L Journal of Alloys and Compounds 298 (2000) 285–290 www.elsevier.com / locate / jallcom Determination of the material parameters of a melt-textured YBCO superconductor by DC magnetization analysis ¨ ~ * ¨ S. C¸ elebi , A. Ozturk, E. Yanmaz, A.I. Kopya Department of Physics, Faculty of Science and Arts, Karadeniz Technical University, 61080 Trabzon, Turkey Received 12 July 1999; accepted 12 October 1999 Abstract We report the isothermal magnetization curves of a YBCO superconducting sample prepared by a flame-quench–melt-growth (FQMG) method. Hysteresis loops have been calculated and analyzed in the framework of a critical state model including the Meissner current. The bulk critical current and Meissner current as a function of applied field were extracted, and the volume fraction of the grains, f , has been g estimated to be 0.64 from a comparison of the theoretical and experimental M–H hysteresis loops. The lower critical field, m H , was 0 c1 found to be 14.4 mT.  2000 Elsevier Science S.A. All rights reserved. Keywords: High-Tc superconductor; YBCO; Critical state model; Critical current density; Meissner current 1. Introduction critical field H . Some workers [9] proposed an exponen- c1 tial model who have treated the critical state model with Over the years many workers have employed mag- the equilibrium magnetization. Clem [10] proposed a netization data to infer the current carrying capability of critical state model including the potential barrier [11] at type II superconductors. It is well known that the critical the surface which requires extra field increments DH en current density depends on the flux pinning strength of a (DH ) for the flux entry into (exit from) the sample. ex superconductor. The pinning force depends on the tem- LeBlanc and Lorrain [12] calculated some simple loops perature, flux line density and the nature of the pinning where the critical current density j , independent of B, and c sites caused by the various types of defects such as voids, DH 5DH 5DH 5constant. Chen et al. [13] developed a ex en dislocations, grain boundaries impurities, etc. The larger phenomenological model for interpreting the magnetic the pinning strength, the larger the critical current, hence properties of hard type II superconductors which includes the wider the hysteresis envelope. According to the the effects of the bulk pinning, the thermal equilibrium equilibrium Meissner contribution to their magnetic be- magnetization and the surface barrier. Tochihara et al. [14] haviour, nonideal type II superconductors can be classified and Yasuoka et al. [15] published the calculations of the as semi-reversible and hysteretic. initial magnetization and full hysteresis loops within the The critical state model introduced by Bean [1] and framework of the modified Kim–Anderson critical state London [2] has been exploited in the analysis of magnetic model. Recently C¸ elebi et al. [16] reported on the magnetic behaviour of type II superconductors by many workers. In responses of a semi-reversible YBCO superconductor with this simple model, the critical current density j is assumed numerical calculations of the magnetization based on the c to be independent of B, the local magnetic flux density. critical state model including the Meissner current contri- The model of Kim et al. [3,4] included a dependence of bution. critical current on B. Irie and Yamafuji [5] and Green and In this work, we determine some material parameters of n Hlawiczka [6] proposed a power law model, j 5 k / B our melt-textured YBCO sample by the same model c where k and n are positive constants, where n 50.5 was calculation as in Ref. [16]. proposed by Yasukochi et al. [7,8]. All these models neglected the effect of flux exclusion below the lower 2. Experimental *Corresponding author. Fax: 190-462-325-3195. E-mail address: celebi@risc01.ktu.edu.tr (S. C¸ elebi) The sample used in this work was prepared by a flame- 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388(99)00644-1