The contactless measurement of the electrical resistivity

Physicists are interested in understanding the processes of nature. Within the field of Solid State Physics the characterisation of materials and the measurement of their properties is the first step towards identifiying new and interesting areas of scientific activities. The electrical resistivity of conductive materials is an important property which provides information about the electronic behaviour of the material. An elegant method to determine this characteristic is the measurement without using electrical contacts. Such a method avoids a whole set of experimental problems connected with the physics of electrical contacts to the sample. This Master-thesis gives an introduction into this experimental technique. A detailed theoretical description is developed. The experimental activity has involved the design, construction and testing of the apparatus. In the process of testing the method novel aspect emerged: The measurement at resonance point. These measurements can yield separate values for two different physical quantities: the electrical resistivity e and the magnetic susceptibility x. The innovation of this project is the simultaneous characterisation of both values for the material under investigation. The report starts by giving the theoretical background within which the first part of the detailed theoretical predictions are discussed. The second part contains experiments and a description of the experimental set-up. This design is the result of a long period of optimisation and testing. The working of the apparatus is demonstrated by the measurement of some samples

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

Domain wall formation and spin reorientation in finite-size magnetic systems

We investigate the formation of stable one-dimensional N\'eel walls in a ferromagnetic slab with finite thickness and finite width. Taking into account the dipolar, the exchange and the uniaxial anisotropic crystalline field interactions, we derive an approximative analytical self-consistent expression that gives the wall width in terms of ratios between the three different energy scales of the problem. We also show that, even when the crystalline anisotropy does not favour the formation of domain walls, they can yet be formed due to the dipolar interaction and the finiteness of the system. Moreover, using a Stoner-Wohlfarth approach, we study the magnetization reorientation inside the domains under the action of an external magnetic field and obtain the respective hysteresis loops, showing that their shapes change from squared to inclined as the width of the slab varies. Finally, we discuss possible applications of this model to describe qualitatively some recent experimental data on thin films of MnAs grown over GaAs substrates.Comment: 11 pages, 10 eps figure

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

Manifesto įpėdinių kalba (Dėl kai kurių dabartinės vokiečių kalbos reiškinių)

Marksas ir Engelsas laikė kalbą priemone, nuo kurios pritaikymo priklauso minčių poveikis masėms.20-ame amžiuje spaudoje pasirodo eilė straipsnių apie nesveikų reiškinių vokiečių kalboje vartojimą. Tai liečia žodžių darybą, sudaiktavardintus žodžius ir sutrumpinimus. Nors šie reiškiniai yra kritikuojami, jie yra paplitę, kadangi jie surišti su mokslo ir technikos išvystymu ir pagrįsti kalbos ekonomijos principu