Spin Reorientation Driven Renormalization of Spin-Phonon Coupling in Fe4GeTe2
Abstract
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 strong 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 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 linewidth narrows, consistent with reduced anharmonicity, while across the spin reorientation transition at TSR they display anomalous softening, linewidth broadening, and a peak in lifetime, which are signatures of strengthened spin-phonon coupling. 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.
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