Unified theory of spiral magnetism in the harmonic-honeycomb iridates α, β, γ Li2IrO3
Abstract
A family of insulating iridates with chemical formula Li2IrO3 has recently been discovered, featuring three distinct crystal structures α,β,γ (honeycomb, hyperhoneycomb, stripyhoneycomb). Measurements on the three-dimensional polytypes, β- and γ-Li2IrO3, found that they magnetically order into remarkably similar spiral phases, exhibiting a non-coplanar counter-rotating spiral magnetic order with equivalent q=0.57 wavevectors. We examine magnetic Hamiltonians for this family and show that the same triplet of nearest-neighbor Kitaev-Heisenberg-Ising (KJI) interactions reproduces this spiral order on both β,γ-Li2IrO3 structures. We analyze the origin of this phenomenon by studying the model on a 1D zigzag chain, a structural unit common to the three polytypes. The zigzag-chain solution transparently shows how the Kitaev interaction stabilizes the counter-rotating spiral, which is shown to persist on restoring the inter-chain coupling. Our minimal model makes a concrete prediction for the magnetic order in α-Li2IrO3.
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