Magnetic ground state of superconducting Eu(Fe0.88Ir0.12)2As2: A combined neutron diffraction and first-principles calculation study

The magnetic order of the localized Eu2+ spins in optimally doped Eu(Fe1-xIrx)(2)As-2 (x = 0.12) with superconducting transition temperature T-SC = 22 K was investigated by single-crystal neutron diffraction. The Eu2+ moments were found to be ferromagnetically aligned along the c direction with an ordered moment of 7.0(1) mu(B) well below the magnetic phase transition temperature T-C = 17 K. No evidence of the tetragonal-to-orthorhombic structural phase transition was found in this compound within the experimental uncertainty, in which the spin-density-wave (SDW) order of the Fe sublattice is supposed to be completely suppressed and the superconductivity gets fully developed. The ferromagnetic ground state of the Eu2+ spins in Eu(Fe0.88Ir0.12)(2)As-2 was supported by the first-principles density functional calculation. In addition, comparison of the electronic structure calculations between Eu(Fe0.875Ir0.125)(2)As-2 and the parent compound EuFe2As2 indicates stronger hybridization and more expanded bandwidth due to the Ir substitution, which together with the introduction of electrons might work against the Fe-SDW in favor of the superconductivity.