Enhancement of giant magnetoresistance effect in the Ruddlesden-Popper   phase Sr3Fe2-xCoxO7-d: Predominant role of oxygen nonstoichiometry and   magnetic phase separation
  The magnetic and magnetotransport properties of the Sr3Fe2-xCoxO7-d system (0.2 <= x <= 1.0) were systematically investigated. This oxide system exhibits a giant magnetoresistance (GMR) effect at low temperatures, reaching up to 80% in 7 T at 5 K. Ac-susceptibility measurements show that there exists a strong competition between ferromagnetic (F) and spin glass states, and the balance between these two magnetic states can be controlled by varying cobalt (x) and/or oxygen contents (d). Importantly, the MR effect is closely related to the magnetic property: the development of magnetic disordering leads to enhancement in the negative MR effect. It is suggested that the compound segregates into F clusters embedded in a non-F matrix, being a naturally occurring analog of the artificial granular-GMR materials, as in the doped perovskite cobaltites, La1-xSrxCoO3 (x < 0.18). 
T. Motohashi, B. Raveau, M. Hervieu, A. Maignan, V. Pralong, N.   Nguyen, and V. Caignaert