Strongly stabilized magnetic phase transitions in vdW multiferroic NiI2 under hydrostatic pressure

Multiferroics with coupled ferroelectric and magnetic orders have been the subject of intense interest for their functional magnetoelectric properties. The van der Waals (vdW) material NiI₂ has emerged as a new platform to explore multiferroicity in the 2D limit, enabling a plethora of potentially new physical phenomena due to reduced dimensionality. In this case, frustration of intralayer exchange interactions on the triangular magnetic lattice results in two magnetic phase transitions, first to an antiferromagnetic state at T_N1 = 75K and then to an incommensurate, helimagnetic and multiferroic phase at T_N2 = 60 K. These critical temperatures decrease with decreasing number of layers, revealing the importance of interlayer exchange interactions to stabilize the helimagnetism. In this vein, hydrostatic pressure is a powerful tool that can be used to tune the interlayer distance in vdW materials and, hence, enhance the interlayer exchange, as well as to perturb the delicate balance of intralayer exchange interactions which result in non-collinear magnetism. Here we investigate the magnetic and structural phase transitions in bulk NiI2 with high-pressure temperature-dependent (30K-298K) Raman spectroscopy, optical linear dichroism (LD) and x-ray diffraction (XRD). We obtain evidence for a significant and concurrent enhancement of both the magnetic (T_N1) and multiferroic (T_N2) transitions at a rate of ~ 14K/GPa. Our results reveal a general path to obtain high-temperature type-II multiferroicity via high pressures in vdW materials, allowing for a deeper understanding of the role of the intra- and inter-layer exchange parameters to guide the design of novel multiferroic vdW materials.