Layer dependance of Kondo temperature and linear magnetoresistance of 2D Ferromagnets: Fe₅Ge₂Te₂ and Fe₃GeTe₂

Two-dimensional (2D) metallic ferromagnets including Fe₅Ge₂Te₂ (FG2T) and Fe₃GeTe₂ (FGT) exhibit a variety of fascinating physical properties. In particular, recent experimental studies on FGT have shown evidence of strong electron correlation indicative of f-electron-like characteristics. In this work, we focus on the low-temperature transport and magneto-transport behaviors in both FGT and FG2T nano-devices over a range of thicknesses. We fabricate FG2T and FGT nanodevices by exfoliating flakes from crystals grown by solid-state reaction down to the monolayer thickness. Most FGT and FG2T devices are protected by a thin layer of sputtered Al2O3. For some thin FGT devices, the FGT flake is picked up by BN and transferred onto pre-patterned electrodes inside glovebox to avoid further lithography and air exposure. In devices thicker than 3 layers, we find that the resistivity shows a clear minimum, followed by an upturn which leads to a saturation at lower temperatures. This overall low-temperature behavior can be very well fitted by the Kondo effect with a nearly constant Kondo temperature value of 34 K, independent of the number of layers in both FG2T and FGT. In these devices, the magnetoresistance is found to be linear up to 14 T. The resistivity minimum in thin devices (3 layers and below) is shifted to much higher temperatures due to an apparent insulating temperature-dependent background. Discussions of the low-temperature transport and magneto-transport data will be presented.