ABSTRACT
The magnetic properties of Ge:Mn thin films have been extensively investigated, especially some comprehensive studies [13, 34, 46, 49]. In the first report of Ge:Mn magnetic semiconductors in 2002, Park et al. [5] reported a Tc up to 116 K from a Ge:Mn thin film with 3.5% Mn and they demonstrated that the ferromagnetism was hole mediated. In contrast, Li et al. [35] observed two different ordering temperatures Tc and Tc* (Tc < Tc*) in the secondary-phasefree Ge:Mn thin films with 5% Mn. A Tc of 12 K was attributed to the global ferromagnetic ordering, while a Tc* of 120 K showed the onset of local ferromagnetism associated with isolated spin clusters, indicating that Ge:Mn has inferior properties to be a candidate for high-Tc DMSs. Although the debate about the origin of the ferromagnetism is still continuing, most results indicate that high-temperature ferromagnetism originates from Mn-rich precipitates. In a detailed study of magnetic properties of Ge:Mn thin films, Jaeger et al. [49] claimed that the Ge:Mn film exhibits a spin-glass-like behavior with two transition temperatures, Tf = 12 K and Tb = 250 K. Their origins were attributed to the blocking or freezing transitions of two different kinds of superparamagnetic precipitates (Mn-rich clusters and Mn5Ge3 precipitates), as shown in Fig. 19.24. Indeed, two transition temperatures were also observed, with Tb1 ~ 22 K being related to the coherent Mn-rich nanoclusters and Tb2 ~ 250 K being attributed to Mn5Ge3 precipitates, as shown in Fig. 19.25 for the tadpole samples. Similar results were also observed in the nanocolumn samples [44]. On the other hand, Devillers et al. [34] identified four different magnetic phases in their Ge:Mn thin films: diluted Mn in Ge matrix, low-Tc nanocolumns (Tc <= 170 K), high-Tc nanocolumns (Tc >= 400 K), and Mn5Ge3 precipitates (Tc ~ 300 K). In particular, they claimed the successful growth of Ge-rich Ge:Mn nanocolumns with Tc over 400 K. It is of interest to note that although the thin films were grown at 130°C with nominal 6% Mn doped, no magnetic precipitates, such as Mn5Ge3, were observed or indirectly detected. Unfortunately, the growth window for such high-Tc nanocolumns is fairly narrow, and normally a low density of Tc nanocolumns were observed [35, 44].
