Chromium tri-iodide (CrI₃) is a prototypical ferromagnetic van der Waals insulator with its genuinely two-dimensional (2D) long-range magnetic order below 45 K demonstrated recently. The underlying magnetic anisotropy has not been completely understood while both the Dzyaloshinskii-Moriya (DM) interaction and the Kitaev\(-\Gamma\) type interaction have been considered as the relevant magnetic Hamiltonian. In addition, the relation between the crystal structure and the magnetic order needs to be further elucidated concerning their possible coupling in few-layer samples and in the topmost surface layers of bulk samples. Here, we investigate these issues via temperature- and magnetic field-dependent terahertz spectroscopy on bulk CrI₃ single crystals, focusing on the dynamics of ferromagnetic resonances (FMRs) and optical phonons in the terahertz (THz) region from 4 to 120 cm⁻¹ (from 0.5 to 15 meV). We narrow down the possible ranges of the interaction parameters such as the off-diagonal symmetric terms and the single-ion anisotropy. The accurate values of these parameters significantly constrain the magnitude of possible Kitaev\(-\Gamma\) exchange interaction and the topological magnon gap. Moreover, the structure-magnetism relationship was critically analyzed based on the temperature- and field-dependences of two \({{\rm{E}}}_{{\rm{u}}}\) in-plane optical phonon modes, which shows that the commonly believed structural phase diagram of CrI₃, derived from surface-preferential data, has to be seriously modified.

三碘化铬 (CrI ₃ ) 是一种典型的铁磁范德华绝缘体，最近展示了其低于 45 K 的真正二维 (2D) 长程磁序。虽然 Dzyaloshinskii-Moriya (DM) 相互作用和 Kitaev\(-\Gamma\) 型相互作用都被认为是相关的磁哈密顿量，但潜在的磁各向异性尚未完全被理解。此外，晶体结构和磁序之间的关系需要进一步阐明它们在少层样品和块体样品的最顶层表面层中可能的耦合。在这里，我们通过大块 CrI ₃ 单晶上与温度和磁场相关的太赫兹光谱研究这些问题，重点关注太赫兹 (THz) 区域中的铁磁共振 (FMR) 和光学声子的动力学。 4 至 120 cm ⁻¹ （0.5 至 15 meV）。我们缩小了相互作用参数的可能范围，例如非对角对称项和单离子各向异性。这些参数的准确值极大地限制了可能的 Kitaev\(-\Gamma\) 交换相互作用和拓扑磁振子间隙的大小。此外，基于两个\({{\rm{E}}}_{{\rm{u}}}\)面内光学声子模式的温度和场依赖性，对结构-磁性关系进行了批判性分析，这表明普遍认为的 CrI ₃ 结构相图（源自表面优先数据）必须进行认真修改。