Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis The Growth and Characterization of Fe/Tao Multilayers for Spintronics Applications(Izmir Institute of Technology, 2008) Tokuç, Hüseyin; Tarı, Süleyman; Tarı, Süleyman; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn this thesis, the effect of Ta buffer layer and the thickness of the Ta2O5 barrier layer on the structural and magnetic properties of Fe/Ta2O5/Co multilayers have been studied. XRD and AFM techniques were used for structural investigations and VSM was used for investigation of magnetic properties. Refractive index of the barrier layer was determined by ellipsometry technique. In this study, magnetic tunnel junctions have also been fabricated by using photolithography technique and then electrical and magnetoresistance measurements were done.The structural investigations showed that Ta under layer increases the crystalline quality of Fe layer and causes a change on magnetic parameters of Fe films. The AFM results showed that the range of the roughness for all layers is between 1.7 A and 6.3 A. When the thickness of the oxide layer was 4 nm, magnetic decoupling appears. Clear differences between the coercive fields of the ferromagnetic layers were observed in further increase of the barrier layer thickness. The effect of annealing on the Fe/TaOx/Co multilayer was studied and it was found that only the coercivity of Fe film increases with increasing temperature up to the 250C. Then, annealing at 400C showed a sharp decrease in the coercivity of Fe film indicating an intermixing at the interface of Fe/TaOx. Co minor loops showed that the magnetostatic coupling is large for thin barriers and decreases with increasing the barrier thickness. Electrical measurements showed that conduction occurs via tunneling electrons. However, no TMR ratio has been observed after magnetoresistance measurements.Master Thesis Structural and Magnetic Properties Os Si(100)/Ta Multilayers for Spintronics Applications(Izmir Institute of Technology, 2007) Vahaplar, Kadir; Tarı, Süleyman; Tarı, Süleyman; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyThis thesis is concerned with the structural and magnetic properties of Si(100)/Ta/Co single and multilayer thin films grown by DC magnetron sputtering technique. The structural properties of the films have been studied by X-Ray Diffractometer (XRD), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). This study revealed that a single Co film grows amorphous on silicon substrate up to 50 nm at room temperature. After this thickness, Co starts crystallizing in hexagonal (002) plane. The same crystallinity was also observed for 25 nm amorphous Co which was annealed at 4500C at high vacuum for 30 minutes. The presence of a single crystalline tetragonal Ta phase (-Ta) with the orientation along (002) has been observed for 40 nm Ta growth on silicon substrate. The Si(100)/Ta/Co bilayers and multilayers show good crystallinity for both Ta and Co films. SEM and AFM results show that all the single and multilayers grew uniform, continuous and with very low surface roughness. The magnetic properties of the films were investigated using Vibrating Sample Magnetometer (VSM), by measuring hysteresis loops. The effects of the thickness and growth pressure on the magnetic properties of Co films were studied. The easy magnetization axis of the samples is found to be parallel to the Co film plane. As the Co film thickness increased from 4 nm to 15 nm, the coercivity (Hc) decreased from 72 G to 20 G and after a threshold thickness it increased almost linearly up to 180 G for 100 nm film while the magnetization decreased. Moreover, it has been observed that as the Co growth pressure increases, the Hc value of Co films increases. Finally, we obtained two different Hc values for our MTJ sandwich with the structure of Si(100)/Ta/Co/TaOx/Co/Ta.Master Thesis Spin Polarized Tunneling in Large Area Mesas of Superconducting Bi2sr2cacu2o8+d for Terahertz Emission(Izmir Institute of Technology, 2010) Türkoğlu, Fulya; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyThere is an increasing interest in science and technology of electromagnetic waves in terahertz frequency range (0.1-10 THz) because of their emerging application areas including physics, biology, chemistry, astronomy, medicine etc. The observation on generation of THz radiation emitted from lateral dimension of high temperature superconductor (HTS) Bi2Sr2CaCu2O8+.. (Bi2212) and responses to THz waves increase the importance of these HTSs. Single crystal of HTS Bi2212 forms natural superconductor-insulator-superconductor (SIS) layered junctions, which are called intrinsic Josephson junctions (IJJ). The stacks of IJJs in Bi2212 can be used such a voltage-frequency converter and their large energy gap allows the emissions at THz frequency range. Recently, it has been