Deposition of YBaCo4O7+δ thin films on (001)-SrTiO3 substrates by dc sputtering

The structure, magnetic and magnetotransport properties of YBaCo4O7+δ thin films deposited by high oxygen-pressure dc magnetron sputtering on (001)-oriented SrTiO3 substrates have been studied. The film growth was carried out at three different substrate temperatures (750 °C, 850 °C and 870 °C) in order to determine the influence of this parameter on the crystalline quality of the films deposited on SrTiO3 substrates. Within these limits, the growth quality, with respect to the preferential growth orientation, changed significantly improving notably with increasing substrate temperature. The enhanced intensity of (203), (220) and (110) reflections stemming from films grown at 850 °C suggests that the as-grown films are polycrystalline. In turn, the dependence of the resistivity on temperature shows a semiconducting-like behavior, without any distinguishable structure, in the temperature range measured (400-120 K). The analysis of the experimental data shows that the transport mechanism in the films is well described by using the Mott variable range hopping (VRH) conduction model. From the fitting procedure, a temperature scale T∗∼107 K is obtained which ended up being similar to that found in other transition metal oxides. The fact that the conductivity of oxygen-rich YBaCo4O7+δ samples features a VRH nature implies itinerant electron states with the apparent insulating behavior due to magnetic frustration. Interestingly, the YBaCo4O7+δ thin films show a positive magnetoresistance over the studied temperature range without evidence for transition to negative values as observed in ferromagnetic cobaltites. The origin of the positive magnetoresistance effect in the studied samples is difficult to explain by the known magnetoresistance models as the samples don't show evident magnetic transition in the entire temperature region from low temperature to room temperature. Nevertheless, the main features of the positive magnetoresistance seem to be explained by Zeeman splitting of the localized states that suppresses the spin dependent hopping paths in the presence of on-site Coulomb repulsion.