  The thermal behavior of a one-dimensional antiferromagnetic chain doped by
donor impurities was analyzed. The ground state of such a chain corresponds to
the formation of a set of ferromagnetically correlated regions localized near
impurities (bound magnetic polarons). At finite temperatures, the magnetic
structure of the chain was calculated simultaneously with the wave function of
a conduction electron bound by an impurity. The calculations were performed
using an approximate variational method and a Monte Carlo simulation. Both
these methods give similar results. The analysis of the temperature dependence
of correlation functions for neighboring local spins demonstrated that the
ferromagnetic correlations inside a magnetic polaron remain significant even
above the N\'eel temperature $T_N$ implying rather high stability of the
magnetic polaron state. In the case when the electron-impurity coupling energy
$V$ is not too high (for $V$ lower that the electron hopping integral $t$), the
magnetic polaron could be depinned from impurity retaining its magnetic
structure. Such a depinning occurs at temperatures of the order of $T_N$. At
even higher temperatures ($T \sim t$) magnetic polarons disappear and the chain
becomes completely disordered.
