First-principles study of magnetic interactions in 3d transition metal-doped phase-change materials

摘要

Recently, magnetic phase-change materials have been synthesized experimentally by doping with 3d transition metal impurities. Here, we investigate the electronic structure and the magnetic properties of the prototypical phase-change material Ge_2Sb_2Te_5 (GST) doped with V, Cr, Mn, and Fe by density functional calculations. Both the supercell method and the coherent potential approximation (CPA) are employed to describe this complex substitutionally disordered system. As regards the first approach, we consider a large unit cell containing 1000 sites to model the random distribution of the cations and of the impurities in doped cubic GST. Such a large-scale electronic structure calculation is performed using the program KKRnano, where the full potential screened Korringa-Kohn-Rostoker Green's function method is optimized by a massively parallel linear scaling (order-N) all-electron algorithm. Overall, the electronic structures and magnetic exchange coupling constants calculated by KKRnano agree quite well with the CPA results. We find 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. The ferromagnetic interaction is particularly strong in the case of Cr. As a result, high Curie temperatures close to room temperatures are obtained for large Cr concentrations of 15%.