No description available.

Complexity, a fundamental concept in physics, encompasses phenomena spanning atomic to cosmic scales. The natural emergence of complexity can be explained by self-organized criticality. In this work, two complexity measures in magnetic systems are explored. The multiscale structural complexity (MSC) and spin temperature both capture complexity but are fundamentally different in nature and hence behave differently when subject to various temperature profiles. The MSC is extended to incorporate time correlations and compared to the time-averaged static MSC for examining spin glasses and bcc Fe at different temperatures. The spin glass transition temperature is determined with an accuracy of 1 K using the time-extended MSC, outperforming similar estimates based on the heat capacity in terms of accuracy, computational cost, and efficiency. Future work includes the optimization of coarse-graining scales in spin glasses, the investigation of transient magnetization dynamics, and the influence and loss of information of averaging magnetic unit cells before computing complexities.

Graphene, the first experimentally isolated atomically thin crystal has displayed numerous superlative properties for quantum and spin-based electronics, as evidenced by research results of more than a decade. The scalable form of graphene, produced by the chemical vapor deposition (CVD) method has been increasingly attracting scientific and technological interest, as outstanding properties are combined with large scalability and high quality. The high-performance devices based on large-scale polycrystalline graphene growth capabilities with efficient charge and spin transport make it prospective for practical implementation into future spintronic and quantum integrated circuits. While CVD graphene presents unlimited prospects for exploring spin currents, there exist challenges along the way in terms of scalability of efficient performance, and reliability. Deformations, wrinkles, and structural (electronic) modifications caused at the interfaces with contacts remain key concerns for device performance. In particular, oxide-based interfaces with graphene are central to both graphenes electronic and spintronic devices. For high-performance scalable devices, it is of crucial significance to understand the details of these interfaces and how devices of CVD graphene with polycrystallinity respond to high current limits. In this thesis, we discuss a systematic study of the effect of e-beam evaporated ultra-thin titanium oxide (TiOx) and aluminum oxide (AlOx) on graphene; which are conventionally used as tunnel barriers in spintronic and nanoelectronics devices. Characteristic topographic features of both metal oxides on the graphene surface were revealed by atomic force microscopy. To estimate the impact of these oxides on graphene, electrical measurements were performed on graphene spin devices with and without metal oxides on the same devices. These measurements show significant p-type doping for both metal oxides, with sustained sheet conductance (σ0) and mobility (μ) values. Strikingly, Raman spectroscopy and X-ray photoelectron spectroscopy show the emergence of significant sp3 carbon for AlOx on graphene, in sharp contrast to TiOx. Our results and observations, together with theoretical calculations provide new insights into how sp3 carbon for AlOx can lead to new memristive mechanisms and explicate enhanced spin relaxation into graphene with AlOx devices, which was widely attributed to the presence of interface pinholes. Here we also investigate how CVD graphene-based devices respond to high current stress to understand their stability and robustness. Despite the grainy and wrinkled structure, we observed the highest till-date current density of 5.2 × 108 A/cm2, remarkably higher than previously reported values for multilayer graphene and graphene nanoribbons. The recorded reversible regime (~108 A/cm2) for device operation allows reliable spin transport measurements with an observable spin signal up at such high current density. Furthermore, our investigation also encompasses cyclical current-voltage electrical measurement, to unveil the stability of graphene/ultra-thin oxide interfaces in graphene devices. Overall, these results present significance for CVD graphene device engineering for nanoelectronics and spintronics.

This report presents the work done on an optical setup combining Raman and BLS spectroscopy and discusses the achieved improvements of the system. After some general optimization and minor redesigning, a new alternative beam path was implemented. This new beam path allows to freely adjust all three volume Bragg grating filters in the setup, which are used to suppress the elastic line and which feature an exceptionally narrow filter window. This enables measurement of BLS signal closer to the elastic line, with a frequency separation of only 30 GHz from it achieved during the project. For measuring below this threshold, additional suppression is needed. As a first technique to achieve this, the Fabry-Pérot interferometer provided by the BLS spectrometer was partially put to use. At the point of this report, this has not been completed successfully and the setup also suffers from other adverse effects in the form of signal fluctuations and spurious measurements close to the elastic line. Nonetheless, the setup has showed promising intermediate results and can be expected to perform well once these challenges have been overcome.

No description available.

