**7. Conclusion**

In summary, MFM is a relatively new technique for imaging magnetization patterns with high resolution and minimal sample preparation. The technique is an offspring of AFM and employs a sharp magnetic tip attached to a flexible cantilever. The tip is placed close to the sample surface (from some nanometres to a few micrometres) and interacts with the stray field emanating from the sample. The image is formed by scanning the tip laterally with respect to the sample and measuring the force (or force gradient) as a function of position. The interaction strength is then determined by monitoring the motion of the cantilever using a sensor. Although a lot of effort has been done in order to get quantitative information, MFM is still predominantly a qualitative characterization technique. In the present work, MFM proved to be particularly suitable to study the magnetic properties of Si and Ge-based magnetic semiconductors. In this context, the technique is very efficient to detect magnetic activity in the form of vortices in sub-micrometre structures. As well, a combination of the MFM and SQUID techniques can be very convenient to probe the magnetic properties of microsized (or sub-microsized) isolated structures.

## **8. Acknowledgments**

The authors are indebted to Professor Antonio Ricardo Zanatta (Instituto de Física de São Carlos, Universidade de São Paulo, Brazil) for the support with the deposition and characterization of the Si and Ge samples. This work was financially supported by the Brazilian agencies FAPESP and CNPq under CEPOF/INOF and INEO.
