We present evidence that electrical transport studies of epitaxial p-type GeMn thin films fabricated on highresistivity Ge substrates are severely influenced by parallel conduction through the substrate, related to the large intrinsic conductivity of Ge due to its small band gap. Anomalous Hall measurements and large magnetoresistance effects are completely understood by taking a dominating substrate contribution as well as the measurement geometry into account. It is shown that substrate conduction persists also for well-conducting, degenerate, p-type thin films, giving rise to an effective two-layer conduction scheme. Using n-type Ge substrates, parallel conduction through the substrate can be reduced for the p-type epilayers, as a consequence of the emerging pn-interface junction. GeMn thin films fabricated on these substrates exhibit a negligible magnetoresistance effect. Our study underlines the importance of a thorough characterization and understanding of possible substrate contributions for electrical transport studies of GeMn thin films.

The spin-polarized scanning tunneling microscopy (sp-STM) is demonstrated between a magnetized STM tip and a magnetic substrate at the solid/liquid interface under electrochemical conditions at room temperature. Thereby, sp-STM at the solid/liquid interface is accomplished using the differential magnetic mode established by Wulfhekel and Kirschner for investigations at the solid/vacuum interface. The implementation of the sp-STM setup into the electrochemical STM is an important part of this thesis. Special attention is paid to the magnetostriction effect of possible magnetic STM tips. As a model system extended Co islands are electrochemically grown on an Au(111) single crystal. An appropriate electrochemical environment is required to ensure metal Co islands on Au(111), since Co deposition occurs at low potentials where hydrogen evolution and dissolved oxygen contributes to electrochemical reactions at the substrate. After sample preparation extended Co islands on Au(111) up to four atomic layers in height are observed with bare Au(111) areas in between, which is an ideal sample surface for sp-STM at the solid/liquid interface showing ferromagnetic and diamagnetic areas. Sp-STM is employed in-situ at the solid/liquid interface to observe not only sample topography but also sample magnetization of the model system Co/Au (111)