Low-field vortex patterns in the multiband BaFe2−xNixAs2superconductor (x=0.1, 0.16)

Abstract

The distribution of vortices at low magnetic fields in single crystals of multiband BaFe${}_{2\ensuremath{-}\mathrm{x}}$Ni${}_{\mathrm{x}}$As${}_{2}$ (x $=$ 0.1 and x $=$ 0.16) superconductors is studied by Bitter decoration. Highly inhomogeneous vortex patterns, including vortex stripes and vortex clusters, are observed. The origin of these inhomogeneous vortex arrays is likely to be due to strong flux pinning, as suggested by magnetization measurements performed at high fields. Alternative possible scenarios, such as type-1.5 superconductivity due to multiband effect, are also discussed.