The aim of the study, which was carried out in two main parts, was to investigate the atomic and magnetic ordering in various full Heusler alloy systems.In the theoretical part of the thesis, investigations based on the statiscothermodynamical theory of ordering by means of Bragg-Williams-Gorsky (BWG) method combined with electronic theory in the pseudopotential approximation have been conducted to model order-order (L21&amp / #8596 / B2) and order-disorder (B2&amp / #8596 / A2) phase transformations in Ni-Mn-C (C=Ga, In, Sb) and A2BGa (A=Fe, Ni, Co / B=Ni, Mn, Co, Fe) Heusler alloys. The partial ordering energies, calculated according to the electronic theory of alloys in pseudopotential approximation for the first two coordination spheres were utilized as input data for the theoretical superlattice formation models based on BWG approximation. Furthermore, the trends of L21&amp / #8596 / B2 transition temperatures with electron concentration at A, B and C atomic sites were determined. In the experimental part of the thesis, structural and magnetic properties of Ni-Mn-Ga and Ni-Mn-Al Heusler alloys were investigated. For the Ni-Mn-Ga Heusler alloy system, it was found that the martensitic (TM) and Curie temperatures (TC) merge for Ni54Mn20Ga26 and Ni56Mn18Ga26 alloys by compositional tuning. For the Ni-Mn-Al Heusler alloy system, it was found that ferromagnetism was introduced into these alloys by low temperature aging, as a result of the stabilization of the L21 phase. In addition to that, magnetocaloric effect (MCE) was determined in the vicinity of the first order magneto-structural transition for Ni-rich Ni-Mn-Ga alloys and near the second order magnetic transition for the Ni2MnAl alloy.

Multilayer structures of AlxIn1-xAsySb1-y/GaSb (0.37 £ x £ 0.43, 0.50 £ y £ 0.52), grown by molecular beam epitaxy on GaSb (100) substrates were characterized using variable temperature Hall and Shubnikov-de Haas techniques. For nominally undoped structures both p and n-type conductivity was observed. The mobilities obtained were lower than those predicted by an interpolation method using the binary alloys; therefore, a detailed analysis of mobility versus temperature data was performed to extract the appropriate scattering mechanisms. For p-type samples, the dominant mechanism was ionized impurity scattering at low temperatures and polar optical phonon scattering at higher temperatures. For n-type samples, ionized impurity scattering was predominant at low temperatures, and electron-hole scattering dominated for both the intermediate and high temperature range. Analyses of the Shubnikov-de Haas data indicate the presence of 2-D carrier confinement consistent with energy subbands in GaAszSb1-z potential wells. Epilayers of GaAs1-xSbx (0.19<x<0.71), grown by MBE on semi-insulating GaAs with various substrate orientations, were studied by absorption measurements over the temperature range of 4-300 K. The various substrate orientations were chosen to induce different degrees of spontaneous atomic ordering. The temperature dependence of the energy gap (Eg) for each of these samples was modeled using three semi-empirical relationships. The resulting coefficients for each model describe not only the temperature dependence of Eg for each of the alloy compositions investigated, but also for all published results for this alloy system. The effect of ordering in these samples was manifested by a deviation of the value of Eg from the value of the random alloy. The presence of CuPt-B type atomic ordering was verified by transmission electron diffraction measurements, and the order parameter was estimated for all the samples investigated and found to be larger for the samples grown on the (111) A offcut orientations. This result strongly suggests that it is the A steps that contribute to the formation of the CuPt-B type ordering in the GaAs1-xSbx alloy system.

Ni-based superalloys are the important materials for gas turbines in advancedaeroplane engines . The addition of refractory elements to these superalloys,such as rhenium and tungsten, can significantly improve the hightemperatureperformance by so-called solid-solution hardening. Although thestrengthening effect of refractory elements in Ni-based superalloys have beenknown for a long time, the effective interactions among alloying componentsas well as the atomic ordering in the alloy systems are still under investigationand even under debate. In this work, we study these interactions and thisordering for two binary alloys, Ni-rich Ni-Re and Ni-rich Ni-W, by means ofab initio simulations and statistical mechanics simulations based on the IsingHamiltonian. For the Ni-rich Ni-Re alloys, we show that the effective cluster interactionsvary substantially depending on the temperature, concentration of the componentsand the magnetic state of the matrix. The strain-induced interactionshave large contribution to the nearest-neighbor pair-interactions and some multisitecluster-interactions in the ferromagnetic and nonmagnetic states. Theordering tendency of binary Ni-Re alloy systems can be predicted in terms ofordering energy and enthalpy of formation. We show that the D1a orderedstructure should be stable at the concentration of 20 at.% Re in the Ni-rich Ni–Re alloy system. The Monte Carlo simulations of Ni-Re random alloysshow the existence with the D1a-Ni4Re ordered structure at low temperatures. We also calculated lattice parameters for different compositions of Ni-rich Ni-W alloys, and we find that lattice parameters of random Ni-W alloys increaselinearly with the concentration of W. This is in good agreement withthe Vegard’s law predictions and experimental data. We investigated phasestability of Ni-rich Ni-W alloys in terms of the enthalpies of formation andordering energies. We find the chemical pair interactions are sensitive to themagnetic state and concentration. The calculated strain-induced interactionsare quite large for the first coordination shell, which is due to a large sizemismatch of Ni and W. Taking local lattice relaxation into account, the Ni-Wsystems were modeled by Monte Carlo method. The D1a-Ni4W ordered structurecan be observed up to 22 at.% W. In higher concentrations of W, in ourMC calculations, the DO22-Ni3W and Pt2Mo-Ni2W ordered structures can beobserved in Ni-25 at.% W alloy and Ni-33 at.% W alloy, respectively. / <p>QC 20160721</p>