To study the effect of carbon interstitials in austenitic steels on plastic deformation mechanisms is the main goal of the present thesis. Using first-principlesmethods, the generalized stacking fault energy (GSFE) of C-alloyed γ-Fe is firstcalculated. The GSFE curve includes several prominent stacking fault energiesthat are fundamental for, e.g, predicting critical twinning stress and twinnability. The C effect was previously investigated in γ-Fe assuming nonmagnetic(NM) state. However, paramagnetic (PM) state with local magnetic momentson each site and total magnetization equal to zero is a more appropriate description for austenites. The Exact Muffin-Tin Orbitals (EMTO) method is capableof modelling the PM state together with the Coherent Potential Approximation (CPA). We also compare the NM GSFEs of C-alloyed γ-Fe obtained fromEMTO and Vienna Ab initio Simulation Package (VASP) to evaluate the performance of EMTO on handling the C-interstitial structure. The EMTO resultsare verified to fit reasonably well with VASP results so the GSFE calculationfor the C-alloyed γ-Fe is further extended to the PM state.The influence of C interstitials on the GSFE for PM γ-Fe is significantly different from what is predicted for NM γ-Fe. Though the GSFE is increased byC addition for both NM and PM γ-Fe, the C-driven change on the GSFE ascompared to pure γ-Fe at the PM state deviates from that at the NM state:paramagnetism significantly weakens the C impact on the intrinsic stacking faultenergy while strengthens it on the unstable stacking fault energy as comparedto the hypothetical NM case. The different behaviours uncovered for the intrinsic and unstable stacking fault energies due to the presence of local magneticmoments is illustrated by the magnetic structures of the Fe-C alloys as a function of volume, which mainly emerged from the suppression effect of C on themagnetic moments of its adjacent Fe neighbours.Using the generalized stacking fault as an approximation for the partial dislocation core, we investigate the minimum energy path (MEP) for C diffusionin the dislocation core (i.e., for various displacement vectors ) for NM γ-Feusing VASP. In contrast to the common assumption of stationary interstitialatoms during the passage of fast-moving dislocations, a pair of partial dislocations moves C atoms forward on the slip plane by one full Burgers vector. Thisdissociated dislocation-mediated transport mechanism for C is a strain inducedprocess, which is present even when the normal thermally activated diffusion isinoperative. Moreover, at the stacking fault ribbon and especially near the partial dislocation core, the in-plane diffusion energy barriers for C are significantlyreduced compared to that in bulk, opening a fast diffusion pathway for C. Themagnetic effect is also indirectly considered for the in-plane C diffusion energybarrier by calculating the MEP in high-spin ferromagnetic (HS-FM) Fe and ferromagnetic (FM) Ni. It is concluded that the presence of magnetic couplingdoes not change this trend. Therefore, contrary to the previously suggestedmechanism based on the reorientation of Mn-C short range order, our resultsreveal that the fast pipe diffusion of C at the dissociated dislocations is primarily responsible for the dynamic strain aging (DSA) in Fe-Mn-C steels and themechanism for DSA-mediated formation of deformation twinning is proposed to understand the strain rate dependence of deformation twinning in the presenceof DSA.

Superconductors are important to study due to their many applications. Superconductivityarises when electron pairs, so-called Cooper pairs, form in the material.To understand the mechanism behind the formation of the Cooper pairs, the pairingsymmetry, described by the order parameter, must be found. One method to experimentallydetermine the order parameter is through quasi-particle interference.Quasi-particle interference (QPI) is a real space perturbation due to a defect andcan be experimentally imaged using scanning tunneling microscopy (STM), wherean atomic sized tip is scanned over a surface at a bias measuring the tunneling currentthrough the sample surface. QPI can also be theoretically calculated using atight-binding model describing the electronic structure and then using an interpolationbetween unit cells using wave functions to carry intra-unit cellular information,so-called Continuum QPI (CQPI). This report studies the influence of different superconductingorder parameters in a one-band cuprate-like material on theoreticallycalculated phase-referenced Fourier transformed CQPI (PRFT-CQPI). The resultsshow that this method gives qualitatively distinct results depending on the ordersymmetry, suggesting that it can be used to experimentally determine the superconductinggap symmetry. The results are also compared with experiment and withpreviously published CQPI calculations.

No description available.

