MAGNETIZATION OF FINE-GRAINED YBACUO NEAR LOWER CRITICAL-FIELD

Drastic changes in the behavior of the experimental magnetization curv es of fine-grained YBaCuO are observed al high temperatures where the grain sizes D are comparable with penetration depth lambda. Drawing, o n the analogy between the HTSC samples at high temperature and an isot ropic type II superconductor the initiation of a vortex structure in t he case of D similar to lambda considered on the basis of a model taki ng into account both vortex-vortex and vortex-surface interactions. Th e fields of the first seven vortices penetration as well as the magnet ization curves M(H) are calculated for a wide range of D/lambda ratio. It is shown that the fields of penetration not only of the very first , but also of the following vortices increase considerably as the grai n size is decreased down to D similar to lambda. This effect allows us to explain the peculiarities in the experimental curves M(H) of fine- grained HTSC samples.