Electronic Phase Separation in Pr sub 1 sub x (Ca, Sr) sub x MnO sub 3 subdelta and Tunneling Magnetoresistance in Sr sub 2 FeMoO sub 6

In this work, we approach two aspects of the physics of magnetic perovskites presenting colossal magnetoresistance (CMR). Firstly, we go deeply into the phase separation (PS) between the ferromagnetic (FM) metallic state and the antiferro-magnetic (AFM) charge ordered (CO) insulating state, in manganites of the type Pr sub 1 sub x (Ca,Sr) sub x MnO sub 3 subdelta. Secondly, and more briefly, we analyze the problem of the influence of the grain boundary insulating barriers on the tunneling magnetoresistance in the Sr sub 2 FeMoO sub 6 double perovskite.The use of different measurement techniques allowed us to make a complete characterization of the PS state in the compounds Pr sub 0.65Ca sub 0 sub . sub 3 sub 5 sub - sub y Sr sub y MnO sub 3 for 0 ) keeping constant the carrier concentration (x = 0.35). This material presents an evolution from a CO insulating phase for small (y =0), towards the FM metallic phase for large (y = 0.20). In