Robustness of Basal-Plane Antiferromagnetic Order and the $J_{\mathrm{eff}}\mathbf{=}1/2$ State in Single-Layer Iridate Spin-Orbit Mott Insulators

The magnetic structure and electronic ground state of the layered perovskite ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ have been investigated using x-ray resonant magnetic scattering. Our results are compared with those for ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, for which we provide supplementary data on its magnetic structure. We find that the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds and that the electronic ground state in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, deduced from an investigation of the x-ray resonant magnetic scattering $L_3/L_2$ intensity ratio, is consistent with a $J_{\mathrm{eff}}=1/2$ description. The robustness of these two key electronic properties to the considerable structural differences between the Ba and Sr analogues is discussed in terms of the enhanced role of the spin-orbit interaction in $5d$ transition metal oxides.

Figure 1

The left-hand panel shows the crystal structure of ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. Perovskite ${\mathrm{IrO}}_6$ layers, where the Ir atoms (grey) lie at the center of corner sharing oxygen (red) octahedra, are separated by Ba atoms (light green). The right-hand panel shows the basal-plane antiferromagnetic structure of the ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, where the magnetic moments are pointing along the [1 1 0] direction.

Figure 2

(a) $L$ scans across the ($\frac{1}{2}\frac{1}{2}10$) magnetic reflection, $T=50\mathrm{K}$ in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. (b) The temperature dependence of the ($\frac{1}{2}\frac{1}{2}10$) magnetic reflection in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. The solid blue line is a fit to a power law. (c) Resonant enhancement of the ($\frac{1}{2}\frac{1}{2}8$) magnetic reflection across the $L_{2,3}$ edges at $T=50\mathrm{K}$ in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. The solid black line shows the XANES spectra normalized to the number of initial states. The blue spheres and red triangles show the intensity of the ($\frac{1}{2}\frac{1}{2}8$) reflection. The black dashed line demarcates the integrated white line used to calculate the branching ratio.

Figure 3

The azimuthal dependence of the ($\frac{1}{2}\frac{1}{2}10$) magnetic reflection (solid blue spheres) in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. The lines are the azimuthal dependencies calculated for the allowed moment orientations in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. The azimuthal angle $\Psi$ is defined with respect to the reference vector [1 1 0] in the $I4/mmm$ space group.

Figure 4

Reciprocal space $L$ scan (a) and energy dependence (b) of the XRMS intensity of the (1 0 24) reflection in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. The solid blue line is a fit to a Lorentzian peak shape. The azimuthal dependence of the same reflection (c) is compared with a calculation for collinear moments along [1 0 0]. The azimuthal angle $\Psi$ is defined with respect to the reference vector [1 0 0] in $I{4}_1/acd$, which corresponds to the [1 1 0] in the $I4/mmm$ space group, as demonstrated in the inset by means of a two-dimensional projection onto the basal plane of the unit cell.