Transport properties of La_1-xSr_xCoO_3-δ films (0.15 ≤ x ≤ 0.5)

The magnetic and transport properties of La_1-xSr_xCoO₃ films prepared by laser ablation have been investigated. The metal-insulator transition was observed in the films with decreasing Sr²⁺ concentration (if x ≤ 0.25). Analysis of the experimental results revealed that the transport of carriers in the metallic state is governed by electron-electron scattering if the nature of a small energy gap in the spectrum of electron excitations is taken into account. An oxygen deficiency in the aged La_0.5Sr_0.5CoO_3-δ film leads not only to a change in the Co³⁺/Co⁴⁺ ratio, but also to a structural disorder in the Co-O-Co conduction channel due to the accumulated oxygen vacancies. A growth of the resistivity with decreasing temperature for the aged La_0.5Sr_0.5CoO_3-δ film is explained in the framework of the Coulomb-interaction model. Analysis of ρ(T) data shows that the transport of carriers at high temperatures (above the temperature of magnetic transition) in the insulating state (x ≤ 0.25) is governed by the Mott-like variable-range hopping (VRH) rather than motion of the lattice polarons. However, the VRH model needs to be modified for compounds of this type. The ρ(T) dependence is fairly described based on the two parallel-resistor model in the low-temperature range, where one resistor corresponds to the ferromagnetic metal-cluster network and the other to the insulating matrix (IM). Two mechanisms of magnetoresistance (MR) are involved in the investigated films. In the metallic state (x > 0.25) the MR is provided by a change in the electron scattering rate, influenced by the magnetic field. This process is reflected in the MR temperature dependence as a small peak at the Curie temperature. In the insulating state (x ≤ 0.25) the phase separation into the ferromagnetic metallic clusters and the IM is formed and an applied magnetic field induces a growth of the ferromagnetic metallic phase that can be possibly described by the double-exchange mechanism.