Investigation of magnetic properties of topological insulators and topological superconductors with integrated probe microscope (SPM) techniques

Nanotechnology is an interdisciplinary science proposed by Feynman in 1959 to research, model, and functionalize the atomic and molecular level phenomena. The nanoscale characterizations are common on the investigation of new generation materials' properties. The bulk magnetic properties and the surface magnetic properties of materials show different characteristics in nanoscale systems. Thus, the mechanisms of magnetism are being investigated extensively in nano-scale. Among the Scanning Probe Microscopy (SPM) techniques; Magnetic Force Microscopy (MFM) and Scanning Hall Probe Microscopy (SHPM) techniques are widely used in nanomagnetism research. The Scanning Tunneling Microscope (STM) and the Atomic Force Microscope (AFM) were invented in 1982 and 1986, respectively. These techniques, known as Scanning Probe Microscopes (SPM), are characterization techniques frequently used in surface science and nanotechnology research. MFM which is a scanning mode of AFM, is a qualitative technique that is widely used to investigate the magnetic properties of materials. In the MFM technique, the magnetic phase map of the surface is generated using a magnetic sensor. The advantage of the MFM technique is that it can achieve at a sensitivity of 10 nm, depending on the size of the sensor tip. Another type of microscope used in magnetism research is the SHPM. The magnetic field on the surface can be quantitatively measured by the Hall device employed as sensor. The major advantage of the SHPM technique is that it is non-invasive. These microscopic techniques can be integrated with low temperature systems operating in the range between 300K-1K environment for nanomagnetism researches such as Low Temperature-Magnetic Force Microscope (LT-MFM) and Low Temperature-Scanning Hall Probe Microscope (LT-SHPM). Superconductors are conductive materials whose electrical resistance is completely zero below a certain critical temperature and external magnetic field. Due to its zero resistance, they are a potential material to remove energy losses from electrical resistance. Until now, there has been no superconducting material used in ambient temperature and pressure yet, but there are superconductors and application examples when the temperature is low enough. In the superconducting phase; superconductors characteristically expel the magnetic field, this phenomenon is called the Meissner effect. Type II superconductors have inherent characteristics in which some of the magnetic field is expelled and some of the magnetic field locally penetrates, when they are cooled below the critical temperature value under a magnetic field. At that points where the magnetic field penetrates into the superconductor, the magnetic flux forms vortices and passes through the opposite side. These magnetic vortices formed in Type II superconductors could be investigated by LT-MFM and LT-SHPM techniques. Topological Insulators are technological quantum materials, which are mass insulators with a conductive boundary on the surface of a single atomic layer. In this top conductive atomic layer only the electrons with the same spin could be positioned, because the electrons on topological insulators can not backscatter inherently. With these special characteristic topological insulators are natural material candidates for spin polarization research. Any electron has a characteristic magnetic field based on the principle of physical Hall Effect due to minus charge and spin motion. This magnetic field, which the electron possesses, is called the spin hall effect and can be measured. When electric current is applied from the topological insulator materials, a certain magnetic field is formed due to the polarization of the spins on the surface, and this magnetic field can be measured by magnetic measurement techniques. In this thesis, the magnetic vortex structures in Meissner state in the FeSe superconductor were investigated by the LT-MFM technique and the magnetic vortex structures in Meissner state in the FeSeTe superconductor were investigated by the LT-SHPM technique. On topological insulator Bi2Se3 the Spin-Hall effect was experimentally investigated with LT-MFM at 300K and 150K. In the first additional study to the main work, 10nm Co/Pd ferromagnetic nanoislands fabricated by low energy proton irradiation technique with the selective phase transformation method were characterized with MFM technique. In the second additional study, the use of dielectric substrates and graphene growth mechanisms were investigated in the study of growth of nano-graphene on SrTiO3(111) substrate with chemical vapor deposition technique. In the scope of the thesis, the magnetic vortices of FeSe superconductor in Meissner state were investigated