Large orbital moment in VI₃ as evidenced by x-ray magnetic circular dichroism

Van der Waals (vdW) magnets are currently intensely investigated due to their long-range magnetic order in low dimension. VI₃ is a vdW semiconductor with ferromagnetic order below 50K. In this system, layers composed of edge-sharing VI₆ octahedra arranged in a honeycomb lattice are bound by vdW forces. It exhibits a large magnetic anisotropy and the measured total magnetic moment shows discrepancies to the expected spin-only value. These experimental results supported by a few theoretical works about the V ground state lead to the suggestion of a large unquenched orbital moment (OM). Below ~ 36K various experimental techniques evidenced a qualitatively different ferromagnetic phase, and the co-existence of two inequivalent V sites was proposed. In this work, we have used x-ray magnetic circular dichroism at the V L_3,2 edges in VI₃ to quantify the V OM. Using sum rule analysis of the XMCD spectrum we confirm an exceptionally large unquenched OM. With the help of density functional theory and ligand field multiplet simulations we propose a model for the V ground state where there is a co-existence of two V sites with different orbital occupation.