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Ce travail étudie le rôle de la densité de trous à l'équilibre sur la dynamique des charges et de la norme de l'aimantation de (Ga,Mn)As pour des densités de manganèse et d'impuretés fixées indépendamment. Des expériences " pompe-sonde " mettent en relation les dynamiques de réflectivité et d'angle de rotation Kerr. Deux relaxations sont mises en évidence. La première traduit un échauffement variable du gaz de trous entre 1ps et 100ps. La seconde traduit une diffusion-recombinaison des charges entre 100ps et 1500ps et évolue en fonction du rapport entre extension spatiale d'états d'impuretés, piégeant les électrons photo générés, et vitesse de Fermi. Pour compléter l'approche, une étude numérique de l'état fondamental des échantillons par la théorie de la fonctionnelle de la densité relie aimantation, température et densité de trous. Elle interprète la dynamique de la norme de l'aimantation à partir d'un diagramme de phase statique correspondant aux données publiées pour (Ga,Mn)As, qui est fonction de la température et de la densité de trous. Cette dynamique se ramène à celle de la réflectivité. Ceci permet de préciser les contributions de la norme et de l'orientation de l'aimantation dans le signal dynamique de rotation Kerr.

Spin-polarized electron beam has not yet been produced from an unpolarized electron beam using Stern-Gerlach type spin filter, because of the Lorentz force and Heisenberg uncertainty principle. At present, electron spin detectors and filters work on the basis of spin-dependent scattering of an electron beam from crystal surfaces. Single channel efficiencies of all the spin detectors for electrons are orders of magnitudes lower than the ideal one. Specular reflection type spin-polarized low energy electron diffraction (SPLEED)-spin detectors are having higher single channel efficiencies compared to the conventional Mott detectors. Moreover, multichannel detection can be realized from specular reflection type SPLEED-spin detectors. They have higher effective efficiency than the ideal spin detector. In order to develop specular reflection type spin filter, it is important to develop a spin-polarized low energy electron diffraction system. In addition, SPLEED system allows us to study the spin-orbit and exchange scattering at crystal surfaces. The general direction of the thesis has been the development of spin-polarized low energy electron diffraction (SPLEED) system. This system has been used to investigate the spin-orbit interactions on Ir(100) surface and exchange interactions of Fe grown on Ir(100). The thesis is organized into chapters as follows. Chapter 1 introduce the reader to some of the basic concepts of polarized electrons and the evolution of spin-polarized electron sources and detectors. Sources of polarized electrons are discussed with emphasis on photocathodes such as GaAs and strained GaAs. Widely used spin detector is the Mott detector which works in the higher energy range. The working principle of the Mott detector is discussed. Commonly used spin detector in the lower energy range is the LEED detector. The concept of the LEED detector is also discussed. Working principle and recent developments of specular reflection type SPLEED spin filters are introduced. Evolution of electron spin detector is discussed towards the end of the chapter. Chapter 2 discusses about the two instruments designed and developed during the course of the thesis. The first one is a spin-polarized low energy electron diffraction system working in the reflected electron pulse counting mode in UHV. This system is capable of measuring spin asymmetries due to spin-orbit interaction and exchange interaction. This instrument is useful in understanding structure and magnetism at surfaces as well as helps to develop new spin polarimeter based on SPLEED by evaluating spin asymmetries from different surfaces. All instruments connected to SPLEED system, measurement protocol and controlling software are discussed with some details. Along with this, standard characterization tools such as X-ray diffraction and magneto-optic Kerr effect measurements are discussed. The second instrument is a novel quadratic magneto-optic Kerr effect measurement system using permanent magnets, which is simple, compact and cost-effective. We have used rotating field method to extract QMOKE component in saturation. So there is no need for precise real-time measurement of magnitude and direction of the magnetic field as in the case of vector magnet. This instrument can easily quantify QMOKE coefficients for ferrimagnetic and ferromagnetic thin films and single crystals. Chapter 3 discusses SPLEED experiments carried out on Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. The surface structure and surface preparation techniques are discussed. The stability of the Ir(100)-(1×5)-Hex surface is evaluated by monitoring the spin asymmetry as the function of time. Within 25 hours after the surface preparation, the profile of the spin asymmetry and the reflected electron count for Ir(100)-(1×5)-Hex surface resembles that of hydrogen adsorbed Ir(100)-(1×5)-H surface. The electron energy-angle of incidence landscape of reflectivity, spin asymmetry and figure of merit are recorded for Ir(100)-(1×2+2×1)-O surface. Many