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 Sr3Fe1.8Co0.2O∼7 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 Co-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 Sr3Fe1.8Co0.2O∼7 is interpreted in terms of covalency-driven electron transfer processes between the eg orbitals of adjacent transition metal ions which stabilize ferromagnetic alignment of the spins of the more localized t2g electrons.