Pinning mechanisms in bulk high-T-c superconductors

Volume pinning forces are determined for a variety of bulk high-T-c superco nductors of the 123 type from magnetization measurements. By means of scali ng of the pinning forces, the acting pinning mechanisms in various temperat ure ranges can be identified. The Nd-based superconductors and some YBCO cr ystals exhibit a dominating pinning of the delta T-c type (i.e. small, supe rconducting pinning sites). This is ascribed to the presence of an Nd-rich phase with weaker superconducting properties, leading to a spatial scatter of T-c, which can also be provided by oxygen vacancy clusters. In contrast to this, the addition of insulating 211 particles provides pinning of the d elta l type. Measurements of the field-cooled magnetization show that the N d-based superconductors exhibit two-step transitions if cooled/warmed in fi elds above 4 T. This secondary transition can be correlated to the peak eff ect. This suggests that the peak effect is an unique property of the superc onducting matrix (i.e. oxygen vacancy clusters), whereas the 211 particles provide effective pinning in the entire temperature range acting as a quasi -'background' pinning mechanism for the peak effect. Based on these observa tions we construct a pinning force diagram for bulk high-T-c superconductor s.