Incommensurate Counterrotating Magnetic Order Stabilized by Kitaev Interactions in the Layered Honeycomb α-Li₂IrO₃

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The layered honeycomb magnet α-Li₂IrO₃ has been theoretically proposed as a candidate to display novel magnetic behaviour associated with Kitaev interactions between spin-orbit entangled j 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 β and γ-Li₂IrO₃, 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 α-Li₂IrO₃, indicative of universal Kitaev physics across all three members of the harmonic honeycomb family of Li₂IrO₃ polytypes.