Orbital mixing as key ingredient for magnetic order in a van der Waals ferromagnet

Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light 3$\mathit{\text{d}}$-transition metal elements, combined with chalcogenides or halogens. Despite the high technological promise in the field of spintronics, the connection between the $\mathit{\text{d}}$-orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet CrI\textsubscript{3}, via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk CrI\textsubscript{3} in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the mechanism of stabilization of ferromagnetism is attributable to orbital mixing between I $\mathit{\text{p}}$ and Cr $\mathit{\text{e}}\mathit{\text{textsubscript}}\mathit{\text{g}}$ states. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.