Neste trabalho foram utilizadas técnicas de difração de raios-x para estudar propriedades químicas e estruturais de ilhas de Ge:Si(001). Através de experimentos de difração por incidência rasante foi realizado um mapeamento estrutural da relaxação de strain dentro de pirâmides e domos de Ge. Alterando-se a energia dos raios-x próximo à borda K do Ge – em medidas de difração anômala – foi possível determinar a composição química dos dois tipos de ilhas. A energia elástica, obtida correlacionando-se estes dois resultados, provou ser um dos fatores responsáveis pelas transições morfológicas neste sistema. Uma extensão dos resultados, com o uso de um novo método de análise, permitiu um completo mapeamento tri-dimensional da estrutura e estequiometria dos domos de Ge. Por último, foi observada a existência de uma liga ordenada de SiGe dentro dos domos, indicando o importante papel da cinética de crescimento na incorporação de Si nas ilhas.

In dieser Arbeit werden das Wachstum mittels Molekularstrahlepitaxie und die Eigenschaften der Ferromagneten Fe und Fe_3Si auf niedrig-symmetirschen GaAs(113)A-Substraten studiert. Drei wichtige Aspekte werden untersucht: (i) Wachstum und strukturelle Charakterisierung, (ii) magnetische Eigenschaften und (iii) Magnetotransporteigenschaften der Fe und Fe_3Si Schichten auf GaAs(113)A-Substraten. Das Wachstum der Fe- und Fe_3Si-Schichten wurde bei einer Wachstumstemperatur von = bzw. 250 °C optimiert. Bei diesen Wachstumstemperaturen zeigen die Schichten eine hohe Kristallperfektion und glatte Grenz- und Oberflächen analog zu [001]-orientierten Schichten. Weiterhin wurde die Stabilität der Fe_(3+x)Si_(1-x) Phase über einen weiten Kompositionsbereich innerhalb der Fe_3Si-Stoichiometry demonstriert. Die Abhängigkeit der magnetischen Anisotropie innerhalb der Schichtebene von der Schichtdicke weist zwei Bereiche auf: einen Beresich mit dominanter uniaxialer Anisotropie für Fe-Schichten = 70 MLs. Weiterhin wird eine magnetische Anisotropie senkrecht zur Schichtebene in sehr dünnen Schichten gefunden. Der Grenzflächenbeitrag sowohl der uniaxialen als auch der senkrechten Anisotropiekonstanten, die aus der Dickenabhängigkeit bestimmt wurden, sind unabhängig von der [113]-Orientierung und eine inhärente Eigenschaft der Fe/GaAs-Grenzfläche. Die anisotrope Bindungskonfiguration zwischen den Fe und den As- oder Ga-Atomen an der Grenzfläche wird als Ursache für die uniaxiale magnetische Anisotropie betrachtet. Die magnetische Anisotropie der Fe_3Si-Schichten auf GaAs(113)A-Substraten zeigt ein komplexe Abhängigkeit von der Wachstumsbedingungen und der Komposition der Schichten. In den Magnetotransportuntersuchungen tritt sowohl in Fe(113)- als auch in Fe_3Si(113)-Schichten eine antisymmetrische Komponente (ASC) im planaren Hall-Effekt (PHE) auf. Ein phänomenologisches Modell, dass auf der Kristallsymmetrie basiert, liefert ein gute Beschreibung sowohl der ASC im PHE als auch des symmetrischen, anisotropen Magnetowiderstandes. Das Modell zeigt, dass die beobachtete ASC als Hall-Effekt zweiter Ordnung beschreiben werden kann. / In this work, the molecular-beam epitaxial growth and properties of ferromagnets, namely Fe and Fe_3Si are studied on low-symmetric GaAs(113)A substrates. Three important aspects are investigated: (i) growth and structural characterization, (ii) magnetic properties, and (iii) magnetotransport properties of Fe and Fe_3Si films on GaAs(113)A substrates. The growth of Fe and Fe_3Si films is optimized at growth temperatures of 0 and 250 degree Celsius, respectively, where the layers exhibit high crystal quality and a smooth interface/surface similar to the [001]-oriented films. The stability of Fe_(3+x)Si_(1-x) phase over a range of composition around the Fe_3Si stoichiometry is also demonstrated. The evolution of the in-plane magnetic anisotropy with film thickness exhibits two regions: a uniaxial magnetic anisotropy (UMA) for Fe film thicknesses = 70 MLs. The existence of an out-of-plane perpendicular magnetic anisotropy is also detected in ultrathin Fe films. The interfacial contribution of both the uniaxial and the perpendicular anisotropy constants, derived from the thickness-dependent study, are found to be independent of the [113] orientation and are hence an inherent property of the Fe/GaAs interface. The origin of the UMA is attributed to anisotropic bonding between Fe and As or Ga at the interface, similarly to Fe/GaAs(001). The magnetic anisotropy in Fe_3Si on GaAs(113)A exhibits a complex dependence on the growth conditions and composition. Magnetotransport measurements of both Fe(113) and Fe_3Si(113) films shows the striking appearance of an antisymmetric component (ASC) in the planar Hall effect (PHE). A phenomenological model based on the symmetry of the crystal provides a good explanation to both the ASC in the PHE as well as the symmetric anisotropic magnetoresistance. The model shows that the observed ASC component can be ascribed to a second-order Hall effect.