The hyperfine interaction between carrier spins and nuclear spins is an important component in exploring spin-dependent properties in materials with strong spin orbit interaction.However hyperfine interaction has been less studied in bismuth (Bi), a heavy element exhibiting a strong Rashba-like spin-orbit interaction in its two-dimensional surface states due to the broken spatial inversion symmetry. In this dissertation we experimentally explore the carrier spin polarization due to transport under strong spin-orbit interaction and the nuclear polarization resulting from the relatively unexplored hyperfine interaction on Bi(111) films.The carrier and nuclear spin polarizations are expected to dynamically interact, a topic with ramifications to other materials where surface states with noteworthy properties play a role.To achieve this goal, an optimized van der Waals epitaxy growth technique for Bi(111) on mica substrates was developed and used, resulting in flat Bi surfaces with large grain sizes and a layered step height of 0.39±0.015 nm, corresponding to one Bi(111) bilayer height. A comparison between Bi(111) films grown on three different substrates (mica, InSb(111)B, and Si(111)) is discussed, for which scanning electron microscopy and atomic force microscopy are applied to obtain the structural and morphological characteristics on the film surface. Magnetotransport measurements are carried out to extract the transport properties of theBi(111) films. Using the high quality Bi(111) film deposited on mica, we develop quantum magnetotransport techniques as delicate tools to study hyperfine interaction. The approach is based on measuring quantum corrections to the conductivity due to weak antilocalization, which depend on the coherence of the spin state of the carriers. The carrier spin polarization is generated by a strong DC current in the Bi(111) surface states (here called the Edelstein effect), which then induces dynamic nuclear polarization by hyperfine interaction. Quantum transport antilocalization measurements in the Bi(111) thin-films grown on mica indicate a suppression of antilocalization by the in-plane Overhauser field from the nuclear polarization, and allow for the quantification of the Overhauser field, which is shown to depend on both polarization duration and the DC current magnitude. Various delay times between the polarization and the measurement result in an exponential decay of the Overhauser field, driven by relaxation time T1. We observe that in the Bi surface states, the appreciable electron density and strong spin-orbit interaction allow for dynamic nuclear polarization in the absence of an external magnetic field. / Doctor of Philosophy / This dissertation focuses on the heavy element bismuth (Bi), a semimetal with strong spin-orbit interaction at its two-dimensional surface. Given the challenge to grow high qualityBi(111) films, we present an optimized van der Waals epitaxy technique to grow Bi(111)films on mica substrates, which show a flat surface with large grain sizes and a layered step height of 0.391±0.015 nm, corresponding to one Bi(111) bilayer height. To demonstrate the high quality of the Bi(111) surface, a comparison of surface morphology was conducted among Bi(111) films deposited on three different substrates (mica, Si(111), and InSb(111)B),along with a comparison between their electronic transport properties. By applying a DC current on the high quality Bi(111) film on mica, a carrier spin polarization is established via mainly what we here call the Edelstein effect, which then induces dynamic nuclear polarization by hyperfine interaction and generates a non-equilibrium nuclear spin polarization without externally applied magnetic field. We quantified the Overhauser field from the nuclear polarization all-electrically by conducting quantum transport antilocalization experiments, which showed a suppression of antilocalization by the in-plane Overhauser field.Comparative measurements indicated that the magnitude of the Overhauser field depends onthe spin-polarizing DC current magnitude and the polarization duration. The experiments also show that antilocalization forms a sensitive probe for hyperfine interaction and nuclear polarization.

The main goals of this research were to fabricate magnetic cobalt nanoparticles and study their structural, crystal structure, optical, and magnetic properties. Cobalt nanoparticles with average particle size 8.7 nm were fabricated by the method of high temperature reduction of cobalt salt utilizing trioctylphosphine as a surfactant, oleic acid as a stabilizer, and lithium triethylborohydride as a reducing reagent. Energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the formation of cobalt nanoparticles. High resolution transmission electron microscopy images show that Co NPs form both HCP and FCC crystal structure. The blocking temperature of 7.6 nm Co NPs is 189 K. Above the blocking temperature, Co NPs are single domain and hence showed superparamagnetic behavior. Below the blocking temperature, Co NPs are ferromagnetic. Cobalt nanoparticles with a single-domain crystal structure support a sharp plasmon resonance at 280 nm. Iron nanoparticles with average particle size 4.8 nm were fabricated using chemical reduction method show plasmon resonance at 266 nm. Iron nanoparticles are ferromagnetic at 6 K and superparamagnetic at 300 K.

