This work targeted the growth of gadolinium (Gd)-doped gallium nitride (GaN) thin films (Ga₁₋ₓGdₓN) by metal organic chemical vapor deposition (MOCVD). Characterization and evaluation of these Ga₁₋ₓGdₓN thin films for application in spintronics/optoelectronics devices also formed part of this work. This work presents: (1) the first report of stable, reproducible n- and p-type Ga₁₋ₓGdₓN thin films by MOCVD; (2) the first Ga₁₋ₓGdₓN p-n diode structure; and (3) the first report of a room temperature spin-polarized LED using a Ga₁₋ₓGdₓN spin injection layer. The Ga₁₋ₓGdₓN thin films grown in this work were electrically conductive, and co-doping them with Silicon (Si) or Magnesium (Mg) resulted in n-type and p-type materials, respectively. All the materials and structures grown in this work, including the Ga₁₋ₓGdₓN-based p-n diode and spin polarized LED, were characterized for their structural, optical, electrical and magnetic properties. The spin-polarized LED gave spin polarization ratio of 22% and systematic variation of this ratio at room temperature with external magnetic field was observed.

The static and dynamic properties of spin distributions within domain walls(DWs) confined by Permalloy nanowire conduits are investigated by numerical simulations and high-speed magneto-optic polarimetry. Phase boundaries and critical points associated with DW spin distributions of various topologies are accurately determined using high-performance computing resources. Field-driven mobility curves that characterize DW propagation velocities in 20 nm thick nanowires are calculated with increasing the width of nanowires. Beyond the simple one-dimensional solution, the simulations reveal the four distinct dynamic modes. Oscillations of the field-driven DW velocity in Permalloy nanowires are observed above the Walker breakdown condition using high-speed magneto-optic polarimetry. A one-dimensional analytical model and numerical simulations of DW motion and spin dynamics are used to interpret the experimental data. Velocity oscillations are shown to be much more sensitive to properties of the DW guide structure (which also affect DW mobility) than the DW spin precessional frequency, which is a local property of the material. Transverse bias field effects on field-driven DW velocity are studied experimentally and numerically. DW velocities and spin configurations are determined as functions of longitudinal drive field, transverse bias field, and nanowire width. For a nanowire that supports vortex wall structures, factor of ten enhancements of the DW velocity are observed above the critical longitudinal drive-field (that marks the onset of oscillatory DW motion) when a transverse bias field is applied. The bias-field enhancement of DW velocity is explained by numerical simulations of the spin distribution and dynamics within a propagating DW that reveal dynamic stabilization of coupled vortex structures and suppression of oscillatory motion in the nanowire conduit resulting in uniform DW motion at high speed. Current-driven and current-assisted field-driven domain wall dynamics in ferromagnetic nanowires have thermal effects resulting from Joule heating, which make difficult to separate the spin-torque effects on DW displacements. To understand the thermal effects on DW dynamics, the temperature dependence of field-driven DW velocity is explored using high-bandwidth scanning Kerr polarimetry. Walker critical fields are decreased with increasing temperature and temperature-induced dynamic mode changes are observed. The results show that Joule heating effects are playing an important role in current-driven/current-assisted field-driven DW dynamics. / text

The concept of Berry phase, since its proposition in 1984, has found numerous applications and appears in almost every branch of physics today. In this work, we study several physical effects in ferromagnetic metal materials which are manifestations of the Berry phase. We first show that when a domain wall in a ferromagnetic nanowire is undergoing precessional motion, it pumps an electromotive force which follows a universal Josephson-type relation. We discover that the integral of the electromotive force over one pumping cycle is a quantized topological invariant equal to integer multiples of h/e, which does not depend on the domain wall geometry nor its detailed dynamic evolution. In particular, when a domain wall in a nanowire is driven by a constant magnetic field, we predict that the generated electromotive force is proportional to the applied field with a simple coefficient consisting of only fundamental constants. Our theoretical prediction has been successfully confirmed by experiments. Similar effect known as spin pumping