Angular-dependent magnetic torque in iron-pnictide BaFe2−xNixAs2

Angular-dependent torque measurements have been performed on the electron doped iron-pnictide superconductors BaFe2−xNixAs2 with series of Ni concentrations. In the superconducting state, an irreversibility, as the evidence for the presence of vortex, is observed between the torque measured with increasing and decreasing angle. Our results in underdoped samples x = 0.065 show that the irreversible torque signal survives up to a temperature Tirr well above the superconducting transition temperature Tc, suggesting the existence of superconducting vortices above Tc, which is likely to originate from superconducting fluctuations.