Revisiting the Kitaev material candidacy of ${\mathrm{Ir}}^{4+}$ double perovskite iridates

Quantum magnets with significant bond-directional Ising interactions, so-called Kitaev materials, have attracted tremendous attention recently in the search for exotic spin liquid states. Here we present a comprehensive set of measurements that enables us to investigate the crystal structures, ${\mathrm{Ir}}^{4+}$ single-ion properties, and magnetic ground states of the double perovskite iridates ${\mathrm{La}}_{2}B{\mathrm{IrO}}_6$ ($B=\text{Mg,}\text{Zn}$) and $A_2{\mathrm{CeIrO}}_6$ ($A=\text{Ba,}\text{Sr}$) with a large nearest-neighbor distance $>5$ Å between ${\mathrm{Ir}}^{4+}$ ions. Our neutron powder diffraction data on ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$ can be refined in the cubic space group $Fm\overline{3}m$, while the other three systems are characterized by weak monoclinic structural distortions. Despite the variance in the noncubic crystal field experienced by the ${\mathrm{Ir}}^{4+}$ ions in these materials, x-ray absorption spectroscopy and resonant inelastic x-ray scattering are consistent with $J_{\mathrm{eff}}=1/2$ moments in all cases. Furthermore, neutron scattering and resonant magnetic x-ray scattering show that these systems host $A$-type antiferromagnetic order. These electronic and magnetic ground states are consistent with expectations for face-centered-cubic magnets with significant antiferromagnetic Kitaev exchange, which indicates that spacing magnetic ions far apart may be a promising design principle for uncovering additional Kitaev materials.

Figure 1

A schematic of the ideal fcc crystal structure showing how NN Kitaev exchange can arise in this case. The colored bonds correspond to bond-directional Ising interactions between the different spin components $S_x, S_y$, or $S_z$. For simplicity, the bonds going through the cube are not shown.

Figure 2

Neutron powder diffraction data, indicated by the solid symbols and collected with a neutron wavelength 1.54 Å at a temperature $T=4$ K, are shown for (a) ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$, (b) ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$, (c) ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$, and (d) ${\mathrm{Sr}}_2{\mathrm{CeIrO}}_6$. The best structural refinements are superimposed on the data as solid curves, the difference curves are shown below the diffraction patterns, and the expected Bragg peak positions are indicated by ticks.

Figure 3

X-ray absorption spectra collected at the Ir $L_3$ edge (left) and the Ir $L_2$ edge (right) for the double perovskite iridates ${\mathrm{La}}_2{\mathrm{MgIrO}}_6, {\mathrm{La}}_2{\mathrm{ZnIrO}}_6, {\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$, and ${\mathrm{Sr}}_2{\mathrm{CeIrO}}_6$. Note the dramatic intensity difference between the sharp “white-line” features observed at the $L_3$ and $L_2$ absorption edges.

Figure 4

Resonant inelastic x-ray scattering spectra collected at the Ir $L_3$ edge ($E_i=11.215$ keV) for (a) ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$, (b) ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$, (c) ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$, and (d) ${\mathrm{Sr}}_2{\mathrm{CeIrO}}_6$. Note the presence of a strong elastic line at an energy transfer $\hslash \omega =0$, two peaks at energy transfers of $\hslash \omega \sim 0.6$–0.7 eV corresponding to intraband $t_{2g}$ crystal-field transitions, and higher-energy excitations centered at $\hslash \omega \sim 3.5$ eV corresponding to interband $t_{2g}$ to $e_g$ crystal-field transitions.

Figure 5

Resonant magnetic x-ray scattering results on single crystalline ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$. (a) The structurally forbidden Bragg peak (1.5 1.5 0) only appears in the $\sigma -\pi$ scattering channel. (b) The same (1.5 1.5 0) peak shows a resonant enhancement just below the Ir $L_3$ absorption edge, while the (3 3 1) structural peak exhibits typical intensity modulation near the absorption edge. (c) The azimuthal intensity dependence of the (0.5 0.5 0) Bragg peak is best described by A-II AFM order with the Ir moments aligned very close to the crystalline $c$ direction. (d) A schematic of the proposed magnetic structure for ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$. It is anticipated that the Ir moments cant away from the $c$ axis to track the small ${\mathrm{IrO}}_6$ octahedral rotations.

Figure 6

Bulk characterization and neutron powder diffraction (NPD) measurements on polycrystalline ${\mathrm{Sr}}_2{\mathrm{CeIrO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$. (a),(b) Magnetic susceptibility measurements as a function of temperature reveal ordering temperatures $T_N=21$ and 17 K for the Sr and Ba analogs, respectively, which is in good agreement with previous work [40, 41]. (c),(d) The magnetization of both materials increases linearly with field. (e),(f) Neutron powder diffraction data are presented for both materials at $T=4$ and 30 K. Two different refinement results are superimposed (solid and dotted curves) on the ${\mathrm{Sr}}_2{\mathrm{CeIrO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$ data that are consistent with the field dependence of the magnetization. The A-II AFM model provides superior agreement with the NPD data in both cases. (g),(h) Order-parameter plots for the most intense magnetic Bragg peak observed in each case. The $T_N$ values are consistent with the magnetic susceptibility results. Power-law curves are superimposed on the data as a guide to the eye.

Figure 7

Color contour plots of the $E_i=15$ meV HYSPEC data for ${\mathrm{Ba}}_2{\mathrm{CeIrO}}_6$ at (a) $T=1.5$ K and (b) $T=30$ K. Similar plots are shown in (c) and (d) for ${\mathrm{Sr}}_2{\mathrm{CeIrO}}_6$. In sharp contrast to previous work on ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ [38], no clear magnetic excitations are visible in these spectra.