Electronic, magnetic, and magnetoresistance properties of the n=2 Ruddlesden-Popper phases Sr₃Fe_2-xCo_xO_7-δ (0.25 ≤ × ≤ 1.75)

A series of oxygen-deficient n = 2 Ruddlesden-Popper phases, Sr₃Fe_2-xCo_xO_7-δ (0.25 ≤ × ≤ 1.75), were prepared by solid-state reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for x ≥ 0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at ∼200K, 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 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 ∼-65% for x = 0.25) is observed for all phases. The magnetic susceptibility, Mössbauer 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 Sr₃Fe_2-xCo_xO_7-δ system is compared to the Sr₃Fe_2-xMn_xO_7-δ series, where d localization dominates the properties.