The origin of ferromagnetism in transition-metal doped Ge by the first principles calculations

Recently, Ge-based DMSs have attracted much attention due to many advantages for practical applications and consistencies with the general Si-SMOS technology. However, there has been great discussion about ferromagnetic mechanisms in Fe- and Mn-doped Ge-based DMSs. In this work, we investigate the electronic structures, structural stabilities, magnetic exchange coupling constants, and Curie temperature, and clarify origins of the ferromagnetism, by the density functional theory calculations. In both the (Ge,Fe) and (Ge,Mn) cases, the inhomogeneous distribution of the magnetic impurities plays an important role to determine the magnetic states. By the spinodal nano-decomposition, the Fe impurities in Ge gather together with keeping the diamond structure, so that the number of the first-nearest-neighbor Fe pairs with strong ferromagnetic interaction increases. Therefore, the Curie temperature drastically increases with the progress of the annealing. On the other hand, in (Ge,Mn) case, the other ferromagnetic ordered phases with different crystal structures tend to occur and might be an origin of high Curie temperature.