57 research outputs found
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
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
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
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