Lattice Dynamics, Phonon Chirality, and Spin–Phonon Coupling in 2D Itinerant Ferromagnet Fe₃GeTe₂

Abstract

Fe 3 GeTe 2 has emerged as one of the most fascinating van der Waals crystals due to its two-dimensional (2D) itinerant ferromagnetism, topological nodal lines and Kondo lattice behavior. However, lattice dynamics, chirality of phonons and spin-phonon coupling in this material, which set the foundation for these exotic phenomena, have remained unexplored. Here we report the first experimental investigation of the phonons and mutual interactions between spin and lattice degrees of freedom in few-layerFe 3 GeTe 2 . Our results elucidate three prominent Raman modes at room temperature: two A 1g (Γ) and one E 2g (Γ) phonons. The doubly degenerate E 2g (Γ) mode reverses the helicity of incident photon, indicating the pseudo-angular momentum and chirality. Through analysis of temperature-dependent phonon energies and lifetimes, which strongly diverge from the anharmonic model below Curie temperature, we determine the spin-phonon coupling in Fe 3 GeTe 2 . Such interaction between lattice oscillations and spin significantly enhances the Raman susceptibility, allowing us to observe two additional Raman modes at the cryogenic temperature range. In addition, we reveal laser radiation induced degradation of Fe 3 GeTe 2 in ambient conditions and the corresponding Raman fingerprint. Our results provide the first experimental analysis of phonons in this novel 2D itinerant ferromagnet and their applicability for further fundamental studies and application development.

Thickness and Spin Dependence of Raman Modes in Magnetic Layered Fe3GeTe2

“…194), which corresponds to point group. [ 24,25 ] Because the unit cell of the bulk has 12 atoms in total, it possesses 36 phonon normal modes at the Γ point, which can be decomposed as Raman active modes correspond to non‐degenerate A 1 g , twofold degenerate E 1 g , and twofold degenerate E 2 g . Their Raman tensors are shown in Table 1, as determined by group theory.…”

“…Since most of experimentally observed Raman peaks in Fe 3 GeTe 2 reside above 100 cm −1 , [ 24,25 ] our study is focused on Raman modes above 100 cm −1 , where there are two A 1 g modes and three pairs of E 2 g modes, as shown in Table 2 . The calculated atomic displacement patterns of these five Raman modes are illustrated in Figure .…”

“…At present, there are limited studies on the lattice dynamics in Fe 3 GeTe 2 , [ 24,25 ] where the focus was on the bulk and thick samples with only the interlayer FM order considered. In this work, we carry out a systematic computational study of the lattice dynamics in the bulk, bilayer and monolayer Fe 3 GeTe 2 with both interlayer FM and AFM orderings taken into account, so we can reveal the delicate effects of the layer number and interlayer spin ordering on the Raman modes.…”

van der Waals Magnets: Material Family, Detection and Modulation of Magnetism, and Perspective in Spintronics

“…[ 72 ] From temperature‐dependent Raman measurements shown in Figure a, it can be seen that the phonon frequency of A 1g mode in Fe 3 GeTe 2 shows a clear deviation from the fitting line when the temperature is below T C , implying an abnormal softening of phonon with the onset of magnetism. [ 117 ] In Fe 3 GeTe 2 , the lattice vibrations will modulate the superexchange interaction, leading to strong spin‐phonon coupling and later the unusual decrease of phonon frequency below T C . In addition, the presence of magnetism is also identified by the change of polarization dependence of specific Raman modes.…”

“…Reproduced with permission. [ 117 ] Copyright 2019, Wiley‐VCH. b) The polar plots of A 1g mode in monolayer CrI 3 at 60 K (left panel) and 15 K (right panel) with moments aligned upward (red) and downward (blue).…”