| 1. |
- Simonov, Konstantin, 1988-
(författare)
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Effect of Substrate on Bottom-Up Fabrication and Electronic Properties of Graphene Nanoribbons
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Taking into account the technological demand for the controlled preparation of atomically precise graphene nanoribbons (GNRs) with well-defined properties, the present thesis is focused on the investigation of the role of the underlying metal substrate in the process of building GNRs using bottom-up strategy and on the changes in the electronic structure of GNRs induced by the GNR-metal interaction. The combination of surface sensitive synchrotron-radiation-based spectroscopic techniques and scanning tunneling microscopy with in situ sample preparation allowed to trace evolution of the structural and electronic properties of the investigated systems.Significant impact of the substrate activity on the growth dynamics of armchair GNRs of width N = 7 (7-AGNRs) prepared on inert Au(111) and active Cu(111) was demonstrated. It was shown that unlike inert Au(111) substrate, the mechanism of GNRs formation on Ag(111) and Cu(111) includes the formation of organometallic intermediates based on the carbon-metal-carbon bonds. Experiments performed on Cu(111) and Cu(110), showed that a change of the balance between molecular diffusion and intermolecular interaction significantly affects the on-surface reaction mechanism making it impossible to grow GNRs on Cu(110).It was demonstrated that deposition of metals on spatially aligned GNRs prepared on stepped Au(788) substrate allows to investigate GNR-metal interaction using angle-resolved photoelectron spectroscopy. In particular intercalation of one monolayer of copper beneath 7-AGNRs leads to significant electron injection into the nanoribbons, indicating that charge doping by metal contacts must be taken into account when designing GNR/electrode systems. Alloying of intercalated copper with gold substrate upon post-annealing at 200°C leads to a recovery of the initial position of GNR-related bands with respect to the Fermi level, thus proving tunability of the induced n-doping. Contrary, changes in the electronic structure of 7-AGNRs induced by the deposition of Li are not reversible. It is demonstrated that via lithium doping 7-AGNRs can be transformed from a semiconductor into a metal state due to the partial filling of the conduction band. The band gap of Li-doped GNRs is reduced and the effective mass of the conduction band carriers is increased.
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| 2. |
- Akansel, Serkan, 1983-
(författare)
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Magnetization Dynamics in Ferromagnetic Thin Films : Evaluation of Different Contributions to Damping in Co2FeAl and FeCo Film Structures
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Static and dynamic magnetic properties of Co2FeAl and Fe65Co35 alloys have been investigated. Co2FeAl films were deposited at different temperatures and the deposition parameters were optimized with respect to structural and magnetic properties. As a result, a film with B2 crystalline phase was obtained without any post-annealing process. A lowest magnetic damping parameter of was obtained for the film deposited at 573K. This obtained low value is comparable to the lowest values reported in research literature. After optimizing the deposition parameters of this alloy, different seed layers and capping layers were added adjacent to the Co2FeAl layer and the effect of these layers on the magnetic relaxation was investigated. In addition to adding nonmagnetic layers to Co2FeAl, the dependence of the magnetic damping parameter with respect to the thickness of Co2FeAl was investigated by depositing films with different thicknesses. A temperature dependent study of the magnetic damping parameter was also performed and the measured damping parameters were compared with theoretically calculated intrinsic Gilbert damping parameters. Different extrinsic contributions to the magnetic damping, such as two magnon scattering, spin pumping, eddy-current damping and radiative damping, were identified and subtracted from the experimentally obtained damping parameter. Hence, it was possible to obtain the intrinsic damping parameter, that is called the Gilbert damping parameter.In the second part of the thesis, Fe65Co35 alloys were investigated in terms of static and dynamic magnetic properties. Fe65Co35 films were deposited without and with different seed layers in order to first understand the effect of the seed layer on static magnetic properties of the films, such as the coercivity of the films. Then the films with seed layers yielding the lowest coercivity were investigated in terms of dynamic magnetic properties. Fe65Co35 films with different rhenium dopant concentrations and with ruthenium as the seed and capping layer were also investigated. The purpose of this study was to increase the damping parameter of the films and an increase of about ~230% was obtained by adding the dopant to the structure. This study was performed at different temperatures and after subtraction of the extrinsic contributions to the damping, the experimental values were compared with theoretically calculated values of the Gilbert damping parameter. During the thesis work, magnetic looper and superconducting quantum interference device magnetometers set-ups were used for static magnetic measurements and cavity, broadband in-plane and broadband out-of-plane ferromagnetic resonance set-ups were used for dynamic measurements.
