No description available.

No description available.

With growing interest in devices that utilize the spin degree of freedom of the charge carriers, there is an extensive research effort into materials with high spin polarization. Two types of materials that have attracted particular attention are the half metallic (HM) ferromagnets and dilute magnetic semiconductors (DMS). I report on a series of experiments which probe the level spin polarization in HM CrO2 and the DMS Ga1−xMnxAs. In order to accurately determine the spin polarization, P, of CrO2 in a realistic device structure I have developed a method to chemically modify the surface of CrO2 to obtain a consistent and reproducible barrier, which preserves the bulk spin polarization. Using this method I have been able to produce high quality CrO2 based planar junctions with either superconducting (SC) or ferromagnetic (FM) counter electrodes. Analysis of both zero field and Zeeman split conductance data from CrO2-SC junctions consistently yield P values close to 100%, providing unambiguous evidence that the high P of CrO2 is maintained at and across an artificial barrier in a realistic device structure. Magnetic tunnel junctions (MTJ) fabricated with CrO2 and Co electrodes display a low field inverse magneto-resistance with a maximum magneto-resistance (MR) of -24% occurring at 5K. The origin of this inverse sign is discussed in terms of selective spin transport due to variations in the type of interfacial bonding between the electrodes and the barrier. A strong linear bias dependence of the MR, similar to what is seen in the CrO2-SC junctions, is observed. This linear background is attributed to a continuum of inelastic states in the barrier region. Measurements of the MR as a function of temperature display a rapid decrease in MR as temperature increases. Additionally we have carried out the first direct measurement of the degree of spin polarization of the magnetic semiconductor Ga1−xMnxAs using Andreev reflection spectroscopy. Analysis of the conductance spectra of high transparency Ga0.95Mn0.05As/Ga junctions consistently yields an intrinsic value for P greater than 85%. Our experiments also revealed an extreme sensitivity of the measured spin polarization to the nature and quality of the interface for this material. / Dissertation / PhD

<p>Localization and magnetic properties of electrons in a thin, cyclic quasi one-dimensional GaAs wire with a central potential barrier were studied using the Hartree-Fock and LSDA (Local Spin Density Approximation, exchange only) and compared to more time consuming Quantum Monte-Carlo calculations. Within LSDA, evidence of true localization was found as well as evidence for the existence of both ferromagnetic as well as anti-ferromagnetic states. Also signs of two-dimensional spin localization was found, without associated localized electrons.</p>

Localization and magnetic properties of electrons in a thin, cyclic quasi one-dimensional GaAs wire with a central potential barrier were studied using the Hartree-Fock and LSDA (Local Spin Density Approximation, exchange only) and compared to more time consuming Quantum Monte-Carlo calculations. Within LSDA, evidence of true localization was found as well as evidence for the existence of both ferromagnetic as well as anti-ferromagnetic states. Also signs of two-dimensional spin localization was found, without associated localized electrons.

No description available.

No description available.

Mesoscopic devices, such as nano-wires, are of interest for the next step in creating spintronic devices. With the ability to manipulate electrons and their spin, spintronic devices may be realised. To that end the different effects found in low-dimensional devices must be studied and understood. In this thesis  the influence that lateral spin-orbit coupling (LSOC) has on a nanowire, with asymmetrical confinement potential, is studied. The nanowire is studied through a numerical approach, using the Hartree-Fock method with Dirac interactions to solve the eigenvalue problem of an idealised infinite nanowire. The nanowire has a split-gate that generates the electrostatic asymmetrical confinement potential. It is found that the lateral spin-orbit coupling has little to no effect without any longitudinal effects in the wire, such as source-drain bias. The electrons will spontaneously create spin-rows in the device due to spin polarization. The spin polarization is triggered by using LSOC, numerical noise or from a weak magnetic field. / Mesoskopiska anordningar, som nano-trådar, tros vara ett viktigt steg för att skapa spinnelektronik. För att kunna skapa spinnelektronik behövs kunskap om hur elektroner kan manipuleras. Generellt måste därför existerande fenomen i nanoelektronik studeras. I denna avhandling studeras hur ''lateral spin-orbit koppling'' (LSOC) influerar en nanotråd som har en asymmetrisk potentialbarriär. Hartree-Fock metoden, med Dirac potential för elektron-elektron interaktioner, användes för att beräkna energinivåerna för en idealisk, oändligt lång nanotråd. Nanotråden har en split-gate som alstrar den elektrostatiska, asymmetriska potentialbarriären. "Lateral spin-orbit koppling" visar sig ha minimal effekt då longitudinella effekter, exempelvis spänning, saknas. Elektronerna placerar sig spontant i spinn-rader i tråden vid spontan spinn polarisation. Spinn polarisationen sätts igång av LSOC, numeriska störningar eller från svagt pålagt magnetfält.

