Terahertz evidence of electromagnon excitations in the multiferroic van der Waals insulator NiI2

We report on our terahertz spectroscopic investigation of the van der Waals insulator NiI2, exhibiting antiferromagnetism below TN1 <^> 78 K and multiferroicity below TN2 <^> 59.5 K. Two electromagnon modes were detected at 34 and 37 cm-1 below TN2 where the material is in the helimagnetic-multiferroic phase. Our transmission measurement shows that the electromagnon resonance modes redshift with increasing temperature (at zero magnetic field) but blueshift with increasing magnetic field (at 1.5 K). A separate reflection measurement confirms the electric dipole active nature of the two electromagnon modes. The polarization, temperature, and magnetic field dependences show that these electromagnon modes are closely linked with the helimagnetic ordering in the multiferroic phase of NiI2. The electromagnon energies are also consistent with the energy scale of the two-magnon sideband excitation around the Zhang-Rice exciton mode recently discovered in NiI2.