Effects of Pressure on Electronic and Magnetic Properties of Bulk NiI₂

Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI₂ is a centrosymmetric magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent breakthrough experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the evolution of the electronic structure and magnetic exchange interactions in NiI₂ as a function of external pressure using first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest neighbor intralayer exchanges, which determines the magnetic propagation vector q of the helimagnetic ground state, monotonically increases in magnitude with pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature indicating that pressure is an effective means to tune the magnetic ground state of NiI₂.