Spin wave dispersion in helically spin ordered SrFeO3-δ and CaF3O3

One of the most interesting problems in condensed matter physics is the metal- insulator (Ml) transition driven by strong electron correlations. The cubic perovskites SrFe03, and CaFe03, are isoelectronic to the manganite system and exhibit colossal magneto resistance effects. But in contrast, the ferrates show a helical instead of a collinear spin structure. Furthermore, perfectly cubic SrFe03, shows no charge order and is metallic whereas pseudocubic CaFeO3 shows a MI-transition at the charge ordering transition of 290 K. Therefore, both compounds are right at the borderline between itinerant and strongly correlated systems. This results in a detailed interplay between competing long-range ferromagnetic and short- range antiferromagnetic interactions which leads to the formation of the helical spin structure. We have determined the static and dynamic spin properties by neutron scattering. The extracted parameters of the spin Hamiltonian are a big step towards the understanding of the mechanisms behind the helical spin order and the charge order in the ferrates and provide furthermore a perspective for the understanding of the cycloidal spin structure in multiferroic materials.