Particular Transport Properties of NiFe₂O₄ Thin Films at High Temperatures

Abstract

NiFe2O4 (NFO) thin films were deposited on quartz substrates by rf magnetron sputtering, and the influence of the deposition conditions on their physic-chemical properties was studied. The films structure and the high temperature transport properties were analyzed as a function of the deposition temperature. The analysis of the total conductivity up to 800 °C in different pO2 containing atmospheres showed a distinct electronic behavior of the films with regard to the bulk NFO material. Indeed, the thin films exhibit p-type electronic conductivity, while the bulk material is known to be a prevailing n-type electronic conductor. This difference is ascribed to the dissimilar concentration of Ni3+ in the thin films, as revealed by XPS analysis at room temperature. The bulk material with a low concentration of Ni3+ (Ni3+/Ni2+ ratio of 0.20) shows the expected n-type electronic conduction via electron hopping between Fe3+–Fe2+. On the other hand, the NFO thin films annealed at 800 °C exhibit a Ni3+/Ni2+ ratio of 0.42 and show p-type conduction via hole hopping between Ni3+–Ni2+.