Pinning in Twinned Single Crystals of Superconducting REBa2Cu3O7−x Investigated by Torque Magnetometry

Two types of single crystals REBa2Cu3O7-x which are grown by different methods and which differ in their content of twin boundaries are investigated in the vicinity of Tc with respect to rotational hysteretic losses. Two types of frictionlike losses may be differentiated, one of which is obviously connected with the presence of twin boundaries. Losses are found to be vanishing at a temperature below Tc which may be explained by the existence of an irreversibility line. In dependence on the minor loop amplitude a transition from reversible to irreversible reversal behaviour is observed. Results may be understood by the assumption of an angular lag between vortex lines and the external magnetic field. From the field dependence of this angular lag within the ab-plane a characteristic field is derived which has the same order of magnitude as the lower critical field. The results are discussed qualitatively in terms of the pinning potentials for the vortices.