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| 3. |
- Andersson, Cecilia, 1976-
(författare)
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Exploring the Magnetism of Ultra Thin 3d Transition Metal Films
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis the magnetic and structural properties of ultra-thin 3d transition metals films have been investigated, in particular Fe, Ni and Co films. X-ray Magnetic Circular Dichroism (XMCD) has provided element specific spin (ms ) and orbital (ml ) moments per atom by utilizing the magneto optic sum-rules. Element specific hysteresis curves have been measured by means of X-ray Resonant Magnetic Scattering (XRMS), and the local crystallographic structure has been investigated using Extended X-ray Absorption Fine Structure (EXAFS).By performing XMCD on Fe/Ag(100) we observe a spin reorientation from in-plane to out-of-plane as the Fe thickness is lowered. At temperatures below 300K it occurs around 5-7 mono layers (ML) of Fe. While reorienting the magnetization out-of-plane the orbital moment increases with 125% but only a minor increase (5%) of the spin moment is observed. Extended X-ray Absorption Fine Structure (EXAFS) measurements indicate that films 6 ML and thicker have a bulk-like bcc structure. For the thin out-of-plane films, the local crystallographic structure is more complicated.The spin reorientation of the Au/Co/Au tri-layer system has been studied as a function of temperature, Co layer and Au cap thickness. An unexpected behavior of the orbital moment upon spin reorientation is found in these systems. An ex-situ prepared sample shows a smooth spin reorientation from an in-plane to an out-of-plane easy magnetization direction as the temperature is lowered from 300K to 200K. In-situ prepared samples have also been investigated and a novel phase diagram has been identified. The Au/Co interface has been explored during the Au capping by means of photoemission measurements.In the bi- and tri-layer system of Fe and Ni we have been able to manipulate the spin reorientation by varying the Fe and Ni thickness. A novel non-collinear interlayer exchange interaction for 3d ferro magnets in direct contact has been discovered for a set of samples. This exchange interaction is found to be strongly dependant on the preparation conditions.
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| 4. |
- Belotcerkovtceva, Daria
(författare)
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Intricacies, Endurance, and Performance Enhancement in Graphene Devices : Towards 2D electronic and spintronic circuits
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene, the atomically thin material of carbon atoms, first isolated experimentally in 2004, exhibits remarkable properties and holds potential for applications in quantum, electrical, and spin-based devices. The chemical vapor deposition (CVD) method enables graphene production on a large scale, merging its exceptional characteristics with scalability and high-quality implementation. Despite the extraordinary promise of CVD graphene with structural imperfections, the main challenge for graphene electronics and spintronics lies in achieving reliability at the device and circuit levels with scalable materials and interfaces. To address these, it is essential to understand the intricacies, endurance, and performance issues in graphene devices. In this thesis, to understand graphene interfaces in devices, we first explored a critical aspect of graphene's interaction with metal oxides, particularly titanium oxide (TiOx) and aluminum oxide (AlOx), and their implications for graphene-based nanoelectronic and spintronic devices. Investigating the electrical characteristics of graphene, both with and without oxides, uncovers the distinct behaviors of TiOx and AlOx when interfaced with graphene, highlighting the charge transfer-induced p-type doping and the formation of sp3 defects, traps, and impurities, especially at the AlOx/graphene interface. These findings bring new insights for graphene spintronic devices while opening possibilities for novel functionalities such as hybrid resistive switching devices. Advancing further towards van der Waals heterostructures in these studies, we could also observe the impact of monolayer MoS2 on graphene’s properties. Next, we explored how CVD graphene devices withstand high current stress to elucidate device durability and resilience. We examine the impact of extreme electric currents on channel structures and resistive tunnel barrier interfaces, focusing on their feasibility for high-capacity electronic and spintronic applications. Here, despite the polycrystalline nature of CVD graphene, we could observe the highest current density of 5.2×108 Acm-2 in graphene on Si/SiO2 substrates, elevating it further to 1.7×109 Acm-2 on diamond substrates, remarkably exceeding previous reports. Performing systematic cyclic electrical measurements, with a gradual increase in the applied high current, we could determine the limits of the reversible regime for safe device operation of both channels and contacts. This knowledge of high current limits and oxide interfaces with graphene leads to an innovative current-treated passive graphene (CTPG) system, where we passivated graphene with metal oxide and applied high current to enhance quality. This method addresses the challenge of interfacial defects and remarkably improves carrier mobility, thereby reducing Coulomb scattering while mitigating electromigration issues. The CTPG presents a scalable platform for stable nanoelectronic and spintronic circuits. The experiments and systems studied in this thesis open possibilities for the exploration of temperature-dependent charge and spin transport measurements via new heterostructures and interfaces with different material combinations.
