FINE-GRAINED YBACUO - THE FORMATION OF THE INITIAL VORTEX LATTICE AND THE MAGNETIZATION CURVES

Drastic changes in the experimental magnetization curve behaviour of fine-grained YBaCuO were observed when the grain sizes D became commensurable with penetration depth lambda. In particular, the magnetic field H-max (when M(H) reached its maximum) increased significantly when the grain size decreased. A theoretical study of initial flux line structure formation in the case of D similar to lambda was carried out, drawing an analogy between HTSC samples at high temperatures and an isotropic type II superconductor on the basis of a model taking into account both vortex-vortex and vortex-surface interactions. The formation fields of the first seven vortices as well as the calculated magnetization curves M(H) were obtained for a wide range of D/lambda ratio. It was shown that a significant increase in the fields of penetration, not only for the first but also for the subsequent vortices, took place. This effect allowed us to explain peculiarities of experimental M(H) curves for fine-grained HTSC samples.