Electronic, Magnetic, and Magnetoresistance Properties of the n=2 Ruddlesden-Popper Phases Sr3Fe2-xCoxO7-DELTA (0.25

A series of oxygen-deficient n=2 Ruddlesden-Popper phases, Sr3Fe2-xCoxO7-delta (0.25< or =x< or =1.75), were prepared by solid-state reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for x> or =0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at ~200 K, and a transition to a cluster-glass state is observed for all compositions below ~45 K. The temperature variation of resistivity for all the compounds shows variablerange hopping behavior with two different localization energy scales: one for T<40 K and another for T>80 K. Large negative magnetoresistance (the largest MR ~-65% for x=0.25) is observed for all phases. The magnetic susceptibility, M?ssbauer and X-ray absorption nearedge spectroscopy data indicate that the formal oxidation state of Fe is close to 4+. The key role of d delocalization in the Sr3Fe2-xCoxO7-DELTA system is compared to the Sr3Fe2xMnxO7-DELTA series, where d localization dominates the properties.