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This thesis reports magnetotransport measurements in graphene Hall bar devices. Graphene samples fabricated from different techniques (epitaxial growth on silicon carbide, exfoliation, and CVD) are measured and compared. Measurements are taken primarily using a 21T magnet, at liquid Helium 4 temperatures. The first three chapters present the background for the work. Chapter One details the motivation for the thesis, and gives a general background to carbon and the state of carbon research. Chapter Two covers the theoretical background of graphene, including the anomalous quantum Hall effect and weak localization. Chapter Three covers the synthesis of graphene and a typical procedure undertaken for device fabrication. The next three chapters report experimental results. Chapter Four presents measurements of the energy loss rates in exfoliated graphene. The mechanism of carrier energy loss is investigated, and compared to theory. Further, the breakdown of the quantum Hall effect in the device is investigated, demonstrating peak current densities far in excess of those found in the literature for exfoliated graphene. Chapter Five shows measurements comparing the carrier energy loss rates in graphene derived from the epitaxial, exfoliated and CVD fabrication techniques. An unconventional method for measuring the energy loss rate based on measuring the weak localization peak is developed, and trends in the energy loss rates with carrier density are investigated for a wide range of devices. Chapter Six reports a comparison of the decomposed weak localization scattering lengths from graphene devices derived from the epitaxial and CVD methods, and compares these to measurements from the literature. Further, a previously reported saturation of the weak localization in graphene is investigated, and demonstrated to be an experimental artefact. This thesis provides a development of the understanding, and an experimental verification, of several aspects of heat transfer in graphene. An understanding of heat transfer is of critical importance to proposed high-density nano-electronics, and bolometry applications. The high breakdown currents and observed trends in carrier density are also of significant assistance in the design of low-cost resistance metrology devices based on graphene.

AN ABSTRACT OF THE THESIS OFABHIYAN OLI, for the Master of Science degree in Applied Physics, presented on August 10, 2023 at Southern Illinois University Carbondale. TITLE: STUDY OF PHASE TRANSITION AND MAGNNETOCALORIC EFFECT FOR THE SYSTEM Ni-Mn-In-Bi MAJOR PROFESSOR: Dr. Saikat Talapatra We experimentally investigate the Heusler alloys Ni50Mn35In12Bi3 and Ni47Mn35In15Bi3 on their different magnetic properties: structural, magnetic, magnetocaloric and magnetotransport properties by using room-temperature X-ray diffraction (XRD), and magnetization measurements in the temperature interval of 10 -380K and field up to 5T. This alloys shows both high temperature austenite phase (AP) and martensite phase (MP). The alloy Ni47Bi3Mn35In15 crystallize in primitive Cubic structure with space group Fm-3m and Ni50Mn35In12Bi3 with the crystal structure of Tetragonal L21 type with space group I4-3m. Alloy Ni47Bi3Mn35In15 show two phase transition FOPT from Ferrimagnetic/AFM to FM and SOPT from FM to PM towards higher temperature and its result will be discussed here mainly. The martensitic transition (TM) takes place around 200K and Curie temperature (TC) 313K in presence of 100Oe field. The saturation magnetization (Ms) at 10K was found to be increasing at lower field and stabilized at higher field indicating ferromagnetic behavior. The Ni47Bi3Mn35In15 shows high magnetocaloric effects (ΔSM = -47.36 Jkg-1K-1) and Relative Cooling Power (RCP = 222.12 J/Kg) in the vicinity of its Curie temperature (TC =313K). Magnetotransport measurement is done by using a standard four-probe method from 10-380 K temperature in presence of zero field and 50 kOe field.

In the present work, the magnetotransport measurement technique was developed and various materials, exhibiting resistances from 1 mOhm up to several tens of kOhm, were investigated in pulsed magnetic fields of up to 60 T. Phase diagrams of irreversibility and upper critical fields for pure and Zn-doped YBa2Cu3O_7-x high-temperature superconductors were measured. A high-field study of the electronic properties of the two semimetals LaBiPt and CeBiPt were presented. Magnetoresistance of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 thin films were investigated.

