Speaker
Description
Magnetic materials containing octahedrally coordinated Ir$^{4+}$ ions can give rise novel J$_{eff}$ =$\frac{1}{2}$ magnetic moments due to the interplay of strong spin-orbit coupling, onsite Coulomb repulsion and crystalline electric field. The exchange interaction between such moments depends on the geometry of the exchange paths between the magnetic ions and could be highly anisotropic such as the Kitaev exchange in 2D honeycomb lattice. This could lead to a rich variety of magnetic ground states with exotic excitation as has been proposed theoretically and also observed experimentally in several real materials. (NH$_4$)$_2$IrCl$_6$ retains its cubic symmetry (fcc) down to very low temperatures and offer best possible condition for the cubic crystalline electric field to realize genuine J$_{eff}$ =$\frac{1}{2}$ state. The crystal and magnetic structures of the (NH$_4$)$_2$IrCl$_6$ single crystal have been studied using neutron diffraction, synchrotron X-ray diffraction and resonant inelastic X-ray scattering techniques. The study shows that the interplay of geometrical frustration and the bond dependent exchange frustration stabilizes a type-III collinear AFM ordering at $T_{\rm N}$=2.1 K with propagation vector (1 $\frac{1}{2}$ 0). Thus the bond dependent Kitaev interaction in the fcc lattice may oppose the magnetic frustration which is in sharp contrast to the Kiteav interaction in honeycomb lattices promoting quantum spin-liquid ground states.
