Opposite pressure effects on magnetic phase transitions in NiBr2

TL;DR

This study investigates how hydrostatic pressure distinctly influences magnetic phase transitions in NiBr2, revealing a sharp increase in Neel temperature and suppression of helimagnetic order, highlighting the importance of interlayer interactions.

Contribution

The paper uncovers the contrasting pressure responses of magnetic phases in NiBr2 compared to NiI2 and identifies key interlayer exchange interactions driving these effects.

Findings

Neel temperature of NiBr2's collinear phase increases at 20 K/GPa.

Helimagnetic phase in NiBr2 is suppressed above 0.8 GPa.

Interlayer exchange interaction j2' stabilizes the collinear AFM phase.

Abstract

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 Neel 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.