Effect of twin planes in the magnetization hysteresis loops of NdBa2Cu3O7 single crystals

The angular dependence of the critical currents is investigated in a modera tely twinned NdBa2CU3O7-delta (NdBCO) single crystal in the temperature ran ge 30 less than or equal to T less than or equal to 90 K. In intermediate f ields around the fishtail peak a reduction of critical current density due to twin structure is observed up to 90 K. This effect implies that vortices retain their three-dimensional character practically up to T-c. The effect of twin planes disappeared at external fields declined more than theta app roximate to 15 degrees from the c axis (twin planes). In this case, the hys teresis loops exhibited a clear fishtail shape. The intermediate peak obser ved previously in NdBCO single crystal at theta =0 degrees is identified wi th the low-field limit of the twin plane activity. [S0163-1829(98)51946-7].