Solid State Communications, Vol. 78, No. 3, pp. 237-239, 1991. Printed in Great Britain. 0038-1098/91 $3.00 + .OO Pergamon Press plc MAGNETIC PROPERTIES AND CRITICAL CURRENT DENSITIES OF YBa,Cu,O, OF LARGE CRYSTALS M. Guillot, B. Souletie and J.L. Tholence S.N.C.I. and C.R.T.B.T. C.N.R.S. B.P. 166X, 38042 Grenoble and Ctdex, France H. Noel, J.C. Levet, M. Potel and P. Gougeon Chimie Minerale B C.N.R.S., Avenue du General Leclerc, 35042 Rennes Ctdex, France (Received 15 November 1990 by P. B&et) Large crystals of YBa,Cu,O, (3 x 3 x 2mm3) have been grown by the flux method (r, N 89 K). High magnetic field (up to 20Tesla) magnetization measurements have been used to determine the critical current densities Jc (H, r) from the Bean model. The same features as for smaller samples are observed: magnetic jumps at low temperature, a maximum in the field dependence of the critical current density. The several effects (smaller Tc irreversibility line H(T) and field where J, has a maximum) are associated with a small content of Sr impurities. 1. INTRODUCTION MAGNETIC measurements are a convenient way of obtaining important characteristics of superconduc- tors [l-3]: H,, , Hr2, irreversibility field, critical current densities, etc . . . In the case of strongly anisotropic superconductors however, the determination of the different components of the anisotropic properties can only be made through a careful study of the effect of the sample dimensions [ 1,4,5]. For instance the initial current density in the a, b plane, for a field direction along the c-axis J& can be easily determined assuming Jj = Jh(‘. For field directions along the a or b direc- tions, the induced current flows successively along the b or a direction, then, along the c direction, and for thick (along c) crystals it is limited by the smaller critical current density along the c axis: J$‘. We have previously observed: magnetization jumps at low tem- perature, a well marked maximum of J(H) which we attributed to pinning centers due to oxygen defects, an exponential decrease of J(T), a exp - @T/20 for both field directions up to - 50 K and an anisotropy ratio zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ J:.h I J:,&e - 4, observed in this temperature range [I], keeping in mind that this ratio is thickness depen- dent since ~$~a~e tends to JCah for thick samples and to Jhu.;f for thin films [I, 41. We focus here on the magnetization of a single crystal (3 x 3 x 0.8mm3) for a field applied along the c-axis. (Superconducting current in the a, b plane giving J&). The single crystal is different from those previously studied [l] since it has been prepared by a flux method (instead of a mineralization process), but it is of comparable large size. However, due to the poor quality of starting materials a small amount of Sr impurities is present, and it is shown to strongly modify properties like the flux pinning in a field. 2. MAGNETIZATION MEASUREMENTS The magnetization has been measured at constant temperatures, in a steady field (Bitter coil) which varied by steps of 0.2 T up to 4 T and of 0.4 T or 0.5 T between 4 and 20T. As usual, large magnetization jumps are observed (Fig. 1) in such large crystals below N 10 K, due to the large sample sizes and local heating [ 11. At higher temperature, a maximum in the change of magnetization, AM = M+ - Mm, taken for increasing or decreasing field is observed [6] (it can present a “butterfly shape” if the high field is not large enough). In large field and at high temperature AA4 tends to zero in a region between the irreversibility line Hi,,,(T) and HC2 (obviously AM = 0 in the normal state too). The critical current density can be deduced from AM or from M, using, J(H) = 3AM/s or J(0) = 6M,/a where a is the diameter of a disk perpendicular to the field (or the edge of a a x a square or a = 3a, (1 - u,/3a,) for a rectangle with u2 < u,, AM or M, being in Am-‘, a in m and J in Am-‘. As previously observed large critical current densities Jacb (of the order of 10” Am-‘) are observed 237