Spin-reorientation as a switch for electronic topology in van der Waals ferromagnets

Abstract

The interplay between spin reorientation and topological electronic structure in two-dimensional (2D) van der Waals (vdW) ferromagnets is central to understanding how magnetic anisotropy shapes charge transport. Although spin-reorientation transitions (SRTs) are common in 2D metallic ferromagnets, their impact on electronic-topology-driven thermodynamic and transport properties remains largely unexplored. Here we investigate this issue in Fe$_4$GeTe$_2$ (F4GT), a room-temperature quasi-2D vdW ferromagnet, using temperature-dependent magnetization, specific heat, magnetotransport, and thermoelectric measurements. Magnetization and specific heat establish a reorientation of the magnetic easy axis near $T_{\mathrm{SRT}} \sim 100$~K, in addition to ferromagnetic ordering at $T_C \sim 270$~K. Across the SRT, the Seebeck coefficient and anisotropic magnetoresistance show clear anomalies, indicating Fermi-surface reconstruction. The magnetoresistance exhibits a two-step field dependence: a low-field enhancement near the SRT associated with scattering from canted spins and evolving domains, followed by a higher-field negative response as spin fluctuations are suppressed. The simultaneous sign change of the ordinary Hall coefficient $R_0$ and the sharp anomaly in the anomalous Hall resistivity $ρ^{A}_{yx}$ further point to a temperature-driven modification of the underlying band topology. Analysis of the anomalous Hall conductivity $σ^{A}_{xy}$ and the scaling of $ρ^{A}_{yx}$ shows that the Berry-curvature-driven anomalous Hall response below $T_{\mathrm{SRT}}$ is strongly modified above the transition. Our results identify spin reorientation as an internal control parameter for switching between distinct topological transport regimes in a 2D vdW ferromagnet, providing a symmetry-controlled route to engineer spin-polarized electronic states and domain-texture-driven functionalities.

Figures (5)

• Figure 1: Crystal structure and temperature-dependent thermodynamic properties. (a) Crystal structure of F4GT. (b) Temperature-dependent dc magnetization (M) in the zero-field-cooled mode for applied magnetic field H = 500 Oe in both directions. (c) The temperature dependence of longitudinal resistivity ($\rho_{xx}$) of F4GT single crystal at 0 T and 5 T with applied magnetic field parallel and perpendicular to $ab$-plane (current directions, I $\parallel$$ab$). The inset shows the first-order derivative of $\rho_{xx}$ as a function of temperature. Arrows indicate the FM ordering temperature and the spin reorientation transition (SRT) temperature. (d) The temperature dependence of specific heat C$_p$. The inset shows the magnetic contribution to the heat capacity. (e) Measured magnetic anisotropy K$_{eff}$, calculated shape anisotropy K$_{sh}$, and resulting estimated uniaxial magnetocrystalline anisotropy K$_{u}$ as a function of temperature. (f) Calculated anisotropy field as a function of temperature.
• Figure 2: Thermoelectric response of Fe$_4$GeTe$_2$. (a) Temperature dependence of the Seebeck coefficient, $S(T)$, of Fe$_4$GeTe$_2$ single crystal. (b) Temperature derivative $dS/dT$, highlighting pronounced anomalies near the spin-reorientation transition and the ferromagnetic ordering temperature, indicative of a modification of the Fermi surface that manifested in transport response.
• Figure 3: Magnetoresistance (MR) measurements. MR of Fe$_4$GeTe$_2$ single crystal for the applied magnetic field (a) parallel to $ab$-plane and (c) along the $c$-axis, ranging from -6 to +6 T at a few representative temperatures in the range of 20-200 K. (b,d) Enlarged views of (a,c) around the low field. The magnetoresistance value is positive along H$\parallel$ab around $SRT$, but for the H$\parallel$c, the negative $MR$ value is higher than at other temperatures. (e) Resistivity as a function of H at various angles $\theta$ at 100 K. Angle-dependent MR (e) and enlarged views (f) around the low field. The MR value changes from positive to negative with increasing angle (a schematic diagram of sample orientation is shown in the inset).
• Figure 4: Anisotropic magnetoresistance (AMR) and inferred domain dynamics. (a) The MR for the applied magnetic field along $ab$ the plane (blue) and $c$-axis (red) at 20 K. (b) Evolution of $MR_x$ with temperature for the two different magnetization directions. (c) The volume fraction of in-plain domains $x$ is shown on the left axis. The right axis shows the $AMR$ ratio as a function of temperature. (d-e) The contour plot of $\frac{d(MR)}{d(\mu_0H)}$ for H$\parallel ab$ and H$\parallel c$ as a function of $H$ and $T$. (f) Observed $T$ dependence of the field $H_{max}$ (where the field derivative $\frac{d(MR)}{d(\mu_0H)}$ shows a maximum).
• Figure 5: Hall resistivity and the anomalous Hall effect. (a) Magnetic field-dependence of Hall resistivity ($\rho_{yx}$) at different temperatures. (b) Temperature dependence of the ordinary Hall coefficient (R$_0$). (c) Temperature-dependent carrier density $n$. (d) Anomalous Hall resistivity ($\rho^{A}_{yx}$) as a function of temperature. (e) Plot of ln $\rho^{A}_{yx}$ vs ln $\rho_{xx}$; the solid red line is the fit using the relation $\rho^{A}_{yx}$$\propto$$\rho^{\alpha}_{xx}$. (f) Temperature dependence of anomalous Hall conductivity ($\sigma^{A}_{xy}$).