Magnetic and Transport Properties of Quasi Two-Dimensional Ferromagnet Sr3Fe2-xCoxO7-δ (x=0.2 and 0.5)

We have grown single crystals of layered perovskite oxides Sr3Fe2-xCoxO7-delta with x = 0.2 (delta = 0.11) and x = 0.5 (delta = 0.15), and investigated the anisotropic magnetic and transport properties. The x = 0.2 crystal undergoes a semiconductor-insulator transition around 40 K, below which the magnetization exhibits cluster-glass like behavior. On the other hand, the x = 0.5 crystal shows two-dimensional (2D) ferromagnetic (FM) behavior below similar to 150 K, and quasi metallic transport is observed in the FM ab-plane. The negative magnetoresistance (MR) is observed both parallel and perpendicular to the FM plane. A contribution of tunneling process, like the case of the isostructural 2D ferromagnet La2-2xSr1+2xMn2O7 with x = 0.3, might be present in the out-of-plane MR.