Volume pinning forces are determined for avariety of bulk high-T_c superconductors of the 123 type frommagnetization measurements. By means of scaling of thepinning forces, the acting pinning mechanisms in varioustemperature ranges can be identified. The Nd-basedsuperconductors and some YBCO crystals exhibit a dominatingpinning of the #delta#T_c type (i.e. small, superconductingpinning sites). This is ascribed to the presence of an Nd-richphase with weaker superconducting properties, leading to aspatial scatter of T~, which can also be provided by oxygenvacancy clusters. In contrast to this, the addition of insulating211 particles provides pinning of the #delta#l type.Measurements of the field-cooled magnetization show that theNd-based superconductors exhibit two-step transitions ifcooled/warmed in fields above 4 T. This secondary transitioncan be correlated to the peak effect. This suggests that the peakeffect is an unique property of the superconducting matrix (i.e.oxygen vacancy clusters), whereas the 211 particles provideeffective pinning in the entire temperature range acting as aquasi- 'background' pinning mechanism for the peak effect.Based on these observations we construct a pinning forcediagram for bulk high-Te superconductors.
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