The quasi-1D material MoI₃ consists of dimerized chains with distorted octahedral coordination weakly coupled to adjacent chains in a triangular lattice [1].  The dimerized chains exhibit easy-plane antiferromagnetic exchange coupling with the two exchange coupling constants J₁ and J₂ differing in magnitude by a factor of 5. As the hole filling is increased to 0.5 per unit cell (16 atoms), the magnitude of J₂ decreases, goes to zero, and then switches sign to become ferromagnetic coupling.  Once J₂ switches sign, the magnonic spectrum becomes gapped, and a smooth variation of the magnitude of J₂ causes the gap to close and re-open. For the larger magnitudes of J₂, the gapped magnonic bandstructure is in a non-trivial topological phase qualitatively similar that of a Su-Schrieffer-Heeger model. The antiferromagnic interchain coupling J₃ gives rise to a helical spin pattern in the a-c plane perpendicular to the chains. Depending on the relative strengths of J₂<0 and J₃>0, the spin pattern along the chains is either AFM or FM coupled AFM dimers. The magnetic ground state and its excitations are analyzed both analytically and numerically with exchange and anisotropy parameters extracted from density functional theory calculations.

1. F. Kargar et al., Appl. Phys. Lett., 121, 221901 (2022).