Coexistence of long-range magnetic order and superconductivity from Campbell penetration depth measurements
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 a sufficiently high frequency, the TDR was used to probe the 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 (~0.3 K). In contrast, weak ferromagnetic as well as antiferromagnetic orders can coexist with bulk superconductivity and may even lead to an enhancement of vortex pinning. By analyzing the Campbell penetration depth we find a 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.