FePS₃ is a layered van der Waals (vdW) Ising antiferromagnet that has recently been studied in the context of true 2D magnetism and emerged as an ideal material platform for investigating strong spin-phonon coupling, and non-linear magneto-optical phenomena. In this work, we demonstrate an important unresolved role of spin-orbit coupling (SOC) in the ground state and excitations of this compound. Combining first-principles calculations with linear flavor wave theory (LFWT), we find strong mixing and spectral overlap of different spin-orbital single-ion states. Low-lying excitations form hybrid spin-orbit exciton/magnon modes. Complete parameterization of the low-energy model requires nearly half a million coupling constants. Despite this complexity, such a model can be inexpensively derived using local many-body-based approaches, which yield quantitative agreement with recent experiments. The results highlight the importance of SOC even in first-row transition metals and provide essential insight into the properties of 2D magnets with unquenched orbital moments.

FePS ₃是一种层状范德华 (vdW) 伊辛反铁磁体，最近在真正的二维磁性背景下进行了研究，并成为研究强自旋声子耦合和非线性磁光现象的理想材料平台。在这项工作中，我们证明了自旋轨道耦合（SOC）在该化合物的基态和激发中的重要的未解决的作用。将第一性原理计算与线性风味波理论（LFWT）相结合，我们发现不同自旋轨道单离子态的强烈混合和光谱重叠。低位激发形成混合自旋轨道激子/磁振子模式。低能量模型的完整参数化需要近五十万个耦合常数。尽管存在这种复杂性，但可以使用基于局部多体的方法以廉价的方式导出这样的模型，该方法与最近的实验产生了定量的一致性。研究结果强调了 SOC 的重要性，即使在第一行过渡金属中也是如此，并为具有未淬火轨道矩的二维磁体的特性提供了重要的见解。