Ac irreversibility line of bismuth-based high temperature superconductors
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Ac irreversibility line of bismuth-based high temperature superconductors A. Mehdaoui Laboratoire de Physique et de Spectroscopie Electronique, URA 1435, Facult´e des Sciences, Universit´e de Haute Alsace, 4, rue des Fr`eres Lumi`ere, 68093 Mulhouse Cedex, France
J. Beille Laboratoire Louis N´eel, CNRS, BP 166, 38042 Grenoble Cedex 9, France
D. Berling and B. Loegel Laboratoire de Physique et de Spectroscopie Electronique, URA 1435, Facult´e des Sciences, Universit´e de Haute Alsace, 4, rue des Fr`eres Lumi`ere, 68093 Mulhouse Cedex, France
J. G. Noudem and R. Tournier EPM– MATFORMAG, Laboratoire d’Elaboration par Proc´ed´e Magn´etique, CNRS, BP 166, 38042 Grenoble Cedex 9, France (Received 1 February 1996; accepted 7 April 1997)
We discuss the magnetic properties of lead doped Bi-2223 bulk samples obtained through combined magnetic melt texturing and hot pressing (MMTHP). The ac complex susceptibility measurements are achieved over a broad ac field range (1 Oe , hac , 100 Oe) and show highly anisotropic properties. The intergranular coupling is improved in the direction perpendicular to the applied stress and magnetic field direction, and an intragranular loss peak is observed for the first time. A comparison is made with other bismuth-based compounds and it is shown that the MMTHP process shifts the ac irreversibility line (ac IL) toward higher fields. It is also shown that all the ac IL’s for quasi 2D bismuth-based compounds show a nearly quadratic temperature dependence and deviate therefore strongly from the linear behavior observed in quasi 3D compounds and expected from a critical state model.
I. INTRODUCTION Improving the critical current jc at liquid nitrogen temperatures is one of the main goals for technological applications of high temperature superconductors (HTS). If one excepts the case of thin films, jc values at 77 K remain unfortunately at rather low values in bulk samples (102 Aycm2 , 103 Aycm2 ), owing to the granular structure obtained through classical sintering techniques. The grains, characterized by their own critical parameg g ters (Tcg , Hcl , Hc2 , jcg ), are embedded in a medium whose j j characteristic parameters (Tcj , Hcl , Hc2 , jcj ) exhibit lower values, and which act as an intergranular network of weak links. Therefore it turns out that the upper limit of the bulk critical current corresponds to the intergranular value jcj . Since the discovery of the first HTS, improved critical currents have been achieved either through new fabrication routes (sol-gel techniques, use of precursors, etc.) and/or through appropriate working atmospheres (low O2 partial pressure, fluorination, etc.) which act on the size of the grains and on the nature of the links (amorphousness, oxygen diffusion, and penetration depth). Thermomechanical treatments such as partial melting, zone melting, quench and melt growth (QMG), or melt processed melt growth (MPMG) have been introduced 2226
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J. Mater. Res., Vol. 12, No. 9, Sep 199
