Observation of spin freezing and relaxation at microwave frequencies in the spin ladder compound $S{r}_{14-x}C{a}_{x}C{u}_{24}{O}_{41}$

Abstract

We report the observation of a frequency ($\omega$) and temperature (T) -dependent loss peak in ${\chi }^{\prime \prime }$ and accompanying dispersion in ${\chi }^{\prime }$ from microwave (2-18GHz) measurements of the complex susceptibility $\tilde{\chi }(\omega ,T)={\chi }^{\prime }+i{\chi }^{\prime \prime }$ of $S{r}_{14-x}C{a}_{x}C{u}_{24}{O}_{41}$. 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 $SrCu{O}_2$ and $S{r}_{2}Cu{O}_3$.

4 pages, 5 figures included in ps format