La magnétite (Fe3O4) est un candidat prometteur pour des applications dans des dispositifs en spintronique. Ce ferrimagnétique avec une température de Curie élevée a été théoriquement prévu pour être un demi-métal avec un canal conducteur pour les spins minoritaire et un semi-conducteur pour les spins majoritaires, résultant en 100% de polarisation en spin au niveau de Fermi. Cependant, jusqu'à présent, aucune preuve expérimentale claire sur ce sujet n'a été faite. Cette thèse présente des études en photoémission résolue en angle et en spin sur la structure électronique et magnétique de couches minces de Fe3O4 (001) épitaxiées sur MgO(001) . Un calcul de la structure de bande utilisant l'approximation du gradient généralisé (GGA + U) est proposé pour expliquer les résultats expérimentaux. Bien que l'intensité de photémission au niveau de Fermi soit très faible en raison du rôle joué par les polarons, une dispersion de la bande Fe 3d-t2g est observée. Le comportement global de cette bande est en bon accord avec le calcul de la structure état électronique représentant état fondamental. Pour simuler les spectres de photoémission, nous avons utilisé l'approximation de l'électron libre à l'état final, tout en ignorant les éléments de matrice de la transition électronique. Dans la simulation, les bandes calculées sont convoluées respectivement par la lorentzienne et la gaussien pour tenir compte de la durée de vie et des effets de couplage électron-phonon. En intégrant l'intensité spectrale sur un intervalle d'énergie de 100 MeV au niveau de Fermi, nous avons obtenu la première preuve expérimentale de la surface de Fermi. Détermination de la polarisation de spin des électrons est un test ultime des calculs de bandes et des spectres de photoémission modélisés. Dans nos expériences de photoémission résolue en spin, nous avons utilisé des photons de 4.65 et 6.20 eV. Le même échantillon comme pour la photoémission intégré en spin a été mesuré, nécessitant son transfert par l'air dans une autre chambre. L'échantillons n'a pas été soumis à un nettoyage avant les mesures résolues en spin ce a conduit à une réduction de la polarisation en spin à cause de la présence d'une couche polluée sur la surface. Néanmoins, une polarisation de spin de - 50% et -72 % a été mesurée au voisinage de EF respectivement pour les photons de 6.20 et de 4.65 eV. Nous en concluons que Fe3O4 peuvent être décrits par un modèle de bande et en particulier qu'il est demi-métallique. Nous avons également utilisé des impulsions femtoseconde laser dans une expérience pompe-sonde pour étudier la dynamique ultra-rapide à l'échelle atomique. Nos résultats montrent que la durée de vie des électrons excités dans Fe3O4 est beaucoup plus longue que dans un métal «ordinaire». L'analyse de la polarisation en spin des électrons excités montre que la désaimantation ne se produit pas dans le domaine de la femtoseconde, ce qui est compatible avec des propriétés demi-métalliques de la magnétite . / Magnetite (Fe3O4) is a promising candidate for application in spintronic devices. This ferrimagnet with a high Curie temperature has been theoretically predicted to be a half-metal with a conductive minority-spin (↓) channel and a semiconductive majority-spin (↑) channel, resulting in 100 % spin polarization at the Fermi level. But up to now, any clear experimental evidence is lacking. This thesis presents spin- and angle-resolved photoemission studies on the magnetic and electronic structure of Fe3O4 (001) epitaxially grown on MgO (001). A band structure calculation using generalized gradient approximation plus U (GGA+U) to the density functional theory (DFT) is proposed to explain the experimental results. Although the PES intensity at Fermi level is very low because of the role played by polarons, a dispersion of the Fe 3d-t2g states is observed. The overall behaviour of these bands is in good agreement with the calculation of ground state electronic structure. In order to simulate the spectra, we used the free electron approximation for the final states, ignoring the matrix elements. Calculated ground state data are convoluted by Lorentzian and Gaussian functions to account for the lifetime and electron-phonon coupling effects, respectively. By integrating the spectral intensity over an energy interval of 100meV at Fermi level, we obtained the first experimental evidence of the Fermi surface plot. Determination of electron spin polarization is an ultimate test of both the band calculations and our model of Fe3O4 photoemission spectra. In our spin-resolved photoemission experiments 4.65 and 6.20 eV photons were used. The same sample was used as for spin-integrated ARPES, requiring its transfer through air to another chamber. It was not subject to any cleaning prior to the SRPES measurements that lead to a reduction of the spin polarization as a consequence of the presence of a dead layer on the surface. Nevertheless the spin polarization close to EF reaches - 50% and -72% for 6.20 and 4.65 eV photons respectively. We conclude that Fe3O4 can be described within a band model and in particular that it is half-metallic. We also used femtosecond laser pulses in pump-probe experiments to investigate ultrafast dynamics on atomic scale. Our results show that the lifetime of excited electrons in Fe3O4 is much longer than in an “ordinary” metal. From the spin analysis of excited electrons, we deduce that the demagnetization does not occur in the femtosecond range, which is compatible with half-metallic properties of magnetite.</dcterms:abstract> <dc:type xsi:type="dcterms:DCMIType"

The main topic of this project is the so called Edelstein effect. This recently discovered effect consists in the possibility of converting an electric field (a current) into a magnetization in materials that fulfill specific characteristics, more specifically materials where an effective Rashba spin-orbit coupling is present. The Edelstein effect is appealing to the scientific community from the fundamental physics point of view as well as from the technological point of view. In fact the possibility of efficiently converting an electric signal into a magnetic signal could revolutionize the current information storage technology. In this project, after a study of basic concepts of solid state physics: crystal structure, Bloch's theorem, spin-orbit coupling; we addressed the study of the basics of a powerful numerical tool, called density functional theory (DFT), for predicting the electronic properties of solids. This tool provides us with all the needed quantities for numerically calculating any kind of linear response, which we show that the Edelstein effect is a specific form of. Using a specific implementation of DFT, called augmented spherical wave (ASW), we calculate the Edelstein effect in iron and copper (where no effect is expected) and manganese silicide (where the effect is expected to appear). We also perform a systematic study on how the Edelstein effect depends on the symmetry of the material and the magnitude of the spin-orbit coupling. The calculations showed promising results from which we concluded that the numerical methods used could clearly distinguish between the presence of the Edelstein effect or not in mentioned materials.