  Application of a tunnel-diode resonator (TDR) technique for studies of the
vortex response in magnetic superconductors is described. Operating at very
small excitation fields and sufficiently high frequency, TDR was used to probe
small-amplitude linear AC response in several types of single crystals where
long-range magnetic order coexists with bulk superconductivity. Full local -
moment ferromagnetism destroys superconductivity and can coexist with it only
in a narrow temperature range ($\sim 0.3$ K). In contrast, weak ferromagnetic
as well as antiferromagnetic orders can coexist with bulk superconductivity and
may even lead to enhancements of vortex pinning. By analyzing the Campbell
penetration depth we find sharp increase of the true critical current in the
vicinity of the magnetic phase transitions. We conclude that critical magnetic
fluctuations are responsible for this enhancement.
