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.

Figures (6)

• Figure 1: Structural charcterization of bulk Fe$_5$GeTe$_2$ single crystals. (a) Schematic of the crystal structure of Fe$_5$GeTe$_2$ (b) Room temperature XRD of Fe$_5$GeTe$_2$ (c) FESEM image of bulk Fe$_5$GeTe$_2$ showing the vdW layers.
• Figure 2: Temperature-dependent magnetization measurements of bulk Fe$_5$GeTe$_2$ crystal. (a) Magnetization versus temperature M(T) for H $\parallel$$ab$ and H $\parallel$$c$ under an applied field H = 50 Oe. (b) M(T) curves for H $\parallel$$ab$ at different magnetic fields. (c) Temperature-dependent Field cooled cooling (FCC) and Field cooled warming (FCW) magnetization for H $\parallel$$ab$.
• Figure 3: (a) Isothermal magnetization curves M(H) for H $\parallel$$ab$ and H $\parallel$$c$ at 3 K. Inset shows the M(H) curve at 300 K. (b) Temperature dependence of the uniaxial magnetic anisotropy $K_{\text{eff}}$. The red line is a straight line joining the data points above 125 K.
• Figure 4: (a) Reciprocal molar susceptibility vs temperature for H = 50 Oe applied parallel to the $ab$ plane. The red line indicates the Curie-Weiss fitting in the temperature range 310$-$390 K. (b) M(T) curve above saturation magnetic field H = 20 kOe fitted by standard Bloch spin-wave model in the low temperature region (5$-$80 K). The inset shows the Bloch fitting in the wider temperature region (5$-$250 K) [solid red line represents the fitting].
• Figure 5: Raman spectroscopy of bulk Fe$_5$GeTe$_2$ crystal. (a) Temperature dependent Raman spectra from 80 to 300 K. The black dash-dotted lines shows the position of the peaks at 300 K. It is clearly visible that the peak position is shifted towards higher energy at low temperatures. (b) and (d) Temperature evolution of Raman modes E$_{2g}$ and $A_{1g}$, respectively. Red solid lines represent the fitting using a model for anharmonic phonons as mentioned in the text and dashed line is extrapolation to lower temperatures. (c) and (e) Temperature dependence of peak-widths (FWHM) of E$_{2g}$ & $A_{1g}$ peaks respectively.