Dynamics at T -> 0 in the half-integer isotropic high spin molecules

We investigate the dynamical spin-spin auto-correlation function of the isotropic high spin molecules CrCu$_{6}$ (S=9/2), CrNi$_{6}$ (S=15/2) and CrMn$_{6}$ (S=27/2), using magnetization, $\mu$SR and NMR measurements. We find that the field autocorrelation time $\tau$ of the molecule's spin at zero and low fields is nearly temperature independent as $T\to 50$ mK. The high temperatures $\tau$ is very different between the molecules. Surprisingly, it is identical ($\sim$ 10 nsec) at low temperature. This suggests that $\tau$ is governed by hyperfine interactions.