1 8 Se p 20 08 Coexistence of Long - Range Magnetic Order and Superconductivity from Campbell Penetration

Application of a tunnel-diode resonator (TDR) technique for studies of the vortex response in magnetic superconductors is described. Operating at very small ex-citation 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 (∼ 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. ‡ On the occasion of his 80th birthday, we dedicate this paper to Alexey Alexeevich Abrikosov who discovered two types of superconductors and described vortices in type-II superconductors [1, 2].