Coexistence and Competition of the Short-Range Incommensurate Antiferromagnetic Order with the Superconducting State ofBaFe2−xNixAs2

Superconductivity in the iron pnictides develops near antiferromagnetism, and the antiferromagnetic (AF) phase appears to overlap with the superconducting phase in some materials such as ${\mathrm{BaFe}}_{2\ensuremath{-}x}{T}_{x}{\mathrm{As}}_{2}$ (where $T=\mathrm{Co}$ or Ni). Here we use neutron scattering to demonstrate that genuine long-range AF order and superconductivity do not coexist in ${\mathrm{BaFe}}_{2\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{As}}_{2}$ near optimal superconductivity. In addition, we find a first-order-like AF-to-superconductivity phase transition with no evidence for a magnetic quantum critical point. Instead, the data reveal that incommensurate short-range AF order coexists and competes with superconductivity, where the AF spin correlation length is comparable to the superconducting coherence length.