Orbital mixing and strong Hund's coupling stabilize spin order in the van der Waals ferromagnet CrI 3

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 d transition metal elements, combined with chalcogenides or halogens. Despite the great technological promise in the field of spintronics, the connection between the d -orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet CrI 3 , via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk CrI 3 in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the stabilization of ferromagnetism is attributable to orbital mixing between I p and Cr e g states, and to the presence of strong Hund's coupling. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.