Magnetization studies of Tl0.6Pb0.4Sr1.7Ba0.3CaCu2Oy single crystals

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Abstract

Magnetic measurements for fields up to 12 T have been performed on Tl0.6Pb0.4Sr1.7Ba0.3CaCu2Oy single crystals in order to investigate the effect of high pressure annealing in a N2 atmosphere on the peak effect of the magnetic hysteresis curves. After annealing, X-ray diffraction revealed a reduction of the distance between the superconducting CuO2 planes which we relate to a decrease of the anisotropy γ. As a consequence we observe a shift of both the magnetization peak and irreversibility lines towards higher fields, which can be accounted by considering plastic deformations of the flux line lattice above the peak. At low temperatures, where relaxation is weak, the magnitude of the shielding currents is significantly reduced after annealing, in agreement with the predictions of the anisotropic collective pinning theory.

Introduction

The high temperature superconductors (HTSC) are layered materials and this confers on them a large electronic anisotropy [1]. Included in this category are the Tl containing compounds, which form two homologous series Tl(Ba/Sr)2Can−1CunO2n+3 and Tl2Ba2Can−1CunO2n+4 consisting of n perovskite-like slabs separated by single and double layers of thallium oxide, respectively. The possibility of changing the thickness of the rocksalt-type layers containing the Tl cations makes these materials very interesting as far as the effect of anisotropy on the superconducting properties is concerned.

A striking and still actively investigated feature of the hysteretic magnetization curves of the HTSC is the second peak i.e., the non-monotonic field dependence of the shielding currents. This phenomenon has been analyzed in several studies and has been attributed to various mechanisms like a field induced increase of pinning due to sample inhomogeneities [2], crossover from surface barrier to bulk pinning [3], relaxation effects [4], 3D to 2D transitions [5]or to a crossover from elastic to plastic creep of the flux line lattice 6, 7. However in the case of the Tl containing superconductors even after several experimental studies on various members of this family 8, 9, 10, 11, 12, 13, 14, little insight has been attained into the origin of the second magnetization peak, whereas the effect of anisotropy on it is still far from being elucidated.

In this study we performed magnetization measurements on a Tl0.6Pb0.4Sr1.7Ba0.3CaCu2Oy single crystal in order to investigate the effect of high pressure annealing in a N2 atmosphere on the pinning of the flux line lattice. With the annealing treatment we succeeded in reducing the c-axis parameter and hence the spacing of the cuprate sheets, which leads to a decrease of the electronic anisotropy γ=(mc/mab)1/2 [15]. The obtained results demonstrate the strong influence of γ on both the magnitude and field dependence of the shielding currents and are consistent with recent predictions 1, 7, 16.

Section snippets

Experimental details

Tl-1212 single crystals were grown from a mixture with the nominal composition Tl0.6Pb0.4Sr1.7Ba0.3CaCu2Oy. Initially CaCO3, CuO, BaO2 and SrCO3 (99.9% pure) were mixed with a composition of (Sr1−x, Bax):Ca:Cu=2:1:2, x=0.3 and sintered at 930°C for 48 h. Subsequently ceramic mixtures of (Sr1−x, Bax)CaCu2Oy, PbO and Tl2O3 according to the ratio (Sr1−x, Bax)CaCu2Oy:Tl:Pb=1:0.6:0.4 were formed. The mixture was wrapped with a gold foil and put into a tube furnace and heated at 950°C for 3 h. Then

Results

Until this investigation the pinning properties of the Tl-based compounds were studied by using samples of various compositions and origins. In this way the magnetic response of different members of the family of Tl-containing superconductors was investigated and some qualitative conclusions about the effect of anisotropy on their magnetic behaviour have been inferred [13]. However, comparison of different crystals can lead to inaccurate conclusions due to unavoidable differences in their

Discussion

Annealing of the single crystal in N2 is an oxygen reducing procedure and can effect the vortex dynamics in various ways. Firstly, the carrier density can be reduced resulting in a change of the intrinsic parameters of the superconductor. However, in our case this effect should not be important considering the independence of the critical temperature on the annealing procedure. Furthermore, removal of oxygen leads to an increase of the number of oxygen vacancies and thus of pinning centers. As

Conclusions

We have found that high pressure annealing of T1-1212 single crystals in flowing N2 results in a reduction of the distance between the CuO2 layers and apparently decreases the anisotropy γ. It was demonstrated that all the observed changes in the magnetic properties could be mainly attributed to the change of γ. At low temperatures (T≲10 K) the annealing treatment reduces the value of the current density for all fields studied (up to 12 T). This can be explained within the framework of the

Acknowledgements

This work is part of a project supported by the EPSRC (UK). S.K. acknowledges financial support from the University of Southampton.

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    On leave from the Moscow Institute of Radioengineering, Electronics and Automation, 117454, Moscow, Russia.

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