Opposite pressure effects on magnetic phase transitions in NiBr2

NiI₂ and NiBr₂ are archetypal van der Waals (vdW) triangular-lattice multiferroics that host incommensurate helimagnetic order at the lowest temperatures and undergo a transition to collinear antiferromagnetic order upon heating. Focusing on NiBr₂, we reveal that both antiferromagnetic phases exhibit a pronounced sensitivity to hydrostatic pressure. The Néel temperature of the collinear phase increases steeply at ~20 K/GPa, reaching ~100 K at 3 GPa without any indication of saturation, whereas the helimagnetic phase is completely suppressed only above ~0.8 GPa. This behavior contrasts sharply with NiI₂, in which both helical and collinear phases are strengthened until a moderate pressure of ∼6 GPa, above which the helical phase instantly disappears. Ab initio calculations identify the second-nearest interlayer exchange interaction j₂′ as the primary driver stabilizing the collinear AFM phase in NiBr₂. In addition, the in-plane exchange ratio renders the helical order in NiBr₂ considerably more fragile, enabling its suppression under relatively small pressures. These results underscore the dominant role of interlayer interactions in governing the distinct pressure responses of the magnetic phases in NiBr₂ and NiI₂. NiBr₂ and NiI₂ are van der Waals multiferroics with magnetic phase transitions from incommensurate helimagnetic order at low temperatures to collinear antiferromagnetic order upon heating. Here, the authors demonstrate that hydrostatic pressure significantly affects the antiferromagnetic phases in NiBr₂, with ab initio calculations highlighting interlayer interactions as key to stabilizing the collinear order, and reveal a contrasting pressure response in NiI₂, offering insights into pressure-induced magnetic phase control.