Fig. 1: Magnetic ordering and properties of layered NiI2.

a Specific heat capacity and magnetic susceptibility of NiI2 as a function of temperature. FC stands for field cooling. b Schematic illustration of the transition of magnetic structure in NiI2, shifting from helimagnetic (HM) to antiferromagnetic (AFM) states due to enhanced interlayer coupling. Lower figure represents the dominate interlayer exchange interactions J1 and J3 at ambient pressure and the perpendicular effective exchange coupling \({J}_{2}^{\perp }\) at high pressure.