Magneto-Optical Study of Chiral Magnetic Modes in NiI$_{2}$: Direct Evidence for Kitaev Interactions

TL;DR

The study provides direct evidence that Kitaev bond-dependent interactions dominate the magnetic dynamics in NiI$_2$, a van der Waals multiferroic. By combining magneto-optical measurements (magneto-transmission, Faraday rotation, and magnetic circular dichroism) with LSWT/KPM simulations of a Kitaev Hamiltonian augmented by an in-plane anisotropy term $A_{zz}$, the authors show that two electromagnon modes at $34$ cm$^{-1}$ and $37$ cm$^{-1}$ arise from a single magnon branch split by Kitaev exchange and $A_{zz}$, with opposite chirality and field-dependent behavior consistent with Kitaev physics. The observed zero-field dichroism and nonzero Faraday responses, along with the inferred weak ferromagnetic moment, point to intrinsic time-reversal symmetry breaking that selects a chiral spin texture, while the results imply that NiI$_2$ is a promising platform for Kitaev-driven topological spin phenomena, including high-order skyrmion lattices, tunable by strain or doping. Overall, the work advances understanding of bond-dependent interactions in real materials and highlights NiI$_2$ as a versatile system for exploring chiral magnons and topological magnetism in two dimensions.

Abstract

Bond-dependent magnetic interactions, particularly those described by the Kitaev model, have emerged as a key pathway toward realizing unconventional magnetic states such as quantum spin liquids and topologically nontrivial excitations, including skyrmions. These interactions frustrate conventional magnetic order and give rise to rich collective behavior that continues to challenge both theory and experiment. While Kitaev physics has been extensively explored in the context of honeycomb magnets, direct evidence for its role in real materials remains scarce. Magnetic van der Waals (vdW) materials have emerged as a versatile platform for exploring low-dimensional electrical, magnetic, and correlated electronic phenomena, and provide a fertile ground for potential applications ranging from spintronics to multiferroic devices and quantum information technologies. Here, we demonstrate, through magneto-transmission, Faraday angle rotation, and magnetic circular dichroism measurements, that the magnetic excitation spectrum of NiI$_2$, a van der Waals multiferroic material, is more accurately captured by a Kitaev-based spin model than by the previously invoked helical spin framework.

Figures (5)

• Figure 1: Magneto-transmission and chiral dynamical simulations of NiI$_2$.a, Magneto-optical transmission in the far-infrared spectral range measured at $T = 4.2$ K for zero and maximum applied magnetic field $B$. b, Evolution of the magnon mode with magnetic field. The solid lines show theoretical calculations incorporating anisotropy, as described in the text. c,e, FTIR intensity and chiral dynamical spin structure factor simulations of NiI$_2$ using linear spin-wave theory (LSWT). d,f, Simulations of NiI$_2$ using only Kitaev interaction ($K$) or easy-plane single-ion anisotropy ($A_{zz}$).
• Figure 2: Faraday rotation and dichroism in NiI$_2$.a, Faraday rotation angle as a function of frequency at $T = 4.2$ K and an applied magnetic field of $B = 7$ T, estimated using the fast protocol described in the text. b, Left-handed ($\sigma_{-}$) and right-handed ($\sigma_{+}$) optical conductivities derived from the Faraday rotation data in the FIR range at $T = 4.2$ K and $B = 7$ T. c, Imaginary part of the transverse optical Hall conductivity, $\text{Im}[\sigma_{xy}]$, in the FIR range at $T = 4.2$ K and $B = 7$ T. d, Dichroism of right-circularly polarized light ($\sigma_{+}$) and left-circularly polarized light ($\sigma_{-}$). Solid lines indicate linear fits to the data. e, Absorption spectra of NiI$_2$ measured with left-circularly polarized (LCP) terahertz radiation at 1.5 K, showing the magnetic field dependence of the 34 cm$^{-1}$ and 37 cm$^{-1}$ electromagnon modes. f, Corresponding spectra measured under right-circularly polarized (RCP) radiation.
• Figure 3: Supplementary Fig. S1 | Evolution of magnon central frequency and transmission in the Faraday configuration.a, Evolution of the magnon mode with magnetic field. The solid line represents a fit using the easy-axis model, as described in the text. b, Spectral weight of the imaginary part of the transverse optical Hall conductivity as a function of magnetic field at $T = 4.2$ K. The solid line shows a linear fit to the data. c–f, Peak positions of circularly polarized absorption spectra for the lower (34 cm$^{-1}$) and upper (37 cm$^{-1}$) magnon modes under left- and right-circularly polarized (LCP/RCP) terahertz radiation. The chiral polarization, defined as $(I_{\mathrm{LCP}} - I_{\mathrm{RCP}})/(I_{\mathrm{LCP}} + I_{\mathrm{RCP}})$, $I$ is the intensity.
• Figure 4: Supplementary Fig. S2 | Transmission in the Voigt configuration.a–d, Magneto-optical response of NiI$_2$ in the far-infrared spectral range at $T = 4.2$ K in the Voigt geometry ($B \perp E$).
• Figure 5: Fig. 4 | Hidden ferromagnetic moments in NiI$_2$.a, Field-dependent magnetization measurements of NiI$_2$ at $T = 1.8$ K with $H \parallel c$. b, Temperature dependence of magnetization in NiI$_2$, measured after field cooling under a finite magnetic field with $H \parallel c$. Each color represents a distinct magnetic field. c, d, Bloch sphere representation of the magnetic moments in NiI$_2$. c shows the ideal magnetic structure of NiI$_2$ without ferromagnetic moments, while d depicts the helical magnetic order with weak ferromagnetic moments. The blue arrow indicates the direction of the weak ferromagnetic moments.