Realistic Spin Model for Multiferroic NiI2

A realistic first-principle-based spin Hamiltonian is constructed for the type-II multiferroic NiI2, using a symmetry-adapted cluster expansion method. Besides single ion anisotropy and isotropic Heisenberg terms, this model further includes the Kitaev interaction and a biquadratic term, and can well reproduce striking features of the experimental helical ground state, that are, e.g., a proper screw state, canting of rotation plane, propagation direction, and period. Using this model to build a phase diagram, it is demonstrated that, (i) the in-plane propagation direction of (11 over bar 0) is determined by the Kitaev interaction, instead of the long-believed exchange frustrations and (ii) the canting of rotation plane is also dominantly determined by Kitaev interaction, rather than interlayer couplings. Furthermore, additional Monte Carlo simulations reveal three equivalent domains and different topological defects. Since the ferroelectricity is induced by spins in type-II multiferroics, our work also implies that Kitaev interaction is closely related to the multiferroicity of NiI2.

Automated summary

A realistic first-principle-based spin Hamiltonian is constructed for the type-II multiferroic NiI2, which includes single ion anisotropy, isotropic Heisenberg terms, Kitaev interaction, and a biquadratic term. The model can reproduce the experimental helical ground state features and establish the relationship between Kitaev interaction and the multiferroicity of NiI2. Additionally, Monte Carlo simulations reveal three equivalent domains and different topological defects.