Cryst. Res. Technol. 39, No. 10, 926 – 931 (2004) / DOI 10.1002/crat.200410278

Growth, structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10-y crystals E. Giannini*1, V. Garnier1, N. Clayton1, N. Musolino1, R. Gladyshevskii2, and R. Flükiger1 1 2

DPMC, University of Geneva, 24, quai E.-Ansermet, CH-1211 Geneva 4, Switzerland Dept. Inorg. Chem., Ivan Franko National University of L'viv, Kyryla i Metodiya str. 6, UA-79005 L'viv, Ukraine

Received 24 February 2004, revised 10 April 2004, accepted 20 April 2004 Published online 1 September 2004 Key words crystal growth, Bi-2223, high Tc superconductivity. PACS 74.72.H, 81.10.Fq Large and high-quality single crystals of both Pb-free and Pb-doped high temperature superconducting compounds (Bi1-xPbx)2Sr2Ca2Cu3O10-y (x = 0 and 0.3) were grown by means of a newly developed "VapourAssisted Travelling Floating Zone" technique (VA-TSFZ). This modified zone-melting technique was realised in an image furnace and allowed for the first time to grow Pb-doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm3 were successfully grown. Postannealing under high pressure of O2 (up to 10 MPa at T = 500°C) was undertaken to enhance Tc and improve the homogeneity of the crystals. Structural characterisation was performed by single-crystal X-ray diffraction (XRD) and the structure of the 3-layer Bi-based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb-free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (∆Tc ≤ 1 K). In optimally doped Pb-free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2·105 A/cm2 were measured at T = 30 K and µ0H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40-50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1-5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d-wave superconductivity in Bi-2223.

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1

Introduction

In spite of the remarkable improvements in (Bi,Pb)2Sr2Ca2Cu3O10-y (Bi,Pb-2223) tape and wire fabrication for industrial applications [1], structure details, fundamental superconducting features and intrinsic pinning properties of this material are still unknown, due to the lack of high-quality single crystals and thin films. Single crystals of both Bi-2223 and Bi,Pb-2223 are necessary for fundamental investigations as well as a better knowledge of these phases, with the aim of improving the performances of Bi-based tapes and wires. Few Bi2223 crystals have been successfully grown in the recent past by the alkali-chloride flux [2-5] and by the travelling solvent floating zone method [6-7]. Large (up to 4×2×0.01 mm3) Pb-free Bi-2223 crystals were grown, but quite broad superconducting transitions were reported. The Bi-2223 crystals generally contain a small proportion of the Bi-2212 phase which could perturb measurements of the intrinsic properties of the Bi2223 phase. ____________________

* Corresponding author: e-mail: [email protected] © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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By means of the newly developed Vapour-Assisted Travelling Floating Zone Technique (VA-TSFZ), suitable for crystal growth in the presence of volatile elements such as Pb, we succeeded in growing large (up to 2×3×0.1 mm3) and pure single crystals of both Pb-free and Pb-doped Bi-2223. The Bi-2223 structure was refined for the first time on these crystals. The magnetic characterisation is also reported together with the study of the superconducting properties of these compounds. The irreversibility line was measured and interpreted as a transition from an entangled vortex glass to a vortex liquid. The measurement of the lower critical field Hc1 was performed and allowed us to obtain the temperature dependence of the penetration depth, λ. The linear dependence of λ(T) is a signature of d-wave superconductivity in Bi-2223.

Fig. 1 Sketch of the image furnace employed for the TSFZ crystal growth.