occurs in magnetic multilayer heterostructures, where a precessing free magnetic layer pumps a spin current into its adjacent normal metal layers. Based on this effect, we propose two magnetic nanodevices that can be useful in future spintronics applications: the magnetic Josephson junction and the magneto-dynamic battery. The magnetic Josephson junction has a drastic increase in resistance when the applied current exceeds a critical value determined by the magnetic anisotropy. The magneto-dynamic battery acts as a conventional charge battery in a circuit with well-defined electromotive force and internal resistance. We investigate the condition under which the power output and efficiency of the battery can be optimized. Finally we study the side jump contribution in the anomalous Hall effect of a uniformly magnetized ferromagnetic metal. The side jump contribution, although arises from disorder scattering, was believed to be independent of both the scattering strength and the disorder density. Nevertheless, we find that it has a sensitive dependence on the spin structure of the disorder potential. We therefore propose a classification scheme of disorder scattering according to their spin structures. When two or more classes of disorders are present, the value of side jump is no longer fixed but depends on the relative disorder strength between classes. Due to this competition, the side jump contribution could flow from one class dominated limit to another class dominated limit when certain system control parameter changes. Our result indicates that the magnon scattering plays a role distinct from the normal impurity scattering and the phonon scattering in the anomalous Hall effect, because they belong to different scattering classes. / text

The interfacial electronic structure of the organic material- tris(8-hydroxyquinolinato)aluminum (Alq3) forming an interface with cobalt metal has been investigated in this research. The primary characterization method used in this research was near-edge X-ray absorption fine structure (NEXAFS) spectroscopy which probes the unoccupied molecular orbitals of a material. Density functional theory (DFT) calculations have also been employed to calculate the partial density of states (PDOS) of all constituent elements present in Alq3 molecule. The DFT calculations helped to determine the molecular orbital structure of Alq3 and to understand how the orbital structure is influenced by forming an interface with ferromagnetic Co layer. The experimental NEXAFS spectra measured in total fluorescence yield (TFY) showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 states of Alq3 were not affected by the presence of Co when Co is deposited onto Alq3. On the other hand, a charge transfer between Co and Alq3 led the loss or reduction of LUMO+2 state for a Co(top)/Alq3 bilayer sample when compared to pristine Alq3 reference sample (without Co deposition). This selective effect of Co on the orbital configuration of Alq3 suggests that Co atoms diffuse into Alq3 and interact with preferred sites in Alq3. By comparing the spectral change in the experimental NEXAFS spectra to the calculated PDOS of Alq3, the preferred interaction sites between Co and Alq3 could be successfully determined. This work suggests that the spectroscopic approach using synchrotron-radiation X-ray spectroscopy can serve as a powerful means for studying the interfacial electronic structure between magnetic metals and organic semiconductors and can contribute to the research and development of high performance organic spintronics.

Mesoscopic effects in ferromagnets could be different from mesoscopic effects in normal metals. While normal metals with a short mean-free-path do not exhibit classical magnetoresistance, weakly disordered ferromagnets with a similar mean-free-path display magnetoresistance including domain wall resistance (DWR) and anisotropic magnetoresistance (AMR). Magnetoresistance could lead to novel mesoscopic effects because the wave function phase depends on the scattering potential. In this thesis, we present our measurements of mesoscopic resistance fluctuations in cobalt nanoparticles and study how the fluctuations with bias voltage, bias fingerprints, respond to magnetization-reversal processes. The resistance has been found to be very sensitive to the magnetic state of the sample. In particular, we observe significant wave-function phase shifts generated by domain walls, and it is explained by mistracking effect, where electron spins lag in orientation with respect to the moments inside the domain wall. Short dephasing length and dephasing time are found in our Co nanoparticles, which we attribute to the strong magnetocrystalline anisotropy.

