Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI 3 .

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI 3 . The structural transition at T S1 = 79 K is accompanied by a large splitting of polar phonon modes. Below T S1 , strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below T C = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature T S2 of the second structural transition associated with the order-order magnetic phase transition. Below T S2 , a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI 3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered the exclusive domain of antiferromagnets.