The crystal surface was investigated by polarized light optical microscopy (PLOM) and by scanning electron microscopy (SEM) (LEO, model EVO 50), equipped with electron back-scattered (EBS) detector. The compositional analysis
Figure 2. PLOM image along the a–c plane of the sample bc11. The picture shows a lamellar pattern typical of the eutectic solidification. The lamellar period is on the order of 30 µm. Table 2. Compositional Results Obtained by EDS Analysisa
a
sample
phase 1
phase 2
bc09 bc10 bc11
Sr3Ru2.04O7.85 Sr3Ru2.09O7.16 Sr3Ru1.96O7.06
Sr4Ru3.07O10.72 Sr4Ru3.00O10.34 Sr4Ru3.05O9.88
The identified phases are Sr3Ru2O7 and Sr4Ru3O10.
was carried out by energy dispersive spectroscopy (EDS), while the crystal microstructure and the local orientation of the two phases were studied by means of electron back scattered diffraction (EBSD). The structure and crystalline qualities were assessed by a high-resolution X-ray diffractometer (Philips, model X′Pert MRD) with the same arrangement described in ref 22. We measured the transport properties of the Sr3Ru2O7–Sr4Ru3O10 eutectic crystals by the standard four point contact technique.
Results and Discussion To check the phases constituting the crystals, XRD measurements on crushed crystals were performed. From the results reported in Table 1, it is evident that the Sr3Ru2O7–Sr4Ru3O10 eutectic forms in a very narrow n′ range, compared with other members of the R-P series.34 These measurements indicated the presence of SrRuO3 in the crystals grown from a rod with a value of n in the range 1.8–2, by changing V1 in the range 11–15 mm/h. Since the intergrowth of SrRuO3 is due to an excess of Ru that does not evaporate during the crystal growth, we tried to grow crystals starting with less Ru in the polycrystalline material.
Eutectic Sr3Ru2O7–Sr4Ru3O10 Crystals
Crystal Growth & Design, Vol. 7, No. 12, 2007 2497
Figure 3. A BSE image of a polished a–b plane of Sr3Ru2O7–Sr4Ru3O10 crystal revealing the presence of two phases. On the top right corner is the phase map indicating the presence and location of the two phases.
As shown in Table 1, the most appropriate parameters to obtain eutectic crystals of Sr3Ru2O7–Sr4Ru3O10 are n in the range 1.7–1.65 and a crystal growth speed of 13 mm/h. In Figure 1, a SEM image of a cleaved surface of the sample bc10 shows an evident stacked structure. PLOM inspection showed that the optimized Sr3Ru2O7– Sr4Ru3O10 eutectic crystals have a lamellar arrangement of the two phases whose period is on the order of 30 µm. Along the a–c planes, crystals with n′ ) 1.50 and 1.48 showed lamellae of Sr3Ru2O7 included in a main matrix of Sr4Ru3O10, while the crystal with n′ ) 1.44 displayed a Sr3Ru2O7 matrix with Sr4Ru3O10 lamellae. We observed that the lamellae extend over areas as large as 500 µm. In Figure 2, a PLOM image on a polished surface of the crystal with n′ ) 1.44 cut perpendicularly to the cleaved plane (a–c plane) is shown. Backscattered electron (BSE) images confirmed the presence of two phases only. To ascertain the composition of the two phases, we have performed EDS analysis of crystals with different n′. In Figure 2, the dark areas indicate the Sr3Ru2O7 phase, while the bright ones indicate the Sr4Ru3O10. The composition of each measured crystal, summarized in Table 2, was derived by averaging all EDS data acquired over different areas by normalizing to the stoichiometric value of Sr ) 4 or Sr ) 3. EBSD was used to further characterize the microstructure of our eutectic crystals and to identify, possibly, the presence of grains having discontinuous orientation change at the interfaces. In Figure 3 we have shown a BSE image of the polished a–b plane of the sample bc11 on which EBSD mapping was performed. In the EBSD micrograph (right corner of Figure 3) a good matching can be inferred between Sr3Ru2O7 and Sr4Ru3O10 at the interface. EBSD also revealed the absence of spurious phases at the interfaces. This feature is also supported by BSE images and EDS analysis. Moreover, EBSD orientation mapping demonstrated a matching of the two phases along the three crystallographic axis as observed by XRD measurements. High resolution XRD pattern taken on a cleaved and polished surface of the crystals is consistent with the presence of only two phases (Figure 4). All the diffraction peaks can be identified
Figure 4. X-ray diffraction pattern of a cleaved surface of the sample bc09. All the peaks correspond to reflections due to Sr4Ru3O10 and Sr3Ru2O7 phases.