[Truncated abstract] Although exchange bias was discovered more than four decades ago, a satisfactory understanding of every instance of exchange bias observed in experiment has not yet emerged. Understanding exchange bias is complicated by many factors. For example, details of the antiferromagnet interface structure set up during the initial field cooling, thermal activation processes in the ferromagnet and antiferromagnet, and domain formation and domain wall movement in the antiferromagnet are all important in determining features associated with exchange bias. Two exchange bias systems are investigated in this thesis. One is a disordered system: a single layer Co/CoO film with random interfaces prepared by a reactive RF sputtering technique. The other is a ‘model’ system of Fe/KFeF 3 bilayers with compensated interfaces prepared by molecular beam epitaxy (MBE). The central theme of this work is to understand exchange bias and other related magnetic properties in these two very different systems. The Co/CoO exchange bias system studied here is different in structure from conventional exchange bias systems such as bilayer and multilayer systems where interfaces between ferromagnet and antiferromagnet are reasonably well defined. In this Co/CoO system, the Co and CoO is in the form of particles distributed randomly in a sputtered film. The interfaces between the Co and CoO are randomly distributed and may not be continuous over a large length scale. More importantly, the interface area is dependent on the shape and size of the particles and on their distribution. Many unique magnetic properties are related to the random interface in this system. For example, exchange bias and coercivity obtained at low temperatures are very large due to the large interface area between Co and CoO particles. The interface area can be controlled by changing the Co/CoO mass ratio in the film. Unlike in bilayer systems, film thickness in this single layer Co/CoO system turns out not to be critical for exchange bias and coercivity. The independence of film thickness may be technically important. More interestingly, because the interface is random, exchange bias can be setup by field cooling in any direction. Both training and magnetic viscosity effects were studied and provided evidence of thermal activation processes in this Co/CoO system. Training is explained as formation of a domain wall in the CoO with motion limited locally due to limited continuity of Synthesis and Characterization of Magnetic Thin Films - Exchange Bias Systems interfaces between the Co and CoO. Specific magnetization measurements over time were made and studied using viscosity theory. The magnetic viscosity was found to be strongly temperature dependent. There is a broad distribution of blocking temperatures which might be due to a broad distribution of Co particle sizes

Thesis (M.S.)--University of Alabama at Birmingham, 2008. / Description based on contents viewed May 30, 2008; title from title screen. Includes bibliographical references (p. 50-51).

Dans cette thèse, on analyse la possibilité de changer un courant de charge dans des jonctions supraconductrices par une manipulation des propriétés de spin en utilisant la résonance magnétique. On considère deux jonctions différentes: Premièrement, une jonction Josephson non-conventionnelle entre un supraconducteur conventionel de type s et un supraconducteur non-conventionel de type px. Deuxièmement, une jonction entre un demi-métal et un supraconducteur conventionel. La jonction spx contient deux états liés d'Andreev qui sont 2pi-periodiques. Ils donnent lieu à une aimentation spontanée à l'équilibre. Ceci ouvre la possibilité de manipuler l'occupation des niveaux d'Andreev en utilisant un champ magnétique dépendant du temps. On demontre que ce champ induit des oscillations de Rabi cohérentes entre différents états de spin de la jonction. Ces oscillations se manifestent comme des résonances dans la relation courant-phase de la jonction. Pour un champ polarisé circulairement, on trouve une règle de sélection de spin qui autorise des oscillations de Rabi seulement dans un certain interval de phases dans la relation courant-phase permettant une éventuelle détection du spin. De plus, le champ induit des transitions non-cohérentes qui nécessitent la présence d'une quasiparticule dans le continuum d'états. Ces transitions agissent comme processus de recharge et d'ionization pour les niveaux d'Andreev. Pour un champ polarisé circulairement, ces processus induits par le champ ne donnent pas lieu à un mécanisme de relaxation pour les oscillations de Rabi à cause des contraintes en spin et en énergie. Pour un champ polarisé linéairement, il n'y a pas de règle de selection de spin et la largeur des