Resolving Intrinsic Versus Anomalous Raman Modes in 2D Ferromagnet Fe _{3– <i>x</i>} GeTe ₂ with Application to Regulating the Surface Oxide

Electron correlations in Fe_3-<i>x</i>GeTe₂ give rise to numerous emergent phenomena, notably two-dimensional (2D) itinerant ferromagnetism with application in spintronics. Lattice dynamics play an integral role in Fe_3-<i>x</i>GeTe₂'s novel properties, yet a complete understanding of them has been confounded by conflicting reports of the frequencies of its Raman modes. Here, we present time-dependent Raman scattering measurements of Fe_3-<i>x</i>GeTe₂ that resolve its intrinsic versus anomalous modes. The anomalous modes emerge from Te precipitates that form when Fe and Ge atoms at the surface oxidize and release Te from its original bonding. The anomalous modes become measurable when Fe_3-<i>x</i>GeTe₂ is exposed to air for more than 20 h, and they grow in intensity until they obscure the intrinsic modes after more than 38 h in air. We control the onset of these modes with both air exposure and laser irradiation. These results provide a reliable spectral reference for Raman scattering measurements to probe spin-phonon coupling, interlayer spin ordering, and phonon chirality in Fe_3-<i>x</i>GeTe₂. As the Raman scattering cross section of Te precipitates is large, we find that their modes provide a nondestructive, highly sensitive means to track the growth of a surface oxide in the range of 1-14 nm thick. This paves the way for the controlled design of an Fe_3-<i>x</i>GeTe₂/oxide heterostructure with a ferromagnetic/antiferromagnetic interface within which properties such as an exchange bias effect, enhanced perpendicular magnetic anisotropy, and skyrmions, among others, are found.