The field of spintronics emerged from the search for innovative concepts to comply with the ever increasing need for larger data storage. One major subject of spintronics are devices based on pure spin currents. Such devices usually rely on a combination of two materials: first, a magnetic insulator, to carry the spin information, and second, a spin Hall active metal, to convert from spin to charge information and vice versa and thus effectively act as electrical interface. While previously, such bilayers have been studied in detail in various planar structures, lately, curved and/or three dimensional (3D) geometries have gained more and more attention. Especially for magnetic systems, it has been reported that curvature can lead to the materialization of additional interactions such as curvature-induced anisotropy and the Dzyaloshinskii-Moriya-interaction. The latter is closely related to the magnetic topography of the system, which can be deduced from its essential role in the formation of skyrmions. To systematically study curvature- or topology-based effects, artificially designed systems with a controllable curvature are essential. Consequently, providing an experimental platform, which allows the realization of a variety of different geometries is a key step towards accessing this innovative field of magnetism research. In this thesis, we used atomic layer deposition (ALD) to establish such a platfrom. ALD is a chemical vapor deposition technique which enables the conformal coating of arbitrarily shaped objects while maintaining an excellent thickness control of the layers. In conjunction with 3D (nano)printed resist structures, which can be designed in a multitude of different designs, ALD serves as a powerful platform to fabricate 3D micro- or nanostructures. One of the most popular material combination in spintronics consists of yttrium iron garnet (YIG) and platinum. On the one hand, the ferrimagnetic insulator YIG is the ideal candidate for any kind spin transport experiments, due to its very low magnetic damping. Pt, on the other hand, has a large spin-orbitinteraction, which is necessary for an efficient spin to charge conversion. Therefore, the fabrication of 3D spintronic devices from YIG and Pt is highly desirable. In this thesis the successful fabrication of YIG and Pt by ALD is outlined. We validate the usability of the ALD-Pt layers for 3D spintronics by showing that the ALD-Pt layers are spin Hall active with a electrical quality comparable to other deposition techniques. For the fabrication of ALD-YIG, a nanolaminate approach was used, which is based on the alternate deposition of ultra thin layers of two binary ALD processes: ALD-Fe2O3 and ALD-Y2O3. Therefore, these two processes were optimized beforehand to establish the growth conditions. Furthermore, asdeposited ALD layers are often non-crystalline. To characterize, how this reflects in magnetotransport experiments, we used a model system of non-crystalline sputtered YIG and Pt. By rotating the magnetization in three rotation planes, we find the fingerprint of the spin Hall magnetoresistance. We demonstrate that 3D nanoprinted resist templates can be made fit for the deposition of ALD bilayers even at temperatures where the resist is not stable on its own. To enhance the stability of the resist templates, a layer of low temperature ALD-Al2O3 is used. Finally, the deposition of ALD-YIG is described. A subsequent annealing step is used to promote crystallization of the nanolaminates into YIG. Upon characterizing the structural properties, we find that our process is extremely stable with respect to changes in the stoichiometry within the nanolaminate. From additional measurements of the static and dynamic magnetic properties, we conclude that our ALD-YIG are of good quality comparable to other deposition techniques. By enabling the fabrication of high quality YIG and Pt via ALD, we lay the groundwork for studying the electrical and magnetic properties of systems with curvature and/or a nontrivial topography - effectively advancing the research field of 3D spintronics.

Magnetoresistive effects are powerful tools for studying the intricate structure of solid state electronic systems, and have many applications in our current information technology. In particular, the electronic system reflects the crystal symmetry and the orbital structure of the atoms of a given solid, and thus is crucial to understanding magnetism, superconductivity and many other effects which are of key interest to current solid state research. Consequently, studies of the electrical transport properties of solid state matter allow to evaluate this imprint and in turn draw conclusions about the interactions within a material. In this thesis, we will exploit the capabilities of magnetotransport measurements to infer the properties of a multitude of magnetic systems. In turn, this allows us to push the understanding of transport phenomena in magnetic materials. The first part of this work is focused on the magnetoresistance observed in spin Hall active metals in contact with a magnetic insulator. In such bilayers, the interfacial spin accumulation caused by the spin Hall effect in the metal can interact with the magnetic insulator, giving rise to interesting magnetoresistive effects. In the framework of this thesis, bilayers with several magnetic insulators are studied, including antiferomagnets, ferrimagnets and paramagnets (disordered magnets). For the disordered magnetic insulators, we find that the established spin Hall magnetoresistance framework does not allow to consistently describe the observed transport response. Consequently, we propose an alternative explanation of the magnetoresistance in such heterostructures, using the Hanle magnetoresistance and assuming an interface which has a finite electrical conductivity. This alternative model can serve to generalize the theory of the spin Hall magnetoresistance, providing addition information on the microscopic picture for the loss of the transverse spin component. Additionally, by partly removing the magnetic insulator and studying the ensuing changes, we verify that magnons are crucial for the observation of a non-local magnetoresistance in bilayers of a magnetic insulator and a metal. Finally, the local and non-local spin Seebeck effect (i.e. the electric field generated by a thermally driven pure spin current) is investigated in bilayers of Cr2O3 and Pt where the occurrence of a spin superfluid ground state was reported. In our sample, however, the transport response is consistent with the antiferromagnetic spin Seebeck effect mediated by the small magnetic field induced magnetization also reported for other antiferromagnet/metal heterostructures. As such, we cannot verify the presence of a spin superfluid ground state in the system. In the second part of this thesis, the topological properties of the electronic system and the related changes of the magnetoelectric and magnetothermal transport response are investigated. To that end, we first demonstrate a novel measurement technique, the alternating thermal gradient technique, allowing to separate the relevant thermovoltages from spurious other voltages generated within the measurement setup. We employ this novel technique for measuring the topological Nernst effect in Mn 1.8 PtSn and show the possibility to combine the magnetoelectric and magnetothermal transport response to evaluate the presence of topological transport signatures without requiring magnetization measurements. Additionally, we show that the anomalous Nernst effect in the non-collinear antiferromagnet Mn3Sn is connected to the antiferromagnetic domain structure: Using spatially resolved measurements of the anomalous Nernst effect, direct access to the antiferromagnetic domain structure is demonstrated. Additionally, a thermally assisted domain writing scheme is implemented, allowing the preparation of Mn3Sn into a defined antiferromagnetic domain state.

In this thesis the properties of graphene and its few-layers are engineered to make them highly conductive. Two different approaches were implemented to achieve this goal. One approach was to increase the concentration of charge carriers by intercalation of acceptor FeCl3 molecules between graphene planes. This resulted in a highly conductive yet transparent material which can be useful for applications. Another approach was to increase the mobility of carriers by means of removing surface contamination in the current annealing process. Optimal annealing parameters were found and a reproducible cleaning method was suggested.