Materialintegration von Halbleitern mit magnetischen Werkstoffen

This work investigates the growth and characterization of magnetic MnAs layers on semi-insulating GaAs substrates. The single-crystalline MnAs layers are deposited on GaAs by molecular beam epitaxy. A detailed analysis of the MnAs growth reveals the influence of the growth conditions on the structural quality of the layers. A phase diagram showing four stoichiometry dependent reconstructions is obtained by a systematic analysis of the MnAs surface structure. The nucleation of MnAs during growth of the first monolayers is examined by in-situ reflection electron diffraction and reflectance difference spectroscopy measurements. The structural properties of the MnAs layers are analyzed by transmission electron microscopy. It is found that the lattice mismatch is accommodated at the MnAs/GaAs interface by a coincidence lattice along one direction and by misfit dislocation along the perpendicular direction. The discussion of the magnetic properties covers the macroscopic as well as the microscopic scales. Magnetometry results provide important knowledge about thin film magnetism in MnAs layers. Extensive magnetic force microscopy investigations of the magnetic domains in MnAs illustrate the complicated relationship between surface topography and magnetic structure. Imaging of magnetic domains as a function of the applied magnetic field gives new insights on magnetization behavior on a microscopic scale. The MnAs phase transition at 43 C is investigated by combined X-ray diffraction and magnetization measurements. The MnAs structure changes from the ferromagnetic to the paramagnetic state, accompanied by a crystal structure change from hexagonal to orthorhombic. A detailed analysis of the phase transition provides important information concerning crystal growth and allows optimization of the fabrication conditions.Available from: http://dochost.rz.hu-berlin.de/dissertationen/schippan-frank-2000-12-01/PDF/Schi ppan.pdf / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Elastic and magnetic properties of epitaxial MnAs layers on GaAs

We have investigated the elasto- and magneto-optical properties of MnAs layers epitaxially grown on (001) GaAs for temperatures around the structural (hexagonal/orthorhombic) and magnetic (ferromagnetic/paramagnetic) phase transition of MnAs at T(c)similar to40 degreesC. The phase transition is accompanied by a large variation of the MnAs lattice parameter a of similar to1%, which induces a strong and anisotropic strain field in the MnAs/GaAs heterostructures. The latter was measured by detecting the optical anisotropy induced on the GaAs substrate by means of polarization-sensitive light transmission measurements. The experimental results show clear evidence for the quasi-uniaxial strain induced on the GaAs substrate during the phase transition, which extends over a temperature range of similar to30 degreesC in the MnAs/GaAs heterostructures. The strain levels are well reproduced by an elastic model for the heterostructures which assumes that the strain is transferred across the MnAs/GaAs interface without relaxation. The elastic properties during the phase transition were compared to the average magnetization probed using a SQUID magnetometer and to the magnetization near the front and the back surfaces of the MnAs films detected using the magneto-optical Kerr effect. The smaller temperature range of the phase transition observed in the magneto-optical Kerr effect measurements indicates a lower stability of the ferromagnetic phase near the surface of the MnAs layers.652

Spin-polarized Zener tunneling in (Ga,Mn)As

We investigate spin-polarized inter-band tunneling through measurement of (Ga,Mn)As based Zener tunnel diode. By placing the diode under reverse bias, electron spin polarization is transferred from the valence band of p-type (Ga,Mn)As to the conduction band of an adjacent n-GaAs layer. The resulting current is monitored by injection into a quantum well light emitting diode whose electroluminescence polarization is found to track the magnetization of the (Ga,Mn)As layer as a function of both temperature and magnetic field.Comment: 11 pages, 4 figures. Submitted, Physical Review B15 Rapid Communication

Point Contact Spin Spectroscopy of Ferromagnetic MnAs Epitaxial Films

We use point contact Andreev reflection spin spectroscopy to measure the transport spin polarization of MnAs epitaxial films grown on (001) GaAs. By analyzing both the temperature dependence of the contact resistance and the phonon spectra of lead acquired simultaneously with the spin polarization measurements, we demonstrate that all the point contacts are in the ballistic limit. A ballistic transport spin polarization of approximately 49% and 44% is obtained for the type A and type B orientations of MnAs, respectively. These measurements are consistent with our density functional calculations, and with recent observations of a large tunnel magnetoresistance in MnAs/AlAs/(Ga,Mn)As tunnel junctions.Comment: 5 Figure

Umgegend V. Dem Amalgamirwerke U. Der Schmelzhütte Auf Halsbrücke

UMGEGEND V. DEM AMALGAMIRWERKE U. DER SCHMELZHÜTTE AUF HALSBRÜCKE Umgegend V. Dem Amalgamirwerke U. Der Schmelzhütte Auf Halsbrücke ( -

Religious influences on human capital variations in imperial Russia

Historical legacies, particularly imperial tutelage and religion, have featured prominently in recent scholarship on political regime variations in post-communist settings, challenging earlier temporally proximate explanations. The overlap between tutelage, geography, and religion has complicated the uncovering of the spatially uneven effects of the various legacies. The author addresses this challenge by conducting sub-national analysis of religious influences within one imperial domain, Russia. In particular, the paper traces how European settlement in imperial Russia has had a bearing on human development in the imperial periphery. The causal mechanism that the paper proposes to account for this influence is the Western communities’ impact on literacy, which is in turn linked in the analysis to the Western Christian, particularly Protestant, roots, of settler populations. The author makes this case by constructing an original dataset based on sub-national data from the hitherto underutilised first imperial census of 1897