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| 5. |
- Kumar, P. Anil, et al.
(författare)
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All-alkoxide based deposition and properties of a multilayer La0.67Sr0.33MnO3/CoFe2O4/La0.67Sr0.33MnO3 film
- 2021
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Ingår i: European Journal of Inorganic Chemistry. - : John Wiley & Sons. - 1434-1948 .- 1099-1948 .- 1099-0682. ; 2021:18, s. 1736-1744
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Tidskriftsartikel (refereegranskat)abstract
- Single and multilayer films of La0.67Sr0.33MnO3 and CoFe2O4 were deposited by spin-coating. The all-alkoxide precursors allowed inorganic gel films of extreme homogeneity to be formed and converted to phase pure complex oxides at low temperatures. La0.67Sr0.33MnO3 films were made with La- and Ca-methoxy-ethoxides and Mn19O12(moe)(14)(moeH)(10) as precursors at 800 degrees C. The CoFe2O4 films were obtained at extremely low 275 degrees C, using a new CoFe2-methoxyethoxide precursor. The decomposition and microstructural development on heating was described by TG, TEM, XRD and IR spectroscopy. XRD showed no spurious phases and the unit-cell dimensions coincided quite well with literature values of the targeted phases. The structural, magnetic and electronic properties of these films established their phase purity and high quality with physical properties comparable to films deposited by physical deposition methods. The magnetic and magneto transport results are presented for single, bi- and tri- layer structures. The magnetically soft La0.67Sr0.33MnO3 layer was exchange coupled to the magnetically hard CoFe2O4 layer, giving rise to interesting switching behaviour in magnetism and magneto-transport properties.
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| 6. |
- Malik, Rameez Saeed, 1987-
(författare)
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Ultrafast Magnetization Dynamics : Element-selective studies of magnetic alloys using ultra short XUV pulses
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, I investigate the ultrafast magnetization dynamics in 3d ferromagnets and their alloys with ultrashort laser pulses. The high harmonics generation (HHG) setup provides extreme-ultraviolet photons with energies 35-72 eV, which is the energy range where 3d metals have their M2,3 absorption edges. By employing HHG with the transverse magneto-optical Kerr effect, the magnetization of multiple elements in a magnetic system is probed and their dynamics are resolved separately on femtoseconds time scales.The magneto-optical response of elemental Fe and Ni during demagnetization is investigated. This magneto-optical response is measured as an asymmetry in the intensity of reflected light for two opposite sample magnetization directions. Experiment and density functional theory calculations show that for Fe, the asymmetry is strongly dependent on the particular type of magnetic excitation. However, for Ni, it is relatively insensitive to the magnetic excitation. Next, the element-specific magnetization dynamics of FeNi alloys are investigated. A time delay in the Ni demagnetization relative to Fe is observed for all alloy compositions. This Ni-delay depends on the alloy composition and is related to changes in the exchange interactions.Co2FeAl (CFA) Heusler alloys are unique due to their peculiar electronic structure and because they can exhibit very low damping. Experimentally, CFA films show a decreased damping with an increase in structural ordering. The demagnetization times of Fe and Co in CFA samples with different amount of ordering are similar for all samples. However, the remagnetization times exhibit a dependence on the structural ordering. Both the theoretical and experimental damping parameters correlate well with the remagnetization times. In FeCo alloys, the damping can be changed by doping with heavy metals. Here, the magnetization dynamics of Fe65Co35 films as a function of Re doping are investigated. We find no observable change in the demagnetization times for samples with increased damping. However, when increasing the Re doping and the damping, the remagnetization time becomes faster. Also, a fast increase of the asymmetry signal is observed at the Ru-edge during the demagnetization of FeCo. This effect is attributed to a super-diffusive spin current going from the FeCo layer to the Ru capping layer.Last, the magnetization dynamics of a ferrimagnetic insulator is studied. The NiFe2O4 asymmetry shows oscillatory dynamics after an ultrashort laser pulse excitation. With 1.55 eV pump, these oscillations are strong. For 3.1 eV pump, demagnetization becomes dominant and the oscillations diminish.
