Non-collinear order and spin-orbit coupling in Sr₃ZnIrO₆

Sr3ZnIrO6 is an effective spin one-half Mott insulating iridate belonging to a family of magnets which includes a number of quasi-one dimensional systems as well as materials exhibiting three dimensional order. Here we present the results of an extensive investigation into the magnetism including heat capacity, a.c. susceptibility, muon spin rotation (μSR), neutron diffraction and inelastic neutron scattering on the same sample. It is established that the material exhibits a transition at about 17K into a three-dimensional antiferromagnetic structure with propagation vector k = (0, 1/2, 1) in the hexagonal setting of R3¯c and non-collinear moments of 0.87μB on Ir4+ ions. Further we have observed a well defined powder averaged spin wave spectrum with zone boundary energy of ∼5 meV at 5K. We stress that a theoretical analysis shows that the observed non-collinear magnetic structure arises from anisotropic inter- and intra-chain exchange which has its origin in significant spin.orbit coupling. The model can satisfactorily explain the observed magnetic structure and spin wave excitations.