  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$_2$.
