Transport and optical measurements in van der Waals materials Fe3GeTe2 and WTe2
Creators: name: Tan, Cheng
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510404
subject: Electronic and magnetic properties of condensed matter; superconductivity

description: Two dimensional (2D) van der Waals (vdW) materials have attracted great attention since the discovery of graphene. The invention and development of various techniques, such as mechanical exfoliation, heterostructure transfer methods, electron-beam lithography, focused ion beam, etc., have greatly broaden the research of 2D vdW materials. The studies in the field of 2D materials have a great potential for the future application in electronics, optics, optoelectronics and spintronics. Within vdW materials regime, Fe3GeTe2 (FGT) and WTe2 are two interesting vdW materials with unique properties. The electron transport and optical measurements on them will be presented in this dissertation. <br><br>FGT is a vdW ferromagnet with a Curie temperature of over 200 K. Bulk FGT crystal shows tiny remanence and small coercivity when measuring, which indicates its limited prospect for further study in the field of vdW materials and ferromagnetism. However, for a vdW magnet, its domain structure, which determines the magnetic behaviour, is strongly dependent on its thickness. In our recent research, FGT single crystals were exfoliated into thin nanoflakes and showed dramatically different magnetic behaviours when the anomalous Hall effect measurements were carried out on them. Different from its bulk single crystal, nanoflake FGT showed large coercivity and high remanence. Layer-dependent Curie temperatures were also observed, which is originated from the changing interlayer magnetic couplings. Moreover, we also performed angle-dependent Hall measurement and successfully fitted the data with a modified Stoner-Wohlfarth model. The experimental and fitting outcomes well-indicated the strong magnetic anisotropy in FGT, demonstrating its promising role in the study of vdW spintronics.<br><br>After the confirmation of superior magnetic properties in FGT, we tried to design vdW heterostructures based on FGT in order to explore vdW spintronics. We stacked FGT/Graphite/FGT heterostructures and performed transport measurements on them. Amazingly, these heterostructures exhibited antisymmetric magnetoresistance (MR) effects, which are rarely observed before on ferromagnetic metal/nonferromagnetic metal/ferromagnetic metal heterostructures. The MR in these vdW heterostructures features distinct high, intermediate and low resistance states. This unique phenomenon is suggestive of underlying physical mechanisms that differ from the traditional giant magenetoresistance effect which is based on the similar structure. The theoretical calculations attributed the three resistance behaviour to a spin momentum locking induced spin polarized current at the graphite/FGT interface. This work indicates that assembled ferromagnetic vdW heterostructures can exhibit substantially different properties from those exhibited by similar heterostructures grown in vacuum, which highlights the potential for new physics and new spintronic applications to be discovered via vdW heterostructures. <br><br>WTe2 is another focus point of this thesis. Monolayer WTe2 has been confirmed to be a 2D topological insulator. Moreover, WTe2 has also been confirmed to be a type II Weyl semimetal, which has a linear of bands crossing the Fermi level. Weyl semimetals are potential candidates for future electronic and optoelectronic applications. Surface plasmon polariton (SPP) is one of the potential fields for the application of Weyl semimetals. Thus, exploring SPP in WTe2 should be an interesting issue. Hence, by performing focused ion beam (FIB) assisted nanostructure fabrication and microspectrophotometry measurements, we managed to observe SPP within visible wavelengths in the type II Weyl semimetal WTe2. The plasmonic resonance peaks can be evidently observed in the WTe2 nanograting arrays fabricated by FIB. The resonance peaks vary with the width of the nano-gratings in each array. The research on plasmonic in Weyl semimetal will pave the way for plasmonic control and devices in high frequency region.