Ultrasonic studies of flux-line pinning in superconducting Bi2Sr2CaCu2O8+y

Measurements have been made of ultrasonic shear wave attenuation and sound velocity in highly textured polycrystalline samples of Bi2Sr2CaCu2O8+y in magnetic fields up to 9 T and at temperatures down to 4.2 K. Magnetic field dependent attenuation peaks and velocity enhancements were in good agreement with theoretical predictions based on the assumptions of the thermally assisted flux-flow model of the mixed state. Flux-line pinning activation energy obtained from the data showed a weak anisotropy, similar to 160 K for field parallel and similar to 240 K for field perpendicular to the c-axis. The flux-line lattice was found to be fully pinned at lowest temperatures. Experiments were also performed in low magnetic fields (up to 9 mT), but showed no evidence of reported ultrasonic effects that could be attributed to weak superconductivity in grain boundaries.