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| 7. |
- Mukherjee, Soham, et al.
(författare)
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Structure and Electronic Effects from Mn and Nb Co-doping for Low Band Gap BaTiO3 Ferroelectrics
- 2021
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:27, s. 14910-14923
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the doping-induced local structural and electronic effects in the recently developed low band gap room temperature ferroelectric Mn-Nb co-doped BaTiO3. Experimental and theoretical Raman spectroscopies are utilized to quantify the Ti off-centering, identified to be the intrinsic origin of ferroelectricity in these systems. Mn and Nb exhibit contrasting doping behaviors that have remarkable effects on BaTiO3 functionality. Jahn-Teller distorted Mn3+ is primarily associated with lowering of the bulk band gap, while charge-compensating Nb5+ off-centers within the O-6 octahedra, creating a polar mode that stabilizes the ferroelectric ground state. The charge neutral aliovalent Mn3+-Nb5+ pair effectively couples to the inherent ferroelectric instability of the BaTiO3 lattice, restoring some spontaneous polarization lost by doping Mn3+ (d(4)) ions at Ti4+ (d(0)) sites.
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| 8. |
- Phuyal, Dibya
(författare)
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An X-ray Spectroscopic Study of Perovskites Oxides and Halides for Emerging Devices
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates the electronic structures on several perovskite oxide and halide materials with a focus on light harvesting applications. The systematic study of the electronic properties of the transition metal oxides and post-transition metal halides is a key point if one is to understand their properties. The element and site selective nature of several x-ray based spectroscopic techniques are given special emphasis in order to obtain a complete picture of the electronic properties of the compounds in question. Much of the experimental studies are accompanied by ab initio calculations that corroborate with our experimental results.In the oxide portion of this work, a new class of metallic oxides based on doping of an antiferromagnetic LaFeO3 was synthesized and systematically studied with x-ray absorption, x-ray emission, and photoemission spectroscopies. The compound’s electronic structure is complex, having itinerant as well as localized components that give rise to a unique physical state where antiferromagnetism, metallicity and charge-disproportionation coexist. Our resonant photoemission results establish that the Fe states in both magnetically ordered oxides show insulting properties, while the Mo states provide an itinerant band crossing the Fermi level. An excitation energy-dependent RIXS investigation on LaFe1-xMoxO3 and the double perovskite Sr2FeMoO6 revealed a double peak structure located in proximity to the elastic peak that is identified to purely d-d excitations, attributed to the strongly correlated nature of these transition metal compounds.The growth of high-quality thin film ferroelectric based on BaTiO3 grown epitaxially by means of pulsed laser deposition were investigated. We systematically reduce the band gap of the ferroelectric thin film while retaining its polarization at ambient conditions in spite of the aliovalent doping. The electronic structure is studied by several x-ray techniques that show how the ferroelectricity persists as well as the effective reduction of the band gap through hybridized states.In the post-transition metal halides, the valence and conduction bands were mapped using x-ray absorption, emission, and photoemission spectroscopies. The spectroscopic results identify the constituent states that form the valence band as well as the band energy positions, which is an imperative parameter in optoelectronic devices. In addition, x-ray based spectroscopy was used to demonstrate the stereochemical activity of lone-pair states (5s2 and 6s2) for several different halide compounds and their influence on the chemical, structural, and electronic properties of the material. Nanostructured halide perovskites are also explored. The position of iodine p states and valence band states in reduced dimensional lead-based compounds were examined, as their states are found to be confined in one crystallographic direction in contrast to their three-dimensional counterpart. This information highlights the interesting material properties and their use in current third generation solar cell research.
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