Probing antiferromagnetism in exfoliated FeGeTe using magneto-transport measurements.

Among the material class of van der Waals magnets, FeGeTe (FGT) has emerged as one of the most studied owing to features such as its relatively high Curie temperature, metallic nature, and large spin polarization. Though most studies only investigate its explicitly ferromagnetic properties, FGT is also predicted to have an antiferromagnetic phase in the out-of-plane direction emerging at temperatures below 150 K, leading to a blend of ferromagnetic and antiferromagnetic ordering. Here, we explore the emergence of this phase and its effects in FGT/h-BN heterostructures using magneto-transport measurements. The devices' anomalous Hall and magnetoresistance responses exhibit a complex trend with temperature that is consistent with multiple magnetic phases. In addition to the usual out-of-plane sensing, we also rotate the applied field to the in-plane direction and observe behavior resembling the planar topological Hall effect. Intriguingly, this response follows a similar temperature trend to the out-of-plane response. We also use the out-of-plane anomalous Hall response to show that, at sufficiently low temperatures, both positive and negative field-cooling results in an increased saturation Hall resistance. Such a field-cooling divergence is consistent with antiferromagnetic ordering resulting in a spin-glass like state in the sample. In addition to providing insight into one of the most exciting candidate materials for 2D magnetic devices, our work demonstrates the power of magneto-transport measurements to probe complex behavior in vdW magnets where common magnetometry techniques used on bulk samples may not be viable.