  In this article, we examine the superconducting properties of low- and
high-$T_c$ magnetic superconductors in magnetic fields close to the first
penetration field. Attention is paid to the properties that relate to the
interactions between antiferromagnetism and superconductivity. It is suggested
that several features characterizing the interplay between magnetic and
superconducting subsystems in low-$T_c$ superconductors can also be present in
high-$T_c$ materials, however, they have not been observed for any
non-substituted antiferromagnetic superconductors of the Y123 type. For the
Gd$_{1+x}$Ba$_{2-x}$Cu$_3$O$_{7-\delta}$ compound, a peak in the temperature
dependence of the ac susceptibility has been found for $x = 0.2$ near the
N\'{e}el temperature of the Gd sublattice. This peak is attributed to the
suppression of superconducting persistent currents due to the pair breaking
effect that results from the enhanced magnetic fluctuations in the vicinity of
the phase transition temperature. This observation indicates that the
interaction between magnetic and conducting electrons is present for the
composition with $x = 0.2$, where magnetism is enhanced and superconductivity
diminished.
