CU(2) MAGNETIC CORRELATIONS IN YBA2CU3OX (6.0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-7.0) STUDIED BY MOSSBAUER-SPECTROSCOPY ON SUBSTITUTED YB-170(3+) .2. DYNAMIC PROPERTIES

We extend the method used to measure the internal (molecular) field on Yb-170(3+) diluted for Y3+ in YBa2Cu3Ox described in the preceding paper (Part 1) to the study of dynamical effects. (The method exploits the field dependence of the Yb-170(3+) electronuclear levels and the measurements are made by Mossbauer spectroscopy. ) By studying the fluctuation rates of the Cu(2) derived molecular field acting on the Yb-170(3+) probes, we follow the fluctuation rates of the short range correlated Cu(2) moments as a function of oxygen content (6.0 less-than-or-equal-to x less-than-or-equal-to 7.0) and temperature (less-than-or-similar-to 70 K). When x approximately 6.0 the correlated Cu(2) moments are static or fluctuate at frequencies below the lower limit of the accessible frequency window. For x congruent-to 6.35, the average fluctuation rate shows a strong thermal dependence decreasing through the accessible range of 10(12) to 5 x 10(8) s-1 as the temperature is lowered to 1.4 K and the system shows spin glass-like behaviour. At 1.4 K as x is increased above about 6.5, we observe that a fraction of the probes no long show spin glass-like behaviour and show no evidence of any fields fluctuating in the accessible frequency range. We identify these probes as being situated in the superconducting regions of the sample where at temperatures much lower than the gap there are not expected to be any Cu(2)-based magnetic excitations. From these local probe measurements we find that the amount of the sample that is superconducting depends strongly on x increasing from essentially zero at x congruent-to 6.50 to about 50% at x congruent-to 6.56, to about 75% at x congruent-to 6.66, and reaches 100% as x approaches 7.0. The remaining fraction of the probes which shows a concomitant decrease in importance, continues to show spin glass-like behaviour with a strong thermal dependence of the average local fluctuation rates.