Magnetic Frustration in Iridium Spinel Compound ${\mathrm{CuIr}}_2{\mathrm{S}}_4$

We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ is not the presumed spin-singlet state but a novel paramagnetic state, showing a quasistatic spin glasslike magnetism below $\sim 100\mathrm{K}$. Considering the earlier indication that ${\mathrm{IrS}}_6$ octahedra exhibit dimerization associated with the metal-to-insulator transition below 230 K, the present result suggests that a strong spin-orbit interaction may be playing an important role in determining the ground state that accompanies magnetic frustration.

Figure 1

Octamer configuration associated with charge order in ${\mathrm{CuIr}}_2{\mathrm{S}}_4$. The exchange interaction $S_i^{\gamma }{S}_j^{\gamma }$ for each ${\mathrm{Ir}}^{4+}$ pair is shown by lines along the respective $\gamma \gamma$ bond ($\gamma =x$, $y$, $z$). The octupolar manifold at each ${\mathrm{Ir}}^{4+}$ site represents the spin density profile in a hole of isospin-up state [11]. The bond length of the (1)-(4) and (2)-(3) ${\mathrm{Ir}}^{4+}$ pairs is reported to shrink by $\sim 15\%$ upon charge ordering and the associated structural transition [7].

Figure 2

(a) Time-dependent $\mu \mathrm{SR}$ spectra observed at several temperatures in powder sample of ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ ($x=0$) under zero external field, with solid curves showing fits by Eq. (2). (b) $\mu \mathrm{SR}$ spectra at 2 K under longitudinal fields of 1, 2, 5, 10, 20, and 40 mT, where the solid curves are fits using the Lorentzian Kubo-Toyabe relaxation function for the $f$ component. (c) Temperature dependence of the partial $\mu$-e decay asymmetry, one for the component showing fast exponential damping ($A$) and the other showing Gaussian damping ($B$). (d) Depolarization rate of the respective components [with ($A$) and ($B$) representing the same in (c)] as a function of temperature. (e) $\mu \mathrm{SR}$ spectra in ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Ir}}_2{\mathrm{S}}_4$ under zero external field with $x=0.01$, and (f) $x=0.1$, where solid curves show fits by Eq. (2). (g) Relative amplitude of the component showing fast exponential damping ($A$) as a function of temperature. (h) Magnetic susceptibility ($\chi$) of ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Ir}}_2{\mathrm{S}}_4$ samples used for $\mu \mathrm{SR}$ measurements (measured at 0.1 T). The inset shows the temperature of metal-insulator transition ($T_{\text{MI}}$) determined by $\chi$. [Vertical axes for (a), (b), (e), and (f) are scaled by $A_0$ in Eq. (2) for the ease of comparison by eye.]