We imitated the sandwich structure of TMR(Tunneling Magnetoresistance) to apply to CMR(Colossal Magnetoresistance) material . We choose one of the Colossal Magnetoresistance material La0.67Sr0.33MnO3(113) to be the Ferromagnetic(FM) layers as top and bottom layer in sandwich structure and use La0.67Sr0.33MnO3 ¡P SrO(214) to be middle layer which have Antiferromagnetic(AFM) property to form FM-AFM-FM structure. The FM and AFM layer can match their lattice in interface joint. What its purpose is to use this structure to enhance SPT (Spin Polarization Tunneling) effect and let spintronics can periodical spin-flip in supper lattice structure of antiferromagnetic. Upon this compose we try to show increase the LFMR (Low Field Magnetoresistance) by use CMR. The experiment result shows maybe the film structure damage occurred in our made TMR tunneling device process (ex. Ion etching process), so we should improvement the process to get the exactly experiment data. Additional, due to the alignment of the moment of La0.67Sr0.33MnO3 is unstable, the back and forth hysteresis loop can¡¦t overlap, So that afterwards we can use the more stable material La0.67Ca0.33MnO3 which have more stable moment.

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We correlated experimental results with theoretical estimations of the dielectric function ε(ω) in two contexts: the effect of an electric field in quantum wells and that of the spin polarization of an interacting electron-hole plasma in bulk semiconductors. In the first part, we recorded photoreflectance spectra from Ge/GeSi quantum wells of different widths but having comparable builtin electric fields caused by doping. The reason why the spectra differed in overall shape was difficult to understand by conventional methods, for example, by calculating the allowed transition energies or by fitting the data with lineshape functions at each transition energy. Instead, we computed the photoreflectance spectra from first-principles by using the confined electron and hole wavefunctions. This method showed that the spectra differ in overall shape because of the experimentally hitherto unobserved trend in quantum well electro-optical properties, from the quantum confined Franz-Keldysh effect to the bulk Franz-Keldysh effect, as the well width is increased. The second part develops a threeband microscopic theory for the optical properties due to spin-polarized carriers in quasiequilibrium. We show that calculations based on this theory reproduce all the trends observed in a recent circularly polarized pump-probe experiment reported in the literature. To make the computation less intensive, we proposed a simplified, two-band version of this theory which captured the main experimental features. Besides, we constructed a cw diode laser-based pump-probe setup for our own optical studies of spin-polarized carriers by Kerr rotation. We achieved a sensitivity of detection of Kerr rotation of 3 x 10¯ 8 rad, corresponding to an order of magnitude improvement over the best reports in the literature. The efficacy of our setup allowed for the demonstration of a pumpinduced spin polarization in bulk GaAs, under the unfavorable conditions of steady-state and room temperature.

