Optical absorption and transverse Kerr effect spectra, resistivity, and magnetoresistance of La 1Ϫx Sr x CoO 3 (xϭ0.15,0.25,0.35) films have been studied. The temperature dependences of the optical and magneto-optical properties of the films exhibit features which can be attributed to the transition of the Co 3ϩ ions from the low-spin state (Sϭ0) to the intermediate-spin state (Sϭ1) and to orbital ordering of the Co 3ϩ ions in the latter state. The evolution of the properties influenced by doping with Sr is interpreted on the basis of the phase separation model.
The transport and magnetic properties of as-deposited and long-aged La0.5Sr0.5CoO3−δ films prepared by pulsed laser deposition have been investigated. It was found that an oxygen deficiency in the aged film leads not only to a change in the Co3+:Co4+ ratio, but to the formation of a structural disorder in the Co–O–Co conduction channel due to the accumulated oxygen vacancies. In addition to the usual ferromagnetic transition at TC≃250 K, the second magnetic transition was observed at TM≈50 K, which is treated as a freezing of the spin-glass state provided by the weak disorder in the oxygen arrangement.
The magnetic and transport properties of La 1Ϫx Sr x CoO 3 films prepared by laser ablation have been investigated. It was found that a metal-insulator transition is observed at a Sr doping of xр0.25. The analysis of experimental results shows that the transport of carriers in the metallic state is governed by the electronelectron scattering, taking into account the origin of a small energy gap in the spectrum of electron excitations. In the insulating state the transport of carriers is governed by the Mott-like variable-range hopping. Possible mechanisms for the negative magnetoresistance observed for the films are also discussed.
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