Magnetic order and spin dynamics of <i>j</i>_eff = ½ Ir⁴⁺ moments on the fcc lattice in La₂<i>B</i>IrO₆ (<i>B </i>= Mg<i>, </i>Zn)

We have performed bulk characterization, neutron diffraction, and inelastic neutron scattering experiments to characterize the magnetism in polycrystalline samples of the weakly-distorted fcc double perovskite iridates La₂ZnIrO₆ and La₂MgIrO₆. We find that these materials are spin-orbit-assisted Mott insulators with A-type antiferromagnetic ground states. Furthermore, the powder inelastic neutron scattering data on these systems provides clear evidence for gapped spin-wave excitations with very weak dispersion. The INS results and thermodynamic data on these materials can be reproduced by a conventional Heisenberg-Ising model with significant uniaxial Ising anisotropy and sizeable second-neighbor ferromagnetic interactions, or perhaps more tantalizingly, with a Heisenberg-Kitaev model featuring a dominant antiferromagnetic Kitaev exchange. Our findings highlight how conventional magnetic orders in heavy transition metal oxides may be driven by highly-directional exchange interactions rooted in strong spin-orbit coupling.