Transport study on two-dimensional ferromagnetism and heterostructure devices
Creators: name: Algarni, Meri Ahmed O
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510404
subject: Electronic and magnetic properties of condensed matter; superconductivity

description: Since the separation and optimisation of graphene in 2004, many van der Waals (vdW) ferromagnetic (FM) materials have gained considerable attention and experienced substantial development. Two-dimensional (2D) ferromagnetism has been confirmed in several monolayer vdW materials, such as CrI3, Cr2Ge2Te6, Fe3GeTe2 and Fe5GeTe2. In the presence of weak coupling between 2D vdW layers, vdW heterostructures can assemble different lattice mismatched.  By stacking, artificial multilayers can be constructed that cannot exist in nature or be synthesised using conventional chemical processes based on high temperature. The result of these artificial heterostructures has been various exciting physical phenomena that have been observed and predicted in the application of electronics and spintronics. The main goal of this thesis is to study the fundamental properties of vdW FM materials, as well as to investigate the electronic transport properties of heterostructure devices and explore their potential applications for the next generation of electronic devices.
 
As described in Chapter 4, Mn1/3TaS2 single crystals were mechanically exfoliated with different thicknesses from the bulk crystals. They exhibited dramatically different magnetic behaviours when anomalous Hall effect measurements were taken. A large topological Hall effect and anomalous Hall effect were tuned, which modulated the carrier density of the system using a protonic gate at low temperatures. By controlling the carrier density using a protonic gate, the maximum topological Hall effect of 97% was reached, which is almost double the ungated value. In addition, anomalous Hall effect amplitudes were increased to up to 216% of the ungated value.  Theoretical analysis indicates that a large topological Hall effect originates from the chiral spin textures due to the sizeable Dzyaloshinskii–Moriya interaction. This interaction arises from broken spatial inversion symmetry and strong spin–orbit coupling, giving rise to a non-trivial topological Hall effect. Hence, our findings establish that the magnetic and transport properties in chiral magnetic Mn1/3TaS2 can be controlled by both carrier density and thickness. These methods of control may enable increased opportunities for investigating and manipulating topological spin textures.

Next, Fe3GeTe2 /graphite/Fe0.26TaS2 devices were fabricated using a pick-up transfer technique and transport measurements were conducted (see Chapter 5). In this work, we investigated the multichannel transport properties in two different kinds of vdW heterointerfaces. Our magnetic heterointerfaces consist of FM layers with two opposite anomalous Hall effects. A multichannel transport simulated both positive and negative topological Hall effects without involving any topological charges, which is termed an artificial topological Hall effect. In our work, artificial topological Hall effects were also observed at FM-metal heterointerfaces, even without the presence of two opposite anomalous Hall effects; this was attributed to multichannel transport. Our results contribute to an understanding of the multichannel transport properties in artificial heterointerfaces and provide an alternative explanation for topological Hall effects at heterointerfaces.

Finally, as explained in Chapter 6, vdW magnet Cr1.2Te2 devices were fabricated and subjected to measurements. A protonic gate technique was used to tune the magnetism of Cr1.2Te2 nanoflakes at room temperature with a large electron doping concentration of up to 3.8 × 1021 cm-3. In a protonic gated Cr1.2Te2 device, the magnetic phase transition from FM to antiferromagnetic was successfully realised at room temperature. In these experiments, square-shaped anomalous Hall effect loops were observed over 50 K, indicating a perpendicular magnetic anisotropy. Under different gate voltages, the anomalous Hall effect resistivity and coercivity were successfully tuned. This effective control of the magnetism by a gate voltage represents a significant step in broadening the range of applications for spintronics.