Observation of spin freezing and relaxation at microwave frequencies in the spin ladder compound $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$

Abstract

We report the observation of a frequency ($ω$) and temperature (T) -dependent loss peak in $χ''$ and accompanying dispersion in $χ'$ from microwave (2-18GHz) measurements of the complex susceptibility $\tildeχ(ω, T)=χ'+iχ''$ 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 $Sr_{2}CuO_{3}$.