STRUCTURE AND RELAXATION OF THE TRAPPED MAGNETIC-FLUX IN Y-BA-CU-O SUPERCONDUCTORS IN MAGNETIC-FIELDS BELOW THE FIRST CRITICAL-FIELD H-C1

Relaxation of the remanent (MR) and isothermoremanent (MIR) magnetic moment, having the same initial value, is investigated in small superconducting Y-Ba-Cu-O particles in magnetic fields below the first critical field Hc1. It is found that the relaxation rate depends strongly on the cooling procedure. A noticeable relaxation of MIR is observed after cooling the sample in zero magnetic field and then applying the magnetizing field. Instead, the value of MR obtained after cooling the sample in a field proved to be very stable. This result indicates that the structure of the magnetic flux trapped in superconducting particles is different in these two cases. We attribute MR to a single or a few vortices trapped in the center of superconducting particles, while MIR consists of many short interacting vortices trapped near the surface. © 1995 The American Physical Society.