Incommensurate Counterrotating Magnetic Order Stabilized by Kitaev Interactions in the Layered Honeycomb ${\alpha}$-Li$_2$IrO$_3$

TL;DR

This study reveals an incommensurate counter-rotating magnetic order in $ ext{Li}_2 ext{IrO}_3$, driven by Kitaev interactions, suggesting universal Kitaev physics in honeycomb iridates.

Contribution

It provides experimental evidence of a novel magnetic structure in $ ext{Li}_2 ext{IrO}_3$ stabilized by Kitaev interactions, extending understanding of magnetic behaviors in honeycomb iridates.

Findings

Incommensurate magnetic order with counter-rotating moments observed

Magnetic structure cannot be explained by isotropic Heisenberg couplings

Kitaev interactions are dominant in $ ext{Li}_2 ext{IrO}_3$

Abstract

The layered honeycomb magnet $\alpha$-Li$_2$IrO$_3$ has been theoretically proposed as a candidate to display novel magnetic behaviour associated with Kitaev interactions between spin-orbit entangled $j_{\rm eff}=1/2$ magnetic moments on a honeycomb lattice. Here we report single crystal magnetic resonant x-ray diffraction combined with powder magnetic neutron diffraction to reveal an incommensurate magnetic order in the honeycomb layers with Ir magnetic moments counter-rotating on nearest-neighbour sites. This type of magnetic structure has not been reported experimentally before in honeycomb magnets and cannot be explained by a spin Hamiltonian with dominant isotropic (Heisenberg) couplings. The magnetic structure shares many key features with the magnetic order in the structural polytypes $\beta$ and $\gamma$-Li$_2$IrO$_3$, understood theoretically to be stabilized by dominant Kitaev interactions between Ir moments located on the vertices of three-dimensional hyperhoneycomb and stripyhoneycomb lattices, respectively. Based on this analogy and a theoretical soft-spin analysis of magnetic ground states for candidate spin Hamiltonians, we propose that Kitaev interactions also dominate in $\alpha$-Li$_2$IrO$_3$, indicative of universal Kitaev physics across all three members of the harmonic honeycomb family of Li$_2$IrO$_3$ polytypes.