Amplitude Dependent Internal Friction and Magneto-Acoustic Effects in YBCO Ceramics

Abstract
The paper is concerned with effects in damping and Young's modulus observed in YBaCuO ceramic superconductors vibrating ultrasonically either in or without magnetic field. The amplitude and time dependencies which arise as a result of high amplitude vibrational excitation are presented for coarse-grained and fine-grained specimens. Weak magneto-acoustic effects were observed, and found to be strongly influenced by vibration amplitude and by thermal pre-history of the sample. Analysis of the results shows that, in the vicinity of liquid helium temperature, high amplitude ultrasonic stresses induce structural modifications in these YBaCuO ceramics. Dislocation point defect interaction mechanisms are proposed for explaining the micro-plastic behaviour of the YBaCuO samples observed in the ultrasonic experiments. The magneto-acoustic effects are discussed in the framework of flux line pinning models.