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             Enhanced quantum fluctuation in low dimensions often engenders unusual properties of two-dimensional (2D) magnets. Mermin-Wagner theorem states that any long-range magnetism is forbidden in the isotropic 2D system owing to the strong spin fluctuation. In this context, magnetic anisotropy is a critical ingredient to stabilize the magnetic orderings in 2D systems, and understanding its microscopic origin is of paramount importance in the study of 2D magnets.

             Recently, the true 2D ferromagnetism down to the monolayer limit is realized in Cr-based van-der-Waals magnet CrI₃. In this system, the ⁴A₂ ground state of Cr³⁺ with quenched orbital moment cannot contribute to the single ion anisotropy by LS coupling. Instead, the strong spin-orbit coupling of ligand 5p orbital is identified as the origin of strong c-axis magnetic anisotropy in CrI₃.

             In this work, we investigated the origin of magnetic anisotropy in recently discovered V-based 2D ferromagnet VI₃, where both single ion anisotropy of V³⁺ and spin-orbit coupling from ligand 5p orbital contribute to the magnetic anisotropy. Using the synergetic combination of X-ray absorption spectroscopy, X-ray magnetic circular dichroism, and Cluster many-body calculation, we resolved the ground state of V³⁺ ion and successfully disentangled the contributions to the magnetic anisotropy. Our study provides fresh insights into the microscopic origin of long-range magnetic orderings in 3d-transition metal-based 2D magnets.