demonstrated that rectangular IJJ mesa structures of Bi2212 can be used as a source of continuous, coherent and polarized THz radiation. It was shown that all THz emitting mesas are below a certain underdoped level, which has relatively small critical current in contrast to optimally doped and overdoped Bi2212. In this work, rectangular Au/Co/Au/Bi2212 mesa structures with large areas and high thicknesses were fabricated on as-grown Bi2212 single crystals using standard photolithograph and Ar ion beam etching techniques. In order to characterize the mesas, c-axis resistance versus temperature (R-T) and current-voltage (I-V) characteristics were investigated. During I-V characterization, Si composite bolometer was used to detect the emission. We obtained small critical current from as-grown mesas due to injection of spin polarized current. We observed THz emission peak for one of the mesas which has low quasiparticle conductivity and low dissipation due to its small critical current density. It means that the adjustment of doping level can be eliminated for THz emission by the injection of spin polarized current through the c-axes of the asgrown mesas.Master Thesis Calculations of Electric and Magnetic Properties of Triangular Graphene Fragments Using Density Functional Theory: Effects of Edge Functionalization and Electric Field(Izmir Institute of Technology, 2013) İyikanat, Fadıl; Senger, Ramazan Tuğrul; Senger, Ramazan Tuğrul; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyThe triangular graphene flakes (N-TGFs) we consider have equilateral triangular shapes with zigzag edges, where N denotes the number of edge hexagonal cells in one side of the triangle. Termination of these N-TGF structures with several elements (of the first two rows of the periodic table) and application of electric field to these flakes alter their electronic and magnetic properties. In accordance with previous studies [1, 2], it is found that bare flakes have large spin magnetic moment values of 4(N − 1) μB, whereas they reduce to (N − 1) μB for full saturation of edges with Hydrogen, Lithium, Beryllium or Flour atoms. Moreover we have studied possible termination of TGF with other elements like Boron, Carbon and Nitrogen. Hydrogen and Flour atoms prefer to bind at the top of an edge Carbon atom. Unlike Hydrogen and Flour, the other atoms prefer to bind at the bridge sites. Recent studies [3, 4] have shown that themagneticmoments of triangular graphene flakes can be controlled by applied electric field. We show that the value of total spin polarization of triangular graphene flakes can be changed by tuning an applied in-plane external field. We demonstrate that, in these flakes total spin polarization can be reduced stepwise with the applied field. The electric field control of ferromagnetism in TGFs promises a new route for spintronic applications.Master Thesis Electronic, Spintronic and Transport Properties of Carbon Based Nanowires(Izmir Institute of Technology, 2011) Arı, Ozan; Arı, Ozan; Senger, Ramazan Tuğrul; Senger, Ramazan Tuğrul; 01. Izmir Institute of Technology; 04.05. Department of Pyhsics; 04. Faculty of ScienceIn this thesis, properties of carbon based nanowires are studied by ab-initio calculations. The aim is to gain a thorough understanding of the electronic, spintronic, transport properties in nanowires and how they are affected by different geometric formations, defects and adatom adsorptions. To this end the non-equilibrium Green's function formalism with first principles pseudopotential density functional theory calculations have been used to describe spin-polarized systems. Firstly, different geometric formations of Cobalt-Benzene nanowires are investigated. Systems with ferromagnetic ordering are calculated as half-metallic while systems with antiferromagnetic ordering behave as metallic. Also the results of the spin polarized current calculations indicate that one of the spin components of current is dominant for the antiferromagnetic systems while both spin components of current are dominant in different bias windows of a specific total applied bias. As second case, alkali atom termination of the zigzag graphene nanoribbons (ZGNR) are studied. In particular, using sodium atoms for the saturation of ZGNR edges at half the concentration of edge-carbon atoms make it a one dimensional, perfect semimetal, where the valance and conduction bands meet at only a single, Dirac-like point. Unlike pristine graphene, the Dirac-"cones" of Na-ZGNR is not symmetric with respect to wave vector, but rather it is tilted. Finally, adsorption up to the graphenic sheets with periodic 5-8 defects is studied. Especially, electronic structure of the V adsorption into 5-8 defects induced graphenic sheets are calculated as half-metallic while formation of linear bands crossing at the Fermi level which form a tilted Dirac cone.