Inelastic neutron scattering experiments with a large set of comounted ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ crystals reveal the low-energy magnon dispersion in this candidate material for Kitaev physics. The magnon gap amounts to 1.7(1) meV and can be interpreted similarly to the sister compound $\alpha \text{−}{\mathrm{RuCl}}_3$ to stem from the zone boundaries in the antiferromagnetic zigzag structure. The neutron experiments find no evidence for low-energy excitations with ferromagnetic character, which contrasts with the findings in $\alpha \text{−}{\mathrm{RuCl}}_3$. Our results are consistent with a recently proposed microscopic model that involves an antiferromagnetic Heisenberg nearest-neighbor exchange in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ in contrast to the ferromagnetic one considered for $\alpha \text{−}{\mathrm{RuCl}}_3$. Although the magnetic response shows the signatures of bond-directional anisotropy in both materials the different relative signs of Kitaev and Heisenberg interaction result in different deviations from the initial Kitaev model. Low-energy ferromagnetic fluctuations cannot be considered as a fingerprint of ferromagnetic Kitaev interaction.

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• Antiferromagnetism
• Magnons
• Spin dynamics
• Antiferromagnets
• Transition metal oxides
• Inelastic neutron scattering
• Kitaev model