This thesis describes experimental investigation of thin films made of diluted Pt 1-x Ni x ferromagnet alloy and Nb-Pt 1-x -Nb Josephson junctions. Such Hybrid Superconductor-Ferromagnet (S-F) Structures are of significant interest because of the new physics involved and possible applications in low temperature and spintronic devices. In many cases, such devices require components with small monodomain ferromagnetic layers, which requires development of specific nano-fabrication techniques. Pt 1-x Ni x alloy is used as the ferromagnet layer due to very good solubility of the two components which results in homogeneous diluted ferromagnet. Systematic analysis of both chemical composition, and ferromagnetic properties of Pt 1-x Ni x thin films for Ni concentrations ranging between 0 and ~70 at.% is performed. The energy-dispersive X-ray spectroscopy (EDS) technique is employed to study chemical composition of Pt 1-x Ni x thin films. To eliminate possible errors during EDS characterization, EDS is used with different electron beam energies, different electron beam incident angles and on the free standing Pt 0.59 Ni 0.41 flakes. Ferromagnetic properties of Pt 1-x Ni x thin films are analyzed by studying the anomalous Hall effect. The Curie temperature of Pt 1-x Ni x films decreases in a non-linear manner with the Ni concentration and has the onset at ~27 at.% of Ni. It is observed that the critical concentration of Ni is lower and the Curie temperature is higher than it had been observed early for the bulk PtNi alloys. The 3D Focused ion Beam Nanosculpturing is used to fabricate nanoscale S-F-S Josephson junctions providing the uniform, monodomain structure of the ferromagnet layer within the junction. The detailed studies of S-F-S Josephson junctions are carried out depending on the size of junction, thickness and composition of the ferromagnet layer. The obtained Fraunhofer modulation of the critical current as a function of in-plane magnetic field serves as evidence for uniformity of the junction properties and monodomain structure of ferromagnet layer. The junction critical current density decreases in spin glass state with increasing Ni concentration. In the ferromagnetic state the maximum current density of the junction starts to increase. The latter is attributed to switching into the pi state as a function of Ni concentration. Simultaneously it is observed that the critical current can completely disappear presumably as the result of stray fields from the F layer in contact leads. The Josephson junction is used as a phase sensitive detector for analysis of vortex states in mesoscopic superconductors. By changing the bias current at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states. A mesoscopic superconductor can thus act as a memory cell in which the junction is used both for reading and writing information (vortex).

Transparent conducting oxides (TCOs) are an important class of materials with many applications such as low emissivity coatings, or transparent electrodes for photovoltaics and flat panel displays. Among the possible TCO materials, Al-doped ZnO (AZO) is studied due to its relatively low cost and abundance of the raw materials. Thin films of AZO are commonly produced using physical vapour deposition techniques such as magnetron sputtering. However, there is a problem with the homogeneity of the films using reactive direct current magnetron sputtering (DCMS). This homogeneity problem can be related to the bombardment of the growing film with negative oxygen ions, that can cause additional acceptor defects and the formation of insulating secondary phases. In this work AZO films are deposited by high power impulse magnetron sputtering (HiPIMS), a technique in which high instantaneous current densities are achieved by short pulses of low duty cycle. In the first part of this thesis, the possibility to improve the homogeneity of the deposited AZO films by using HiPIMS is demonstrated. This improvement can be related to the high instantaneous sputtering rate during the HiPIMS pulses, so the process can take place in the metal mode. This allows for a lower oxygen ion bombardment of the growing film, which can help to avoid the formation of secondary phases. Another problem of AZO is the stability of the properties in humid environments. To assess this problem, the degradation of the electrical properties after an aging procedure was investigated for films deposited by both DCMS and by HiPIMS. A method was proposed, to restore the properties of the films, using a low temperature annealing under N2 atmosphere. The improvement of the electrical properties of the films could be related to a diffusion process, where water is diffusing out of the films. Then, the influence of the substrate temperature on the properties of AZO films deposited by HiPIMS was studied. The electrical, optical and structural properties were found to improve with increasing substrate temperature up to 600 ◦C. This improvement can be mostly explained by the increase in crystalline quality and the annealing of defects. Finally, the deposition of AZO films on flexible PET substrates was investigated. The films are growing as a thick porous layer of preferentially c-axis oriented columns on top of a thin dense seed layer. The evolution of the sheet resistance of the films after bending the films with different radii was studied. There is an increase in the sheet resistance of the films with decreasing bending radius, that is less pronounced for thicker films.