During the last decade, the interest in spintronics has been growing up fast particularly when considering the physics of spin transport in a semiconductor. Indeed, very long spin relaxation and dephasing times as well as their control by electric fields are predicted in semiconducting nanostructures. In addition, ballistic spintronics devices were suggested, as initiated by the seminal work by Datta and Das on the gate-controlled spin transistor. Most of these new schemes require efficient spin injection in a semiconductor. Suitable materials for spin injection are ferromagnetic semiconductors (FMS) because most of them are predicted to be half-metals and they do not raise the problem of conductivity mismatch as in the case of ferromagnetic metals. At the SP2M, we have recently fabricated a new FMS compatible with silicon technology and exhibiting high Curie temperatures: Mn doped germanium.
Fig. 1: Top: TEM observations of a Ge0.89Mn0.11 film. (a) cross section and (b) plane view. Bottom: High resolution TEM pictures of Mn-rich nanocolumns grown in different conditions: (c) Ge0.99Mn0.01 grown at 100°C, (d) Ge0.94Mn0.06 grown at 130°C and (e) Ge0.89Mn0.11 grown at 150°C.
Up to now, semiconductor spintronics has been mainly based on diluted magnetic semiconductors (DMS), in which magnetic atoms randomly substitute the semiconductor atoms. DMS's containing secondary phases (metallic inclusions, semiconducting ferromagnetic phase or concentration modulation due to spinodal decomposition) were seldom studied and usually not well controlled. However, the presence of secondary phases or inhomogeneities is believed to increase the critical temperature since the magnetic atom concentration can be locally very high.
We have used low temperature Molecular Beam Epitaxy (MBE) in order to dope germanium films with Mn. This out-of-equilibrium technique allows increasing the Mn solubility in germanium. Co-evaporating Ge and Mn on Ge(001) substrates leads to the formation of self-assembled Mn-rich nanocolumns as a result of 2D spinodal decomposition (figure 1). These nanocolumns are observed in a wide range of growth temperatures (Tg=80°C to 180°C) and Mn concentrations (1% to 30%). Due to the very small size of nanocolumns, we have used a set of high resolution or spectroscopic techniques in order to derive their structural and magnetic properties. In agreement with TEM observations, X-ray diffraction performed at the ESRF confirmed the diamond-like structure of nanocolumns (with a sizeable structural disorder) grown at Tg<150°C despite the high Mn content (up to 30%).
Fig. 2: Top: Zero Field Cooled / Field Cooled (ZFC / FC) measurements performed at 0.015 T. (a) Ge0.89Mn0.11 grown at 100°C, (b) Ge0.94Mn0.06 grown at 130°C and (c) Ge0.89Mn0.11 grown at 150°C. Bottom: (d) Magnetoresistance and (e) Anomalous Hall Effect measurements performed on a high-TC sample.
For Tg<100°C, we observed very small columns fully strained on the Ge matrix and from SQUID measurements, we concluded that they are superparamagnetic with low-TC (<150 K) and weak magnetic anisotropy (figure 2). Close to Tg=130°C, pairs of dislocations at the column/matrix interface start to appear either along  or [1-10] directions. Nanocolumns are ferromagnetic exhibiting very high Curie temperatures (>400 K). EXAFS analysis performed on these columns showed their complex local structure with a building block in the form of Ge-3Mn tetrahedron (also observed in Ge3Mn5) in epitaxy on the diamond crystal. In parallel ab-initio calculations and magnetic simulations were devoted to the study of the local structure and magnetic properties of these nanocolumns by considering the stability of different Mn clusters in Ge supercells. Finally, for Tg>150°C, nanocolumns are amorphous and exhibit low TC and weak magnetic anisotropy. Magneto-transport measurements performed on high-TC samples were very promising (figure 2). All the (Ge,Mn) films grown on Ge(001) substrates are p-type. We found a very strong positive magnetoresistance (up to 7000% at 30 K and 9 T) as well as a strong anomalous Hall Effect demonstrating the influence of Mn-rich nanocolumns on hole spins. These results show that (Ge,Mn) films may be used in future spintronic applications
maj : 13-10-2011 (749)