résonances de Rabi dans la relation courant-phase est déterminée par les processus d'ionization induits par le champs. Dans la jonction entre le demi-métal et le supraconducteur conventionel, il n'y a pas de courant pour une aimentation statique, puisque la polarization parfaite en spin du demi-métal interdit les processus de réflexion d'Andreev à l'interface. On demontre que pour une géométrie de point contact, un courant d'Andreev passe, si le demi-métal est soumis à la résonance ferromagnétique. La précession de la direction de l'aimentation dans le demi-métal donne lieu au mécanisme de spin-flip nécessaire. Le courant est forcé par la précession de la direction de l'aimentation qui crée une situation hors équilibre pour les porteurs de charge. De plus, dans un matériau ferromagnétique avec une densité de porteurs minoritaires non-nulle, le courant est réduit et disparaît si les densités majoritaires et minoritaires sont égales. On considère, par ailleurs, une géométrie d'interface étendue, plus réaliste. Pour une jonction ballistique, le courant est augmenté par rapport à la géometrie de point contact, en raison du nombre plus élevé de canaux. De plus, on demontre que le désordre est le plus important dans le matériau ferromagnétique. Le courant d'Andreev à travers la jonction désordonnée est beaucoup plus grand que le courant à travers la jonction ballistique dans la même géométrie. / In this thesis we investigate the possibility to change the charge current in superconducting junctions by manipulating the spin properties using magnetic resonance. We consider two different junctions: First, an unconventional Josephson junction between a conventional s-wave superconductor and an unconventional px-wave superconductor and second a half-metal/conventional superconductor junction. The spx junctions hosts two spin-polarized Andreev bound states, which are 2pi-periodic, giving rise to a spontaneous magnetization in equilibrium. This opens the possibility to manipulate the occupations of the Andreev levels using a time-dependent magnetic field. We show that the field induces coherent Rabi oscillations between different spin states of the junction that appear as resonances in the current-phase relation. For a cicularly polarized magnetic field, we find a spin selection rule, giving Rabi oscillations only in a certain range of superconducting phase differences, which provides a spin detection scheme. In contrary, for a linear polarization, there is no spin constraint on the Rabi oscillations. The field also induces non-coherent transitions including continuum states that act as refill and ionization processes for the Andreev levels. For a circularly polarized field, these field-induced processes do not provide a decay mechanism for Rabi oscillations, due to spin and energy constraints. For a linear polarization, the width of the Rabi resonances in the current-phase relation is determined by the field-induced ionization processes. In the half-metal/conventional superconductor junction no Andreev current may flow for a static magnetization direction, since the perfect spin polarization of the half-metal forbids Andreev reflection processes at the interface. We show that an Andreev current flows, if the half-metal is subject to ferromagnetic resonance. The precessing magnetization direction in the half-metal provides the necessary spin-flip mechanism. The current is driven by the precession of the magnetization direction that creates a non-equilibrium situation for the charge carriers. We also show for a point contact geometry that in a ferromagnet with non-zero minority carrier concentration the current is reduced and vanishes at equal minority and majority carrier concentrations. Additionally, we consider a more realistic, extended interface geometry. For a ballistic junction, the current is enhanced compared to a point contact geometry due to the larger number of transport channels. Furthermore, we show that disorder is most important in the ferromagnet. The Andreev current through the disordered junction is much larger than the current through a ballistic junction in the same geometry.

