Magnetic excitations in the Kitaev material Na$_2$IrO$_3$ studied by neutron scattering

TL;DR

This study uses neutron scattering to analyze magnetic excitations in Na$_2$IrO$_3$, revealing magnon dispersion and gaps that support an antiferromagnetic Heisenberg model, contrasting with related compounds and challenging previous assumptions about ferromagnetic Kitaev interactions.

Contribution

It provides the first detailed neutron scattering analysis of Na$_2$IrO$_3$, demonstrating the dominance of antiferromagnetic Heisenberg exchange over ferromagnetic Kitaev interactions.

Findings

Magnon gap of 1.7 meV observed.

No evidence of ferromagnetic low-energy excitations.

Results support an antiferromagnetic Heisenberg model.

Abstract

Inelastic neutron scattering experiments with a large set of comounted Na$_2$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 $\alpha$-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 to the findings in $\alpha$-RuCl$_{3}$. Our results are consistent with a recently proposed microscopic model that involves an antiferromagnetic Heisenberg nearest-neighbor exchange in Na$_2$IrO$_3$ in contrast to the ferromagnetic one considered for $\alpha$-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.