(Ru-1212) grown by top seeded melt

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Materials Science and Engineering B 163 (2009) 165–169

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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Physical properties and characterization of RuSr2 GdCu2 O8 (Ru-1212) grown by top seeded melt textured technique S. Uthayakumar a,b,∗ , P. Santhosh a , M. Gombos b , M. Ramesh Babu c , R. Jayavel c , A. Vecchione b , S. Pace b a b c

Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Department of Physics “E.R. Caianiello”, University of Salerno, via S. Allende I-84081, Baronissi (SA), Italy Crystal Growth Center, Anna University, Chennai 600 025, India

a r t i c l e

i n f o

Article history: Received 9 September 2008 Received in revised form 27 May 2009 Accepted 6 June 2009 Keywords: Top seeded melt growth Ruthenocuprates Magnetic superconductors Microstructure Electrical and magnetic properties

a b s t r a c t Efforts have been made in optimizing growth conditions for bulk textured growth of RuSr2 GdCu2 O8 (Ru-1212) employing top seeded growth technique (TSG). Thermal stability and peritectic temperature (Tp ) have been determined by TG–DTA analysis. The study facilitates appropriate thermal treatments to yield good quality sample. The structural study of the as grown sample has been carried out by Xray diffraction technique. Morphological study by polarized optical microscope (PLOM) and scanning electron microscope (SEM) reveals the growth mechanism. Subsequently, the stoichiometry has been conﬁrmed by energy dispersive X-ray analysis (EDAX). The observed characterization results show the inﬂuence of superconducting property on growth technique and allow one to correlate between the physical properties and experimental technique. © 2009 Elsevier B.V. All rights reserved.

1. Introduction There has been a renaissance of research on Ru-1212 which has been spurred due to the co-existence of superconductivity and magnetism [1]. The possibility in co-existence property was predicted for the ﬁrst time in the lanthanum–gadolinium compound [2]. Since 1958, the co-existence of such antagonistic relationship is a very long standing puzzle in solid state physics. It has been well established that the magnetic impurities are effective in suppressing superconductivity. In our previous study on Bi-2212 [3], we explored the suppression of Tc as a result of pair breaking mechanism upon magnetic substitution. In analogous, the presence of ‘Gd’ also expected to affect the superconductivity nature through distortions in the CuO2 planes. It is well known that Gd3+ ion possesses very strong magnetic moment at room temperature and is expected to affect the superconducting property; nevertheless it does not inﬂuence the superconductivity property. Existing reports on Ru-1212 predicts that the superconductivity originates from eg states of Cu in CuO2 layer and magnetic moment arises from t2g states of Ru in RuO plane. The magnetic moment of Ru t2g electron and superconducting Cu eg states are separated

∗ Corresponding author at: Department of Physics, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom. Tel.: +49 7116891372; fax: +49 7116891389. E-mail address: [email protected] (S. Uthayakumar). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.06.003

by insulating ‘SrO’ layers and as a result, superconductivity is preserved [4]. The co-existency develops in one thermodynamical phase and hence, it is highly essential to have good quality samples of Ru-1212 [5]. Numerous studies on growth, substitution and characterization end up with the scattered results due to polycrystalline nature of the sample. From the studies carried out so far, it becomes obvious that the preparation method plays a vital role on superconducting property. Wide range of techniques are employed to produce good quality samples of superconducting materials such as slow cooling (SC), traveling solvent ﬂoating zone (TSFZ), top seeded solution growth (TSSG) and top seeded melt textured growth (TSG) [6–9]. Among these techniques, slow cooling technique is the most viable method, since large number of single crystals can be grown in one single run. Indeed, prolonged thermal treatments with intermediate grinding ends with non-homogeneous sample which is due to high evaporation rate of RuO [10]. Encrusting growth complexities, TSG technique was opted for the growth of Ru-1212. 2. Experimental Although fundamental understanding with regard to melt processing is highly essential for optimization of thermal processing conditions, detailed phase diagram informations, particularly for solid–liquid phase relations, are very limited. To overcome such a problem, DTA–TGA analysis was made to optimize the thermal proﬁle for the growth of top seeded growth technique.

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Fig. 2. Heat treatment pattern for the TSG growth.

Fig. 1. Thermal behaviour (DTA–TGA) of Ru-1212.

Differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA) for Ru-1212 samples were carried out using SETARAM system. This system is equipped with attachments for DTA and TGA. The scan rate has been set up at 5 ◦ C/min, and alumina crucibles have been used as sample holders. The use of DTA/TGA facilitates the optimization of the TSG growth by selecting the appropriate thermal proﬁle of the sample to be crystallized. From Fig. 1, it is evident that the structural decomposition of the Ru-1212 phase occurs at 1073 ◦ C. Such a behaviour infers that the phase decomposition of 1212 takes place in oxygen. This in turn implies that high temperature annealing performed during growth process leads to a phase separation into FM and SC phases [11]. No other peaks were observed other than the strong endothermic peak at 1073 ◦ C which reinforces the purity of the sample processed through two step solid state reaction (explained in the subsequent part). The TGA curve shows a sharp slope at 830 ◦ C and this weight loss has been assigned to the ‘Ru’ evaporation. On careful reviewing of DTA/TGA results, the polycrystalline Ru-1212 precursor powder was obtained through two step solid state reaction methods. The two step process encompasses the synthesis of RuSr2 GdO6 (Gd-1210) followed by the impregnation of CuO [12]. Gd-1210 was synthesized by direct mixing of RuO2 , SrCO3 and Gd2 O3 with 3N purity according to the nominal composition. The individual stoichiometric mixtures were grinded in an agate mortar using acetone as a binding agent. This mixture was calcined twice at 960 ◦ C for 24 h with intermittent grinding. The phase pure Gd-1210 powders were mixed with CuO in right stoichiometry and subjected to thermal treatment in a single zone mufﬂe furnace with an oxygen ﬂow at 1020 ◦ C for 72 h. The synthesized Ru-1212 and Ru-1210 powders was taken in a molar ratio of 0.8:0.2, grinded well and pelletized into a cylindrical pellet. Single crystal MgO seed of 1 mm × 1 mm × 1 mm was placed at the center of the pellet and then placed in the furnace to undergo the top seeded growth. Fig. 2 displays the heat treatment patterns followed in the present investigation. The samples were heated to 850 ◦ C at the rate of 300 ◦ C/h and then to 1155 ◦ C at 50 ◦ C/h with a dwell time of 5 h to accomplish complete melting. Then the crystal growth was started by cooling the sample at the rate of 50 ◦ C/h down to 1095 ◦ C, where it was soaked for 5 h, then cooled at 1 ◦ C/h to 1080 ◦ C, with a dwell time of 5 h to ensure a complete growth. In the ﬁrst sector, the sample temperature was raised above the peritectic temperature and the semi-solid state pellets were then rapidly cooled below the peritectic temperature. The whole process was performed in air. From the as grown sample, a small pellet of size 4 mm × 4 mm × 4 mm has been selected for characterization studies. The structural property of the as grown Ru-1212 was studied using X-ray diffraction technique and resistivity was measured

using standard four-probe method. Morphology and composition was studied by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). 3. Results and discussion 3.1. X-ray diffraction studies X-ray diffraction patterns were obtained on a Philips PW1700 diffractometer using Ni-ﬁltered CuK␣ radiation. Diffraction patterns were collected at room temperature over the 2 range (20–75◦ ) with a step of 0.05◦ . Fig. 3 shows the XRD analysis performed both on Ru-1210 and on Ru-1212 precursors. Both the patterns of Ru-1210 and Ru-1212 precursor powders conﬁrm a phase pure nature which has to be used for the top seeded growth. Further, Rietveld analysis was carried out on TSG grown Ru-1212 and is shown in Fig. 4 (red: experimental points, black: calculated curve, green: expected Bragg position; and blue curve: difference between the experiment and calculation). It is clearly evident that all the peak positions are ﬁtted well and Rietveld analysis of the XRD data reveals that the structure is tetragonal in P4/mmm space group with lattice parameters a = 3.8338 Å and c = 11.5418 Å. During the synthesis process, most of the growth accompanies with the formation of impurity phases but our optimized thermal proﬁle succeeded in suppressing such impurities and leads to a phase pure Ru-1212 as substantiated from our structural study. Further, it is evident from the ﬁgure that there is a signiﬁcant change in the intensity ratio of (1 1 0) and (2 0 0) reﬂections, which conﬁrms a partial orientation of the TSG sample along the (l 0 0) direction. However, the intensity of the (1 1 0) peak and appearance of other reﬂections con-

Fig. 3. X-ray diffraction patterns of RuSr2 GdO6 and RuSr2 GdCu2 O8 precursors.

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Fig. 6. M–T measurements of Ru-1212 at different magnetic ﬁelds.

