Quasi-two-dimensional magnon identification in antiferromagneticFePS3via magneto-Raman spectroscopy

Recently it was discovered that van der Waals bonded magnetic materials retain long range magnetic ordering down to a single layer, opening many avenues in fundamental physics and potential applications of these fascinating materials. One such material is $\mathrm{FeP}{\mathrm{S}}_{3}$, a large spin $(S=2)$ Mott insulator where the Fe atoms form a honeycomb lattice. In the bulk, $\mathrm{FeP}{\mathrm{S}}_{3}$ has been shown to be a quasi-two-dimensional-Ising antiferromagnet, with additional features in the Raman spectra emerging below the N\'eel temperature $({T}_{\mathrm{N}})$ of approximately 120 K. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that one of these Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of $\ensuremath{\approx}3.7\phantom{\rule{0.16em}{0ex}}\mathrm{THz}\phantom{\rule{4pt}{0ex}}(122\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$. Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the antiferromagnetic magnon as a function of temperature and magnetic field, respectively, where we determine the $g$ factor to be $\ensuremath{\approx}2$. In addition, the symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of $\mathrm{FeP}{\mathrm{S}}_{3}$.