Pressure-enhanced helimagnetic order in van der Waals multiferroic NiI₂ in bulk and the 2D limit

Multiferroics – materials exhibiting coexisting ferroelectric and magnetic orders- have been the subject of intense interest for their functional magnetoelectric properties. The van der Waals (vdW) type-II multiferroic NiI₂ has emerged as a candidate for exploring non-collinear magnetism and magnetoelectric effects in the 2D limit. Frustration of intralayer exchange interactions on the triangular magnetic lattice results in two magnetic phase transitions, first to an antiferromagnetic state at T_N,1= 75 K and then to an helimagnetic ground state, with spin-induced improper ferroelectricity at T_N,2 = 60 K. Here we investigate the magnetic and structural phase transitions in bulk and atomically thin-layers of NiI₂, using high-pressure Raman spectroscopy, optical linear dichroism, and x-ray diffraction. We obtain evidence for a significant pressure enhancement of the antiferromagnetic and helimagnetic transition temperatures, at rates of ∼15.3/14.4 K/GPa in the bulk. Complementary high-pressure optical measurements on atomically-thin samples confirm the pressure enhanced magnetic phase transitions in the 2D limit. The large enhancement of the transition temperatures is attributed to a cooperative effect of interlayer and third-nearest-neighbor intralayer exchange through comparison to first-principles calculations. Finally, the close correspondence between the multiferroic and trigonal-to-monoclinic structural transitions establishes for the key role of interlayer stacking symmetry for driving the ground state spin structure leading to multiferroicity. These results reveal a general path for obtaining high-temperature type-II multiferroicity via high pressures in vdW materials.

* Occhialini, C.A. and Pimenta Martins, L.G. contributed equally to this work.