Magnetic structure of superconducting Eu(Fe 0.82 Co 0.18) 2 As 2 as revealed by single-crystal neutron diffraction

The magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2 is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu2+ moments along the c direction is revealed below the magnetic phase transition temperature TC=17 K. In addition, the antiferromagnetism of the Fe2+ 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 TN=70 K and TS=90 K, respectively, compared to the parent compound EuFe2As2. 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(Fe0.82Co0.18)2As2. Moreover, the comparison between Eu(Fe1-xCox)2As2 and Ba(Fe1-xCox)2As2 indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.