Coupling phenomena and critical currents in superconducting/magnetic superlattices

In order to study the interplay between superconductivity and magnetism in proximity systems, we measured the critical temperature T-c, the critical field H-c2 and current I-c of [Nb(400 Angstrom)Fe(t(Fe))](5) superlattices. The critical temperature T-c decreases continuously with increasing t(Fe) up to t(Fe) = 15 Angstrom. At higher t(Fe) values T-c suddenly drops due to a larger pair breaking effect when the Fe layers change from the nonmagnetic to the ferromagnetic regime. Measurements of H-c2(T) show that the superconducting Nb layers are already decoupled for t(Fe) = 12 Angstrom; magnetization measurements however indicate that the Fe layers are only ferromagnetic for t(Fe) > 15 Angstrom. Critical current measurements indicate the presence of; a matching effect for fields applied parallel to the layer due to the penetration of a row of vortices in the middle of the multilayer. The interaction of flux lines with magnetic dots is studied in more detail in Pb films with a lateral superlattice of magnetic Co dots (2D artificial modulation). Matching anomalies in the magnetization curves (M(H)) indicate the stabilization of specific stable vortex lattices at integer and rational multiples of the first matching field. The influence of the stray field of the magnetic dots on the pinning properties will be discussed.