Observation of Superconducting Fluctuations above Tc in underdoped BaFe2-xNixAs2

Angular dependent torque measurements have been performed on the electron doped iron pnictide superconductors BaFe$_{2-x}$Ni$_x$As$_2$ with a series Ni doping (0.03 $\leq$ \emph{x} $\leq$ 0.3). In the superconducting state, an irreversibility, as the evidence for the pinning of vortex, is observed between the torque measured with increasing and decreasing angle. Our results in underdoped sample (\emph{x} = 0.065) show that the irreversible torque signal can survive up to a temperature $T_{irr}$ well above the superconducting transition temperature $T_c$, suggesting the existence of superconducting fluctuations (SCF) above $T_c$. The Ni doping dependent phase diagram both for $T_{irr}$ and $T_c$ with a strong SCF region in the underdoped samples, is summarized based on our results.