Spin wave stiffness and exchange stiffness of doped permalloy via ab initio calculations

The way doping affects the spin wave stiffness and the exchange stiffness of permalloy (Py) is investigated via ab initio calculations, using the Korringa-Kohn-Rostoker (KKR) Green function formalism. By considering various types of dopants of different nature (V, Gd, and Pt), we are able to draw general conclusions. To describe the trends of the stiffness with doping it is sufficient to account for the exchange coupling between nearest neighbors. The polarizability of the impurities is not important for the spin wave stiffness. Rather, the decisive factor is the hybridization between the impurity and the host states as reflected by changes in the Bloch spectral function. Our theoretical results agree well with earlier experiments.