Magnetic order of Jeff =1/2 moments in the antifluorite (NH4)2IrCl6 Mott insulator

Magnetic materials containing Ir4+ ions can give rise novel Jeff=1/2 magnetic moments due to the strong spin-orbit coupling. Their magnetic interactions depend on the exchange bonds between the magnetic ions and could be highly anisotropic depending on the geometry of the crystal structure. This leads to rich magnetic phases and exotic excitations in such materials. Unlike other related Ir4+ ion based materials, (NH4)2IrCl6 is believed to have genuine Jeff=1/2 magnetic moment, because it could retain its cubic symmetry (fcc lattice) down to very low temperature. To establish that, we want to determine its crystal structure at a very low temperature, well inside its magnetically ordered state. The fcc lattice, where geometrical frustration and exchange frustration coexist, has been predicted to host rich magnetic phases. So, we also want to determine the magnetic structure of (NH4)2IrCl6.