Magnetic frustration in iridium spinel compound CuIr${}_2$S${}_4$

We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound, CuIr${}_2$S${}_4$, is not the presumed spin-singlet state but a novel paramagnetic state, showing a quasistatic spin glass-like magnetism below ~100 K. Considering the earlier indication that 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.