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Flux dynamics of single crystals in the presence of disorder caused by substitution

We report on the temperature and field dependences of the resistance of single-crystal (NdBCO) in a fields up to 7 T. The dissipation is found to be of the thermal activation type in the presence of defects due to Nd-Ba substitution sites. Transport measurements using the six-terminal configuration together with out-of-plane voltage reveal the vortex velocity correlation along the c-direction. Such a correlation in the vortex state is generally explained as due to the twin planes acting as correlated disorder. The voltage-current behaviour predicts nearly a vortex-glass phase at low temperature. The irreversibility field in was found to be higher in the twinned crystal in comparison with the detwinned one. A small amount of vortex channelling was observed in the angle-dependent magnetization when the field was applied parallel to the c-axis of the crystal. Our magnetic relaxation both in fields and in remnant magnetization shows three distinct regions and a dominant plateau in region II from 30 to 60 K. The existence of this plateau can be explained well using collective pinning theory. The relaxation process is found to be influenced by the shape of the hysteresis loop. The results have been analysed taking into account other relevant theories of the time decay of nonequilibrium magnetization. The magnetic phase diagram from ac susceptibility experiments suggests the validity of the vortex-glass phase transition from the field-temperature scaling. The pinning centres having a size of several nm to several tens of nm are viewed by STM and are primarily responsible for high-field pinning. The pinning ability of Nd-Ba substitution sites is significantly enhanced by the presence of twin boundaries, especially at high field.