Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of spin freezing and relaxation at microwave frequencies in
  the spin ladder compound $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$   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 $Sr_{2}CuO_{3}$.

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The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
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The categories of this paper are: q-alg
16
The categories of this paper are: stat
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The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
36
The categories of this paper are: cmp-lg
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The categories of this paper are: quant-ph
8
The categories of this paper are: hep-ex
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
3
The categories of this paper are: physics