Ce travail est dédié à l'étude de l'effet des champs magnétiques sur les propriétés magnétiques et le comportement à la solidification d'alliages à base de Cobalt en surfusion sous champ magnétique intense. Les alliages à base Co sont d'excellents candidats pour obtenir une surfusion en dessous ou proche du point de Curie sous champ intense en raison du faible écart entre ce point de Curie et la température du liquidus. Dans cette étude, un dispositif haute température de surfusion intégrant une mesure magnétique a été construit dans un aimant supraconducteur, et est utilisé pour la mesure in situ de l'aimantation de liquides surfondus et pour l'étude du sur-refroidissement et de l'évolution de la microstructure de solidification en champ intense. Le cobalt liquide en surfusion est fortement magnétique sous champ, et son aimantation est même supérieure à celle du solide au chauffage à la même température. L'aimantation de l'alliage proche eutectique Co-B en surfusion dépend de la température de surchauffe, tandis que le Co-Sn en surfusion est toujours paramagnétique. La surfusion moyenne et l'étendue de la recalescence de différents métaux et alliages est affectée par un champ externe. En champ magnétique uniforme, la surfusion du Cuivre est amplifiée, tandis que la surfusion du Cobalt et de Co-Sn reste identique. Cependant, l'étendue de la recalescence du Cobalt et de Co-Sn est réduite, et l'effet est d'autant plus important pour des teneurs supérieures en Cobalt. Le champ magnétique promeut la précipitation de la phase dendritique a-Co et la formation d'eutectique anormal dans la microstructure des alliages Co-Sn surfondus. Les processus d'évolution de la microstructure sont affectés par le champ magnétique, et dépendent de l'intensité du champ et de la surfusion. Ce travail offre de nouveaux horizons dans l'étude des propriétés magnétiques d'alliages métalliques en forte surfusion et dans l'étude de la solidification hors équilibre sous champ magnétique intense. / This work is devoted to the investigation of the magnetic field effect on the magnetic properties and solidification behavior of undercooled Co based alloys in high magnetic field. Co based alloys are promising candidates to be undercooled below or approaching their Curie point in strong magnetic field due to their small temperature difference between liquid line and Curie point. In this dissertation, a high temperature undercooling facility with magnetization measurement system is built in a superconducting magnet, and is used for in situ measurement of the magnetization of the undercooled melts and study the undercoolability and solidification microstructure evolution in magnetic field. The deep undercooled Co melt is strongly magnetized in magnetic fields, and its magnetization is even larger than the magnetization of heated solid at the same temperature. The magnetization of undercooled Co-B near eutectic alloy is related with overheating temperature while the undercooled Co-Sn melt is always in paramagnetic state. Mean undercooling and recalescence extent of different metals and alloys are affected by external field. In uniform magnetic field, the undercooling of Cu is enhanced while the undercoolings of Co and Co-Sn keep constant. However, the recalescence extents of Co and Co-Sn alloys are reduced, and with the increasing Co content, the effect becomes larger. Magnetic field promotes the precipitation of αCo dendrite phase and the formation of anomalous eutectics in solidified microstructure of undercooled Co-Sn alloys. The microstructure evolution processes are affected by magnetic field depending on the field intensity and undercooling. This work opens a new way to investigate the magnetic properties of deeply undercooled metallic melts and non-equilibrium solidification in strong magnetic fields.

Dans ce travail, différent systèmes ont été étudiés par des méthodes de microscopie électronique en transmission (TEM) : nanoparticules (NPs) de Pt sur du carbone amorphe, boîtes quantiques (QDs) de Ge, l'incorporation du Mn dans les QDs de Ge and des nanocolonnes (NCs) GeMn dans une matrice de Ge pure ou de GeSn. Le mûrissement de NPs de Pt sur un film de carbone amorphe a été étudié par TEM haute résolution (HRTEM) après des recuits à des températures comprises entre 200 °C et 300 °C pour des durées allant jusqu'à 160 h. Une augmentation significative de la taille moyenne des particules est observé en augmentation la durée du recuit pour toutes les températures étudiées. Une expérience de recuit in-situ a révélée deux étapes de mûrissement. La première est dominée par le mûrissement de Smoluchowski tandis que la seconde est dominée par le mûrissement d'Oswald de surface. La dépendance de type Arrhenius du coefficient de transport de masse de surface donne une énergie d'activation de Ed = 0.84 ± 0.08 eV/atome pour la diffusion des atomes de Pt sur un substrat de carbone amorphe. Des méthodes de TEM avancée ont été utilisé pour déterminer directement des profiles de concentration à l'échelle atomique et grand champ de vue par corrélation de signaux de champ sombre annulaire à grand angle (HAADF) et de spectroscopie de perte d'énergie d'électron (EELS). Cette méthode a été appliquée à l'étude de la concentration de Ge à l'échelle atomique dans le system SiGe. Le profile de concentration le long de la direction de croissance est expliqué par la