Title: Tuning the electronic and magnetic properties of NiBr$_2$ via pressure

Abstract: Transition metal dihalides (MX$_2$, M= transition metal, X= halide) have attracted much attention recently due to their intriguing low-dimensional magnetic properties. Particular focus has been placed in this family in the context of multiferroicity -- a common occurrence in MX$_2$ compounds that adopt non-collinear magnetic structures. One example of helimagnetic multiferroic material in the dihalide family is represented by NiBr$_2$. Here, we study the evolution of the electronic structure and magnetic properties of this material under pressure using first-principles calculations combined with Monte Carlo simulations. Our results indicate there is significant magnetic frustration in NiBr$_2$ due to the competing interactions arising from its underlying triangular lattice. This magnetic frustration increases with pressure and is at the origin of the helimagnetic order. Further, pressure causes a sizable increase in the interlayer interactions. Our Monte Carlo simulations show that a large (3-fold) increase in the helimagnetic transition temperature can be achieved at pressures of around 15 GPa. This indicates that hydrostatic pressure can indeed be used as a tuning knob to increase the magnetic transition temperature of NiBr$_2$.