Fig. 4. Rietveld plot showing the observed, calculated and difference pattern for Ru-1212. The Bragg reﬂections for both the phases are indicated by vertical bars.

ceals that only small regions of the sample are oriented, being the whole of it of multi-crystalline in nature. The appearance of unidentiﬁed peaks in the X-ray pattern might be due to the superstructure of the tilted RuO6 [13,14]. 3.2. Resistivity and magnetization studies The temperature dependence of resistivity of Ru-1212 was measured using a standard four-probe method, Fig. 5. The data clearly discerned a sharp superconducting transition at 54 K. Normally, the superconductivity transition is sharper when the inter-grain structure is homogenous. There are several reports which assert a broader transition width Tc (∼15–20 K) (Tc = Tc(onset) − Tc(zero) ) compared to other superconductors [15]. In general, there are several reasons for the broadness of the transition. One among them is granularity and various intra-grain links. But in the case of Ru1212, it has been ascribed to motion of self-induced vortices. The TSG grown Ru-1212 yields sample with a narrow transition width (Tc ) of about 6 K. It should be emphasized that the best quality sample resides in the range (∼8.5 K) [16]. This clearly manifests the superior quality of the as grown sample which in turn refers a stronger inter-grain coupling, probably due to coarsening of the Ru-1212 grains during the TSG growth.

Fig. 5. Temperature dependence of resistivity behaviour of as grown Ru-1212.

Fig. 6 presents the M–T measurements of Ru-1212 at different magnetic ﬁelds. It is evident from the ﬁgure that the ferromagnetic phase appears at T = 150 K. The obtained broad transition clearly shows the behaviour which is normally observed for the high Tc superconductors. Moreover, one should clearly note that the applied ﬁeld does not changes the transition temperature (Tc ) but slightly broadens the transition width. This might be due to the orientation of the spontaneous magnetization of the individual domains with respect to applied ﬁeld. Furthermore, the preservation of zero resistance state at different magnetic ﬁeld clearly demonstrates the better quality of the as grown sample. 3.3. Morphological study The microstructures of the as grown sample were observed under polarized light optical microscope (PLOM) and scanning electron microscopy (SEM) studies. Optical microscopic studies show striation pattern, Fig. 7. The observation of striations is believed to a non-steady state convective motion due to different temperature zones during the growth process. Despite the observation of striations, it indirectly expresses non-homogeneity of melt sequence in the growth processes. The later growth pattern of the as grown sample observed through scanning electron microscope is shown in Fig. 8. This demonstrates that the growth takes place by means of a two-dimensional nucleation mechanism [17]. A dark line oriented along a and b axes could be seen from SEM image (Fig. 9), which shows the presence of oriented domains with

Fig. 7. Polarized light optical microscope image of Ru-1212 showing striation.

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Fig. 8. SEM image of Ru-1212 showing layer growth pattern.

Fig. 10. SEM image of Ru-1212 showing platelet morphology.

a strong domain border. The differences between the two regions across the domain border might be due to insufﬁcient melting of Ru-1212 preventing from a uniform melt growth. The crystals of YBa2 Cu3 O7−ı were well known for its platelet morphological characteristic nature. A similar feature was observed for the as grown Ru-1212 which conﬁrms the resemblance to YBCO, Fig. 10. We believe that when the single crystal seed (MgO) lies at the top of the Ru-1212 pellet which contains supersaturated solution, the convection currents produced in the liquid encourages growth along the plane parallel to the seed, as a consequence of platelet morphology. It is well known that the grain growth of the samples depends on temperature and dwell time. Our morphological study exploits the absence of high density cracks, which is due to the slow cooling rate during the melt processing. This yields a good quality material with minimum defect. 3.4. Compositional analysis In some cases, a dark grey matrix is noticed during morphological investigations, which might be due to the variation of chemical composition or speciﬁc crystallographic orientation or even some other surface defects [18]. To distinguish among these, the sample was subjected to EDAX analysis and the EDAX spectrum of Ru-1212 is shown in Fig. 11. The EDAX spectrum shows the existence of all

Fig. 11. EDAX spectrum of Ru-1212.

Table 1 EDAX results of Ru-1212. Elements

Starting composition (wt%)

Crystal composition (wt%)

Ru Sr Gd Cu

14.68 25.45 22.83 18.46

15.88 27.44 23.11 20.94

the constituent elements and the results corresponding elements in weight percentage are given in Table 1. From the table it is inferred that, there is no much deviation in the composition of the as grown Ru-1212. 4. Conclusions We have successfully carried out the growth of Ru-1212 by top seeded growth technique. Our result demonstrates that the thermal proﬁle and the growth technique are important factors concerning the superconductivity property in Ru-1212. Further improvement in the texturing and increase of superconducting transition temperature (Tc ) is expected to be feasible by varying the molar ratio of RuSr2 GdO6 (Gd-1210) and RuSr2 GdCuO8 (1212) with an appropriate thermal proﬁle. References

Fig. 9. SEM image of Ru-1212 showing domain border across the melt region.

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