Multiband superconductivity in Co-doped SrFe2As2 investigated using local magnetic imaging

High resolution scanning Hall probe microscopy and Hall magnetometry have been used to investigate the magnetic properties of superconducting Co-doped SrFe2-xCoxAs2 single crystals. We resolve rather disordered field-cooled vortex structures at all values of the applied field studied, present due to the strong vortex pinning arising from substitution of Fe with Co. We have fitted the profiles of well-isolated vortices as a function of temperature in order to extract mesoscopic estimates of the temperature dependence of the magnetic field penetration depth, lambda. These data were then used to calculate the superfluid density (rho(s)) over the full temperature range and fitted to a two-band alpha-model with two full gaps. The results suggest that the superfluid density is shared almost equally between hole and electron pockets and that the larger (electron pocket) gap appears to have an approximately isotropic s-wave order parameter. Displacements of the vortices from an ideal triangular lattice have been used to estimate the low field pinning forces which are several orders of magnitude smaller than values estimated by other means for similar superconducting crystals.