Magnetoresistance, temporal evolution, and relaxation of the electrical resistivity in the re-entrant semiconducting La0.80Ba0.20CoO3 perovskite

We report here a study on the electrical and magnetic properties of La1-xBa xCoO3 in the re-entrant semiconducting region (x = 0.20). We find that in t his material: (i) the insulator-metal-insulator sequence is unstable and ev olves toward a purely semiconducting behavior; the initial rho versus T cur ve can be reinstated upon appropriate annealing treatments; (ii) there are relaxation effects that can be seen by changing the polarity of the electro des; (iii) there is a negative magnetoresistance Delta rho/rho similar to 2 -3%, for a field as low as 9 kOe, especially at the metal-insulating transi tion temperatures; and (iv) there are important fluctuations in the electri cal resistivity. Taking into account these experimental observations, we ca n interpret this material as an inhomogeneous system where two thermodynami c phases, one semiconducting and the other metallic and ferromagnetic, coex ist, although they are crystallographically indistinguishable.