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Large increase of the anisotropy factor in the overdoped region of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ as probed by fluctuation spectroscopy

We study the diamagnetism induced by thermal fluctuations above the superconducting transition of the iron pnictide Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ with different doping levels. The measurements are performed with magnetic fields up to 7 T applied in the two main crystal directions. These data provide double information: first, they confirm at a quantitative level the applicability to these materials of a 3D-anisotropic Ginzburg-Landau approach valid in the finite field regime. Then, they allow to determine the doping-level dependence of the in-plane coherence length and of the superconducting anisotropy factor, $γ$. Our results provide a stringent confirmation of the large increase of $γ$ with the doping level, as recently proposed from magnetoresistivity measurements. The implications of the applicability of the model used to a multiband superconductor are discussed.