wide regions with a large figure of merit are identified in the E- landscape. Chapter 4 reports SPLEED experiments carried out on Ir(100)-(1×5)-H surface. The comparison between asymmetries evaluated for the Ir(100)-(1×5)-Hex surface after 25 hours and Ir(100)-(1×5)-H surface suggests that Ir(100)-(1×5)-Hex surface is transforming to Ir(100)-(1×5)-H surface, in 25 hours. This can be due to the adsorption of more than four Langmuir of residual hydrogen during this time. The energy-angle landscape of reflectivity, asymmetry and figure of merit are recorded for Ir(100)-(1×5)-H surface in an energy range 20 eV to 100 eV and angle range 10 to 60 . Many regions are identified as the working point for specular reflection type spin filter based on SPLEED. The surface structure and surface preparation techniques are discussed. The stability of the surface is also evaluated. Chapter 5 investigates the growth and magnetic properties of Fe(100) film on Ir(100)-(1×1), Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. LEED, MEED, LMOKE and QMOKE studies were presented. The growth is found to be layer-by-layer at least up to 20 monolayers (ML) at room temperature. At higher deposition temperature, the MEED oscillations disappear around 3-5 ML. Magnetic anisotropy of the Fe(100) film grown on Ir(100)-(1×2+2×1)-O surfaces is evaluated using LMOKE measurement using Kerr microscope. Simultaneous in-situ LMOKE and MEED measurements were carried out during the deposition. Ferromagnetic ordering with an in-plane easy axis starts above 4.5 ML at room temperature. The Kerr rotation normalized by thickness is evaluated in the pseudomorphic regime and strain relaxed regime. The probing depth of the MOKE is found to be around 14 nm in Fe(100)/Ir(100). An antisymmetric component is observed in the re-magnetization loop measured using MOKE. This antisymmetric loop arises due to the quadratic magneto-optic coupling which is separated by symmetrization and antisymmetrization procedure. The observed quadratic magneto-optic coupling suggests that the analysis based on the assumption that the magneto-optic coupling is linear in magnetization has to be modified. In order to quantify the quadratic magneto-optic coupling parameters, a QMOKE measurement system is developed and measurements were carried out. Chapter 6 discusses SPLEED experiments carried out on various thicknesses of Fe(100) film. Fe(100) films grown on Ir(100) substrate with the thickness less than or equal to 4 ML is not ferromagnetic with in-plane easy axis at room temperature. The non-zero exchange asymmetry observed for 5 ML and above indicates the presence of ferromagnetic ordering. A difference in the profile of exchange asymmetry is observed between pseudomorphic and strain relaxed regime. Large spin-orbit asymmetry is observed for 1 ML and 2 ML Fe(100) which is unexpected from a low atomic number (Z) material. The reason for large spin-orbit asymmetry is still unknown. The energy-angle landscape of reflectivity, exchange asymmetry, spin-orbit asymmetry and figure of merit were evaluated for 21 ML of Fe(100). Many working points were identified for different types multichannel spin filter based on exchange interaction Finally, the various results are summarized and a broad outlook is given.

This thesis presents HELIOS, an in-house laboratory for time-resolved pump-probe spectroscopy with extreme-ultraviolet (XUV) probe radiation. A wide span of pump wavelengths can be generated using commercial laser equipment while XUV probe radiation is generated via a high harmonic generation process in a noble gas delivering probe photons with energies between 20 eV and 72 eV. The XUV beam path features a time-preserving monochromator and was constructed and built in-house. HELIOS features an overall time resolution of about 50 fs when using 800 nm pump and 41 eV probe photons. An energy resolution of 110 meV at 41 eV photon energy can be achieved. HELIOS features two beamlines. One µ-focus beamline with an XUV focal size of about 20 µm can be used with experiments that require such a small XUV focal size as well as with different end stations. The other beamline features a semi-permanently mounted end station for angle-resolved photoelectron spectroscopy under ultra-high vacuum conditions. Experiments demonstrating the usability of HELIOS and the two beamlines are presented. A pump-probe measurement on graphene demonstrates the capability of determining a large part of the k-space in only one measurement due to the use of an ARTOF angle-resolved time-of-flight electron spectrometer. A non-angle-resolved pump-probe measurement on the conducting polymer PCPDTBT demonstrates the high signal-to-noise ratio achievable at this beamline in non-angle-resolved photoelectron-spectroscopy pump-probe measurements. The usability of the µ-focus beamline is demonstrated with time-resolved measurements on magnetic samples employing an in-house-designed spectrometer. These experiments allow the retrieval of element-specific information on the magnetization within a sample employing the transversal magneto-optical Kerr effect (T-MOKE). Additionally, a Fourier transform spectrometer for the XUV is presented, the concept was tested at a synchrotron and it was used to determine the longitudinal coherence of the XUV radiation at HELIOS.