Nonlinear acoustic effects in yttrium-barium-copper-oxygen (YBCO) ceramics with different grain sizes were investigated by means of the piezoelectric ultrasonic composite oscillator technique (PUCOT) at frequencies of about 100 kHz, corresponding to longitudinal resonance vibration. It has been shown that the acoustic parameters (decrement and resonance frequency) exhibit hysteretic behavior at temperatures in the range between 6.5 K and 290 K. Irreversible changes occurring in these acoustic parameters during measurements of their amplitude dependence at 6.5 K were found to disappear as a result of thermal annealing at 65 K. Furthermore, an unusual crossover behavior in the amplitude dependent damping curves obtained at low temperatures has been observed for the first time. This crossover effect depends strongly on grain size and disappears at temperatures above 100 K; it has never been, so far, observed at room temperature. It appears that the crossover effect is related to an unusual time dependence of the nonlinear damping: at high vibration amplitudes, the decrement decreases, simultaneously with a decrease of the resonance frequency, that is, an increase of the Young's modulus defect. This observation can be understood by considering separately the effects of high-amplitude vibration on the acoustic parameters at both low and high strain amplitude.
Analysis of the results suggests that the hysteretic phenomena observed in YBCO ceramics may be ascribed to crystal dislocations vibrating in the internal stress fields of various types of point defects, some of which remain mobile down to liquid helium temperature. High-amplitude excitation introduces (quasi-permanent) changes in the microstructure, that is, in the dislocation-pinning point configuration, which are responsible for the enhanced dissipation of mechanical energy and thus for the crossover effect.
Finally, some preliminary results of magnetomechanical measurements are presented, demonstrating the usefulness of the PUCOT for studying the relationship between the magnetic properties and the stress-induced structural modifications occurring in YBCO ceramics.