Antiferromagnetic resonance in the cubic iridium hexahalides ( NH 4 ) 2 IrCl 6 and K 2 IrCl 6

We report on high-field electron spin resonance studies of two iridium hexahalide compounds ${({\mathrm{NH}}_{4})}_{2}{\mathrm{IrCl}}_{6}$ and ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$. In the paramagnetic state, our measurements reveal isotropic $g$ factors $g=1.79(1)$ for the ${\mathrm{Ir}}^{4+}$ 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 ${({\mathrm{NH}}_{4})}_{2}{\mathrm{IrCl}}_{6}$ and ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$, respectively. Based on that and using linear spin-wave theory, we estimate the nearest-neighbor exchange couplings and anisotropic Kitaev interactions ${J}_{1}/{k}_{B}=10.3\phantom{\rule{4pt}{0ex}}\mathrm{K},$ $K/{k}_{B}=0.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$ for ${({\mathrm{NH}}_{4})}_{2}{\mathrm{IrCl}}_{6},$ and ${J}_{1}/{k}_{B}=13.8\phantom{\rule{4pt}{0ex}}\mathrm{K},$ $K/{k}_{B}=0.9\phantom{\rule{4pt}{0ex}}\mathrm{K}$ for ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$, revealing the nearest-neighbor Heisenberg coupling as the leading interaction term, with only a weak Kitaev anisotropy.