Electronic, magnetic, and magnetoresistance properties of the n=2 Ruddlesden-Popper phases Sr3Fe2-xCOxO7-δ (0.25≤x≤1.75)

A series of oxygen-deficient n = 2 Ruddlesden-Popper phases, Sr3Fe2-xCoxO7-delta (0.25less than or equal toxless than or equal to1.75), were prepared by solidstate reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for xgreater than or equal to0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at similar to 200 K, and a transition to a cluster-glass state is observed for all compositions below similar to 45 K. The temperature variation of resistivity for all the compounds shows variable-range 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 similar to -65% for x = 0.25) is observed for all phases. The magnetic susceptibility, Mossbauer and X-ray absorption near-edge 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 Sr3Fe2-xMnxO7-delta series, where d localization dominates the properties. (C)2002 Elsevier Science (USA).