2

Experimental

Both Pb-free and Pb-doped Bi-2223 single crystals were grown by a modified Travelling Solvent Floating Zone technique in a home-made image furnace equipped with two 400 W halogen lamps, in which an internal source of Pb-vapour was added (Vapour-Assisted TSFZ). Details of the precursor and sample preparation are reported elsewhere [8]. In the presence of a volatile element, such as Pb in the Pb-doped Bi-2223 samples, losses occur at high temperatures. The internal source of PbO, kept at 700-750°C inside the image furnace, supplies a Pb vapour flow which compensates for the Pb-losses in the molten zone and allows us to grow Pbdoped crystals [8]. A sketch of the furnace is shown in Fig. 1. A fast pre-melting was performed at 25 mm/h in order to densify the feed rod. Both the feed and the seed rods were counter-rotating at ω = 0.14 s-1. The crystal growth process was performed at the optimised travelling velocity of 50-60 µm/h. In order to lower the precursor melting temperature, a reduced oxygen partial pressure was employed during growth (93%Ar-7%O2). The thermal gradient at the liquid-solid interface was measured to be 50°C/mm and the thickness of the molten zone be ~3 mm. Post-annealing in pure O2 at high pressures (1-100 bar) was applied in order to increase and homogenise the oxygen content. Single crystal X-Ray Diffraction was performed in a STOE Image Plate Diffraction System using MoKα radiation. Superconducting properties of our crystals were investigated by means of A.C. susceptibility and SQUID magnetometry.

3

Results

3.1 Pb-free Bi2Sr2Ca2Cu3O10 single crystals As-grown crystals, cleaved out from the seed rod, have a size up to 2×3×0.1 mm3 and exhibit quite a low critical temperature (Tc onset ≤ 103 K) and very broad superconducting transitions. After annealing under high © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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E. Giannini et al.: Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10-y crystals

pressure of pure oxygen, the critical temperature increases up to Tc = 111 K (at the optimal doping, after 20 h annealing in pO2 = 20 bar at T = 500°C) and the width of the transition is reduced to 5-6 K. After further O2 annealing, the critical temperature slightly decreases (Tc = 109 K) but the transition becomes very sharp (∆T < 1 K), indicating the high homogeneity of the oxygen content inside the crystals. The superconducting transition measured after annealing in pO2 = 100 bar at T = 500°C for 100 h is shown in Fig. 2. In the insert, a picture of the same crystal is shown.

Fig. 2 ZFC susceptibility of the Pb-free Bi-2223 single crystal shown in the insert. The crystal was annealed in pure oxygen (pO2 = 100 bar) for 100 h at 500°C and shows a very sharp superconducting transition.

It is worth noting that the value -1 of the low temperature susceptibility in Fig. 2 is directly obtained from the measured magnetic moment after taking into account sample mass, theoretical density of Bi-2223 ( = 6.6 g/cm3 ) and demagnetising coefficient. Single Crystal X-ray Diffraction measurements confirmed the good quality of these crystals. The diffraction patterns in Fig. 3 clearly show that the Bi-2223 structure is modulated, as marked by the diffraction satellites in the [0kl] and [k0l] directions. The refined supercell is orthorhombic with a space group A2aa and lattice parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. The a-axis modulation is not commensurate, the short cell vector being q~0.21a*. If Bi-2212 intergrowths are present in our crystals, their amount is below the sensitivity of our XRD and magnetic measurements.

Fig. 3 Diffraction patterns of a Pb-free crystal acquired with an Image Plate. Satellites are present in [0kl] and [k0l] directions, indicating a modulated superstructure in the ab plane.

Magnetisation measurements have shown the presence of a second peak in the m(H) hysteresis loops at intermediate temperatures [8]. The second peak in Bi-2223 is found to be weaker than in Bi-2212 [9]. The critical current density, as obtained from the hysteresis loops by using the Bean model, is jc = 2·105 A/cm2 (at © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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T =30 K, µ0H = 0 T) and decreases rapidly above T =40 K. As a general remark, the magnetic behaviour of the Bi-2223 phase exhibit several analogies with the Bi-2212 phase.

3.2 Pb-doped (Bi,Pb)2Sr2Ca2Cu3O10-y single crystals By compensating for the Pb-losses occurring at high temperatures, Pb-doped crystals were grown by the VATSFZ method as described above. Typical 2×3×0.1 mm3 crystals are obtained with an average composition Bi2.16Pb0.26Sr2.08Ca1.95Cu2.55Ox as measured by EDX. The superconducting transition of these crystals is shown in Fig. 4.

