Particular Transport Properties of NiFe2O4 Thin Films at High Temperatures

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 degrees C in different pO(2) 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 degrees C exhibit a Ni3+/Ni2+ ratio of 0.42 and show p-type conduction via hole hopping between Ni3+Ni2+.