Localized magnetic excitations of coupled impurities in a Heisenberg ferromagnet: Optical defect modes

AbstractA Green's function method is used to obtain the spectrum of excitations of two neighboring impurities in a semi-infinite ferromagnet. The equations of motion for Green's functions are determined in the framework of the Heisenberg model. The energies of non-resonant localized modes are calculated as a function of the interaction parameters for the exchange coupling between impurity–spin pairs, host–spin pairs, and impurity–host neighbors, as well as the effective field parameter at the impurity sites. With two impurities the system is less symmetric and has more localized modes when compared with a single impurity case