Vortex motion in Nb/PdNi/Nb trilayers: New aspects in the flux flw state.

We study the dynamics of vortex lines in Superconductor/Ferromagnet/Superconductor (SFS) heterostructures. Microwave techniques are used to gain information in the small-oscillation regime, which does not perturb significantly the static vortex ordering. Wideband (1-20 GHz) Corbino disk and single-frequency (8 GHz) dielectric resonator techniques are combined to study the temperature (2.7 KTc) and field (up to Hc2) dependence of the vortex-state parameters (flux-flow resistivity ρff, depinning frequency) in Nb/PdNi/Nb trilayers with different thickness of the F (PdNi) layers. We stress that the high-frequency technique is able to disentangle pinning from dissipation, thus yielding the true flux flow resistivity. The heterostructures have been grown by UHV dc magnetron sputtering. They are composed by Nb layers of nominal thickness dS=15 nm, and a ferromagnetic PdNi layer of thickness 1 nm <dF< 9 nm. TEM and EXAFS analysis were employed to assess the morphology and the local structure. From the microwave complex response we obtained the true flux flow resistivity and the depinning frequency as a function of the field and temperature, and the resulting data were compared to data obtained in pure Nb. We find that the depinning frequency is reduced with increasing thickness of the F layer. A striking result comes from the analysis of the flux-flow resistivity: for very thin F layers ρff closely follows the expression obtained in the GL framework (different however from the simple Bardeen-Stephen model), in complete analogy to the results in pure Nb. However, at large dF the flux-flow resistivity exceeds the theoretical expectations. In particular we find ρff > ρn H/Hc2. However, it is still true (at large F thickness) that ρff is a scaling function of the reduced field H/Hc2 (where Hc2 has been directly measured). Interestingly, at intermediate dF the scaling is lost. Our results indicate that Hc2 is the only relevant field scale at small dF, where the magnetic effects are confined to a depression of Tc and the response is identical to a pure Nb sample, and at large dF, where we argue that the ferromagnet has a significant influence on the underlying superconducting state. At intermediate dF there are (at least) two different field scales that determine the flux-flow resistivity. Work partially supported by a MIUR-PRIN 2007 project.