Antiferromagnetic resonance in the cubic iridium hexahalides (NH4)2IrCl6 and

We report on high-field electron spin resonance studies of two iridium hexahalide compounds, (NH_4_2rCl_6and K_2rCl_6 In the paramagnetic state, our measurements reveal isotropic gfactors g = 1.791) for the Ir^{4+}ions, in agreement with their cubic symmetries. Most importantly, in the magnetically ordered state, we observe two magnon modes with zero-field gaps of 11.3 and 14.2 K for (NH_4_2rCl_6and...