by LT-MFM technique. As a result of the experiments the vortices are positioned randomly compatible with the theoretical studies for unconventional superconductors. Vortices have been observed as special lines that called vortex chain structure. The chain structure could form on the twin planes in the FeSe crystal structure. Vortices could position selectively on twin planes because of twin plane's different energy levels. Additionally, individual vortices have been observed except the chain structure. In addition to the findings of the experiments, phase separation zones were detected at room temperature, and partial regions with different magnetic phase structures were observed by LT-MFM technique. From all the findings it could be said that FeSe structure has the basic properties of the iron chalcogenide superconductor family. The vortex structure of FeSeTe superconductors in Meissner state has been investigated with LT-SHPM technique. From the experiments vortex-antivortex structure were observed. On the vortex-antivortex structure +1.5G and -1.5G equivalent magnetic fields symmetrically located at opposite directions were measured with LT-SHPM sensor. In other experiments on FeSeTe, the regular magnetic vortex structure in the Meissner state was examined. It is thought that the addition of Te to the FeSe structure could create crystal defects and that the vortex structure could become more regular than the formation of the vortex chain in FeSe. The Spin-Hall effect in Bi2Se3 topological insulator was experimentally observed with LT-MFM for the first time with this technique. The Bi2Se3 sample was patterned as bar structures with Focused Ion Beam (FIB). Through each arm of the bar structures alternately +10mA and -10mA current was passed respectively during scanning with MFM, while the magnetic sensor was tuned at the resonance frequency and brought closer to the surface. On the sample surface, a certain magnetic field is formed due to electrons' Spin-Hall effect. Due to this magnetic field, the resonance frequency of the MFM sensor is shifting. When the direction of current is reversed, electrons with opposite spins accumulate on the surface, so that the magnetic field caused by the Spin-Hall effect also reverses. Since the direction of the magnetic field affecting the MFM sensor changes, the resonance frequency shifts in the opposite direction. The magnetic fields that spin-up and spin-down electrons generate are attractive or repulsive. This causes an attractive or repulsive effect on the MFM sensor. Thus, indirectly, Spin-Hall effect on Bi2Se3 material was observed for the first time in the literature with this technique at 300K and 150K with MFM technique. In the first additional study in the scope of the thesis collaborated with Y. Hyunsoo's group. In the study 10nm Co/Pd ferromagnetic nanoislands patterned by selective phase transformation method with low energy proton irradiation technique were characterized with high sensitive MFM technique. According to the results obtained, 10nm sized magnetic storage devices have been produced. In the technique of selective lithography with the proton irradiation technique, a magnetic data storage device could be produced without damaging the base material with shortening the lithography steps. The second additional study within the scope of the thesis carried out jointly with S. Karamat. In the study, the benefits of the use of dielectric substrates in the production of nano-graphene by chemical vapor deposition on SrTiO3(111) substrate were discussed. According to the results obtained, it is possible to selectively produce the graphene plates with smooth edge structures by forming active nucleation zones on the growth of the SrTiO3 dielectric substrate. The graphene structures that were produced in the study were characterized by AFM in the thesis and worked on growth mechanisms. The results obtained at the end of the thesis could help to develop theories and understand the properties of topological insulators and topological superconductors. The Spin Hall effect in the Bi2Se3 topological insulator observed at 300K and 150K for the first time in the literature by LT-MFM technique. Since it is the first time that the MFM method is used in the characterization of the spin-hall effect, it is thought that spintronic researchers may use this technique in different applications in the future. Surface magnetic properties and vortex structures of FeSe and FeSeTe superconductors have not been investigated previously with LT-MFM and LT-SHPM techniques. The investigation of the vortices of FeSe and FeSeTe superconductors may help to understand the characteristics of the iron superconductor family. Studies carried out within the scope of this thesis will undoubtedly help to understand the complex structures of topological insulators and topological superconductors.