Fig. 4 ZFC susceptibility of the as-grown Pb-doped Bi2223 single crystal shown in the insert.

One should notice the higher value of Tc and the narrower transition compared to the as-grown Pb-free crystals. Pb-doped crystals grow more homogeneous and closer to the optimal doping, indicating that the presence of Pb homogenise the liquid, enlarges the crystallisation field of the Bi,Pb-2223 phase and changes the growth conditions. Structural investigations are in progress in order to elucidate the effect of the Pb doping. Pb-doped Bi,Pb-2223 single crystals also exhibit a weak second peak at intermediate temperatures. The critical current values are of the same order of magnitude as in Pb-free samples (the value strongly depending on the sample) and the jc(H) dependence is also similar in both kinds of crystals.

3.3 Superconducting properties of Pb-free Bi-2223 crystals: vortex phase transition and unconventional superconductivity The irreversibility line of these crystals is obtained by measuring the ZFC and FC branches of the magnetic moment vs temperature curves art various applied fields up to µ0H = 5 T. The irreversibility line, Hm(T), generally interpreted as the onset of the bulk pinning, is plotted in Fig. 5. The transition is marked by a kink in the magnetic moment vs. temperature curve and become more pronounced as the magnetic field increases. The transition from the irreversible to a reversible state can be interpreted as the melting of an entangled vortex glass into a vortex liquid [10]. We used the Lindemann criterion fro melting coupled with the expression for two-dimensional fluctuations to account for this transition and we obtained the following equation: H m (T ) ≈

  φ02 cL2 s φ0 exp   2 2 2 s γ  8πλ ab ( 0 ) k BT 

(1)

where cL is the Lindemann number, s is the interlayer spacing, γ is the anisotropy, λ is the penetration depth, φ0 is the magnetic flux quantum, kB the Boltzmann constant and T the temperature. The fit to the experimental data using the eq. 1 is plotted in Fig. 5 (solid line). The best fit is obtained with cL = 0.11, λ(0) = 1220 Å and γ = 270. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 5 Irreversibility line for Bi-2223 single crystals, measured with H//c. The solid line is a fit to the experimental data using a Lindemann criterion for melting.

Fig. 6 Penetration depth, λ, as a function of temperature as obtained from the measurements of the lower critical field. The linear dependence of λ(T) at low temperature is a signature of d-wave superconductivity in Bi-2223.

One essential point never investigated so far is the nature of the order parameter in the Bi-2223 phase. We have measured the lower critical field, Hc1(T), in order to obtain the temperature dependence of the in-plane penetration depth, λ. In order to avoid spurious effect on the Hc1(T) measurements due to surface barrier effects, we perform these measured at a very low field sweeping rate (dH/dt < 10-4 Oe/s). No upturn of the Hc1(T) curve at low temperature was observed, which proves that no surface barrier effect are affecting our results [10]. The temperature dependence of the penetration depth was obtained from the Hc1(T) by using the Ginzburg-Landau expression Hc1 = (φ0/4πλab2)lnk, (where k is the Ginzburg-Landau parameter) and the results are shown in Fig. 6. For T < 30 K, a linear dependence is found, which is consistent with d-wave symmetry of the order parameter in Bi-2223 [10, 11]. These are the first measurements of λ(T) for Bi-2223 single crystals from Tc down to 10 K.

4

Conclusions

We succeeded in growing large (up to 3×2×0.1 mm3) and pure single crystals of both Pb-free and Pb-doped Bi2223, by means of a newly developed Vapour-Assisted Travelling Floating Zone technique. Our method allows us to compensate for the Pb-losses occurring at high temperatures. The crystal structure of the Pb-free Bi-2223 phase was refined for the first time. A incommensurate modulated superlattice was found, with space group A2aa. Superconducting investigations have shown several features of Bi-2223, such as the phase transition from an entangled vortex glass into the vortex liquid, and the evidence of d-wave superconductivity in Bi-2223, given by the linear state of λ(T) at low temperatures.

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