Local antiferromagnetic correlations in the iron pnictide superconductors LaFeAsO1-xFx and Ca(Fe,Co)2As2 as seen via normal-state susceptibility

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We have studied the interplay of magnetism and superconductivity in LaFeAsO₁₋ₓFₓ and Ca(Fe₁₋ₓCoₓ)₂As₂. While antiferromagnetic spin density wave formation is suppressed and superconductivity evolves, all samples show a doping independent strong increase of the normal state susceptibility upon heating which appears a general feature of iron pnictides. The data provide evidence for robust local antiferromagnetic correlations persisting even in the superconducting regime of the phase diagram.