Unusual magnetotransport and anomalous Hall effect in quasi-two-dimensional van der Waals ferromagnet Fe$_4$GeTe$_2$

Abstract: Fe$4$GeTe$_2$, an itinerant vdW ferromagnet (FM) having Curie temperature (T$_C$) close to room temperature ($\sim 270$ K), exhibits another transition (T${SR}$ $\sim$ 120 K) where the easy axis of magnetization changes from in-plane to the out-of-plane direction in addition to T$C$. Here, we have studied the magnetotransport in a multilayer Hall bar device fabricated on 300 nm Si/SiO$_2$ substrate. Interestingly, the zero field resistivity shows a negligible change in resistivity near T$_C$ unlike the typical metallic FM, whereas, it exhibits a dramatic fall below T${SR}$. Also, the resistivity shows a weak anomaly at T $ \sim $ 38 K (T$Q$), below which the resistivity shows a quadratic temperature dependence according to the Fermi liquid behavior. Temperature-dependent Hall data exhibits important consequences. The ordinary Hall coefficient changes sign near T${SR}$ indicating the change in majority carriers. In a similar manner, the magnetoresistance (MR) data shows significantly large negative MR near T${SR}$ and becomes positive below T$_Q$. The observations of anomaly in the resistivity, sign-change of the ordinary Hall coefficient and maximum negative MR near T${SR}$, together suggest a possible Fermi surface reconstruction associated with the spin reorientation transition. Furthermore, analysis of the Hall data reveals a significant anomalous Hall conductivity (AHC) from $\sim 123 \Omega^{-1}$ cm$^{-1}$ (at T $\approx$ 5 K) to the maximum value of $\sim 366 \Omega^{-1}$ cm$^{-1}$ near T$_{SR}$. While the low-temperature part may originate due to the intrinsic KL mechanism, our analysis indicates that the temperature-dependent AHC is primarily appearing due to the side-jump mechanism as a result of the spin-flip electron-magnon scattering. Our study demonstrates an interplay between magnetism and band topology and its consequence on electron transport in Fe$_4$GeTe$_2$.