Second order nonlinear optical spectroscopy study of a van der Waals multiferroic NiI₂

Despite belonging to a well-studied family of transition metal halides such as CrI₃, a-RuCl₃ and TiCl₃, NiI₂ has only begun to gain attention relatively recently for various attractions. First, recent studies have shown that chemically grown monolayer NiI₂ hosts both an antiferromagnetic order and a ferroelectric order. Second, it is also a candidate material for realizing unique magnetic excitons that are possibly similar to those found in NiPS₃. Despite the significance of magnetism in both attractions mentioned above, it remains understudied thus far. In this presentation, we will leverage the high sensitivity of our electric-quadrupole second harmonic generation (EQ SHG) to investigate the magnetic phase in NiI₂ and its coupling to the ferroelectric phase. In contrast to the electric dipole (ED) SHG used in the literature of NiI₂ that only shows up in the noncentrosymmetric ferroelectric phase, the EQ SHG is present in the centrosymmetric paramagnetic and antiferromagnetic phases. We will show the thorough temperature dependence of the rotation anisotropy (RA) of EQ SHG above the ferroelectric transition and the superposition of EQ and ED SHG below the ferroelectric transition. Finally, we will show the SHG imaging of both the antiferromagnetic phase and the ferroelectric phase.