LINE-SHAPE OF THE FIELD-DEPENDENCE OF THE TRAPPED MOMENT IN Y-BA-CU-OSUPERCONDUCTING POWDERS IN LOW MAGNETIC-FIELDS - MANIFESTATION OF VORTEX POLARON

The structure of trapped magnetic flux is investigated in small superc onducting Y-Ba-Cu-O particles, attributing the trapped moment M-pol to an ensemble of vortex polarons which consists of vortices surrounded by diamagnetic matter. To distinguish between the contributions of the vortices and their diamagnetic surrounding to the magnetic moment, me asurements of the field dependence of M-pol are made with a SQUID magn etometer around zero magnetic field H. It is found that the value of M -pol grows with increasing field and has a narrow dip at H = 0. A simi lar line shape but with opposite sign is found in the field dependence of the zero field cooled susceptibility confirming the polaron nature of the trapped flux. (C) 1998 Elsevier Science B.V. All rights reserv ed.