Physical properties of epitaxial La2NiO4+δ thin films

Epitaxial thin films of layered nickelates La2NiO4+δ were grown on single crystal substrates by metalorganic chemical vapour deposition with the (1 1 0) plane of the nickelate parallel to the surface of substrate. High resolution transmission electron microscopy (HRTEM) confirms the epitaxial relationship between film and substrate and reveals planar structural faults.Magnetization and resistivity of the films have been measured as a function of temperature. A surprising ferromagnetic behaviour is observed until 400 K which is the highest temperature to be reached; this ferromagnetic component does not come from the nickelate film but most probably from an iron oxide impurity which has been evidenced by X-ray energy dispersive spectroscopy. The temperature dependence of the resistivity, measured from room temperature down to 30 K, corresponds to a semiconductor. In order to examine the electrical conduction mechanism, data have been analysed according to different conduction models; the best description is obtained in the case of a variable range hopping mechanism; this form of conductivity is discussed in relation with the disorder of the structure observed by HRTEM.