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Micromagnetic study of self-organizes magnetic nanostructures

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Abstract (Summary)
In the present thesis the micromagnetic structure, as well as the magnetization reversal, of epitaxial MnAs films on GaAs substrates is studied. Over a wide temperature range, MnAs films show a coupled magneto-structural phase transition. As a result of the strain imposed by the substrate, a phase coexistence of the ferromagnetic ?-phase and the nonmagnetic ?-phase is observed. The two phases arrange themselves in a regular fashion and the ferromagnetic areas show a large variety of micromagnetic structures. The shape and distribution of the self-organized phase pattern is determined by the substrate orientation. For anisotropic substrates, a stripe pattern is found whereas isotropic substrates lead to an island structure with three-fold symmetry. In the focus of this work is the systematic investigation of the micromagnetic structure of anisotropically strained MnAs films on GaAs(001). The domain structure of MnAs films was investigated with MFM (magnetic force microscopy). Earlier investigations at room temperature revealed a meanderlike pattern, which is due to antiparallel domains separated by a 180? Bloch wall. The investigations have been expanded to a large variety of film thicknesses, as well as to different phase compositions which can be tuned by the temperature, revealing a multitude of MFM contrast patterns aside from the meander-like structure. So far, their micromagnetic origin has not been understood. To completely characterize the micromagnetic properties of the self-organized stripe structure, I expanded the temperature-controlled MFM setup by a variable magnetic field assembly using a permanent magnet. Now, the magnetic structure can be investigated with high lateral resolution as a function of applied magnetic field without undesired sample heating. This gives us access to the different magnetization reversal processes of MnAs films on a microscopic scale. A classification of the domain structure has been derived from MFM experiments. Since the MFM signal does not allow the unambiguous determination of the domain structure, I derived a simple model on the basis of uniaxial bar magnets that allows the calculation of the stray field - and thus the MFM contrast - for the classified domain types. The stray field calculations agree qualitatively with the measured contrast. Up to three basic domain types are found that are characterized by the number of subelements perpendicular to the ferromagnetic stripe. More complex domain structures can be explained by a combination of the basic domain types. Subsequent
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School:Oberlin College

School Location:USA - Ohio

Source Type:Master's Thesis

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