Magnetic impurities in spin-spiral multiferroic materials

We investigate the effect of magnetic impurities in the magnetic-spiral-induced multiferroic phases using an effective multiferroic Hamiltonian. We model the impurities as a two- level system and consider the regimes when the impurity spins relax both slowly and fast. Using realistic material parameters we study the effect of impurities on ferroelectricity for varying impurity strength. We find that when the impurity strength is weak the electric polarization is not affected. However as the impurity strength is increased the amplitude of the host spin components is reduced and the ferroelectricity suppressed. We also discuss the role of impurities on a realistic multiferroic spin Hamiltonian for the rare-earth- metal Mn perovskites.