Thesis presented November 28, 2008

Abstract:
The growth of magnetic semiconductors with Curie temperature above room temperature is one of the major challenges of spintronics. In this manuscript, we describe our results on MBE grown Ge:Mn layers. We first investigate structural properties with transmission electron microscopy, electron and X-ray diffraction and EXAFS spectroscopy. We have observed that Mn segregation in GeMn thin films results in the formation of Mn-rich phases. For low growth temperature (~100°C), a spinodal decomposition occurs which leads to the growth of Mn-rich nanocolumns embedded in a germanium matrix. For temperature higher than 200°C, Ge₃Mn₅ clusters were evidenced. The magnetic properties of these phases were also studied. Superparamagnetism was observed in Mn-rich nanocolumns, and under precise growth conditions, ferromagnetism up to 400K was demonstrated. By combining GeMn nanocolumns with various materials (GaAs, Ge, MnO), we have evidence phenomena such as exchange bias or columns self-organization which are of great interest for spintronics devices. The results presented in this manuscript are very promising for spintronics since they open a way for spin injection into semiconductors such as Ge, Si, and GaAs.

Keywords:
semiconductor, ferromagnetism, DMS, nanotechnology, spintronics, germanium, manganese, GeMn, self-organization, self-assembling, MBE, SQUID, TEM

On-line thesis.