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Fe₃GeTe₂ and Fe₅GeTe₂ are layered and exfoliable compounds that display what seemingly are ferromagnetic (FM) transitions below T_c ~ 220 and 280 K, respectively. At low fields, both compounds display an anomalous Hall effect (AHE) that scales with the magnetization. For FM materials, this AHE has been described using Berry phase concepts or argued to be topological in nature [1, 2], with a specific scenario [3] proposed for Fe₃GeTe₂. However, measurements of the magnetic torque reveal a second metamagnetic transition observed in both compounds in fields exceeding 10 T. These indicate that neither compound can be considered as a simple collinear ferromagnet [4], in agreement with Ref. [5], and hence that their AHE is not intrinsic. For both compounds, the AHE remains oblivious with respect to the occurrence of these metamagnetic transitions, indicating that an unconventional scenario is required to explain it.

[1] Nagaosa, N. et al., Rev. Mod. Phys. 82, 1539 (2010).
[2] Onoda, M. and Nagaosa, N. J. Phys. Soc. Jpn. 71, 1 (2002).
[3] Kim, K. et al. Nat. Mater. 17, 794 (2018).
[4] May, A. F., Calder, S., Cantoni, C., Cao, H. and McGuire, M. A. Phys. Rev. B 93, 014411 (2016).
[5] Jieyu, Y. et al., 2D Mater. 4, 011005 (2017).