with the expected (0 0 l) Bragg reflections coming from Sr4Ru3O10 and Sr3Ru2O7. The c-axis parameters evaluated from the spectrum are c ) 28.622(7) Å for Sr4Ru3O10 and c ) 20.738(4) Å for Sr3Ru2O7. These values are in agreement with the reported ones for Sr4Ru3O10 and Sr3Ru2O7 single crystals.14,35 The presence of only (0 0 l) reflections is evidence that the crystals cleave in the a–b plane. The XRD analyses performed on other cleaved crystals gave similar results. In Figure 5, the pole figures obtained from sample bc09 are shown. To perform the pole figure measurements, the sample was tilted in Ψ and rotated in Φ. The data in Figure 5a were collected by using the (1 0 7) reflection of Sr3Ru2O7 obtained at a fixed value 2θ ) 38.177°, while in Figure 5b the (2 0 14) reflection of Sr4Ru3O10 at 2θ ) 66.253° is shown. The appearance of four poles for (1 0 7) of Sr3Ru2O7 and four poles for (2 0 14) of Sr4Ru3O10 at the same Φ indicates an in-plane alignment. The value of Ψ depends on the particular analyzed reflections. For (1 0 7) of Sr3Ru2O7 Ψ ) 36.40°, while (2 0 14) of Sr4Ru3O10 has Ψ ) 46.31°. These are the expected values for a surface perpendicular to the c-axis. From XRD measurements, we deduce that the Sr3Ru2O7 and Sr4Ru3O10
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promising candidate for exploring novel quantum critical phenomena specific to itinerant magnetism systems. Acknowledgment. Fruitful discussions with Y. Maeno, D. Zola, M. Salvato, A. Romano, M. Cuoco, and M. Gombos are gratefully acknowledged. We acknowledge B. Sagliocca and C. D’Apolito for technical support.
References
Figure 5. X-ray pole figures of the sample bc09. The fixed 2θ angle was (a) 38.177° corresponding to Sr3Ru2O7 (1 0 7) planes, (b) 66.253° corresponding to Sr4Ru3O10 (2 0 14) planes.
Figure 6. Resistance versus T2 for Sr3Ru2O7–Sr4Ru3O10 eutectic crystals. The current was 1 mA, and the dimension of the crystals was 4 × 2 × 0.5 mm. The electrical leads were put on a polished a–b plane. The solid line is a Fermi liquid T2 fit to the data between 4 and 26 K.
phases in our eutectic crystals have not only parallel c-axis but also parallel a- and b-axes. To check the transport properties of our eutectic crystals, as well as the behavior related to the presence of interfaces between Sr3Ru2O7 and Sr4Ru3O10, resistance versus temperature measurements were performed using a standard four probe technique with the leads on the a–b plane. The T2 dependence of the resistance shown in Figure 6 (sample bc10) in the temperature range of 4–26 K gives evidence of the same Fermi liquid behavior observed at low temperature in single crystals of both Sr3Ru2O7 and Sr4Ru3O10, thus supporting the picture of a goodquality interface. The good crystal matching and the variable percentage of the two phases in our crystals should allow the study of the peculiar anisotropic magnetic properties of Sr3Ru2O7 and Sr4Ru3O10 phases and their mutual interaction. Conclusions We showed that eutectic single crystals of Sr3Ru2O7–Sr4Ru3O10 can be grown by using the FFFZ method. BSE images and EDS compositional analysis confirmed the presence of the desired phases only, while PLOM investigations ruled out a different percentage of Sr3Ru2O7 and Sr4Ru3O10 in our crystals. The samples grew with a lamellar arrangement of the two phases. Concerning structural properties, X-ray analyses showed that the Sr3Ru2O7 and Sr4Ru3O10 phases are aligned not only outof-plane but also in-plane. The Sr3Ru2O7–Sr4Ru3O10 eutectic crystal provides a material for investigating natural junctions of metamagnetic–ferromagnetic type. Hence, it represents a
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CG070180P
