Magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2as revealed by single-crystal neutron diffraction

The magnetic structure of superconducting Eu(Fe${}_{0.82}$Co${}_{0.18}$)${}_{2}$As${}_{2}$ is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu${}^{2+}$ moments along the $c$ direction is revealed below the magnetic phase transition temperature ${T}_{C}=17$ K. In addition, the antiferromagnetism of the Fe${}^{2+}$ moments still survives and the tetragonal-to-orthorhombic structural phase transition is also observed, although the transition temperatures of the Fe spin-density-wave (SDW) order and the structural phase transition are significantly suppressed to ${T}_{N}=70$ K and ${T}_{S}=90$ K, respectively, compared to the parent compound EuFe${}_{2}$As${}_{2}$. We present microscopic evidence for the coexistence of the Eu ferromagnetism and the Fe SDW in the superconducting crystal. The superconductivity competes with the Fe SDW in Eu(Fe${}_{0.82}$Co${}_{0.18}$)${}_{2}$As${}_{2}$. Moreover, the comparison between Eu(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ and Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.