Vortex creep perpendicular to the c-axis in crystalline and grain-oriented YBCO superconductor

Magnetization measurements were carried out on crystalline and grain-oriented YBa₂Cu₃O_7-δ samples. We have measured the creep of magnetic flux lines along the a-axis (perpendicular to the c-axis) of the crystalline sample and perpendicular to the c-axis of a c-axis-grain-oriented polycrystalline sample. The measurements were done at 4.2 K and for magnetic fields in the range 1 kOe < H < 10 kOe. The magnetization M was found to decrease linearly with ln t in both samples for all the applied magnetic fields. With the initial increase of the applied magnetic field, the vortex pinning potential, U_p, was found to decrease sharply in the grain-oriented sample, followed by a slow (almost linear) rate of decrease. In the crystalline sample, the rate of decrease of U_p was found to be slow and linear for all applied magnetic fields.