ANALYSIS OF MAGNETIC-RELAXATION IN TYPE-II SUPERCONDUCTORS

Three different methods for the analysis of magnetic-relaxation data, M(t, T), are presented: In one model a spectrum, p(U), of activation e nergies U is considered, whereas the other model is based on the exist ence of a single barrier, permitting arbitrary dependences U(j - M), w here j denotes the current density. These two models are applied to ex perimental relaxation data, carried out on Bi2Sr2CaCu2O8+delta single crystals. The results are not in contradiction with each other but pro be different aspects of the creep process. In consequence the time dec ay of the magnetization, M(t, T), can be interpreted either by the exi stence of a spectrum p(U) of activation energies or as being due to a single activation energy, U(M(t)), that increases with time. In a thir d, unique model, the combination of a spectrum p(U) assuming a linear relation U approximately j is considered. From theoretical investigati ons of demagnetization effects the conclusion is drawn, that magnetic relaxation of samples with extreme geometries, i.e. thin films, has to be considered carefully, because there are stray-field effects contri buting to the relaxation function, that do not necessarily follow the same time law.