Supercond. Sci. Technol. 13 (2000) 183–186. Printed in the UK PII: S0953-2048(00)07485-6 Temperature dependence of filament-coupling in Bi-2223 tapes: magneto-optical study A V Bobyl†‡, D V Shantsev†‡, T H Johansen†, M Baziljevich†, Y M Galperin†‡ and M E Gaevski‡ † Department of Physics, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway ‡ Ioffe Physico-Technical Institute, Polytechnicheskaya 26, St Petersburg 194021, Russia Received 1 September 1999 Abstract. Coupling through random superconducting bridges between filaments in a multifilamentary Ag-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ tape has been investigated by magneto-optical imaging at temperatures from 20 K up to T c . Magnetic flux distributions have been measured on the surface of an intact tape in the remanent state on applying a strong perpendicular magnetic field. The flux distributions observed at low temperatures reflect the arrangement of individual filaments. At high temperatures, the distribution becomes more similar to that for a uniform monocore tape, indicating that superconducting connections appear between the filaments. To discuss the relative contributions of the intra- and inter-filament currents, a simple model based on the Bean critical state was proposed and applied to analyse the temperature dependent behaviour. The inter-filament coupling, increasing with temperature, reaches at 77 K a point where the currents flowing in large inter-filament loops are roughly equal to the intra-filament currents. 1. Introduction There is great interest in making use of high-T c supercon- ducting multifilamentary tapes in power applications. An important issue concerning multifilamentary tapes is the pos- sible presence of superconducting connections between the filaments. Such a connection may appear through forma- tion of random inclusions of the superconducting material, making up superconducting ‘bridges’ between neighbouring filaments. On the one hand, the coupling between filaments may be useful for maintaining the current flow when it en- counters a structural defect within a single filament. on the other hand, however, this also leads to a substantial increase in ac loss, important for many applications, such as power transmission lines or ac magnets. Filament bridging was observed in Ag-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223) tapes and its effect on the mag- netization loops was analysed in [1]. For multifilamentary samples, the measured susceptibility is considerably larger than that in isolated filaments, being although somewhat smaller than the value expected for a monocore tape. Studies of ac loss in a Bi-2223/Ag tape have shown that the filaments behave as if they were connected into bundles, a typical bun- dle being composed of eight filaments [2]. Although the filament bridging strongly affects the magnetic properties of a tape, its existence can be established unambiguously only by space-resolved studies. Indeed, these techniques have proved to be very successful in investigations of field and current distribution in mono- and multifilamentary tapes [3–7]. Hall-probe measurements of the remanent state after passing a transport current [5] and magneto-optical (MO) imaging at various applied fields [4] have both revealed the existence of superconducting bridges between filaments in Bi-2223/Ag tapes. In the present work we used the MO imaging method to investigate the temperature dependence of filament coupling. Since there are different mechanisms limiting the current inside individual filaments and the current leaking through the random superconducting interconnections, one would expect different temperature dependences of the corresponding critical currents. In order to separate the two contributions experimentally we constructed a model which can be fitted quantitatively to the MO images. Good agreement is achieved over a wide range of temperatures which allows the behaviour of both the intra- and inter-filament critical currents to be determined. 2. Experiment The object of study was a 55 filament Bi-2223 tape, prepared by the powder-in-tube method with subsequent drawing and rolling [8]. The tape width, including the Ag sheath, was 3.7 mm; the critical current was 45 A at 77 K. An optical image of the tape cross section is shown in figure 1(a). MO images of the magnetic field distributions were taken using a Faraday-active Bi:YIG indicator film with in-plane magnetization. The indicator, grown by liquid phase epitaxy 0953-2048/00/020183+04$30.00 © 2000 IOP Publishing Ltd 183