Antiferromagnetic resonance in the cubic iridium hexahalides (NH$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$

TL;DR

This study uses high-field electron spin resonance to investigate antiferromagnetic resonance in cubic iridium hexahalides, revealing weak Kitaev interactions and quantifying exchange couplings in (NH$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$.

Contribution

It provides the first detailed quantification of exchange interactions and anisotropies in these iridium hexahalides using ESR and spin-wave theory.

Findings

Observation of two magnon modes with zero-field gaps of 11.3 and 14.2 K.

Nearest-neighbor Heisenberg coupling is dominant with weak Kitaev anisotropy.

Isotropic g-factors consistent with cubic symmetry.

Abstract

We report on high-field electron spin resonance studies of two iridium hexahalide compounds, (NH$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$. In the paramagnetic state, our measurements reveal isotropic $g$-factors $g = 1.79$(1) 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$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$, respectively. Based on that and using linear spin-wave theory, we estimate the nearest-neighbor exchange couplings and anisotropic Kitaev interactions, $J_1/k_B=10.3$ K, $K/k_B=0.7$ K for (NH$_4$)$_2$IrCl$_6$, and $J_1/k_B=13.8$ K, $K/k_B=0.9$ K for K$_2$IrCl$_6$, revealing the nearest-neighbor Heisenberg coupling as the leading interaction term, with only a weak Kitaev anisotropy.