  We report the observation of a frequency ($\omega $) and temperature (T)
-dependent loss peak in $\chi''$ and accompanying dispersion in $\chi'$ from
microwave (2-18GHz) measurements of the complex susceptibility
$\tilde{\chi}(\omega, T)=\chi'+i\chi''$ of $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$. We
associate this phenomenon with a rapid decrease of spin disorder and
corresponding spin relaxation rate, representing a ``spin freezing transition''
accompanying charge ordering which occurs at temperatures $\sim 250K$. Our
results enable direct quantitative measurements of the spin relaxation rate,
and yield information on spin dynamics in these materials and the related
compounds $SrCuO_{2}$ and $Sr_{2}CuO_{3}$.
