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

TL;DR

This study demonstrates that spin reorientation transitions in a 2D van der Waals ferromagnet can switch electronic topology, affecting charge transport and Berry curvature, thus enabling control over topological electronic states.

Contribution

It reveals how spin reorientation acts as an internal switch for electronic topology in a 2D ferromagnet, linking magnetic anisotropy to topological transport properties.

Findings

Fermi-surface reconstruction across the spin reorientation transition

Sign change in Hall coefficients indicating band topology modification

Modification of Berry-curvature-driven anomalous Hall response below and above the transition

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 $\rho^{A}_{yx}$ further point to a temperature-driven modification of the underlying band topology. Analysis of the anomalous Hall conductivity $\sigma^{A}_{xy}$ and the scaling of $\rho^{A}_{yx}$ 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.