Observation of Raman anomaly and characterization of magnetic phases in van der Waals ferromagnet Fe$_5$GeTe$_2$

Abstract

Two-dimensional (2D) van der Waals (vdW) ferromagnet Fe$_5$GeTe$_2$ has garnered significant interest due to its high Curie temperature (T$_C$), large saturation magnetization, and complex magnetic behavior arising, in part, from multiple inequivalent iron sites and vacancies. While several aspects of its complex magnetic and structural characteristics have been examined through careful experiments and first principles studies, much of it remains debatable. In this study, we present one of the first comprehensive temperature-dependent Raman spectrum for bulk Fe$_5$GeTe$_2$ and in the process reveal an interesting peak shift anomaly at 150 K. We discuss the possible relationship of this Raman anomaly with the anomalous lattice expansion reported earlier for this material at around 110 K. The impact of the anomalous lattice expansion on the magnetic anisotropy in this van der Waals material is also revealed by an isothermal magnetization analysis. These findings will prove crucial for the use of Fe$_5$GeTe$_2$ in high-performance spintronic devices.