ségrégation de surface des atomes de Ge pendant la croissance avec un modèle d'échange à deux états. L'incorporation de Mn dans les boîtes de Ge a été effectuée par croissance par jets moléculaire (MBE) de GeMn. Des précipités de SiMn sont formés pour des températures de croissance de 380 °C. La diminution de la température de croissance à 220 °C permet de limiter la ségrégation latérale de Mn et d'incorporer le Mn dans les QDs de Ge. Les compositions chimiques absolues obtenues par STEM-EELS prouvent que la densité atomique totale dans les NCs de GeMn est presque deux fois supérieure par rapport à la matrice de Ge. Des études structurales par HRTEM montrent les NCs cristallines sont très désordonnées. Les observations expérimentales peuvent être modélisées par une structure de phase α modifié, si des variants sont introduits pour annuler des réflexions de Bragg et des atomes de Ge sont substitués par des atomes de Mn. Les propriétés structurales et magnétiques de films GeSnMn croît par MBE à basse température (LTMBE) ont été étudiées. De manière similaire aux films GeMn, les atomes de Mn diffusent pendant la croissance et s'agrègent pour former des NCs de quelques nanomètres de diamètre, alignées verticalement et riche en Mn. Les observations TEM en vue plane montrent clairement que l'incorporation de Sn n'est pas homogène avec des concentrations en Sn dans les NCs inférieures à la limite de détection de l'EELS. La matrice présente une solution solide tandis qu'une coquille riche en Sn est formée autour des NCs de GeMn. La magnétisation dans les couches de GeSnMn est plus élevée que dans celles de GeMn. L'augmentation du moment magnétique dans les couches de GeSnMn est probablement due à la modification de la structure électronique des atomes de Mn in the NCs par la coquille de Sn. / In this work, different system have been studied using transmission electron microscopy (TEM) methods: Pt nanoparticles (NPs) on amorphous carbon, Ge quantum dots (QDs), Mn incorporation in Ge QDs and GeMn nanocolumns (NCs) embedded in Ge or GeSn matrix. The coarsening of Pt NPs on amorphous carbon film was studied by high resolution TEM (HRTEM) after annealing at temperatures between 200°C and 300°C for periods of up to 160 hours. A significant increase of the average particle size is observed with increasing annealing time for all investigated temperatures. An in-situ annealing experiment reveals two coarsening stages. The first coarsening stage is dominated by Smoluchowski ripening whereas the second coarsening stage is dominated by surface Ostwald ripening. The Arrhenius-type dependence of the derived surface mass-transport coefficients yields an activation energy Ed = 0.84 ± 0.08 eV/atom for the surface diffusion of Pt atoms on an amorphous carbon substrate. Advanced TEM methods have be used to obtain direct determination of composition profiles with atomic resolution and large field of view by correlation of high angle annular dark field (HAADF) and electron energy loss spectroscopy (EELS) signals. This method was used to obtain a direct and precise quantification of Ge concentration at the atomic level for the SiGe system. The Ge concentration profile along the growth direction was explained by Ge surface segregation during the growth with a two-state exchange model. The incorporation of Mn in Ge QDs have been performed by molecular beam epitaxy (MBE) growth of GeMn. At growth temperature of 380°C, SiMn precipitates are formed. Lowering the growth temperature at 220°C allows limiting the lateral segregation of Mn in Ge and incorporating Mn in Ge QDs. Absolute chemical composition by STEM-EELS evidenced that the total atomic density in Ge(Mn) NCs is almost two times higher than in the Ge matrix. Structural analysis by HRTEM shows that the crystalline NCs exhibit a high degree of disorder. Experimental observation can be model with a modified α-phase structure if variants are introduced to cancel reflexions and Ge atoms are substituted by Mn atoms. The structural and magnetic properties of GeSnMn films grown on Ge(001) by low temperature MBE (LTMBE) have been studied. Like in Ge(Mn) films, Mn atoms diffuse during the growth and aggregate into vertically aligned Mn-rich NCs of a few nanometers in diameter. TEM observations in plane view clearly indicate that the Sn incorporation is not uniform with concentration in Mn rich vertical NCs lower than the EELS detection limit. The matrix exhibits a GeSn solid solution while there is a Sn-rich GeSn shell around GeMn NCs. The magnetization in GeSnMn layers is higher than in GeMn films. This magnetic moment enhancement in GeSnMn is probably related to the modification of the electronic structure of Mn atoms in the NCs by the Sn-rich shell, which is formed around the NCs.