In the ferrates SrFeO3 and CaFeO3, spin and charge degrees of freedom play an intriguing role. Their detailed interplay results in various electronic and magnetic phases, for example as consequence of charge order. The ferrates are isoelectric to the Jahn-Teller distorted manganite system and exhibit also colossal magnetoresistance effects. But in contrast, the ferrates show a helical instead of a collinear spin structure. Oxygen doping has a dramatic effect on the electronic properties of the ferrates since the oxygen deficiencies order systematically, leading to different well defined crystallographic phase with different electronic properties, e.g. metal-insulator transitions or charge order. Remarkably, our elastic and inelastic neutron scattering experiments have revealed an almost universal magnetic behavior for all the different electronic phases. The spin wave dispersion is comprising upward- and downward-dispersing branches in the form of an hour glass. Such a dispersion is common to compounds with metallic and charge-ordered insulating ground states and closely resembles the extensively studied, universal dispersion of spin excitations in layered copper oxides such has high temperature superconductors. The helical spin arrangement is a consequence a competition between long range double-exchange and short range superexchange interactions. Our theoretical calculations were able to convincingly reproduce the helical spin structure and the experimentally obtained spin wave dispersion in the ferrates SrFeO3 and CaFeO3.
