Aging effect on the magnetic and transport properties of laser-deposited La0.5Sr0.5CoO3-δ films

Magnetic and transport studies are carried out on laser-deposited films of La0.5Sr0.5CoO3-delta and La0.65Sr0.35CoO3. It is shown that prolonged aging (for up to one year) of La0.5Sr0.5CoO3-delta films leads to a decrease of the oxygen concentration to a deficit deltaapproximate to0.08. The oxygen deficit that forms is accompanied not only by a redistribution of the charge between the cobalt ions of different valences but also by the formation of topological disorder in the Co-O-Co conducting channel as a result of the accumulation of oxygen vacancies. The observed growth of the resistivity with decreasing temperature is more accurately described in a model of weak localization of the carriers than by a thermally activated conduction mechanism. In addition to the usual ferromagnetic transition at T(C)approximate to240 K, in the "aged" La0.5Sr0.5CoO2.92 film an additional transition, typical of a magnetic transition in a spin glass, is observed at T(M)approximate to50 K. Analysis of the temperature behavior of the resistivity of La0.5Sr0.5CoO3 and La0.65Sr0.35CoO3 films found in the metallic state show that in addition to the quadratic term proportional to T-2, the temperature dependence of the resistivity contains an exponential term of the form proportional toexp(-T-0/T), which is due to the opening of a spin gap in the conducting channel at low temperatures. (C) 2002 American Institute of Physics.