All-electron calculations of crystalline and amorphous phases in magnetic phase change materials by KKR Green's function method

We perform large-scale density functional theory (DFT) calculations for crystalline and amorphous phases in magnetic phase change materials by all-electron full-potential screened Korringa-Kohn-Rostoker (KKR) Green’s function method. Here, we choose transition metals doped Ge₂Sb₂Te₅ (GST) systems as the typical cases. We consider large unit cells containing 1000 sites for the crystalline phase and 1365 sites for the amorphous phase to model the configurational and structural disorders in the magnetic phase change materials. Such large-scale DFT calculations are performed using the program KKRnano, where a massively parallel linear scaling all-electron algorithm is implemented. We investigate the electronic structures and distance dependent magnetic exchange coupling constants in the crystalline and amorphous phases. It is found, in the crystalline phase, that ferromagnetic states are favorable in the cases of V and Cr doping, due to the double exchange mechanism, whereas antiferromagnetic superexchange interactions appear to be dominant for Fe- and Mn-doped GST. In particular, the Cr doped GST shows strong ferromagnetic interaction and high Curie temperature for both the crystalline and amorphous phases.