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

Abstract: The magnetic order of the localized Eu$^{2+}$ spins in optimally-doped Eu(Fe${1-x}$Ir${x}$)${2}$As${2}$ ($\mathit{x}$ = 0.12) with superconducting transition temperature $\mathit{T_{SC}}$ = 22 K was investigated by single-crystal neutron diffraction. The Eu$^{2+}$ moments were found to be ferromagnetically aligned along the $\mathit{c}$-direction with an ordered moment of 7.0(1) $\mu_{B}$ well below the magnetic phase transition temperature $\mathit{T_{C}}$ = 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 groud state of the Eu$^{2+}$ spins in Eu(Fe${0.88}$Ir${0.12}$)${2}$As${2}$ was supported by the first-principles density functional calculation. In addition, comparison of the electronic structure calculations between Eu(Fe${0.875}$Ir${0.125}$)${2}$As${2}$ and the parent compound EuFe${2}$As${2}$ indicates stronger hybridization and more expanded bandwith due to the Ir substitution, which together with the introduction of electrons might work against the Fe-SDW in favor of the superconductivity.