Magnetic two-dimensional (2D) materials, proposed to exhibit non-trivial electronic topology, have been discovered recently. In particular, a potential for devices based on van der Waals (vdW) ferromagnetic heterostructures was demonstrated for Fe3GeTe2, which displays the same crystallographic structure as Mn3Sn. The opportunity exists to design and fabricate many devices based on vdW materials, where they can be employed in unique spin–orbit torque configurations when stacked with vdW metals that exhibit strong spin–orbit interactions [1, 2]. However, in order to exploit ferromagnetic vdW materials as building blocks for heterostructure-based spintronics, we are tasked to understand the nature of 2D magnetism for compounds of interest. Among all the predicted and experimentally observed vdW ferromagnetic materials, the most promising candidates are part of the Fe-Ge-Te (FGT) family: Fe3GeTe2 (FGT 312) and Fe5GeTe2 (FGT 512). This is due to their relatively high Curie temperatures: near 220 K [3] and 280 K [4] in their bulk state, respectively. At low fields, both compounds display an anomalous Hall effect (AHE) that scales with the magnetization. For ferromagnetic (FM) materials, this AHE has been described using Berry phase concepts depending on the band geometry (or band topology) of the material [5, 6], with a specific scenario [7] proposed for FGT 312. However, measurements of the magnetic torque reveal a second metamagnetic transition observed in both compounds in fields exceeding 10 Tesla. These indicate that neither FGT compound can be considered as a simple collinear ferromagnet [8], in agreement with Ref. [9], and hence suggests that the origins of their AHE may not be trivial. 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 this anomalous Hall response. In this study, we have synthesized crystals of Fe3-xGeTe2 and Fe5-xGeTe2 using Chemical Vapor Transport and Flux methods. For nominal FGT 312, we report the AHE, magnetic torque, planar Hall effect, Righi-Leduc (thermal Hall) effect, heat capacity, and anomalous Nernst data. For nominal FGT 512, we have collected AHE, magnetic torque and heat capacity data. We then detail the fundamental principles of operation and procedures including the methodology in characterization, synthesis and growth.
