Unusual magnetotransport and anomalous Hall effect in quasi-two-dimensional van der Waals ferromagnet Fe_4GeTe_2

Fe_4GeTe_2, an itinerant vdW ferromagnet (FM) having Curie temperature (T_C) close to room temperature (∼ 270 K), exhibits another transition (T_SR ∼ 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 ∼ 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 ∼ 123 Ω^-1 cm^-1 (at T ≈ 5 K) to the maximum value of ∼ 366 Ω^-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_4GeTe_2.