Comprehensive Raman spectroscopy study of MBE-grown off-stoichiometric van der Waals ferromagnet Fe5-xGeTe2 thin film

Abstract

Off-stoichiometric Fe5-xGeTe2 (F5GT) has recently emerged as a promising two-dimensional (2D) magnetic material within the van der Waals (vdW) family, owing to its tunable magnetic properties and high Curie temperature, attractive for potential spintronic applications. Here, we report a comprehensive Raman spectroscopic investigation of a high-quality epitaxial F5GT thin film (similar to 50 nm) grown by molecular beam epitaxy on c-cut Al2O3. The epitaxial nature and structural quality of the film are confirmed by in-situ reflection high-energy electron diffraction (RHEED) and out-of-plane X-ray diffraction (XRD), revealing strong c-axis orientation. For the first time, polarization-resolved Raman measurements using both linearly and circularly polarized light are performed to identify and assign phonon symmetries. We also performed temperature-dependent polarized Raman studies to explore the thermal stability and evolution of the phonon modes as well as the crystal symmetry. Polarization-resolved Raman spectroscopy results were compared and corroborated with calculated optical selection rules based on crystal symmetry-dependent Raman tensors. This comparison indicates the breaking of intrinsic three-fold crystal symmetry, resulting in mixing of in-plane and out of plane vibrational modes, possibly due to Fe deficiency in off-stoichiometric Fe5-xGeTe2. Additionally, to explore temperature-driven anharmonicity in phonon modes, we investigated the temperature-dependent evolution of the primary phonon modes, excited with unpolarized light, both in the absence and presence of applied magnetic field. The observed temperature-induced frequency and linewidth variations are correlated with the temperature-dependent magnetization behavior, providing possible insights into the interplay between lattice dynamics and the magnetic ordering. A high Curie temperature of similar to 307 K and a weak spin-phonon coupling are observed in this off-stoichiometric F5GT film. The present study reveals, based on the first extensive Raman spectroscopy investigation, (i) off-stoichiometry-driven phonon symmetry modification and (ii) existence of magnetic ordering near room temperature in F5GT, validating its potential as a 2D vdW magnet and establishing Raman spectroscopy as a powerful tool for probing 2D vdW magnets.