Anisotropic antiferromagnetic order in spin-orbit coupled trigonal lattice Ca2Sr2IrO6

We used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca2Sr2IrO6. The crystal structure is determined to be $R\bar{3}$ with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below $T_N=13.1$ K. The magnetic wave vector is identified as $(0,0.5,1)$ with ferromagnetic coupling along the $a$ axis and antiferromagnetic correlation along the $b$ axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34${}^{\circ }$ towards the $c$ axis. The ordered moment is 0.66(3) ${\mu }_B$/Ir, larger than other iridates where iridium ions form corner- or edge-sharing $\rm IrO_6$octahedralnetworks\mathrm{.}Thetiltingangleisreducedto$\approx19^\circ$whenamagneticfieldof4.9Teslaisappliedalongthe$c$ axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap $\sim 0.5$~eV.