Spin Reorientation Driven Renormalization of Spin-Phonon Coupling in Fe4GeTe2.

Quasi-2D van der Waals ferromagnet Fe4GeTe2, featuring the simultaneous presence of high Curie temperature (TC ∼ 270 K) and a spin-reorientation transition at TSR ∼ 110 K, is a rare system where the interplay of spin dynamics, lattice vibrations, and electronic structure leads to a wide range of interesting phenomena. Here, we investigate the lattice response of exfoliated Fe4GeTe2 nanoflakes using temperature-dependent Raman spectroscopy. Polarization-resolved measurements reveal that, while only one Raman mode exhibits a purely out-of-plane character, the rest display mixed symmetry, reflecting interlayer vibrational nonuniformity and symmetry-driven mode degeneracies. Below TC, phonons harden, and the line width narrows, consistent with reduced anharmonicity, while across the spin reorientation transition at TSR they display anomalous softening, line width broadening, and a peak in lifetime, which are signatures of strengthened spin-phonon coupling. The associated phonon renormalization and effective spin-phonon coupling strength gradually decrease with decreasing sample thickness but remain finite down to ∼10 nm. Complementary DFT+DMFT calculations and atomistic spin dynamical simulations reveal temperature-dependent spin excitations whose energies overlap with the Raman-active phonons, providing a natural route for the observed magnon-phonon interaction. Together, these insights establish Fe4GeTe2 as a versatile platform for exploring intertwined spin, lattice, and electronic degrees of freedom, with relevance for dynamic spintronic and magneto-optic functionalities near technologically meaningful temperatures.