Influence of aging on magnetic and transport properties of La0.5Sr0.5CoO3-δ films, prepared by laser evaporation

The magnetic and transport investigation of the La0.5Sr0.5CoO3-δ and La0.65Sr0.35CoO3 films prepared by laser pulse deposition are carried out. It is Shown that the long term (up to one year) aging of La0.5Sr0.5CoO3-δ results in a reduction of oxygen content by δ 0.08. The oxygen depletion is accompanied not only by the charge redistribution between the cobalt ions of different valency but by the topological disorder in the Co-O-Co conduction channel due to the accumulation of the oxygen vacancies. The increase of resistivity observed with decreasing temperature is described more precisely by the weak localization model than by the thermal activation mechanism of conductivity. In addition to the usual ferromagnetic transition at TC 240 K, the aged La0.5Sr0.5CoO2.92 film displays another transition at TM 50 K which is typical of the magnetic transitions in spin-glasses. The analysis of the ρ(T) data for the La0.5Sr0.5CoO3 and La0.65Sr0.35CoO3 films in the ferromagnetic state shows that, apart from the usual square term ∝ T2 the temperature-dependent resistivity involves an exponential term ∝ exp (- T0/T) which is responsible for by the occurrence of a spin gap in the conduction channel at low temperatures.