Dans le manuscrit, nous étudions l’impact des défauts sur le transport électronique dépendant du spin dans les jonctions tunnel magnétiques (JTM). Ces études ont été effectuées sur des hétérostructures possédant des barrières tunnel composé de SrTiO3, TiO2 et de MgO. Dans le cas des deux premières structures, nous montrons comment l’oxydation à l’interface induit réduction très prononcée de la magnétorésistance tunnel (TMR). Dans le cas du MgO, des études optiques sur les états induits par les défauts dans la barrière isolante ont été effectué. Nous avons montré qu’il est possible de contrôler la densité de certain type de défaut (lacune d’oxygène) en altérant les conditions de dépôts du MgO. Les études électriques et optiques effectué sur ces échantillons permettent de remonter à l’énergie des défauts au sein de la barrière. Les méthodes d’analyses des mesures électrique Î (I chapeau), qui représente la variation relative ou absolue du courant électrique en fonction de la température, permet de définir différents régime de transport à travers les jonctions CoFeB/MgO/CoFeB. Le régime intrinsèque observé à basse température s’explique par les effets de structure de bande du CoFeB et du MgO (filtrages des électrons de différente symétrie par la barrière de MgO), tandis que le régime « extrinsèque » observé aux températures intermédiaires résulte d’une activation thermique progressive des lacunes d’oxygène et est accompagné d’une réduction de la résistance ainsi que du signal TMR. Nous avons également montré dans des études préliminaires qu’une excitation optique des défauts a un impact sut le magnéto-transport. / In the manuscript it is presented how the spin-polarized transport across magnetic tunnel junctions (MTJ) is affected by presence of structural defects within the barrier and at its interfaces. Studies concern structures incorporating SrTiO3, TiO2 and MgO insulators. In case of the first two structures it is shown how interfacial oxidation results in the drastically reduced value of the tunnelmagnetoresistance (TMR). In case of the MgO barrier, extensive studies on defect sites within the crystalline network of the insulator are performed. It is shown that one may control density of certain types of oxygen vacancies by altering growth conditions of the MgO layer. Further electrical and optical studies give insight into energetical positioning of these defect sites. Extension of the Î magnetotransport analysis method from the ‘absolute’ to the ‘relative’ case reveals several regimes of transport across CoFeB/MgO/CoFeB junctions. The low-temperature ‘intrinsic’ regime is attributed exclusively to the band structure effects of the CoFeB electrodes, whereas the mid- to room-temperature ‘extrinsic’ regimes result from the gradual incidence of thermally activated defect sites on the spin-polarized transport, which is accompanied by an increased reduction of both the resistance and the TMR signal. Initial experiments show that optical excitation of the defect sites has a crucial impact on the magnetotransport.

Phénomènes de spintronique dans les jonctions tunnel magnétiques et des films minces sont très prometteurs des deux points fondamentaux et l'application de vue. Elles sont basées sur l'exploration de spin d'électron en plus de sa charge et comprennent intercalaire couplage d'échange (CEI), l'anisotropie magnétique perpendiculaire (PMA), géante (GMR) et magnétorésistance tunnel (TMR), Couple de transfert de spin (STT), Spin effet Hall (SHE) et même induire du magnétisme dans les éléments non compris d graphène. Cette thèse comprendra premiers principes des études de phénomènes spintronique qui ont été d'un grand intérêt récemment. La première partie est consacrée à intercalaire couplage d'échange à travers les matériaux isolants dont le MgO, SrTiO3, GaAs et ZnSe. La deuxième partie comprendra des études ab initio d'anisotropie magnétique perpendiculaire au Fe | interfaces MgO et MTJ y compris le mécanisme et sa corrélation avec le spin Bloch symétrie Etat fondé de filtrage. Dans les enquêtes troisième partie de l'anisotropie magnétique et la fonction de travail dans les Co | interfaces graphène seront présentés. Ensuite, il sera montré possibilité d'induire et d'optimiser le magnétisme intrinsèque dans nanomeshes graphène. Dernière partie sera consacrée à l'induction de polarisation de spin et le réglage de Dirac point et ordre magnétique dans le graphène à l'aide d'effets de proximité magnétiques substrat. / Spintronic phenomena in magnetic tunnel junctions and thin films are very promising from both fundamental and application points of view. They are based on exploring spin of electron in addition to its charge and include interlayer exchange coupling (IEC), perpendicular magnetic anisotropy (PMA), giant (GMR) and tunnel magnetoresistance (TMR), Spin Transfer Torque (STT), Spin Hall Effect (SHE) and even inducing magnetism in non d elements including graphene. This thesis will include first-principles studies of spintronic phenomena which have been of high interest recently. First part is devoted to interlayer exchange coupling across insulating materials including MgO, SrTiO3, GaAs and ZnSe. The second part will include ab initio studies of perpendicular magnetic anisotropy at Fe|MgO interfaces and MTJs including the mechanism and its correlation to the Bloch state symmetry based spin filtering. In third part investigations of magnetic anisotropy and work function in Co|graphene interfaces will be presented. Next, it will be shown possibility of inducing and optimizing intrinsic magnetism in graphene nanomeshes. Final part will be devoted to inducing spin polarization and tuning Dirac point and magnetic order in graphene by means of magnetic substrate proximity effects.

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