Competing magnetic interactions and large magnetoresistance effects in a layered iron(IV) oxide:: citrate-gel synthesis and properties of Sr3Fe1.8Co0.2O∼7

A large negative magnetoresistance (MR) effect at low temperatures (45% at 5 K and 9 T) has been observed for the iron(IV)-based Ruddlesden-Popper type phase Sr3Fe1.8Co0.2Osimilar to7 which was prepared by a citrate sol-gel technique with final high pressure oxygen annealing. Powder X-ray diffraction data show that the layered tetragonal crystal structure of the parent compound Sr3Fe2O7 is retained on Go-doping. Magnetic susceptibility and isothermic magnetization measurements suggest a cluster-glass-like magnetism with strong ferromagnetic and competing antiferromagnetic interactions. Magnetic and magnetoresistance properties of this semiconducting material are closely correlated. The magnetotransport behaviour of Sr3Fe1.8Co0.2Osimilar to7 is interpreted in terms of covalency-driven electron transfer processes between the e(g) orbitals of adjacent transition metal ions which stabilize ferromagnetic alignment of the spins of the more localized t(2g) electrons. (C) 2000 Elsevier Science Ltd. All rights reserved.