Observation of anisotropic pinning effect in Bi2Sr2CaCu2O8+δ single crystals

The anisotropic pinning effect of magnetic flux lines in Bi2Sr2CaCu2O8+δ single crystals was investigated by torque measurements in a temperature range from 4.2 to 100 K. A sharp cusp in angular dependence of the torque was found near Hparallelab-plane between 15 and 82 K. The magnetic field dependence of the angle yielding the cusp θc1 is clearly determined by the field component perpendicular to the CuO2 planes HcP1 obeying Hcos θc1=HcP1, revealing the intrinsic flux trapping between the CuO2 planes and the start of the stepwise penetration at HcP1. In addition, extra cusps were observed in the torque curves in a narrow temperature range from 21 to 40 K, suggesting a certain change in the vortex state. The relation between fields and angles to give extra cusps θc2 also well fitted the equation Hcos θc2=HcP2. The temperature dependence of HcP1 and HcP2 was determined and the phase transition of the vortex state is discussed. (orig.)