Pinning mechanisms in bulk high-T_c superconductors

Volume pinning forces are determined for a variety of bulk high-T_c superconductors of the 123 type from magnetization measurements. By means of scaling of the pinning forces, the acting pinning mechanisms in various temperature ranges can be identified. The Nd-based superconductors and some YBCO crystals exhibit a dominating pinning of the δT_c type (i.e. small, superconducting 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 δl type. Measurements of the field-cooled magnetization show that the Nd-based superconductors exhibit two-step transitions if cooled/warmed in fields above 4 T. This secondary transition can be correlated to the peak effect. This suggests that the peak effect is an unique property of the superconducting 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 observations we construct a pinning force diagram for bulk high-T_c superconductors.