Direct Magneto-Optical Evidence for Kitaev Interactions in the van der Waals Multiferroic Nil₂

We have performed magneto-optical transmission and Faraday angle rotation measurements on NiI₂, a van der Waals (vdW) multiferroic two-dimensional magnetic material. We observed two collective modes [1,2] around 34 cm⁻¹ and 37 cm⁻¹ at 4.2 K, deep in the helical AFM ground state, well below T_N1≈76 K and T_N2≈59 K. The modes exhibit an anisotropic blueshift with increasing magnetic field up to 16 T. The circular optical conductivity shows a dichroic response of the magnon excitations, which are also opposite in chirality. Our experimental results are interpreted using the Kitaev exchange interaction model in NiI₂, incorporating the intrinsic crystal anisotropy. These findings reveal that the magnetic excitations in the helical phase arise from anisotropic Kitaev interactions and suggest that NiI₂ is a promising platform for realizing high-order skyrmionic lattice states driven by competing exchange and anisotropy effects.

[1] Son et al., Advanced Materials 34, 10 (2022).

[2] Kim et al., Phys. Rev. B 108, 064414 (2023).

[3] Bazyliansky et al., to be submitted in Nature Physics