Thesis presented January 27, 2011

Abstract:
The system of self-assembled Mn-rich GeMn nanocolumns embedded in a Mn-poor germanium matrix is a promising material for spintronics applications. Depending on the growth parameters, coherent GeMn nanocolumns, amorphous GeMn nanocolumns and/or Ge₃Mn₅ nanoclusters can be observed. In this manuscript, we report on the investigation on the electronic, magnetic and structural properties of the GeMn nanocolumns using synchrotron techniques. Measurements using grazing incidence x-ray scattering techniques in samples containing coherent nanocolumns, free from Ge₃Mn₅ precipitates, show some disorder in the nanocolumns. Reciprocal space maps are quantitatively explained by considering the scattering of the Ge matrix strained by the inclusion of the nanocolumns in the matrix and their correlations in position, without requiring the consideration of different additional phases. X-ray absorption spectroscopy and x-ray magnetic circular dichroism allow for the specific probing of the Mn magnetic properties in samples free of Ge₃Mn₅ clusters. The lineshapes of the XAS-XMCD spectra in the nanocolumns are found to be very similar to those in Ge₃Mn₅. The local magnetic moment on the Mn atom possess a small but non-zero orbital component and its total magnitude is much smaller (0.8 +/- 0.1 µB) than that in Ge₃Mn₅ (~2.6 µB) or than that expected for fully substitutional Mn atoms (~3 µB). This points to a different nature of the nanocolumns. The XAS-XMCD spectra have been calculated for several structural models, including simple defects and new crystalline phases, and critical parameters for the calculations have been identified. The best agreement is found for a new Ge₂Mn crystalline phase.

Keywords:
Self-organized nanostructures, Magnetic semi-conductors

On-line thesis.