surface of a Ge0,9Mn0,1 thin film for Te= 100 °C (left) and for Te= 150 °C (right). The Mn-rich columns are seen standing. The insets show that they are crystalline on the left and amorphous on the right.
Ferromagnetic semiconductors are very attractive for spintronics: the spin polarization of carriers is strong and the magnetic properties can be controlled with an electric field. So far, the manganese-germanium alloy (GeMn) grown in thin films is the best candidate because of its Curie temperature of 400 K and its compatibility with silicon technology. We have successfully established a correlation between the magnetic anisotropy and the structural quality of the layers.
GeMn thin films are deposited by molecular beam epitaxy at low temperature (Te). These layers contain Mn-rich zones (40% Mn) that form nanocolumns. At Te=100°C, these nanocolumns are well crystallized and in epitaxy with the Ge substrate. At Te=150°C, they partially relax and become amorphous. An understanding of magnetic anisotropy is important as it determines the behavior of the layer when it is subject to a magnetic field.
Measuring the magnetic anisotropy is difficult as Mn, which carries the magnetic moment, is also present in low quantities in the Ge matrix. By coupling SQUID magnetometry and electronic paramagnetic resonance, we have shown that the structure of the nanocolumns determines the material anisotropy. When they are amorphous, the columnar aspect is predominant and the easy magnetization axis is perpendicular to the thin film. On the contrary, when they are crystalline, the easy axis is in-plane.
Further reading: Jain A et al., Applied Physics Letters 97 (2010) 202502
Maj : 17/02/2014 (922)