Experimental aspects of flux expulsion in type-II superconductors

Experimental aspects of flux expulsion in Nb₃Sn and YBa₂{Cu}₃O₇ type-II superconductors are presented. There is a clear distinction in magnetization between field-cooled measured-upon-cooling (FCC) and field-cooled measured-upon-warming (FCW) results. This thermal hysteresis, predicted in the temperature-dependent critical-state model at low fields by Clem and Hao, was observed for measuring fields up to about 0.5 T. The model explains the observation of increases in diamagnetism after field cooling and thermal cycling. The thermal hysteresis, together with weak links, accounts for the occurrence of a negative peak in FCW magnetization. The FCC-FCW bifurcation observed for a 0.1 mT field down to 5 K might imply that flux lines are not completely frozen below Tc1, the temperature at which the lower critical field is equal to the measuring field, but are expelled from the sample even in the Meissner state.