Resistive Avalanches in La1- xSrxCoO3-δ (x = 0, 0.3) Thin Films and Their Reversible Evolution by Tuning Lattice Oxygen Vacancies (δ)

Abstract

Strong correlations are often manifested by exotic electronic phases and phase transitions. LaCoO_3-δ (LCO) is a system that exhibits such strong electronic correlations with lattice-spin-charge-orbital degrees of freedom. Here, we show that mesoscopic oxygen-deficient LCO films show resistive avalanches of about 2 orders of magnitude due to the metal-insulator transition (MIT) of the film at about 372 K for the 25 W RF power-deposited LCO film on the Si/SiO₂ substrate. In bulk, this transition is otherwise gradual and occurs over a very large temperature range. In thin films of LCO, the oxygen deficiency (0 < δ < 0.5) is more easily reversibly tuned, resulting in avalanches. The avalanches disappear after vacuum annealing, and the films behave like normal insulators (δ ∼0.5) with Co²⁺ in charge ordering alternatively with Co³⁺. This oxidation state change induces spin state crossovers that result in a spin blockade in the insulating phase, while the conductivity arises from hole hopping among the allowed cobalt Co⁴⁺ ion spin states at high temperature. The chemical pressure (strain) of 30% Sr²⁺ doping at the La³⁺ site results in reduction in the avalanche magnitude as well as their retention in subsequent heating cycles. The charge nonstoichiometry arising due to Sr²⁺ doping is found to contribute toward hole doping (i.e., Co³⁺ oxidation to Co⁴⁺) and thereby the retention of the hole percolation pathway. This is also manifested in energies of crossover from the 3D variable range hopping (VRH) type transport observed in the temperature range of 300-425 K, while small polaron hopping (SPH) is observed in the temperature range of 600-725 K for LCO. On the other hand, Sr-doped LCO does not show any crossover and only the VRH type of transport. The strain due to Sr²⁺ doping refrains the lattice from complete conversion of δ going to 0.5, retaining the avalanches.